Advances in Catalysis, Volume 33

ADVANCES IN CATALYSIS VOLUME 33

Advisory Board

M. BOUDART Stanford. California

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

G . A. SOMORJAI Berkeley, California

M. CALVIN Berkeley, California

A. OZAKI Tokyo. Japan

P. H. EMMEIT Portland, Oregon

G.-M. SCHWAB Munich, Germany

R. UGO Milan, Italy

ADVANCES IN CATALYSIS VOLUME 33

Edited by

D. D. ELEY The University Norringham, England

HERMANPINES Norrhwesrern University Evansron, Illinois

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

1985

ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers

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85868788

9 8 7 6 5 4 3 2 I

Contents CONTRIBUTORS .............................................................. PAULHUGHEMMETT,1900- 1985 . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

ix

Homogeneous Nickel-Catalyzed Olefin Hydrocyanation C. A. TOLMAN, R. J. MCKINNEY, W. C. SEIDEL, J. D. DRULINER, AND W. R. STEVENS

I.

II. 111. IV. V.

Introduction and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equilibria Involving Ni(0) Complexes ........................ ........... Dienes and HCN . . . . . , . . . . . . . . . . . . . . . . . . . . . . Olefin lsomerization Monoolefins and HC References .........................................................

2 6 14 22 25 45

Supported Chromium Catalysts for Ethylene Polymerization M. P. MCDANIEL

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

Introduction ........................................................ Hexavalent Cr/Silica . . . . ................ ........... Reduced Cr/Silica . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymerization over Cr/Silica . . . . The Porosity of the Support . . . . . ............... Modifications of CrlSilica . . . . . . . Chromium Oxide on Other Support ................_ Organochromium Catalysis . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48 48 54 59 70

76 87 92 96

Catalysis Controlled by the Constitution of Active Sites KEN-ICHITANAKA 1. 11.

Ill. IV. V.

Introduction . . . . . . . . Isomerization Reaction Hydrogen Exchange Re Hydrogenation Reaction . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V

99 104 128

137 156

157

vi

CONTENTS

Selective Oxidative Dehydrogenation of Butenes on Ferrite Catalysts HAROLDH. KUNCAND MAYFAIRC. KUNC

I. 11. 111.

IV .

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

. . . . . . . . . . . . . . . . 159

Densities of Oxidation Sites . . . . . . . . .

V.

VI. VII . VIII. IX. X. XI.

Kinetics and Mechanism ............................. Effect of Crystallite Size . . . . . . . . . . . . Other Structure-Sensitive Oxidation Reactions

. . . . . . . . . . . . . . . . 196

Conclusions . . . . . .

The Study of Aluminosilicate and Related Catalysts by HighResolution Solid-state NMR Spectroscopy I. M. THOMASAND J. KLINOWSKI 1.

IV. V.

VIII. IX.

Introduction . . . Silica-Alumina Gels . . . . . . . . . . . . . . . Derivatized Surfaces and "Immobilized" Homogeneous Catalysts . . . . . . . . . . . . 327 tal Catalysts by NMR without Utilizing High-Resolution ................................................. 331 re, and Mechanism in Heterogeneous Catalysis and in 333 Chemisorbed States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sheet Silicates and Their Pillared Variants . . . . . . . . . . . . . . . . . . . . . 335 Recent Trends in the Study of Catalytic Solids by . . . . . . . . . . 346

AUTHORINDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUBJECTINDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

375 391

Contributors Numbers in parentheses indicate the pages on which the authors' contributions begin.

J. D. DRULINER,Central Research and Development Department, E. 1. du Pont de Nemours and Company, lnc., Wilmington, Delaware 19898 (1) J . KLINOWSKI,Department of Physical Chemistry, University of Cambridge, Cambridge CB2 IEP, England (199) HAROLDH. KUNG,Chemical Engineering Department and the lpatieff Catalytic Laboratory, Northwestern University, Evanston, Illinois 60201 (159) MAYFAIRC . KUNG,Chemical Engineering Department and the lpatieff Catalytic Laboratory, Northwestern University, Evanston, Illinois 60201 ( 159) M . P. MCDANIEL,Phillips Research Center, Bartlesville, Oklahoma 74004 (47) R. J. MCKINNEY,Central Research and Development Department, E. 1. du Pont de Nemours and Company, lnc., Wilmington, Delaware 19898 (1) W . C. SEIDEL,Central Research and Development Department, E. 1. du Pont de Nemours and Company, lnc., Wilmington, Delaware 19898 (1) W . R. STEVENS,Petrochemicals Department, E. 1. du Pont de Nemours and Company, lnc., Wilmington, Delaware 19898 (1) KEN-ICHITANAKA, The Institute for Solid State Physics, The University of Tokyo, Roppongi, Tokyo 106, Japan (99) J . M . THOMAS,Department of Physical Chemistry, University of Cambridge, Cambridge CB2 lEP, England (199) C. A. TOLMAN,Central Research and Development Department, E. I . du Pont de Nemours and Company, Inc., Wilmington, Delaware 19898 (1)

vii

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Paul Hugh Emmett, 1900-1985 Catalysis has lost a great leader. For over 60 years the creative genius of Paul Hugh Emmett challenged and stimulated the catalysis community worldwide. The techniques he developed and the mechanistic studies he directed have been vitally important in transforming catalysis from an empirical art into a highly sophisticated science. Loved and admired by all who knew him, Paul Emmett is remembered as a pioneer whose guidance will be sorely missed. Born on September 22, 1900 in Portland, Oregon, Emmett graduated from the local Washington High School and received a B.S. degree in chemical engineering from Oregon Agricultural College (now Oregon State University) in 1922. His Ph.D. was earned in physical chemistry from the California Institute of Technology under the direction of Dr. Arthur F. Benton, who had been a student of Sir Hugh Taylor at Princeton. After a year of teaching chemistry at his a h a mater in Oregon, Emmett joined the Fixed Nitrogen Research Laboratory of the U.S. Department of Agriculture (USDA) in Washington, D.C., where he spent eleven of his most productive years. In 1937 he was appointed as the first chairman of the Department of Chemical Engineering at The Johns Hopkins University in nearby Baltimore. For 5 years he served on the National Research Council’s committee on contact catalysis and as a USDA consultant. During the early years of the Second World War, Emmett directed an important National Defense Research Committee project at Hopkins that involved the use of adsorbents in gas masks to remove poison gases. In 1943 he became a division chief in the Manhattan Project, dealing with enrichment by diffusion of uranium isotopes for use in nuclear weapons. From 1945 until his death he was a consultant to the Atomic Energy Commission on peacetime uses of atomic power. For the next eleven years (1944-1955), Emmett directed the Gulf Oil-sponsored Multiple Petroleum Fellowship at the Mellon Institute in Pittsburgh. In 1955 Emmett returned to Hopkins, but this time as the W. R. Grace Professor of Chemistry; there he remained until his retirement in 1971. His last 14 years were spent back in his beloved state of Oregon, where he held the title of Research Professor at the Portland State University. Dr. Emmett died on April 22, 1985. He is survived by his wife, Mrs. Pauline Pauling Emmett. ix

X

PAUL HUGH EMMETT, 1900-1985

Paul Emmett is best known for the leading role he played in developing, along with Steven Brunauer and Edward Teller, the BET theory for measuring the surface area of porous materials. This fundamental technique laid the foundation which ushered in the modem era of catalysis in the mid-1930s. A large fraction of Emmett’s research centered around iron catalysts and the application of both radioactive and stable isotopic tracers in catalysis. His studies of the iron-synthetic ammonia system led to conclusions that remain unchallenged to this day. At the Mellon Institute he applied I4C tracers to examine the behavior of intermediates in Fischer-Tropsch synthesis over iron catalysts. By adding small amounts of radioactively labeled compounds to the CO/H, synthesis gas mixtures, he was able to prove that some of these compounds (e.g., small alcohols) are involved in the initiation step of the chain growth process that leads to larger hydrocarbon products. It was during this era that his associates first placed a catalytic reactor into the carrier gas stream of a gas chromatograph and developed the “microcatalytic pulse reactor,” which is now a standard piece of equipment for mechanistic studies with labeled molecules. While at Mellon Institute Emmett began editing his comprehensive set of seven volumes called Catalysis, which he continued at Hopkins. Nor did catalytic cracking escape the probing attention of Paul Emmett. At Johns Hopkins his students used labeled molecules extensively to examine the nature of secondary reactions in the cracking of cetane over amorphous silicaalumina and crystalline zeolites. They demonstrated that small olefins (e.g., propylene) are incorporated extensively into higher-molecular-weightmolecules, especially aromatics, and are the primary source of coke formation on these catalysts. A member of the National Academy of Sciences, Paul Emmett received numerous honorary degrees, awards, and medals in the United States, Europe, and Japan. His name has been immortalized through the Paul H. Emmett Award in Fundamental Catalysis administered by the Catalysis Society of North America. With over 150 research publications during his lifetime, Emmett was for 10 years an associate editor of the Journal of the American Chemical Society. His membership in the ACS spanned over 60 years, and he served as a Councilor from the Pittsburgh section during the early 1950s. Emmett attended the very first Gordon Research Conference in 1931 and occupied a front row seat at each of the GRC Conferences on Catalysis until his death. Twice he served as chairman of that annual conference. One of the most notable attributes about Paul Emmett was his incredible memory. He was literally a walking encyclopedia of useful references from the chemical literature. This in-depth knowledge caused him to be in great demand as a consultant. Those of us who had the opportunity to study under the tutelage of this creative man will always remember and appreciate the personal interest he

PAUL HUGH EMMETT, 1900-1985

xi

took in our career development. While his death is a time of sadness, Emmett’s life was filled with a multitude of insightful innovations that have greatly expanded the horizons of science and have had a positive impact on all of mankind. JOE W. HIGHTOWER RICE UNIVERSITY HOUSTON,TEXAS

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ADVANCES IN CATALYSIS. VOLUME 33

Homogeneous Nickel-Catalyzed Olefin Hydrocyanation C. A . TOLMAN.* R . J. McKINNEY.* W . C. SEIDEL.* J . D . DRULINER.* AND W. R . STEVENS' 'Central Research and Development Department

and 'Petrochemicals Department E . I . du Pont de Nemours & Company. Inc . Wilrnington. Delaware

I . Introduction and Scope . . . . . . . . . . . . . . . . . . A . Old ADN Technology . . . . . . . . . . . . . . . . B. Early Attempts to Hydrocyanate Olefins . . . . . . . . . . C. The Current ADN Process . . . . . . . . . . . . . . D. Description of Semibatch. Pulse. and Continuous Reactors . . . . E. Scope . . . . . . . . . . . . . . . . . . . . . . I1. Equilibria Involving Ni(0) Complexes . . . . . . . . . . . . . A. NiL. Dissociation . . . . . . . . . . . . . . . . . . B. NiL, and Nitriles . . . . . . . . . . . . . . . . . . C. NIL. and Olefins . . . . . . . . . . . . . . . . . . D. NiL.andH+ . . . . . . . . . . . . . . . . . . . . E. NiL.andHCN . . . . . . . . . . . . . . . . . . . 111. Dienes and HCN . . . . . . . . . . . . . . . . . . . . A . Reaction of Butadiene . . . . . . . . . . . . . . . . . B. Isomerization of 2M3BN to 3PN . . . . . . . . . . . . . C. n-Allylic Ni Intermediates . . . . . . . . . . . . . . . D . Other Dienes . . . . . . . . . . . . . . . . . . . . IV . Olefin Isomerization . . . . . . . . . . . . . . . . . . . A . Butene Isomerization and n-Ally1 Formation from Dienes and HNiL; B. Isomerization of 3PN with HNiLg . . . . . . . . . . . . C. Isomerization of 3PN with NIL.. HCN. and Lewis Acids . . . . V. Monoolefins and HCN . . . . . . . . . . . . . . . . . . A . Unpromoted Hydrocyanations . . . . . . . . . . . . . B. Hydrocyanations Promoted with Lewis Acids . . . . . . . . C . Summary of the Mechanisms for Hydrocyanation of Pentenenitriles . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .

1

2 2 3 4 4 5 6 6 7

8 9 10 14 14 14 15 19 22 22 23 23 25 25 34

40 45

.

Copyrigh! Ci 1985 by Academic Press Inc. All rights of reproduction in any form reserved.

2

c. A.

TOLMAN

et al.

I. Introduction and Scope

A. OLDADN' TECHNOLOGY Adiponitrile (ADN) has been a molecule of considerable industrial importance ever since the development of Nylon 66 by du Pont during the 1930s. Adiponitrile is hydrogenated to hexamethylenediamine which in turn is condensed with adipic acid. Because of the large volume of Nylon 6,6 produced worldwide (6 billion lb/yr), it is not surprising that considerable time and resources have been dedicated to developing the most efficient process for the production of ADN. From a feedstock economics viewpoint, the addition of 2 mol hydrogen cyanide to butadiene to give ADN has always been very attractive. However, the technology for direct addition of HCN to butadiene was unknown until the late 1960s. Prior to that time, an indirect method was utilized. The indirect hydrocyanation of butadiene as practiced by du Pont ( I ) involved the electrolysis of sodium chloride, formation of sodium cyanide from HCN using the NaOH, chlorination of butadiene to give 1,4-dichlorobut-2-ene, chloride displacement with sodium cyanide, and subsequent hydrogenation, as indicated in Eqs. (1)-(5), with the net result of Eq. 6.

electrolysis

2NaCl+ 2H,O

+ ZHCN C,H, + CI, C4H,C12 + 2NaCN C4H6(CN), + H, 2NaOH

Net: C,H,

+ ZHCN

---+

CI,

+ 2NaOH + H,

2NaCN

+ 2H,O

C4H,CI, C,H,(CN),

+ 2NaCI

(1)

(2) (3) (4)

NCC,H,CN

(5)

NCC,HBCN

(6)

Though this process was used successfully for many years, the electrolysis of 2 mol NaCl to make 1 mol of ADN, and the corrosive nature of chlorine made the direct addition of HCN to butadiene highly desirable.

' Abbreviations: A, Lewis acid; ADN, adiponitrile; BD, butadiene; C2M2BN, cis-2-methyl-2butenenitrile; Cp, cyclopentadienyl; CZPN, cis-2-pentenenitrile; Cy, cyclohexyl; DCN, deuterium cyanide; DN, dinitrile; ESN, ethylsuccinonitrile; HCN, hydrogen cyanide; L, a phosphorus ligand; 2M2BN, 2-methyl-2-butenenitrile; 2M3BN, 2-methyl-3-butenenitrile; MGN, 2-methylglutaronitrile; Ph, phenyl; PN, pentenenitrile; 2PN, 2-pentenenitrile; 3PN, 3pentenenitrile; 4PN, 4-pentenenitrile; THF, tetrahydrofuran; TZPN, trans-2-pentenenitrile; T3PN, trans-3-pentenenitrile.

NICKEL-CATALYZED OLEFIN HYDROCYANATION

3

It should be noted that ADN is also synthesized commercially by electrolytic coupling of acrylonitrile (2).

B. EARLY ATTEMPTSTO HYDROCYANATE OLEFINS

The addition of hydrogen cyanide (HCN) to carbon-carbon double bonds activated by electron-withdrawing groups in the presence of a base as a catalyst (a variation of the Michael Reaction) has been known for a long time. Nitriles were also obtained by hydrocyanation of branched olefins, such as isobutylene and trimethylethylene, in vapor phase reactions; in particular the reactions over alumina (3)and cobalt-on-alumina ( 4 ) were reported in the late 1940s and early 1950s. Addition of HCN to conjugated dienes in the presence of cuprous salts (vapor and liquid phase) was reported as early as 1947 (5). The first example of homogeneously catalyzed olefin hydrocyanation was reported by Arthur et al. in 1954 (6). Unactivated monoolefins, as well as conjugated dienes, were hydrocyanated in the presence of dicobalt octacarbonyl. Hydrocyanation of monoolefins appeared to become more difficult as the chain length of the olefin increased. For example, under similar conditions, ethylene, propylene, and 1-butene gave > 65 % conversion to nitriles whereas 1-octene gave only 13 % conversion. Styrene gave > 50 % conversion to 2-phenylpropionitrile. 2-Butene, having an internal double bond, gave only 9 % conversion to 2-methylbutyronitrile; only branched nitriles were formed. The addition of HCN to conjugated olefins such as butadiene and isoprene gave primarily 1,Caddition products-results similar to the copper halide catalyzed reaction. Interestingly, nonconjugated dienes isomerized in situ to allow 1,Caddition. Although some dinitrile was observed in these cases, it was always a branched isomer; no adiponitrile was observed in the reaction of butadiene. The problem of terminal addition (anti-Markovnikov) of HCN to isolated unactivated double bonds was not solved until carbon monoxide-free, lowvalent transition metal complexes became available. During the mid 1960s, W.C. Drinkard allowed 1-hexene to react with HCN in the presence of tetrakis(triethylphosphite)nickel(O) and the resulting product mixture contained a small amount of the terminal addition product n-heptanenitrile, and Drinkard and Lindsey found that the reaction with 3-pentenenitrile produced ADN (7). Irreversible oxidation by excess HCN during batch reactions limited Ni(0) catalyst lifetime and so further work was undertaken to improve catalyst utility. It was reasoned that the addition of NaBH,, a reducing agent, might reduce Ni(I1) in situ back to the active zero-valent form. Addition of NaBH,

4

C. A.

TOLMAN

et al.

along with a very slow feed of the HCN (so-called semibatch feed) did result in a large improvement in catalyst activity and lifetime. However, NaBH, did not function by reducing oxidized nickel. Analysis of the reaction mixture showed that significant dinitrile production did not begin until NaBH, had disappeared; actually HCN, NaBH,, and 3-pentenenitrile were reacting to form trialkylboron compounds (8)!The promotional effect of Lewis acids in the hydrocyanation of monoolefins had been discovered. A number of Lewis acid cocatalysts were found (9) which improved catalyst activity, and the hydrocyanation of 3-pentenenitrile to produce adiponitrile began to appear to have commercial significance. The first plant utilizing this technology began operation in 1971. The thirty years du Pont had spent scouting research in hydrocyanation had finally paid off! C. THECURRENT ADN PROCESS

The current hydrocyanation process can be broken down into two major steps. In the first, HCN is added to butadiene in the presence of an NiL, catalyst to give 3-pentenenitrile (3PN) and 2-methyl-3-butenenitrile (2M3BN) [Eq. (7)]. Fortunately the branched 2M3BN may be isomerized to the linear 3PN isomer [Eq. (8)]. In the second step, a Lewis acid promoter is added to the NiL4 (L = a phosphorus ligand) catalyst to effect the double bond isomerization of 3PN to 4-pentenenitrile (4PN) concurrently with the HCN

+M

.-

m

C

N

+

(7) CN

(8)

-CN CN -CN-

CN

(9)

selective addition of HCN to 4PN [Eqs. (9) and (lo)]. By-products in the second step include 2-methylglutaronitrile (MGN), ethylsuccinonitrile (ESN), and 2-pentenenitrile (2PN) arising, respectively, from Markovnikov addition to 4PN, direct addition of HCN to 3PN, and isomerization of 3PN to its conjugated isomer which is not hydrocyanated.

D. DESCRIPTION OF SEMIBATCH, PULSE,AND CONTINUOUS REACTORS Whereas many nickel-catalyzed olefin hydrocyanation reactions may be run in the batch mode (i.e., all reagents charged to the vessel at the beginning of reaction), it is often preferable to feed one or more components in a

NICKEL-CATALYZED OLEFIN HYDROCYANATION

5

controlled manner. For this reason, three different types of reactor system have been utilized to gather the data described below: semibatch, pulse, and continuous. The semibatch reactor is the simplest. All reagents except the HCN are placed in a thermostated vessel (usually glass). HCN is then fed in a controlled manner by syringe pump as a pure liquid (or more usually as a solution). An even simpler method of adding HCN is vapor transfer; pure liquid HCN is maintained at 0°C in an ice bath and a controlled flow of nitrogen gas bubbled through it. The resulting vapor is about 35 % HCN and may be fed directly into the reaction mixture or more commonly just above the mixture (the HCN is adsorbed from the vapor very efficiently). The reaction may be followed thermally (exothermic reaction), by IR spectroscopy (nitrile bands), or by gas chromatography (GC). Most of the nonspectroscopic results described below were obtained in this manner and indeed most of the scouting and optimizations were carried out this way. However, because kinetic studies are very difficult if not impossible by this method, a pulse reactor system was developed. The pulse reactor method is similar to semibatch in that all the ingredients except HCN are placed in a small, well-mixed vessel in a thermostated bath. Very small amounts of HCN are then rapidly injected into the reaction mixture with vigorous mixing and the exotherm is monitored. Repeated pulses are made only after the reaction mixture has come back to temperature equilibrium with the bath. In this manner, kinetic information may be obtained. Whereas much mechanistic information can be obtained by one of the above methods, any practical applications must be demonstrated under conditions similar to process operation, i.e., continuous flow. Small glass reactors which allow controlled addition of reagents by syringe pump and continuous removal and monitoring (IR spectroscopy) of product mixture have been developed. Much of the information obtained from semibatch operation has been reproduced under these continuous flow conditions.

In this article, we will discuss the chemistry behind the du Pont adiponitrile process from a mechanistic viewpoint (10). It is not intended to be a comprehensive review of the hydrocyanation literature. We will restrict ourselves rather to homogeneous nickel-catalyzed hydrocyanation of olefins and will depend primarily on du Pont studies. Reviews which explore hydrocyanation in a more general way include those of Brown (11), Hubert and Puentes (12), and James (13). A general review of low-valent organonickel chemistry has been published by Jolly and Wilke (14).

6

c. A.

TOLMAN

et al.

Before discussing hydrocyanation chemistry we will explore the interaction of zero-valent nickel phosphite complexes with various independent components of the catalytic system. Then, in turn, we will examine the catalyzed addition of HCN to butadiene, the isomerization of olefins, and the addition of HCN to monoolefins. Finally, a summary of the mechanism as it is now understood will be presented.

II. Equilibria Involving Nickel(0) Complexes

A. NiL, DISSOCIATION

The development of the adiponitrile process has had considerable impact on the process of organometallic chemistry. The discovery that certain zerovalent nickel complexes catalyze the hydrocyanation of butadiene (7)led to extensive studies on the formation and reactions of NiL, complexes. I n particular, a detailed understanding of the solution behavior of tertiary phosphine and phosphite complexes of nickel and their substitutional chemistry was developed at an early stage after it was discovered that the ability of phosphorus ligands to compete for coordination to Ni(0) was dominated by ligand size. This led to a heightened awareness of the general importance of steric effects in organometallic chemistry and Tolman and coworkers (15-20) quantified the steric and electronic factors which affect the reactivity of NiL, complexes for a broad variety of phosphorus ligands; steric factors of a ligand L were defined by cone angle whereas electronic factors were measured by the change in carbonyl vibrational frequency (vco) in Ni(CO), L. Whereas electronic factors contribute to the substitutional reactivity of NiL, complexes, the strengths of the nickel-phosphorus bonds [which range between 32 and 39 kcal/mol in NiL, complexes (ZO)] are dominated by steric effects. For example, in the complexes NiEPPh,], (Ph = phenyl) and Ni[P(O-o-tolyl),], the phosphorus ligands are electronically very different but sterically similar [cone angles of 145" and 141" for PPh, and P(0-o-tolyl), , respectively] and both show extensive ligand dissociation in solution (18). While ligand exchange in the phosphine complex is so rapid that no "P resonance can be observed in the NMR spectrum until one gets to low temperatures (21), the phosphite complex spectrum shows distinct signals for NiL,, NiL,, and L even at room temperature and above (17). The complex Ni[P(OEt),],, with a smaller ligand cone angle of 109", is not dissociated to any detectable extent even in highly dilute solutions at 70°C (Table I). Virtually all substitution reactions of NiL, complexes involve prior

7

NICKEL-CATALYZED OLEFIN HYDROCYANATION

TABLE I Ligand Dissociation from NIL, Complexes""

L

Kd

AHd (kcal/mol)

0 (degrees)

13 24 23

145 141 130 128 109

> 10 4.0 x lo-* 2.7 x 6x < lo-'' at 70°C

PPh, P(O-o-tolyl), P(0-i-Pr), P(0-p-tolyl), P(OEt),

' NiL,

(M)

Kd

* Ref. 18.-

NiL,

L

+ L, in benzene at 25°C.

Not measurable at 25°C.

dissociation of ligand to give a 16-electron complex, followed by association of the incoming ligand, as indicated in Eqs. (11) and (12). NiL, NiL,

+L

-

NiL,

+L

NiL3L

(11) (12)

Rate constants measured for Eq. (11) in benzene are 1.8 x lo-, sec-' for L = P(O-p-tolyl), and 0.8 x sec-' for L = P(O-p-C,H,Cl), at 25°C (22) and 4.9 x lo-, for L = P(OEt), at 36°C (23). Activation energies range between 26 and 29 kcal/mol. The fact that the activation energy for ligand dissociation from Ni[P(O-p-tolyl),], is only slightly larger than the enthalpy of dissociation (Table I) indicates that the activation energy for ligand recombination is small. The observation that dissociation of ligand is required to induce reactivity, in this and many of the cases described below, led to Tolman's proposal that most catalytic processes occur through discreet 16- and 18-electron intermediates (24). This proposal has remained a cornerstone for the study of homogeneous catalysis.

B. NiL,

AND

NITRILES

A variety of ligands other than phosphites are of course also important in hydrocyanation. The isolation of the 16-electroncomplex Ni[P(O-o-tolyl),], (17) provided a remarkable opportunity for the study of how various components of the catalytic system interact with nickel. The addition of nitriles to this complex led to the observation of the first nitrile complex of zero-valent nickel (25). When the phosphite ligands are very bulky, as in the case of P(0-o-tolyl),, a nitrile complex of formula (RCN)NiL, is formed nearly quantitatively even in the presence of excess L, and is identified easily

8

c. A.

TOLMAN

et al.

by spectroscopic means, though not readily isolable. The K for Eq. (12) with L' = CH,CN is 230 M - ' ) at 25°C (25). With less sterically demanding ligands like P(0-p-tolyl), much less of the nitrile complex is formed even when the nitrile is in vast excess; very dilute solutions of NiL, (0.005 M in acetonitrile) are needed to detect significant concentrations of nitrile complex at by 31P NMR; K for Eq. (13) with L = P(0-p-tolyl), is only 1.2 x 55°C. CH,CN

(CH3CN)NiL3 + L

+ NiL4 C. NiL,

AND

(13)

OLEFINS

Olefin complexes of the formula (olefin)NiL, have been isolated and characterized when L is P(0-o-tolyl), (2631), PPh,, (32) PCy, (Cy = cyclohexyl) (33), various other bulky phosphorus ligands (32, 34), or tbutylisocyanide (35). Single crystal X-ray structures have been determined for (C,H,)Ni[P(O-o-tolyl),], and (CH,=CHCN)Ni[P(O-o-tolyl),], (30). It is interesting to note that the isolable (or spectroscopically detectable) olefin complexes are generally 16-electron complexes and as such need not dissociate an additional ligand for further reactivity. It appears that substitution in these 16-electron complexes occurs through associative pathways (27). TABLE I1 Equilibrium ConstantP for Cyanoolefin Reactionsb with Ni[P(O-0-tolyl),], in Benzene at 25°C Cyanoolefin

K

Acrylonitrile ClH, CZPN T2PN 3-butenenitrile 2M3BN 4PN 5-hexenenitrile I-hexene T3PN CH,CN

4.0 x 104 250 17 17 10 6.0 3.6 2.2 0.5 1.7 x

c

435 c c c c c

190 230

Ref. 36.

* olefin + NiL, nitrile + NIL,

K

-

Not determined.

KN

(olefin)NiL, (nitrile)NiL,

+L

9

NICKEL-CATALYZED OLEFIN HYDROCYANATION

The stability of the olefin complexes seems to be determined by the steric and electronic characters of both the phosphorus ligand and the olefin (22). For example, ethylene complexes have only been isolated for the cases with sterically large ligands such as P(0-o-tolyl), and PPh,; however, maleic anhydride forms a stable isolable complex with the smaller P(0-p-tolyl), ligand. The nickel-ethylene bond strength is estimated to be 39 kcal/mol based on values of 36 kcal/mol for 1-hexene and 42 kcal/mol for acrylonitrile [when L = P(0-o-tolyl),] (22). Studies of equilibria in which cyanoolefins are allowed to react with Ni[P(O-o-tolyl),], have shown (36) that olefin coordination of 4PN is preferred over 3PN by a factor of 200, while olefin coordination of 2PN is preferred over 3PN by a factor of 1OOO. (Note the decreasing K for olefin as n complex formation in Table I1 in the series CH,=CH(CH,),CN increases from 0 to 3: 4 x lo4, 10,3.6, and 2.2. A value of0.5 can be estimated for n + 00, based on the K for 1-hexene.) The relative amount of nitrilecoordinated nickel in solutions of cyanoolefins increases with added free phosphorus ligand, because of the different stoichiometries in Eqs. (14) and (15).

-

+ NiL, nitrile + NiL, olefin

-

D. NiL,

(olefin)NiL, (nitrile)NiL,

AND

+L

(14) (15)

Ht

Hydrogen cyanide is a weak acid (pK, = 9) that, in the presence of Lewis acids, can become considerably stronger. The reaction of Ni(0) complexes with strong acids having weakly coordinating anions was first reported in 1969- 1970 (37-39). Tolman (40)studied the formation and decomposition of HNi[P(OEt),]f and found that decomposition resulted through ligand dissociation to give a spectroscopically unobservable HNiLi intermediate which was further attacked by acid. Addition of free ligand strongly inhibited the decomposition path but had no effect on the formation rate. HNi[P(O-p-tolyl),] i has been observed spectroscopically at low temperature (41). Perhaps one of the most important findings was that protonation of NiL, significantly labilizes ligand dissociation. The activation enthalpy of 17 kcal/mol for ligand dissociation from HNiLf [L = P(OEt),] (42) may be compared with a value of 27 kcal/mol for ligand dissociation from the corresponding NiL, complex (23). Protonation causes a dissociation rate increase of > lo7 at 25"C! HNi[P(OEt),]a appears in the proton NMR as a quintet (.IpH= 26.5 Hz at t 24.3) and shows a single type of phosphorus in its "P spectrum (at 135 ppm downfield of H3P04) (40). The most stable structure is probably the

c. A.

10 4

I

TOLMAN et

I

I

al. 1

I

-

l-

2070 2000 2090 Vco(A1) OF Ni(CO),L (crn-l) FIG.1. Equilibrium constants for protonation of NiL, complexes in CH,OH at 0°C.

2060

trigonal bipyramid 1; the equivalence of the phosphorus ligands is a consequence of the rapid intramolecular ligand exchange common in HML, complexes (43, 44).

1

The equilibrium constant of reaction (16) depends on the electronic character of the ligand as shown in Fig. 1. More electronegative ligands [as indicated by increasing vco from Ni(CO),L] give smaller values of log K. NIL,

+ H+

HNiL:

(16)

P(OCH,CI,),, with a vco of 2092 cm-', showed no measurable hydride formation in 1 M H,SO,, implying log K < - 1 (45). E. NiL, 1.

AND

HCN

Without Lewis Acids

Druliner et al. (46)studied the addition of hydrogen cyanide to a variety of NiL, complexes and found that both electronic and steric factors are important in the stability of the HNiL,CN complexes (Table 111). For example, Ni[P(O-o-tolyl),], and Ni[PEtJ4 react immediately and quantitatively with HCN to give HNiL, CN, whereas the equilibrium position of Eq. (17) remains far to the left with Ni[P(O-p-tolyl),],; the equilibrium constant in the latter case has been estimated to be 4 x from visible/UV spectra in benzene at 25°C (47). The corresponding K for L = P(0-o-tolyl),,

11

NICKEL-CATALYZED OLEFIN HYDROCYANATION

TABLE Ill Equilibrium constant^",^ f o r H N i L , C N Formation

PPh(OEt), P(OEt), P(OCH2CH2C1)3 P(0-o-tolyl), P(0-p-tolyl),

0.03 0.005 o.oO01

2113 2120 2130 2126

3 x 1044 4 x 10-4'

Taken from ref. 45 except for L

=

P(0-p-tolyl), from ref.

46. NiL, + HCN HNiL,CN 25°C. ' In benzene. Estimated as described in text. Not detected in the IR.

+ L, in CH,CI,

at

assuming that the increase in K (Table 111) depends only on the difference in NiL, dissociation constants K , (Table I), can be estimated to be 3 x lo4, but is too large to measure directly. NiL,

+ HCN

HNiL,CN

+L

(17)

With sufficiently bulky ligands (or with phosphines basic enough to be protonated by HCN), ligand dissociation from HNiL,CN occurs and the 16-electron HNiL,CN complexes can be observed in solution as triplet hydride resonances (46). In the IR region the CN stretching frequencies are 10-30 cm-' higher in the HNiL,CN than in the corresponding HNiL,CN complexes. In nonpolar solvents such as toluene, the rate of reaction of HCN with NiL, complexes is controlled by the rate of dissociation of L from NiL,; however, in polar solvents such as methanol, the ionic pathway illustrated in Eqs. (1 8)-(20) contributes, and can greatly increase the rate of reaction. Hydride resonances of both HNiLa and HNiL,CN can be observed at high field when HCN is added to Ni[P(OEt),], (46). NiL4

HNIL;

2.

+ HCN

HNiLiCN-

(18)

HNiL:

HNiLi + L

+ CN-

HNiL,CN

(19) (20)

F=====

With Lewis Acids

Lewis acids (A) can affect both the rate of formation of hydridocyanide complexes and their stability, by increasing the acidity of HCN and enhancing the ionic pathway, as well as by coordinating as shown in Eq. (21). The

c. A.

12

TOLMAN et

al. H

H

A

2

3

structures of 2 and 3 have been established with the aid of proton and phosphorus NMR, and IR spectroscopy. In the case where L = P(O-o-tolyl), and A = BPh,, the hydride and ,'P chemical shifts (7 25.4 and 6 118.7) are essentially the same for 2 and 3; however, P-H coupling in the hydride quartet (or "P doublet) decreases on BPh, coordination from 36 to 33 Hz. The frequency of vCN in the IR increases from 2128 to 2184 cm-' (47); a similar increase in vCN occurs for CH,CN upon complexation with BPh, (2254 to 2344 cm- '). HNi[P(O-o-tolyl),],CN-BPh, has been isolated and fully characterized. That the carbon of the cyano group is bound to nickel in HNiL,CN-A complexes is convincingly supported by the 13C-H cou(Fig. pling observed in the proton spectrum of HNi[P(OEt),],CN-ZnC1, 2) when 55 % ',C-enriched HCN was mixed with Ni[P(OEt),], and ZnC1, in tetrahydrofuran (THF). The cationic hydride-without C-H coupling-appears at lower field. IR bands at 2131 and 2165 cm-' in similar solutions made with unlabeled HCN are assigned to vCN in a cyanide ion-ZnC1, complex and HNi[P(OEt),], CN-ZnCl,, respectively (48). Trigonal bipyramidal structures for 2 and 3 are consistent with the spectroscopic data and with the structure of the isoelectronic HCo(PPh,),N, determined by X-ray diffraction (49). Bonding of Lewis acids to the nitrogen lone pair is supported by the crystal structure of CpFe(CO),CN-BPh, (Cp = cyclopentadienyl) (50). Lewis acids differ markedly in their ability to coordinate to a given HNiL,CN complex. While BPh,, B(p-tolyl),, and B(CH, Ph), all have equilibrium constants for reaction (21) (L = P(0-o-tolyl),) that are too large HNiL3CN

+A

(21)

HNiL3CN-A

to measure directly, B(o-tolyl), and BCy, react only partially, leaving a substantial amount of free HNiL,CN. This can be seen in the IR spectra (taken soon after mixing in order to avoid decomposition) shown in Fig. 3. From the fact that only about half of the HNiL,CN reacted with a 1 : 1 ratio of B(o-tolyl), :Ni, an association constant of 10 M - is estimated. Bands at 2086, 2130, and 2184 cm-' are assigned to vCN in HCN, HNiL,CN and HNiL,CN-A, respectively. The band at 2070 cm-' is observed only in the presence of excess HCN, and disappears when BPh, is added in a 1 : 1 A: Ni ratio; it is believed to be due to a hydrogen bonded HNiL,CN HCN complex.

-

a

'

NICKEL-CATALYZED OLEFIN HYDROCYANATION

13

H L$L L L ‘C3L

c

!

L

LA

1

a HI~CN

b 45% H%N 55% H”CN

8 -14

-15

-16

-17

Fic. 2. ‘H-NMR spectra of 0.8 M Ni[P(OEt),], and 0.8 M ZnCI, in THF with (a) 1.4 M H1,CN, or (b) 1.4 M H12CN/H13CN (45,55). (6 in ppm from TMS.) Used by permission of the American Chemical Society.

FIG.3. IR spectra of Ni[P(O-o-tolyl),],, HCN, and BR, in 1 : 1 : 1 ratio in toluene

[L = P(0-o-tolyl),, 4 = phenyl]. Used by permission of the New York Academy of Sciences (10).

c. A.TOLMAN et al.

14

The borates B(OPh), and B(0-o-tolyl), are bound much more weakly than the corresponding boranes. Their weaker Lewis acidity is reflected in low values of vCN of about 2135 cm-' in the HNi[P(O-o-tolyl),],CN-A complexes.

111.

Dienes and HCN

A. REACTIONOF BUTADIENE Hydrogen cyanide smoothly adds to butadiene (BD) in the presence of zero-valent nickel catalysts to give (3PN) and (2M3BN) [1,4- and 1,2addition products, respectively, Eq. (7)]. A variety of Ni[P(OR),], (R = alkyl or aryl) complexes are suitable as catalysts. The reaction may be carried out neat or in a variety of aromatic or nitrile solvents at temperatures from 50-120°C. Whereas in many olefin hydrocyanations it is desirable to keep the HCN concentration very low to protect the nickel from degradation, with butadiene HCN may be added batchwise as long as the HCN concentration is kept near the butadiene concentration. In the case of batch reactions one must be cautious because of possible temperature rises of 50°C or more over a period of a few minutes. Under typical batch conditions, when Ni[P(OEt),],, butadiene, and HCN are allowed to react in a ratio of 0.03: 1.0: 1.0 at 100°C for 8 hr, a 65% conversion to 3PN and 2M3BN (1.5: 1) is observed (7). In the absence of Lewis acids, further hydrocyanation of the monoolefin products does not readily occur. However, the addition of a Lewis acid cocatalyst allows pentenenitriles (PNs) to be hydrocyanated to dinitriles. When BD and 4PN are hydrocyanated together with Ni[P(O-p-tolyl),], and ZnC1, at 80"C, BD hydrocyanates 20 times faster than 4PN.

B. ISOMERIZATION OF 2M3BN TO 3PN In a practical sense, the formation of 2M3BN is undesirable because its direct hydrocyanation cannot give ADN. It is fortunate that 2M3BN can be isomerized to 3PN. Isomerization with a variety of NiL4 catalysts takes place over a temperature range of 90-130°C and is facilitated by the addition of a Lewis acid, e.g., ZnC1,. It is apparent that initial produce selectivity is kinetically controlled; the isomerization of one to the other in the presence of Lewis acids produces a 3PN/2M3BN mixture approaching the thermodynamic ratio of 93 :7. When 2M3BN, deuterium labeled on the methyl group, is isomerized in the presence of Ni[P(O-p-tolyl),], and ZnC1, at 1 10°C, the deuterium label is found in both the methyl and methylene groups of the resulting 3PN (48).

NICKEL-CATALYZED OLEFIN HYDROCYANATION

15

This result coupled with the observation of small amounts of free butadiene in the reaction mixture supports a mechanism involving dehydrocyanation of 2M3BN back to butadiene and HCN [Eq. (22)], though decomplexation of butadiene is not required. An alternative mechanism, involving allylic CN transfer [Eq. (23)], may play a role in the isomerization, but cannot by itself account for the deuterium scrambling.

LHNiLnCN]

CN

C. R-ALLYLICNICKEL INTERMEDIATES The reaction of butadiene, HCN, and Ni(0) compounds to give P N s and 2M3BN, as well as the isomerization of the undesired branched product to the linear one, proceeds through relatively stable n-allylnickel cyanide intermediates. One set of hydrocyanation experiments is particularly instructive and will be described in some detail. A solution of 0.5 M Ni[P(OEt),], and 0.6 M HCN in CH,Cl, gradually turned yellow with the appearance of a new IR band at 2120 cm-', assigned to vCN of the HNiL,CN complex. The ,'P-NMR spectrum showed that about 15 % of the nickel was present in the form of the hydridocyanide, the rest remaining as NiL,. With the addition of butadiene to a concentration of 0.5 M the solution turned orange, then deep red-orange in a period of 4-5 hr at 35°C; optical spectra showed a new absorption maximum at 360 nm whose intensity increased and then decreased with time. IR spectra run on aliquots of the solution at various times showed that the 2120 cm-' band was rapidly replaced by a new one at 2110 cm-', whose intensity increased to a maximum in -5 hr, then decreased, as shown in Fig. 4. (Concentrations were calculated from IR extinction coefficients determined from solutions of known concentrations, or using material balances.) It is clear from the 1 : 1 disappearance of HCN and BD that the intermediate contains the elements of these molecules in a 1 : 1 ratio. The sigmoid shape of the product curve is consistent with product formation from the 21 10 cm-' intermediate, while a mass balance shows that all of the HCN and BD not in products or starting materials is in the intermediate. Except for the first and last 15% of the reaction, the rate is nearly zero order in both

16

c. A.

TOLMAN et

al.

Time at 35'C (hr)

FIG.4. Hydrocyanation of 0.5 M butadiene with 0.5 M Ni[P(OEt),], and 0.6 M HCN in CH2C12. Used by permission of the New York Academy of Sciences (10).

[HCN] and [BD], indicating that collapse of the intermediate, rather than its formation, is rate determining. The fact that the ratio of HCN : BD consumed does not rise above 1 : 1 as long as BD is still present indicates that the intermediate for the addition of the first HCN is much more stable than the intermediate(s) for the second. This is understandable if the intermediate for the first addition is a trihapto n-ally1 complex, while that for the second is a monohapto a-alkyl. That solutions of the intermediate contain a syn- 1methyl-n-ally1 group was shown in a parallel proton NMR experiment (in CD,Cl,). A spectrum taken after -3 hr at 35°C showed the characteristic methyl of 4 as a doublet at 8.45 7 with a coupling to the neighboring allylic H of 6 Hz.[The same resonance in the related complex 5 appears as a doublet at 8.40 7 with J = 6.5 Hz (42).] The proton spectrum was quite complex, showing resonances of free HCN, BD, trans-3-pentenenitrile (T3PN), and 2M3BN-the latter two in a ratio of about 2.5 : 1, indicating preferential coupling of the CN to the unsubstituted end of the n-ally1 group. As in the nally1 solutions prepared from Ni[P(OEt),], and strong acids, there were only two types of phosphite methyl in the spectrum-the stronger one at 8.86 7 (J = 7) due to the NIL,, complex, and the weaker one at 8.75 7 due to all other L's in rapid exchange. This interpretation was confirmed by 31P-NMR experiments. Thus, the value of rn in 4 could not be determined, but is probably 2, which would give the nickel 18 valence electrons. An 18-electron complex is suggested by the visible/UV absorption band at 360 nm, which can be compared to 370 nm in the red complex 5 with n = 3 (42). The yellow 16-electron complex 5 with n = 2 shows only a tailing absorption in the visible.

NICKEL-CATALYZED OLEFIN HYDROCYANATION

17

When an experiment like that shown in Fig. 4 was carried out at 50"C, a similar behavior of the intermediate was observed, except that both formation and decay were six times faster. Experiments were also followed at 50°C in which 0.5 M excess P(OEt), was added initially, or when the intermediate n-ally1 complex was at a maximum. In the latter case, the rate of disappearance of the intermediate was essentially the same as in the control, consistent with rate-determining reductive elimination in 4 with m = 2; the rate of consumption of HCN and BD decreased abruptly, however, when excess L was added. In the experiment with L added initially, the overall rate of product formation was only about half that of the control; the smaller maximum concentration of intermediate is consistent with a greater inhibiting effect of added L on the rate for formation of the intermediate than on its rate of decay. Ni L,CN

NiLn*

4

5

The close relationship between the neutral n-ally1nickel cyanide complexes 4 and the cationic complexes 5 was shown by mixing previously prepared and characterized [(n-C,H,)NiL,]PF, (42) with excess NEt4CN in CH,Cl,. The solution turned red-orange on mixing and the IR spectrum showed intense bands at 21 10 and 2090 cm- assigned to [(n-C,H,)NiL,]CN and cyanide ion, respectively. On standing, P N s formed as expected; however, GC analysis showed that the major products were the conjugated isomers trans-2-pentenenitrile (T2PN) and cis-2-methyl-2-butenenitrile (C2M2BN). Drinkard has shown that NEt,CN is an effectivecatalyst for the double bond isomerization of pentenenitriles, even in the absence of nickel. In another experiment using the [(n-C,H,)NiL,]PF, complex with HCN, reaction with the nickel complex was slow; however after heating for 1 hr at 50°C the only nitrile products detected by GC were T3PN and 2M3BN. Our results on the hydrocyanation of butadiene are consistent with the mechanism shown in Fig. 5. In the L = P(OEt), system, steps 1,5, and 6 are relatively slow; all others are fast. The product distribution is determined by the relative values of the forward rate constants for steps 5 and 6. All steps are reversible, but the reversibility of 5 and 6 is emphasized by double arrows since this feature is essential for skeletal isomerization of 2M3BN to 3PN. As we saw in Section II,C, mixing T3PN or 2M3BN with Ni[P(O-o-tolyl),], at 25°C gives the (olefin)NiL, complexes shown in Fig. 5, along with (nitrile)NiL, complexes, not shown. Addition of ZnC1, causes rapid oxidative addition in the cyanoolefin complexes to give the syn-lmethyl-n-allyl-NiL,CN-ZnC1, complex, all of whose protons could

'

c. A.

18

TOLMAN et

al.

NiL4

I

FIG.5. Mechanism of butadiene hydrocyanation.

be assigned in the 100-MHz NMR spectrum. The IR spectrum showed a strong band at 2168 cm-' assigned to the CN stretch. Similar behavior is observed with AlCI,, but not with BPh,. In the absence of Lewis acid the oxidative addition is much slower, but can be brought about by heating the solution for 15 sec at 100°C. Ally1 cyanide itself has a CN-stretching band at 2258 cm-' in CH,Cl,. The solution will dissolve 1 eq of ZnCI,, giving a new strong band at 2130 cm-' assigned to the Lewis acid adduct CH,=CHCH,CN-ZnCI,. A solution to which (C,H,)NiL,, ZnCI,, and allyl cyanide were added in a 1 : 1 :2 ratio showed the 2168 cm-' band of (?I-C,H,)NiL,CN-ZnCI, and the 2258 cm-' band of free allyl cyanide, but no band at 2130 cm-'. The absence of CH,=CHCH,CN-ZnCI, indicates that all of the ZnC1, is complexed to the nickel cyanide. It is interesting to note the much greater Lewis basicity of the nitrogen lone pair in the NiCN group than in the -CH,CN. The role of Lewis acids in catalyzing the oxidative addition can be understood in terms of reducing the activation energy for breaking the C-CN bond by stabilizing the transition state shown in 6. We might expect that ZnCl,, by microscopic reversibility, would also accelerate the rate of reductive elimination. This would be very similar to the promotional effect observed for Lewis acids in alkyl-to-carbonyl migration reactions, e.g., in CH, Mn(CO), (51,52).

-

6

19

NICKEL-CATALYZED OLEFIN HYDROCYANATION

D. OTHER DIENES A variety of dienes in addition to butadiene can be hydrocyanated at room temperature using HNi[P(O-o-tolyl),],CN, as seen in Table IV. Experiments were done by adding the diene (1 : 1) to the preformed hydridocyanide in toluene, then running IR spectra repeatedly to follow the time course of the reactions. After a day the final solutions were analyzed by GC/MS. In some cases reactions were also followed by proton NMR in toluene-d, . TABLE IV Hydrocyanation of Dienes by Ni[P(O-o-to!yL),], Hydrocyanation products

Diene

Diene

Hydrocyanation products

C8H13CN

N

NC

CHz=C=CHz

*

C

W

h

Q

Nc9-

ldC

6

c. A.

20

TOLMAN et

al.

of the 1-Me group were observed at T 8.41, 8.55, and 8.65. Based on earlier work (53)on cationic mallyl complexes, the structures of the intermediates are proposed to be 7, 8, and 9. On standing, the intermediates gradually Me-Et

n

Et 8

7

9

disappeared to give a mixture of products with vCN at 2240 cm-' in the IR. GC/MS analysis (Table IV) showed the same major C,H,,CN products, presumably 10 and 11, starting with either diene, similar to the results of

MeY-Et CN

+YE' CN

10

11

Keim and coworkers (54). Clearly, rapid double bond migration is possible under the reaction conditions. Reactions of the HNiL,CN complex with 1,3-cyclopentadiene, 1,3-cyclohexadiene, and 1,3-~yclooctadienegave intermediates with decreasing stabilities in that order; the 1,3-cyclooctadiene intermediate was not spectroscopically observable. The cyclohexadiene adduct was shown to be the cyclohexadienyl complex 12 by its proton spectra, with resonances of Ha, H,, and -(CH2),at T 4.53, 6.06, and 8.47, respectively; these values are close to the chemical shifts found earlier (51) for 13: t 4.52,5.86, and 8.48. The reaction of DNi[P(OMe),]d with cyclopentadiene gives 13-d, with addition of D and Ni to the same side of the ring (52). Backvall and Andell (55) have shown, using Ni[P(OPh),], and deuterium cyanide (DCN), that addition of D and CN to cyclohexadiene is stereospecifically cis, as expected for n-ally1 intermediate 12.

+ ---

Ha

--NiL,CN

Hb

12

Ha$--

Ni [P(OMs13]:

"b

13

The nonconjugated 1,4-cyclohexadiene gave the same intermediate as the 1,3- isomer; however 1,5-cyclooctadiene, unlike 1,3-cyclooctadiene, gave a rather stable intermediate with a band at 2145 cm-', assigned to the 1,4,5-trihapto structure 14. A 1,3,4-trihapto structure 15 probably formed in

NICKEL-CATALYZED OLEFIN HYDROCYANATION

14

21

15

the case of norbornadiene. A copius yellow precipitate formed immediately on mixing, but redissolved after a few minutes at room temperature to give two isomers of dicyanonorbornane (identified by GC/MS) with vCN at 2236 cm- While we did not determine the product stereochemistry, we presume that the products are the exo isomers indicated in Table IV, based on the work of Jackson and Love1 (56). Cyclooctenyl and norbornenyl platinum complexes analogous to 14 and 15 have been proposed in the reactions of 1,5-cyclooctadieneand norbornadiene with HPt(NO,)(PEt,), (57). 4-Vinylcyclohexene formed a rather stable intermediate which persisted even after a day at room temperature. The structure is thought to be 16, arising from isomerization of the internal double bond. An alternative possibility, 17, is considered less likely because 1,3- and 1,Ccyclohexadiene gave much more reductive elimination product after a day.

’.

NiLnCN I

oc

Ni Ln CN

16

17

Allene reacted with HNiL, CN to convert about half of the hydridocyanide to an intermediate with vCN = 2146 cm- probably a n-allylnickel cyanide complex. GC/MS analysis, however, did not show the formation of ally1 cyanide. Rather, peaks at m/e 107, 134, and 147 are assigned to C,H,CN, C,H,,(CN),, and C,H,,CN, with likely structures shown in Table IV. These probably arise from allene insertions into the nickel-carbon bond of a n-allylnickel cyanide complex, followed by reductive elimination of nitrile. Further hydrocyanation could give dinitrile products. Analogous insertion of allene into a-allylpalladium acetylacetonate complexes has been reported by Hughes and Powell (58). Dicyclopentadiene readily monohydrocyanates to give two isomers identified by Wu and Swift (59) as exo- and endo-8-cyanotricyclo[5.2.1.02*,]dec-3-ene. Finally, cis,cis,trans-1,5,9-cyclododecatriene gave a reasonably stable intermediate which collapsed to give two major C, 2Hi9CN isomers of undetermined structure. Clearly the hydrocyanation reaction is very general for hydrocarbon dienes.

’,

22

c. A.

TOLMAN et

al.

IV. Olefin lsomerization AND A-ALLYL FORMATION FROM DIENES AND HNiLa A. BUTENEISOMERIZATION

The addition of strong acids HX to solutions of Ni[P(OEt),], gives the cationic hydrides HNiL: X- which are extremely active catalysts for olefin isomerization (60). For 1-butene, kobs= 0.7 sec-' M-' [HNiL:] at 25°C; NiL, and HNiLa were the only spectroscopically detectable nickel species in solutions containing butenes or ethylene, and the isomerization rate was found to be inversely proportional to [L]. In the presence of butenes, butane is formed as the catalyst dies. These observations and the results of kinetic labeling experiments are consistent with the mechanism shown in Fig. 6. With the addition of 1,3-butadiene, the initially yellow hydride solutions turn red with the formation of relatively stable 1-Me-n-allyl-nickel complexes, and olefin isomerization activity stops. By measuring the rate of formation of the n-ally1 complexes in the presence of added P(OEt),, it was possible to measure the rate constant for dissociation of L from HNiLa and show that this is the rate-determining step (42). In a proton NMR experiment in which lP-pentadiene was added to a solution of HNi[P(OMe),]f, it was possible to watch the isomerization of 1,4- to 1,3-pentadiene, followed by formation of 1,3-dimethyl-~-allylcomplexes (53).The observation of Ir-ally1 products in the reaction of the hydride with the conjugated diene, but not in the a-alkyl intermediates involved in isomerization, illustrates the much greater stability of n-ally1 complexes of nickel compared to a-alkyls, a feature which is also observed in the hydrocyanation reactions.

FIG.6. Mechanism of olefin isomerization with HNiLf

NICKEL-CATALYZED OLEFIN HYDROCYANATION

23

B. ISOMERIZATION OF 3PN WITH HNiLa The isomerization of the internal olefin 3PN to the terminal olefin 4PN is a critical step in the hydrocyanation of 3PN to ADN [Eqs. (9) and (lo)]. Unfortunately, there is a loss in yield because the undesirable conjugated isomer 2PN is also produced. Observations discussed below have led us to the belief that cationic nickel-hydride complexes, HNiL:, may be important in the isomerization process. When 3PN solution containing Ni[P(O-p-tolyl),], is treated with trifluoromethylsulfonic acid (1 eq/Ni) at 50"C, rapid isomerization occurs for less than 30 sec before catalyst degrades. During this short burst of isomerization, 4PN and 2PN are produced in a ratio of 70: 1. Similar results are obtained at 40°C and 25°C (61). When a different phosphite ligand is used, the ratio of 4PN to 2PN initially produced is altered significantly; isomerizations with Ni[P(OEt),], and trifluoromethylsulfonic acid at 50°C produce a 4PN :2PN ratio of 17.5 : 1. In contrast, when 2-hexene is treated with this same catalyst system, the initial ratio of 1-hexene to 3-hexene produced is less than 2 : 1. This unprecedented kinetic preference for isomerization of the internal olefin to a terminal olefin is in stark contrast to the strong thermodynamic preference for the conjugated isomer 2PN; the thermodynamic distribution at 50°C is 78.3 :20.1 : 1.5 (2PN : 3PN : 4PN). It should be emphasized that the ratio of 4PN :3PN never goes above the equilibrium ratio of about 0.07 : 1, but arrives at that equilibrium ratio before any significant production of 2PN occurs. This may result from coordination of nitrile prior to olefin, thereby directing the nickel-hydride addition to the olefin as illustrated in 18. In the r

18

case of the smaller triethylphosphite ligand, perhaps nitrile coordination becomes less favored over olefin coordination and allows a pathway involving coordination of olefin to become more important and thereby produce more 2PN. OF 3PN C. ISOMERIZATION

WITH

NiL,, HCN,

AND

LEWIS ACIDS

When HCN is added to 3PN in the presence of NiL, and a Lewis acid, the onset of isomerization to 4PN is observed before any significant hydrocyanation occurs. Observation of this isomerization reaction over the first few

c. A.

24

TOLMAN

et al.

minutes of the reaction before steady-state levels of the PN isomers are obtained allows the extraction of relative rate constants for the formation of 4PN and 2PN from 3PN. Using the thermodynamic ratios of the isomers, the relative rate constants for the back reactions of 3PN from 2PN or 4PN may be calculated. The results so obtained for a variety of Lewis acids are given in Table V. It is apparent that the rate constants change for different

TABLE V Relative Rate of 3PN Isomerization" Relative k Lewis Acid

k,

k2

k,lk,

BPh, ZnCI, Ph,SnO,CCF, SnCI, AICl,

1 .oo 0.42

1.5 x lo-' 5.8 x 1 0 - 3

0.29 0.24 0.1

4.9 1 0 - 3 3.9 x 10-3 1.7 x 10-3

66 72 59 61 59

"3PN

--%

3

N

P

A

4PN 2PN

Reaction conditions: Ni[P(O-p-tolyl),], (0.02 M), P(0-p-tolyl), (0.08 M), Lewis Acid (0.08 M) in 3PN at 50°C.

Lewis acids but note that the ratio k , : k2 is similar (about 65 : 1) for all Lewis acids. This compares favorably with a k l :k2 ratio of 70 : 1 found for the HNiL: catalyst described in Section IV,B. It would appear that Lewis acids may control the concentration of the catalytic species responsible for isomerization but not be directly involved in the isomerization process. In Section II,E, we showed that the addition of HCN to NiL4 in the presence of a Lewis acid gives an equilibrium mixture of complexes as shown in Eq. (24). In the isomerization process, Lewis acid appears simply to control the equilibrium position of Eq. (24). HNiL: CNC-A- is most likely responsible for the majority of isomerization activity, thereby explaining the lack of Lewis acid influence on the rate constant ratios. HNiL,CN-A

+L

HNiLiCN-A-

(24)

NICKEL-CATALYZED OLEFIN HYDROCYANATION

V.

A.

25

Monoolefins and HCN

UNPROMOTED HYDROCYANATIONS

1. Alkenes and Styrene

The title olefins form complexes with Ni(0) with equilibrium constants for formation decreasing in the order ethylene > styrene > propylene N l-hexene > disubstituted alkenes (28). With ethylene and styrene the (olefin)NiL, complexes have been isolated with L = P(0-o-tolyl), . Addition of HCN to solutions of the pure olefin complexes results in rapid and complete conversion to alkylnickel cyanide intermediates which are spectroscopically detectable; subsequent C-C coupling gives the observed nitrile products: propionitrile from ethylene and (predominantly) 2-phenylpropionitrile from styrene (47). The same alkyl intermediates are formed when ethylene and styrene are added to HNiL,CN [L = p(0-o-tolyl),]. Addition of other alkenes to the hydridocyanide gives much less stable alkyls, and most of the nickel remains in the form of HNiL,CN even in the presence of excess olefin. Hydrocyanation does occur, however, at 25°C. Propylene gives n-butyronitrile and i-butyronitrile in a 70 :30 ratio; 1-hexene gives a similar distribution of linear and branched products and, in addition, a small amount of the internal nitrile product 2-Et-valeronitrile (Table VI), presumably by isomerization of 1- to 2-hexene, followed by HCN addition. Isobutene gives exclusively 3-Me-butyronitrile, resulting from CN addition to the unsubstituted end of the starting olefin, presumably because of the extreme steric crowding in a t-butylnickel cyanide precursor. A similar regioselectivity has been reported by Backvall (62) for t-butylethylene. Cyclopentene,cyclohexene, and cyclooctene all give the expected cycloalkylnitrile products (Table VI). The hydrocyanation of ethylene is a model for many monoenes and has been studied in some detail (47, 63). Upon addition of HCN in the presence of excess ethylene at -40"C, the ,lP-NMR resonance of (C, H,)Ni[P(O-o-tolyl),], at 141.4 ppm is quantitatively replaced by four new singlets at 129.8 (uncoordinated ligand), 118.1, 117.7, and 116.9 ppm with areas in a relative ratio of 1.00:0.14 :0.80: 0.06 (63).These same signals are produced when HNi[P(O-o-tolyl),],CN is treated with excess ethylene at - 50°C (47). These signals persist until HCN and/or ethylene is consumed. In the 'H-NMR spectrum at -50°C five Ni-C,H, protons appear as a single broad resonance at z 9.39, due to accidentally similar chemical shifts; the methyl triplet may be observed separately from the methylene near O"C, but at that temperature the rate of reductive elimination becomes appreciable.

c. A.

26

TOLMAN

et al.

TABLE VI Olefns Hydrocyanated in Unpromoted Reactions Using Ni[P(O-o-tolyl),],

Olefin

Hydrocyanation products

Ethylene

/CN

Propy 1ene

1-Hexene

Isobut ylene

A C N

V

Cyclopentene

CN

OCN

Cyclohexene

CyCN

Cyclooctene f

PhWCN

Styrene Ph&N

CH,=CHSiMe,

NC-SiMe3

When DCN is substituted for HCN, propionitrile is formed in which deuterium is found in both methyl and methylene groups, indicating that the insertion of ethylene into the nickel-hydrogen bond is reversible and occurs rapidly compared to the irreversible coupling of Et and CN to give propionitrile. A singlet at t 7.97 is assigned to coordinated ethylene. 13C NMR shows that the carbons of the ethyl group do not exchange rapidly on the NMR time scale. Figure 7 shows a portion of a 13C{'H} spectrum obtained using singly "C-labeled ethylene in an experiment at - 50°C. The singlet at 14.1 ppm is assigned to the methyl carbon (confirmed in undecoupled spectra) and the doublet at 11.7 ppm is assigned to the methylene carbon, split by 35 Hz by a trans phosphorus. A singlet at 58.9 ppm is assigned to coordinated ethylene. As with the phosphorus and proton spectra, in the presence of excess HCN and C,H,, the carbon resonances of the intermediate remain unchanged while propionitrile resonances grow in the spectrum, as shown by the repeated sweeps in Fig. 8. (The methyl and

NICKEL-CATALYZED OLEFIN HYDROCYANATION

-

27

20

25

PPM

FIG.7. I3C-NMR spectrum of I3C 12CH,Ni[P(O-o-tolyl)3](13C I2CH,)CN, 19a, (ppm vs. TMS) at - 50°C.

methylene carbons of propionitrile accidentally have the same chemical shift.) Based upon the above data, the intermediate species are assigned the isomeric structures 19a-19c with 19a being the predominant isomer, based on CHZ=CH,

I L-Ni-C,H, I C N

19a

CH,

1)

CHZ

L -

CH,=CH,

-Ni-C,H,

NC-Ni-C2HS

I

I

C N 19b

I

I

L 19c

the carbon-phosphorus coupling constant. Whether all isomers participate in catalysis is unclear, but equilibrium among isomers appears to be faster than the catalytic reaction. Kinetic studies reveal that the rate of propionitrile formation is first order in the concentration of 19 and of free ligand, i.e., d[EtCN]/dt = k[19][L], with activation parameters of A H f = 8.9 & 0.9 kcal/mol and ASf = - 32 k 4 eu. These data suggest that 19 and free ligand, L, recombine in a slow step, generating a five-coordinate 18-electron complex which then rapidly eliminates EtCN and regenerates (C,H,)NiL,. Figure 9 shows a complete catalytic cycle, starting with (C,H,)NiL2. [This figure is different from the one published earlier ( 4 9 , and reflects more recent results (63).]

c. A. TOLMAN et al.

28

I80 I50

-c -

120

I

90

E

w

k

60 J<

A-

A

30

.

A

;6

A r -

L . .

1

l'0

-

. A ~

~

~

12

14

* 1

l

"

'

I

"

'

l

'

8

10

PPM

FIG.8. H,"C12CH, hydrocyanation at -50°C in toluene-ds.

I

/ I

L' I,/$4 L- NI -Et

CH2=CH2

I

L2-Ni--H

I

i:

4

N

L

CH2=CHp

I

L-Ni-Et

CH2=CHz

I

L-Ni--H

FIG.9. Mechanism of ethylene hydrocyanation.Dashed arrows imply irreversible reactions.

NICKEL-CATALYZED OLEFIN HYDROCYANATION

29

Associative reductive elimination has been well characterized in only two other cases-both with nickel. Yamamoto et al. (64, 65), have found that reductive elimination from some L,NiR, complexes is facilitated by attack of L' (L' = olefin, PMe,). In a less kinetically characterized system, Favero et al. (66) found that L,Ni(Ph)CN(L = PEt,) reductively eliminates PhCN when treated with P(OEt),. Tatsumi et al. (67) have provided a theoretical explanation for why reductive elimination from a five-coordinate nickel system is preferred. It is our belief (yet to be proven) that in systems which contain weaker binding olefins and smaller better coordinating phosphites, e.g., P(0-p-tolyl),, an intermediate in which the olefin is replaced by phosphite is likely. In order to allow for this possibility, in subsequent complex structures and mechanistic schemes L, may be taken as either phophite or olefin or a combination of these two ligands, and as such a general form of complexes 19 is expressed below.

L,Ni

/ \

Et CN

19

When a solution of 19 and excess HCN was warmed to 25"C, rapid formation of propionitrile was observed along with a small amount of ethane. Ethane formation is accompanied by irreversible oxidation of 19 (vCN = 2152 cm - ') to nickel dicyanide complexes, which precipitate from solution and give a new broad IR band at -2170cm-'. Product propionitrile (in CH,Cl,) appears at 2252 cm-' with a shoulder at 2240 cm-' due to (C,H,CN)NiL,. Most likely, this oxidation arises by attack of HCN on the 16-electron intermediate 19. In view of the much greater tendency to form alkyl intermediate from ethylene than from propylene, it is not surprising that the hydrocyanation rate of ethylene is much higher. Figure 10 shows an experiment in which initially nearly equal concentrations of the two olefins were allowed to react with HNi[P(O-o-tolyl),],CN and the concentrations of various species were followed with time by NMR at 0°C. On addition of the olefins, the HNiL,CN was quantitatively converted to EtNiL,CN, and formation of EtCN began. After about 15 min, the free C2H4 was consumed, the ethyl intermediate began to decline at the same rate that EtCN was being formed, and HNiL,CN began to reappear; no propylnickel intermediates were observed, and butyronitriles were observed with certainty only when all free C, H4 was gone. The maximum rate of propionitrile formation at 0°C was about eight times as great as the rate of formation of the butyronitriles (shown by

30

c. A.

TOLMAN et

al.

d r 0°C

0

10

20

At t 25°C

I

30

FIG.10. Competitive hydrocyanation of ethylene and propylene with 0.175 M Ni[P(O-o-tolyl),], in 75 % toluene-d,/25% CH,CI,, followed by proton NMR. Used by permission of the New York Academy of Sciences.

subsequent GC analysis to be 72 % linear and 28% branched). On warming to 25°C the remaining HCN was quickly consumed. Styrene, unlike propylene, produces an alkyl intermediate which is stable enough to be readily detectable, and the branched nitrile product 20 is strongly favored over the linear one. This unusual behavior can be attributed to stabilization of intermediate 21 through donation of ring electrons to the

20

21

coordinatively unsaturated Ni center (47). Support for the involvement of ring electrons is the deep red color of intermediate 21. The ethylnickel cyanide intermediate 19 is pale yellow. Crystal structures of re(22) and lated compounds [(r13-CH,CHC6H,)CpRh(PMe3)]PF6 (~3-CH2C6H,CH,)MoCp(CO)2 (23)have been reported in the literature (68, 69).

'-Rh-PMe3 I CP

22

CP

23

31

NICKEL-CATALYZED OLEFIN HYDROCYANATION

The branched structure of 21 is indicated by the doublet of its CH, group observed in the proton NMR spectrum (in toluene-d, at -25°C) at 9.16 T ( J = 6 Hz); the corresponding doublet of the major product 20 appears at 9.00 T. 2. Olejns with Heteroatoms Some monoolefins with heteroatoms can also be hydrocyanated at 25°C using Ni[P(O-o-tolyl),], as the catalyst, as shown in Table VI. As with t-butylethylene, a single product was observed with trimethylsilylethylene. Trifluorometh ylethylene, unlike propylene (and trimethylsilylethylene) forms a rather stable (alkyl) NiL,CN intermediate, with a half-life of 1 hr at 25°C. The intermediate CF,CH,CH,NiL,CN (24) shows a single broad 19F resonance (at 58.2 ppm), and a broad methylene-lH resonance rather than a methyl doublet expected for a branched intermediate; the C N stretch is at

-

TABLE VII Olejns Not Hydrocyanated Using Ni[P(O-o-tolyl),], Olefin C2F4 CH,=CHF CH,=CHCI CH,=CHOCOCH, CH,=CHOBu CH,=CHCOOCH, CH,=CHCOCH, CH,=CHCHO CH,=CHCN CH,CH=CHCN C2PN CH,=C(CH ,)CN 1-Cyanocyclobutene CH,=C(CF,)CN trans-NCCH=CHCN Maleic anhydride CH,=CHCH,CI CH,=CHCH,OH CH,=CHCH,SiMe,

Other productsb

Dimer

Dimer and trimer

T2PN Dimer

RNiL,CN", vCNC 2163 2143 2158 2154 2140 2152 2149 2125 2164 2164 2163 2162 2164

d

-

NCCH2CHzCN

-

e

2150

In 24 hr at 25°C. Detected by GC/MS. 'IR frequency in toluene (cm- '). A trace of hydrocyanation product was found by GC/MS. A trace of hydrocyanation product was suggested by IR spectrosCOPY. @

c. A.

32

al.

TOLMAN et

2152 cm-', the same as found in 19, consistent with a distal placement of the CF, group as indicated in 24. The "F-NMR spectrum of the organic product shows an AA'BB'X3 pattern (at 67.9 ppm upfield from external CFCI,, in C6D6),consistent with CF,CH,CH,CN (25). We found that many olefins containing heteroatoms (as shown in Table VII) were not readily hydrocyanated at 25°C. They contain halogen, oxygen, ester, ketone, aldehyde, or -CN directly attached to the olefinic carbon, or halogen, oxygen, or trimethylsilyl in an allylic position. The hydrocyanations fail for a variety of reasons. In most cases examined, stable alkylnickel cyanide complexes rapidly formed, as indicated by IR spectra, but they were so stable that they failed to reductively eliminate product. The high NiCN stretching frequencies (near 2163 cm-') in the case of the conjugated cyanoolefins suggest that the alkylnickel cyanide complexes have structures 26 and 27, i.e., all contain a-cyanoalkyl groups. NC

\

NC CHNiL,CN

'R 26

NiL,CN

6 21

R = CH3-, CH,CHI-, CH,CHzCH,CH,-

In support of this idea the organic CN stretch always appears around 2230 cm - ',considerably lower than in alkyl cyanides, suggesting a strong perturbation by the nearby metal atom (70, 71). Compounds 26 and 27 are difficult to characterize because they decompose rapidly, losing phosphite ligands to form insoluble, presumably polymeric, olive yellow solids. One such material (26 with R = CH,-) was precipitated from CH,CI, by the addition of HCN to a solution of (CH, = CHCN)NiL,. Washing with methanol and drying gave an olive solid with broad IR bands at 2155 and 2231 cm-' (in KBr), with an elemental analysis which suggests that some olefin and L have been displaced. The addition of acrylonitrile to an active ethylene hydrocyanation rapidly precipitates all of the nickel, stopping any further reaction. In pentenenitrile hydrocyanation, relatively stable a-cyanoalkyl complexes form from 2PN (which is not hydrocyanated). However, the isomerization of cis-2pentenenitrile (C2PN) to T2PN (Table VII) indicates that formation of the alkyl is reversible. With fumaronitrile and maleic anhydride, IR spectra, after adding olefin and HCN to NiL,, showed only the very stable (olefin)NiL, complexes. In these cases the oxidative addition of HCN is evidently suppressed by the strongly electron withdrawing character of the olefins. Small amounts of the reduction products were detected by GC/MS after the solutions had stood for a day, however. The failure of ally1 chloride to hydrocyanate is no doubt due

NICKEL-CATALYZED OLEFIN HYDROCYANATION

to the rapid CC-CI Section II1,C.

33

bond cleavage and oxidative addition described in

3. Cyanoolejns Cyanoolefins in which the CN group is separated from the olefinic carbon by at least one carbon can be readily hydrocyanated at 25°C using Ni[P(O-o-tolyl),], with the results shown in Table VIII. In all cases the RNiL, CN intermediates were stable enough to be readily observed; all showed NiCN stretching bands at -2144 cm-'; the organic CN stretch at 2245 cm- ' grew in intensity with time as more C-CN bonds were formed. While the ratio of linear to branched product is only about 2 in the case of 3butenenitrile, it is about 16 in the case of 2M3BN, probably because of the increased steric crowding in the intermediate in the latter case. 2-Methyl-2butenenitrile (2M2BN) gave only one detectable product as shown, though the yield was low. This is the only case where a conjugated cyanoolefin was hydrocyanated; in this case the methyl group on the a-carbon must significantly destabilize the alkyl with nickel on the same carbon. 4PN and 3PN TABLE VIII Cyanoolejins Hydrocyanated in Unpromoted Reactions Using Ni[P(O-o-tolyl),],

Olefin

Hydrocyanation products

c. A.

34

TOLMAN

et al.

give a very similar mixture of the three isomeric products; 3PN gave slightly more ESN. The results indicate that double bond isomerization is fast, but not instantaneous, relative to hydrocyanation in this system. The enhanced stability of intermediates NC(CH,),NiL,CN (28, rn = 3, 4) (from ally1 cyanide and 4PN) relative to CH,(CH,),NiL,CN (29,n = 3, 6 ) (from propylene and 1 -hexene) may be due to the inductive electron-withdrawing effect of the CN group both at the end of the alkyl chain and on a coordinated olefin (recall the effect on olefin complex equilibria in Section 11,C). B. HYDROCYANATIONS PROMOTED WITH LEWIS ACIDS The unpromoted hydrocyanations of monoolefins discussed so far generally involved only a few catalytic cycles on nickel. The development of a practical commercial process depended on getting many cycles. Certain Lewis acids are quite remarkable in increasing (1) catalyst cycles, (2) the linearity of products obtained, and (3) the rates of reaction. The effects depend on the Lewis acid, the phosphorus ligand used, and the olefin substrate (72). 1.

Propylene, 1 -Hexene, and Styrene

,

The half-life of propylene hydrocyanation by HNi[P(O-o-tolyl),] CN in the absence of Lewis acid is -60 min at 0°C. Addition of the Lewis acids AICI,, ZnCI,, or BPh, gives dramatically different results; no alkyl intermediates were detected. The results are shown in Table IX.AlCl, tremendously accelerates the rate, ZnC1, has little effect, and BPh, slows it down. Perhaps more surprising was the increase in percentage of linear product with BPh,, which was not observed with the other Lewis acids. The beneficial product distribution effect of arylboranes was confirmed with 1-hexene and TABLE IX Lewis Acid Effects Hydrocyanation of Propylene in 75 % TolueneJ25 % CD,CI, with P(0-o-tolyl), Approximate t , , , (min) Lewis acid AICI, ZnC1, None None BPh,

- 25°C

0°C

+ 25°C

10

% Linear product

>7

72 70 72 70

> 60

89

<4

60

35

NICKEL-CATALYZEDOLEFIN HYDROCYANATION

B(p-tolyl),, where the linear heptanenitrile increased from '67 % to 91 % of hydrocyanated product. When styrene is added to HNi[P(O-o-tolyl),],CN, the solution goes from yellow to red and the hydride is quantitatively converted to alkyl complex. However, addition of excess styrene to HNiL,CN-B(p-tolyl), causes a color to change to orange and leaves most of the nickel as hydride complex, as shown by both IR and NMR spectroscopy. Thus, the Lewis acid decreases the equilibrium constant for reaction (26) relative to reaction (25). PhCH=CH, PhCH=CH,

+ HNiL,CN

+ HNiL,CN-A

CH,CH(Ph)NiL,CN

+L

L CH,CH(Ph)NiL,CN-A

+L

(25) (26)

A weak -CH, doublet in the proton spectrum at 9.16 T is assigned to CH,CH(Ph)NiL, CN-B(p-tolyl),; the shift from 9.32 T in the parent compound found in the absence of Lewis acid indicates that B(p-tolyl), is part of the alkyl complex. The borane greatly slows the rate of product formation, increasing the half-life from minutes to hours at 25"C, probably because of the decrease in the concentration of alkyl intermediates. GC analysis after completion of reaction showed an increase in linear 3phenylpropanenitrile from 9 % without to 19% with Lewis acid. While we cannot prove it spectroscopically,the change in product distribution suggests that B(p-tolyl), decreases the percentage of branched alkyl intermediate at equilibrium, i.e., the equilibrium constant for reaction (28) is smaller than for reaction (27). PhCH,CH,NiL,CN PhCH2CH2NiL2CN-A

PhCH(CH,)NiL2CN PhCH(CH,)NiL,CN-A

(27) (28)

2. Cyanoolefins Addition of some arylboranes to solutions containing HNi[P(O-o-tolyl),], CN affects the course of the reaction of 4PN similarly: the steady state concentration of alkyl intermediates is reduced, the rate of product formation is reduced, and the product distribution is shifted in favor of linear product, as shown in Table X. As with styrene, the results indicate that the arylborane decreases reaction (30) relative to reaction (29), and reaction (32) relative to reaction (31). In 4 PN

4 PN

+ HNiL,CN

+ HNIL,CN-A

NC(CH2),NiL2CN NC(CH,),NiL,CN-A

F =

NC(CH,),NiL,CN

+L

NC(CH,),NiL,CN-A

+L

.-== NC(CH,),CH(CH,)NiL,CN .-== NC(CH,),CH(CH,)NiL,CN-A

(29) (30) (31) (32)

c. A.

36

TOLMAN et

al.

TABLE X Lewis Acid Effects on Product Distribution' % Linear Nitrile Product Lewis acid

From 4PN

From styrene

99 98 8ob 11 14

14 33 9 9 9 11 1 10

12 10

66

Determined from peak heights in GC/MS traces. Percentages are probably good to f 3 %, in most cases. Overlap of MGN and bibenzyl makes this number uncertain.

addition, the reaction solution remains homogeneous. This latter observation is very important and provides a possible key for explaining why many more catalyst cycles are possible in the presence of the Lewis acid. By coordinating to the NiCN group in the a-cyanoalkylnickel cyanide complexesformed from 2PN, as shown in Eq. (33), the Lewis acid ties up the nitrogen lone pair and prevents it from binding to another nickel center to form insoluble polymer. CH,(CH,),CH(CN)NiL,CN

+A

CH,(CH,),CH(CN)NiL,CN-A

(33)

The question naturally arises-what properties of the Lewis acids are responsible for changing the distribution? Table X shows the percentage of linear product in experiments starting with 4PN or styrene and a variety of boron-containing Lewis acids. It can be seen that the best distributions with 4PN or styrene are obtained with BAr,, and the worst with B(OAr),, the latter giving somewhat poorer linearity than no Lewis acid. B(CH, Ph), , B(o-tolyl),, and BCy, are not distinguishably better than no Lewis acid. Spectroscopic experiments by IR and NMR spectroscopy, as described in Section II,E,2, showed that B(o-tolyl), and BCy, have much smaller equilibrium constants than BPh, in reaction (34), and so in these cases the Lewis HNiL,CN

+A

K

HNiL3CN-A

(34)

acid-free pathway dominates the product distribution. A different explanation is required for the relationship between BAr,, B(CH,Ph),, and B(OAr),. Continuing studies suggest both steric and electronic effects by

37

NICKEL-CATALYZEDOLEFIN HYDROCYANATION

TABLE XI Lewis Acid Eflects on Product Distribution Using Ni[P(O-ptolyl),], in 3PN"

Percent product CN

Lewis acid

NC -CN

CN CN

BPh3 ZnCI, AICI,

4 16

96

82 50

<1

2 11

39

'Conditions: Ni[P(O-p-tolyl),], (0.02 M),P(0-ptolyl), (0.08 M ) , Lewis acid (0.02 M ) at 50°C.

Lewis acids on product distributions; this will be the topic of subsequent publications. 3. Isotopic Labeling Experiments Table X I shows the distribution of dinitrile products using Ni[P(O-p-tolyl),],catalyst and the Lewis acids AlCl,, ZnCl,, or BPh, in a pentenenitrile solvent. The ability of AlCl, and ZnC1, to induce oxidative addition of allylic cyanides to give a-ally1complexes (Section II1,C) suggested that MGN might form by skeletal isomerization of 3PN to 2M3BN, followed by addition of HCN to the latter, as shown in Fig. 11. This was of particular concern, since in competition experiments with equimolar 2M3BN and 4PN at 60°C [using Ni[P(O-p-tolyl),], and ZnCl,] ADN was formed only after W

C

N

--L

W

4PN

C 3PN

N

C

z,

2Me3BN

YCN J 2% -0%

FIG.11. Pathways for isotopically labeled products from P N s and H13CN. Used by permission of the American Chemical Society (48).

-

c. A. TOLMAN et al.

38 m

C

m

N

C

KT N

c

L

C

N

D C T

ADN-d

&CN

MGN-d

N

1

CN

\

~

C D MGN-d

&CN N CN ESN-d

ESN-d

FIG.12. Pathways for isotopically labeled products from P N s and DCN. Used by permission of the American Chemical Society (48).

most of the 2M3BN was hydrocyanated to MGN. This order of rates, i.e., 2M3BN > 4PN > 3PN, parallels the order of stability constants in Table 11. After addition of H',CN to 3PN in the presence of Ni[P(O-p-tolyl),], and AICI, or ZnCI,, I3C{'H} NMR of the recovered MGN showed that all of the MGN came via H13CN addition to either 4PN or 3PN (48). To distinguish between 4PN or 3PN precursors to MGN, further experiments were done using DCN, followed by analysis of dinitrile products and recovered 3PN by GC/MS and 'H NMR. The simplest results were obtained with ZnCI,, where little or no deuterium incorporation in the recovered 3PN was observed. The ability of this NiL,/DCN/ZnCI, catalyst system to isomerize double bond migration without deuterium incorporation in the isomerized olefin is reminiscent of the isomerization of l-butene with Ni[P(OEt)3]4/D2 SO,, where the ratio of isomerization to deuteration rates was 170 (at OOC). Figure 12 shows the possible pathways to DN-d, products from PN-do precursors. Essentially all of the MGN comes from wrong-way addition of DCN to 4PN, rather than from 3PN. This is consistent with rate measurements which show that addition of HCN to 4PN is 500-600 times faster than addition to 3PN (47). Too little ESN was formed using ZnC1, to analyze it reliably. With AICI,, however, the only ESN-d, product detected was that arising from 3PN, rather than 2PN (48). With BPh, the rate of PN olefin-insertion/deinsertionand isomerization of 3PN to 4PN is so fast relative to C-C coupling, that only statistically scrambled deuterium was observed in recovered P N s and DN's. 4. Kinetic Order of Constituents Evaluation of the kinetic order of the various constituents in a PN hydrocyanation was carried out by monitoring the temperature after injecting small portions of HCN into a pulse reactor (Section I,D) containing NiL,

39

NICKEL-CATALYZED OLEFIN HYDROCYANATION

r

0

1

I

2.o TIME (min)

1.o

I

I

3.0

4.0

FIG.13. Typical pulse reactor hydrocyanation exotherms at 60°C.

catalyst, excess L, and Lewis acid (A) in a PN/DN solvent mixture. A mixture of 4PN and 3 P N s was used in a ratio of about 1 :20; in this system the steady-state ratio is close to the equilibrium value. Figure 13 shows typical temperature profiles. The decay of the reactor temperature back to the bath temperature at the completion of reaction is characterized by a heat transfer rate constant 6 measured separately in our reactor to be about 0.9 min- ' or 0.015 sec-'. Provided that the reaction rate is first order with respect to a limiting reagent-HCN in this case-the temperature as a function of time is given by Eq. (35), where C is a constant which depends on the initial HCN concentration, the enthalpy of the reaction, and the heat capacity of the solution and reactor. Knowing 6, one can solve for the rate constant k using Eq. (36) by measuring the time t, for the temperature to reach a maximum after addition of a pulse of HCN. Equation (36) is obtained by setting the time derivative in Eq. (35) to zero at t = t,. dT/dt = C[ke-k' - de-"] ke-kl

= de-al

(35) (36)

That the reaction is indeed first order in HCN can be seen from the fact that the exotherm is proportional to the charge of HCN, while t, is independent of the charge (Fig. 13). By varying in turn the concentrations of nickel, 3PN, Lewis acid (A), and added L, it was possible to obtain the rate expression (37). Linear plots of kobs against [Nil, [A], and [4PN] can be obtained from experiments where each is varied independently, holding the other concentrations fixed. Varying only [L], a plot of log kobsagainst log [L] gives a slope of -2, as shown in Fig. 14. Rate = kobs[HCN] [Nil [3PNI [A]/[L]

(37)

c. A.

40

0.11 t

TOLMAN

I t t n '

lo-'

I

1

et al.

I I

IIII

I .o tL1 ( M I

FIG.14. Effect of ligand concentration on hydrocyanation rate. (Numberscorrespond to the

[L] :[Nil ratio at each point.) OF THE MECHANISMS FOR HYDROCYANATION OF C. SUMMARY PENTENENITRILES

The mechanism shown in Fig. 9 for the hydrocyanation of ethylene with (C, H,)Ni[PO-o-tolyl),], is inconsistent with the kinetic data described above for 4PN with Ni[P(O-p-tolyl),], and Lewis acid (A). This is not unreasonable when we remember that the equilibrium constant for binding of ethylene to Ni(0) is 70 times greater than that for binding of 4PN (Table 11), whereas P(0-p-tolyl), is preferred over P(0-o-tolyl), by a factor of lo8 (Table I)! This leads to the possibility that an intermediate such as 19 is much less important in the 4PN/P(O-o-tolyl), system. How the Lewis acid changes the mechanism is also still not clear. Figure 15 shows a mechanism which is consistent with kinetic results for the hydrocyanation of 4PN, starting with NiL,, and shows two isomeric alkylnickel cyanide complexes RNiL,CN and R'NiL, CN arising from antiand Markovnikov addition of HCN across the double bond. Irreversible C-C coupling steps 7 and 8 are slow and rate determining, as indicated with

NICKEL-CATALYZED OLEFIN HYDROCYANATION

41

N)L4

HNiL&N

/ FIG.15. The mechanism of 4PN hydrocyanation in the absence of a Lewis acid. RNi = NC(CH,),Ni, R’Ni = NC(CH,),(CH,)CHNi.

dashed arrows; all other steps in the loop are rapid and reversible. Whereas reductive elimination from the four-coordinate 16-electron complex RNiL,CN appears inconsistent with the ethylene results (Fig. 9) which indicate that reductive elimination occurs from a five-coordinate 18-electron complex; we cannot discount the possibility that the nitrile group at the end of the alkyl group coordinates in a chelate fashion. The ratio of ADN to MGN in the mechanism is given by k , K 5 / k , K 6 . If one assumes that k, and k , are similar, then the product distribution is controlled by the relative stability ( K 5 / K 6 )of the linear and branched alkyl intermediates. Possibly the linear alkyl is more stable because of less steric crowding, and this may account largely for the ADN: MGN ratio of about 3: 1 observed in the absence of a Lewis acid. Figure 15 has been oversimplified in not showing pathways for olefin coordination to NiL, prior to HCN addition, or protonation of NiL, prior to ligand loss. It also does not show coordination of 4PN as a nitrile, double bond isomerization, or catalyst deactivation. Figure 16 presents a more complete mechanism that includes reactions of HNiL,CN with 3PN and 2PN. The reversibility of steps 4-6 and 13-15 provides a means of isomerizing 3PN to 4PN. Step 20 is drawn dashed to show its irreversibility; we have seen no evidence for isomerization of 2PN, Reversibility of steps 19 and 21 does however provide a means to isomerize C2PN to T2PN. We propose that the ratio ADN:MGN:ESN in the products is controlled primarily by the relative equilibrium concentrations of RNi, R’Ni, and R”Ni intermediates. The addition of Lewis acids further complicates the picture, since each NiCN group in Fig. 16 can provide an electron pair for coordination to a Lewis acid A. This is shown schematically in Fig. 17, where each NiCN group in the A, plane has a corresponding NiCN-A group directly below it in the

42

c. A.

TOLMAN

et al.

FIG. 16. The mechanism of PN isomerization/hydrocyanation in the absence of a Lewis acid. R”Ni = NCCH,(CH,CH,)CHNi. R”’Ni = NC(CH,CH,CH,)CHNi.

A, plane. For clarity, only some of the pathways connecting the two planes are shown. If we number the steps in the A , plane the same as in the A,, plane, but identify them by primes, and assume (1) that all reactions except C-C coupling are rapid and reversible (in agreement with rate and isotopic labeling studies discussed earlier with L = P(O-p-tolyl), and A = BPh,), (2) that NiL4 is the major nickel species in solution, and (3) that [2PN] is negligible, we can readily derive Eq. (38) for the rate.

+ K3K13(K14k8 + Kl5kl6)C3PNI

+ KA[A]K;K;(K;k; + Kbk’,)[4PN] + KA[A1K;K;3(K;4k’8 + K;,k;6)[3PNI)

(38)

The first two large terms in braces are for an unpromoted reaction, the second two for Lewis acid promoted reaction. The fact that the rate dependence (Section V,B,4) is linear with [A] is consistent with a very large KA (as indicated in Section II,E) and means that the first two terms make a negligible contribution in this case. We also find that the last term is very

NICKEL-CATALYZED OLEFIN HYDROCYANATION

43

FIG.17. The mechanism of PN isomerization/hydrocyanationin the presence of a Lewis acid A. The A, plane, described in more detail in Fig. 18, has no Lewis acid. Each complex in the A, plane contains one Lewis acid in the form NICN-A.

small with most Lewis acids (very little ESN formed), so that Eq. (38) reduces to Rate = k

[HCN][NiL,] [A][4PN] CL1

(39)

where k = K K , K , K ; K i ( K ; k; + Kkk‘,), in agreement with the form of Eq. (37). The inverse square dependence on [L] is a consequence of the fact that the composition of the transition state for the rate-determining step has two L‘s less than the major form of the catalyst in solution (RNiL,CN versus NiL,) ( 73). The role of Lewis acid in improving the distribution of linear product, and in improving the number of catalyst cycles, may be explained by referring to Fig. 18. The straight line in each plane is drawn at the level of the free energy of the linear alkylnickel complex. In the A, plane, the hydride is less stable than the linear alkyl (it is converted to the linear alkyl on adding 4PN); in the A, plane the hydride is more stable. The free energies increasing in the order RNiL, CN < R’NiL, CN < R”NiL, CN, attributed to increasing steric crowding, are reflected in decreasing concentrations of these intermediates, and in decreasing rates of formation of product from each. The stability of R”’NiL,CN is attributed to the electronic effect of the electronegative a-cyano group on the R”’ alkyl group. The addition of a bulky Lewis acid, like BPh,, causes increased steric crowding around the nickel, destabilizing the

44

c. A.

TOLMAN

et al.

A ~ N

F]iL$N*A

FIG.18. Free energy diagrams for PN isomerizationfiydrocyanation:(a) unpromoted A, plane, (b) promoted A, plane.

branched alkyls relative to the linear isomer (and the latter relative to hydride). This has the effects of slowing ESN and MGN formation relative to ADN formation, while at the same time increasing the barrier to R"'NiL2CN-A formation. The relative poor distributions observed with ZnC1, and AICIJ are due in part to the small steric requirements of these Lewis acids, and in part to the fact that they accelerate C-C coupling so much that the steady state ratio of [4PN] to [3PN] is significantly less than the equilibrium ratio. This is particularly true of AlCl,, with which significant amounts of ESN are formed (Section V,B,2). Note that under these conditions the simple treatment used to obtain Eq. (38) no longer applies, and both forward and reverse rate constants in steps 5', 6 , 14, and 15' must be explicitly taken into account. The regiospecificity of DCN addition using ZnCl,, and the fact that very little deuterium is found in recovered 3PN, strongly suggest that, at least in this system, the back reactions in steps S, 6 , 14, and 1 6 are slow and olefin isomerization is catalyzed by cationic nickel hydrides, as shown in Fig. 6. This may also occur with other Lewis acids. ACKNOWLEDGMENTS

We wish to acknowledge the large number of people in the Petrochemicals Department and the Central Research and Development Department whose work, though not recognized through publication, has contributed to our understanding of olefin hydrocyanation. In

NICKEL-CATALYZED OLEFIN HYDROCYANATION

45

particular, we wish to thank those whose work has been particularly relevant to the present discussion of mechanism: W. C. Drinkard for his pioneering studies in all aspects of hydrocyanation; L. Scott for his development of the pulse reactor system, which has provided most of the available kinetic data; C. M. King for isolation and characterization of HNi[P(O-o-tolyl),],CNBPh,; and D. N. Marks for thermodynamic equilibrium data on PN isomers.

REFERENCES 1. Parshall, G. W., “Homogeneous Catalysis,” p. 219. Wiley, New York, 1980. 2. Baker, M., J. Electrochem. SOC.111,215 (1964); CHEMTECH, pp. 161-164 (1980); Daly, D. E., CHEMTECH, p. 302 (1980). 3. Harris, C. R., and DeAtley, W. W., U.S. Pat. 2,455,995 (1948). 4. O’Neill, T. G., and Kirkbride, F. W., U. K. Pat. 687,014 (1953). 5. Schulze, W. A., and Mahan, J. A., U.S. Pat. 2,422,859 (1947). 6. Arthur, P., Jr., England, D. C., Pratt, B. C., and Whitman, G. M., J. Am. Chem. SOC.76,5364 ( 1954).

7. 8. 9. 10. 11.

12. 13. 14.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

Drinkard, W. C., and Lindsey, R. V., Jr., US.Pat. 3,496,215 (1970). Drinkard, W. C., US.Pat. 3,496,218 (1970). Drinkard, W. C., and Kassal, R. J., US. Pat. 3,496,217 (1970). A shorter discussion of the mechanism of hydrocyanation may be found in Seidel, W. C., and Tolman, C. A., Ann. N.Y.Acad. Sci. 415,201 (1983). Brown, E. S., in “Organic Synthesis via Metal Carbonyls” (J. Wender and Pino, eds.), Vol. 2, Wiley (Interscience), New York, 1977. Hubert, A. J., and Puentes, E., “Catalysis in CI Chemistry” (W. Keim, ed.), pp. 219-242. Reidel, Dordrecht, 1983. James, 8. R., in “Comprehensive Organometallic Chemistry” (G. Wilkinson, F. G. A. Stone, and E. W. Abel, eds.), pp. 353-360. Pergamon, Oxford, 1982. Jolly, P. W., and Wilke, G., in “The Organic Chemistry of Nickel,” Vol. 1. Academic Press, New York, 1974. Tolman, C. A., J. Am. Chem. SOC.92,2953 (1970). Tolman, C. A., J. Am. Chem. SOC.92,2956 (1970). Gosser, L. W., and Tolman, C. A., Inorg. Chem. 9, 2350 (1970). Tolman, C. A., Seidel, W. C., and Gosser, L. W., J. Am. Chem. SOC.96, 53 (1974). Tolman, C. A., Reutter, D. W., and Seidel, W. C., J. Organomef.Chem. 117, C30 (1976). Tolman, C. A., Chem. Rev. 77, 313 (1977). Mynott, R., Mollbach, A., and Wilke, G., J. Organomef. Chem. 199, 107 (1980). Tolman, C. A., Seidel, W. C., and Gosser, L. W., Organomeralfics2, 1391 (1983). Meier, M., Basolo, F., and Pearson, R. G., Inorg. Chem. 8, 795 (1969). Tolman, C. A., Chem. SOC.Rev. 1, 337 (1972). Tolman, C. A,, Inorg. Chem 10, 1540 (1971). Seidel, W. C., and Tolman, C. A., h f g . Chem. 9, 2354 (1970). Tolman, C. A., and Seidel, W. C., J. Am. Chem. SOC.96,2774 (1974). Tolman, C. A., J. Am. Chem. SOC.%, 2780 (1974). Tolman, C. A., Riggs, W. M., Linn, W. J., King, C. M., and West, R. C., Inorg. Chem. 12,2770 (1973).

Guggenberger, L., 1nOt-g. Chem. 12,499 (1973). Tolman, C. A., English, A. D., and Manzer, L. E., Inorg. Chem. 14, 2353 (1975). Wilke, G., and Hermann, G., Angew. Chem. Inr. Ed. Eng. 1, 549 (1962). Jolly, P. W., Jones, K., Kruger, C., and Tsay, Y.-H., J. Organornet. Chem. 33, 109 (1971). Hermann, G., ph.D. Thesis, Aachen, Rheinisch-Westfalische Technische Hochschule, 1963. 35. Ittel, S . D., Inorg. Chem. 16, 2589 (1977). 30. 31. 32. 33. 34.

46 36. 37. 38. 39.

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et al.

Tolman, C. A., Organometallics 2, 614 (1983). Schunn, R. A., Inorg. Chem. 9, 394 (1970). Green, M. L. H.,and Saito, T., J. Chem. SOC.D, 208 (1969). Drinkard, W. C., Eaton. D. R., Jesson, J. P.. and Lindsey, R. V., Jr., Inorg. Chem. 9, 392

( 1970). 40. Tolman, C. A,, J. Am. Chem. SOC.92,4217 (1970).

41. Eaton, D. R., McGlinchey. M. J., MofTat, K. A., and Buist, R. J., J. Am. Chem. SOC.106,8110 (1984). 42. Tolman, C. A., J. Am. Chem. SOC.92,6777 (1970). 43. Meakin, P., Jesson, J. P., Tebbe, F. N., and Muetterties, E. L., J. Am. Chem. SOC.93, 1977 (1971). 44. Meakin, P., Muetterties, E. L., and Jesson, J. P., J. Am. Chem. SOC.94, 5271 (1972). 45. Tolman, C. A,, Inorg. Chem. 11, 3128 (1972). 46. Druliner, J. D., English, A. D., Jesson, J. P., Meakin, P., and Tolman, C. A,, J. Am. Chem. SOC. 98, 2156 (1976). 47. Tolman, C. A., Seidel, W. C., Druliner, J. D., and Domaille, P. J., Organometallics 3, 33 ( 1984). 48. Druliner, J. D., Organometallics 3, 205 (1984). 49. Davis, B. R., Payne, N. C., and Ibers, J. A., Inorg. Chem. 8,2719 (1969). 50. Laing, M., Krueger, G., DuPreez, A. L., J. Organomet. Chem. 82, C40, (1974). 51. Richmond, T. G., Basolo, F., and Shriver, D. F., Inorg. Chem. 21, 1272 (1982). 52. Butts, S. B., Strause, S. H., Holt, E. M., Stimson, R. E., Alcock, N. W., and Shriver, D. F., J. Am. Chem. SOC.102,5093 (1980). 53. Tolman, C. A., J. Am. Chem. SOC.92,6785 (1970). 54. Keim, W., Behr, A,, Luhr, H.-O., and Weisser, J., J. Catal. 78, 209 (1982). 55. Backvall, J.-E., and Andell, 0. S., J. Chem. Soc., Chem. Commun., 260 (1984). 56. Jackson, W. R., and Lovel, C. G., Aust. J. Chem. 35,2053 (1982). 57. Deeming, A. J., Johnson, B. F. G., and Lewis, J., J. Chem. Soc., Dalton Trans., p. 1848 (1973). 58. Hughes, R. P., and Powell, J., J. Organomet. Chem. 60,409 (1973). 59. Wu, C. Y., and Swift, H. E., A.C.S. Symp. Homog. Catal., Div. Pet. Chem., Houston, p. 372, 1980. 60. Tolman, C. A., J. Am. Chem. SOC.94,2994 (1972). 61. McKinney. R. J., Organometallics 4, 1142 (1985). 62. Backvall, J. E., and Andell, 0. S.,J. Chem. Soc., Chem. Commun., p. 1098 (1981). 63. McKinney, R. J., and Roe, D. C., J. Am. Chem. SOC.107,261 (1985). 64. Yamamoto, T., Yamamoto, A., and Ikeda, S., J. Am. Chem. SOC.93, 3360 (1971). 65. Abe, Y., Yamamoto, A., and Yamamoto, T., Organornetallics 2, 1466 (1983). 66. Favero, G., and Turco, A., J. Organomet. Chem. 105, 389 (1976); Favero, G., Gaddi, M., Morvillo, A., and Turco, A., J. Orgunomet. Chem. 149,395 (1978); Favero, G., Morvillo. A., and Turco, A., J. Organomet. Chem 162, 99 (1978). 67. Tatsumi, K., Nakamura, A., Komiya, S., Yamamoto, A., and Yamamoto, T.,J. Am. Chem. SOC.106, 8181 (1984). 68. Werner, H., and F e w , R., J. Organomet. Chem. 232, 351 (1982). 69. Cotton, F. A., and LaProde, M. D., J. Am. Chem. SOC.90,5418 (1968). 70. Ariyaritne, J. K. P., and Green, M. L. H., J. Chem. Soc., p. 2976 (1963). 71. Halpern, J., and Wong, L-Y., J. Am. Chem. SOC.90,6665 (1968). 72. Taylor, B. W., and Swift, H. E., J. Coral. 26, 254 (1972). 73. Tolman, C. A., and Faller, J., in “Homogeneous Catalysis with Metal Phosphine Complexes” (L. H. Pignolet, ed.), Chapt. 2. Plenum, New York, 1983.

ADVANCES IN CATALYSIS. VOLUME 33

Supported Chromium Catalysts for Ethylene Polymerization M . P. McDANIEL Phillips Research Center Bartlesuille. Oklahoma Introduction . . . . . . . . . . I . Hexavalent Cr/Silica . . . . . . . A. Chromate versus Dichromate . . . B. Hydroxyl Replacement by Chromium . C . Chromyl Chloride . . . . . . . D . Reaction with HCL . . . . . . E. Saturation Coverage . . . . . . F . Conclusion . . . . . . . . . I1. Reduced Cr/Silica . . . . . . . . A . Valence of the Active Site . . . . . B. Cr(V)-The y-Phase Resonance . . . C. The Cr(I1) Species . . . . . . . D . Trivalent Chromium . . . . . . E . Organochromium Catalysts . . . . F . Conclusions . . . . . . . . . 111. Polymerization over Cr/Silica . . . . . A . Modes of Polymerization . . . . . B. Mechanism of Polymerization . . . C . Molecular Weight Control . . . . D . Branching . . . . . . . . . E. Influence of the OH Population . . . F. Molecular Weight Distribution . . . G . Active Site Concentration . . . . . IV . The Porosity of the Support . . . . . A . Activity versus Pore Volume . . . . B. Critical Pore Diameter . . . . . C. Fragmentation . . . . . . . . D . The Kinetic Profile . . . . . . E. Molecular Weight versus Porosity . . V. Modifications of Cr/Silica . . . . . . A. Promotion by Titania . . . . . . B. Anhydrous Impregnation of Chromium C. Dehydration by Chemical Means . . D . Reduction/Reoxidation . . . . . VI . Chromium Oxide on Other Supports . . . A. Alumina . . . . . . . . . . B. Aluminum Phosphate . . . . . . 41

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48

M. P. MCDANIEL

VII. Organochromium Catalysis . A. Attachment to the Support B. Kinetics of Polymerization C. Termination Mechanism . D. Support Effects. . . . References . . . . . .

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Introduction

Surely the Phillips Cr/silica polymerization catalyst, discovered by J. P. Hogan and R. L. Banks in the early 1950s (I),can claim one distinction-to have been one of the most studied and yet most controversial systems ever. Today we seem to be debating the same questions posed over thirty years ago, being no nearer to a common view. There is enough blame to go around for the confusion. Industrial scientists have necessarily been secretive due to the commercial importance of the discovery and to the cutthroat competition between companies. Most of this great mountain of commercial research has focused not so much on understanding the catalyst, as on exploiting it, and the bulk of this information remains secret. In contrast, the academic community has been astonishingly prolific on what might be considered an industrial topic. Researchers groping in this industrial darkness rarely refrained from drawing sweeping conclusions about the commercial catalyst from a few vacuum line experiments. And there has been a tendency to regard Cr/silica as monolithic, whereas in fact an industry has developed around its diversity. So in this climate there is little reason to expect this paper will do any more than add to the confusion. Nevertheless an industrial perspective has often been missing from the controversy, and perhaps that can be remedied to some degree here. I. Hexavalent Cr/Silica

A. CHROMATE VERSUS DICHROMATE The Phillips Cr/silica polymerization catalyst is prepared by impregnating a chromium compound onto a wide pore silica and then calcining in oxygen to activate the catalyst. This leaves the chromium in the hexavalent state, monodispersed on the silica surface. Chromium trioxide (CrO,) has been impregnated mast commonly, but even a trivalent chromium salt can be used since oxidation to Cr(V1) occurs during calcining.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

49

Although bulk CrO, begins to decompose above 200°C into 0, and Cr,O,, a certain amount is stabilized on silica even up to 900°C. This is due to the formation of a surface chromate or perhaps dichromate ester in which each chromium atom is directly linked to the support.

Chromate

Dichromate

Whether the chromium atoms are isolated as chromate, or exist in pairs as dichromate, has proven a difficult problem despite its importance to our understanding of the catalyst. Some researchers believe polymerization can occur only when the chromium atoms are paired, and elaborate polymerization mechanisms are proposed involving two chromium atoms (2-2Z). This was advanced as the reason why unsupported Cr(V1) esters have failed to exhibit activity, because they are always chromate-like structures (2). Dichromate is the main species in CrO, solutions. But that does not necessarily imply bonding to the silica as dichromate, as some have suggested (5). Even in acid solution an equilibrium exists between the two species, and insoluble chromates are often precipitated from these “dichromate” solutions. Catalysts made from ammonium chromate or ammonium dichromate or CrO, behave identically after being calcined. Spectroscopy has not proven to be very conclusive in solving this problem. Similarities between the visible spectrum of the calcined catalyst and that of bulk dichromates have been noted (5,22-14). In the end, however, there is always doubt about the interpretation of spectra because no adequate reference data exist for these surface bound species (26). Krauss and coworkers have carefully studied the luminescence of Cr/silica and concluded that at least a portion of the chromium is present as chromate (25). Others have examined the geometry of the chromate and dichromate ions and compared then to models of the silica surface (22). Again, the uncertainty is high because we have very little idea what the silica surface really looks like. Hydroxyl spacings derived from a particular face of cristobalite or tridymite may have little relevance to amorphous, high surface area silica gel. B. HYDROXYL REPLACEMENT BY CHROMIUM Several researchers have measured the change in hydroxyl population on binding of the chromium. A chromate species should react with two hydroxyls per chromium, whereas dichromate displaces only one. Results

50

M. P. MCDANIEL

1.6

a 1.0 \

-

I 0

0

E

0.6

ACTIVATION TEMPERATURE ('C) FIG.1 . Aqueous impregnation ofCrO, on silica. Chromium lowers the hydroxyl population found on silica after calcining. The number of hydroxyls lost per chromium attached is not constant but depends on the calcining temperature.

from this approach have not been very conclusive. Hogan, working from gravimetric measurements at 150"C, concluded that CrO, attaches mainly as chromate (17). Others, working at 500-600, reported that dichromate was the dominant species (12, 13). These seemingly contradictory results can be rationalized. Figure 1 shows the OH populations on three silica samples containing various loadings of chromium after calcination between 200 and 800°C (18). Notice that the chromium does displace hydroxyls at all temperatures, but more hydroxyls are displaced at 200°C than at 800°C. Therefore, the AOH/Cr replacement ratio is not a constant 2 or 1 as is usually expected for chromate or dichromate. Instead it varies from near 2 at 200"C, in agreement with Hogan, to near 1 at 500"C, which also agrees with earlier reports. And at higher temperatures it drops even below 1 which does not correspond to either species. These data are probably artifacts of the experiment. For illustration, Fig. 2 shows a hypothetical portion of the surface, call it region A, which before calcining contains two OH groups. Region B is identical to region A except that the two OH sites have been occupied by a chromate species. At the start of the experiment region A contains two more OH groups than region B, and AOH/Cr = 2. During calcining, however, region A dehydroxylates normally up to 500°C at which point it contains only half of its initial O H population and AOH/Cr = 1, even though the chromium is still present as chromate. At higher temperatures the ratio drops even lower. The most that can be concluded from such experiments is that CrO, attaches initially to the hydrated surface as chromate. Hierl and Kraus, in

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

51

PROMOTED

UNPROMOTED

FIG.2. Two hypothetical portions of surface shown before and after calcining: Virgin (unpromoted) and with chromate attached (promoted). The number of hydroxyls apparently displaced by a chromate species is not always 2, but rather depends on the extent and pattern of dehydroxylation during calcining.

another type of gravimetric study using CO reduction, also concluded that chromate is the dominant species (19). There is a basic uncertainty associated with all such measurements of hydroxyl replacement by Cr, however, because a correct interpretation depends on knowing what sites are occupied and how this affects later dehydration of the silica. C. CHROMYL CHLORIDE

In a more direct approach, calcined silica can be treated with chromyl chloride vapor at 200°C (18). Attachment as chromate should leave no chloride on the catalyst, but if the spacing between hydroxyls does not allow chromate, only the chlorochromate species (Si-0-Cr0,C) can form, leaving one C1 per Cr attached. Formation of dichromate should not be possible under these conditions.

OH OH

L

OH

L

CrO CI 200-c

A

/- \

/

U L Chromate

0

Chlorochromate

When the silica has been calcined a 400°C or lower, most or even all of the chromyl chloride attaches as chromate, losing two chlorides per Cr. All of the chromium remains hexavlent. These chromate catalysts exhibit nearly identical activity to CrO, on silica activated at the same temperature. The kinetics

52

M. P. MCDANIEL

of polymerization, polymer properties, and even the UV-visible reflectance spectra are all similar. However, when the silica has been calcined at 800°C or above only chlorochromate is formed from chromyl chloride. This mimics the behavior of other reagents such as chlorosilanes,TiCl,, or BCl,, which have been used to determine the extent of hydroxyl pairing on silicas (20-23). The chlorochromate species does not polymerize ethylene.

D. REACTION WITH HCl The reverse reaction has also been studied (18). Treating the calcined CrO,/silica with dry HCl strips off the chromium as chromyl chloride vapor leaving one OH for each former point of attachment. The silica base is otherwise unaffected. Hydroxyl measurements before and after the stripping give a direct indication of the bonding of Cr(VI), because chromate leaves two hydroxyls per chromium removed and dichromate leaves only one. 0

0

' c t

/ \

Chromate

0 0

I1 I1

O=CrOCr=O I I 0 0

HCI 130°C

+ CrO,Cl, OHOH

L

Dichromate

Despite scatter in the data it appears that both temperature and chromium loading are important (18). At the lower loadings chromate is the main species found, even after calcination at 700-900"C, but as the loading is increased evidence for some dichromate is also found, particularly at the higher temperatures. A typical commercial catalyst of 1 % Cr would progress from nearly pure chromate at 400°C or below, to perhaps as much as 25 "/, dichromate at 800-900"C, if the data can be trusted. One interesting result of stripping off Cr(VI), whether chromate or dichromate, is that it leaves pairs of hydroxyls, even if the catalyst has been calcined previously at 800°C to remove OH pairs. Despite being paired, these hydroxyls do not condense easily when heated; a temperature of 800°C is required to remove all of them. However, they do react with chromyl chloride vapor at 200°C to yield a good deal of chromate. (Ordinarily silica calcined at 800°C forms no chromate when it reacts with chromyl chloride.) This suggests that even at 800°C Cr/silica may contain a considerable amount of chromate.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

53

Cr(Vl)/nm2 AFTER ACTIVATION

2.0

0 1 2 3 4 5 TOTAL Cr IMPREGNATED (Cr/nm')

FIG.3. As the chromium loading increases, all is stabilized in the hexavalent state until a certain saturation coverage is reached, which depends on calcining temperature. Beyond this limit the excess is converted mainly to Cr,O,.

E. SATURATION COVERAGE Figure 3 shows what happens to CrO,/silica during calcining when the chromium loading is increased (18,24).At first all of the chromium remains hexavalent, but at some point, as the loading increases, saturation occurs and beyond that the excess CrO, merely decomposes to the trivalent oxide, a chromia, This limit is often a sharply defined one and varies only with temperature, not with the total chromium loading. A fully hydrated silica contains about 4.5 O H nm-2 (20),so saturation of sites by chromate would yield no more than 2.2 Cr nm-*, whereas dichromate could give up to 4.5 Cr nm-2. The highest saturation coverage actually found was at 425"C, i.e., 1.8-2.3 Cr(V1) nm-2. This suggests mainly chromate. However, saturating the surface with chromyl chloride yields only 1.6 chromates nrnw2.So again we conclude that some dichromate may also exist under crowded conditions. F. CONCLUSION

It seems likely that the chromate species can exist on the silica surface and acts as parent for an active site. Thus, pairing of chromium atoms is not a requirement for polymerization. Chromium trioxide (CrO,) probably binds to the silica as chromate initially, at least at the ordinary 1% loading. But some rearrangement to dischromate a t high temperatures may occur. If so, it could account for the change in color from yellow to orange, and even to red in some modified catalysts.

54

M. P. MCDANIEL

II. Reduced Cr/Silica OF A. VALENCE

THE

ACTIVESITE

The Phillips catalyst contains hexavalent chromium a..;r calcining, but the early discoverers quickly realized that reduction takes place in the reactor on contact with ethylene, leaving chromium in a lower oxidation state as the active species. A worldwide debate has continued to this day about the valence of this reduced species. Chromium in every valence state from Cr(I1) to Cr(V1) has been proposed as the active site, either alone or in combination with another valence. The question has received far more attention than it probably deserves, undoubtedly at the expense of more fundamental issues, like the polymerization mechanism. B. Cr(V)-THE Y-PHASE RESONANCE Most of the early work concentrated on an ESR signal that appears when the hexavalent catalyst is partially reduced by H, or CO. This y-phase resonance is generally attributed to a Cr(V) surface species (25,26), although some insist that it involves a combination of Cr(V1) and Cr(II1) (10,11,27). Usually these studies found either a correlation or a reverse correlation between this signal and some measure of polymerization activity, often from a catalyst bearing little resemblance to those used commercially. This was indirect evidence at best, and the issue was always clouded by the simultaneous presence of several oxidation states. This work has already been summarized adequately (28-30). C. THECr(I1) SPECIES Krauss and Stach (31) demonstrated that the hexavalent catalyst can be quantitatively reduced by CO at 350°C to divalent chromium. This material has no y-phase resonance but is active for ethylene polymerization, indicating that Cr(I1) is definitely an active valence.' These results have since been confirmed by several other laboratories, including this one (30). In fact, Hogan concluded, as early as 1959 from similar reduction studies, that the active species must be divalent. The CO-reduced catalyst polymerizes ethylene in a high-pressure autoclave much like its hexavalent parent, and produces almost identical polymer. Since the polymer properties are extremely sensitive to the catalyst pretreatment, this is a strong endorsement for the conclusion that Cr(I1) is probably also the active species on the commercial catalyst after reduction by ethylene. We speak of the active valence before alkylation, since the addition of ethylene could further change the formal oxidation state. In fact, oscillation between two states, e.g., Cr(I1) and Cr(III), during polymerization is considered likely in many schemes.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

55

Further evidence is the x-ray photoelectron spectrum of the catalyst, which is about the same whether the reduction is carried out in C O or in ethylene (30). However, more positive evidence comes from Baker and Carrick (32) who measured the valence on a catalyst exposed to ethylene at 125"C, a typical polymerization temperature. Within a few minutes they obtained 85-96 % conversion to Cr(I1). Formaldehyde was the by-product. The divalent catalyst is highly coordinatively unsaturated and therefore exhibits some unusual chemistry (33-42). It has a light green color but quickly truns blue when exposed to N,, indicating a weak chemisorption. Carbon monoxide is adsorbed to yield a violet color, and of course it poisons the polymerization. U p to two molecules can be adsorbed. Olefins also adsorb in a 2:l ratio, and acetylene is converted to benzene. Polar compounds like alcohols, ethers, or amines are strongly held. Nitric oxide (NO) attaches in a 2: 1 ratio. A few experimenters have found IR bands which they assign to adsorbed CO bridging two chromium atoms (2-4). If correct this would indicate that at least some of the chromium atoms have close neighbors. These assignments are very uncertain, though, and even if correct do not necessarily imply a paired active site, or even a dichromate parent. Similar claims are also made for chromocene/silica, in which pairing seems most unlikely. Oxygen is adsorbed by the divalent catalyst with a brilliant flash of chemiluminescence, converting the chromium back to its original orange hexavalent state (15, 17,41,42). The ease with which this reversal occurs suggests that there is little rearrangement during reduction at 350°C.

However, higher temperatures do seem to cause a rearrangement even though no further reduction is observed (16,30,43,44).The reoxidation to Cr(V1) can be less complete when the catalyst has been reduced in CO at 800-900"C, sometimes also yielding Cr(II1). The results of this rearrangement are decreased coordinative unsaturation, shown in Fig. 4 by decreased chemisorption of CO, and diminished activity for ethylene polymerization (Fig. 5). What happens during rearrangement at high temperatures is unknown. A clustering of the chromium may occur, although no CrO crystallites have been detected by X-ray diffraction ( 4 4 , or perhaps the chromium finds a better position on the surface so that it can coordinate with more oxide or hydroxyl ligands. The Turin group has detected four distinct types of Cr(I1) that they suggest differ in the number of coordinated oxides (16,43).

56

M. P. MCDANIEL 2.0

1.5 \ u)

W

-I

3

0 1.0 W

A

0

z 0.5

-

0

300 400 500 600 700 aoo CO TREATMENT TEMPERATURE (‘C)

FIG.4. The coordinative unsaturation of Cr(II)/silica, measured by the chemisorption of CO pulses at - 78“C, declines with increasing temperature of reduction in a series of samples calcined first in air at 850”C, then in CO at the temperature shown. 6r

0

20

40

60

80

100

POLYMERIZATION TIME

FIG.5. Ethylene pollymerization kinetics (kg polyethylene g- catalyst hour- ’) over Cr/silica, (A) calcined in air at 850”C, then reduced in CO at (B) 3WC, (C) 5WC, or (D) 700°C.

Reduction in CO at 350°C generated mainly “exposed” Cr,(CN = 2) and Cr,(CN = 3) sites (CN = coordination number), which were highly reactive. But Cr, was deactivated on further heating to “unexposed ” Cr,(CN = 4). Krauss has also detected several types of Cr(I1) species which differ in their reactivities (15). He notes that hydroxyls on the silica can interfere with the reduction to Cr(I1) and diminish the “quality” (that is the coordinative

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

57

unsaturation) of the Cr(I1) that does form (36).Hogan observed that H, is a poorer reducing agent than C O because it forms water, which can distabilize Cr(V1) or oxidize Cr(I1) (17).

D. TRIVALENT CHROMIUM Surprisingly, the direct evidence of Cr(I1) has not ended the controversy, Several groups have since proposed Cr(II1) as the active valence. The most direct evidence comes from Kazansky (45) and more recently from Lunsford and co-workers (46,47). Trivalent chromium salts, notably CrCI,, impregnated onto silica and calcined under vacuum, were found to polymerize ethylene. In fact, similar catalysts made from the divalent salts were not active. Unsupported chromia has also been reported to polymerize ethylene weakly (48). However, the attachment of amorphous Cr,O, to the silica (30,46,49) probably does not resemble that of the Cr(V1) and Cr(I1) species discussed in Section 11, C. It may have little relevance to the commercial catalyst, even if Cr(II1) is active. These trivalent catalysts have been purported to have equal (47) or much better ( 2 , 3 )activity than the standard Cr(VI)/silica recipe. This would be a real boon to commercial producers who always look for better activity and prefer, for environmental reasons, not to handle Cr(V1) when possible. We have tried to confirm these findings many times, but if the trivalent catalysts have any activity at all in the high-pressure autoclave, it is infinitesimal by commercial standards. In fact, even Kazansky’s reported activities at high pressure are microscopic in comparison (45). To our knowledge, no other producer has been able to use such a trivalent catalyst either. Some explanation for this conflict may lie in the choice of polymerization temperature (50). Commercial runs are usually made at 80-1 10°C. Lowering the temperature also lowers the activity of Cr(II)/silica; depending on the pressure, in the range 0-25°C it is barely active. Some proponents of Cr(II1) have carried out their experiments at 25°C in a vacuum line (46,47). Another argument advanced in support of Cr(II1) is based on optimum reduction (45). The activity of the hexavalent catalyst passes through a maximum with increasing severity of reduction, and finally yields an overreduced inactive Cr(1I). This could imply that the active valence is an intermediate one, i.e., Cr(II1). But this is indirect evidence again, obtained from catalysts having several oxidation states. We already know that Cr(I1) becomes deactivated by overreduction. Finally, the most nagging argument advanced by supporters of Cr(II1) is not unreasonable. They suggest that all of the activity comes from an undetectably tiny portion of the chromium (47). Of course this reasoning could be applied to other valences too. Unfortunately it is true that we know

58

M. P. MCDANIEL

very little about the active site population. This subject will be addressed in the next section.

E. ORGANOCHROMIUM CATALYSTS It is curious that during 30 years of interminable debate about valence, almost no mention has been made of organochromium compounds that also make active catalysts. As early as 1961 Walker and Czenkusch at Phillips showed that diarene-Cr(0) compounds polymerize ethylene when deposited on silica or silica-alumina (51). We now suspect that the Cr(0) is oxidized by silanol groups to Cr(I), implying that Cr(1) is also an active valence. Such catalysts, however, do not resemble Cr(VI)/silica. The kinetics and polymer obtained are entirely different. Many other organochromium compounds have since been synthesized and found to be active, including those with chromium exhibiting every valence up to Cr (IV). Chromocene is a well-studied example of an active divalent compound (52-55). Pentadiene-Cr(I1) (56)is another, along with allyl-Cr(I1) (52,57). Allyl-Cr(II1) is also active (52, 57-61). /3-Stabilized alkyls of Cr(I1) and Cr(IV) such as trimethylsilylmethyl-Cr(IV), which also polymerizes ethylene when supported on an oxide carrier, have been synthesized and tested in this laboratory (57,62). All these organochromium catalysts are comparable in activity to the Cr(VI)/silica standard.

F. CONCLUSIONS It is incorrect to regard only one particular valence state of chromium as the only one capable of catalyzing ethylene polymerization. Active catalysts have been made from organochromium compounds with every valence from Cr(1) to Cr(1V). On the commercial Cr(VI)/silica catalyst the predominant active valence after reduction by ethylene is probably Cr(II), but other states, particularly Cr(III), may also polymerize ethylene under certain conditions. It is ridiculous to speak of a particular valence state as if it were monolithic. Surely a Cr(I1) or Cr(II1) species formed by reduction of Cr(V1) is not equivalent to one formed by impregnating CrC1, or CrCl,. In fact, the Turin group has found four distinct types of Cr(I1) formed by reduction, some active and some not. Furthermore, despite a wide range of valences the organochromium catalysts resemble each other more than they resemble chromium oxide catalysts. This indicates that the environment of the chromium, i.e., its type and arrangement of ligands, may be more important that the formal valence.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

111.

59

Polymerization over Cr/Silica

A. MODESOF POLYMERIZATION

Three processes are used commercially to make linear polyethylenesolution, slurry, and gas phase. All are called “low-pressure” processes (< 50 atm) to distinguish them from the free radical or “high-pressure” process that makes highly branched polyethylene. In the solution mode a hydrocarbon solvent at 125-170°C dissolves the polymer as it forms. The reaction usually slows as the solution becomes viscous because it becomes difficult to stir ethylene into the liquid phase. In contrast, The slurry process uses a poor solvent and low temperature (60-1 10°C) to prevent dissolving or even swelling of the polymer. Each catalyst particle creates a polymer particle several thousand times larger than itself. There is no viscosity limitation in the slurry method; the diluent serves to transfer heat and to keep the catalyst in contact with ethylene and other reactants. Finally, the gas-phase process is much like the slurry method in that polymer particles are formed at similar temperatures. A bed of catalyst/polymer is fluidized by circulating ethylene, which also serves as a coolant. Whatever the merits of each process in a continuous commercial operation, the slurry process is very convenient for batch polymerization studies in the laboratory. The diluent permits precise control of the temperature and serves to dissolve ethylene and other reactants that must contact the catalyst during polymerization. Most of the work reported here was done in a slurry reactor. This raises the question of whether diffusion plays a role in the kinetics of slurry polymerization. Certainly there is no limitation across the gas-liquid interface; doubling the catalyst also doubles the polymer yield, but increasing the stirring rate does nothing. Diffusion through the polymer particle is a more troubling issue. There are times when the polymerization clearly becomes diffusion limited, or “fouled,” due to solvation of the polymer, but this is rarely a problem if the temperature is kept down and the molecular weight up. Several facts suggest that diffusion through the polymer particle normally does not affect slurry kinetics: (1) Small catalyst particles (< 40 pm) are no more active than large ones (> 500 pm).(2) The polymer is highly porous. (3) The activity of a catalyst can be varied widely through chromium concentration or calcining without changing the kinetic profile. (4) Ziegler catalysts are often ten times more active than the best Cr/silica catalysts due to a higher concentration of active metal. (5) Many Ziegler and organometal catalysts show constant activity form the start of the run when there is no polymer to the end when great mountains of polymer surround the catalyst.

60

M. P. MCDANlEL

On the other hand, there is no doubt that diffusion through the silica does affect polymerization activity (although probably not the kinetic profile). This is discussed in Section IV. Finally, the great bulk of published polymerization studies have been conducted on a vacuum line at 25°C and c 1 atm. Such activity measurements may be the most accurate of all, in their own way. Unfortunately, the experimenters often draw conclusions about the commercial world, to which such studies have little connection. Polymerization is not just the addition of ethylene to a chain. It involves a complicated series of reactions-reduction, alkylation, propagation, and termination-all of which are pressure and temperature dependent. Therefore, vacuum line techniques are not always reliable indicators of a catalyst’s performance under commercial conditions.’ To study polymerization, there is no substitute for making polymer.

B. MECHANISM OF POLYMERIZATION Cr(VI)/Silica develops polymerization activity only gradually when exposed to ethylene at 100°C in a slurry autoclave. An example is shown in Fig. 5, which depicts an experiment in which the catalyst was not immediately active upon introduction into the reactor, but first underwent a dormant period or induction time. The rate of polymerization then increased during the remainder of the experiment. This is thought to be due to the slow reduction of Cr(V1) by ethylene to the Cr(I1) active site, or perhaps to the desorption of by-products such as formaldehyde (32). Thus, the concentration of active sites is probably not constant but increases with time. Below 100°C the induction time becomes longer until at about 60°C there is almost no activity. Conversely, increasing the temperature shortens the induction time. At 150°C the catalyst exhibits an immediate and constant activity in solution phase polymerization. Prereducing the catalyst in CO at 300°C to make Cr(II)/silica also eliminates the induction time, as shown in Fig 5. Such catalysts exhibit polymerization activity at a temperature as low as 25°C. Reducing agents such as the alkyls of aluminum, boron, or zinc also eliminate the induction time of Cr(VI)/silica when added to the reactor at l W C , but as shown in Fig. 5, even prereduced catalysts often show a gradual rise in activity during the run. This may be due to an initiation raction in which the active site is alkylated by ethylene. The incorporation of the first ethylene is poorly understood, but it is probably different and slower than incorporation of succeeding monomer.

* For example, Cr(VI)/silica, the commercial catalyst itself, is inactive at 25°C and < 1 atm because it does not reduce.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

61

One possible initiation mechanism involving an allyl species is shown in scheme 1 (63).One can imagine chain growth at the Cr-ally1 bond when the allyl is in one of the monohapto forms. (The first chain would likely be different form later ones.) The extra hydrogen atom could be attached to the chromium as hydride as shown, so that it too might be the starting species, or to the accompanying oxide as a proton. Many other schemes are also conceivable, and there is little evidence to favor one over another, with one exception: Hydroxyls on the silica are not required (9), because completely dehydroxylated preparations still make excellent catalysts. It is even possible that several initiation mechanisms may operate, depending on conditions. For example, Krauss found methyl ketone groups at the end of the first chain produced by Cr(VI)/silica, which he attributed to a combined reduction/ alkylation step (64), but this would not be applicable to CO-reduced catalysts.

Cr-H or

Ethylene

H H Cr-CH-CH H

F

Propagation

Cr-CH-chain H

H H Cr-CH-CH-chain

H H Cr-CH-CH-chain

Cr-H

H HC=CH-chain

by p-Elimination

SCHEME 1

Propagation and termination are also part of polymerization and again there are several plausible mechanisms. Some researchers have suggested that the propagation may involve a carbene species ( 6 3 6 6 ) . There is little evidence to support or deny this, but I3C-NMR spectroscopy indicates that there is no hydrogen scrambling during propagation (67),which is expected from some carbene mechanisms. Infrared or NMR analysis of homopolymers nearly always indicates one terminal vinyl and one methyl group per chain. Other structures with branching or internal unsaturation occur very infrequently. One can imagine schemes in which the vinyl group forms first (68), but termination by flhydride elimination seems more likely, at least until evidence to the contrary

62

M. P. MCDANIEL Molecular Weight XI Melt Index

Inherent Viscositv

I

m-2.1)

. 1.8 ' 1.2 \

*

0.8

.0.4

im

lo5

110

I 115

Reactor Temperature (73

FIG.6. A primary means of regulating polyethylene chain length is through control of reactor temperature. Increasing the temperature enhances termination, probably by destabilizing the Cr-chain bond, resulting in shorter chains.

is presented. fl-Elimination fits the known chemistry of metal alkyls, and quick-kill experiments have found methyls formed early in the catalyst history, but not vinyls (9,69).

c.

MOLECULAR WEIGHT CONTROL

The rate of termination of chains, relative to propagation, determines the average chain length, i.e., the molecular weight (MW) of the polymer. Another indication of MW is the melt index (MI), which is a measure of the molten polymer fluidity and is therefore inversely related to its MW. For these systems the MI typically vanes with the inverse fourth power of the MW. Thus, the higher the MI the higher the relative termination rate. Manufacturers and customers of polyethylene tend to be more concerned with MI than M W because it gives an indication of the flow of the molten polymer during processing. Once the initiation has occurred and polymerization has begun, the MW of the product can be controlled by several reactor variables. For example, the effect of reactor temperatrue is shown in Fig. 6. Raising the temperature of polymerization greatly enhances the rate of termination, i.e., it makes the metal-polymer bond less stable and more inclined to undergo fl-elimination. But its effect on the propagation rate is minor in comparison, so the result is shorter chains and increased MI.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

63

FIG.7. The polymerization activity of Cr/silica exhibits almost first-order dependence on ethylene concentration,while the terminationrate is less affected. The result, as ethylene pressure increases, is longer chains.

Changing the ethylene concentration has the opposite effect. The propagation step is highly dependent on ethylene pressure, as shown in Fig. 7 by the near first-order increase in activity, but the rate of termination is less dependent on ethylene. So increasing the ethylene concentration increases propagation without greatly affecting termination and the net result is longer chains, hence the declining MI in Fig. 7. Hydrogen can also be used to regulate the molecular weight of the polymer. With Ziegler catalysts it shortens the chains by hydrogenation and is the main method of MW control. The result is a saturated polymer containing few, if any, vinyl end groups. Ti-CH,-chain

H, Termination by hydrogenation

b

Ti-H

+ CH,-chain

On Cr/silica catalysts, however, the effect of hydrogen is minor in comparison. And surprisingly, there is no evidence of hydrogenation on Cr/silica catalysts, Hydrogen must shorten the chains in some other still mysterious way.

D. BRANCHING To introduce short branches into the otherwise linear polymer, a-olefins are sometimes copolymerized with ethylene. The random branching breaks up the crystallinity of the polymer, imparting more flexibility. In addition to

64

M. P. MCDANIEL

the usual terminal vinyl unsaturation the polymer now exhibits vinylidene as well. This suggests that the a-olefin is selectively oriented when copolymerized because internal unsaturation is almost entirely absent. R Cr-CH,-CH-chain Cr-chain

+ CH,=CHR

-

Cr-H

R

+ CH,=C-chain Common

Termination

\

R

Cr-CH-CHI-chain

__*

Cr-H

+ R-CH=CH-chain Rare

The amount of vinylidene found in the polymer exceeds that expected on a statistical basis, indicating that copolymerization encourages termination, Decreased molecular weight confirms it. Hogan (17) believes a tertiary hydride is more easily removed during termination than the usual secondary hydride. This effect is especially pronounced on some modifications of Cr/ silica. The selective orientation may indicate a steric hindrance from the branch. Internal olefins like 2-butene tend to be poisons. They adsorb strongly but do not copolymerize to any significant extent. 2-Methylpropene is not very reactive either. In the absence of ethylene, a-olefins can be polymerized over Cr/silica but their reactivity is much lower than that of ethylene. Sometimes adding a-olefins to the reactor will improve activity, not because they are more reactive monomers, but because they are better reducing agents. Long chain branching (LCB) is a normal consequence of the preparation of polyethylene over Cr/silica. Although the branched chains are present in only trace amounts, they exert enormous influence on polymer properties. Branching probably occurs when the terminal vinyl of one chain becomes incorporated into another growing chain. Long chain branching varies with chromium loading and calcining temperature. Probably anything which increases the active site density, i.e., that decreases the distance between sites, increases the probability of LCB (70). Methyl branching also occurs in the preparation of many polymers. Ordinarily methyl branches are present in only trace amounts, barely detectable by the most sensitive 13C-NMR(69), and they have no effect on polymer properties. But if the polymerization is carried out at atmospheric pressure or less, methyl branching becomes much more common (69). It may originate from isomerizarion at the active site. This isomerization is probably not favored but does occur very rarely (Scheme 2). Low pressure decreases the propagation rate, allowing each chain to spend more time around the active site, thus increasing the probability of such side reactions. Internal

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

65

unsaturation, which is also found occasionally in very minute amounts, could arise from the same mechanism. H HCH

H CH

H H Cr-CH-CH-chain

H-Cr

Cr-H

\

CH-chain

I

Cr-CH-chain

H H I Cr-CH-CH-CH-chain

CH=CH-chain SCHEME 2

E. INFLUENCE OF

THE

OH POPULAT~ON

Cr/silica does not catalyze the polymerization of ethylene until it has been calcined between about 300 and 1000°C. While heat is necessary to cause the esterification of CrO, to the silica surface, this is probably not the only purpose of the high temperature calcining step. Figure 8 shows how the activity develops as the calcining temperature increases. A respectable activity does not appear until about 50O0C, whereas the esterification of Cr(V1) is complete at 300°C (16). Furthermore, other sources of chromium behave similarly in activity even though the particular mechanism and

REACTION TIME ( m i d

FIG.8. The polymerization kinetics of Cr/silica depends on the temperature of calcining, which controls the surface hydroxyl population.

66

M. P. MCDANIEL

FIG.9. Increasing the calcining temperature, which dehydroxylatesthe surface, enhances the polymerization activity and especially the termination rate up to 925"C,where sintering destroys the silica base.

temperature of binding must vary somewhat. Therefore, the calcining step must achieve some other necessary purpose in addition to the formation of a surface chromate or dichromate ester. Figure 9 shows more completely the relationship between activity and calcining temperature. Here activity is defined as the inverse of the time needed to make 5000g of polymer per gram of catalyst. Activity increases with increasing calcining temperature up to a maximum at around 925"C, and then declines as sintering destroys the surface area and porosity of the silica base. Krauss has shown that the coordinative unsaturation of Cr(II)/ silica follows a similar trend (36). Thus, although the chromium is fully attached by 3 W C , the activity continues to increase right up t o the point of sintering. During this range, all of the chromium ramains hexavalent and no obvious changes are observed in the crystallinity or porosity of the support, just a gradual dehydration of the silica surface as hydroxyl groups condense to release water. The hydroxyl population is plotted in Fig. 1 as a function of calcining temperature. Notice that when heated in air the hydroxyls are never completely stripped off even at 1OOO"C some remain. This suggests that these hydroxyls may somehow interfere with the active site. Even more sensitive to the calcining temperature is the rate of termination.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

67

Figure 9 shows how the weight average MW (M,)varies with calcining temperature. The more dehydrated the catalyst, the lower the MW of the polymer produced, up to 925°C. Then the trend reverses as the silica begins to sinter. Thus the rate of termination of chains behaves much like the overall rate of polymerization, but in a more exaggerated way. (If propagation and termination increased proportionally, no change in MW would occur.) This is also apparent from the relative melt index potential (RMIP) of the catalyst, which is plotted in Fig. 9. Melt index increases with calcining temperature up to 925"C, indicating that the rate of termination relative to propagation varies in the same manner. Catalysts generally change from yellow to a deeper orange color as the calcining temperature is raised. On exposure to moisture, however, the orange samples immediately revert back to yellow and lose their activity. A second calcining step then restores the activity and RMIP. All of these facts indicate a strong reverse correlation beteen the hydroxyl population on the silica surface and the catalyst activity and termination rate. Possibly these hydroxyls coordinate to the active center and kill or at least retard it. Groeneveld et al. have reported that on barely activated samples, protons from surface hydroxyls later appear in the polymer (8,9).This may be evidence of interference by hydroxyls. Or perhaps the hydroxyls are not directly involved at all, but merely reflect some other important change such as the strain introduced onto the surface by their condensation. Whatever the reason, this relationship is used commercially to control MW and many other important polymer properties.

F. MOLECULAR WEIGHT DISTRIBUTION The amorphous nature of silica probably creates a wide diversity of active sites on the catalyst. Different types of Cr(I1) have been detected from chemisorption and chemiluminescence experiments (15,16,43). Probably each site is influenced by the local environment and therefore has its own characteristic propagation and termination rates. The sensitivities of both polymerization and termination rates to the calcining history of the catalyst suggest this. More evidence can be seen in Fig. 10, where the MI of the polymer being produced is seen to decrease with time. The first sites to come on stream, i.e., those most reactive, seem to have a naturally high termination rate because at first a high MI is obtained; as more sites come alive, however, the MI drops, suggesting that these slower, less reactive sites tend to make longer chain^.^ Probably for the same reason, reducing a catalyst with CO, The lifetime of each chain is only a fraction of a second, so the drop in MI over an hour cannot be ascribed to simple chain growth with time.

68

M . P. MCDANIEL

FIG.10. Chain length increases with polymerization time, suggesting (since the activity also increases) that the first sites to come alive have a naturally higher termination rate than those which develop later.

which brings all sites to life quickly, also gives slightly lower MI polymer at equivalent yields. This diversity of sites explains why the molecular weight distribution (MWD) of polymers produced by Cr/silica is broad (71). Model calculations 2: but in which assume a single type of active site usually predict M , / M , reality M , / M , = 6-15 is common, and 20-30 can be achieved with catalyst modifications. The distribution is also broader than that generally obtained from Ziegler catalysts, for which M J M , = 3-6 under similar conditions. Experience with organometallic compounds suggests that a broad MWD may be a general feature of catalysts which terminate by p-elimination. Most polymer properties are extremely dependent on the MWD. Since each application has its own requirements, it becomes important to control the distribution. The sensitivity of Cr/silica to preparation variables thus becomes very useful. N

G. ACTIVESITECONCENTRATION Only a fraction of the chromium is probably active for polymerization. Commercial catalysts contain about 1% Cr, but cutting that in half has little effect on the activity. Decreasing the chromium concentration further will eventually hurt the overall catalyst activity, but the activity based on chromium climbs until it reaches a maximum near 0.01% C r (17). If we assume at this point that every chromium is active, then about 10-30% of the The ratio of weight average ( M , ) to number average (M,) molecular weights.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

69

chromium on the ordinary 1 % Cr catalyst is active. Poisoning experiments with triethylamine yield similar figures (17). Both give only an upper limit. Poisoning experiments usually show very selective adsorption. As the first increments of poison are added, activity declines rapidly. Then with later increments the effect of the poison is less severe, probably because the adsorption is less discriminating between active and inactive chromium. This suggests that the active site population is that portion of the reduced chromium which, due to surface heterogeneity, is most coordinatively insaturated. Presumably this would correspond to some of the Cr, population identified by the Turin group, which under favorable conditions comprised almost half the total chromium (16,40). Sometimes experimenters are tempted to determine the number of chains formed during polyerization and assume each site makes one chain, but a site terminates and reinitiates chains continuously, making this approach invalid except at very short reaction times. Quick-kill experiments in this laboratory (69) tend to confirm Hogan’s number (13,but to actually see the first chain growing with time, the polymerization must be artificially slowed by using noncommercial conditions, and the results are not very reproducible. Zakharov et al. have used a radio tagging technique to measure the active site density in which polymerization is killed with labeled methanol (72, 73). They found only about 1 % or less of the chromium to be active, or about one tenth of Hogan’s number. But because they calcined Cr/silica at only 400-50O0C, their catalyst was probably only one-tenth as active. So the two studies are not necessarily in conflict. As expected, the active site density found by tagging increases with time during a polymerization run. Surprisingly, the alkoxy group (and not the proton) from the methanol was found in the killed polymer. This suggests a polarity in the site-polymer bond which is opposite to that generally imagined. The polymer would be more positive than the site. Yermakov and Zakharov (72, 73) proposed that the polymer may actually grow from an oxide rather than from the Cr. Before any hard conclusions can be drawn, however, more must be known about the kill reaction itself, i.e., whether it really is the true kill mechanism and not just a side reaction connected with oxidation of Cr(I1) by methanol, and whether it really is quantitative. One other approach has been tried in this laboratory (69).A CO-reduced catalyst was permitted to adsorb 13C-enriched ethylene at - 78°C where no polymerization occurs. The excess was pumped off and replaced by natural ethylene. When the temperature was raised, polymerization began and presumably the enriched ethylene started the first chain. Later, when the polymer was analyzed by 13CNMR,the enriched end groups stood out from those caused by natural I3C abundance because their resonance was split by coupling of the 13C pairs. Initial results indicated that about 10% of the chromium was active.

70

M. P. MCDANIEL

IV. The Porosity of the Support

Polymerization poses a special problem in catalysis because the polymer product is often hundreds of times longer than the average pore diameter of the catalyst. Commercial polyethylene having a weight average M W of 100,000-200,000 is typical, with a good fraction of the chains in the millions. Thus, it is difficult to imagine how these long chains could diffuse from the interior of a large catalyst pellet. Clogging of internal pores likely would stop the reaction except on the exterior surface.

A. ACTIVITY VERSUS POREVOLUME The polymerization reaction can sometimes cause a fracturing of the catalyst that exposes fresh exterior surface (65,66). Therefore, it is critical to choose the right silica, because in addition to having a high surface area, it must be fragile enough to fragment during polymerization, yet sturdy enough to handle during processing. This generally requires a high pore volume, and most commercial silicas sold as adsorbents or for other catalytic applications cannot be used for polymerization because their pore volume is too low. This is illustrated in Table I, where four commercial silicas and one made in house are compared. Although all have a high surface area, only two are suitable for polymerization. The LPS silica is prepared by extracting the water from a low solids hydrogel with an organic solvent to avoid the compression of aqueous surface tension. This leaves a fragile catalyst of very high pore volume. Such preparations often provide dramatic examples of the effect of pore volume because the same gel dried directly in an oven will frequently be less active or even completely dead. Invariably, nitrogen sorption shows the inactive silica to contain a low pore volume mainly inside small pores, e.g., less than 60 A diameter. In contrast, the active sample dried by extraction usually has equal or greater volume inside small pores, and, in addition, considerable volume TABLE I Porosity versus Activity for Some Commercial Silicas

Silica type

Surface area

(m2g-')

Pore volume (cc g-')

Davison 35 Davison 81 Davison 59 Davison 952 LPS

740 600 340 280 320

0.43 0.70 1.15 1.60 3.20

Activity (g g-' h r - l ) 0 30 100

1250 1400

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

71

DRIED IN OVEN IOIAD CATALVSTI

d(V0L.) dlDIA.)

DRIED BY EXTRACTION IVERY ACTIVE CATALYITI

100

100

ow

400

600

PORE DIAMETER (A)

FIG.11. Nitrogen sorption isotherm of two catalysts made from the same hydrogel, i.e., one very active, the other completely dead. The active sample was specially dried by extraction with an organic solvent of low surface tension to protect the larger pores.

inside the large pores too. An example of these two extremes is shown in Fig. 11, where the extracted sample had an activity of 6000 g g- h-’ and its oven dried counterpart was dead. Table I1 provides another illustration of this effect. In these experiments analysis was done by mercury intrusion, which is sensitive to a broader range of pores. In this series of experiments. samples of hydrogel were washed in various alcohol-water mixtures to yield pore volumes ranging from high, for the sample washed in pure alcohol, to low, for the sample washed in pure water. This procedure does not affect the surface area, which remained constant at 375 m’lg-’. The activity of the finished catalyst increases with the pore volume. Notice that all samples have about the same volume inside TABLE 11 Activity versus Pore Size Distribution Liquid in gel during drying: water :alcohol

(% : %)

Volume (cc g-’) inside pores of diameter: Activity” (g g - ’ 0.5 hr-I)

30-100A l00-lOWA 0.1-lpm

1-10pm

>lOpm

0.17 0.25 0.25 0.37 0.46 0.60

1.53 1.85 2.10 2.05 2.14 2.00

_ _ _ _ ~

100:O

80 :20

6o:m 40:60 25 : 75 0:100 (I

842 1724 2445 3230 4125 4600

0.66 0.69 0.75 0.71 0.69 0.65

Surface area 375 m 2 g-’ for all samples.

0.12 0.21 0.25 0.26 0.51 0.64

0.03 0.07 0.09 0.14 0.24 0.31

72

M. P. MCDANIEL

-=

small pores (diameter 100 A). Apparently these pores, which contain 95 % of the surface area, are strong enough to withstand drying, even from pure water. In contrast, it is the middle range pores, 100 A to 10pm, that are most susceptible to crushing by surface tension during drying. The activity of the catalyst correlates nicely with the volume inside these pores. The very large pores, those > 10pm in diameter, probably are not pores at all but spaces between particles. Hence their volume rarely correlates with anything.'

B. CRITICAL POREDIAMETER Another drying experiment, shown in Table 111, narrows the critical range of porosity still further. In this series, one sample of another hydrogel was dried in an oven from water, while a second was dried by azeotropic TABLE 111 Activity versus Pore Size Distribution Drying process Oven Organic Extract

Activity (gg-lhr-')

Surface area

Pore volume (cc g- I ) inside pores of diameter:

(m2)

30-l00A

100-lOOOA

lOOOAto1pm

1-10pm

300

364

0.87

0.22

0.03

0.01

2000

370

0.82

0.79

0.07

0.04

extraction with isopentanol. Again the latter sample was active and the former almost dead despite similar volumes inside the small pores (diameter, 30-100 A). In this example, due to a special preparation, neither silica had much volume inside the largest pores of diameter > lo00 A. So we conclude that the midrange of diameters (100-10oO A) is most critical to the development of activity. Still another type of experiment provides an example in which the main change occurs in pore diameter rather than in total pore volume. Commercial silica producers often make, by acidic gellation, a silica of extremely high surface area known as RD hydrogel. Although quite useful as an adsorbent, this material makes a bad polymerization catalyst, despite its high surface area, because it contains mainly very small pores. However, it can be converted into the more porous ID hydrogel by aging the gel under certain conditions. This transition, monitored by mercury intrusion, is shown in The activity measurements listed in this section are not always comparable from one table to the next since the activation processes sometimes varied.

73

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

Table IV. The total pore volume is only a little changed, and the surface area is diminished a good deal. Nevertheless, activity greatly improves because large diameter pores are created in place of the small ones. This undoubtedly makes the gel more fragmentable during polymerization, and the more open pore structure permits an easier escape of the polymer chains from the fragments. Note (Table IV) that most of the pore volume of the active ID hydrogel is in pores with diameters in the range 100-loo0 8. TABLE 1V Eflect of Aging o n Hydrogel Activity

Activity Catalyst" ( g g - ' hr-')

RD ID a

1100 4Ooo

Surface Pore volume (cc g- ') inside pores of diameter: area (m') 30-60A 60-100A 100AtoO.lpm 0.1-1 pn I-IOpm

909 288

0.77 0.11

0.38 0.48

0.47 1.33

0.04 0.23

0.08 0.25

Both hydrogels dried by extraction with an organic solvent.

From a commercial viewpoint the catalyst should be porous enough to be active, yet still rigid enough to handle easily and give a high polymer bulk density. This is because the polymer particles usually replicate the catalyst particles. So, a dense rigid catalyst is desirable. Quite often, as shown in Table 11, most of the volume of a dried catalyst is not in pores critical to activity, but in the very large pores. These are also the weakest pores, permitting them to be selectively crushed in a process used to densify and harden commercial catalysts. This hardening process does not decrease the activity, because it works mainly on the largest pores. An example is shown in Table V. Two catalysts were made from the same alcohol washed stock, which had a high volume inside the large pores. However, one catalyst was then crushed and cemented into rigid pellets, using the hardening process mentioned above. Notice in Table V that the process selectively crushes the largest pores, those TABLE V Compression Destroys Large Pores without Hurting Actioity Pore volume (cc g- ') inside pores of diameter:

Before hardening After hardening

30-100 A

1CH-3OO A

3OD-lOOo A

0.1-1pm

1-10 pn

0.77 0.84

0.64

0.54

0.85 0.32

0.65 0.07

0.70 0.19

74

M. P. MCDANIEL

greater than lo00 A in diameter, without doing appreciable damage to the critical smaller ones. The result is a hard dense particle having high attrition resistance but little loss in activity. Finally, while the pore volume for pores in the diameter range 100- lo00 A provides a useful estimate of activity within a family of catalysts (where other parameters are constant), it cannot serve as an absolute guide between families. For example, glass beads can be etched to yield the critical range of pores, but such catalysts are still inactive. Actually the activity of any catalyst is probably governed by a complex combination of several physical characteristics-surface area, fragility, fragment size, and fragment porosity-all related to the initial porosity of the catalyst. C. FRAGMENTATION That the catalyst really does fracture during polymerization can be easily demonstrated. When a coarse Cr/silica, e.g., 60-80 mesh (1 50pm), polymerizes ethylene to a yield of lo00 g g- ', each catalyst particle produces a polymer particle of approximately the same shape but about lo00 times larger. Cutting into the polymer granule will not expose the original catalyst particle. Even under a microscope the fragments are too small to be seen easily in the polymer background. Gently burning away the polymer leaves catalyst ash, a loose agglomerate of extremely fine dust. Often the original catalyst shape is still visible, although greatly expanded, but it crumbles at the slightest touch. The 7 pm (65,66). We now average fragment size is measured to be only suspect that even this particle is probably not the real fragment size, but a weak reagglomeration of fragments. These small fragments are almost impossible to see in the polymer, even under a microscope. However, evidence of the fracturing is seen by examining the ash via mercury intrusion. An example is shown in Table IV, where Davison Grade 952 Cr/silica was allowed to polymerize ethylene to various yields. The polymer was then burned away, and the porosity of the catalyst ash determined by mercury intrusion. Presumably the fracturing occurs along the weakest faults, which are no doubt the largest pores. So, it makes sense that we see in Table VI little change in the small pores between the original catalyst and the fragmented ash. The volume inside pores with 30-100 A diameter remains more constant as the productivity increases. Likewise in the range 100-300 A and even in the range 300-1000 A the volume changes very little. Above lo00 A however, there is a great increase in pore volume, measured as the mercury crushes a loose reagglomeration of the fragments. Since particles of any size tend to pack together to make holes of a similar size, the fragments may be 0.1- 1 pm in diameter, or even smaller.

-

15

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERlZATlON

TABLE VI Porosity of Catalyst Ash ajier Burning the Polymer Away

Volume (cc g-l) inside pores of diameter: Yield" (gg-')

30-100A

100-300A

300AtoO.Ipm

0.1-1pm

1-10pm

0 12 40 250 1270

0.57 0.73 0.81 0.60 0.57

0.74 0.82 0.93 0.96 0.74

0.21 0.40 0.55 0.58 0.52

0.17 0.53 1.24 1.53 1.40

0.07 0.67 1.08 0.84 1.30

a

Polymer yield before being burned away.

D. THEKINETIC PROFILE Since the ability of a silica to fragment limits its polymerization activity, some restarchers think fracturing also determines the kinetic profile, giving the characteristic rise in polymerization rate as new surface is gradually exposed. Several facts argue to the contrary. Ziegler or organometal catalysts on the same silica exhibit no such rise in rate. Catalyst ash, presumably already fragmented, can be recovered and reactivated, but its second kinetic profile is just like the first. Furthermore, the kinetic profile is dependent on temperature and pressure, and adding alkylmetal reducing agents to the reactor often eliminates the rise in rate. Therefore, the rise in rate is more likely due to reduction or alkylation, not fragmentation.

E. MOLECULAR WEIGHT VERSUS POROSITY Cr/silica catalysts also exhibit a strong correlation between their porosity and the molecular weight of the polymer produced. The larger the average pore diameter of the silica, the lower the MW (67). Figure 12 shows the results of experiments in which a silica hydrogel was dried by extraction with several organics varying in surface tension. All these catalysts had surface area of 400-410 m2 g-', but the porosity varied from about 0.8 cc g-' to over 3 cc g-'. All catalysts were allowed to polymerize ethylene to a yield of 5000 g g- under the same conditions. Notice that the melt index, which is inversely related to MW, increases with porosity. The reason for this behavior is not entirely clear. Diffusion of ethylene into the pores is the most obvious explanation. However, the trend actually runs opposite to what would be expected, because starving the catalyst of ethylene, which might happen in small pores, is known to decrease MW, not increase it.

76

M. P. MCDANIEL

Catalyst Pore V o h n e (cc/g) FIG.12. The porosity of the catalyst determines not only its activity but also the chain length. Here melt index (MI) varies with catalyst pore volume in a series in which a common hydrogel was dried by extraction with diRerent organic solvents to achieve variations in porosity.

Another explanation is that long chain branching becomes more likely in small pores because the active sites are closer together. That is, the vinyl group terminating one chain may more easily become incorporated into another chain growing from a neighboring site.

V.

Modifications of Cr/Silica

A.

PROMOTION RY TITANIA

Although titania is not itself a good carrier for Cr(VI), its presence in small amounts on Cr/silica catalysts does have a promotional effect on both activity and termination rate (74, 75). The beneficial effect probably results from a change in the electronic environment on the chromium, which possibly becomes linked to the titania during calcining. Two ways of incorporating titania onto Cr/silica catalysts are used, and each has certain advantages. In the simplest method the silica surface is coated with a layer of titania by allowing a titanium ester to react with the hydroxyl groups.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

77

POLY MERlZATlON RATE Ikg POLYMERg'CATALYST HOUR-')

7 6 -

5 4 -

3 2 -

1 -

0

4 0 60 80 100 120 140 POLYMERIZATION TIME (MINI

20

FIG. 13. Titania, cogelled into the silica base, serves to promote the polymerizationactivity of chromium. The members of this series of catalysts, calcined at 7 W C , differ in the amount of titania added.

Unreacted ester groups are then burned away during the calcining. Owing to its simplicity this method permits commercial silicas to be treated with any amount of titania up to 5-6% Ti, at which point saturation is usually reached.6 The method increases melt index and tends to broaden the molecular weight distribution, which can be useful in some applications such as blow molding. The second method of incorporating titania onto the catalyst consists of coprecipitating hydrous titania along with the silica gel. This can be accomplished by adding a water soluble titanium salt to the silicate solution before gellation. Although this method gives a higher degree of dispersion throughout the catalyst bulk, some of the titania is exposed on the surface and during later calcining chromium may attach to it. Specialty catalysts have been developed that impart a narrow MWD and extremely high environmental stress crack resistance (ESCR) to the polymer. Figure 13 demonstrates the promotional effect of titania on the activity of Cr/silica catalysts. These samples were made by coprecipitation. The chromium was then added and each sample was calcined at 760°C to form surface attached Cr(V1). For comparison, titania concentrations are expressed as Ti atoms per square nanometer of surface, even though a good part of the titania may actually be in the bulk. It is clear in Fig. 13 that titania increases the activity of the catalyst, first by shortening the induction time, and then by allowing higher polymerization If the silica has been predried at 200°C and has a surface. area of about 280 m2 g-', this corresponds well to the expected reaction with 2.8 paired and 1.1 single OH/nmZ (23).

78

M. P. MCDANIEL +-

MELT INDEX

0

0.5

1.o

TlTANlA CONCENTRATION(Ti/nm

1.5 2,

FIG.14. The melt index of the polymer, which reflects the catalyst termination rate, is also promoted by titania, at least at the lower activation temperatures.

rates. The shortened induction time suggests that titania makes Cr(V1) more easily reducible because the lower valent active sites come to life more quickly. The faster increase in polymerization rate also suggests this, and the higher maximum rates may indicate an increase in the active site population, although this is not certain. That titania increases the termination rate also can be seen in Fig. 14. Here the melt index, which reflects the termination rate of some coprecipitated samples, is plotted against the titania concentration. At 650°C and 760°C calcining temperatures the melt index increases with titania content, but at 870°C a peak in melt index is obtained, followed by a sharp drop. This is due to sintering, which can be considered as the earliest stages of melting. Sintering destroys the surface area and porosity of the catalyst. Although Cr/ silica itself does not sinter at 870"C, the added titania does promote sintering, as impurities often lower the melting point of solids. Both activity and MI potential are diminished by sintering, and the more titania added, the more easily the catalyst sinters. This is apparent in Fig. 15, where the surface area of silica-titania cogels is plotted against calcining temperature. Pure silica catalyst exhibits very little drop in surface area, but when titania is added the surface area becomes unstable. The more titania added, the lower the temperature needed to cause sintering. This explains the melt index behavior of coprecipitated silica-titania catalysts which is shown in Fig. 16. With each catalyst, the MI rises with increasing calcining temperature until sintering begins, then it drops. The

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

79

SURFACE AREA (rn*/g)

1

700

I

I

800

900

ACTIVATION TEMPERATURE IN AIR

I

1000

('c)

FIG.15. A drop in surface area marks the onset of sintering in a series of cogelled Cr/ silica-titania catalysts. Titania decreases the thermal stability of the catalyst.

temperature for optimum MI depends on the thermal stability of each catalyst, which in turn depends on the titania content. If we avoid sintering by staying at only 700°C then the more titania added, the higher the MI. This sintering is associated with a tendency toward phase separation between silica and the titania. X-ray photoelectron spectroscopy (XPS) indicates that the titania tends to migrate to the surface. This is shown in Table VII, where the XPS intensity ratio Ti/Si is listed for a coprecipitated silica-titania sample calcined at various temperatures. As the temperature increases, the intensity ratio also increases. Since XPS is a surface technique, this indicates more titania near the surface. Notice also in Table VII that this is not true of the first method of titania incorporation. When the titania is applied as a surface layer it does not promote sintering, and according to the XPS intensity it does not migrate. Catalysts containing a surface layer of titania usually exhibited about twice the Ti/Si XPS intensity ratio as coprecipitated samples containing the same overall titania content. This confirms that cogellation does leave a good portion of the titania in the bulk. This may explain why even at equivalent Ti surface concentration, the two types of catalysts still produce slightly different

80

M. P. MCDANIEL -RMlP

1 1.

0.9 Ti/nm'

A

FIG.16. The relative melt index potential (RMIP) of a series of cogelled Cr/silica-titania catalysts rises and then falls with calcining temperature, indicating first dehydroxylation then sintering. However, the more titania in the catalyst, the more easily it sinters and therefore the lower the temperature at which peak RMIP develops.

TABLE VII Ti/Si X P S 2 Intensity Ratio

Coprecipitated

Surface layer

0.0338 (600°C) 0.0389 (800°C) 0.0411 (870°C)

0.0461 (450°C) 0.0437 (750°C) 0.0465 (900°C)

XPS, X-ray photoelectron spectroscopy.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

81

polymers.' The presence of titania in the bulk may change the geometry of the surface somewhat. Many experiments suggest that the promotional effect of titania is due to formation of Ti-0-Cr bonds, and that this is very dependent on subtle variations in the preparation of the catalyst. For example, most titanium salts are ineffective as promoters when impregnated onto CR/silica as an aqueous solution, but anhydrous titanium esters, which react with silanols, are often highly effective. A uniform monolayer of titania is probably not achieved in the first case. The following experiment provides another example of preparation subtleties. Three catalysts of identical composition can be made by impregnating chromium and titanium. In one preparation the Cr(V1) is first anchored to the silica by calcining. Then the titanium is added and the sample calcined again. Little promotional effect is noticed, probably because the chromium remains attached to the silica and does not link up with titania. In a second preparation, however, the order of impregnation is reversed, i.e., titania first, then chromium. A huge promotional effect is observed this time, suggesting that much of the chromium is deposited onto the titania. Finally, simultaneous impregnation of chromium and titania yields an intermediate effect. Thus the promotional effect of titania seems to parallel the probability of forming Ti-0-Cr bonds. B. ANHYDROUS IMPREGNATION OF CHROMIUM

Ordinarily the chromium binds to the silica by reacting with hydroxyls on a fully hydrated surface, because chromium is impregnated aqueously onto the silica and then calcined. However, a different catalyst results if the chromium attaches instead to a surface already dehydrated by calcining. A large promotional effect, particularly on the termination rate, is obtained (76). To do this the silica is first dehydrated at 9 W C , for example, then impregnated with chromium anhydrously so that the surface is not rehydrated. A secondary calcining step at some lower temperature such as 300-600°C then fixes the chromium to the silica. The method is especially effective if the support also contains titania. Any anhydrous chromium solution will work. CrO, in acetonitrile or chromate esters in hexane are good candidates. Lower valent compounds, like diarenechromium in hexane, can also be used because they are oxidized to the same Cr(V1) surface species during the second calcining in air. Even chromyl chloride vapor can be used if enough surface hydroxyls are left to

' Seemingly minor changes in the molecularweight distributioncan have a powerful influence on such polymer properties as warpage, impact strength, ESCR, and flow.

82

M. P. MCDANIEL

TABLE VIII Anhydrous Impregnation of Chromium

Secondary calcining temperature'

Melt index

160°C 3 15°C 425°C 540°C 650°C 760°C 870°C Aqueous 870°C

Dead 0.74 0.88 1.20 0.87 0.56 0.55 0.54

a Silica-titania dehydrated at 870"C, then impregnated with 0.5% Cr from hexane, and calcined again in air at the temperature listed.

permit initial attachment. Residual chloride is burned off easily during the second calcining, which no doubt rearranges the chromium. The cause of the promotional effect is not clear. Probably the chromium attaches somewhat differently to a dehydrated surface than to one which is fully hydroxylated. A higher concentration of dichromate, instead of chromate, is one possible cause since the remaining hydroxyls are more widely spread. The promotional effect could also be due to a strained attachment which is frozen to the surface by the low binding temperature. Heating these anhydrously prepared catalysts at 900°C destroys the promotional effect, leaving only an ordinary aqueous type catalyst. Probably the increased mobility at 900°C relaxes the strain or rearranges the Cr(V1) back to its usual mode of attachement. An example is shown in Table VIII. In these experiments silica-titania dehydrated at 870" was impregnated with 0.5 % Cr as dicumenechromium in hexane and calcined in air at the temperatures listed. Notice that melt index, which ordinarily increases with calcining temperature, instead goes through a maximum at about 540°C for the anhydrously impregnated catalyst. After being calcined at 870°C,however, the promotional effect vanishes and the MI reverts back to that expected for an aqueously impregnated catalyst. The position of the peak varies with the support, occurring as low as 300°C in some cases. C. DEHYDRATION BY CHEMICAL MEANS As noted in Section III,E the hydroxyl population on the catalyst seems to exert a powerful influence on the activity and melt index potential, which are

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

83

ACTIVATION TEMPERATURE cc)

FIG.17. The hydroxyl population on silica depends on the temperature and atmosphere during calcining. Reducing agents like CO and carbon-sulphur compounds tend to depress the OH level further.

enhanced by raising the temperature of dehydration. But even at 900°C where sintering begins, the OH population does not drop to zero; some residual hydroxyls remain. However, a further lowering of the OH population can be accomplished by calcining not in air, but in carbon monoxide (76). This is shown in Fig. 17. The effect is probably due to the water-gas shift reaction in which water (hydroxyls) is converted to hydrogen and carbon dioxide. 2Si-OH

+ CO

-

Si-0-Si

+ CO, + H2

Sulfur compounds like COS or CS, are even more effective than CO for removal of hydroxyl groups. Reducing agents which do not contain carbon, such as H,, S, SO,, or H,S, are ineffective. Again this is probably due to a variation of the water-gas shift reaction, since sulfur is not left on the support, but the exact mechanism is unknown. Table IX shows the effect of dehydration by chemical means. Silica samples were calcined at 870°C in various gases, impregnated anhydrously with 0.5 % Cr, and finally calcined in air at 650"C, producing Cr(V1). Only the silica (not the chromium) was exposed to the reducing treatment. Even so, CO more than doubled the melt index potential and COS improved it by a factor of 60. Activity was also increased. Hydroxyl groups can also be replaced by halide. Fluorided catalysts show improved activity at low calcining temperatures. The electronic environment is probably altered considerably because termination rates are always depressed. The M W D is sometimes narrowed, suggesting a more uniform

84

M. P. MCDANIEL TABLE IX The Eflect of Dehydration by Various Chemical Means Treatment'

Relative melt index

Air Carbon monoxide Carbonyl sulfide co I,, 0, CO Br,, 0,

2.5 6.0 150.0 12.0 21.0

+ +

'Silica-titania was calcined at 870°C in the composition shown, impregnated with 0.5% Cr as dicumenechromium in hexane, then calcined in air at 315-600°C. environment. The other halides tend to kill the catalyst, implying that like hydroxyls they somehow interfere with the active site. It is possible to burn off most of the surface bromide or iodide with oxygen at 6WC, leaving a partially dehydrated support (77). This treatment gives a powerful promotional effect on melt index, which is also shown in Table IX. Some of the iodide is oxidized to a highly reactive surface species that strangely is paramagnetic. But whether it affects the polymerization center is unknown. Calcining the support in carbon monoxide benefits the catalyst in another way, by protecting it from sintering. Apparently the hydroxyls, or minute amounts of moisture in the gas stream, encourage sintering, because when the silica or silica-titania is calcined in carbon monoxide instead of predried air or nitrogen, very little sintering is observed. This is shown in Fig. 18, where a loss in surface area indicates sintering. The protective effect of C O is most pronounced on samples containing high levels of titania as shown in Table X, where pore volume serves as a measure of sintering. The porosity of high TABLE X Calcining in CO Protects High-Titania Catalysts from Sintering Titania content (Ti/nm2)

Pore volume (cc g - ' ) Initial

CO (870°C)

Air (870°C)

0.4 0.9 1.1 1.5

2.64 2.90 2.58 2.55

2.48 2.76 2.31 2.33

2.43 2.61 1.78 1.59

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

i

-

85

SURFACE AREA (rn 2/g)

I

250

-

200

X

I

150

I

'

750

I

I

I

I

800

850

900

950

1000

ACTIVATION TEMPERATURE ('C)

FIG.18. A drop in surface area marks the onset of sintering in a series of Cr/silica-titania catalysts calcined in dry air or CO. Sintering is less severe in CO.

titania samples is badly damaged by heating in air at 870°C, but not by heating in CO.

D. REDUCTION/REOXIDATION Thus a drier, less sintered support is obtained by calcining in C O rather than air. Unfortunately the chromium will not tolerate such a severe reduction treatment, and unless Cr is added secondarily [as in the above (Section V,B) examples] a dead catalyst results. This is shown in Fig. 5, where it is apparent that reduction at 350°C improves activity, whereas higher temperatures destroy it. This does not indicate overreduction, as some have claimed. Rather, it indicates a rearrangement of the Cr(I1) into a less coordinatively unsaturated form-possibly aggregates. The problem is solved by reoxidizing the deactivated Cr(I1) back to Cr(V1) (44). If a low temperature is used for reoxidation, e.g., 300-60O0C, then the attachment to the surface is like that obtained by anhydrous impregnation. A strong promotional effect is obtained in addition to that expected from the dehydration. This effect is shown in Fig. 19. Catalyst series A was simply calcined in air at the temperatures shown. Series B was calcined in CO, then air, at the same temperatures. The large promotional effect on MI is due only to dehydration by CO since the temperatures of attachment are identical.

86

M. P. MCDANIEL

600 600 700 000 000 1000 ACTIVATION TEMPERATURE PC)

FIG.19. The termination rate, plotted here as relative melt index potential (RMIP),reflects the extent of surface dehydroxylation in two series of Cr/silica-titania catalysts, calcined in air or ( 0 )CO and then air to reoxidize the chromium, both at the temperatures shown. The third series (+) shows the additional benefit of low-temperatureattachment. It was calcined in CO at the temperatures shown, then air at a lower temperature (760°C).

(v)

Finally, series C was calcined in CO at the temperature shown, then reoxidized in air at a lower temperature, 760°C. It exhibits a double promotional effect, caused by dehydration in CO and also by the low temperature of attachment. The optimum reoxidation temperature, shown in Fig. 20, is about 600°C. This depends to some extent on the type of support. Highly titanated samples produce peak melt indexes at only 300°C.Probably the temperature is needed to bind the chromium to the dehydroxylated support. Above 600°C the activity remains high, but the promotional effect of low temperature attachment is lost, and the MI drops. Again (as mentioned in Section V,C) sulfur compounds perform better than CO, as can be seen in Fig. 20, because they are better dehydrating agents. When Cr/silica is reduced by COS or CS, a black chromium sulfide forms. Reoxidation then converts it back to the hexavalent oxide. The catalyst retains no sulfur, but it often takes on a new reddish hue and the activity is greatly improved. This is probably an extension of the trend already observed in Fig. 10, which shows both activity and termination to increase as the catalyst is dehydrated. Perhaps the color change from yellow to orange, and finally to red for sulfided catalysts, indicates a transition from chromate to dichromate, or maybe just less coordination to hydroxyls. Adding water vapor to a sulfided catalyst completely reverses the benefit. Although much less effective,even nitrogen, which is also shown in Fig. 20, can be considered a reducing treatment. If one starts with a trivalent catalyst,

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

87

rp t

l5

450 600 750 900 REOXIDATION TEMPERATURE F C )

300

FIG.20. After being reduced at 870°C. three series of Cr/silica-titania catalysts yield highest termination rates (RMIP)after reoxidation at 600°C. Catalysts reduced in CS, display best results because CS, is the most effectivedehydroxylatingagent. Carbon monoxide is second best. Trivalent samples calcined in N, also show the benefit of low-temperature reoxidation, but without the effect of increased dehydroxylation.

calcining in nitrogen at 900°C leaves the chromium as amorphous clumps of Cr,O, (49). Then air at 600°C oxidizes it to the hexavalent state and gives the promotional effect expected from low temperature attachment onto a dehydrated surface. However, since nitrogen does not assist in the dehydration of silica as does CO, a smaller promotional effect is obtained. If the experiment is repeated with hexavalent Cr/silica, calcining as before in nitrogen at 900°C again produces Cr,O,, because Cr(V1) is not very stable in the absence of oxygen. However in this experiment large crystallites of 0: chromia are obtained because the crystallization is encouraged by even a trace of Cr(VI), which acts as a flux. These crystallites are more difficult to oxidize and consequently the optimum promotional effect is now found at 750°C rather than 600°C. The higher temperature, of course, diminishes the magnitude of the effect.

VI.

Chromium Oxide on Other Supports

Unlike Ziegler catalysts, chromium oxide based catalysts are extremely sensitive to minor changes in the preparation or calcining history. The active sites no doubt respond to the local electronic environment, which determines the molecular weight distribution of the polymer. Therefore, replacing the

88

M. P. MCDANIEL

silica by another support has profound consequences for the polymer. For some applications silica is not the best support. A. ALUMINA Alumina will also bind CrO, and stabilize it to 900"C, and it can polymerize ethylene when reduced to Cr(I1). High surface area y alumina can be made having the porosity necesssary for good activity. Besides the electronic differences between Si-0-Cr and AI-0-Cr bonds, such alumina catalysts typically have 50- 100 % more hydroxyl groups than silica at normal calcining temperatures. This is clear in Fig. 21, which shows the hydroxyl populations of three different supports. The hydroxyl concentration was measured by reaction with methylmagnesium iodide. The polymerization behavior of Cr/alumina seems to reflect the higher hydroxyl population. More surface hydroxyls also means more sites available to support chromium, and alumina does stabilize about twice as much Cr(V1) as silica. However, the higher chromium levels do not yield a more active catalyst. Cr/alumina is typically only one tenth as active as Cr/silica. Termination rates are also extremely depressed on Cr/alumina. Both effects could be attributed to the extra hydroxyls, which are thought to interfere with polymerization. However, the depressed termination rate is not always a disadvantage. For example, Cr/silica is not well suited to make ultrahigh molecular weight polymer. Molecular weight can be increased by lowering the calcining temperature, reactor temperature, and porosity, etc., but even under the best conditions it is difficult to achieve an inherent viscosity (IV) much higher

0'

2M)

'

400

'

MX,

'

IKX,

Activation Temperature PC)

FIG.21. Hydroxyl population on silica, y alumina, and aluminum phosphate having similar porosity. (Measured by reaction with methylmagnesium iodide.)

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

89

AMMONIUM BlFLUORlDE (%)

FIG.22. Flouride greatly improves polymerization activity in a series of Cr/alumina catalysts calcined at W C , but too much fluoride sinters the catalyst and kills the activity.

than 6 or 7. In contrast, Cr/alumina naturally produces such a high MW range of polymers that IV = 17 is attainable. Cr/alumina can be modified like Cr/silica. Adding titania is not particularly useful, but replacing the hydroxyls with fluoride does boost the activity by as much as 10-fold (62). An example is shown in Fig. 22, where activity is plotted versus the amount of fluoride impregnated onto a highly porous alumina. Too much fluoride tends to sinter the alumina and destroy the activity. Other modifications which improve the activity of Cr/alumina include adding chloride, sulfate, boria, phosphate, or 1-5 % silica (62, 78).

B. ALUMINUM PHOSPHATE Silica and aluminum phosphate have much in common. They are isoelectronic and isostructural, the phase diagrams being nearly identical even down to the transition temperatures. Therefore, aluminum phosphate can replace silica as a support to form an active polymerization catalyst (79,80). However, their catalytic properties are quite different, because on the surface the two supports exhibit quite different chemistries. Hydroxyl groups on AlPO, are more varied (P-OH and AI-OH) and more acidic, and of course the P=O species has no equivalent on silica. The presence of this third species seems to reduce the hydroxyl population, as can be seen in Fig. 21, so that Cr/APO, is somewhat more active than Cr/silica at the low calcining temperatures, and it is considerably more active than Cr/alumina. Of the many contrasts between Cr/silica and Cr/AIPO,, the most peculiar is the sensitivity to hydrogen. While Cr/AlPO, itself tends to produce very high MW polymers, a small amount of hydrogen added to the reactor

90

M. P. MCDANIEL

reverses this. Thus, a wide range of polymers can be made by varying the hydrogen concentration in the reactor. Such M W control by hydrogen is characteristic of Ziegler catalysts, but it is unknown to other chromium oxide based catalysts. The termination, however, is not by hydrogenation, because the polymers contain the usual vinyl unsaturation. Instead, /?-elimination seems to be accelerated by hydrogen through an unknown mechanism. Hydrogen does not retard the activity of Cr/AlPO, as it does on catalysts which hydrogenate the chain. The usual boost in termination by olefins is also greatly magnified on Cr/AlPO,. This sensitivity is almost certainly due to the influence of the phoshpate, perhaps to the formation of P-0-Cr bonds. Alumina can be treated with phosphoric acid to produce the effect, and the greater the phosphate content, the greater the sensitivity. Even silica can be impregnated with phosphate to improve its sensitivity. The kinetics of polymerization over Cr/AlPO, are also quite unlike that of Cr/silica, as shown in Fig. 23. Typically, Cr(VI)/silica undergoes a dormant period before the initiation of polymerization. In contrast, Cr/AlPO, polymerizes ethylene immediately on contact. Usually the activity peaks during the first 30 min and then declines during the next hour or more. The lack of an induction time suggests that reduction and/or desorption occurs more easily when the chromium is attached to aluminophosphate rather than to silica. That the rate so quickly reaches its maximum, in contrast to Cr/silica, suggests that the initial alkylation also occurs more readily on Cr/AlPO,. Stopped flow experiments indicate the decay in activity is due to a chemical change on the catalyst, and not to polymer buildup.

W LXXAAVVAAL I N T C A TTAALLVVSSTT

0

10 20 30 40 60 60 POLYMERIZATION TIME (mln)

FIG.23. The polymerization activity of Cr/AIPO, develops immediately on contact with ethylene, then peaks and declines rapidly, quite unilke Cr/silica. Samples reduced either by CO at 300°C or by triethylborane (TEB) in the reactor behave similarly but are more active.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

91

Like Cr/silica catalysts, the activity of Cr/AlPO, is improved with increasing calcining temperatures up to about 800°C. Again this probably reflects the condensation of surface hydroxyls, which are believed to interfere with polymerization. Alkylmetals (MR) have long been added to the reactor to increase the polymer yield of Cr/silica catalysts. They can be strong reducing agents or scavengers to remove the usual redox by-products (scheme 3). The induction time of Cr(VI)/silica is usually eliminated but the maximum activity is not greatly affected.

SCHEME 3

The polymer yield of Cr(VI)/AIPO, is also enhanced by adding a small amount of alkylmetal to the reactor, but not in the same way. Since there is no induction time to eliminate, the kinetics are not greatly affected, but the rate of polymerization is increased. These results are shown in Fig. 23. The diluent in one run contained triethylborane (TEB); a maximum polymerization rate quickly developed followed by a slow decline. However, the peak activity was about five times higher in the presence of TEB. Other alkyls, like those of zinc, aluminum, lithium, or magnesium, are less effective. The kinetic profile of Cr/AlPO, fits that expected from a transient species in a series of consecutive reactions.

“41 Oxidized Precursor

CBI

2 Reduced species

k

CD1

[CI Alkylated species (active)

k3

Dead species

The first step, reduction (kl), probably occurs so quickly on AlPO, that it has little effect on the development of activity. Instead, alkylation (k,) must be the controlling step (k, > k 2 ) . This would explain why a catalyst prereduced in CO has about the same kinetic profile as its oxidized parent (see Fig. 23). TEB probably reduces the catalyst but does not alkylate it. Thus, it also gives the same kinetic profile. Adding TEB to a CO-reduced catalyst has no effect,

92

M. P. MCDANIEL

again suggesting that TEB cannot alkylate. That both TEB and prereduction improved the overall activity suggests that they create a new population of reduced chromium available for alkylation. Applying this kinetic model with an iterative approach indicates that both k , and k 2 depend on ethylene concentration, whereas k , is affected only by reactor temperature. Activity develops most quickly at high pressure, and lasts longest at low temperatures. The propagation constant is, as usual, near first order in ethylene. These same factors probably also determine the quite different kinetic profile of Cr/silica, whose sites are slower to form and slower to die. The reduction assumes more importance on silica, controlling the development of activity. Alkylmetals accelerate the reduction but do not create a vast new population of sites. Just why the active sites are less stable on AlPO, than on silica is not clear. The decay in activity may indicate an oxidation of the site by the support. If so, the more acidic hydroxyls (P-OH) might be responsible, but replacing them with fluoride or sodium does not seem to help. Another possibility is oxidation by the phosphate group itself. VII.

Organochromium Catalysts

A number of organochromium compounds also form highly active polymerization catalysts when deposited on an oxide carrier. Usually the carrier does play an essential role, because without it such compounds rarely exhibit any activity. In most respects the organochromium catalysts are quite different from their oxide counterparts. A.

ATTACHMENT TO

THE

SUPPORT

Although organochromium catalysts are not well characterized, organochromium compounds are thought to bind to the support by reaction with surface hydroxyls as other types do. When Cr(allyl), or Cr(allyl), is used, propylene is released (59,60). Chromocene loses one ring (52-55), and 8stabilized alkyls of chromium lose the alkane (81). OH

+ CrR,

1

-----+O-CrR

+ RH

1_

Diarenechromium(0) compounds may be somewhat different because hydrogen is released, suggesting an oxidation-probably to Cr(1). Whether a ring is also lost is unknown. Double attachment to the support by reaction with a pair of hydroxyls is also possible, yielding a variety of surface species (5860).

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

93

Many of the organochromium compounds exist as dimers, e.g. diallylCr(II), and one exists as a tetramer i.e., Cr(II),tmsm, (82, 83)., This is interesting in view of the assertion by some (2) that paired chromium is necessary for polymerization. In fact, neither species is very active for ethylene polymerization until it has been supported on a carrier. The monomeric organochromium compounds behave in about the same way. It seems likely that these polymeric chromium compounds react with the support to form isolated monomeric surface species, thus becoming coordinatively unsaturated. When monomeric compounds react with the surface the loss of a ring or other multicoordinate ligand probably also leaves vacancies in the coordination sphere.

B. KINETICS OF POLYMERIZATION The valence of the starting organochromium compound has been varied from Cr(0) to Cr(IV), but seems to make little difference. All species are quite active, and all initiate polymerization rapidly in comparison to the oxide catalysts. There is no induction time, since the chromium is already reduced, and no gradual rise in rate. Polymerization usually starts immediately on contact with ethylene and either holds steady or slowly declines during a 1 hr run. The rapid start could be attributed to the chromium already being alkylated. However, it seems unlikely, at least for the ring compounds, that the ligand starts the first chain. Otherwise there would be no difference between many of the catalysts, which in fact exhibit a great deal of individuality. Instead, the alkylation reaction may be similar to that on the oxide catalysts, only accelerated. In this case also the initiation mechanism is very poorly understood at this point. C. TERMINATION MECHANISM The organochromium compounds are usually more sensitive to hydrogen as a molecular weight regulator than the oxide catalysts. They can be classed into two broad groups, (A) those which terminate by hydrogenation, including chromocene and its derivatives, and (B) those which undergo /?elimination, including most other compounds like diarene-Cr(O), chromium allyls, and /?-stabilized alkyls of chromium. All generate some unsaturation in the polymer, but for chromocene at 80-90°C it is barely detectable. /?Elimination only becomes significant as the temperature is raised. Adding hydrogen to the reactor with a chromocene catalyst shortens the chains, but

* tmsm = trimethylsilylmethyl.

94

M. P. MCDANIEL

there is no increase of vinyl unsaturation in the polymer. In contrast, the other organochromium compounds, such as diarene-Cr(0) or dipentadienylCr(II), can be even more sensitive to hydrogen, but the polymer is full of terminal vinyl unsaturation. Thus, on these catalysts hydrogen also shortens the chains by some mechanism other than hydrogenation. Like Zeigler and other catalysts which hydrogenate, the chromocene catalysts tend to make polyethylene having a narrow MWD, that is M JM, = 3-6. This suggests that the support's wide diversity of site environments, which for the oxide catalysts produces a broad MWD, has little effect on the hydrogenation mechanism. In fact, the chemistry of the chromocene site is probably dominated by the remaining ring. The activity of the chromocene catalysts is diminished considerably by hydrogen. The polymer tends to have some methyl branching, probably caused by isomerization (see Section III,D), but otherwise it is linear. In contrast, the other organochromium catalysts, which terminate mainly by /&elimination,produce extremely broad M W D polyethylene. In fact, the range of products is so broad that in addition to high polymers, a good portion of the product is also oligomeric. The activity is not diminished by hydrogen. Side reactions must occur easily on such catalysts because the polymers frequently are considerably branched (all types) and have some internal unsaturation as well. D. SUPPORTEFFECTS The best supports for organochromium catalysts seem to be those that have isolated acidic hydroxyls, like aluminum phosphate, some silicaaluminas, and fluorided alumina. The less acidic supports, like silica, are often not as reactive. For example, dicumene-Cr(0) is highly active on aluminum phosphate, but nearly dead when deposited on silica at 25°C. Some activity develops after it is warmed in nitrogen at 100-200°C to encourage reaction with surface hydroxyls. If the silica is first impregnated with an acidic compound such as aluminum, phosphoric acid, or even sulfuric acid, then it reacts with dicumenechromiumat 25°C to yield an active catalyst. Numerous other reagents also activate the silica in this way. Even certain acidic carbon blacks form active catalysts when treated with dicumenechromium. More reactive compounds, like chromocene, are active on silica but aluminum phosphate is often better. High calcining temperatures, such as 500-800"C, are usually preferred, probably because isolated hydroxyls remain. But this depends to some extent on the particular compound. Alumina is usually the worst choice of support, probably due to its larger hydroxyl population.

CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

95

LOG MW

FIG.24. Two types of sites are visible in the molecular weight distribution of polymers from three organo-Chromium catalysts run with hydrogen. Only the support was different. The low MW peak probably results from chromium associated with phosphate, while other sites are associated with aluminum.

The organochromium catalysts display the same sensitivity to the support as chromium oxide. Compounds which undergo B-elimination are much more sensitive to hydrogen when supported on aluminum (i.e., A1,0, or AlPO, but not SiO,). Chromium associated with phosphate exhibits generally higher termination rates and high response to TEB. Evidence for two types of active site on aluminum phosphate can be seen in the molecular weight distribution. Figure 24 shows the size exclusion chromatographs of three polymers obtained from dimethylpentadienyl-Cr(I1) deposited on different supports (56). Hydrogen was added to the reactor in each case. The first curve, for which the support is AlPO, (P/Al = 0.9), contains two peaks. However, the low MW peak diminishes when phosphated alumina is the support (P/Al = O.l), and nearly disappears when the support is just alumina. This suggests that the low MW peak comes from sites associated with phosphate, whereas the high MW peak comes only from the aluminum. The active site concentration on the organochromium catalysts may be higher than that of the oxide catalysts. The activity usually assumes a more linear increase with chromium loading than on the oxide catalysts, at least up to 2 % Cr. Yermakov and Zakharov, studying allyl-Cr(III)/silica catalysts, stopped the polymerization with radioactive methanol, and found that the kill mechanism is different from that on the oxide catalysts (59). The proton of the methanol, and not the alkoxide, became attached to the polymer. This suggests a polarity opposite to that of the oxide catalysts, with the site being more positive than the chain.

96

M. P. MCDANIEL REFERENCES

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28. Eley, D. D., Rochester, C. H., and Scurrell, M. S.,Proc. R. SOC.London, Ser. A 329,361,375 (1972);J . Cutul. 29,20 (1973). 29. Clark, A., Curd Rev. 3(2), 145 (1969). 30. Merryfield, R., McDaniel, M.. and Parks, G., J. Curd 77, 348 (1982). 31. Krauss, H. L., and Stach, H., Inorg. Nucl. Chem. Lett. 4,393 (1968);Z . Anorg. Allg. Chem.

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CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION

97

35. Krauss, H. L., and Weisser, B., Z. Anorg. A&. Chem. 412(1), 82 (1975). 36. Krauss, H. L., Rebenstorf, B., and Westphal, U., 2. Anorg. Allg. Chem. 414(2), 97 (1975). 37. Krauss, H. L., Westphal, U., Z. Nuturforsch., B: Anorg. Chem., Org. Chem. 33B(11), 1278 (1978). 38. Zecchina, A,, Garrone, E., Ghiotti, G., and Coluccia, S., J. Phys. Chem. 79(10), 972 (1975). 39. Zecchina, A,, Garrone, E., Morterra, C., and Coluccia, S., J. Phys. Chem. 79(10), 978 (1975). 40. Garrone, E., Ghiotti, G., and Coluccia, S., Zecchina, A,, J. Phys. Chem. 79(10), 985 (1975). 41. Morys, P., Gerritzen, R., and Krauss, H. L., Z. Nuturforsch., B: Anorg. Chem., Org. Chem. 318(6), 774 (1976). 42. Moeseler, R., Horvath, B., Lindenau, D., Horvath, E. G., and Krauss, H. L., Z. Naturforsch., B : Anorg. Chem., Org. Chem. 31B(6), 22 (1976). 43. Ghiotti, G., Garrone, E., Della Gatta, G., Fubini, B., and Giamello, E., J. Curd 80, 249 (1983). 44. McDaniel, M. P., and Welch, M. B., J. Catul. 82, 98 110 (1983). 45. Przhevalskaya, L. K., Shvets, V. A., and Kazansky, V. B., J. Catul. 39,363 (1975). 46. Beck, D. D., and Lunsford, J. H., J. Cutul. 68, 121 (1981). 47. Myers, D. L., and Lunsford, J. H., J. Cutul., in press. 48. Burwell, R. L., Haller, G. L., Taylor, K. C., and Read, J. F., Ado. Cutal. 20, 1 . (1969). 49. Cornet, D., and Burwell, R. L., J . Am. Chem. Soc. 90,2489 (1968). 50. Lunsford J. H., J. C a r d , in press. 51. Walker, D. L., and Czenkusch, E. L., US. Pat. 3,157,712 (November 1967). 52. Karol, F. J., and Johnson, R. N., J . Polym. Sci., Polym. Chem. Ed. 13, 1607 (1975). 53. Karol, F. J., Brown, G. L., and Davison, J. M., J. Polym. Sci., Polym. Chem. Ed. 11, 413 (1973). 54. Karol F. J., Karapinka, G. L., Wu, C., Dow, A. W., Johnson, R. N., and Carrick, W. L., J . Polym. Sci., Part A - 1 10, 2621 (1972). 55. Karol, F. J., Wu, C., Reichle, W. T., and Maraschin, N. J., J. Cutul. 60,68 (1979). 56. Smith, P. D., unpublished results. 57. Smith, P. D., unpublished results. 58. Ballard, D. G. H., Adu. Cutal. 22, 263 (1973). 59. Yermakov, Y., and Zakharov, V., Adu. Cutul. 24, 173 (1975); Cutul. Reu.-Sci. Eng. 19(1), 67 (1 979). 60. Yermakov, Y.,Cutal. Rev.-Sci. Eng. 13(1), 77 (1976). 61. Ballard, D. G . H., Jones, E., Pioli, A. J. P., Robinson, P. A., and Wyatt, R. J., US. Pat. 3,840,508 (October 1974). 62. McDaniel, M. P., Smith, P. D., Norwood, D. D., U S . Patent, submitted (1984). 63. Krauss, H. L., and Hums, E., Z . Naturforsch., B: Anorg. Chem., Org. Chem. 34B,1628 (1979); Z . Nuturforsch., B: Anorg. Chem., Org. Chem. 35B(7), 848 (1980); Z . Naturforsch., B: Anorg. Chem., Org. Chem. 388(11), 1412 (1983). 64. Krauss, H. L., Symp. Mech. Hydrocarbon React., 1973. Slofok Hungury, 1973. 65. Ivin K. J., Rooney, J. J., Stewart, C. D., Green, M. L. H., and Mahtab, R., J. Chem. SOC.. Chem. Commun., Com. 323, p. 604 (1978). 66. Ghiotti, G., Garrone, E., Coluccia, S., Morterra, C., and Zecchina, A,, J . Chem. Soc., Chem. Commun., Com. 801. p. 1032 (1979). 67. McDaniel, M.P., J . Polym. Sci., Polym. Chem. Ed. 21, 1217 (1983). 68. Hogan, J. P., “Applied Industrial Catalysis,” Academic Press, New York, 1982. 69. Hsieh, E. T., Randall, J. C., and McDaniel, M. P., unpublished results. 70. Hogan, J. P., unpublished results. 71. Clark, A., and Bailey, G. C., J. Catal. 2, 230, 241 (1963). 72. Zakharov, V. A., and Yermakov, Y. I., J. Polym. Sci.,Part A-I 9, 3129 (1971): Proc. Int. Congr. Cutul., 4th, 1968, pap. 16, p. 232.

98

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73. Zakharov, V. A., Yermakov, Y. I., and Kushnareva, E. G., Kinet. Catal. (Engl. Transl.) 8(6), 1181 (1967); J. Polym. Sci., Part A-1 9, 771 (1971). 74. Pullukat, T. J., Hoff. R. E., and Shida, M., J. Polym. Sci., Polym. Chem. Ed. 18,2857 (1980); Symp. Transit. Met. Catal. Polym., Mich. Mol. Inst. (August 1981);J. Appl. Polym. Sci. 26, 2927 (1981). 75. McDaniel, M. P.. Welch, M. B., and Dreiling, M. J., J. Catal. 82, 118 (1983). 76. McDaniel, M. P., and Welch, M. B., J. Catal. 82, 98, 110 (1983). 77. McDaniel, M. P., J. Phys. Chem. 85, 532,537 (1981). 78. McDaniel, M. P., US. Pat. 4,397,765 (August 1983). 79. Hill, R. W., Kehl, W. L., and Lynch, T. J., U.S.Pat. 4,219,444 (August 1980). 80. McDaniel, M. P., and Johnson, M. M., U.S. Pat. 4,364,842, 4,364,854, and 4,364,855 (December 1982). 81. Smith, P. D., and McDaniel, M. P.,unpublished results. 82. Williams-Smith, D. L., and McGlinchey, M. J., J. Am. Chem. SOC.95,3337 (1973). 83. Smith, P. D., unpublished results.

ADVANCES IN CATALYSIS, VOLUME 33

Catalysis Controlled by the Constitution of Active Sites KEN-ICHI TANAKA The Institute for Solid State Physics The Uniuersity of Tokyo Tokyo, Japan

I. Introduction . . . . . 11. Isomerization Reaction . . 111. Hydrogen Exchange Reaction IV. Hydrogenation Reaction . V. Conclusion . . . . . References. . . . . .

1.

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

. . . . .

. . . . .

. . . . .

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. . . . .

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. . . . .

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99

. 104 . 128 . 137 . 156 . 157

Introduction

A catalytic reaction is in general composed of a series of elementary processes, and the surface atoms are necessarily involved in making intermediates and/or activated complexes, as well as in the adsorption of reactant and product molecules. For this reason, the participation of surface atoms is indispensable in all of the elementary processes accomplishing the catalysis, but the kinetics are mainly influenced by the manner of the participation of surface atoms in the rate-determining steps. Accordingly, in order to make clear the roles of the surface atoms in catalysis, it should be clarified how the surface atoms participate in the rate-determining steps as well as in the other elementary processes. Hydrogenation of olefins is a good example for demonstrating the roles of the surface atoms in catalysis. The orbital symmetry rule in chemical reactions suggests that the highest occupied molecular orbital (HOMO) of one reaction partner and the lowest unoccupied molecular orbital (LUMO) of the other should meet the symmetry requirements. In this respect, a concerted addition of an H, molecule to the double bond of an olefin, that is, a molecular addition reaction, is a forbidden process. Adsorption of olefin on transition metal surfaces undoubtedly changes the population of electrons in the HOMO (nu)and the LUMO ($) as shown schematically in Fig. 1. In spite of such perturbation of the electron densities of the HOMO and the 99 Copyright Q 1985 by Academic Pms,Inc. All rights of reproduction in any form rcservcd.

100

KEN-ICHI TANAKA

FIG. 1. Symmetry forbidden approach of a hydrogen molecule to adsorbed olefin.

LUMO, the symmetry restriction may not be circumvented. In other words, the role of the catalyst is not to circumvent the symmetry restriction but to provide some of the elementary processes which avoid the reaction route of the symmetry restriction. Wood and Wise ( 1 ) demonstrated these circumstances in their study of the hydrogenation of cyclohexene over a gold film which was intimately bonded to the outer side of a Pd-Ag thimble, as described in Fig. 2. The thimble is semipermeable to hydrogen diffusion, that is, hydrogen diffuses from the Pd-Ag side to the Au side, but the diffusion from the Au side to the Pd-Ag side is negligible. Such characteristic semipermeability is responsible to the deficient ability of the Au surface for

,eThermocouple

Hydrogen Permeable Thimble

FIG.2. Hydrogen semipermeable reactor (I).

101

ACTIVE-SITE CONTROL OF CATALYSIS

*% 8

0

-! X

Hydrogen

I

P,

rJ

8 4 c

U

$2

I_,<-

Cyclohexane

n

0 0

600

1200

1800

time (rnin)

FIG.3. Hydrogenation ofcyclohexene on Au at 125°C. H, was admitted at time 1300 min to Pd/Ag thimble (I).

the dissociation of the hydrogen molecule. When a mixture of H, and cyclohexene was added to the Au side of the reactor, a small increase of cyclohexane was observed at 125"C, but the hydrogenation reaction commenced suddenly upon filling the Pd-Ag side with hydrogen, as shown in Fig. 3. This result indicates that the dissociation of H, is an indispensable process for the catalytic hydrogenation of olefins with H,,and that the catalytic hydrogenation reaction can proceed even on a surface being inactive for the dissociation of the H, molecule if hydrogen atoms are provided on the surface through different routes. This result suggests that the elementary processes are not necessarily accomplished on an identical site if the reaction intermediates encounter each other efficiently. This is quite an important concept in the design of efficient catalysts, because the functions required for the active sites to promote a desired reaction are more easily fulfilled by the cooperation of several sites with different catalytic abilities. For example, the dissociation of the H, molecule on a single crystal surface of Pt(s)[6( 11 1) x (1 1 l)] occurs at the steps in preference to the terrace (2). This does not mean necessarily that the hydrogenation reaction proceeds entirely on the steps, because hydrogen atoms are efficiently formed on the steps with kinetic facility but are rapidly diffused onto the terrace by thermodynamic facility. As a result, the hydrogenation of olefins proceeds predominantly over the terrace and the overall rate is insensitive to the step densities, although the steps are indispensably involved in the dissociation of hydrogen molecules, That is, the catalytic hydrogenation reaction requires steps but the rate of the hydrogenation reaction is influenced little by the step densities. From this viewpoint, catalytic activities depending on alloy composition such as observed in the dehydrogenation of cyclohexane to benzene on

102

KEN-ICHI TANAKA

Ni-Cu alloy, (3)

and the dehydrogenation of propane to propene over Pt-Au alloy (4), i.e., C-C-C + C-C=C + H,, are interesting phenomena. It is reasonable that these two dehydrogenation reactions require the same functions for active sites because they are composed of similar types of elementary processes. In spite of the expectation of the similarity of these two reactions, specific activity of the alloys for the two reactions is influenced differently with respect to the composition of alloys, as shown in Fig. 4. Such a

0

5

10

15

Pt (at */a)

y 5 Cyclohexane

a 4

.

\

-g ol 0

20

40

Cu

60 80

100

(at%)

FIG.4. Specific activity of Pt-Au alloy for the dehydrogenation of propane to propene at 360°C (4, and that of Ni-Cu alloy for the dehydrogenation of cyclohexane to benzene at 316°C (3).

ACTIVE-SITE CONTROL OF CATALYSIS

103

remarkable contrast may be rationalized by the different manner of the participation of surface atoms in the rate-determining steps. That is, all of the elementary processes except the desorption of benzene in the dehydrogenation of cyclohexane may take place only on Ni-sites, but the desorption of benzene occurs on Ni as well as on Cu atoms. Then, if the desorption of benzene is the rate-determining step of the reaction, the reaction rate will be rather insensitive to the composition of Ni atoms. In contrast to this, the ratedetermining step of the propene formation reaction involves Pt atoms, and Au is an entirely inert component for this reaction. In other words, the apparent catalytic activity of alloys does not necessarily reflect the composition or the structures of active sites required for catalysis. This is quite analogous to the influence of the crystallographic or clusterized structures of active sites on catalytic activity. Accordingly, in order to understand the roles of active sites in catalysis, we should clarify not only the intermediates but also the functions of active sites in relation to elementary processes. For this reason, it may be an interesting question whether the two reactions taking place via the same kind of intermediates occur on the same or different active sites. One good example is the isomerization of olefins via alkyl intermediates and their subsequent hydrogenation. In the Horiuti and Polanyi mechanism, complete overlapping of the intermediates, as well as the reaction routes, was tacitly assumed as described in Eq. (1). In this reaction scheme, step (1) and step (1’) are the

SCHEME1. Hydrogenation and isomerization of olefin via common intermediates.

same type of elementary reaction. As a result, if step (1) is reversible on a given site, the isomerization and/or hydrogen exchange of olefin will proceed in general on that site. As will be discussed later, the elementary processes involve, in general, physical processes such as surface migration and rotational motion of intermediates. Accordingly, if internal or external rotation of o-bonded alkyl species is prohibited, the isomerization reaction scarcely occurs although the alkyl intermediates are reversibly formed on the surface. It is clear that the hydrogenation of olefins requires subsequent reaction of the alkyl species with hydrogen in step (2). Thus, if step (2) occurs only on sites with additional functions, the alkyl intermediates for the isomerization and those for the hydrogenation reaction will not overlap each other and the two reactions will proceed independently on different sites.

104

KEN-ICHI TANAKA

If a catalytic cycle composed of several elementary processes is promoted on an isolated single site, we could make distinctions about the function of the active sites. For example, some metal complexes which are active for the isomerization reaction of olefins via alkyl intermediates are not effective catalysts for the hydrogenation reaction, and such differences in catalytic ability of the metal complexes is explained by the numbers of coordinatively unsaturated sites which are available for the reactions as described schematically in Scheme 7. If two such types of site having different catalytic abilities coexist on a catalyst surface, the isomerization and hydrogenation reactions of olefins could proceed simultaneously but on different sites. As will be discussed later, it is rather reasonable that real catalysts involve active sites with different abilities. For this reason, the Horiuti and Polanyi mechanism which assumes a priori common alkyl intermediates for the isomerization and hydrogenation reactions, appears less logical. Taking into account the concept that the active sites are indispensable partners of the intermediates, the present review focuses on the intermediates and the controlling mechanisms of active sites in heterogeneous catalysis. II. lsomerization Reaction

The catalytic isomerization reation of olefins is caused by either an associative or a dissociative mechanism. The associative mechanism involves either o-alkyl or alkyl ion intermediates and the dissociative mechanism involves allylic intermediates, as described in Scheme 2, where M-H and M represent active sites. In the associative mechanism, the reaction of olefin with metal hydride produces o-alkyl species while the reaction with proton produces alkyl cations. In contrast to the covalent bonding of o-alkyl species, alkyl cations are bound to catalytic sites by ionic force. In the dissociative mechanism, adsorbed olefin loses a hydrogen atom or ion which results in formation of one of the three allylic species described in Scheme 2. These allylic species are bound on sites by either o- or A-bonding or by ionic force. Any catalytic isomerization reaction of olefins occurs through any one of the six reaction routes in Scheme 2. A characteristic feature of the associative mechanism is that the isomerization reaction is necessarily accompanied by intermolecular hydrogen exchange, while the dissociative mechanism is accompanied by no intermolecular hydrogen exchange provided that a dissociated hydrogen atom does not migrate from one site to the other sites. The distinctive features of the associative and the dissociative mechanisms for the isomerization of olefins are described in Scheme 3, where a 1 : 1 mixture of 1-butene-doand 1-butene-d, is adopted as a

105

ACTIVE-SITE CONTROL OF CATALYSIS

XZ.Ht

SCHEME 2. Probable reaction mechanisms and intermediates of catalytic isomerization reactions.

&..\ CH

+ CDfl D -C QC D3

' c$.

CHj=CH-CHfiH3

/-A-xJ

i'CH-CH3

y-H

CD /....\ ),C D ;

CDCD, X-D

-

2-b~tene-d~

-

2-butene-dg

SCHEME 3. Isomerization and intermolecular and intramolecular hydrogen exchange reactions of I-butene.

106

KEN-ICHI TANAKA

model reaction. In this reaction scheme, n-butyl species are excluded from the reaction scheme because they may not participate in the isomerization reaction. The same is true for the cis to trans isomerization reaction. If the cis to trans rotation occurs via sec-butyl intermediates as described in Scheme 4, coisomerization of a 1:1 mixture of cis-2-butene-do and cis-2-butene-d, will

SCHEME 4. Cis to trans isomerization through stereospecific hydrogen addition (A) and elimination (E) processes on 2MH-site.

produce trans-do, -dl -d,, and -d, in equal probability. By taking multiexchanged products into account, Hightower and Hall ( 5 ) derived a generalized equation giving the average number of exchanged hydrogen atoms per molecule. That for n-butene is given by H atoms exchanged per molecule =

4

7

i= 1

i=5

1 iNi + c(8- i)Ni

(2)

where N , is the mole fraction of each species containing i deuterium atoms. The isomerization of n-butenes on MoS, catalyst is a typical reaction for which this relationship holds. As shown in Fig. 5, neither the double bond migration reaction nor the cis to trans isomerization reaction of n-butenes occurs on MoS, in the absence of hydrogen, but these reactions are markedly enhanced by the addition of H,. Such a hydrogen promoting effect reveals the formation of sec-butyl species on the MoS, catalyst. This speculation was substantiated by the coisomerization of cis-2-butene-do and cis-2-butene-d, in the presence of a 1: 1 mixture of H, and D,. A clear relation suggesting the associative mechanism was obtained (6) as shown in Fig. 6. The average number of exchanged hydrogen atoms per trans-2-butene molecule is very close to 0.5, which indicates that one hydrogen atom is necessarily exchanged in each double bond migration. This fact implies that the hydrogen addition and elimination processes in the formation of sec-butyl species on MoS, proceed in a stereospecific manner as described in Scheme 4. However, the average number of exchanged hydrogen atoms per 1-butene molecule is

107

ACTIVE-SITE CONTROL OF CATALYSIS

20

I

-

H p 3.5 torr added

(a

trans-2-Butane

1 ln ln

a n

0

50

100

Time

(min)

Time

(rnin)

150

FIG.5. Hydrogen promoting effects on the cis to trans isomerization (a) and the double bond migration (b) of n-butenes on MoS, catalyst at room temperature (36).

undoubtedly larger than 0.5, which may be caused by a rapid hydrogen exchange of 1-butene via n-butyl intermediates. If the cis to trans rotation proceeds in accordance with the mechanism described in Scheme 4, the coisomerization of cis-2-butene-do and cis-Zbutene-d, will produce mainly trans-2-butene-2-d, and trans-2-butene-2-h,. Microwave spectroscopic analysis is one of the best methods for identifying the location of the deuterium atom in d,-olefins. As the dipole moment of trans-Zbutene is too small to do microwave spectroscopic analysis, cis-2-butene-d1, formed by the reverse isomerization of trans-Zbutene, was subjected to the analysis. That is, trans-Zbutene-do will change to either cis-2-butene-do or cis-Zbutene-2-dl, and trans-2-butene-2-d, will change to cis-Zbutene-do, cis-2-butene-2-d1,

108

KEN-ICHI TANAKA

1.4 1.2

0.8

I

1-Butene

trans-z-~utene.

-a

0.6

/

/'

- 0 - 4

0.4 -

-

0.2

0 - 3 0

10

0

L

30

20

40

50

60

conversion ("lo) FIG.6. Average number of exchanged hydrogen atoms per molecule in the coisomerization of cis-2-butene-do and cis-2-butene-d, ( 1 : 1) on MoS, in the presence of a mixture of H, and D, ( 1 :1 ) at room temperature (6).

and cis-2-butene-2,3-d2 by the reverse isomerization. It was proved that cis-2-butene-d, is composed of 100% cis-Zbutene-Zd,, as shown in Table 1 (6). This fact implies that trans-2-butene-2-d, is formed in the coisomerization of cis-Zbutene-do and cis-2-butene-d8, and the hydrogen promoting effect such as observed in Figs. 5-a and 5-b is evidence for the formation of alkyl intermediates. On the other hand, it was found that if a mixture of cis-2-butene and 2methyl-1-butene was added to MoS, powder at room temperature, the TABLE I Geometrical Isomers of cis-2-Butene d , Formed in the Coisomrrization of cis-2-Butene-do and cis-2-Butene-d, on MoS," Conversion to trans-2-butene

cis-2-butene-24,

cis-2-butene-I-d,

66%

100 %

0%

Room temperature; From ref. 6.

109

ACTIVE-SITE CONTROL OF CATALYSIS

0

10

20

30

4 0 5 0 60 time (min)

70

80

FIG.7. Segregated isomerization of cis-2-butene and 2-methyl-1-butene on MoS, at room temperature (7,44).

double bond migration of 2-methyl-1-butene was catalyzed in the absence of hydrogen, as shown in Fig. 7. When hydrogen was added, however, the isomerization of cis-2-butene was markedly enhanced although the isomerization of 2-methyl-I-butene was influenced very little. If hydrogen atoms or ions preexist on the catalyst surface, the isomerization of olefins may occur without hydrogen addition. Accordingly, it was speculated that the surface of MoS, has acidic hydrogen which can promote the isomerization of 2-methyl1-butene selectively, while the isomerization of cis-2-butene proceeds substantially via a-bonded sec-butyl intermediates. An important feature of alkyl cations is that they are held to the conjugate base sites, which have negative charge, by ionic force. Ionic bonding or interaction is less sensitive to the bond length or bond angle because of its long range force. As a result, the stability of alkyl cations depends substantially on the proton activity of the Bronsted sites but is less sensitive to the structures of the active sites. This is in contrast to the formation of covalently bonded a-alkyl species on active sites, which is markedly influenced by the bond length and/or bond angle. For this reason, the results in Fig. 7 are especially interesting because the isomerization reaction via a-alkyl intermediates and that via alkyl cation intermediates occur concomitantly on MoS, (7). X-Ray diffraction showed that the molybdenum disulfide powder used in this experiment has a hexagonal layer structure. A remarkable feature of such layer compounds is that the powder is composed of small single crystal particles. In view of these facts, an interesting question arises as to whether

110

KEN-ICHI TANAKA

(ioto I FIG.8. Stoichiometricstructures of (OOOl), (1 lzO), (TOIO), and (1070) surfaces of MoS, (11).

such distinctive isomerizations as those of cis-Zbutene and 2-methyl-lbutene occur concomitantly on the same crystal faces or on different crystallographic surfaces of the MoS, powder. To answer this question, a series of elaborate experiments were performed on MoS, single-crystal catalysts. The crystallographic structure of MoS, is schematically shown in Fig. 8, where the (OOO1) surface (basal plane) is composed of close-packed sulfur ions and the edge surfaces corresponding to (1070) and (1120) planes expose molybdenum ions as described in the figure. The fully coordinated molybdenum ion is surrounded by six sulfur ions in a prismatic form, but the molybdenum ions exposed on the edge surfaces of MoS, have some coodinative vacancies. By cleaving a single crystal of MoS,, a set of wafers of MoS, single crystals which have approximately equal basal plane areas were prepared. If we cut one of the wafers along the C-axis, we can enlarge the edge surface area without causing an appreciable change of the basal plane, as shown schematically in Fig. 9. If we compare the catalytic activity of these two forms of single crystal faces we can easily deduce which is active for a given reaction. The isomerization reactions of cis-Zbutene and 2methlyl-l-butene on cut and uncut MoS, single-crystal catalysts are shown in Fig 10. The isomerization of cis-Zbutene obviously occurs preferentially on the cut catalyst. On the other hand, the isomerization of 2-methyl-l-

ACTIVE-SITE CONTROL OF CATALYSIS

111

Twin Reactor

Uncut

cut

FIG.9. Sketch of cut and uncut single crystal wafers of MoS,, and a twin reactor used for activity test.

butene occurs on either the cut or the uncut catalyst with nearly equal rates (8).These results provide convincing evidence that the isomerization of cis-2butene proceeds preferentially on the edge surface of the MoS, crystal while the isomerization of 2-methyl-1-butene occurs on the basal plane of the MoS, crystal. It is noteworthy that the isomerization of cis-Zbutene taking place on the edge surface required a hydrogen cocatalyst, but the isomerization of 2methyl-1-butene on the basal plane occurred in the absence of hydrogen. The hydrogen-exchange reaction between C2H4 and C2D4 via ethyl intermediates is the same type of reaction as the cis-trans isomerization reaction of Zbutene, because these reactions both involve the elementary processes of alkyl intermediate formation followed by internal rotation around C-C bonds, as described in Scheme 5. Then the hydrogen exchange

SCHEME5. Hydrogen exchange reaction of ethylene on ,MH-site.

reaction of C, H4 and C,D, and the isomerization of 2-methyl-1-butenewere compared on five different meshes of single crystal catalysts, as summarized in Table 11. It is clear that the hydrogen-exchange reaction of C2H4 and C2D4 is sensitive to the particle size of the MoS, crystal, but that the isomerization of 2-methyl-1-butene is independent of particle size.

112 60

KEN-ICHI TANAKA

c'c-

-

50

CF

-

(a)

c'c-c,c

p

-

5 40

Ut

;io

.s 30 10 O

-

O

d

G

'~

-

-

*

.

mcut L 7

0

20

40 60 80 time (rnin)

100

FIG.10. Isomerization of cis-2-butene (a) and of 2-methyl-1-butene (b) on cut and uncut single crystal wafers on MoS, at 100°C (8,44). 0 1978 American Chemical Society.

The preceding facts lead to a conclusion that the basal plane of the MoS, crystal has a proton which can catalyze the isomerization of 2-methyl-lbutene by making tertiary carbonium cation intermediates, while the isomerization reaction via a-alkyl species as well as the hydrogen exchange reaction of olefins via a-alkyl species are brought about on the edge surface of the MoS, crystal by forming alkyl species in the presence of hydrogen. It is reasonable to assume that the catalytic cycle involving alkyl cation intermediates is definitely controlled by the proton activity of acid sites and the proton affinity of the olefins. The stability of carbonium ions decreases rapidly in the sequence tertiary > secondary > primary cations. In fact, the isomerization of 2-methyl-1-butene via a tertiary carbonium ion is brought about on the basal plane of the MoS, crystal, as shown in Fig. lob, but the isomerization of 3-methyl-l-butene, as expressed by Eq. (3), does not procced TABLE I1 Effects of Crystal Size of MoS, on the Isomerization of 2-Methyl-I-butene and the Hydrogen Exchange between C,H, and C,D,"

Mesh of MoS, crystal Process

10

10-20

Isomerization of 2-me th yl- 1 -butene Hydrogen exchange of C,H,-C,D,

1 lo-,

1

a

From ref. 37.

5

20-42

42-100

1

2

10-3

100

4 1

ACTIVE-SITE CONTROL OF CATALYSIS

C C-C-C=C

I

+ H+(S),

-

113

C

I

C-C-C--C

+

\

(S), C

I

C-C-C=C

C

.*

+ H+(S), Jt, C-J-Y-C', 6);

C-C=C-C

+ H+(S),T

,,**'

H '

(3)

on the basal plane. That is, the proton activity on the basal plane of MoS, is sufficient to make a tertiary carbonium ion by reacting with 2-methyl-lbutene but is not sufficiently high to make a secondary carbonium ion form 3-methyl-1-butene as describe schematically in Eq. (3). This fact is in remarkable contrast to the formation of both n-alkyl and sec-alkyl species on the edge surface of MoS, in the presence of hydrogen; that is, the hydrogenexchange reaction between C, H4 and CzD4 proceeds through ethyl intermediates and the isomerization of n-butenes occurs via sec-butyl intermediates on the edge surface of MoS,. Furthermore, a-olefins such as propene and 1-butene provide both n-alkyl and sec-alkyl species in the presence of hydrogen on the edge surface of the MoS, crystal. As mentioned above, such a hydogen promoting effect as observed in Fig. 5 has been explained by the formation of alkyl intermediates. However, if D, was added instead of H,, deuterium-free 2-butene was mainly formed on the MoS, catalyst as shown in Fig. 1la, while concomitant hydrogenation of 1-butene yielded butane1, 2-d, in > 80 % yield as shown in Fig. 11b. If the hydrogenation reaction as well as the isomerization reaction of 1-butene proceeds via half-hydrogenated intermediates on the MoS, catalyst, the results in Figure 1l a and 1l b indicate that the isomerization of 1-butene proceeds through deuterium-free sec-butyl species (sec-butyl-do) while the hydrogenation reaction occurs via monodeutero butyl intermediates, i.e., either n-butyl-d, or sec-butyl-d,, as described in Scheme 6. Such distinctly different deuterium distribution of the intermediates for the two reactions can not be explained by the traditional Horiuti and Polanyi mechanism, which assumed overlapping of the intermediates for the hydrogenation and isomerization reactions, as described Scheme 1. In a manner similar to the cis-trans isomerization reaction of 2-butenes, the hydrogen-exchange reaction between C, H4 and C, D4 is evidently promoted by the edge surface of the MoS, crystal in the presence of hydrogen, as shown in Fig. 12. It is known that the H,-D, equilibration reaction is also catalyzed by the edge surface of the MoS, crystal, as shown in Fig. 13.

114

KEN-ICHI TANAKA

I-Butene

+

J \[

[ sec-butyl-do]

D2

n-buryl-dl or see-butyl-dd

1

1

2-Butene-do (

lsomerlzotion

Butane- 1,2-d2 )

(

Hydrogenotlon 1

SCHEME6. Plausible deuterium content of the intermediates of the isomerization and deuteration of I-butene on MoS, catalyst.

cis-2-butsnedr

_---~ I____---S - 2 -butene-dc

20-

butane-

4

Y

e

40 20-

00

+5

10

1i

conversion ( % )

FIG.11. Deuterium distributions in trans- and cis-Zbutenes (a) and in butane (b) formed in the reaction of I-butene with D, on MoS, at room temperature.

ACTIVE-SITE CONTROL OF CATALYSIS

0' 0

20

6 0 80 100 time ( m i n )

40

115

120

FIG.12. Hydrogen mixing of ethylene (CzH4-CzD4) on cut and uncut single crystal catalysts of MoS, at 100°C in the presence of hydrogen (14,37).

If these results are true, a question arises as to why hydrogen exchange between olefin and D, does not occur during the isomerization reaction on the edge surface of MoS,. To answer this interesting question, the H,-D, equilibration reaction and the hydrogen scrambling of C,H,-C,D, were carried out together on an MoS, powder catalyst which was treated with H,S at 300°C to suppress the activity. A mixture of 10 torr of C2H4 and C2D4 in a ratio of 3:2 and 20 torr of H, and D2 in a ratio of 2:3 were added together to the MoS, powder catalyst at room temperature (9). The total number of hydrogen and dkuterium atoms in the mixture of C,H, and C,D, was adjusted to be equal to those of the mixture of H, and D,. The deuterium 50

Z 30 Y

cl

= 20 1

0

k

funcut

'

0 0

20

40

60 80 100 time(min)

120

FIG.13. The H,-D, equilibration on cut and uncut single crystal catalyst of MoS, at -40°C (37).

116

KEN-ICHI TANAKA

0

20

40 60 t i m e (hr)

80

100

FIG.14. Segregated hydrogen mixing in ethylene and in hydrogen on MoS, at room temperature (9),where C,H,/C,D, = 3/2 and H,/D,= 2/3.

fraction in ethylene (4; = 2/5) was different from that in hydrogen (4; = 3/5). Accordingly, we can monitor the following three types of hydrogenexchange reactions simultaneously under exactly the same conditions: (a)

(b) (C)

C,H,

+ C,D, H, + D ,

C,H,+ D,

-

+ C,D,H

(4:

=

2/5)

2HD

(4:

=

3/51

C,H,D + H D

(4; < $3

C,H,D

The results are shown in Fig. 14, where both the H,-D, exchange reaction and the hydrogen-scrambling reaction of C2H4 and C2D4 proceed at appropriate rates but the deuterium fractions in ethylene (4: = 2/5) and hydrogen (4; = 3/5) do not show a trend toward approaching each other. This fact implies that the two hydrogen-exchange reactions proceed independently on the edge surface of MoS,. This result is consistent with the predominant 2-butene-do formation in the isomerization of 1-butene on MoS, in the presence of D, (Fig. lla); that is, the isomerization reaction taking place on the edge surface on MoS, requires hydrogen as a cocatalyst but such hydrogen does not participate in the H,-D, equilibration reaction. These results reveal that the active sites for the isomerization and/or the hydrogen-exchange reaction of olefins are isolated from the sites for the H,-D, equilibration and for the hydrogenation reaction of olefins. In other words, active sites with different catalytic abilities coexist on the edge surface of MoS,, and each catalytic reaction proceeds precisely on the sites for which prerequisites are fulfilled. The prerequisites for an active site are sometimes fulfilled by the cooperation of several sites, but if a catalytic cycle is accomplished on an isolated single site, the roles and/or functions of each active site can be clarified more easily.

117

ACTIVE-SITE CONTROL OF CATALYSIS

Siege1 (ZO) proposed that two kinds of active site coexist on oxide catalysts, i.e., one for hydrogenation and the other for isomerization. He supposed that if a catalytic cycle revolves on an isolated single site, the requisites for active sites might be the same as those for mononuclear metal complexes. Mononuclear complexe catalysts require an adequate number of coordination vacancies or labile coordination sites in catalysis, and it is known that complexes active for the isomerization and/or hydrogen-exchange reaction of olefins have two degrees of coordinative unsaturation where one hydrogen atom is bound, while the hydrogenation-active catalysts require three available coordinative sites. The reactions on an isolated single site might be the same as described in Scheme 7, where ,MH-site denotes the monohydride of the site having two degrees of coordinative vacancy, and 3M-site denotes the site having three degrees of coordinative vacancy.

isomerizat ion

hydrogenation

SCHEME7. Catalytic cycles of the isomerization on 'MH-site and of hydrogenation on 3M-site.

If we consider the broken edge surface, the fresh surface might expose molybdenum ions having one or two coordinative vacancies, as visualized in Fig. 8. However, this structure might be stabilized by rearranging sulfur ions, and Fig. 15 shows a plausible rearranged structure of (1010) and (1070) surfaces as proposed by Farragher (ZZ). By measuring the angle-resolved photoemission from the edge surface of MoS,, the electronic band structures of the edge surface could be elucidated. Very recently, Murata et al. (12) succeeded in obtaining angle-resolved photo emission spectra from the edge surface of MoS, which was prepared by cutting a single crystal in a vacuum of lo-'' torr. As described in Fig. 15, the He-I light was beamed onto the surface at an angle of 30" from the normal surface. Peaks A and B of the spectrum are assigned to the emission from the d-band of molybdenum ions on the edge surface, and peaks C, D, and E are a result of the emission from

118

KEN-ICHI T A N A K A

D

0=7oa 9=55'

845'

8-15'

833 tioio)

8-0'

tioio) Binding Energy (ev)

FIG.15. Angle-resolved photo emission from the edge surface of MoS, single crystal (12). and imaginary sketch of reconstructed (7010) and (1070) surfaces. Incident angle of He-I was fixed at 30°C.

the p-band of sulfur ions. It is a remarkable feature of the edge surface that binding energy of these peaks changes very little with the emission angles. This feature is in strong contrast to the photoemission angles from the basal plane of MoS, given by McGoven, (13) where the binding energy of the electrons in the d-band varied in a range of 1 eV depending on the emission angles. These results suggest that the edge surface has a molecular or amorphous-like specific structure on which the dangling bonds of surface molybdenum ions are free from the coordination of sulfur ions. Accordingly, it seems reasonable to assume that the evacuation of MoS, at 430°C may produce some sites having three degrees of coordinative unsaturation upon which the hydrogenation reaction proceeds as shown in Scheme 7. The edge surface stabilized by the evacuation may undergo further reconstruction by the adsorption of some kinds of gases. In this respect, it is worthy of note that the isomerization reaction of cis-2-butene in Fig. 10a has an apparent induction time but that of 2-methyl-1-butene on the basal plane in Fig. 10b occurs with no induction time. Furthermore, it is curious that this

119

ACTIVE-SITE CONTROL OF CATALYSIS

-s v

501

P

40

P

E 10

0

60

120

180

240

I0

300

tirnecrnin)

FIG.16. Induction time for the isomerization of 1-butene on cut single crystal catalyst of MoS, (14.37). I-Butene (25 torr) was added first, followed by the addition of H, (75 torr) at 100°C.

induction time is not shortened by contracting the edge surface with either olefin or hydrogen alone, as shown in Fig.16: that is, the edge surface is activated only by exposing it to a mixture of olefin and hydrogen. Based on a series of reactions performed on the single-crystal catalyst of MoS,, it was deduced that the induction time was observed only on the reactions involving internal rotation of bulky groups (14). Accordingly, it may be concluded that the 'MH-site has the ability to produce alkyl species, but the bulky groups in the alkyl species bound to 2M-sites are restricted from rapid rotation by the surroundings of the sites. The coordination of alkyl species, however, induces reconstruction of the *M-sites during the induction time, and as a result, the reactions involving the rotation of bulky groups start. It should be emphasized here that the rotational barriers for the internal rotation of bulky groups are lowered by the reconstruction of active sites during the induction time, but the rotation around the Mo-C bond of sec- or tert-alkyl species is still restricted (6).As a result, the double bond migration reaction of 1-butene which requires the rotation of sec-butyl species around the Mo-C bond as described schematically in Eq. (4),proceeds much slower than the cis to trans isomerization although both reactions proceed via sec-butyl species .as described by Eqs. (4)and (5). C=C-C-C+H-Mo

-

H'

c-c-c-c S

---Mo

C=C

/

+H-Mo

H

---Mo I

MO

H '

C-C=C-C+H-Mo

-' c-cec-c

C

d'

---

rolamn

>-

c\

c-c-c-c *b

c\

C=C

\

C

+H-Mo

(4) (5)

120

KEN-ICHI TANAKA

The formation of monohydride sites (’MH-site) during the isomerization reaction in Scheme 7 might be nearly irreversible, because the isomerization of 1-butene in the presence of D, produces mainly 2-butene-do, as described in Figure 1 la; that is, the hydrogen on ,MH-sites participates in the isomerization reaction as a cocatalyst and a catalysis cycle revolves numerous times before the H atom on the ,MH-site is replaced with a D atom from D,. Such slow isotopic exchange of the hydrogen atom with D, is responsible for the difficulty of hydrogen dissociation on ,MH-sites, because the dissociation of a hydrogen molecule requires two coordinative vacancies, but the ,MH-site has only one vacancy.

In contrast to the irreversible formation of monohydride sites by a heterolytic (olefin) \

dissociation of H, on

Mo(S), in Eq. (6),a homolytic dissociation of , H, may occur reversibly on an 3MH-site, which follows the H,-D, equilibration as displayed in Scheme 8. The H,-D, equilibration on an isolated single site requires the involvement of at least three hydrogen atoms, so that trihydride sites might be the intermediates of H,-D, equilibration on MoS, as outlined in Scheme 8. In a homogeneous system, trihydride complexes have been prepared by the addition of H, to monohydride complexes as demonstrated in the case of iridium trihydride complex (15). With respect to the formation of mono- and dihydride sites, Schrock and Osborn (16) observed an interesting equilibration between a rhodium monohydride complex and a rhodium dihydride complex as expressed by Eqs. (7) and (8). I

[RhL,SZ]+

+ H2

[RhH,L,S,]

+

[RhH2L3S,lf p==&

RhHL,S,

+H

+

(7) (8)

ACTIVE-SITE CONTROL OF CATALYSIS

121

H2-D2 equilibration SCHEME 8. Catalytic cycle of the H,-D, equilibration reaction on 3MH-site.

where L represents ligands such as PPhMe,, PPh,Me, and PPh,, and S is a solvent molecule. This finding seems to represent the mechanisms of homolytic and heterolytic dissociation of H, over heterogeneous catalysts. In conformity with an octahedral coordination of Rh complexes, the dihydride species in Eq. (7) may correspond to the 3MH,-site and the monohydride species in Eq. (8) to the 'MH-site, respectively, The RhH, complex in Eq. (7) is a good olefin hydrogenation catalyst but is poor in the olefin isomerization reaction. In contrast, the RhH complex in Eq. (8) is an excellent catalyst for olefin hydrogenation as well as for olefin isomerization reactions. An interesting fact is that the RhH, complex and RhH complex are in equilibrium during the olefin hydrogenation reaction, and both contribute to the hydrogenation of olefins. However, if the reactant molecule coordinates strongly with the Rh complex, the situation undergoes significant change. In the case of the hydrogenation of norbornadiene (NBD), [Rh(NBD)L,] + is formed first and is followed by the reaction with H,; that is, the active species is a dihydride complex instead of a monohydride complex. Interestingly, this is similar to the homolytic dissociation of H, forming dihydride sites during the hydrogenation of olefins or dienes on MoS, (19,where the reaction rate is first order in hydrogen pressure and zero order in olefin pressure. This means that the 'M-site on the edge surface of MoS, is occupied by olefin or diene during hydrogenation, and a hydrogen molecule undergoes homolytic dissociation to yield (Olefin)MoH,, as described in Scheme 7. On the other hand, if the 'M-site is exposed to hydrogen in the absence of olefins, and trihydride sites might be formed as described in Scheme 8. There is no convincing evidence as to whether either homolytic or heterolytic dissociation prevails in the adsorption of H, on 'M-sites. If adsorption of hydrogen on the 'M-site prefers homolytic to heterolytic dissociation, the 'M-site changes to a 'MH,-site first, and this is followed by the heterolytic dissociation of H, to yield a trihydride site, i.e., the 3MH,-site. In contrast to the hydrogenation reactions on dihydride sites, isomerization and/or hydrogen

122

KEN-ICHI TANAKA

exchange reactions of olefins occurring by the associative mechanism are catalyzed by either monohydride sites or Bronsted acid sites. If we compare the isomerization reactions taking place on monohydride sites with those taking place on Bronsted acid sites, the former might be considered examples of structure-requirement type catalysis with the latter being examples of structure-nonrequirement type catalysis. The isomerization and/or hydrogen exchange reaction occurring through a dissociative mechanism can also be classified into nonionic and ionic type of reactions, i.e., the reaction via 6-or n-allylic species and that via ally1 anion or cation. The former type of reaction may require a certain degree of coordinative unsaturation for active sites, but the latter type of reaction is influenced essentially by the proton or hydride affinity of the surface. The isomerization of olefins on C r 2 0 3may correspond to the former case, where the active site requires a certain degree of coordinative unsaturation. In fact, the reactions on C r 2 0 3 are inhibited by the addition of 0, (18). In contrast to this, the isomerization and/or hydrogen exchange of olefins via alkylallyl carbanions might not require coordinative vacancies for active sites, and a typical example is the isomerization of n-butene on Na/Al,O, observed by Pines and Haag (19). In homogeneous systems, Q- and ~3-alkylallylcomplexes, as well as alkylallyl carbanions, have been prepared. Furthermore, a reversible interconversion of the q3-allyl complex to the corresponding olefin complex was demonstrated by using’NMR spectroscopy. Equation (9) shows an equilibration between the q3-allylhydridotrifluorophosphinenickelcomplex and xbonded propene complex (20). Brennemann affirmed also that the q3-allyldeuterio nickel complex changes exclusively to propene-l -dl or propene-34, by the reversible operation of Eq. (9).

Direct evidence for the formation of allylic species on a solid surface was given by Dent and Kokes (21) for zinc oxide by using labeled propene, as expressed in Eqs. (10) and (11). In the IR absorption spectra, an 0-D

123

ACTIVE-SITE CONTROL OF CATALYSIS

and band appears at 2653 cm-', from the adsorption of CH,=CH-CD,, an 0 - H band appears at 3593 cm-', from the adsorption of CD,=CH-CH,. The frequencies of OD and OH bands formed from propene are about 69cm-' and 104cm-' higher than the OD and OH formed by the adsorption of D, and H, an ZnO, respectively. These facts suggest that the hydrogen atom eliminated from propene might be trapped close to the allylic species. If a hydrogen atom eliminated from olefin returns to a different carbon atom in the original olefin, the olefin undergoes the isomerization with substantial intramolecular hydrogen transfer. Accordingly, an evidence for the dissociative mechanism is intramolecular hydrogen transfer in the isomerization reaction. For this reason the isomerization of n-butene on carbon doped with potassium carbonate is an interesting example demonstrating a dissociative mechanism. Potassium carbonate deposited on a carbon surface undergoes decomposition at around 650°C, and is reduced to potassium atoms. The potassium atoms formed by the reductions are dissolved into the carbon by heating in a vacuum. If oxygen is added onto this evacuated surface at room temperature, the dissolved potassium is reaccumulated on the surface (23). Such reversible dissolution and reaccumulation of potassium were proved by Auger electron spectroscopy and by catalysis. That is, the catalyst evacuated at 740°C is inactive for the isomerization of 1-butene but is markedly activated by the addition of 0,, as shown in Fig. 17 (22). This fact is in contrast to the poisoning effect of 0 , on the isomerization on Cr,O, (18). tx

at r.t. Evac. at 650.C

trans-2- butene 0-

0

20

40

60 80 100 time (min)

120

140

291 293 235 297

(ev)

FIG.17. Isomerization of I-butene promoted by the addition of 0, over a potassium carbonate doped carbon at room temperature (r.t.), and the X-ray photoelectron spectra indicating surface reaccumulation ol potassium by 0, (22.23).

124

KEN-ICHI TANAKA

TABLE I11 Coisomerization of cis-2-hutene-do and cis-2-hutene-d, on Preevacuated K,CO,/C in the Presence of 0, at Room Temperature" do

Composition (%) Reactants: cis-2-butene 92.0 trans-Zbutene 8.0 1-butene 0

}

Products: 1-butene 5.2 trans-2-butene 7.9 cis-2-butene 86.9

}

d,

d,

dLl

45.0

0

4.1

50.9

-

-

-

-

55.4

0.5

4.0

40.1

41.7

0

4.1

54.2

From ref. 24.

Such an oxygen promoting effect may suggest the formation of n-alkylallyl carbanion by proton abstraction with oxide ion. To confirm this mechanism, the coisomerization of cis-2-butene-do and cis-Zbutene-d, was performed on this potassium-containing carbon in the presence of 0,. 1-Butene was preferentially formed from cis-2-butene and was mainly 1-butene-do and 1-butene-d,, as shown in Table 111 (24). This result indicates that a proton is abstracted from cis-2-butene by oxide ion, and the resulting I-methylallyl anion is bound to potassium ion, and that the proton on the basic oxide ion is readded to the 1methylally1 carbanion. The Z / E values of 1-alkylallylpotassium complexes in Table IV (25) indicate that the aikylallyl anion prefers the 2-conformation in general if the alkyl substituent is either methyl or ethyl. Delocalization of n-electrons in allyl carbanion is influenced in general by the countercations, and less electron-withdrawing countercations prefer the Z conformation of alkylallyl carbanion, as Schlosser and Hartmann (26) showed. On the other hand, the larger the resonance stabilization of allyl carbanion the higher the rotational barrier about the carbon-carbon bond of TABLE IV Relative Ratio of Z/E of I-Alkylallyl Anions" C ,Cfi--yc ( Z )

C

R\ C k - h c ( E ) R

Me Et i-Pr t-Bu From ref. 25.

IO / O 86/14 65/35 0/1O

ACTIVE-SITE CONTROL OF CATALYSIS

125

the allyl anion. In comformity with the fact that the resonance energy of the allyl carbanion is 14.5 kcal/mol(27), while that of the allyl radical is 9.5 kcal/ mol(28), the 2 conformation of 1-methylally1anion formed on cationic sites might be retained in the isomerization reaction if cationic sites are less electron withdrawing. In fact, the rotational barrier of allyl alkali metal complexes increases in the order: 10.7 kcal/mol for (allyl)-Li+, 16.7 kcal/mol for (allyl)-K+, and 18.0 kcal/mol for (allyl)-Cs+ in tetrahydrofuran (THF) solution. Accordingly, the rotation of alkylallyl carbanion from 2 to E conformation might be a rather slow process on basic catalysts, In this case, the ratio of Z / E conformation of the alkylallyl carbanion may be reflected in the ratio of cis/trans in the products, as described in Eq. 12.

R-CH

-CH=CH2

rotation

The selective isomerization of 1-butene to cis-2-butene in Fig. 17 and its reverse process, cis-2-butene to 1-butene (Table III), is caused by the kinetic facility of the formation of (Z)-methylaIlyl carbanion on a potassiumcontaining carbon (rz % r,; reaction rates) in the presence of 0,, and the slow internal rotation of (Z)-methylallyl carbanion to E conformation on this catalyst. Furthermore, it is evident that the proton removed from olefin by an oxide ion does not migrate to the other oxide ions, but undergoes intramolecular hydrogen transfer as shown in Table 111. On the other hand, if the removed hydrogen ion migrates from one oxide ion to the other ions, intermolecular hydrogen exchange of olefins may occur during the isomerization reaction. Accordingly if a dissociative mechanism of 1-butene involves both intramolecular and intermolecular hydrogen transfer, 2-butene-do, 2-butene-1-d,, and 1-butene-34, are formed from 1-butene-do as described in Scheme 9. The formation of 1-butene-34, is a characteristic product of the dissociative mechanism, because the associative mechanism through n- and sec-butyls will provide 2-butene-1-d,, 2-butene-do, l-butene1-d,, and l-butene-2-d1, but no 1-butene-3-d,. With respect to the function of active sites, the isomerization and hydrogenation of olefins via a-alkyl intermediates have different prerequisites for

126

KEN-ICHI TANAKA

D1 ssociot i v e Mechanism

CH2D-CH=CH-CHj CHrCH-CHz-CH3

+

+

(M-X)

X-H or X-D CHTCH-CHZ-CH~

Assoclot I v e Mechonl sin

-.M

CH~ICH-CHZ-CH~

: +

(M-H)

or (M-D)

\\I

j!-CHD-CHz-CH3

CHTCD-CH

2-

CH

3

n... SCHEME9. Deuterium distribution in n-butenes formed by dissociative and associative mechanisms.

active sites as described in Scheme 7. Taking account of this fact, the prerequisites for active sites of the isomerization of olefins through alkylallyl intermediates and that of the hydrogenation of conjugated dienes via alkylallylic intermediates is an interesting subject. The formation of q3- and o-ally1 species requires definite numbers of coordinative vacancies on metal ions, and the formation of q3-allyl species might require at least two degrees of coordinative vacancies as described in Eq. (13), where a hydrogen atom is removed by oxide ion. In fact, it has been shown that the conversion of (a-allyl)Co(CN): - to (q3-allyl)Co(CN):- complexes is brought about by removing a CN- ligand (29). It is worthy of note that the double bond migration via a-ally1 species might require the a-n interconversion on the sites. If this is the case, two coordination vacancies are required on a metal ion, as described in Eq. (13). Accordingly, active sites for the isomerization of

ACTIVE-SITE CONTROL OF CATALYSIS

127

SCHEME10. Hydrogenation of olefin caused by heterolytic dissociation of hydrogen.

olefins via the dissociative mechanism may have a total of three available sites-one on oxide ion and the other two on metal ion as is expressed by Eq. (13). As described in Scheme 7, the hydrogenation reaction by a stepwise addition mechanism requires in general three available sites to promote the hydrogenation cycle. Taking into account these facts, the active sites for the isomerization reaction of olefins by the dissociative mechanism seem to fulfill the prerequisite for the hydrogenation reaction of conjugated dienes if hydrogen from heterolytic dissociation can participate in the reaction as described in Scheme 10. This supposition has not been confirmed in general, but the hydrogenation of ethylene and a dissociative adsorption of propene occur on the same sites of ZnO (30).

The isomerization of olefins via a-alkyl species is a structure-requirement reaction, while the isomerization via alkyl cation is a structure-nonrequirement reaction. By the same reasoning, the isomerization via allyl carbanion or allyl carbonium ion is expected to be a structure-nonrequirement reaction. In fact, the formation of alkylallyl carbanions on basic surfaces seems to

128

KEN-ICHI TANAKA

require no specific structures for active sites. It is worthy of note that such catalysts as potassium-containing carbon (Fig. 17) (22,23),CdO (31), MgO (32), alkali metal hydrides (33), and lithium aluminum hydride ( 3 4 , which are active for the formation of allyl carbonions, are not active for the hydrogenation of olefins but are active for the hydrogenation of conjugated dienes. These findings reveal that the hydrogenation via allyl carbanion on basic catalyst might be a structure-nonrequirement type hydrogenation reaction, as will be discussed in Section 111.

111.

Hydrogenation Exchange Reaction

With respect to the function of active sites, the isomerization of olefins via a-alkyl species and the hydrogenation of olefins through a-alkyl intermediates require different functions for active sites as described in Scheme 7. To understand specific roles and functions of active sites for a given reaction, we should identify first the intermediates of each reaction. Tamaru and coworkers (35) proposed a promising method by which we can estimate the intermediates of the hydrogen-exchange reaction of propene. Propene is a convenient olefin for diagnosing the interaction of olefin with the surface, and the hydrogen-exchange reaction of propene can be precisely followed by microwave spectroscopic analysis. Hydrogen exchange through different intermediates gives different patterns of deuterium distribution in propene, which enables us to identify the following six inter), sec-propyl (CH 3-C*H-CH ), mediates : n-propyl (CH 3-CH2-C*H n-propenyl (CH3-CH=C*H), sec-propenyl (CH,-C*=CH,), o-ally1 (CH,=C-C*H,), and n-ally1 (CH,-CH-CH,). Among these intermediates, n-propenyl, sec-propenyl, and a-ally1 species can be ruled out as intermediates of the isomerization reaction of olefins, but these three species should be considered as intermediates for the hydrogen-exchange reaction. If we compare the isomerization to the hydrogen exchange, the dynamic motions of intermediates are slightly different in these two reactions. As has been pointed out, the hydrogen exchange as well as the isomerization of olefins taking place on a settled active site should accompany the rotation and/or the migration of intermediates. By focusing on the dynamic properties of the intermediates, we can deduce more precisely the roles and functions of active sites. For this purpose, a series of hydrogen-exchange reactions of propene and 1-butene have been performed on MoS, powder (36) and on MoS, single-crystal catalysts (14,37). As was mentioned in the preceding section, hydrogen exchange between D, and propene proceeds slowly on the edge surface of MoS,, although the intermolecular hydrogen

,

129

ACTIVE-SITE CONTROL OF CATALYSIS

01 0

1

GO

8.0

12.0

16.0

I

20.0

FIG.18. Geometrical isomers of propene-d, formed by hydrogen exchange of propene and D, on MoS, at room temperature (36). D H /CH3 /CH3 \ \ ,C=C (Z-I-d,), (E-Id,), CH,=CD-CH, (2-dI) H ‘H CH,=CH-CH,D

=‘’

D (34,)

exchange of propene occurs rapidly on the edge surface. Then, the D atom in the propene-d, formed by the slow exchange with D, is reshuffled many times through rapid intermolecular hydrogen exchange and the original position of the D atom is entirely erased. In fact, a deuterium atom in the propene-d, is almost equally distributed throughout the vinyl group (CH,=CH-), as shown in Fig. 18. It should be noted that the formation of propene-3-d, is unexpectedly slow compared with the formation of propene-1-d, and propene-24, in random distribution. If propene- 1d, and propene-34, are formed via sec-propyl intermediates, the ratio 1-d,/3-d1 is expected to be 2/3 provided that the two methyl groups in a sec-propyl are equivalent on the active sites, as described in Eq. (14). This is not the case, but the results in Fig.

CH3-CH-CH,D

CH,=CH-CH,D (propene-3-d, )

I Mo--CH3-CH=CHD (propene-l -d,)

18 definitely indicate unequal hydrogen elimination from the two methyls. In order to understand the mechanism of this unexpectedly slow formation of

130 :

o

KEN-ICHI TANAKA

o

l

FIG.19. Geometrical isomers or propene-d, formed in the hydrogen exchange between C,H, and C3D, on MoS, at room temperature (6). 0 1976 American Chemical Society.

propene-3-d, on MoS, catalyst, the initial position of the exchanged D atom should be established. It was found that the propene-d, formed in an intermolecular hydrogen exchange between propene-do and propene-d, involves little hydrogen reshuffling. In fact the propene-d, formed in the initial stage of the intermolecular hydrogen-exchange reaction is composed of 70 propene-2-d1, 30 % propene-1-d,, and a very small amount of propene3-d,, as shown in Fig. 19, which is in remarkable contrast to the random distribution of D atoms throughout the vinyl group indicated in Fig. 18. Provided that the contribution of the dissociative mechanism is negligible, propene-2-d, is formed through n-propyl species, and (Z)-propene-1 -d and (E)-propene-1-d, are formed through sec-propyl species as shown in Eq. (15). From a constant ratio of propene-1-d, and propene-2-d, in the initial stage of

CH,=CH-CH,

+ D-Mo

' 24 ",/

CH,--CH-CH,D

I

Mo--1

: rotation

-

CH,--CH=CHD

(Idl)

131

ACTIVE-SITE CONTROL OF CATALYSIS

TABLE V GeometricalIsomers of Propene-d and I-Butene-dI Formed by Intermolecular Hydrogen Exchange Reactions on MoS, Powder and on Single Crystal Catalysts" Z-I-d,

E-l-d,

2-d,

3-d1

Powderb Propene I-butene

12.3 15.9

15.6 16.0

70.2 68.1

1.9 0

Single crystal (1WC)C Propene 1 butened

14.2

14.5

63.7 71.3

8.9 0

27.4

4-d1

~

0 -

0

Ref. 6 .

* Room temperature. Propene-h, formed by a hydrogen exchange between propene-d, and I -butene-d,. ddl-l-buteneat 20 min in Fig. 21.

the hydrogen exchange reaction in Fig. 19, the relative contribution of npropyl and sec-propyl species can be estimated to be 76% and 24%, respectively. An interesting fact is that the same ratio was obtained in the hydrogenexchange reaction between 1-butene-do and 1-butene-d, on MoS, powder, i.e., l-butene-l-dl/l-butene-2-d, = 3/7, as shown in Table V. This fact reveals that the amount of n-alkyl species relative to sec-alkyl species is not appreciably influenced by the size of the alkyl species. A more surprising fact is that the hydrogen exchange between 1-butene-doand 1-butene-d, on MoS, single-crystal (cut) catalyst at 100°C gave the same ratio of 1-d, to 2-d, as listed in Table V. If both n-alkyl and sec-alkyl species were formed from a-olefins on the same ,MH-site, the ratio of n-alkyl to sec-alkyl species is expected to change with temperature and with the size of the alkyl species, but this is not the case on MoS,, as shown in Table V. The z-electron density in a-olefins is higher on the a-carbon than on the B-carbon (38),therefore nucleophilic sites may prefer the formation of sec-alkyl species. The results in Table V giving a constant ratio of n-alkyl to sec-alkyl species perhaps suggests the existence of two types of ,MH-site with different nucleophilicity, one for sec-alkyls and the other for n-alkyls, and a constant ratio of these sites on the edge surface of MoS, perhaps results in giving a constant value of n-alkyllsec-alkyl. If propene-1-d, is formed via sec-propyl species on Mo-H sites, slow formation of propene-34, in Figs. 18 and 19 may be responsible for a restricted rotation of sec-propyl species around the Mo-C bond as described in Eq. ( 1 5).

132

KEN-ICHI TANAKA

In order to establish this supposition of a restricted rotation around the coordination bond, it should be assured that the contribution of the dissociation mechanism is negligibly small in the hydrogen-exchange reaction. For this purpose, a 1 : 1 mixture of (Z)-propene-1-d, (composition: do, 3.6%; 2-1-dl, 86.1 %; E-l-d,,7.5 %; 2-1,2-d,, 1.4%; E-l,2-d2,0%; and 1, 1-d,, 1.4 %) and propene-d, (purity, propene-d,, 96 % and propene-d,, 4 %) was admitted to the MoS, powder catalyst at room temperature, and HD gas (98 %) was added as a cocatalyst. In this experiment, the amounts of Mo-H and Mo-D sites are approximately equal during reaction. The primarily exchanged products formed by associative and dissociative mechanisms are described respectively in Scheme 11. (I) Associative Mechanism. (I[) Dissociative Mechanism,

+ do -D

I

SCHEME1 I . Geometrical isomers formed by the hydrogen exchange of (Z)-propene-I-d,.

If (Z)-propene-1-dl undergoes hydrogen exchange through the dissociative mechanism, (2)-propene-1, 2-d2 should be formed from 2propenyl (CHD=C*-CH,) and propene- 1, 1-d, from 1-propenyl (C*D=CH-CH,). This is not the case as shown in Fig. 20. Initial compositions of the propene-d, isomers and that of the propene-d, isomers were evaluated by graphical extrapolation to zero time as summarized in Table VI. The formation of (2)-propene-1, 2-d, (Table VI) was attributable to (E)-propene-I-d, (7.5 %) contained in the starting (Z)-propene-1-d,. By

133

ACTIVE-SITE CONTROL OF CATALYSIS

dl -propane

1001

dg

'

- propane

0

°

7

c

0

=so-

- 0

0

s

10

IS

20

Time ( m i d

FIG.20. Intermolecular hydrogen exchange between (2)-propene-I-d, and propene-d, in the presence of HD on MoS, at room temperature (36).

making corrections for the minor compounds in the starting (Z)-propene- 1-d,, the propene-d, isomers from (Z)-propene- 1-d, were evaluated to be 81.6% of (E)-propene-1, 2-d, and 18.4% of propene-1, 1-d,. If (Z)-propene- 1- d , undergoes hydrogen exchange via either n-propyl or sec-prop91 species, the relative contribution of n-propyl and sec-propyl species is reasonably expected to be 76 % and 24 % as described in Eq. (15). Based on this relative contribution of n- and sec-propyl species the isomers formed from (Z)-propene- 1-d, were calculated to be 82.4 % (E)-propene- 1, 2-d2 and 17.6 % propene-1, 1-d,, in excellent agreement with the experimental values of 81.6% (E)-propene-1,2-d2 and 18.4% propene-1, 1d2.The yields of all isomers obtained in this experiment are in good agreement with the calculated values as listed in Table VI, and the participation of I-propenyl-1-d,(CH,-C*H=CHD)and that of the 2-propenyl-1-d, TABLE VI Geometrical isomers Formed by the Hydrogen Exchange of (Z)-Propene-1-d, and Propene-d, on MoS,, and Calculated ValuePb Isomer

Experimental (%) Calculated (%)

do

E-I-dl

2-d,

E-I, 2-d,

Z-I, 2-d2

3.0 7.5

51.0 50.0

0 0

34.0 35.0

4.5 0

At room temperature, based on an n-alkyl: sec-alkyl ratio of 76%: 24%. From ref. 36.

I, I-d, 7.5 7.5

134

KEN-ICHI TANAKA

-

C H D ~ C H D - C H ~ H, Mo--I

/‘CD

CH~

(E-12-dt )

n-Propyl

‘;C-C< D I

cy3

D

/c=c(

H

H

Mo-D

‘>C=C< D

+

CH3

(E-1-dl )

H

cH3

(1,l-dz )

H

0 H,C= ’ CHCH3 ‘H ( E-1-d1 )

sec-Propyl

Slow Rotation

r

CHDt

,CH3

P ,J Mo---

CDHz ,CHI ‘CH

cc,

MO---

4CHDrCH-CHz

-

(3-dz)

CHzPCH=CHz ( 3 - d l )

SCIiEME 12. Rotational motions required for the hydrogen exchange of (Z)-propene-1-d, by the associative mechanism.

(CH,-C*=CHD) in the hydrogen-exchange reaction of propene on MoS, catalyst are confidently ruled out. From these facts, we can conclude confidently that the hydrogen-exchange reaction of olefins on MoS, catalyst proceeds entirely through the associative mechanism as described in Scheme 12, and the slow formation of propene-3-d,, which corresponds to slow double bond migration in the isomerization reaction, is caused by a restricted rotation of sec-propyl species around the Mo-C bond. This fact indicates that internal and/or external rotation of intermediates on active sites is undoubtedly one of the indispensable elementary processes for some catalytic reactions. I t may be reasonable to suspect that the covalent adsorption bond might be more resistive to stretching of the bond length than the ionic adsorption bond. As a result, covalently bound adsorbed species may experience more steric hindrance in rotational motion and it may be a case of restricted rotation of bulky groups in sec-alkyl species bound on the edge surface of MoS,. A similar phenomenon is observed in some organic molecules with bulky substituent groups, in which rotational barriers are sometimes greater than 20 kcal/mol for rotation around a single bond (38).

ACTIVE-SITE CONTROL OF CATALYSIS

0

20

40

60

80

135

I00

time (min)

FIG.21. Hydrogen exchange between I-butene-do and I-butene-d, and their isomerization on cut single crystal catalyst of MoS, at 1oO"C, where "mw" indicates sampling for microwave spectroscopic analysis (14.37).

The interaction of adsorbed species with active sites will be made more clear if such well-established reactions as described above are performed on a single-crystal catalyst of MoS,. For this purpose, a coisomerization of I-butene-do and 1-butene-d, was performed on a cut catalyst of MoS, single crystal in the presence of D, at 100°C.As shown in Fig. 21, the intermolecular hydrogen exchange reaction and the isomerization reaction occur simultaneously. However, the isomerization reaction has an apparent induction time while the hydrogen exchange reaction occurs without acy induction time. In Section 11, it was supposed that such characteristic induction time for the isomerization reaction on the single-crystal catalyst might correspond to reconstruction of 'M-sites, which is induced by the coordination of alkyl species. That is, during the induction time the surface is active for the hydrogen-exchange reaction of 1-butene but is inactive for the reactions accompanying the rotation of sec-butyl species. In order to compare the hydrogen-exchange reaction during the induction time with that after the induction time, the 1-butene-d, formed in the induction period was subjected to microwave spectroscopic analysis. As listed in Table V, the I-butene-d, formed in the induction period on single-crystal catalyst (at 20 min in Fig. 21) was exactly the same as the 1-butene-d, isomers formed on MoS, powder catalyst. Furthermore the ratio of n-alkyl to sec-alkyl species formed on the surface did not change in and after the induction time. This fact seems to support the speculation that there are two types of 'MH-sites on the edge surface of MoS,, and that the preference of active sites for either n-alkyl or sec-alkyl species is not affected by the reconstruction of active sites. From these facts, it may be concluded that only the dynamic behavior of the intermediates differs on these two surfaces during and after the induction time. The reconstruction of 'M-sites induced by the coordination of alkyl

136

KEN-ICHI TANAKA

species may lower the barrier for the internal rotation of the ethyl group in sec-butyl, but the rotation of sec-alkyl species around the coordination bond of Mo-C is still restricted from free rotation as is evident from the slow formation of propene-3-d, and the slow double bond migration of n-butenes. It is noted that the internal rotation of n-alkyl species and of methyl groups are less restricted even during the induction period. As a result, the hydrogen exchange reaction between C2H4 and C2D4 occurs with no induction time on the edge surface of MoS,, as shown in Fig. 12. The cis to trans isomerization reaction, which requires the rotation of an ethyl group, has a clear induction time, as shown in Fig. 10a. If a mixture of C2H4, C2D4 and cis-2-butene-do is added to the cut catalyst of MoS, single crystal, the hydrogen exchange between C2H4 and C,D4 occur selectively during the induction time of the isomerization reaction, as shown in Fig. 22. After the induction time, the isomerization reaction and the additional hydrogen exchange between cis-Zbutene and C2D4 are concomitantly enhanced. Such a segregated rotation of the double bonds in ethylene and 2butene on the edge surface of MoS, might be caused by different feasibility of rotation of the methyl and ethyl groups as described in Scheme 13. From these results, it may be concluded that the reactions that proceed through covalently bound intermediates are controlled precisely by the structures of active sites, but the reactions that occur via ionic intermediates are less affected by the structures of active sites. As a result, in some reactions, some specific mode of the dynamic motion of intermediates will be an important factor for selective catalysis.

120,

20

0 0

30

90 120 time (min)

60

150 180

FIG.22. Selective hydrogen mixing between C2H4 and C2D, during induction time for the isomerization of cis-2-butene-do, and superimposed hydrogen scrambling between C, D, and cis-2-butene-do after the induction time on cut single crystal catalyst of MoS, at 100°C (8, 14).

137

ACTIVE-SITE CONTROL OF CATALYSIS

P H D Mo ...

H

H‘ “C=c<

+

Mo-H

H

SCHEME 13. Rotational motion required for the hydrogen exchange of ethylene and for the isomerization of cis-2-butene.

IV. Hydrogenation Reaction

So far the intermediates of the hydrogenation of olefins have been estimated by a hydrogen exchanged between D, and olefins during the hydrogenation reaction, where the steps of the alkyl formation in Scheme 14 are assumed to be reversible and to be the main route of the hydrogenexchange reaction. A historical reaction of olefins with D, on nickel catalyst

138

KEN-ICHI TANAKA R

H

\ i. H/c=c\

D

\

R\ ,H H-C-C;H *

H,c=c’ R, H k R,

H

D

1-

Al kane-d,

H SCHEME 14. Deuterium distribution expected from the Horiuti-Polanyi mechanism.

is characterized by the prominent formation of nondeutero alkanes in the early stage of the reaction at room temperature (39), and the hydrogenation of ethylene with either an equilibrated or unequilibrated isotope mixture of H, and D, produces ethane with the same isotope distribution (40). These phenomena have been explained by rapid hydrogen exchange between adsorbed D atoms and olefins in Scheme 14. If this reaction scheme were true, the hydrogenation reaction of a-olefins through sec-alkyl species would accompany the hydrogen exchange on the terminal carbon atom of the olefin but no hydrogen exchange would occur on the second carbon atom in the olefin. However, if the hydrogen reshuffling occurs through a route independent of the hydrogenation reaction, this method leads us to the wrong conclusion about the intermediates of the hydrogenation reaction. In fact, it has been proved that the hydrogen exchange of olefins on such catalysts as transition metal oxides (10,41,42) and on sulfided nickel (43) proceeds via alkyl intermediates independent of the hydrogenation reaction. Furthermore, the hydrogen-exchange reaction of a-olefins on the edge surface of MoS, proceeds preferably via n-alkyl species (76 %), as discussed in Section 111, although the hydrogenation reaction proceeds substantially through sec-alkyl species (44), as will be discussed. The first clear example proving independent hydrogen exchange of olefin during the hydrogenation reaction was observed in the hydrogenation of ethylene on Co, 0, (42). The reaction of C2H4 with a mixture of H, and D, on c0304 yielded mainly ethane-do and ethane-d,, while the hydrogenation of a mixture of C,H4 and C2D4 was found to undergo rapid isotope mixing of ethylene during the hydrogenation reaction. These phenomena are very hard to explain by a traditional mechanism such as described in Scheme 14. Siege1 (10) succeeded in explaining these curious phenomena by proposing two kinds of sites with different catalytic abilities, one active for the hydrogenation reaction and the other merely effective for the hydrogen-exchange reaction via alkyl species.

139

ACTIVE-SITE CONTROL OF CATALYSIS

He postulated further that such different catalytic abilities are connected with different degrees of coordinative unsaturation of active sites that behave in a manner similar to catalytically active mononuclear complexes. If this speculation is acceptable, a traditional procedure estimating intermediates of the hydrogenation reaction from data on the hydrogen exchange of olefins loses scientific justification. Direct detection of the intermediates with spectroscopic methods is undoubtedly the best way in general, but this is difficult for the hydrogenation of olefins on heterogenous catalysts at the present time. As a result, we necessarily refer to the kinetic method to estimate the intermediates. As has been pointed out above, the trace for the real intermediates of the hydrogenation reaction of olefins would be retained in hydrogenated products but not by olefins. A novel method using ideas along this line has been proposed (16). This method is limited only to a hydrogenation reaction which maintains the molecular identity of hydrogen in the products; the reaction of olefin with a mixture of H, and D, yields alkane-do and alkane-d,, and the reaction with HD gives alkane-d,, selectively. So far, the reaction of olefins with D, on such catalysts as Cr,O, (45), Co,0,,(42), ZnO (45,47), MgO (32), CdO (31), and MoS, (48) has been proved to yield the d, adducts predominantly. If such selective alkane-d, formation is caused by a stepwise mechanism through either n-alkyl or sec-alkyl species as described in Scheme 14, the first step of forming n-alkyl-d, or sec-alkyl-d, should be far from equilibrium because the reverse process causing the hydrogen exchange between olefin and D, is negligible. On the other hand, the hydrogen molecule dissociates on active sites in either a homolytic or heterolytic way, and the reactivity or selectivity of the dissociated hydrogen atom with olefins is strongly influenced by whether the dissociation mode is homolytic or heterloytic. In conformity with the dissociation modes of hydrogen, the dissociation of H,, D,, and H D molecules are described in general as follows: H, D, HD

-

H(I) + H(I1) D(1)

+ D(I1)

> H(1) + D(I1)

2 \ .D(I) + H(I1) In homolytic dissociation, the adsorbed hydrogen in states (I) and (11) is identical and fi = 1. In contrast to this, the adsorbed hydrogen in states (I) and (11) differs in heterolytic dissociation and may lead to 1. Eischens and co-workers (49) proved an example of a heterolytic dissociation of H, or D, on ZnO giving (fn 2) orn:( E), and Kokes and co-workers

+

I40

KEN-ICHI TANAKA

(50) found that the adsorption of the HD molecule on ZnO brings about a

(Fn

substantial orientation in a form of g) at room temperature but the reverse orientation at -40°C; that is, the p in Eqs. (16c) and (16d) is undoubtedly far from unity in the adsorption of HD on ZnO. If the dissociation of H2, D2, and HD is described by Eq. (16), the reaction of dissociated hydrogen atoms with a-olefin is described in general as follows. If the hydrogen molecular identity is maintained in the hydrogenation reaction, that is, no isotopic mixing occurs, the reaction with H2 or D 2 gives only do or d 2 adducts in either reaction route. 1. n-Alkyl intermediate:

H’

‘H(I1)

(c) HD: H\ H

/

R

C-C-H

/*

H\ A-C?

’(‘I)

\

*

alkane-1-d,

(17e)

+ alkane-2-d,

(17f)

H(U

/

R H(l)

ACTIVE-SITE CONTROL OF CATALYSIS

R / C-C-H H/* \ D(I) H

\

H\

/

R

C-C-H

\

H/*

H(ll) +

141 alkane-2-d,

( 1 7g)

a alkane-I-d,

HW)

2. sec-alkyl intermediate:

(18a)

H / C-C-H H/* \ D(IU

R\

H(l)

+ alkane-I-d,

142

KEN-ICHI TANAKA

R

H / /*C - C r H

\

H(l’’

3

R\ C-C-H/H H

/*

\

+

alkane-1-d,

(18g)

alkane-2-d,

(18h)

HW)

In a series of reactions, the addition of HD gives either 1-d, or 2 4 , isomers, and the ratio of these two isomers is strongly affected by whether the intermediate is an n-alkyl or a sec-alkyl species. In the adsorption of H, or D,, the following relations may be established for the amount of each state of adsorption: [H(I)]

=

[H(II)]

and

[D(I)]

=

[D(II)],

(19)

and the adsorption of HD in accordance with Eqs. (16c) and (16d) will give the following relations:

The orientation in the addition of HD to a-olefin is caused by the isotope effect of the reaction with H and D so that we could estimate the intermediates of the hydrogenation reaction by comparing the isotope effect in the addition of H, and D2 and the orientation in the HD addition (16). If the rate-determining step is the formation as sec-alkyl species in Eq. (18), the ratio of alkane-d,/alkane-d, in the reaction with H, and D, is given by the following equation:

where l/a = kH(l)/kH(II)= kD(l)/kD(ii)and the relations in Eq. (20) are assumed as a satisfactory approximation. The value a is the relative reactivity of H(I) and H(I1) or D(I) and D(I1) with olefin, and a case of heterolytic dissociation into Ddfand D6- is schematically shown in Eq. (22). The species Ddt and Dd- correspond to D(I) and D(II) in Eqs. (18c) and (18d), respectively.

ACTIVE-SITE CONTROL

143

R-$-C,

R-2-C

\

D

In the reaction of a-olefin with HD, the ratio alkane-2-dl/alkane-l-d, is represented by the following equation:

where the value of a represents the reactivity of H(I) relative to H(I1) or D(1) relative to D(I1) in the reaction with a-olefin as described above, and 8 indicates the orientation in the dissociative adsorption of the HD molecule as described in Eqs. (16c) and (16d) and Eq. (20). From Eqs. (21) and (23), we can derive an equation which expresses the orientation in the HD addition via sec-alkyl intermediates as follows: alkane-24, [alkane-1-d,]

(1 = (a

+ afl)kH(,, + B)kD(,,

In the same way, if the hydrogenation reaction of a-olefins proceeds according to Eq. (1 7) via n-alkyl intermediates, the following relations are derived: alkane-2-d1 - (a + B)k&,, alkane-do ~ _ _-~ - k&,, _ and (25) alkane-d, alkane-1-d, (1 + a/?)kh,,, . Accordingly, we can derive a relation which may be established in the hydrogenation reaction via n-alkyl species as follows:

If the hydrogen molecule undergoes homolytic dissociation, a = 1 and

fl = 1 are exactly established. Accordingly, the relations of Eq. (24) for sec-alkyl intermediates and of Eq. (26) for n-alkyl intermediates are reduced to the following simple forms: sec-alkyl: n-alkyl:

alkane-2-d1 - alkane-do alkane-1-d, alkane-d, ’ alkane-2-d1 - alkane-d, alkane-1-d, alkane-do ’

It is worthy of note that Eqs. (27) and (28) are inverses of each other.

(27)

144

KEN-ICHI TANAKA

Taking into account that the isotope effect for the addition reaction, k,(,,/kDo, or k",,)/k',,,,, is larger than unity, the value of the ratio in Eq. (27) should be larger than unity but that in Eq. (28) should be smaller than unity. Accordingly, if the ratio 2-d,/l-d, takes a value larger than unity, the hydrogenation reaction may proceed through sec-alkyl intermediates. Contrary to this, if the ratio 2-d1/l-d, is smaller than unity, the intermediates may be n-alkyl species. Furthermore, if the ratio 2-dl/l-dl correlates with neither d,/d, nor d,/d,, the reaction might take place by the heterolytic dissociation of hydrogen, because (a + 8)/(1 + ap) in Eqs. (24) and (26) might not be unity. This method is also applicable to the 1,Zaddition of hydrogen to 1,3dienes. This method has been applied to the hydrogenation of a-olefins as well as that of 1,3-butadiene on catalysts such as MoS, (26,51,52,53), ZnO (16,54), and MoO,/TiO, (55). On these catalysts, the hydrogen molecular identity is maintained, and Fig. 23 shows a typical result of the reaction of

2 1

v

?

8 20

- ..

0

0

0

10

20

30

40

conversion (%)

10

50

20

60

;2

50 conversion (X)

30

40

0

10

1

20

30

40

50

60

conversion (%)

FIG.23. Changes in deuterium distribution during deuteration of butadiene on MoS, at room temperature (52): (a) butadiene, (b) I-butene, (c) hydrogen.

ACTIVE-SITE CONTROL OF CATALYSIS

145

FIG.24. Isotope effect for the 1.2-addition of H, and D, to butadiene and orientation in the (3 - dl)/ 1.2-addition of HD to butadiene on MoS, at room temperature. ( O ) , d,/d,; (0). (4 - di) (51).

1,3-butadiene with D, on MoS, powder catalyst at room temperature (52). On MoS, catalyst, the addition of D, to 1,3-butadiene undergoes exclusively l,2-addition, which results in yields of l-butene-3,4-d2 of nearly 100 % (Fig. 23b) in the initial stage of the reaction. However, the isotopic exchange between butadiene and D, proceeds concomitantly and butadiene-d, and H D are formed as shown in Figs. 23a and 23c, which results in lowering the content of 1-butene-d, as the reaction proceeds in Fig. 23b. As has been discussed already, the estimation of the intermediates of hydrogenation of 1,3-butadiene from the position of the D atom in butadiene-d, neccessarily involves some risk because it is difficult to prove that the hydrogen exchange between butadiene and D, occurs substantially by the process which is the reverse of the hydrogenation of butadiene. However, the relations in Eqs. (27) and (28) offer us a practical and reliable method for estimating the intermediates of the hydrogenation reaction. For this reason, the hydrogenation of butadiene was performed with an equilibrium mixture of H,, HD, and D, (26.8,46.7, and 26.5 % respectively), and the isotope effect in the addition of H, and D, and the orientation in the addition of H D were measured coincidently. Figure 24 shows the orientation in the addition of the HD molecule, i.e., l-butene-3-d1/ l-butene-4-dl, and the isotope effect in the addition of H, and D,, i.e., l-butene-doll-butene-d,.The ratio 3-d,/4-d1 in the H D addition obviously takes a value larger than unity, which agrees well with the isotope effect in the addition of H, and D, (52). These facts indicate that the hydrogenation of 1,3-butadiene proceeds via sec-butenyl intermediates [C=C-C*-C-HI, and that the dissociation of

146

KEN-ICHI TANAKA

hiin

c+c-c=c D2 ::mo

'jl

c=c-c=c ::Q0

4 ,\c+c-c=c

c-c-c-c

D,M~

D

,c- c- c-c I ... .Mo

c=c-c=c

x-c-c=c

(b)

H

.;:Ao

l-tut ene-d,

SCHEME 15. Hydrogenation of butadiene on (a) 'MH,-site and (b) 'MH-site.

hydrogen occurs in a homolytic manner. In other words, the hydrogenation of 1,3-butadiene proceeds on the dihydride sites via sec-butenyl intermediates as described in Scheme 15a. If the hydrogenation of butadiene proceeded on an isolated monohydride site, mainly 1-butene-d, would be formed, as described in Scheme 15b. However, this was not the case; the reaction with a mixture of H, and D, yielded mainly 1-butene-do and 1-butene-d, (48). It is note-worthy that both the hydrogenation reaction and the H,-D, equilibration reaction require three degrees of coordinative unsaturation for active sites, and that the H,-D, equilibration should involve a heterolytic dissociation to yield trihydride sites as described in Scheme 8. Taking into account all these facts, it is proposed that homolytic dissociation of hydrogen may prevail on 3M-sites on which one coordination site is occupied by olefin or diene, while heterolytic dissociation prevails on 'M-sites or 'MH,-sites. To confirm the validity of this method, it was applied to the hydrogenation of 1,3-butadiene on ZnO, where heterolytic dissociation of hydrogen has been established. It is known that the adsorption of the H D molecule on ZnO takes a definite orientation of either (fn g) or (,", E) depending on adsorption temperature (50), with the former conformation preferred at room temperature and the latter at -40°C. The orientation in the adsorption of H D at room temperature is explained by the thermodynamic facility, and the orientation at -40°C is referable to kinetic facility in the adsorption. In agreement with these facts, the hydrogenation of 1,3-butadiene with H,, D,, and HD was performed on ZnO at room temperature and at -40°C (16). Contrary to expectation, the reaction of butadiene with H D on ZnO resulted in giving exactly the same orientation at room temperature and at -4O"C, as shown in Fig. 25 (16). The isotope

ACTIVE-SITE CONTROL OF CATALYSIS

0

0

5

10

Converslon

15

147

20

(%I

FIG.25. Isotope effect for the 1,2 -addition of H, and D,, and the orientation in the addition of H D to 1.3-butadiene on ZnO at room temperature (R.T.) and -40°C (17).

effect in the addition of H, and D, is also plotted in Fig. 25. It is clear that the orientation in the HD addition does not agree with the isotope effect on ZnO. This fact suggests that the hydrogenation reaction involves heterolytic dissociation of hydrogen, and that hydrogen dissociation is not in equilibrium during reaction. Taking account of the fact that a n-allylic anion might be one of the most probable species on ZnO, the following equation was proposed, but the reason for the preferential formation of 1-butene-3-d, remains to be solved (16).

The validity of this new method was assured in the hydrogenation reaction of 1,3-butadiene on the two different types of catalysts, MoS, and ZnO, so that it was extended to the hydrogenation of a-olefins, propene and 1-butene, on MoS, and MoO,/TiO, catalysts. The ratios of alkane-24, to alkane-1-d, obtained in the reaction of propene and I-butene with HD molecule on MoS, catalyst at room temperature, and the isotope effect in the reaction with H, and D, are plotted in Fig. 26. In these experiments, the analysis of propane- 1-d and propane-2-d, was carried out by microwave spectroscopy

,

148

KEN-ICHI TANAKA

-

0

0

2

4

6

8

1

0

1!

Conversion (Hydrogenation) (x) FIG.26. Isotope effect for the 1,Zaddition of H, and D, to a-olefins and orientation in the addition of HD on MoS, catalyst at room temperature (53). 0 1977 American Chemical Society.

but that of butane-1-d, and butene-2-6, was performed by a mass spectroscopic analysis by combining the deuterium atom distribution in the parent ion and in the fragment ion formed by splitting off the methyl group. The ratio of the two geometrical isomers formed in the reaction with HD, 2-dl /l-dl, is undoubtedly larger than unity and agrees reasonably well with the isotope effect (53).In agreement with these facts, it was concluded that the hydrogenation of a-olefins proceeds substantially via sec-alkyl intermediates with homolytic dissociation of hydrogen. This method was also applicable to the hydrogenation of allene (CH,=C=CH,) and methylacetylene (CH,-C-CH) on MoS,. The reaction of methylacetylene with D, on MoS, yielded (E)-propene-1,2-d2 in about 90 % selectivity and the reaction of allene with D, gave propene-2,3-d2 in about 87 % selectivity (56). These results indicate cis-stereospecific hydrogen addition on the dihydride sites (,MH,-site) and the slow isomerization of allene to methylacetylene or vice versa during the hydrogenation reaction. If the hydrogenations of allene and methylacetylene proceed through the half-hydrogenated species, the hydrogenation of methylacetylene occurs via either 1-propenyl and/or 2-propenyl species as described in Scheme 16, and that of allene occurs via either 6-or nally1 or 2-propenyl species. In conformity with slow hydrogen exchange between allene or methylacetylene and D,, the orientation in the HD addition may reflect the intermediates. It is expected that the reaction of methylacetylene with H D via 2-propenyl species may prevail in the formation of propene-24, while the reaction via 1-propenyl prevails in the formation of propene-1-d,. In the same manner, the hydrogenation of allene via 2propenyl may prevail in the formation of propene-24, but the hydrogenation via u- or n-ally1 intermediates may prevail in the formation of

149

ACTIVE-SITE CONTROL OF CATALYSIS

CH

Xc 4 :

H

0 or ti Ckl-C43i

I

I- Mo',

H

./ \CH

3;c

HorD

-

=c/ H

0H D Propene-l-di , 2 4 1

~ropene-2-dl ,

141

;Ao 0D

H 'HZ

*C J H Z D

Or

~ r o p e n e - a d l , 2-d1

-H

SCHEME 16. Intermediates of the hydrogenation reactions of methylacetylene and allene.

propene-34,. The results are summarized in Table VII, where the values of 1-dl/2-d,for the hydrogenation of methylacetylene and 3-d&d, for the hydrogenation of allene agree well with the isotope effect for the addition of H, and D,. Accordingly, the value of propene-1-dJpropene-2-d,1: 1.33 suggests 1-propenyl (CH3-CH=C*H) intermediates for the hydrogenation N 1.16 for the of methylactylene, and the ratio of propene-3-dl/propene-2-d, hydrogenation of allene indicates o- or n-ally1 intermediates. It is a notable result that the active sites for the hydrogenation reaction (3MH,-site) on MoS, prefer the formation of sec-alkyl (RCH,-C*H-CH3) and o-allylic (CH,=CH-C*H-CH, and TABLE VII Orientation in the H D Addition to Methyl Acetylene and Allene, and Isotope, Effects in the Addition o f H , and D, at Room Temperature on MoS, Propene-d, Reactants C-C=C

C=C=C

+ HD + HD

' From ref. 56.

2-d,

E-1-d,

Isotope effect

Orientation

2-1-dl

dold,

1-dl/2-d,

1.28

1.33

42.9

50.2

6.9

3-d,

2-d,

I-d,

52.4

45.0

2.6

3-d,/2-d,

1.16

1.16

150

KEN-ICHI TANAKA

CH2=CH-C*H2) species while the active sites with two degrees of coordinative unsaturation ('MH-site) prefer the n-alkyl species (RCH2-*CH,) to sec-alkyl species. If the sec-butenyl species formed on the 3MH,-sites undergo 0-R interconversion as described in Eq. (30) and the n-allylic species formed can rotate around the Z-axis, 1,Caddition might D CHz=CH-CH=CHz+

8 MO

\ I

D/

-

CH,=CH

CHZD C ' H'

I

___*

I-butene-3,4-d2

,'

/

2-butene-l,4-d2

u-n conversion

7

(30) occur as described in Eq. (30). This is not the case on the MoS, catalyst, and the subsequent hydrogenation of the sec-butenyl species yields solely 1butene in nearly 100 % yield. In agreement with these facts, slow isomerization of allene to methylacetylene on MoS, during the hydrogenation reaction may also be related to slow u-R interconversion on an MoH-site as described in Eq. (31). I

CH3-CECH

,D

+ MO\

D

-

CH=CD-CH3

I

,' M 0'

\D

Mo\

H

-

CHD=CD-CH3

151

ACTIVE-SITE CONTROL OF CATALYSIS

Another unique property of 3MH,-sites is that the process of alkyl species formation on 3MH,-sites is irreversible, which results in maintaining hydrogen molecular identity in the products. This is in remarkable contrast to the rapid and reversible process of alkyl formation on ,MH, which brings about the isomerization and/or hydrogen exchange reaction. The reason why 1,3hydrogen transfer does not occur on 3MH-sites during the hydrogenation of methylacetylene may be related to the inability of 3MH-sites for C-H breaking as described in Eq. (31). As discussed in Section 111, the isomerization reaction via ionic species such as alkyl cation or a-ally1 carbanion does not require specific structures for the active sites and is controlled by the proton activity or affinity of the surface. If this is a common feature of the reactions via ionic intermediates, the hydrogenation reaction via ionic intermediates might occur on a surface even through the surface does not fulfill the structural prerequisites for the hydrogenation reaction. The hydrogenation of 1,3-butadiene on CdO may correspond to this situation. In this case, 1-butene and cis-2-butene were formed mainly in the initial stage of the reaction, and the reaction with D, gave 1-butene-1,2-d2 and cis-2-butene-1,4-d2 predominantly (31); that is, both the 1,2- and 1,Caddition maintained deuterium molecular identity. It is noted that CdO is active for the hydrogenation of 1,3-butadiene but is not active for the hydrogenation of olefins or for the H,-D, equilibration reaction. If the active sites on CdO fulfill the structural requirements for the hydrogenation reaction the sites may catalyze H, dissociation and olefin hydrogenation would proceed. This is not the case. Another interesting feature of CdO is that the isomerization of 1-butene to cis-2-butene proceeds in the presence of butadiene, and that cis-2-butene is selectively isomerized to 1-butene; that is, the double bond migration prevails over the cis-trans isomerization reaction. These phenomena suggest that the hydrogenation of 1,3-butadiene and the isomerization of n-butenes proceed through a common 3-methyl-n-ally1 carbanion intermediate as described in Eq. (32). This is in remarkable contrast to the structure-requirement type hydrogenation and isomerization reactions.

C=C-C=C+D,

-

D-C

/c=c \

C-D

(32)

c=c-c-c / D

\

D

152

KEN-ICHI TANAKA

The n-electrons in 3-methylallyl carbanion are distributed with more electron density on the a carbon atom than on the y carbon, but the formation of 1-butenefrom cis-2-butene indicates the kinetic feasibility of the attack of a proton on the y carbon of 3-methylallyl carbanion. Accordingly, the hypothesis that the hydrogenation of butadiene through a n-allylic carbanion would prefer a priori the 1,Caddition has no scientific basis (57). So far, a hydrogenation reaction occurring through cationic intermediates has never been proved for heterogeneous catalysis. However, a typical structure-nonrequirement type hydrogenation reaction via alkyl cations was found in super acid media (58), and hydrogen isotope exchange reactions also occur via H i intermediates, as described in Eqs. (33) and (34). These reactions that occur via cationic species are influenced by the proton activity in solution and the proton affinity of the reactant molecules. In fact the reaction of sec-carbonium ion with H, is 105-10'0times faster than that of tertiary carbonium ions (59). In a similar way, the hydrogenation as well as the isomerization reaction occuring via an alkylallyl anion might be controlled by the proton affinity or basicity of the surface. These reactions are in contrast to the hydrogenation reaction that proceeds via covalently bound aalkyl or n- or a-alkylallyl intermediates which are influenced by the structures of active sites. D,

+ H+

- (D,H)+

H D + D+

(33)

Another interesting reaction of importance is the selective partial hydrogenation of acetylene or of dienes on heterogeneous catalysts. The selective partial hydrogenation reaction will be established by one of the following three mechanisms: (1) Selective hydrogenation of dienes or acetylene caused by competitive adsorption of acetylene or dienes with olefins. (2) Selective partial hydrogenation of acetylene or dienes caused by adsorption induced activation and deactivation of the surface. (3) Selective partial hydrogenation of conjugated diene via alkylallyl anion intermediates. The first case is commonly observed on metallic catalysts such as Pd and Ni as well as on Co304, Cr203, ZnO, and MoS,, where the subsequent hydrogenation of olefins is retarded in the presence of acetylene or dienes.

153

ACTIVE-SITE CONTROL OF CATALYSIS

60

total pressure

(a) trap

-

20

100

ol E E u

r

80

t

0

60

ln ln

10

g 40

20 0 0

10

20 t irne ( h r )

30

-

5

0 0 0

'

1

10

20 t i m e (hr)

0

30

FIG.27. (a) Hydrogenation of acetylene on sulfided nickel surface at 119°C and (b) the H,-D, equilibration taking place on an activated surface by acetylene at 120°C (43).

However, if acetylene or diene is hydrogenated to olefins, subsequent hydrogenation of olefins proceeds rapidly. As discussed in this review, the isomerization of olefins via alkyl intermediates proceeds on 'MH-sites, but these sites are not effective for the hydrogenation reaction, and the hydrogenation reaction is brought about merely on 3MH-sites. This fact reveals that if the number of available coordination sites changes from 3 to 2 or vice versa for some reason, the surface loses or gains hydrogenation activity. The partial hydrogenation of acetylene established on sulfided nickel catalyst is a typical example of mechanism (2) above. As shown in Fig. 27a, sulfided nickel catalyst becomes active for the hydrogenation of acetylene after a certain induction time. It is well known that the hydrogenation of acetylene on Ni catalyst brings about subsequent hydrogenation of ethylene to ethane. In contrast, the hydrogenation of acetylene on sulfided nickel is not followed by the hydrogenation of ethylene, which results in establishing the partial hydrogenation of acetylene (43). This is a case in which the sulfided nickel surface is activated by contacting with acetylene and deactivated by removing acetylene; that is, the adsorption of acetylene induces a reversible change of the catalyst surface. This inference was confirmed by performing H,-D, equilibration as a monitor reaction. As shown in Fig. 27(b), the sulfided nickel surface was inactive for the H,-D, equilibration at 118°C. If acetylene was added to a mixture of Hz and

154

KEN-1CHl TANAKA

D,, the hydrogenation reaction was brought about after a certain induction time. After that, if acetylene and ethylene were removed from the gas phase by a liquid nitrogen trap, the H,-D, equilibration was found to occur as shown in Fig. 27b, but was soon suppressed. This fact indicates that the surface is activated by contacting with acetylene, and deactivated by removing acetylene, as expressed by Eq. (35), where Surface-I corresponds to freshly sulfided Surface-I

+c,Hz F=== -C,Hz

Surface-I1

(35)

nickel surface and Surface-I1 is the surface activated by contacting with acetylene. Surface-I is undoubtedly inactive for the hydrogenation reaction, as well as for the H,-D, equilibration reaction, but is active for the isomerization of n-butenes or the hydrogen exchange between CzH, and C,D, in the presence of hydrogen, as shown in Fig. 28. The surface during isomerization was diagnosed to be Surface4 by the hydrogenation of acetylene and the H,-Dz equilibration. An Auger electron spectroscopic analysis of the sulfided nickel surface indicated that the ratio S/Ni was about 0.69 at the surface, which is very close to the ratio in nickel subsulfide (Ni3Sz). In fact, a nickel sulfide having a bulk composition of S/Ni = 0.62 exhibited specific activity for the hydrogenation of acetylene with a certain induction time, while a nickel sulfide (NiS) having a composition of S/Ni = 0.96 showed no activity for the hydrogenation of acetylene. The crystal structure of Ni3S, is rhombohedral, being very close to the cubic form. Nickel ions in Ni3Sz are in a slightly distorted tetrahedral 20,

1

0

rans-2-butene

0 0

2

4

6

time ( h r )

6

0

10 time ( h r )

FIG.28. Isomerization of 1-butene (a) and hydrogen exchange of CzH4--CzD4(b) on sulfided nickel surface in the presence of hydrogen at 121 and 120°C (43).

20

155

ACTIVE-SITE CONTROL OF CATALYSIS

configuration, while nickel ions in NiS are surrounded by six sulfur ions; that is, nickel ions in Ni,S, take two degrees of coordinative unsaturation, which may correspond to 'M-sites. Therefore, it is quite reasonable that Ni,S, is active for the isomerization of olefins but is inactive for the hydrogenation reaction. Accordingly, the activation of the sulfided nickel surface by contacting with acetylene might be expressed by a reversible change of the conformation of surface nickel atoms as speculated in Scheme 17. These phenomena are quite analogous to an oscillation in the catalytic oxidation of CO on Pt( 100) surface caused by adsorption-induced reconstruction of the surface (64).

S

\

Surface I p

S

c

S

-c:c-

:Ce: 5.

,sH SONi\~

I

S

1 s, : .. ,Ni;H

I

s

Surface II

y

H2

H s \ ' H, 7 s,Ni\ I xCx:H

$<'

5

~h SCHEME17. Activation processes of sulfided nickel surface and selective catalyses on Surface-I and Surface-11.

The third type of selective partial hydrogenation is prominent in the hydrogenation of conjugated dienes by alkali metals deposited on alumina (60) and by alkali metal hydrides (33). These catalysts are active for the hydrogenation of conjugated dienes but are not effective for the hydrogenation of olefins, which results in substantial selective partial hydrogenation of conjugated dienes to monoenes. The selective partial hydrogenation of 1,3butadiene on CdO and on MgO is a similar case. That is, the CdO and MgO surfaces may not fulfill the prerequisite for the hydrogenation reaction by making covalently bound half-hydrogenatedintermediates. In fact the hydrogenation of olefins, as well as the H,-D, equilibration reaction, does not occur on these oxides. However, conjugated diene reacts with H, on these oxides, to yeild alkylallyl carbanions which results in a partial hydrogenation of dienes. The formation of alkylallyl carbanion as well as of alkyl anions

156

KEN-ICHI TANAKA

depends merely on the basicity of the surface and is not influenced by the structures of active sites. The difficulty of the formation of alkyl anion compared to alkylallyl carbanion leads to the selective partial hydrogenation of conjugated dienes on these basic oxides.

V. Conclusion

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H,-D, equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H,D)+ or (HD,)' are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. Furthermore, physical processes such as internal and external rotation of intermediates will be indispensable elementary processes in some catalytic reactions. In this review, the discussions are focused on rather simple reactions taking place on isolated single sites, but it may be reasonable to assume that more complex reactions demand more complicated prerequisites for active sites. Such higher order prerequisites could be fulfilled by ensemble operation of several sites. For example, a dimeric cluster of cuprous ions on silica gel is very active for the oxidation of CO with N,O at room temperature, but isolated cuprous ions are entirely inactive for this reaction (60). More interesting selectivity may be found in the reaction of olefins with methylene complexes; the reaction of olefins with mononuclear methylene undergoes an olefin metathesis reaction, but the reaction of ethylene with bridging methylene in p-CH2Co,(CO),(Cp), (61), p-CH,Fe,(CO), (62), and p-CH,-p-ClTi(Cp),Al(Me), (63) (Cp = cyclopentadiene) leads to propene formation (homologation reaction). As shown in this review, precisely selective processes are brought about on the uniform sites. These results arouse the hope that effective catalysts with desired selectivity can be prepared by designing active sites on solid surfaces that fulfill the prerequisites for a given reaction.

ACTIVE-SITE CONTROL OF CATALYSlS

157

REFERENCES 1. Wood, B. J., and Wise, H., J. Caral. 5, 135 (1966). 2. Salmeron, W., Gale, R. J., and Somorjai, G. A,, J. Chem. Phys. 67, 5324 (1977). 3. Sinfelt, J. H., Carter, J. L., and Yates, D. J. C., J. Cutal. 24, 283 (1972). 4. Biloen, P., Dautzenberg, F. M., and Sachatler, W. M. H., J. Curd. SO, 77 (1977). 5. Hightower, J. H., and Hall, K. W., Chen. Eng. Pragr. Symp. Ser. 63. 122 (1967). 6 . Okuhara, T., and Tanaka, K., J. Am. Chem. Sac. 98,7884 (1976). 7. Okuhara, T., Tanaka, K., and Tanabe, K., J. Chem. Sac., Chem. Carnrnun. p. 180 (1977). 8. Okuhara, T., and Tanaka, K., J. Phys. Chem. 82, 1953 (1978). 9. Tanaka, K., Okuhara, T., Sato, S., and Miyahara, K., J. Card. 43, 360 (1976). 10. Siegel, S., J. Curul. 30,139 (1973). 11. Faragher, A. L., Ado. Colloid Interface Sci. 11, 3 (1973). 12. Private communication from professor Y. Murata of The Institute for Solid State Physics, The University of Tokyo. 13. McGovern, I. T., Williams, R. H., and Park, A. W., J. Phys. C 12, 2689 (1979). 14. Tanaka, K., and Okuhara, T., Proc. Inr. Canf 3rd Chem. Uses Molydb., 170 (1979). IS. Harrod, J. F., Hamer, G., and Yorke, W., J . Am. Chem. Sac. 101, 3987 (1979). 16. Shrock, R. R., and Osborn, J. A,, J. Am. Chem. Sac. 98,4450 (1976); J. Am. Chem. Soc. 98, 2143 (1976). 17. Tanaka, K., Aomura, K., Okuhara,T., and Miyahara, K.,J. Chem. Sac. Furuday Trans. 1,77, (1981). 18. Burwell, R. L. Jr., Hailer, G. L., Taylor, K. C., and Road, J. F., A h . Curd. 20, 1 (1969). 19. Pines, H., and Haag, W. O., J. Org. Chem. 23, 328 (1958). 20. Brennemann, H., Angew. Chem. Inr. Ed. Engl. 9, 736 (1970). 21. Dent, A. L., and Kokes, R. J., J. Am. Chem. Sac. 92, 6709,6718 (1970). 22. Yokoyama, S., Tanaka, K., and Seisho. M.,J. Chem. Sac. Chem. Cammun., p. 1061 (1981). 23. Yokoyama, S., Tanaka, K., Toyoshima, I., Miyahara, K., Yoshida, K., and Tashiro, J., Chem. Lerr. p. 599 (1980). 24. Yokoyama, S., Tanaka, K., and Haneda, H.,J. Chem. Sac. Chem Cammun., p. 820 (1982). 25. OBrien, D. H., Russell, C. R., and Hart, A. J.. Tetrahedron Left., p. 37 (1976). 26. Schlosser, M., and Hartmann, J., J. Am. Chem. Sac. 98,4674 (1976). 27. Zimmermann, A. Z., and Brauman, J. I., J. Am. Chem. Sac. 99,3565 (1977). 28. Golden, D. M., Rodgers, A. S., and Benson, S. W., J. Am. Chem. Sac. 91,2136 (1969). 29. Kwiatek, J., and Seyler, J. K., J. Orgunomel. Chem. 3, 421 (1965). 30. Tanaka, K., and Blyholder, G., J. Phys. Chem. 76, 1394 (1972). 31. Okuhara, T., and Tanaka, K., J. Catal. 61, 135 (1980). 32. Hattori, H., Tanaka, Y., and Tanabe, K., J. Am. Chem. Sac. 98,4652 (1976). 33. Slaugh, L. H., J. Org. Chem. 32, 108 (1967). 34. Slaugh, L. H., Tefrahedron22, 1741 (1966). 35. Kondo, T., Saito, S., and Tamaru, K., J. Am. Chem. Sac. 96, 6857 (1974). Tamaru, K., “Dynamic Heterogeneous Catalysis, p. 107. Academic Press, New York, 1978. 36. Okuhara, T., and Tanaka, K., J. Chem. Sac. Faruday Trans. 1, 75, 7 (1979). 37. Tanaka, K., and Okuhara, T., J . Carat. 78, 155 (1982). 38. Pople, J. A,, and Gordon, H., J. Am. Chem. Sac. 89,4253 (1967). 38. Dynamic Magnetic Resonance Spectroscopy, ed. by Juckman, L. M. and Cotton, F. A., Academic Press, New York (1975). 39. Turkevich, J., Bonner, F., Schissler, D. O., and Irsa, A. P., Discuss. Furaduy Soc. 8,352 (1950). 40. Twigg, G. H., Discuss. Furuduy Sac. 8, 152 (1950). 41. Tanaka, K., J. Carat. 37, 558 (1975). 42. Tanaka, K., Nihira, H., and Ozaki, A., J. Phys. Chem. 74,4510 (1970).

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43. Takeuchi, A., Tanaka, K., and Miyahara, K., Chem. Lefr., 171, 411 (1974); J. Caral. 40, 94, 101 (1975). 44. Tanaka, K., Okuhara, T., Catal. Reo. Sci. Eng. IS,249 (1977). 45. Burwell, Jr., R. L., Littlewood, A. B., Cardew, M., Pass, G., and Stoddart, C. T. H., J. Am. Chem. SOC.82,6272 (1960). 46. Conner, W . C., Innes, R. A., and Kokes, R. J., J. Am. Chem. SOC.90,6858 (1968). Conner, W. C., and Kokes, R. J., J. Phys. Chem. 73,2436 (1969). 47. Naito, S., Sakurai. Y., Simizu, H., Onishi, T., and Tamaru, K., BUN.Chem. SOC.Jpn. 43,2274 ( 1970). 48. Okuhara, T., Tanaka, K., and Miyahara, K., J. Chem. SOC.Chem. Commun., p. 42 (1976). 49. Eishens, R. P., Pliskin, W. A., and Low, M. S. D., J. Cafal. I, 180 (1962). 50. Kokes, R. J., Dent, A. L., Chang, C. C., and Dixon, L. T., J. Am. Chem. Soc. 94,4429 (1972). 51. Okuhara, T., and Tanaka, K., J. Chem. SOC.Chem. Commun., p. 199 (1976). 52. Okuhara, T., and Tanaka. K., J. Chem. SOC.Faraday Trans. 1,7S, 1403 (1979). 53. Okuhara, T., Kondo, T., Tanaka, K., and Miyahara, K., J. Phys. Chem. 81,90 (1977). 54. Okuhara, T., Kondo, T., and Tanaka, K., J. Phys. Chem. 81,808 (1977). 55. Tanaka, K., Tanaka, K., and Miyahara, K., Chem. Left. p. 943 (1978). 56. Okuhara, T., Kondo. T., and Tanaka, K., Chem. Left. p. 119 (1977). 57. Imizu, Y., Hattori, H., and Tanabe, K., Shokubai 19.64 (1977). 58. Brouwer, D. M., and Hageveen, H., Prog. Phys. Org. Chem. 9, 179 (1972). 59. Olah, G. A., Shen, J., and Schlusberg, R. H..J. Am. Chem. SOC.92, 3831 (1970). 60. Kakuta, N., Kazusaka, A., and Miyahara. K., Chem. L e f / .p. 913 (1982). 61. Theopold, K. M., and Bergman, R. G.,J. Am. Chem. Soc. 103,2489 (1981). 62. Summer, C. E., Riley, P. E., Davis, R. E., and Pettit, R., J . Am. Chem. SOC.102, 1752 (1980). 63. Tebbe, F. N., Parshal, G. W., and Reddy, G. S., J. Am. Chem. SOC.100,361 1 (1978). 64. Ertl, G., Norton, P. R., and Riistig, J., Phys. Reo. Left. 46, 177 (1982); Behm, R. I., Thiel, C. P. A., Norton, P. R., and Ertl, G., J. Chem. Phys. 78, 7437 (1983).

ADVANCES IN CATALYSIS, VOLUME 33

Selective Oxidative Dehydrogenation of Butenes on Ferrite Catalysts HAROLD H. KUNG AND MAYFAIR C. KUNG Chemical Engineering Department and the Ipatieff Catalytic Laboratory Northwestern University Evanston, Illinois

I. Introduction. . . . . . . . . . . . . . . . . .

. . Role of Adsorbed Oxygen. . . . . . Role of Lattice Oxygen . . . . . . Kinetics and Mechanism . . . . . .

11. Selective Oxidation and Combustion Sites 111. Densities of Oxidation Sites . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

IV. V. VI. VII. Factors Affecting Selectivity: Roles of Crystal Structure, Nature of Transition Metal Ions, and Effect of Promoters . . . . . . . VIII. Effect of Crystallite Size . . . . . . . . . . . . . . IX. Other Structure-SensitiveOxidation Reactions. . . . . . . . X. Catalytic Activity of Iron Hydroxide. . . . . . . . . . . XI. Conclusions. . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

1.

159 163 166 169 177 177 180 185 189 193 196 196

Introduction

Iron oxide is an important component in catalysts used in a number of industrially important processes. Table I shows some notable examples which include iron molybdate catalysts in selective oxidation of methanol to formaldehyde, ferrite catalysts in selective oxidative dehyrogenation of butene to butadiene and of ethylbenzene to styrene, iron antimony oxide in ammoxidation of propene to acrylonitrile, and iron chromium oxide in the high temperature water-gas shift reaction. In some other reactions, iron oxide is added as a promoter to improve the performance of the catalyst. It is easily seen that most of these reactions are selective oxidation reactions. A special feature of these reactions, based on the Mars and van I59 Copyright 0 1985 by Academic Press, Inc. All rights or reproduction in any iorm reserved.

160

HAROLD H. K U N G A N D MAYFAIR C. KUNG

TABLE I Ferrite Catalysts Used in Commercial Processes“ Reaction C,H,CH,CH, + 40,-+ C,H,CH=CH, C,H, + to2-+ C4H6 + H 2 0

CO + H20-COz + H2 CH,OH + to2+ HCHO + H 2 0 C,H, + NH, 3 0 2 CH,=CHCN

+

-+

T(”C)

+H20

+ 3H2O

550-660 600-630 600-630 425 360-510 325 350-530 350-420 400-450 400-450

Catalyst Fe,O,-Cr,O,-K,O Fe--K-Mg-0-0 Fe-AI-0 Fe-Mg-Ce-0 Fe-Mg-P-0 Fe-Cr-Mg-0 Fe-Cr-0 Fe-Mo-0 Fe-Sb-0 Fe-Mo-Te-0

Adapted from Dadyburjur, D. B., Jewur, S. S., and Ruckenstein, E., Catal. Reo., 19,293 (1979); Hucknall, D. J., “Selective Oxidation of Hydrocarbons,” Academic Press, New York, 1974; Dumas, T., and Bulani, W., “Oxidation of Petrochemicals: Chemistry and Technology,” Wiley, New York, 1974; Satterfield, C. N., “Heterogeneous Catalysis in Practice,’’ McGraw-Hill, New York, 1980.

Krevelen mechanism (I) is that the catalyst undergoes an oxidation-reduction cycle during reaction:

to2

reactant

.1

product SCHEME 1

In fact, such a redox cycle for the catalyst is involved in the water-gas shift reaction (2,3). In this mechanism, a reactant molecule interacts with the catalyst at a site containing a cation MY’. During the course of the reaction, lattice oxygen is consumed to form the products, and the M , cation is reduced. The active site and the cation M , are reoxidized by migration of lattice oxygen from another site M,, which in turn is reoxidized by the gas-phase oxygen. The cation M, may or may mot be distinct from M I . Based on this mechanism, a basic requirement can be formulated for an oxidic selective oxidation catalyst: the cation in the oxide must be able to

161

DEHYDROGENATION OF BUTENES

interconvert between two oxidation states with some optimal degree of difficulty; that is, the Gibbs free energy change in this redox must not be too large or too small. If a catalyst is too easily reduced (AG of reduction too negative), the lattice oxygen is too reactive and total oxidation of the reactant is likely. If the catalyst is too resistant to reduction (AG of reduction too positive), the oxide cannot undergo the redox cycle and the oxidation reaction cannot occur. The standard AG of conversion per mole of Fe ion at 600 K and 1 atm of oxygen for the conversion of iron oxide among the various oxidation states is shown below (4). a-Fe,O,

- 25.6

kJ

Fe30,

79.8 kJ

FeO

It can be seen that the first conversion is relatively thermodynamically feasible. Thus, from the discussion above, the Fe ion in a selective oxidation catalyst should be near the fully oxidized state such that it can undergo the necessary reduction-oxidation'cycleeasily. It is significant to note that there is a substantial difference in the thermodynamic driving force (or resistance) between the first and the second reduction. This could be important in controlling the amount and the reactivity of the lattice oxygen in oxidation reactions. In the industrial processes of the selective oxidative dehydrogenation of butene to butadiene, iron oxide is the primary component of the ferrite catalysts used. (Because of the similarity in the chemistry involved, this statement is expected to apply also to the dehydrogenation of ethylbenzene.) Table I1 lists the preformance of some of the ferrite catalysts used in this reaction. A t temperatures below about 35OoC, y-iron oxide yields butadiene with 90 % selectivity in the oxidation products. However, the thermal stability of y-Fe,O, is low, and the oxide is converted to the a-form above about 350°C.a-Fe,O, has a much lower selectivity at high conversions. Since the industrial process operates at about 5OO0C,promoters have been added to obtain the desired selectivity.Common promoters are Zn, Mg, and Cr. The exact roles of the promoters are not fully understood. In addition to the possible chemical effects, it has often been suggested that these are structural promoters-they retard sintering and loss of iron oxide surface area, and they help preserve the crystallographic forms of the iron oxide. The latter point suggests that the selectivity of iron oxide depends on its crystallographic form. Progress has been made toward understanding the relationship between the surface and the solid state properties of the oxide catalysts and their selectivities in this reaction. This is the subject of this review. In general, the oxidation of butene can be schematically represented as two interconnecting parallel pathways (Scheme 2). Path I is the selective oxidation reaction. Path I1 is the combustion reaction. The relative rates of the two

TABLE I1 Reaction Orders and Butadiene Selectivities in Butene Oxidation ouer Ferrite Catalysts ~~

Feed (am) Catalyst

Temperature (“C)

a-Fe,O, Y-Fe203

300-350 300-350 300 325 375 360 436 435 300-400 325 325 325 325 325 325 375 350450 225-390

a-FeCrO, CoFe,O, CuFe,O, MgFe204 MgCrFeO, ZnCrFeO, ZnCr~.25Fe1.7504 ZnCrO.

lFel .go,

ZnFe,O, Bi3(F&,XMoO& Fe promoted Bi-Mo-0 (Fe:Bi:Mo= 1:9:15) Fe/Sb/O Fe/V/O (Fe:V = 1:0.05) Fe/Mo/O (Fe:Mo = 1:O.l) Fe/P/O (Fe:P= 1:0.05) Fe/Bi/P/O (Fe: Bi : P = 1:0.03 :0.1)

300-450 4M-500 300

O2

Butene 0.1-0.4 0.1-0.4

0.1-0.4 0.1-0.4 0.01-0.1

0.01 -0.1 0.4

0.6

0.5

0.5

0.013

0.066 0.066 0.037 0.5-1.0 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.08-0.23

0.066 0.066

0.026 0.4

0.4 0.4 0.4 0.4

0.4 0.4 0.09-0.32

0.0067

0.23 0.23 0.2

0.0067

300-350 270-360

-

300

0.15

~

Order Butene

>o

-

-0

1

>o -1

0, Butadiene

Butene conversion (%)

0

< 10 < 10 12

55 > 88 89

50

84

>o

57 Low

76 90 80 50

0 0

-0 0



LOW LOW

LOW

53 0 0

Butadiene selectivity (%) Reference

64 58

0 0

1 1

0 0

0

0 0


56 46 20 47 <50

> 80 86

7 7 14 19 19 25 25

25

88

15 19 19 19 19 19 19 19

> 95

27

90 91 90 92 89

26

30-40

60

41 41 50

0.2

30-40

80

50

0.01

0.01

25-40

93-83

49

0.01

0.01

40-50

> 83

49

0.15

1

CdHda)

------+

CxHyOAa)

~ 1COz1+ HzO

SCHEME 2

paths determine the selectivity. This scheme is in a general form. Branching into the two pathways could take place on adsorption of butene. In this case, there are two chemically different active sites on the catalyst surface, and their relative densities and reactivities determine the selectivity. Branching could also take place at the adsorbed butene, adsorbed butadiene, and/or other intermediates such as n-allyl. In this case, there is only one type of active site. The selectivity is determined by rate constants of the branching reactions. In attempts to understand the differences in selectivity among different ferrites, it is therefore necessary to determine (1) whether there are separate sites for selective oxidation and combustion, (2) the densities of the two kinds of sites, (3) the possibility of degradation of the surface intermediates in the selective oxidation pathway, and (4) the reaction mechanism and the nature of the slow step. From the answers to these questions, better understanding of the effects of crystal structure, of the nature of the transition metal ion, and of the promoters can be obtained.

II. Selective Oxidation and Combustion Sites

A common method for determining the existence and the densities of different active sites is selective chemisorption. However, ignorance of the nature of the active sites makes it impossible to choose suitable probe molecules. Instead, a variation of the selective chemisorption technique can be used that makes use of the reactivity of the active sites. The technique is temperature programmed desorption and reaction, or more generally, thermal desorption and reaction. The principle of the technique is simple. A reactant molecule is adsorbed at low temperatures on an active site. The temperature is then raised in a controlled manner and the evolution of the reaction products is monitored. If the various products are evolved independent of each other, they must be formed on independent sites. The technique was applied to a-Fe,O, (5). cis-2-Butene was introduced to the catalyst at 25°C for equilibration with the active site. After equilibration and removal of the gas-phase butene, temperature programmed desorption

164

HAROLD H. KUNG AND MAYFAIR C. KUNG

0

100

200

300

400

TEMP ( O C )

FIG.1. The temperature programmed desorption profiles for a-Fe,O,: (a) Blank desorption without adsorbates; (b) cis-2-butene adsorption; (c) butadiene adsorption; (d) cis-2-butene adsorption from a catalyst depleted of selective oxidation sites. From ref. 5, reprinted with permission, copyright 0 1979 by the American Chemical Society.

was commenced that resulted in the desorption profile shown in Fig. 1, curve b. There are two sets of products. Below about 210"C, butene isomers are evolved between 50 and 125"C, and butadiene is evolved between 75 and 175°C. Above 210"C, the combustion products carbon dioxide and water are evolved. From the areas of the desorption peaks, about 30% of adsorbed butene is found to be totally oxidized on desorption. Since adsorbed carbon dioxide would desorb below 210°C (9,the desorption of the combustion products is reaction limited. The temperature programmed desorption profile for the adsorption of butadiene in place of cis-Zbutene is shown in Fig. 1, curve c. Two sets of products are observed. The product below 210°C is unreacted butadiene, and the products above 210°C are carbon dioxide and water. The similarity in the evolution of the combustion products of butene and butadiene is an indication that their combustion proceeds via similar reaction mechanisms. The similarity in the desorption of butadiene suggests that in butene adsorption, butadiene desorption is desorption limited. Indeed, that both butene and butadiene adsorb on the same type of sites has been confirmed by sequential adsorption experiments. The results are shown in Table 111. It was found that if the C, hydrocarbons are adsorbed sequentially without thermal desorption between adsorptions, the amounts of the final desorption products are the same as those in experiments where only the first hydrocarbon

165

DEHYDROGENATION OF BUTENES

TABLE 111 Sequential Adsorption of cis-2-Butene or Butadiene on a-Fe, 0," Adsorption sequence First cis-2-Butene cis-2-Butene Butadiene Butadiene Butadiene

Second None Butadiene None cis-2-Butene Butadiene

Total C, Hydrocarbon adsorbed**'

Amount of CO, normally desorbedb*

3.57 3.69 3.52 4.46 4.12

0.87 1.06 1.90 1.62 1.71

Adsorption at 25°C;the gas phase was purged free of hydrocarbon between adsorptions and before thermal desorption. In 10" molecules g- I . ' Total amount adsorbed before desorption; calculated from the desorption products. Amount of CO, detected divided by four.

is adsorbed. Thus, adsorption of butene or butadiene prevents subsequent adsorption of the other compound, showing that these two molecules adsorb on the same sites. The separation of the two sets of desorption products may indicate that they are from different sites. That is, branching of the selective and nonselective oxidation takes place on adsorption of butene. This can be confirmed if the two sets of products can be varied independently. This is shown by two experiments. The first experiment makes use of the fact that butene and butadiene adsorb on the same sites. Butadiene is first adsorbed onto the catalyst (5). The catalyst is then heated to 210"C, desorbing all of the unreacted butadiene, but leaving on the surface the precursors of the combustion products. Since desorption of the unreacted butadiene does not involve a net chemical reaction, the adsorpton sites involved are not affected. The catalyst is then cooled to 22°C and cis-2-buteneis adsorbed. If selective oxidation and combustion take place on the same site, the adsorbed butene would undergo both reactions. If they take place on separate sites, and butene adsorbs only on the selective oxidation site (because the combustion site is covered by species from butadiene adsorption), the adsorbed butene would form only butadiene. Subsequent desorption yields a profile similar to that for a single adsorption of cis-2-butene (Fig.1, curve b). More importantly, within experimental errors, the amount of butadiene evolved is the same as in a cis-Zbutene adsorption experiment, and the amount of CO, evolved is the same as in a butadiene adsorption experiment. Thus, the adsorbed butene forms only butadiene. These results show that under these experimental conditions (i.e., in the absence of gas-phase oxygen), the production of butadiene and carbon dioxide takes place on separate sites.

166

HAROLD H. KUNG AND MAYFAIR C. KUNG

Results of a variation of this experiment yield the same conclusion (5). A catalyst is first adsorbed with butadiene, and the unreacted butadiene is desorbed by heating to 210°C. Then a pulse of cis-2-butene is passed over the catalyst at this temperature. The production of butadiene from this pulse is the same as that from an untreated catalyst. Thus, the preadsorbed precursors of combustion products do not affect the selective oxidation reaction. The second experiment is to perform adsorption-desorption of butene on a catalyst that is depleted of selective oxidation sites. When 10 pulses of cis-2butene are passed over a catalyst at 210°C (5), which is a temperature too low for the production of CO,, the catalyst is reduced. The number of selective oxidation sites is substantially reduced as is evident by the much lower conversion to butadiene in the last pulse. Then the oxide is cooled to 22"C, and cis-2-butene is adsorbed. The resulting desorption profile is shown in Fig. 1, curve d. Clearly, there is no more butadiene production, while the combustion products are produced in a somewhat larger quantity. These results again support the conclusion that the selective oxidation and the combustion sites are independent. 111.

Densities of Oxidation Sites

Having identified the existence of separate oxidation sites, it is desirable to determine the densities of sites which contribute to the different selectivities among different catalysts. Since the products of the thermal desorption experiments described above are directly correlated with the active sites, it is possible to measure the number of active sites by measuring the quantities of desorbed products, provided that each and every active site produces only one molecule of reaction product. To achieve this condition, it must be established that the adsorbate is fully equilibrated with the surface, that there is no multiple reaction per site during equilibration, that there is no readsorption and reaction of desorbed products, and that all reaction products are quantitatively determined. Full equilibration of butene with the surface is established when the amount adsorbed does not change when the adsorption time and temperature are varied. Figure 2 shows the dependence of the equilibration (or trapping) time for a-Fe,O, at room temperature. The amount of butadiene purged out of the reactor at the end of equilibration increases with increasing trapping time. Thus, butadiene is being continuously produced and released into the gas phase during equilibration. The amount of butadiene thermally desorbed, however, is constant except for the initial 30 min. Thus the total amount of butadiene produced increases linearly with time at long times, suggesting that multiple reaction has occurred on some selective oxidation

167

DEHYDROGENATION OF BUTENES

.

2.0L c ur

. 0

c

c,H, o& 0

.4--

purged

I I

"

"

0

2 0 4 0

I

I

1

I

1

8Q IQO 120. 140 trapping time (min)

60

I

160

I

180

FIG.2. Thermal desorption product distribution in cis-2-butene adsorption and desorption on a-Fe,O, as a function of equilibration time. C4H, purged: butadiene purged out at 22°C; C0,/4: CO, thermally desorbed divided by four; C,H, des: butadiene thermally desorbed; total C,H,: total butadiene produced (equals the sum of that purged out and that thermally desorbed). From ref. 5, reprinted with permission, copyright 0 1979 by the American Chemical Society.

sites. In this case, the number of selective oxidation sites is obtained by extrapolating the total butadiene production to zero time. In contrast, the amount of carbon dioxide produced is constant. This implies that there is no multiple reaction on the combustion sites, and that the amount of carbon dioxide equals the number of combustion sites. If the equilibration temperature is lowered, as is shown in Fig. 3, to below about 200 K, no butadiene is released into the gas phase during equilibration, and the total amount of products thermally desorbed, including butene isomers, butadiene, and carbon dioxide, remains constant, independent of temperature (6). Similarly, constant amounts of products are obtained independent of the time of purging of the reactor after equilibration at 195 K (Fig. 4). The constant values imply that the condition of one product molecule for each and every active site has been achieved. The possibility of readsorption and further reaction of desorbed products was examined by varying the residence time of the products in the reactor, which is achieved by varying the carrier gas flow rate and the size of the catalyst bed. A number of different temperature programming rates from

168

HAROLD H. KUNG AND MAYFAIR C. KUNG

TRAPPING TIME Ihr, PURGING TIME I hr

G I

-

TOTAL PROD., DES.

' C,H,+

--

-

C02/4.MS

TOTAL PROD.

0 0

I00

200

300

(t0

TRAPPING b PURGING TEMP

FIG.3. The amounts of thermally desorbed products from cis-2-butene adsorption on a-Fe,O, versus temperature of adsorption and purging. Data for total products include 1-butene, cis-tbutene, butadiene, and C0,/4; data for total desorption include cis-2-butene as well. From ref. 6, reprinted with permission from Academic Press.

TEMP. -78'C TRAPPING TIME 0.5h TOTAL PROD.

CO,M

0

I

a

-

4

2

3

PURGING TIME ( h )

FIG.4. The amounts of thermally desorbed products from cis-2-butene adsorption on a-Fe,O, versus purging time at -78°C. Data for total products and for total desorption include the same molecules as those in Fig. 3. From ref.6, reprinted with permission from Academic Press.

169

DEHYDROGENATION OF BUTENES

TABLE IV Site Densities on Various Ferrite Catalysts Site densities (10’’m-’) Oxide a- Fe

0 y-Fe203 ZnFe,O,

Selective Oxidation

Combustion

1.9 1.9 2.8

0.9 2.5” 0.5

The uncertainty of this number is higher than the others because of residual carbon left on the surface from the conversion of a- to y-Fe,O, by a CO/CO, mixture.

1.6 K sec- to over 50 K sec- were also examined. From these control experiments, conditions were found such that the quantities of oxidation products detected are independent of these operating parameters. Using this technique, the active site densities of a-Fe,O, (9,y-Fe,O, (7), and ZnFe,O, (8) were measured. They are reported in Table IV. It can be seen that there is no significant variation in the site densities, In fact, the more selective y-Fe203 shows a higher density of combustion sites than the less selective a-Fe,O,. It must be added, however, that it has not been investigated as to whether these values depend significantly on the pretreatment of the samples, such as the extent of preoxidation or prereduction.

IV. Role of Adsorbed Oxygen

The adsorption-desorption experiments described in Sections I1 and 111 establish the route of the combustion reaction in the absence of gas-phase oxygen. However, they do not provide information on the possibility of degradation of intermediates in selective oxidation by gaseous oxygen when it is present. Futhermore, it is of interest to understand how the atomic environment of the selective oxidation site differs from the combustion site. With regard to the latter point, it has been noted that the surface concentration of chemisorbed oxygen on a-Fe,O, is comparable to the density of the active sites ( 9 , I I ) .It follows that the chemisorbed oxygen may be involved in the formation of one of the active sites. Indeed, the participation of adsorbed oxygen in selective oxidation has been postulated on F e 2 0 3 (12) and NiO (13). Above room temperature, oxygen adsorbs on a-Fe,O, in at least three different forms that are characterized by different temperatures of desorption

170

HAROLD H. KUNG AND MAYFAIR C. KUNG

DESORPTION TEMP. ("C)

FIG.5. Cumulative amount of oxygen desorbed from a-Fe,O, as a function of increasing desorption temperature. Adsorption was accomplished by exposure to oxygen at 450°C and then cooling to room temperature in oxygen. From ref. 11, reprinted with permission from Academic Press.

(Fig. 5). The u form desorbs at around 50-lWC, the B form at about 250-300"C, and the y form at about 350-400°C (9,JJ). Above 5WC, desorption of lattice oxygen begins. Adsorption of the various forms of oxygen are activated, and higher adsorption temperature is required for the higher temperature form. It was established that the B form desorbs with second-order kinetics, which suggests that atomic oxygen is involved ( 1J), The weak adsorption of the lowest temperature form may be the result of molecular adsorption. Experiments were then performed to preadsorb the various forms of oxygen on an iron oxide surface. These were then followed by the butene adsorption-desorption experiments to determine the amounts of the two types of oxidation sites. Typical results are shown in Table V (6). The amounts of oxidation products are independent of the presence of preadsorbed oxygen. In fact, even the thermally desorbed isomers are not affected by the preadsorbed oxygen. This absence of effect of the preadsorbed oxygen was observed also in pulse experiments. It was found that the amounts of butadiene, C 0 2 ,and butene isomers formed from a butene pulse passing over the catalyst at 100,200, or 300°C are independent of preadsorbed oxygen (6). While preadsorbed oxygen has no effect, the presence of gaseous oxygen drastically changes the product distribution in the thermal desorption and pulse reaction of butene on u-Fe203,as can be seen from results shown in Table VI (6). On this oxide, thermal desorption in an O2 instead of an He carrier results in a much lower yield of hydrocarbons and a much higher yield of COz.The same is observed in pulse reactions. Thus, on a-Fe,O,, adsorbed butene, adsorbed butadiene, and/or butadiene precursors must be very

TABLE V Effect of Preadsorbed Oxygen on the Oxidation of cis-2-Butene in Adsorption-Desorption ExperimentP. ~

Adsorbed oxygen species N + B + Y

B+Y Y

B.D!

Thermally desorbed products (10’’ molecules/m-z)

Adsorption temperature‘ (“C)

1-Butene

trans-2-Butene

cis-2-Butene

Butadiene

Cod4

B.D. + COZ/4

450, -78 450, -78 450, -78 500, -78 450,240 500,240 450,430 450,430 450,450 450,450 450,450 500,500

0.22 0.22 0.32 0.24 0.26 0.22 0.22 0.23 0.25 0.2 1 0.23 0.2 1

0.65 0.68 0.67 0.94 0.69 0.94 0.79 0.78 0.94 0.85 0.90 1.13

9.0 10.7 8.5 7.9 8.7 7.7 8.8 8.6 8.6 10.0 8.5 6.5

2.16 2.14 2.04 2.28 1.92 2.10 2.11 2.02 2.25 2.13 2.25 2.35

1.21 1.20 1.16 1.39 0.95 1.14 0.97 0.97 1.10 0.95 1.11 1.18

0.64 0.64 0.64 0.62 0.67 0.65 0.69 0.68 0.67 0.69 0.67 0.67

About 40 mg of a-Fe,O, that corresponded to a surface area of 0.7 mZ;standard adsorption and desorption procedures. From ref. 6, reprinted with permission from Academic Press. The catalyst was exposed to oxygen at the first temperature; while in oxygen the temperature was lowered to the second temperature at which the reactor was purged with helium. B.D. stands for butadiene.

172

H A R O L D H. K U N G A N D MAYFAIR C. KUNG

Eflect

of

TABLE V1 Gaseous Oxygen on the Production of Butadienefrom cis-2-Eutene Oxidation products'

Catalyst

Carrier gas

T(T)

Butadiene

C0,/4

2.1 0.05

1 .o 5.4

0.73 0.13 4.77 0.64 4.36 1.60 16.1 12.4

0.0 0.05 0.0 1.04 0.38 8.8 1.3 5.1

Thermal desorptioo a-Fe,O,

He 0,

a-Fe,O,'

He 0, He 0, He 0, He

Pulse reactionb

y-Fe,O,c.d

0 2

100 100 200 200 300 300 300 300

In 10" molecules m-' for thermal desorption, in lo" molecules in pulse reaction. Pulse size of butene is 2.6 x 10'' molecules. Amounts used were 40 mg a-Fe,O, (17 m-' g-'), and 16 mg y-Fe,O, (14 mz g-'). I -butene pulses were used.

sensitive to attack by weakly adsorbed oxygen. In contrast, under identical experimental conditions, the production of butadiene and CO, on y-Fe,03 are not affected by the presence or absence of gaseous oxygen (Table VI). This represents a significant difference between u- and y-Fe,O, that contributes to the observed difference in the butadiene selectivity in a steady state reaction. The insensitivity to oxygen in a pulse reaction on y-Fe,O, has also been reported by Misono et al. (14). That gas-phase oxygen is important in the combustion reaction has also been observed on MgFe,O,, where CO, is formed only in butene pulses with oxygen (15). Since it is not known whether the gas-phase oxygen molecules or weakly adsorbed oxygen (which is present only in the presence of gas-phase oxygen) is responsible for the degradation of the surface intermediates, we shall refer to it generically as gas-phase oxygen. The production of butadiene from butene involves at least three surface intermediates: adsorbed butene, n-allyl, and butadiene. One or more of these may be particularly vulnerable to attack by gas-phase oxygen on u-Fe,O3. From the temperature programmed desorption experiments, it was found that the products of isomerization, selective oxidation, and combustion

DEHYDROGENATION OF BUTENES

173

evolve at different temperatures (Fig. 1). Therefore, it may be possible to perform stepwise thermal desorption with the hope of generating a certain intermediate as the most abundant surface species, and then test for its sensitivity to attack by gas-phase oxygen. Results of such experiments on a-Fe,O, are shown in Table VII (6). In these experiments, cis-Zbutene was first adsorbed onto the catalyst at -78°C. After purging out the gas-phase butene, the catalyst was heated to various temperatures indicated, and the desorbed species were purged out of the reactor by He. Then a pulse of oxygen (or N,O) was passed over the catalyst at various temperatures. After the pulse had completely left the reactor, thermal desorption was resumed, and the desorption products were collected and analyzed. The following results are demonstrated by the data in Table VII. Adsorbed butene, if treated at temperatures below 25"C, is not sensitive to degradation by gaseous oxygen. At 75"C, the isomeric butenes are all desorbed, and the surface species are either precursors of butadiene or precursors of CO,. These precursors of butadiene are very sensitive to degradation by gaseous oxygen. Exposure of these species to a pulse of oxygen at a temperature as low as - 78°C results in significant degradation. The fraction of butadiene degraded is not dependent on the temperature of exposure to oxygen or to the oxygen pulse size, which implies that the residual butadiene observed is from species (probably adsorbed butene) that has not yet formed the oxygen-sensitive intermediate, and is not due to incomplete reaction between this intermediate and oxygen. Since the oxygen pulse size was only 2.5 times the amount of butadiene precursors and not all of the oxygen was consumed, the reaction between the intermediate and oxygen is only the first step in the degradation process, and CO, is not yet formed, In fact, under these experimental conditions, CO, from butadiene degradation and from butene combustion on the combustion sites is desorbed simultaneously above 210°C. Degradation of the butadiene precursor by N,O was also investigated by using a pulse of N,O instead of 0,. The results are also shown in Table VII (6). No degradation results from the N,O pulse at -78°C. A separate experiment showed that a pulse of 50 x 10'' molecules of N,O would result in 3.8 x 10l6 atoms of oxygen adsorbed on the iron oxide. If each 0 atom results in the degradation of one butadiene precursor, the decrease in butadiene yield would be detectable. If the 0 atoms first recombine to form 0, molecules which then react with the butadiene precursors, the decrease in butadiene yield would be within experimental uncertainties. When the N,O pulse was passed over the catalyst at a temperature of 75"C, degradation of butadiene took place but the extent was about half as much as that for 0, pulses. The results again can be attributed to the decomposition of N,O into N, and O,, and the 0, then causes degradation of butadiene precursors.

TABLE VII Reaction of Butadiene Precursors with Gas-Phase Oxygen and N 2 0 on a-Fe203*.b Catalyst desorption temperature (“C)

- 78 25

75

75

a

Gas P

h

0 2

0 2

0 2

N2O

Pulsing temperature (“C) None -78 None -78 25 None -78 -78 75 None -78 - 78

Thermal desorption products (10’’ molecules m-2)

Pulse size (10’’ molecules)

1-Butene

None 4.2 None 4.2 42 None 4.2 5oc 4.2 None 4.2

0.23 0.20 0.07

5w

0.05

0.03 0 0

0 0 0 0 0

trans-2-Butene

cis-2-Butene

Butadiene

0.90

9.0 9.8 0.64 0.46 0.38

2.21 1.97 1.81 1.78 1.53 1.47 0.41 0.30 0.27 1.47 1.33 1.40

0.91 0.68 0.33 0.29 0.02 0

0 0

0 0 0.02 0 0

0 0 0 0 0

About 40 mg of catalyst for a surface area of 0.7 m2;standard pretreatment and adsorption of cis-2-butene. From ref. 6, reprinted with permission from Academic Press. These pulses were trapped in the reactor at - 78°C for 0.5 hr. The reactor was then purged, and thermal desorption followed.

CO,/r 1.05 1.01 1.13 1.57 1.88 1.13 2.07 2.08 2.31 1.13 1.37 1.36

Total C, 13.4 13.9 4.33 4.19 4.11 2.62 2.48 2.38 2.58 2.62 2.70 2.76

DEHYDROGENATION OF BUTENES

175

The difference in reactivity of the butadiene precursor toward 0, and N 2 0 is interesting. N,O is known to be active in selective oxidation (16). For example, on molybdenum oxide (16) and cobalt magnesium oxide (13,N,O decomposes at room temperature to form an 0-adsorbed species which is very active in the oxidative dehydrogenation of ethane. The results presented above suggest that the degradation of butadiene precursor on a-iron oxide requires an 0, and not an 0- species. This implies that the degradation proceeds via a peroxide intermediate. While the results establish the fact that adsorbed butene is insensitive to degradation, it is not clear whether the n-ally1 intermediate or adsorbed butadiene is sensitive. An attempt to elucidate this by repeating the oxygen pulse experiments on adsorbed butadiene instead of butene shows that adsorbed butadiene that has been heated to 75°C can be degraded by gaseous oxygen in much the same way as adsorbed butene (6). Thus, the oxygen attack could be on adsorbed butadiene. However, the results do not rule out attack on the Ir-ally1 intermediate. The results also show that on Fe203,CO, can be formed directly from butene and from subsequent reaction of butadiene. This has also been confirmed using I4C-labeled butadiene on MgFe,O, (15). These results for a-Fe,O, can be summarized as follows. Referring to Scheme 2, butene adsorbs on two independent sites that lead to selective oxidation and combustion (paths I and 11). Along the selective oxidation pathway, adsorbed butene is not attacked by gas-phase (or weakly adsorbed) oxygen. Thus, path I11 is not important. However, other adsorbed intermediates such as n-ally1 and butadiene are attacked by gas-phase oxygen, and paths IV and V are important, The observed selectivity in a steady-state reaction is then determined by the relative rates of paths IV, V, VI, and VII. Based on the results described thus far, this competition can be schematically represented as in Fig. 6. The degradation reaction has a very weak temperature dependence and occurs readily at low temperature. The evolution of butadiene has a mild temperature dependence, and it proceeds at a reasonable rate beginning at medium temperature. The evolution of CO, has the strongest temperature dependence, and it proceeds readily only at elevated temperature, Below T,, there is little reaction observed because very little reaction product desorbs. But the surface is covered mainly by combustion products if gas-phase oxygen is present, and they can be detected by thermal desorption. Between TI and T,, a-Fe203appears to be a selective catalyst because desorption of the combustion products is slow. Above T,, when desorption of the combustion products is fast, the selectivity of the catalyst decreases rapidly. This phenomenological description can be used to rationalize the difference between a- and y-Fe,03 (18). As shown in Table VI, the butadiene yield

176

HAROLD H. KUNG AND MAYFAIR C. KUNG

w

c

a

K

TEMP

FIG.6. Schematic representation of the competitive processes in butene oxidation for (a) a-Fe,O,, (b) y-Fe,O,. Deg: degradation of surface intermediates; BD: production and desorption of butadiene; CO,: production and desorption of CO,.

on y-Fe,O, is not decreased by the presence of gaseous oxygen in a pulse experiment. Similar insensitivity toward oxygen degradation is also observed in the adsorption-desorption experiments for y-Fe,O,, shown in Fig. 7 (6). Thus the n-ally1 and adsorbed butadiene intermediates on y-Fe,O, are much less sensitive to gas-phase oxygen attack. Furthermore, Fig. 7 also shows that butadiene desorbs at a lower temperature on y- than on a-Fe203,while the desorption of CO, takes place at the same temperature. The consequences of

0

100

200

300

DESORPTION TEMPERATURE

400

500

(OC)

FIG.7. The fraction of butadiene and CO, precursosr left adsorbed on a-Fe,O, and y-Fe20, as a function of desorption temperature in a butene adsorption experiment. Filled circles are for adsorption-desorption experiments without oxygen, open circles are for those in which the desorption process was interrupted for passage of an oxygen pulse over the catalyst at the indicated temperatures. From Ref. 18, reprinted with permission from Academic Press.

DEHYDROGENATION OF BUTENES

177

these differences are shown in Fig. 6.The evolution of butadiene on y-Fe203 has a higher rate and a weaker temperature dependence than on a-Fe,03. The evolution of CO, from y-Fe203has a strong temperature dependence,as on a-Fe203.The degradation reaction is practically nonexistent for y-Fe203. On this catalyst, for temperatures below T3,evolution of butadiene is much faster than evolution of CO, and the catalyst is very selective. The selectivity declines rapidly above T3 which may be so high that y-Fe,03 is not stable and converts to a-Fe,O,. V.

Role of Lattice Oxygen

Unlike propene oxidation to acrolein or butene oxidation to maleic anhydride, oxygen is not incorporated into the selective oxidation product butadiene. However, water is formed together with butadiene, and it could conceivably be formed with lattice oxygen. There have been no isotopelabeling experiments to elucidate this. Similarly, it is not known whether the formation of any of the combustion products involves lattice oxygen. For the ferrite catalysts that have been investigated at sufficiently high temperatures (14,15,19-21), the production rates of butadiene and CO, are about the same in the presence and absence of gas-phase oxygen, until the catalyst is overly reduced at which point the rates of the oxidation reactions decline rapidly. On ZnCrFeO, (19) and a- and y-Fe,03 (14,20), it was found that an equivalence of about 2 layers of lattice oxygen can be removed before the catalysts are overly reduced. This number of 2 layers happens to be the same as the amount of lattice oxygen that exchanges rapidly with CO, (21-23). At higher temperatures, when the diffusion of Fe ions is rapid, a reasonable rate of butadiene production is maintained on Fe,03 until the oxide is reduced to a form close to Fe,O,.

VI.

Kinetics and Mechanism

The reaction orders of the butene oxidation reaction have been determined for a number of ferrites. They are listed in Table 11. In general, the reaction has a zero order or small positive order in oxygen, a positive order in butene, and a negative order in butadiene. The zero order in oxygen is not too surprising because oxygen is only indirectly involved in the production of butadiene, a product that does not contain oxygen. The pressure of oxygen could affect the degradation of butadiene precursor and the reoxidation of the catalyst. For a selective catalyst, the degradation process is a

178

HAROLD H. K U N G A N D MAYFAIR C. K U N G

minor process, and moderate changes in its rate would not affect the total rate of butene oxidation significantly. The rate of reoxidation, together with the rate of reduction, determines the oxidation state of the catalyst at steady state. It will affect the catalytic activity if the activity depends sensitively on the oxidation state of the catalyst. In the cases examined, on a- and y-Fe,O, (5,14,20,24), ZnCrFeO, (19), and MgFe,O, ( 1 9 , at the temperatures used in Table 11, the butadiene production rate, measured in experiments using butene pulses without oxygen, does not decrease sharply in consecutive pulses. This could mean that the catalytic activity does not depend sensitively on the oxidation state of the catalyst, or that the rate of reoxidation of the surface by diffusion of lattice oxygen from the bulk is rapid to maintain the oxidation state of the surface. Both cases would result in a zero order dependence in oxygen partial pressure. For the latter possibility, it has been shown that CoFe,O, is maintained at close to the fully oxidized state under the reaction conditions (25). Measurements of the order in butene are complicated by the negative order in butadiene, which makes the rate of reaction dependent on the conversion for a given butene partial pressure. Nonetheless, the positive order in butene appears general. This order in butene depends both on the temperature and the pressure. It usually decreases with increasing butene pressure or decreasing temperature (14,20), which can be interpreted as the dependence of the rate on the surface coverage of butene, and that the slow step involves butene and not the reoxidation of the catalyst. (The term slow step is used here instead of rate limiting step because from the available data, it appears that none of the reverse rates of the intermediate steps in the selective oxidation pathway is significant.) This will be further discussed later. The negative order in butadiene is also known as product inhibition. It results from the strong adsorption of butadiene relative to the other reactants, particularly butene. This is consistent with the temperature programmed desorption results described earlier-below 200"C, butadiene evolution is desorption limited. Because butadiene desorption has a higher activation energy than the reactions leading to the formation of adsorbed butadiene, the temperature dependence of its rate is also higher. At sufficiently high temperatures, some other rates in the reaction sequence may become equally slow. The activation energies for butene oxidation were determined for the ironpromoted bismuth molybdates (26). Below 4WC, the value is 160 kJ molwhich decreases to 40kJmol-' above 400°C. A similar change in the activation energy from 209 to 75 kJmol-' is also observed on Bi,(FeO,)(MoO,), (27). The higher value at lower temperatures is attributed to the inhibition effect of butadiene. The activation energies for u- and y-Fe203 are about 25 and 15 kJ mol- respectively, at about 300°C (14).

',

',

DEHYDROGENATION OF BUTENES

179

The molecular mechanism of the selective oxidation pathway is believed to be the one shown in Scheme 2 (Section I). Adsorbed butene forms adsorbed n-ally1 by H abstraction in much the same way as a-ally1 is formed from propene in propene oxidation (28-31). A second H abstraction results in adsorbed butadiene. Indeed, IR spectroscopy has identified adsorbed ncomplexes of butene and n-ally1 on MgFe,O, (32,33). On heating, the n-complex band at 1505cm- disappears between 100-200"C, and the n-ally1 band at 1480 cm-' disappears between 200-300°C. The formation of butadiene shows a deuterium isotope effect. The ratio of the rate constants for normal and deuterated butenes, k,/k,, is 3.9 at 300°C and 2.6 at 400°C for MgFe,O, ( 1 3 , 2.4 at 435°C for CoFe,O,, and 1.8 at 435°C for CuFe,O, (25). The large isotope effects indicate that the breaking of C-H (C-D) bonds is involved in the slow reaction step. The involvement of C-H bond breaking in the slow step is also illustrated by comparing the oxidation rates of various substituted butenes (14). On a-Fe203,the relative rates at 270°C are 1.3:1.0:0.9:0.7 for C-C-C(C)-C, C=C-C-C, C=C(C)-C-C, and C-C(C)=C-C. On y-Fe203, the relative oxidation rates at 180°C for the first two compounds are also 1.3: 1.0. When the relative reactivities are normalized to one hydrogen atom at the allylic position, they are found to be 1,4, and 12 for primary, secondary, and tertiary C-H. Thus, the formation of the allylic cation or radical intermediate is involved in the slow step. While abstraction of H to form the a-ally1 radical is one reaction step, the reverse reaction of hydrogenation of n-ally1 to form adsorbed butene probably does not take place. This is because when a mixture of trans-2-C4H, and trans-2-C,D8 was oxidized over MgFe,O, (15)and CoFe,O, (25),there was no H/D isotope mixing found in butadiene. The reverse of H abstraction would cause H/D mixing in the hydrocarbon, unless the active sites are well separated and the surface mobility of H and D is slow. The active site densities are indeed low (see Table IV). But at around 350 to 400°C, the mobility of adsorbed H and D should be high. Therefore it is more feasible to assume that the H-abstraction step is not reversible. This assumption implies that isomerization of butene is a pathway separate from selective oxidation. Some experimental data are in support of this. Using substituted butenes, it was found that tertiary substitution at the allylic position suppresses the isomerization rate (14). This suggests that isomerization proceeds via an allylic anion, although this anionic allylic assignment is inconsistent with the fact that cis- and trans-2-butene are formed from 1-butene with a ratio of about unity (7,14,25). When a mixture of C,H8 and C,D, is used, isomerization is accompanied by a small amount of H/D exchange in the butene isomers, unlike oxidation (14). The deuterium isotope effects on the isomerization rates are usually smaller than those on oxidation (15,25).

180

HAROLD H. KUNG AND MAYFAIR C. KUNG

Finally, it has been found that isomerization generally proceeds more rapidly when a ferrite catalyst is too reduced to be active in oxidation. It has also been shown that even on oxidized ZnCrFeO, (19) and MgFe,O, ( 1 9 , isomerization is faster in the absence than in the presence of gaseous oxygen. From these results, it has been suggested that a surface Fez+ ion is the active site for isomerization. Less is known about the combustion pathway. Using IR spectroscopy, formate, carboxylate, bidentate carbonate, monodentate carbonate, and acetate bands have been observed when butene is adsorbed on a-Fe2O3and MgFe,O, in the absence and presence of 0,(32,33). Surface degradation (oxidation) of butadiene has also been observed on u-F~,O, at room temperature (34).

VII.

Factors Affecting Selectivity: Roles of Crystal Structure, Nature of Transition Metal Ions, and Effect of Promoters

An inspection of Table I1 shows that the selectivity for butadiene can be increased by converting ct-Fe203 to y-Fe203,or by adding other oxides to ct-Fe,O,. Addition of oxides such as ZnO, COO, MgO, Cr,03, Bi,O,, Sb205,V,O,, MOO,, and P 2 0 5 have resulted in an increase in selectivity, and in most cases activity as well. In the attempt to understand what causes the higher selectivity, several questions have to be answered. What aspects of the butene oxidation reaction are affected by the crystal structure? Why is Fe,03 the most selective catalyst among the first period transition metal oxides? And what are the functions of the promoters? Selectivity is determined by a competition between the rate of butadiene production and the rate of degradation of butene and other hydrocarbon intermediates. Various investigators have considered factors affecting selectivity (31,35-37). Generally speaking, along the selective oxidation pathway, the first abstraction of H to form A-ally1 can be viewed as a combination of an acid-base and a redox reaction; R- -H+ R-

+ 0'+ Mn+

R-

+ OH-

R+M(n-z)+

The acid-base reaction involves the lattice 0'-.For a given hydrocarbon, Indeed, the proton transfer rate is enhanced by an increased basicity of 0'-.

181

DEHYDROGENATION OF BUTENES

in the ammoxidation of propene, the production rate of acrylonitrile increases with basicity of the lattice oxygen among the promoted antimony oxides, when the basicity is measured by the kinetic energy of the Auger electrons of oxygen (38). This is the basis for discussing the promoter effects in terms of the acid-base properties of the oxide. The redox component of the H-abstraction reaction is schematically shown as a two-electron transfer step to a metal ion to result in an allyl cation. It could well be that only one electron transfer occurs and an allyl radical is formed, or that the twoelectron transfer is to two adjacent metal ions, In fact, allyl radicals have been detected in propene oxidation over bismuth molybdates (39). In either case, the redox properties of the oxide will affect this step. The H-abstraction reaction, however, does not determine selectivity entirely. Depending on the experimental conditions, as discussed earlier, butadiene desorption could be one important step. In this case, the electronic properties of the transition metal ions that determine the interaction with the unsaturated hydrocarbon have to be considered. The combustion reaction has been proposed to be due to electrophilic attack of the carbon-carbon double bond by an adsorbed electrophilic oxygen species, such as 0-or Oz-, to form the highly reactive peroxide, aldehyde, or similar species. The ability of the oxide to activate oxygen to these forms is important. Based on the data on butene oxidation on ferrite catalysts, a comparison can be made among several ferrite catalysts using data obtained under similar conditions, as is shown in Table VIII. Referring to the table, it is seen that the butadiene desorption temperatures are about the same for cl-FezO, and TABLE VIII Comparison of Different Ferrite Catalysts in Butene Oxidation"

Oxide

Crystal structure

a-Fe,O, FeCrO, y-Fe,O, ZnFe,O,

Corundum Corundum Spinel Spinel

C4H6 desorption temperature (°C)b

Sensitivity to 0, pulse

C4H6 formed lst/2nd pulse'

88

Yes

-

-

4.3 4.4 1.7" 1.6

-

28 88'

No -

From ref. 18, reprinted with permission from Academic Press. Temperature at the midpoint of the butadiene evolution curve in the thermal desorption of butene, as in Fig. 7. Ratios of the amount of butadiene formed in experiments where pulses of butene were passed over the catalyst at 200°C in a stream of He. Ratio of the first to the tenth pulse; from ref. 14. From ref. 8.

182

HAROLD H. KUNG AND MAYFAIR C. KUNG

ZnFe,O,, but lower for y-Fe,O,. The Fej' ions in the former two catalysts are in octahedral sites, while those in y-Fe,03 are distributed in tetrahedral and octahedral sites. It could be that surface-exposed tetrahedral Fe3 ions bind hydrocarbons less strongly than octahedral Fe3+ions, and that they are the active sites in y-Fe,03 for butadiene production. On the other hand, the exposed octahedral Fe3+ ions are the active sites on a-Fe,O, and ZnFe,O,. A rough correlation has also been observed between the fraction of tetrahedral Fe ions and the production rate of butadiene on the Cr substituted MgFe,O, (19). Since the binding of hydrocarbons involves the properties of the surface ion only, the effect of the bulk structure is only indirect. However, the decrease in butadiene production with pulse number in a pulse reaction experiment at 200°C can be directly correlated with the bulk structure. This indicates that the amount of butadiene produced in subsequent pulses depends on the rate of regeneration of the surface sites consumed in the previous pulse. Such regeneration involves reoxidation of the site by diffusion of lattice oxygen from the bulk to the surface; its rate depends on the crystal structure. In an ideal spinel structure, alternate rows of octahedral holes are vacant, permitting rapid diffusion of the Fe cations from the surface to the bulk (which is equivalent to surface oxidation). Similar channels are not available in the corundum structure for cation diffusion. It is interesting that among the first period transition metal oxides at their highest oxidation states, iron oxide is the most selective catalyst for the production of butadiene (20,40). Indeed, isomerization of butene is the predominant reaction on TiO,, Cr,O,, and ZnO that are low in oxidation activity. On oxides that have high oxidation activity, MnO, and CuO primarily catalyze combustion, while NiO and Co,O, are somewhat selective, and Fe,03 is the most selective. It has been suggested that such a trend can be correlated with the heat of adsorption of butene: isomerization takes place on catalysts that adsorb butene weakly, nonselective oxidation (combustion) takes place on catalysts that adsorb butene strongly, and selective oxidation takes place on catalysts that adsorb butene moderately strongly or, in some cases, on sites of different strengths (41). Unfortunately, heats of adsorption of butene on these oxides have not been measured. However, some data are available on the IR band shifts of the C=C bond on adsorption. Since the magnitudes of the shifts can be correlated to the heats of adsorption (42), a correlation between the C=C stretching frequency shift and the selectivity can be attempted. IR data of butene adsorption are only available for ZnO (43) and Fe,03 (32,33).A more extensive set of data is available for propene adsorption. Due to the similarity between these two compounds, the data for propene are used. Even then, data for only a limited number of oxides are found, and different investigators do not necessarily agree. Nonetheless, some information can be obtained using self-consistent +

DEHYDROGENATION OF BUTENES

183

sets of data (such as those obtained by the same investigator). Davydov et al. reported C=C bond band shifts of 14 cm-' for TiO,, 15 cm-' for COO in MgO-MOO,, 20 cm-' for NiO in MgO-MOO,, and 25 cm-' for ZnO (42,44,45).Isomerization is the primary reaction on TiO,, COO,and ZnO. It is also significant on NiO, although it also shows some selective oxidation activity. In another set, the shifts are found to be 30 cm-' on ZnO (46), 31 cm-' on TiO,, and 46 cm-' on a-Fe,O, (34). It is clear that the largest shift is observed on the most selective oxidation catalyst. It would be nice if a more extensive set of consistent data were available to test this correlation more vigorously. A successful correlation is obtained for the combustion reaction. For the first period tansition metal oxides, it is found that the combustion activity increases with decreasing heat of reduction (per atom of 0) of the oxide to the next lower oxidation state (40). This correlation implies that lattice oxygen participates in the combustion reaction. Similar successful correlations have been observed for the combustion of CO, H,, CH,, and propene (47). The reaction of butene has also been investigated as a function of the degree of reduction of the transition metal oxide, by passing successive pulses of butene over the oxide. On MnO, at 3WC, both the 1-butene conversion and the CO, production decrease with increasing pulse number, while the butadiene production and isomerization increase (40). On Co,O, at 400°C, both butadiene and CO, production decrease with pulse number, while isomerization increases rapidly. The behavior of a-Fe203 depends on the temperature. Below 2OO0C, the butadiene production rate decreases with pulse number (24). Above 275"C, the butadiene production first increases with pulse number and then decreases (20,24). The behavior of Fe,O, at high temperature, and perhaps MnO, also, may indicate that some surface reduction is helpful to limit the number of active lattice oxygen atoms that, if present in an excess number, would result in combustion (36,48). The behavior may also indicate surface atomic rearrangement. In the case of a-Fe,O,, a butene pulse reduces a few layers of the surface to Fe,O,. At high enough temperature, diffusion of Fe ions is rapid and the surface is rapidly reoxidized to y-Fe,O,, which has the same spinel structure as Fe,O,. As described before, the y form of iron oxide is a more selective oxidation catalyst. Thus, the butadiene production rate increases. When the oxide is exposed to a large number of butene pulses, it is so excessively reduced that the surface can no longer maintain the proper oxidation state, and the oxidation activity declines. The above discussion brings out the importance of the crystal structure and of the nature of the transition metal ion. These factors determine the bonding of butene and the redox behavior of the catalyst. They can be used to

184

H A R O L D H. K U N G A N D MAYFAIR C. K U N G

understand the functions of promoters which usually enhance the selectivity of the iron oxide catalyst. A typical list of promoters that have been reported in the scientific literature is shown in Table 11. They include Mg, Cr, Co, Zn, Bi, Mo, P, Sb, and V. The functions of these promoters differ. It has been shown that the addition of Cr, Mg, and Zn increases the resistance to reduction of the ferrite (12). This is important in preserving the desirable crystal structure of the catalyst. Co, Zn, and Mg form spinel ferrites with Fe,03, the spinel being a more desirable structure than corundum. The promoting effect of Mo, V, and P has been interpreted by changes in the acidity and basicity of the iron oxide catalysts. According to Ai (49,50), both butene and butadiene are basic molecules due to their rather large ionization potentials. A selective catalyst should be moderate in both acidic and basic strength. However, it should be sufficiently acidic to adsorb and activate butene, sufficiently basic to allow butadiene to be desorbed readily, yet not too basic to activate adsorbed oxygen that degrades the hydrocarbons. Iron oxide is too basic. Addition of the oxides of Mo, V, and P increases the acidity of the catalyst, thereby enhancing the activation of butene and resulting in a higher activity for butadiene production. Simultaneously, the reduction in basicity reduces the activation of gaseous oxygen by the basic sites that degrade the adsorbed hydrocarbons. The net result is enhanced selectivity. This intrepretation is consistent with the observation that addition of K,O lowers the selectivity and the activity of the catalyst (49), and that basic Bi203 and acidic PzOs tend to cancel the effect of each other (49). It should be mentioned that this model has been advanced to explain reaction rate and selectivity data. There are no independent observations on changes in the adsorption strengths of butene and butadiene due to the addition of promoters, nor are there investigations of new compound formation or distributions of the promoters. Addition of a promoter can result in the formation of a new compound that is active and selective. This is the case for Sb and As promotion (41). FeSbO, and FeAsO, are both selective catalysts. In the Fe-Bi-Mo-0 catalyst, the compound Bi,(FeO,)(MoO,), has been identified and has been found to be active and selective (5I,52). Finally, the effect of the support in supported Fe,O, catalysts has been investigated (53). SiO,, TiO,, Al,O,, and their mixed oxides were used as supports. In general, the activity per gram of catalyst varies widely over two orders of magnitude, but the selectivity for butadiene in oxidation remains in the range between 65-88 %. It was also observed that the oxidation activity, both selective and nonselective, is suppressed drastically by the presence of sulfate ions in the support. The reasons for these observations are not yet known.

185

DEHYDROGENATION OF BUTENES

VIII.

Effect of Crystallite Size

There is yet another way to change the selectivity-by changing the crystallite size of the iron oxide crystallites. Figure 8 shows the dependence of the selectivity on the crystallite size of cr-Fe,O, at low conversions (54). The selectivitiesfor butadiene depend on the temperature and the crystallite size. In general, between 300 and 375"C,they are higher for higher temperature and for smaller crystallite size. Beyond about 10 nm, the selectivity remains constant. These data were obtained using unsupported and silica supported samples that had been calcined in air at 2400°C.In view of the earlier discussion, it would seem necessary to be sure that the variation in selectivity shown in Fig. 8 is not due to other effects, such as the presence of y-Fe,O,, which can be stabilized by: (1) silica up to 500°C ( 5 9 , (2) possible reactions between iron oxide and the support, or (3)variations in the conversions. The dependence of the selectivity on the conversion at low conversions has been investigated by varying the amount of catalysts used and the flow rate of the feed. For conversions below lo%, the selectivity is slightly higher at lower conversion, although the difference is only a few percent. The areal rate (rate per unit surface area of iron oxide) of butene oxidation is slightly higher

-

loo[ 90

50t 0

10

20

30

40

50

60

CRYSTALLITE SIZE (nm)

0,0,

FIG.8. Dependence of the selectivity for butadiene on the crystallite size of or-Fe,O,. a-Fe,O,/SiO, at 300°C and 350°C. respectively: 0 , 0,a-Fe20, at 300°C and 350"C, respectively.

186

HAROLD H. KUNG AND MAYFAIR C. KUNG

at shorter residence time of the reactor, probably because there is a stagnant layer for diffusion of reactants or products into and out of the catalyst particles. The selectivity is not dependent on the residence time. The absence of the effect of pore diffusion was also confirmed when the areal rate and the selectivity were not changed when the most active supported catalyst was crushed into smaller particles. Broadly speaking, the areal rate appears to be higher for catalysts of smaller crystallite sizes. For the catalysts used in Fig. 8, the areal rates vary by a factor of 8 at 30O0C, and 5 at 350°C.The variations are probably larger than actual because of the errors in estimating the iron oxide surface areas. These surface areas are calculated from the average crystallite sizes as determined by X-ray line broadening. The X-ray technique cannot detect very small crystallites which are present in increasingly larger amounts as the crystallite size decreases. This results in an increasingly large underestimation of the specific surface area of iron oxide, which in turn results in increasingly higher areal rates. Therefore, the trend in the areal rate, if it exists at all, is smaller than that reported. The catalysts were characterized by X-ray diffraction, Mossbauer effect spectroscopy, and magnetization to determine the phases of iron oxide and the presence of iron silicate in the samples (56). The X-ray diffraction patterns and the magnetization curves for some typical samples as prepared are shown in Figs. 9 and 10, respectively. For all the samples studied except the smallest size (2.5 nm) sample, the X-ray diffraction patterns can be totally accounted for by u-Fe203.The diffraction pattern of the smallest size sample had poor signal-to-noiseratio and is inconclusive. Similar conclusions are derived from the Mosbauer spectra. For samples 14.5nm and larger, only a six-line magnetically split pattern is obtained. The magnetic hyperfine field, the quadrupole splitting, and the isomer shift are the same as the values for a-Fe203.A central doublet that is rather symmetrical due to superparamagnetic particles is present for the three smallest size samples. The area ratio of the superparamagnetic to the magnetic pattern increases with decreasing crystallite size until only the superparamagnetic pattern is observed for the smallest size sample. The superparamagnetic doublets have isomer shifts of about 0.3 to 0.35mmsec-', quadrupole splitting of about 0.68 to 0.806 mm sec- I , and linewidths at half maximum of 0.5to 0.6 mm sec- '. The presence of a sizable quadrupole splitting argues against y-Fe,O,, which should show zero quadrupole splitting (57-59). The relatively narrow linewidths and the symmetry argue against the presence of a broad mixture of different compounds or structural disorder such as in amorphous iron hydroxide gel. These results, together with the isomer shift that is indicative of Fe3+,do not support the presence of iron silicate. In fact, it has been reported

187

DEHYDROGENATION OF BUTENES c

..

. .

.. .. . .

...

. . * . ..

.*

..

..-. . . c

.".

*.

. . r

I I7

1

I

I

16

15

I+

1

29 FIG.9. X-Ray diffractiondata of a-iron oxide samples in Fig. 8 collected by pulse-counting mode using Mo K, radiation for samples of average crystallite size of (A) 14.5 nm, (B) 25 nm, (C) 7.5 nm, and (D) 9.5 nm. For spectrum C, the region from 13 to 16"was counted for loo0 sec/step. From ref. 56, reprinted with permission, copyright 0 1984 by the American Chemical Society.

188

H A R O L D H. K U N G A N D MAYFAlR C. KUNG

r- F

"0

10

20

~

30

~

MAGNETIC FIELO

o

~

40

(KW

50

FIG.10. Magnetization of oc-iron oxide samples in Fig. 8 at 1.7 K as a function of applied field strength. Sample size: (a) 2.5 nm, (b) 7.5 nm, (c) 9.5 nm, (d) 14.5 nm, and (e) 25 nm. The curve for y-Fe,O, is included for comparison. From ref. 56, reprinted with permission, copyright 0 1984 by the American Chemical Society.

that no silicate is formed after heating silica supported Fe,O, for 24 hr at 950°C (60). That neither y-Fe,O, nor iron hydroxide is present is also supported by the magnetization behavior of the samples (Fig. 10). Bulk y-Fe,O, is ferrimagnetic and exhibits saturation magnetization at relatively low magnetic field, and bulk a-Fe,O, is antiferromagnetic.All the samples larger than 14 nm behave like typical a-Fe,O,. For the smaller crystallite size samples, the magnetization increases with decreasing iron oxide crystallite size. Even for the smallest crystallite size sample, however, the saturation magnetization is only about 15.6 emu per gram of iron oxide, which is very low compared to bulk y-Fe,O,. The magnitude of magnetization suggests that the smallest size crystallites are ferromagnetic. This could be due to incomplete cancellation of the two-spin sublattice in an antiferromagnetic a-Fe,O, crystallite because of an odd number of lattice planes (61).Alternatively, incomplete cancellation could occur at the surface. It could also be due to the possible absence of the Morin transition in very small crystallites since the Morin transition temperature decreases with decreasing crystallite size (62,63). Thus, the magnetization can be explained without y-Fe20,. The shape of the magnetization curves is different from that for iron hydroxide which is sigmoidal(7) (see also Fig. 12). Thus, the data argue against the presence of iron hydroxide. Finally, the smallest crystallite size sample was subjected to severe thermal treatment. After calcining to 450, 500, 600, 780, and 920"C, the catalyst was

DEHYDROGENATION OF BUTENES

189

used in the butene oxidation reaction at 350°C.The specific activity (per unit weight) of the catalyst decreases after calcining at 780°C by over 60 %, but the selectivity for butadiene increases by less than 2 %. A similar observation is obtained for the 9.5 nm crystallite size sample. Since it is unlikely that y-Fe20, or iron hydroxide can be stable up to such high temperatures, these results strongly imply that these compounds are not present to affect the catalytic behavior of the catalysts. It has been reported that by heating in the presence of water vapor, silicon can be incorporated into the lattice of y-Fe,O, (55,64). It is unlikely that this effect occurs under the butene oxidation conditions since a physical mixture of large size iron oxide and silica gel retains its selectivity for butadiene after a prolonged reaction time without any indication of changes in the catalyst. If silicon substitution takes place, the water from the oxidation reaction could catalyze the substitution. In fact, deliberate pretreatment of such a physical mixture with water vapor before reaction does not change the activity or selectivity (54). In summary, the data shown in Fig. 8 indeed illustrate the dependence of selectivity on the crystallite size of a-Fe203. There is no evidence for the presence of y-Fe,03, iron hydroxide, or iron silicate that could affect the data. The reasons for the crystallite size effect are not known. There are several possibilities. If the selective oxidation reaction is crystal face specific, then the size effect is due to the fact that different proportions of various crystal faces are present on crystallites of different sizes. It is known that small crystallites supported on silica are more difficult to reduce than large crystallites. This different reducibility can contribute to the size effect. It is possible that the charge transfer ability of small crystallites is different. Since charge transfer is involved in the activation of gaseous oxygen which is active in degradation of surface intermediates, this could also be a contribution. It is clear that further work is needed to distinguish the possibilities.

IX. Other Structure-Sensitive Oxidation Reactions While the effect of crystallite size has been investigated for reactions on iron oxide, the dependence of the activity and selectivity of other oxidation reactions on the nature of the exposed surface planes has been investigated for reactions on MOO, and V,O, catalysts. A list of these reactions, the catalysts used, and the major conclusions are listed in Table IX. It appears that all the reactions listed are structure sensitive, that is, different crystal faces catalyze different reactions, or special active sites are required.

TABLE IX Structure Sensitive Oxidation Reactions on Transition Metal Oxides

-

Reaction

Temperature (K)

Catalyst

Comments

References

Selectivity for maleic anhydride increases with the thickness of V,O, layers on the support Reaction rate is proportional to the density of surface V=O Selectivity for maleic anhydride increases with the thickness of V 2 0 5 layers on the support V=O on the (010) face has low activity; surface defects have high activity QV on the (010) face is the active site V = O on the (010) face is the active site

65

\o 0

Oxidation of benzene to maleic anhydride

613-723

Oxidation of I-butene and of butadiene to maleic anhydride CO oxidation

713-763

613-745

C2H, oxidation H, oxidation

713-773 713-773

Oxidative dehydrogenation of butene

573-648

V,O,/TiOz, v2°5/A1203

Selectivity for butadiene increases with decreasing Fe,O, crystallite size

65

68 67 66 54

Selective oxidation of propene Ethanol oxidation and dehydration in 0,

648

Ethanol oxidation and dehydration in absence of

488

MOO,

564

0 2

'0

Methanol oxidation and dehydration in 0,

564

MOO,, MoOJgraphite

Methanol oxidation and dehydration in absence of 0, Selective oxidation of o-xylene

488

MOO,, M o o ,/graphite

Acrolein is produced from sites on the (100) face, CO, from the (010) face Oxidative dehydrogenation to acetaldehyde is not face specific, dehydration is on apical (001) and (101) planes Dehydrogenation mostly on basal (010) and apical planes, dehydration on (100) and apical planes Dehydrogenation to formaldehyde on basal (010) and apical planes, dehydration on apical plane, bifunctional to produce methylal on (100) plane Dehydration on (100) and apical planes, bifunctional on apical plane Selective oxidation to phthalic anhydride on (010) plane, combustion on (110) plane

73 69, 71

69, 71

69, 70

69, 70

35

192

H A R O L D H. K U N G A N D MAYFAIR C. K U N G

The structures of MOO, and V205are similar. Both are made up of chains of M 0 6 octahedra that are linked to form a layered structure. Interaction between layers is relatively weak. The surface of these two-dimensional layers forms the basal (010) plane on which isolated terminal V=O or Mo=O groups are found. These terminal groups show characteristic IR absorption frequencies that are easy to follow. In many cases, these groups form the active centers. Among the reactions on V205,it is interesting to note that while the number of terminal V=O groups can be directly correlated with the catalytic activity in benzene, H,, and C,H, oxidations (65-67), thereby suggesting that these V=O groups are the active sites, such a correlation does not exist for CO oxidation. In fact, the data for CO oxidation suggest that sites associated with V=O groups have low activity, and the active sites are proposed to be surface defects (68). In the case of benzene oxidation, although the activity is proportional to the density of V=O groups, the selectivity for maleic anhydride increases with the thickness of the V205layer on the TiO, support. Similar dependence on the V,O, thickness is also found for the selectivity for maleic anhydride in the oxidation of butene and butadiene (65). Clearly, in addition to influencing the properties of V,O, by inducing oriented epitaxial crystal growth to expose the basal (010) plane, the TiO, support affects the nature and reactivity of the V=O groups on these planes. MOO, catalyzes the oxidation and dehydration of methanol and ethanol. A summary of these data suggests that the basal (010) plane is only active for dehydrogenation, and it is much more active in the presence of gaseous oxygen (69-71). The active site is probably associated with the terminal Mo=O groups, In the absence of O,, hydrogen abstraction occurs on this surface, but water is not formed. The H accumulates on the surface to form bronze H,MoO,. The apical (001) and (101) planes are active in dehydrogenation and dehydration both in the presence and absence of gaseous oxygen. Since the fraction of exposed surface being apical planes is small, the reactivity of these faces affects the observed products to only a limited extent. The (100) face catalyzes dehydrogenation in the presence of gaseous oxygenwhen the Mo=O groups exist. In the absence of oxygen, this face catalyzes dehydration where the active sites are the exposed Mo cations which act as Lewis acid sites. The explanation provided for MOO, is entirely phenomenological. Recently, it was proposed that the reactivity of various crystal surfaces can be estimated using the bond-strength model of active sites (72). A different ordering of the activity of the different crystal planes is obtained for propene oxidation than is obtained from the phenomenological approach. It will be interesting to apply this model to the data on methanol and ethanol oxidation.

DEHYDROGENATION OF BUTENES

X.

193

Catalytic Activity of Iron Hydroxide

During the course of studying the effect of crystallite sizes, attempts were made to produce very small unsupported iron oxide powders by lowering the calcination temperature of the iron hydroxyl gel that was precipitated from iron nitrate with ammonium hydroxide. However, catalysts calcined below 300°C still contain hydroxide, and they show high selectivity in butadiene production. For this reason, two catalysts, calcined at 250°C and 300"C, respectively, were studied in more detail. The presence of hydrous ferric oxide (FeOOH) has been confirmed by Xray diffraction, Mossbauer spectroscopy, and magnetization measurements (7).Figure 11 shows the X-ray diffraction patterns for the region of 2 0 between 14 and 18". The two peaks at 15.2 and 16.3"are diffractions from a-Fe,O,, but the large background is from the various forms of hydrous ferric oxide. The lack of fine structure excludes the possibility of identifying

-

I

I

,

I

I

lal

I

I

I

1

1 . 16.5

14.5

18.1

28 FIG.11. X-Ray diffraction patterns of iron hydroxide gel (a) before calcination, (b) after calcination at 250"C, (c) after calcination at 300°C. Also shown are the diffraction lines expected for K- and y-Fe,O,, and various FeOOH.

194

HAROLD H. KUNG AND MAYFAIR C. KUNG

1

20

-

0 '6-

\

Y

c

m

10

20

50

40

10

MAGNETIC FIELD (KOe)

FIG. 12. Magnetization measurements of iron hydroxide gel (a) before calcination, (b) after calcination at 250"C, (c) after calcination at 300°C. Also shown are measurementsfor a-Fe,O,.

which forms are present. It is likely that several forms are present simultaneously with the a-form being dominant. The magnetization curves of these samples are shown in Fig. 12. Characteristic of the hydroxide is the sigmoidal shape of the curves. This shape is retained in the 250°C calcined sample, but has almost disappeared for the 300°C sample. The Mossbauer patterns are consistent with the other results. Although the 250°C sample was not examined, a 200°C calcined sample shows a center superparamagnetic doublet and two sets of magnetically-split six-line patterns. One set of the six-line patterns is identified to be from a-Fe,O,. The other set is probably from a-FeOOH, 6-FeOOH, and/or amorphous iron hydroxide. /3- and y-FeOOH do not exhibit magnetic hyperfine splitting. After calcining at 30O0C, the relative intensity of the six-line pattern of a-Fe,O, grows, while that of the six-line pattern of FeOOH decreases. When the 250°C calcined sample was tested for the butene oxidation reaction at 250"C, butadiene was produced very selectively ( > 80 %). The selectivity decreases from about 90 % initially to 80 % after 12 hr of reaction (Fig. 13). The initial activity is high, but decreases rapidly in the first 2 hr. Regeneration of a catalyst after 13 hr of use by 0, at 250°C recovers the selectivity, but only half of the initial activity. Specific magnetization of the deactivated sample is lower than for a fresh sample, but the shapes of the magnetization curves are the same. Similar behavior was observed for the 300°C calcined sample. The initial activity and selectivity at 300°C are high (Fig. 14). The activity decreases

195

DEHYDROGENATION OF BUTENES

1

TIME (h)

FIG.13. The activity in 1-butene oxidation, and the selectivity for butadiene of a 250°C calcined iron hydroxide gel (7).

slowly with time on stream. The selectivity decreases from 78 % to a steady value of about 55 %. Treatment with 0, at 300°C only partially regenerates the activity and selectivity. The magnetization curve of the used catalyst is very different from that of a fresh catalyst shown in Fig. 12. It can now be described by a combination of 2.5 wt % y-Fe203 and 97.5 wt % a-Fe,03. Clearly, at the higher temperature of reaction, the iron hydroxide is converted to iron oxides.

1

401

O'O

I

1

io

o;

I

I

I

30 TIME (h)

60

50 0

10

FIG.14. The activity in 1-butene oxidation, and the selectivity for butadiene of a 300°C calcined iron hyroxide gel (7).

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HAROLD H. KUNG AND MAYFAIR C. KUNG

These results show that iron hydroxide, although thermally not very stable, is a very active and selective catalyst for butene oxidation. It will be interesting if the thermal stability can be improved such that high conversions can be achieved with high selectivity.

XI.

Conclusions

In this review, we attempt to summarize some of the recent advances in the understanding of factors that determine selectivity in the selective oxidation of hydrocarbons. Although most of the discussions involve the oxidative dehydrogenation of butene to butadiene on ferrite catalysts, the principles are applicable to other oxidation systems. In this regard, it has now been demonstrated that the selectivity depends on a variety of factors ranging from the nature of the transition metal ion which determines the interaction of butene and butadiene on the catalyst and the ability to activate gaseous oxygen to form a highly reactive surface species, the crystal structure which determines the reduction and oxidation properties, the crystallite size, and the orientation of the exposed crystal planes. Depending on the presence or absence of gaseous oxygen and on the temperature of operation, different factors determine selectivity. Therefore, understanding of the adsorbatesurface interaction, the reaction mechanism, and the solid state properties of the catalyst is necessary to explain and predict the catalytic behavior over a wide range of operation conditions. It is to this end that catalysis research has been and will be heading. ACKNOWLEDGMENTS This work has been supported by the Department of Energy, Basic Energy Sciences Division.

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7. 8. 9. 10. 11.

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52. Notermann, T., Keulks, G. W., Skliarov, A., Maximov, Yu., Margolis, L. Ya., and Krylov, 0. V., J. Catal. 39, 286 (1975). 53. Rodenas, E., Iizuka, T., Katsumata, H., and Tanabe, K., React. Kinet. Catal. Lett. 19, 341 (1982). 54. Yang, B. L., Hong, F., and Kung, H. H., J. Phys. Chem. 88,2531 (1984). 55. Lund, C., and Dumesic, J. A., J. Phys. Chem. 85, 3175 (1980). 56. Hong, F., Yang, B. L., Schwartz, L., and Kung, H. H., J. Phys. Chem. 88,2525 (1984). 57. Coey, J. M. D., and Khalafalla, D., Phys. Status Solidi A 11, 229 (1972). 58. Obermeyer, R.T.. Mulay, L. N., Lo, C., Oskovie-Tabrizi, M., and Rao, V. U. S., J. Appl. Phys. 53, 2683 (1982). 59. Hrynkiewicz, A., Pustowka, A., Sawicka, B., and Sawicki, J., Phys. Status Solidi A 9, 607 (1972). 60. Malathi, N., and Puri, S., J. Phys. Soc. Jpn. 31, 1418 (1971). 61. Neel, L., Proc. lnt. Con/: Magn. Crystallogr. 1961,676 (1962). 62. Nininger, R. C., and Schroeber, D., J. Phys. Chem. Solids 39, 137 (1978). 63. Yamamoto, N., J. Phys. Soc. Jpn. 24,23 (1968). 64. Lund, C. R. F., and Dumesic, J. A., J. Phys. Chem. 86. 130 (1982). 65. Mori, K., Inomata, M., Miyamoto, A., and Murakami, Y., J. Phys. Chem. 87,4560 (1983). 66. Mori, K., Miura, M., Miyamoto, A., and Murakami, Y., J. Phys. Chem. 88,5232 (1984). 67. Mori, K., Miyamoto, A., and Murakami, Y.. J. Phys. Chem. 88,2741 (1984). 68. Mori, K., Miyamoto, A., and Murakami, Y., J. Phys. Chem. 88,2735 (1984). 69. Tatibouet, J. M., Germain, J. E., and Volta, J. C., J. Caial. 82, 240 (1983). 70. Tatibouet, J. M., and Germain, J. E., J. Catul. 72, 375 (1981). 71. Tatibouet, J. M., and Germain, J. E., J. Chem. Res. Synop. S, 268 (1981). 72. Ziolkowski, J., J. Catal. 80, 263 (1983). 73. Volta, J.-C.,and Forissier, M., Faraday Discuss. Chem. Soc. 72, 225 (1982).

ADVANCES IN CATALYSIS. VOLUME 33

The Study of Aluminosilicate and Related Catalysts by H igh.ResoIut ion Sol id=State N M R Spectroscopy J . M . THOMAS A N D J . KLINOWSKI Department of Physical Chemistry University of Cambridge Cambridge. England

I. Introduction . . . . . . . . . . . . . . . . . I1. Relevant Spectroscopic Considerations . . . . . . . . . . A . Interactions in the Solid State . . . . . . . . . . . B. Dipolar Interactions . . . . . . . . . . . . . . C. Chemical Shift Anisotropy . . . . . . . . . . . . D. Quadrupolar Interactions . . . . . . . . . . . . E . Cross-Polarization . . . . . . . . . . . . . . F . Multinuclear Applications of NMR . . . . . . . . . I11. Solid-state NMR Studies of Zeolites . . . . . . . . . . A . Outline of the Structure and Properties of Zeolites . . . . . B. 29Si NMR of Silicates and Aluminosilicates . . . . . . . C. Determination of the Composition of the Aluminosilicate Framework Using 29Si MAS NMR . . . . . . . . . D . Silicon-Aluminum Ordering in Zeolites X. Y. and A . . . . E. *’Si MAS NMR of Gallosilicate Zeolites . . . . . . . . F . Highly Siliceous Zeolites . . . . . . . . . . . . G . Resolving Crystallographically Nonequivalent Tetrahedral Sites . H . Factors Determining Resolution. Line Shape. and Relaxation . . I . ”A1 NMR Studies of Zeolites . . . . . . . . . . . J. Decationation and Ultrastabilization . . . . . . . . . K . NMR Studies of Zeolitic Acidity . . . . . . . . . . L . The Mechanism of Dehydroxylation of Zeolites . . . . . . M . Isomorphous Substitution in the Zeolitic Framework . . . . N . Precursors in Zeolite Synthesis . . . . . . . . . . . 0. NMR Studies of Exchangeable Cations . . . . . . . . P. Interactions between Zeolites and the Adsorbed Species . . . Q. Probing the Nature of Zeolitic Microstructure by High-Resolution NMR . . . . . . . . . . . . . . . . . . R . Studies of supported. Finely Dispersed Metal Particles Using 29Xe NMR . . . . . . . . . . . . . . . . . . IV . Silica-Alumina Gels . . . . . . . . . . . . . . . V . Derivatized Surfaces and “Immobilized” Homogeneous Catalysts . . I99

200 202 202 203 204 205 209 212 213 213 218 226 228 233 236 240 251 254 264 272 281 282 289 296 300 308

317 320 327

Copyright 0 1985 by Academic Prcss. Inc. All rights of reproduction in any lorn reserved .

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J. M. THOMAS AND J. KLlNOWSKl

VI. Probing Supported Metal Catalysts by NMR without Utilizing HighResolution Techniques. . . . . . . . . . . . . . . VII. Bond Lengths, Structure, and Mechanism in Heterogeneous Catalysis and in Chemisorbed States . . . . . . . . . . . . . VIII. Sheet Silicates and Their Pillared Variants . . . . . . . . . A. General Comments . . . . . . . . . . . . . . B. Structural Characteristics . . . . . . . . . . . . C. Elucidating the Nature of the Catalytic Performance of Layered Silicates by NMR. . . . . . . . . . . . . . . D. I3C and 'H studies . . . . . . . . . . . . . . E. *'A1 and 29SiStudies. . . . . . . . . . . . . . F. Pillared Clays . . . . . . . . . . . . . . . . 1X. Recent Trends in the Study of Catalytic Solids by NMR . . . . . A. General Comments . . . . . . . . . . . . . . B. NMR Studies of Carbonium Ions . . . . . . . . . . C. Two-Dimensional NMR . . . . . . . . . . . . . D. NMR Imaging . . . . . . . . . . . . . . . E. Zero-Field NMR . . . . . . . . . . . . . . . F. Solid-state NMR as a Means of Elucidating Enzymatic Reactions . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

1.

331 333 335 335 336 339 340 342 345 346 346 347 349 3 54 355 357 361

Introduction

Since its discovery in 1945, there has been an enormous growth in the range of applications of nuclear magnetic resonance (NMR) spectroscopy. Part of this growth is attributable to the increased sensitivity with which NMR spectra may now be recorded. Much of this sensitivity has, in turn, been achieved by sophisticated technical developments such as the construction of superconducting magnets with very high and very uniform magnetic fields; the arrival of low-cost, ultra-fast digital computers; the construction of devices for very rapid spinning of samples in the magnetic field; and, not least, by the development of Fourier transform NMR spectroscopy. Other factors responsible for the pervasive importance of NMR in chemical sciences today are the ingenious ways which have been evolved for recording spectra entailing such features as spin-echo, cross-polarization, multiple pulse techniques, double and triple resonance, two-dimensional (2D) techniques, multiple quantum experiments and, most recently, zero-field NMR. As is well known, the usefulness of NMR in chemistry in general, and in heterogeneous catalysis in particular, rests on the fact that the chemical shifts, intensities, multiplicities, and widths of the magnetic resonance lines of nuclei depend in a sensitive manner on the chemical environment of the nuclei. The precise value of B, the magnetic field which the nucleus experiences, seldom, if ever, equals the applied magnetic field (B = Bo + Blocwhere the two terms on

ALUMINOSILICATE CATALYSTS

20 1

the right-hand side are the applied and local fields at the nucleus). The dynamics of atoms in the material to which nuclei giving rise to NMR belong can be rationalized if, along with line shape and position, we measure the relaxation times of these nuclei. All this is familiar to the practicing student of catalysis. Also familiar is the fact that very broad NMR absorption lines are obtained from solid samples because chemical shift anisotropy as well as dipolar and quadrupolar interactions which in solutions and liquids are precisely averaged by the rapid thermal motion of the molecules are not so averaged in solids. At one time it seemed that solid-state NMR would be of little value as a technique for the study of heterogeneous catalysis. In fact, one of us wrote ( I ) in 1966: “. . . high-resolution NMR will be of comparatively little value in the catalyst field because broad lines observed in solids tend to obscure the shifts of the resonance line. If more sophisticated techniques, such as the mechanical spinning of the sample during resonance measurement, can be developed, it is likely that NMR will become a powerful tool for the study of solid catalysts.” As it happens, many important developments, some of which were cited above, have taken place, one of the most important being the construction of the “magic-angle-spinning” probehead (2) (see Section 11,D).We shall deal with all the relevant background theory in Section 11. Suffice it to say here that the “magic angle” is 5444’8’’ (half the tetrahedral angle). Magic-angle-spinning NMR (MAS NMR)’ occupies (3) a dominant place in present-day high-resolution solid-state NMR, the key point being that the normally broad absorption line associated with a nucleus in a particular magnetic environment in a solid collapses into a comparatively narrow line centered at the isotropic chemical shift value when the sample is rapidly spun at the magic angle with respect to the magnetic field. Information comparable with that obtained from liquid samples can now in principle, and indeed in practice, be obtained from solids. This fact has had an immense impact on our understanding of the catalytic and absorptive behavior of aluminosilicates, ever since it was established (4,5) that the immediate chemical environment of silicon atoms could be probed in hitherto unrivaled AES, Auger electron spectroscopy; ALPO, aluminophosphates; BET, Brunauer, Emmett, Teller; CP, cross-polarization; CPA, carboxypeptidase Aa; CSA, chemical shift anisotropy; DB, deep bed; 2D, two-dimensional; EDTA, ethylenediamine tetraacetic acid; EFG, electric field gradient; EXAFS, extended X-ray absorption fine structure; FID, free induction decay; FT IR, Fourier transform IR; FWHM, full width at half maximum; Hacac, acetylacetone; HREM, highresolution electron microscopy; LEED, low energy electron diffraction; MAS, magic-angle spinning; MTBE, methyl-t-butyl ether; NMR, nuclear magnetic resonance; SAPO, silicoaluminophosphates; SB, shallow bed; SEDOR, spin echo double resonance; SLF, separated local fields; S/N, signal to noise; syngas, synthetic gas; TBA’, tetrabutylammonium ion; TBP’, tetrabutylphosphonium ion; TMA +,tetramethylammonium ion; TMS, tetramethylsilane; TPA’, tetrapropylammonium ion; TPAOH, tetrapropylammonium hydroxide; VASS, variable angle sample spinning; WAHUHA Waugh, Huber, Haeberlen; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction; XRF, X-ray fluorescence; ZSM, Zeolite socony Mobil.

202

J. M. THOMAS AND J. KLINOWSKI

detail via ”Si MAS NMR. The wealth of information derived from multinuclear MAS NMR will be illustrated in later sections: the structure of catalysts, their acidity, the local coordination of certain key elements within the body of or at the surface of catalysts, as well as the nature and properties of derivatized catalyst surfaces have all been advanced by the application of MAS NMR. It must not, however, be forgotten that conventional techniques (e.g., 13C Fourier transform NMR) can be applied to certain solids of catalytic significance, such as sheet silicates since in many of these systems rapid motion of intercalated or otherwise sorbed organic species secures sharp absorption lines which provide much information about the individual atomic environments. Organic species attached to high surface area solids (such as zeolites, silica, alumina, magnesia, as well as other oxides and their mixtures) are specific examples (6).

II. Relevant Spectroscopic Considerations IN A. INTERACTIONS

THE

SOLID STATE

As was mentioned in the introduction, NMR spectra cannot normally be measured in solids in the same way in which they are routinely obtained from liquids and solutions. The reason for this is the existence of net anisotropic interactions which in the liquid state are averaged by the rapid thermal motion of molecules. This is not the case in the solid state; although certain solids have sufficient molecular motion for NMR spectra to be obtainable without resorting to special techniques, in the overwhelming majority of cases involving “true solids” there is little internal motion, and conventional NMR, instead of producing sharp spectral lines, yields a broad signal up to 100 kHz wide which conceals information of interest to the chemist. Although the study of the second moments of such spectra and of various temperature-dependent parameters can provide information on the degree of crystallinity and on molecular motion (“wide-line NMR”), we shall be concerned with ways of achieving high-resolution spectra, i.e., spectra which enable magnetically non-equivalent nuclei of the same spin species (e.g., ”Si) to be resolved as individual resonance lines. Such high-resolution spectra of solids can be obtained only when the anisotropic interactions giving rise to line broadening are removed or at least substantially reduced. These interactions are dipolar coupling, chemical shift anisotropy, and the quadrupolar interactions. We shall consider them in turn together with ways for their removal.

ALUMINOSILICATE CATALYSTS

203

In general, the nuclear spin Hamiltonian is represented by the sum (3,7) X =XZ

+ X R F f XD+ X J +XCS+ XSR+ XQ,

(1)

where 2’ accounts for the Zeeman interaction of the nuclear magnetic moment with the applied field B,; XRF results from the interaction between nuclear spin and the time-dependent radio frequency field B,; ZD, from the direct dipole-dipole internuclear interactions; SJ, from the electron-mediated (indirect) interactions; Xcs,from the chemical shift; ZSR, from the coupling of spins with the magnetic moment produced by the angular momentum of the molecule; and XQ,from the quadrupolar interactions. X z and XRF are under the experimenter’s control, while the remaining terms depend on the nucleus in question and its environment. For the solids under consideration 2’ and XSR are unimportant and will henceforth be disregarded.

B. DIPOLAR INTERACTIONS The dipole Hamiltonian for the coupling between the nuclei i a n d j is ($7)

xD= 1&yjh2r;

3 ( ~ i~ j

3 4 ~ ~ ~C )O (S ~38, - 1)

where y i , y j are nuclear gyromagnetic ratios, rij the internuclear vector, and Oij is the angle between rij and Bo, which is by convention directed along the z-axis. Examination of Eq. (2) reveals the following. 1. As the isotropic average cos’ Oij = f, thermal motion of molecules in fluids averages the dipolar interaction to zero. 2. Dipolar interaction is strongly dependent on the internuclear distance and is therefore important only for nuclei in close proximity. The average distance between “magnetically dilute” nuclei such as 29Si and 13C is large and so homonuclear dipolar interaction is unimportant. In aluminosilicates the same applies to the 27A1-27A1 interaction in view of the fact that the aluminate tetrahedra are never neighbors in the framework. 3. If i and j are different nuclei (for instance I3C and ‘H) the heteronuclear dipolar interaction, which is often strong, can be removed by “dipolar decoupling,” which consists of irradiation nucleus j (say ‘H) at its resonance frequency while observing nucleus i (say 3C).The time-averaged value of the Hamiltonian is then zero. 4. If the sample is rapidly spun at an angle A to the magnetic field, the time-averaged value of the angle between all internuclear vectors and the magnetic field 8;:j = A. When Oij = 54”44‘ (so that 3 cos’ Oij - 1 = 0) the time-averaged value of XDis zero. This technique is known as magic-angle spinning ( 3 , 8 , 9 ) .

204

J. M. THOMAS AND J. KLINOWSKI

In certain cases, notably those involving strong homonuclear proton-proton interactions, the size of the Hamiltonian is such that it cannot be reduced by MAS at rotation speeds achievable at present. Such strong interactions are reduced by multiple-pulse methods (10). C. CHEMICAL SHIFTANISOTROPY

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B,. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B, and therefore “shields” the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as “chemical shift anisotropy.” The chemical shift interaction is described by the Hamiltonian XCs = hy1.(1*B,,

(3)

where (I is a second-rank tensor known as the chemical shielding tensor. In large magnetic fields, (I is symmetric and can be described by three principal values, ol,,az2, 033,and the angles which define the orientation of the principal axes. As is shown in Fig. 1, it is possible to obtain o11, ozz,and 033 experimentally for spin f nuclei directly from the NMR spectrum of the static sample provided dipolar interactions are small. It has been shown (3, 11)that when the sample is spun at the angle b to the magnetic field B, the time-averaged value of the tensor component along B, is

-

3

3

o,, = - o sinZfl

2

1 + -(3 cosZb- 1) 1oijcosz x,, 2

(4)

where o = &gl1 + ozz+ 033)and xi is the angle between the spinning axis and each of the three principal axes. We see that at the magic angle when cosz fi = 3, sin2 f l = 5, and the time-average value of o,, is reduced to the scalar isotropic value o. Chemical shift anisotropy is thus removed. As is seen from Eq. (4), chemical shift (unlike dipolar coupling) is field dependent. This has important consequences for nuclei with large chemical shift anisotropies, such as 13C.While chemical shift anisotropy (CSA) for sp3 carbons is 15-50 ppm, it is 100-200 ppm for sp and spz carbons. In an applied field of 1.41 T, 200 ppm translates as 3.02 kHz, but in the very large field of 11.74 T (in a 500 MHz magnet), it translates as 25.2 kHz. Since the sample must be spun at a rate comparable with the size of the CSA (in hertz) it is

ALUMINOSILICATE CATALYSTS

205

022

0 11

FIG.1. Powder line shapes for an anisotropic chemical shift: (a) arbitrary chemical shift tensor (al # u Z 2# uJJ).u, = $(a,, + uz2+ uaJ);(b) axially symmetric chemical shift tensor (with u1 = uZ2 # uJ3).

,

evident that, in the latter case, chemical shift anisotropy cannot be removed by MAS at rates available at present.

D. QUADRUPOLAR INTERACTIONS

=-

Nuclei with spin I $ possess a quadrupole moment eQ and may interact with electric field gradients (EFG) present in the solid. The EFG is described by the traceless symmetrical tensor (12)

vij= a* v / a x i a x j

(5)

where V is the electric potential and x i and xi are Cartesian coordinates. Taking V', s V,, I V,, the tensor is described by five quantities, three of which specify the orientation and two of which describe the magnitude and shape of the EFG: e9 =

K,,

206

J. M. THOMAS AND J. KLINOWSKI

In the principal axis system of V i i , in which the tensor is diagonal, the quadrupolar Hamiltonian for a single spin is SQ =

e2qQ

41(21 - 1)

+ q(Zl - 131. (7) SQ, ( 2 ) Sz-= SQ, (3) Sz> SQ.

[312 - l 2

Three cases are now possible: (1) SzN No general analytical method is possible for obtaining the eigenstates and eigenvalues of SQin case (l), while case (2) belongs to pure quadrupole resonance. We shall consider the usual “high-field” case in which Sz% XQ, take q = 0 for simplicity (which assumes a field gradient of cylindrical symmetry), and denote

VL =

yBo(1 - 6)/2a,

a = Z(I

(8)

+ 1).

Et),

The various energy levels E g ) , and E!,? (superscripts denote order) of Eq. (7) are obtained using perturbation theory. Instead of a single resonance frequency vL = (Ego’ - E!,?)/h, as in the case of spin l nuclei, there are now several resonance frequencies: vm = VL

+ v g ) + vg).

(9)

In the case of noninteger spin nuclei, such as ”A1 or 23Na,we observe only the central - $- 4 transition, as the other transitions are spread over too wide a frequency range. The first- and second-order frequency shifts are v ( l ) = -12 vQ(m - f ) ( 3 cosz 6 - I), m v (1/2 2)

- -(~$/16vL)(a- $)(l

- COS’

0)(9 C O S ~0 - 1).

(10)

Equations 10 lead to the following conclusions. 1. The first-order frequency shift vanishes for m = $, which means that the central transition for noninteger spin nuclei is not affected to first order by quadrupolar interactions (see Fig. 2). It is clearly advantageous to work with these, as the - 1 -0 and 00 1 transitions for integer spin nuclei are always shifted. 2. The first-order shift is scaled by $(3 cos2 0 - 1) and is therefore always reduced by MAS. 3. The second-order shift is not scaled by (3 cos2 0 - 1). It increases with v$ and is inversely proportional to the magnetic field (through vL). Bearing in mind that the dispersion of the chemical shift, which is normally the

ALUMINOSILICATE CATALYSTS

207

FIG.2. Schematic representation of an Andrew-Beams turbine spinner. Compressed gas enters the space between the stator and the rotor (containing the sample) through jet holes machined at an angle to the conical “mushroom” surface of the rotor. The whole assembly can be placed at any desired angle to the direction of the magnetic field (not shown).

parameter we endeavor to measure, is directly proportional to B,, it follows that it is to our advantage to work at high$elds, where the chemical shifts are larger and quadrupolar effects smaller. As the second-order frequency shift is always present, the feasibility of obtaining useful spectra depends on the magnitude of vQ. The following strategy therefore emerges for the study of quadrupolar nuclei: observe the central transition of nuclei with noninteger spin, use MAS (to remove dipolar coupling, chemical shift anisotropy, and first-order quadrupolar effects), and work at high fields (to minimize second-order effects). It is often found that spin f nuclei have very long TI relaxation times (up to several hours in some cases) particularly in amorphous solids. Quadrupolar nuclei, however, generally relax quite fast, which makes them of special interest in the study of the solid state. The expression for the frequency of the central transition with fast MAS was derived by Kundla et al. (13) and Samoson et al. (14) and rewritten by Kentgens et al. (15): v1/2

= - vL

+ (vi/16vL)(a - :)[&a)

c0s4 p

+ B(a) cos’ p + c(a)],

(1 1)

where A(a) = - $1cos 2a

cos22a,

+ $q2 + $q cos 2a - pq2 cos’ 2a, - +q cos 2a + &q2 cos’ 2a,

B(a) = -3 C(a) =

+ &q’

(12)

20 8

J. M. THOMAS A N D J. KLlNOWSKl

6/2

---

------

----

_ _ _ - -__---

T

"2

- - - _ _ _t $ -%

_ _ - -_ _ - - -

-

FIG.3. Energy level diagram for a spin 3 nucleus showing the effect of the first-order quadrupolar interactionon the Zeeman energy levels. Frequency of the central transition (shown in bold lines) is independent of the quadrupolar interaction to first order, but is subject to second-order quadrupolar effects (see text).

and a and /3 are the polar angles of the spining axis with respect to the principal axes of the quandrupole tensor. As is shown in Fig 4 there are singularities in the calculated powder pattern where i3vl12/i3a= 0 and

avl12/ap= 0. Samoson and Lippmaa (16) used a two-dimensional NMR technique for the study of half-integer spin quadrupole nuclei in powders, without resorting to MAS. They found that central transition excitation spectra provide a useful means of selective determination of quadrupole interaction parameters in correlation with those describing other NMR interactions. It has been shown (17,18) that, for systems in which the second-order quadrupole interaction dominates the breadth of the central transition, optimum line narrowing is achieved by sample rotation at angles other than the magic angle. The theory of variable angle sample spinning (VASS) has been given (17) and the technique may in the future prove to be useful in the study of nuclei with large quadrupolar moments. It has been successfully applied (19) to the 27Al NMR study of the crystallization from a vitreous precursor of synthetic cordierite, Mg,Al,Si,O 18, and in work with various materials where the quadrupolar effects are very large (18). The influence of the various types of interactions on the NMR spectra of rotating solids has been fully discussed in the literature (3,8,9,20). The experimental assembly for MAS of solids is shown schematically in Fig. 2. In summary, theoretical considerations show that the Hamiltonian operator responsible for the central part of the spectrum when line-narrowing techniques are applied is almost identical to that effective in the liquid state.

209

ALUMINOSILICATE CATALYSTS

0

21

5 -A 6

c u - v,

3

’6A 21

O&A

-v

- *z

-v

8 A 9

”A

9

-v

FIG.4. Quadrupolar powder patterns: (a) Spin 3 NMR powder pattern showing that the central - f -f transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions. (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 0 = 90” in Eq. (10). The other features of low intensity correspond to 0 = 0” and 0 = 180”. (c) Theoretical line shape of the f- -4 transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter q (15); A = -( &XV3k)C1(1

+ 1) - $1.

E. CROSS-POLARIZATION Solid-state NMR suffers from two persistent problems: (1) low abundance and/or sensitivity of the observed nucleus, and (2) long spin-lattice relaxation times TI.Both can be remedied with the help of a double-resonance technique known as “cross-polarization” (CP) (21). Consider the concept of “spin temperature” ts with reference to nucleus S of spin i. The population ratio n 1/n t of S nuclei in the upper and lower energy state is

n 1/n 1 = exp( -AE/Rt,) = exp( -hyBo/Rts).

(13)

210

J . M. THOMAS AND J. KLINOWSKI

D Lattice

FIG.5. Thermodynamic picture of I and S spin reservoirs (see text).

It is clear that small population differences (small polarization) are equivalent to high t, and large population differences (large polarization) correspond to low spin temperature. Take a sample containing “abundant” spins I and “rare” spins S. Their spin temperature may be different from one another and from the lattice temperature tL. Polarization of rare spins can be increased by “cooling,” i.e., transferring energy to the lattice-a process characterized by a relaxation time Ty which is normally much longer than the relaxation time TIl of the abundant nucleus. Cross-polarization is an ingenious way of cooling S spins indirectly, by transferring energy to the lattice via the large reservoir of abundant spins. This process is characterized by T,, (cross-relaxation time) which is normally short (see Fig. 5). This enables one to cool the rare spins, thus increasing their polarization, while the larger spin reservoir is only slightly heated (which is described in terms of “large heat capacity”). Very importantly, the whole process is characterized by the short relaxation time T i . Thermal contact between reservoirs is established by applying two separate rf fields B l l and El, to the I and S reservoirs. The energy of spin I in the magnetic field E l l is ylBll;to match it with the energy of the spin S we need to satisfy BIS/Bll = YI/YS,

(14)

known as the Hartmann-Hahn condition (22). This is met by matching the power of the two rf fields so that the process is energy conserving, as required by quantum mechanics. For 13C-lH cross-polarization, yH/yc = 4, which means that the field applied to the carbons must be four times as large as that applied to the protons. Note that the static magnetic field B, does not appear in the equations, as thermal contact is performed in the rotating frame. The sequence of events in cross-polarization is given in Fig. 6, and described as follows. 1. Magnetization of I spins builds up along B,. 2. A 90” pulse of power Ell is applied at resonance along the y’ axis,

21 1

ALUMINOSILICATE CATALYSTS

1-1

irradiation at ’H

Spin Lock

’ Btu

Decouplino

I

frequency

I Ic

I

contact time

I

J

--_ observation

7

time

i -* I

wait period

’ BlC

FIG.6. Cross-polarizationtiming diagram for the ‘H-”C spin system. Upper part, the ‘H channel; lower part, the ‘jC channel.

which sends the spins along the x’ axis. The rf field is then shifted in phase by 90” so that it now lies along the same direction as the spins. This is known as “spin locking.” 3. A 90” pulse of power Bls is applied at resonance to spins S along the y‘ axis aligning them with the x’ axis also. Both spin systems now precess with the same frequency in the x’y‘ plane. Polarization transfer occurs; the time of thermal contact is known as “mixing time.” 4. The Bls field is turned off and free induction decay of spins is observed. During observation time, B,, is still on but serves as the high-power decoupling field to reduce heteronuclear I-S dipolar broadening. 5. Steps (1)-(4) are then repeated. High-resolution spectra are often obtained by a combination of MAS, CP, and high-power decoupling. Polarization of the rare nuclei is increased and the process is carried out with a short recycle time giving a large improvement in sensitivity and signal to noise ratio (S/N). The first combined 13C-’H MAS/CP experiment was performed by Schaefer and Stejskal (23). The double-resonance procedure decouples the strong heteronuclear dipolar interactions and indirect J couplings, while the weak I3C signal is enhanced by proton polarization transfer. The residual spectral width of several kHz arises from 13C chemical shift anistropy and weak 3C-’3Cdipolar interactions between 1.1 % abundant 13Cnuclei. Both of these interactions are removed by MAS. It is important to note that cross-polarization takes place via I-S dipolar interaction. It cannot, therefore, occur in systems with rapid thermal motion

212

J. M. THOMAS AND J. KLlNOWSKl

where the dipolar interaction is averaged to zero. On the other hand, crosspolarization is very efficient in solids. This can be used to discriminate between mobile and immobile components of an adsorbed phase. Furthermore, since the dipolar interaction betwen nuclei depends very strongly on the internuclear distance [see Eq. (2)], so too does the efficiency of crosspolarization. For adsorbed phases, for example, hydrocarbons adsorbed on silica gel, "Si- H cross-polarization is therefore surface selective.

'

APPLICATIONS OF NMR F. MULTINUCLEAR Virtually all chemical elements possess at least one magnetically active isotope, and modern Fourier transform techniques at high fields render even those with very low gyromagnetic ratios observable by NMR. This offers many new possibilities for the study of adsorption and heterogeneous catalysis. The following nuclei are the most important for the study of catalytic substrates: "Si, 27Al,and 1 7 0 for crystalline and amorphous silicas and aluminas, and, in addition, 3'P for aluminophosphates (ALPO); 23Na, 47Ti,'lV, 67Zn,95Mo, '"Rh, '13Cd, ls3W, and '"Pt for metals and metal oxide supports; 'H, 13C, "N, and 31Pfor derivatized surfaces and surfaceimmobilized ligands, as well as for the adsorbed organic molecules. In addition, '"Xe can be used as a sensitive molecular probe for adsorption studies. NMR properties of the nuclei discussed in this review are listed in Table I. Of double resonance methods, cross-polarization was discussed in Section 11, E. Some of the others are based on the Overhauser effect. Yet another double resonance technique, spin echo double resonance (SEDOR), has proved useful in catalytic studies (24-26). SEDOR is highly surface selective and can, for example, observe only those lQ5Ptnuclei on the surface of platinum metal which are in close proximity to the 13Cnuclei in the adsorbed CO gas (26). The principle of the method is as follows. Provided the two nuclei are near enough to be coupled, the resonance of one affects the resonance of the other and the strength of this coupling depends strongly on the internuclear distance. In general, if a 4 2 pulse is followed a time T later by a A pulse, spin echo forms at time 2 2 ; during the first time interval z the spins dephase, but during the second they rephase. When, however, spin I is near to spin S, the latter gives rise to a local field which may increase or oppose the applied field. This has no effect on the resonance of spin I, as dephasing during the first interval T is exactly matched by the rephasing during the second interval T. If, however, spin S is also flipped with a 71 pulse when spin I is given its second A pulse, spin I dephases during the second time interval also, producing a smaller echo at time 2t.

213

ALUMINOSILICATE CATALYSTS

TABLE I N M R Properties of the Nuclei Discussed in This Review

Isotope

Spin

'H 2H

f

7Li IlB I3C lSN

3

Relative sensitivity

99.98 0.015 92.58 80.42 1.108 0.37 0.037 100 100 100 4.7 100 7.28 99.16 15.72

1.00 0.00965 0.29 0.17 0.0159 0.00104 0.0291 0.83 0.0925 0.21 0.00784 0.0663 0.00209 0.38 0.00323 O.ooOO31 1 0.0109 0.0212 0.00994 0.19

1

$ f

1 7 0

5

I9F 23Na

4

$

29Si

5 4

31P

f

47Ti

3 4 ~5

2 7 ~ 1

"V 9

Natural abundance

s

"'Rh 'I3Cd lz9Xe 195Pt

~

$

100

4

12.26 26.44 33.8 70.5

f

+

205~1

NMR frequency Electric in a field quadrupole moment Q of 2.3488 T (in multiples of e x cm2) 100.00 15.351 38.863 32.084 25.144 10.133 13.557 94.077 26.451 26.057 19.865 4.481 5.637 26.289 6.514 3.147 22.182 27.660 21.449 57.708

2.73 x 1 0 - 3 -3 x 1 0 - 2 3.55 x lo-* -

-2.6 x -

0.14-0.15 1.49 x lo-' -

a

-4 x 1.2 x 10-1 ~

-

'Data not available.

Further possibilities are offered by various two-dimensional NMR methods. For example, heteronuclear solid-state correlation spectroscopy (27) is capable of correllating the spectra of abundant and dilute spins in solids, simplifying spectral assignment and permitting determination of shielding tensors. Futhermore, spin diffusion among abundant spins can be directly observed by this method.

111.

A.

Solid-state NMR Studies of Zeolites

OUTLINE OF THE STRUCTURE AND PROPERTIES OF ZEOLITES

The name "zeolite" (from the Greek (EO = to boil and AiOoa = stone) was coined by Cronstedt (28) in 1756 to describe the behavior of the newly discovered mineral stilbite. When heated, stilbite loses water rapidly and thus

214

1.

M. THOMAS AND J. KLINOWSKI

Truncated octahedron

Sodallte

Zeollte A (d)

Faujaelte (zeolite8 X and Y)

FIG.7. (a) The cubooctahedral building block (also known as ”sodalite cage” or “/I-cage”). Tetrahedral atoms (denoted by open circles) are located at the comers of polygons with oxygen atoms (not shown) approximately halfway between them. (b) The structure of zeolite A formed by linking the sodalite cages through double four-membered rings. (c) The structure of sodalite formed by direct face-sharing of four- and six-membered rings in the neighboring sodalite cages. (d) The faujasite structure formed by linking the sodalite cages through double six-membered rings. For clarity, exchangeable nonframework cations are not shown.

215

ALUMINOSILICATE CATALYSTS

seems to boil. Zeolites are a class of framework silicates (other classes include feldspars and feldspathoids) built from corner-sharing SO:- and A10:tetrahedra and containing regular systems of intracrystalline cavities and channels of molecular dimensions (Fig. 7). The net negative charge of the framework, equal to the number of the constituent aluminum atoms, is balanced by exchangeable cations, M"', typically sodium, located in the channels which normally also contain water. The general oxide formula of a zeolite is M,,,(A102),(Si02),~rnH20. It is invariably found that y 2 x. The simplest interpretation of this, given that each silicate and aluminate tetrahedron is linked via oxygen bridges to four other tetrahedra, is that aluminate tetrahedra cannot be neighbors in a zeolitic framework, i.e., that Al-0-A1 linkages are forbidden. This requirement, known as the Loewenstein rule (29),will be discussed in detail later. There are at present around 40 identified species of zeolite minerals [with the Si/AI ratio ( y / x ) = 1-5 depending on the structure] and at least 120 synthetic species with a very wide range of aluminum contents. Classification of zeolites according to secondary building units is given in Table 11. Zeolites are prepared under

Classification Zeolite

Free aperture of channels Classification Zeolite N" (4

S4R

Phillipsite Gismondine

8 8

3.9 x 4.4 2.8 x 4.9

S6R

Erionite Offretite Levyne Mavite Omega (Syn) Losod

8 12 8 12 12 6

3.6 x 5.2 6.9 3.2 x 5.1 7.4 7.4 2.2

Type A(Syn) Type ZK4 (Syn)

8 8

4.2 4.2

Natrolite Scolecite Thomsonite

8 8 8

2.6 x 3.9 2.6 x 3.9 2.6 x 3.9

D4R

4-1

D6R

5-1

4-4-1

Free aperture of channels

N"

(A)

Chabazite 8 Gmelinite 12 Faujasite 12 TypeX(Syn) 12 Type Y(Syn) 12 Type ZK5 (Syn) 8 Type L(Syn) 12

3.7 x 4.2 7.0 1.4 1.4 1.4 3.9 7.1

Mordenite Dachiordite Ferrierite ZSM-5 (Syn) Silicalite (Syn)

12 10 10 10 10

6.7 x 3.7 x 4.3 x 5.4 x 5.2 x

Heulandite Clinoptilolite Stilbite Barrerite

10 10 10 10

4.4 4.4 4.1 4.1

7.0 6.7 5.5 5.6 5.8

x 7.2

x 7.2 x 6.2 x 6.2

216

J. M. THOMAS A N D J. KLINOWSKI

mild (60-400°C) hydrothermal conditions in strongly basic media. The type and concentration of the base are important structure-directing factors and a variety of organic bases are now being used in zeolite synthesis (30). The ZSM series (for Zeolite Socony Mobil) of highly siliceous zeolites is prepared from solutions containing alkylammonium bases. Other elements, such as Ga and Ge, can substitute for Si and Al in the zeolitic framework, and there are claims that many other elements can also do so. New classes of nonsilicate zeolite-type crystalline aluminophosphates (31) and silicoaluminophosphates (SAPO) (65) have been reported but relatively little is known about their chemical behaviour. Zeolites display a number of interesting physical and chemical properties. The three classes of phenomena of greatest practical importance are the ability to sorb organic and inorganic substances, to act as cation exchangers, and to catalyze a wide variety of reactions. The most important aspects of these are described below. The zeolitic channel systems, which may be one-, two-, or three-dimensional and may occupy more than 50% of crystal volume, are normally filled with water. When water is removed (which can usually be done reversibly) other species such as gaseous elements, CO,, CS,, ammonia, alkali metal vapors, hydrocarbons, alkanols, and many other organic and inorganic species may be accommodated in the intracrystalline space. Depending on pore diameter and on molecular dimensions, this process is often highly selective, which gave rise to the alternative name for zeolites: molecular sieves. Thus, dehydrated chabazite, with pore openings less than 5 A wide, can sorb water, methanol, ethanol, and formic acid, but not acetone, ether, or benzene. By contrast, synthetic zeolite omega, with channels more than 7.4 A wide, can sorb molecules smaller than, and including, (n-C,F,),N. Thus, zeolitic sorption is a powerful method for the resolution of mixtures. For example, Ca-exchanged zeolite Linde A can separate n-paraffins from other hydrocarbons. In addition to the effect of pore size alone, polar molecules in general are sorbed selectively in the presence of nonpolar molecules. The reason for this is that zeolitic crystals themselves are usually highly polar. Commercial applications of molecular sieving are many and include thorough drying of organics, separation of hydrocarbons and of N, and 0, in air, and the removal of NH, and CS, from industrial gases. Cations neutralizing the electical charge of the aluminosilicate framework can be exchanged for other cations from solutions. Zeolites often possess high ion-exchange selectivities for certain cations, and this is used for their isolation and concentration. For example, NH: is efficiently removed from solution by clinoptilolite, a zeolite found in large sedimentary deposits. Other applications are the collection of harmful products of nuclear fission (such as 13'Cs and 90Kr), water softening, the treatment of brackish water, and the

ALUMINOSILICATE CATALYSTS

217

recovery of precious elements. Zeolites can act as sieves for ions just as they do for molecules in the course of sorption. For example, in zeolite Na-A sodium can be exchanged for n-alkylammonium cations, but not for branched alkylammonium or tetramethylammonium ions. Finally, molecular sieving properties of zeolites can be modified by ion exchange. Thus Na-A sorbs both N, and O,, while Ca-A sorbs nitrogen preferentially to oxygen. However, it is the ability to catalyze a wide range of reactions, such as cracking, hydrocracking, oxidation, and isomerization of hydrocarbons, that far overshadows all other applications of zeolites. Rare earth-exchanged and hydrogen forms (prepared indirectly by thermal decomposition of the ammonium form) of some zeolites, such as synthetic faujasite, mordenite, gmelinite, and chabazite, have cracking activity which is orders of magnitude greater than that of conventional silica/alumina catalysts. Zeolite-based catalysis was first discovered (32) in 1960 and 2 years later cracking catalysts based on zeolite Y were introduced. They have now almost completely displaced conventional catalysts. The synthetic zeolite ZSM-5, introduced in 1972 (33), is an even more powerful catalyst. Its high silica content (Si/AI is typically 30) gives it high thermal stability, while the channel diameter, ca. 5.5 A, is very convenient for many applications, particularly in the petroleum industry. The 10-membered channels of ZSM-5 are responsible for the quite striking shape selectivity: only certain reactants may penetrate the channel system and only certain products may diffuse out of it. This shape selectivity can be "tuned" even more finely by ion exchange. Catalytic properties of ZSM-5 include the ability to synthesize gasoline from methanol in a single step. Silicalite, a material which is isostructural with ZSM-5 but contains only a small amount of aluminum, is, by contrast to most other zeolites, nonpolar (i.e., hydrophobic) but organophilic. It is used successfully in the removal of dissolved organics from water. Zeolites containing transition metal ions (such as Cr3+,Ag', and C u 2 + ) are active as oxidation catalysts. Comprehensive reviews dealing with various aspects of the structure (34-37) sorption ( 3 3 , catalysis (38-42), and other chemical properties of zeolites are available in the literature. While single crystals of natural minerals readily lend themselves to conventional methods of structural investigation, synthetic zeolites, which are almost always microcrystalline, must of necessity be studied by the less powerful powder X-ray diffractometry. The development of high-resolution solid-state NMR techniques was therefore very timely. Early NMR work with zeolites involved water of hydration and organics sorbed on zeolites using 'H resonance and wide-line spectroscopy and cations such as 7Lif,23Na+,and '05Tl+. Since 1979, 29Si,27A1, and 1 7 0 have been observed in the zeolitic framework using magic-angle spinning. In particular, 29Si studies have provided many new insights into the structure

218

J. M. THOMAS AND J. KLINOWSKI

and chemistry of zeolites. It is fortunate that the relatively short spin-lattice relaxation times of the 29Sinucleus in crystalline aluminosilicatesmade such results possible. Magic-angle-spinning studies of protons in zeolites made progress toward elucidating the nature of catalytic acidity. The most recent advances involve the use of probe molecules, such as various organics and sorbed xenon, and observing 'H, 'H, 13C, 15N, and lZ9Xeresonances, in order to characterize intracrystalline environments. Also 29Sisignals originating from crystallographically nonequivalent silicon atoms can be resolved and related to structural parameters. There is no doubt that many additional problems in zeolite chemistry will be answered with the help of NMR. Some of the most important of these are the following: 1. Si, A1 ordering in the zeolitic framework (known to be intimately related to catalytic activity); 2. The magnetic and crystallographic equivalence, or otherwise, of the various Si and A1 sites; 3. Factors governing the catalytic acidity of zeolites; 4. The mechanism of dealumination of the zeolitic framework and the nature of extra-framework Al; 5. The position and mobility of exchangeable cations; 6. The mobility, diffusivity, and configuration of the adsorbed species; 7. The relationship between NMR spectra and zeolitic structure.

Solid-state NMR studies of zeolites have recently been reviewed (43,44). B. "Si NMR

OF

SILICATES AND ALUMINOSILICATES

29SiNMR studies of solutions are difficult because of the long spin-lattice relaxation times of the nucleus and its negative nuclear Overhauser enhancement. The 29Si-'H dipole-dipole relaxation is inefficient because in most compounds the internuclear distance is large. Fortunately, the problem of relaxation can often be overcome by resorting to cross-polarization (see Section 11,E). The full range of 29Sichemical shifts is over 500 ppm wide, but most shifts are to be found in a narrower range of ca. 120 ppm (45,46). Tetramethylsilane (TMS) is the accepted reference compound. The range of chemical shift anisotropies for 29Siis relatively small as compared with that for I3C, and in silicates the shielding tensor is nearly symmetric. High-resolution 29SiNMR has been extensively used in solution (45,46), enabling the structure of the various silicate anions present in alkali metal silicates, tetraalkylammonium silicates, and solutions of silicic acids to be established. The total range of chemical shifts found is from -60 to- 120 ppm from TMS; this is split up

ALUMlNOSlLlCATE CATALYSTS

219

into well-separated intervals corresponding to silicon atoms in monosilicates, i.e., in isolated SiOi- groups (denoted by Q’), disilicates and chain end groups (Ql), middle groups in chains ( Q 2 ) , chain branching sites (Q3), and fully cross-linked framework sites (Q4). All silicate anions can be described using a combination of Q“ units where the superscript refers to the number of silicon atoms linked, via oxygen bridges, to the central silicon. Conventional NMR studies of solid silicates give spectra with very broad lines. For example, the 29Sisignal from solid Na2Si20, is 208 ppm wide (47). Nevertheless, Holzman et al. (48) were able to measure chemical shifts for silica (- 1 13 ppm from TMS), cristobalite (- 113 pprn), quartz (- 109 ppm), and several multicomponent glasses (- 93 ppm). Gibby et al. (49)determined principal elements of the 29Si chemical shielding tensor for a number of organosilicon compounds at - 186°C using cross-polarization. Later, Grimmer et a1 (50) employed the technique to measure shielding anisotropies in two polycrystalline compounds containing (Si20,)6- and (Si802,)8 anions. Pioneering studies in high-resolution 29Si NMR spectroscopy of solid silicon compounds have been carried out in collaboration by Lippmaa (Tallinn, U.S.S.R.), Engelhardt, (Berlin, German Democratic Republic) and their colleagues. First, they measured spectra of a number of organosilicon compounds using MAS combined with cross-polarization (52). Soon thereafter they carried out a comprehensive study (52) of numerous silicates composed of different types of Q units. In solids of known composition containing more than one type of Q unit, separate lines were observed in the requisite intensity ratio. For instance, the spectrum of xonotlite, Ca,(OH),(Si,O, ,), a double-chain silicate branched at every third silicon,

contains two 29Sisignals, at - 86.8 and -97.8 ppm, in the 2: 1 intensity ratio (Fig. 8). Subsequent work involving a large number of silicates of various types (53)extended the ranges of chemical shift corresponding to each kind of Q“ unit (Fig. 9). The ranges overlap significantly,with the exception of those for Q3 and Q4 units. The “Q notation” used above is not sufficient to describe the basic building units in aluminosilicates. While in framework silicates the environment of each silicon atom is always Q4(4Si), in framework aluminosilicates there are five possibilities described by the formula Q4[nAl, (4 - n)Si], where n = 0, 1,2, 3, or 4. We shall for simplicity denote these five basic units as Si(nA1)

220

J. M. THOMAS A N D J. KLINOWSKI

Xonot lite

Q2

-60 -80 -100 -120

ppm from TYS

FIG.8. High-resolution 29Si MAS NMR spectrum of xonotlite at 39.74 MHz (52).

Q4

I

-60

I

I

-70

I

I

-80

l

l

-90

I

\

-100

I

1

-110

I

I

-120

ppm from TMS

FIG.9. 29Si chemical shift ranges for silicates with different degrees of condensation of SiOa- units, described using Q" notation (see text). The diagram is based on data from the study of 60 silicate minerals (53).The number of materials examined in each Q" range is given in the following list in parentheses after the name of the silicate: Qo, nesosilicates (22); Q', sorosilicates (5); Q'(Si0Si = 180"), sorosilicates with nearly linear Si-0-Si linkages (4); Q2, inosilicates (17); Q3, phyllosilicates (7); Q4. silica polymorphs (5).

ALUMINOSILICATE CATALYSTS

-60

-80

-100

-60

-80

22 1

IOOppmlTMS)

FIG. 10. High-resolution "Si MAS NMR spectrum of thornsonite and natrolite at 39.74 MHz (54). Open circles denote Al atoms, closed circles Si atoms.

or SiC(4 - n)Si]; these formulas express the fact that each silicon atom is linked, via oxygens, to n aluminum and (4 - n) silicon neighbors. Lippmaa et al. (52)found that when one or more Si atoms in a Q4unit are replaced by A1 atoms, a significant paramagnetic shift results, i.e., the "Si chemical shift becomes less negative. In general, the substitution Si[(n - 1)Al] + Si(nA1) brings about a low-field shift of ca. 5 ppm. The spectra of aluminosilicates are again in good agreement with known crystal structures (52,54). For instance, the spectrum of the natural zeolite thomsonite, Na4Ca8(A120Si20080)~24H20, in which all Si atoms are crystallographically equivalent and alternate with A1 atoms in the framework, contains a single signal at - 83.5 ppm, assigned to Si(4Al). The spectrum of natural natrolite, Na2A1,Si,0,,-2H20, which is known to contain two kinds of Si atom, Si(3Al) and Si(2Al), in the 2: 1 population ratio, shows two signals in the same ratio of intensity (Fig. 10). The spectra of zeolites X and Y (synthetic faujasites) showed, depending on composition, all five Si(nA1) signals (Fig. 1 l), while the spectrum ofzeolite A (with Si/Al = 1.00) showed just one signal (Fig. 12), which indicated that the environment of each Si atom is identical, i.e., that there is ordering in the zeolitic framework. The spectrum of zeolite A provided a big surprise in the early 29Si MAS NMR work on zeolites: the signal is found at -89.2 k 1 ppm, and coincides with the Si(3Al) signal in synythetic faujasites, the spectra of which could be unambiguously interpreted in terms of Si(nA1) units. This signal was expected at ca. -84 ppm, which would correspond to Si(4Al) units, i.e., to strict alternation of Si and A1 atoms in the zeolitic framework. Lippmaa et af. (54) concluded therefore that zeolite A consists of Si(3Al) units, which means that each Si atom in the structure is linked to three A1 atoms and one Si atom and vice versa: each Al is linked to three Si and one other Al, thus breaking the Loewenstein rule which forbids Al-0-A1 linkages. This assignment was

222

I. M . THOMAS A N D J. KLINOWSKI Si /A1

A

Si / A l

1.19

A "A

G

135

2 35

256

)\

-A-

-

-00

u -90 -100 -110

-90

-100

-110

p p m from TMS

-80

pprn from TMS FIG.11. High-resolution 29Si MAS NMR spectra of synthetic zeolites Na-X and Na-Y at 79.80 MHz (58). Experimental spectra are given in the left-hand columns; Si(nAl) signals are identified by the n above the peaks. Computer-simulated spectra based on Gaussian peak shapes and corresponding with each experimental spectrum are given in the right-hand columns. Individual deconvoluted peaks are drawn in dotted lines.

-80.2ppm

Zeolite A SI/AI= 1.00

I

-60

I

-80 -100 ppm from TMS

-120

FIG.12. 29Si M A S NMR spectrum or zeolite Na-A at 79.80 MHz.

223

ALUMINOSILICATE CATALYSTS

subsequently shown to be incorrect. The spectrum of zeolite A is considered in Section III,D. It seemed at first, on the basis of the spectra of 14 different zeolites, that the Si(4Al) and Si(3 Al) chemical shift ranges were nonoverlapping, so that even a single-peak spectrum could be assigned from the magnitude of the chemical shift alone. This has since been shown not to be the case. Many more 29Si spectra of natural and synthetic zeolites have been measured by various workers since the pioneering papers by Lippmaa et al. and Engelhardt et al. appeared. The results are listed in Table I11 in two categories. The first category contains spectra which can be satisfactorily interpreted (see Section III,C), and the second contains the few known cases in which such simple interpretation is not possible. Briefly, the reason for this is that some zeolites contain two or more kinds of crystallographically nonequivalent tetrahedral site, each corresponding to a distinct value of the 29Si chemical shift, 6. When the chemical shift difference, Ad, between two such sites, when they are surrounded by the same number of A1 atoms, is comparable to the chemical shift difference, between Si(nA1) and Si[(n & 1)Al] units, spectral lines overlap and cannot be assigned simply. This effect, which has been turned into advantage in structural investigations of zeolites. is discussed fully in Section II1,G. The currently accepted ranges of chemical shift for various Si(nA1) environments in Q4 aluminosilicates are given in Fig. 13. A1

Al

I

-80

Al

Al

SI

0

0

0

0

0

AlOSIOAl

AlOSIOSi

AIOSIOSI

SiOSiOSl

SlOSlOSi

0

0

0

Al

si

0

A1

si

0

Si(4AI)

Si(3Ai)

SNPAI)

SI(1 AI)

SMOAI)

4:O

3:l

22

13

04

I

I

-90

I

I

I

-100

si

1

1

-110

ppm from TMS

FIG.13. Ranges of 29Si chemical shift for Si(nAl) building blocks in framework aluminosilicates.

TABLE 111 Parameters of 29Si MAS N M R Spectra 29Si chemical shifts @pm from TMS) Zeolite

Idealized unit cell composition

Si/AI ratio

Si(4AI)

Si(3AI)

Si(2AI)

Si(lA1)

Si(0AI)

A. Zeolites for which the individual spectral signals can be assigned directly to Si(nAl) structural units Na-A Li-A(BW) Analcime Cancrinite Chabazite Gismondine Gmelinite Laumontite Leucite LOSOd

Mordenite Natrolite Rho Scolecite Sodalite Thomsonite

ZK-5 ZSM-5

CNa,,AI,,Sil,048 ‘27HzOIs Li,A1,Si,Ol,~ 4H,O Nal,AIl,Si3,0,, .16H,O Na,AI,Si6O2,.CaCO3 .2H,O Ca,AI,,Si,,O,, .40H,O Ca,A1,Si80,, . 1 6 H 2 0 Na,A1,Si,,04, .24H,O Ca,AI,Si,,O,,. 16H,O KAISi,O, Na,,AI, ,Si ,04,.18H,O NasA1,Si,,09, .24H,O Na,,AI,,Si,,O,, .16H,O (N& C ~ ) i z A ~ i z ~ i .UHzO 3~09~ Ca,AIl,Si,,O,, .24H,O Na6A1,Si,O,, .2NaCI Na4Ca,AI,,Si,,0,,~ 24H,O Na30A130Si66019~~ 98H,O (x < 27) Na,A1,Si9,_,01,,~16H,0

1.0

1.0 2.0 1.0

2.6 1.0 2.0 2.0 2.0 1.0

-88.9 -80.1 -92

-96.3’

- 101.3

- 108

-94.0

-99.4’

- 104.8

-110

-91.7

-97.1’ -92.4 -91.6

- 102.7

- 108.0

-91.4

- 101.0

- 100 -95.4 -97.2 -95.3

- 105.5

-111.6’

- 102.7‘

- 108.0

-97.6’

- 103.5

- 108.6 -111.8’

-85.4 -89.9 -86.8 -81.0 -88.9

-85.2

5.0

1.5 3.0

-87.7’ -92.5 -86.09-88.6

1.5

1.0 1.0

2.2 31

-84.8 -83.5 -87.5

-920

- 101.8

B. Zeolites for which the individual spectral signals are composites and cannot be assigned to Si(nA[) structural units Idealized unit cell composition

Si/AI ratio

Na,K,AI,Si,,O,, .24H,O Na,AI,Si,,O,, .27H,O Na6A1,Si,,,0,, .8H,O Ca,Al,Si,,O,,. 24H,O K,Na,A19Si,,0,,~21 H,O Na,Al,Si,,O,, .14H,O (TMA),Na,AI,Si,,O,, '28 H,O

5.0 3.0 5.0 3.5 3.0 2.5 5.0

Zeolite'

29Si chemical shifts (ppm from TMS) ~~

Clinoptilolite Erionite Ferrierite Heulandite (K, Na)-L Offretite Omega Silicalited

N N

cn

(si02)96

-

~~~

- 106.9b; - 112.8 -92.4; -97.9; - 102.4; - 107.0; - 112.3 - 100.0; - 105.9b;- 110.5 -95; -99.V; -105.3; -108 -92.6; -96.6; - 101.2b; - 106.5 -93.5; -97.5; - 102.3; - 107.2; - 112.5 -93; -98.3b; -105.7; -113.1 -109.2; -111.3; -112.0; -112.6; -ll3.lb; -113.9; -114.5; -115.3; -116.3 - 100.6;

See text. Taken from ref. 44. Largest peak. ' All zeolites listed in B contain at least two nonequivalent kinds of tetrahedral site for silicon. All resonances from silicalite correspond to nonequivalent Si(4Si) units, but cannot at present be individually assigned to site groups.

226

J. M. THOMAS AND J. KLINOWSKI

C. DETERMINATION OF THE COMPOSITION OF THE ALUMINOSILICATE FRAMEWORK USING29Si MAS NMR

As we have seen in Section III,B and in Figs. 10-12, the 29SiMAS NMR spectra of zeolites consist of one to five signals corresponding quantitatively to different Si(nAl) building blocks. It follows that when a spectrum (1) contains more than one signal, (2) is correctly interpreted in terms of Si(nA1) linkages are present, it must be possible to units, and (3) no Al-0-A1 calculate the Si/Al ratio in the sample from the 29Si spectrum alone. The justification for this conclusion is as follows. In the absence of Al-0-A1 linkages the environment of every Al atom is Al(4Si). Each Si-0-AI linkage in an Si(nA1) unit therefore incorporates 4 Al atom, and the whole unit n/4 Al atom. The Si/Al ratio in the aluminosilicate framework is thus (55-58) :

c 4

(Si/A1)NMR=

n=O

zSi(nAl)/

c4 n4

zSi(nAI)

n=O

where lSi(nAl) is the intensity of the NMR signal attributable to Si(nA1) units. Equation (15 ) is independent of structure and applies to all zeolites provided the assumptions made in its derivation are justified. It can, by implication, serve as a test for the correctness of spectral assignments. The validity of Eq. (15) has been tested in the cases of zeolites X and Y (synthetic faujasites) which can be obtained in a range of compositions (Si/Al from 1.0 to ca. 2.75). Figure 1 1 gives 29Si MAS NMR spectra, obtained at 79.80 MHz, of a series of zeolites X and Y. The spectra were computersimulated using Gaussian peak shapes, and the areas of the individual deconvoluted signals were measured. Recognizing the fact that both the shape and the position of signals are affected by the neighboring signals, this procedure allows “corrected” halfwidths and chemical shifts to be obtained. The results are given in Tables IV and V. As is evident from a comparison of the second and the last columns in Table V, there is very good agreement between the Si/AI ratios obtained by X-ray fluorescence (XRF) and those calculated from the spectra. The individual spectral signals have therefore been interpreted correctly, and the Loewenstein rule is obeyed in the structure. Melchior et al (59) gave an elegant graphical demonstration of this. They pointed out that if the distribution of Si and A1 in the framework were purely random, the average number of A1 neighbors per Si atom would be equal to the average overall linkpopulation: A = 4/(1 + R) where R = Si/Al. If, however, AI-0-A1 ages were forbidden, the average number of Al neighbors would be A = 4/R. Figure 14 clearly shows that the latter is the case.

rABLE f V

Experimental and Corrected Values of 29Si Chemical Shijts in Synthetic Faujasites, and Widths at Half-Height of Simulated Si(nAl) Signals'

Experimental (upper numbers) and corrected (lower numbers) chemical shift Sample number

Si/AI (by XRF)

1

1.19

2

1.35

3

1.59

4

1.67

5

1,87

6

2.00

7

2.35

-

8

2.56

-

9

2.61'

-

10

2.75

-

6mind

6m.16 AdA(ppm) a

Widths at half-height of simulated peaks

Si(4AI)

Si(3AI)

Si(2AI)

Si(lA1)

Si(0AI)

-83.9' -83.9 -84.1' -84.1 -83.9 -83.8 - 83.9 -83.9 - 83.8 - 83.7 -84.0 - 84.0

-88.1 -88.5 - 88.6 - 88.4 -88.1b -88.1 -88.1 -88.3 - 88.3 -88.2 -88.7 - 88.6 -88.5 - 89.0 -88.7 - 88.6 -88.6 - 88.6 -89.6 -89.7

-93.1 -93.4 -94.0 -93.3 -93.0 -93.1 -93.2' -93.2 -93.3' -93.3 -93.7' -93.7 -93.9 -94.1 - 93.6 -93.6 -93.7 -93.8 -94.9 -95.0

- 97.9

- 102.4 - 102.4 - 102.3 - 102.6

-84.4

- 89.6

-95.0

- 83.7

-88.1 1.5

-93.0 2.0

0.7

-98.1 -98.6 - 98.5 - 97.6 -97.7 -97.9 -98.0 -98.1 -98.1 -98.7 -98.6 - 99.6b -99.6 -99.1' -99.1 -99.3' -99.3 - 100.4' - 100.4

(- 101) - 101.9 (- 102) - 102.1 (- 103) - 102.8 - 103.4 - 103.6 - 104.5 - 105.3 - 104.1 - 104.6 - 104.9 - 105.1 - 105.8 - 106.0

- 100.4 - 97.7

- 101.9

2.7

4.1

Si(4AI)

Si(3AI)

Si(2AI)

Si(IA1)

Si(0AI)

1.7

2.4

2.5

1.o

1 .o

2.15

2.95

3.0

3.0

2.2

2.15

3.0

3.1

3.0

3.4

2.2

3.5

3.7

3.3

3.4

2.05

3.1

3.25

3.65

3.5

2.4

2.85

2.9

4.2

3.3

-

3.15

3.0

5.2

2.7

-

3.0

3.2

5.4

2.3

-

3.15

3.0

4.8

2.5

-

2.85

3.4

4.5

3.05

- 106.0

From ref. (58).

' Peaks of highest intensity (cf. Table V), the positions of which are fixed during simulation.

Denotes composition determined by analytical electron microscopy (energy dispersive X-ray analysis). Smi,, S, and A6 denote, respectively, the minimum and maximum values of corrected chemical shifts found for each Si(nA1) signal, and the span of these values (A6 = 6,,, - ami0).

228

J. M. THOMAS AND J. KLINOWSKI

TABLE V High-Resolution 19Si MAS N M R Peak Intensities in Synthetic Faujasites Determined by Computer Simulation, with SiIAl Ratios Determined from Spectra"

Normalized peak intensitiesb Sample number 1 2 3 4 5 6 7 8 9 10

(Si/AI),,, 1.19 1.35 1.59 1.67 1.87 2.00 2.35 2.56 2.61' 2.75

Si(4AI)

Si(3AI)

Si(2AI)

Si(1Al)

Si(0AI)

64.0 33.9 19.9 12.5 8.7 7.5 0 0 0 0

26.5 33.5 36.1 33.5 29.5 24.5 14.7 10.0 12.3 9.0

6.2 21.5 27.5 33.8 36.4 36.5 38.3 34.7 37.7 39.5

1.4 8.9 11.7 15.3 19.9 25.8 41.8 49.2 44.2 43.0

I .9 2.2 4.8 4.9 5.4 5.7 5.2 6.1 5.8 8.5

(Si/AI)NMn 1.14 1.39 1.57 1.71 1.85 1.98 2.46 2.69 2.56 2.69

Ref. 58. For convenience, when handling the experimental results, peak intensities were corrected for spinning sidebands and normalized to 100, i.e.,

"=O

Composition determined by energy-dispersive X-ray analysis.

It now seems that the Loewenstein rule is obeyed by all known zeolites, at least on a spatially averaged basis. It is of course possible that occasional Al-0A1 linkages are present as structural defects. Equation (15) provides the zeolite chemist with a powerful quantitative method for the determination of framework composition of zeolites. By comparing (Si/AI)NMR values with the results of chemical analysis, which gives bulk composition, the amount of nonframework (six-coordinated) aluminum can be calculated. This is of particular value in the study of chemically modified zeolites (see Sections II1,J-II1,M). Equation (1 3) works well for materials with framework Si/AI ratios below ca. 10. For more siliceous zeolites, the 29Si MAS NMR spectrum is dominated by the Si (4A1) signal and the estimation of composition becomes inaccurate. In these cases, the relative amounts of framework and nonframework aluminum can be estimated from 27Al NMR spectra (Section IIIJ).

D. SILICON-ALUMINUM ORDERING I N ZEOLITES X, Y, AND A The considerations of Section III,C prompt the question whether 29Si NMR can be of assistance in determining the ordering (if any) of Si and A1

229

ALUMINOSILICATE CATALYSTS

I

I

1.5

I

2 .o

I

1

2.5

3.0

Si/Al (R)

FIG.14. Plots of the average number, A, of aluminum neighbors for a silicon atom calculated for the “truly random” and the “Loewensteinian”distribution of Si and Al in a range of compositions of synthetic zeolites X and Y (59). Experimental points were calculated from the first moment of the spectra assuming constant half-width and regular spacing of Si(nAl) signals.

atoms in zeolites, beyond the restrictions of the Loewenstein rule. The answer is a qualified yes. First, it should be noted that MAS NMR yields spatially and temporally averaged information, and the spectrum, apart from the cases where there is only one signal, does not by itselfimply long-range order. It does, however, provide valuable subsidiary information. The obvious case to be considered first is that of synthetic faujasites, which come in a range of compositions, and for which a considerable amount of spectral information is available. Evidence of Si, A1 ordering in zeolites X and Y is provided by the presence of discontinuities in the plot of the (cubic) lattice parameter versus the Si/Al ratio (60), which indicates stepwise rather than gradual change in Si, A1 distribution. This effect is even more pronounced in synthetic faujasitic gallosilicates (61). Once the existence of Si, A1 ordering is accepted, the possible ordering schemes may be constructed. The areas under the peaks in the (deconvoluted) NMR spectrum are directly proportional to the populations of the respective structural units in the sample; it is therefore possible to estimate these from the experimental data (see Fig. 11) and to compare with the relative numbers of such units contained in models involving different Si, A1 ordering schemes.

230

1.

M. THOMAS AND J. KLINOWSKI

FIG.15. Two of the possible Si, A1 ordering schemes for zeolite X with Si/AI = 1.18. The ratio of intensities Si(4AI):Si(3AI):Si(2AI):Si(l AI):Si(OAI) corresponding to each scheme is given in the upper right-hand corner. E is the calculated electrostatic energy for the double cage in units of (9e)2/a,where a is the T-0-T distance. The asterisk denotes the scheme preferred by the authors of ref. 58 from which the figure is taken; p # 0 denotes the net dipole moment in a double sodalite cage.

Klinowski et al. (58) examined a great number of such models, and found that for most Si/AI ratios more than one ordering scheme is compatible with the Si(nA1) intensities determined by 29Si MAS NMR. They chose between the various ordering schemes on the basis of three criteria: 1. The degree of agreement between actual spectral intensities and those required by the given model; 2. Compliance with cubic symmetry and the correct unit cell repeat (ao = 24.7 A); 3. Minimum electrostatic repulsion within the aluminosilicate framework.

Figures 15 and 16 show the preferred ordering schemes for Si/AI = 1.18 and 1.67, respectively. The approach described above does offer support for

ALUMINOSILICATE CATALYSTS

23 1

FIG.16. Two of the possible Si, A1 ordering schemes (58) for zeolite Y with Si/AI = 1.67.

the view that a discontinuity in the unit cell parameter should occur at a welldefined Si/A1 ratio: it was found that the electrostatic repulsion energy per number of A1 atoms in the unit cell changes abruptly at Si/Al = 2.0. The question of Si, A1 ordering in zeolites X and Y was considered fully by Melchior et al. (59) and by Engelhardt et al. (55). While the details of their preferred models are sometimes different, the broad conclusions are similar to those reached by Klinowski et al. (58). As was mentioned earlier, 29Si MAS NMR does not by itselfimply Si, A1 ordering in zeolites, except when Si/Al = 1.00. It is therefore interesting to see whether the relative spectral intensities, calculated on the asumption that the distribution of tetrahedral atoms is random but subject to the restrictions of the Loewenstein rule, agree with the actual measured intensities. Calculating the aoerage relative populations of the five Si(nA1) building blocks is equivalent to calculating the expected intensities in the 29Si spectrum. Detailed analysis of 29Si MAS NMR spectral intensities of synthetic faujasites in the light of the “random” model has been given by Klinowski et al. (58), Peters (62), Mikovsky ( 6 3 , and Vega (64). A comparison of the observed and calculated ISi(”*,)intensities reveals a rather poor agreement for 1 R < 2, but, as can be expected, a better agreement as R increases.

-=

232

J. M. THOMAS A N D J. KLlNOWSKl

Unlike for synthetic faujasites, the presence of a single 29Siresonance in the spectrum of zeolite A (for which Si/AI = 1.00) (Fig. 12) does prove that the distribution of Si and A1 atoms in the framework of this zeolite is ordered. Single-crystal X-ray diffraction measurements (66,67) led to the conclusion that the Si and A1 atoms alternate throughout the framework. The magnitude of the chemical shift of the 29Si signal (- 89.2 f 1 ppm from TMS), indicating the non-Loewensteinian Si(3Al) rather than Si(4A1) ordering, was therefore surprising the signal was accordingly assigned (52,68) to Si(3AI) building blocks, and an alternative structure proposed (69) in which each Si atom in zeolite A is linked to three A1 atoms and one Si atom and vice versa. The controversy was finally resolved by the results of the independent 29Si MAS NMR experiments performed by Thomas et al. (56) and Melchior et al. (70),who examined the spectra of zeolite ZK-4, which is isostructural with zeolite A, but has the Si/AI ratio greater than unity. The reasoning behind these experiments was as follows. The 29Si NMR spectrum of ZK-4 must contain more than one resonance, because Si/AI > 1. It had been established earlier that for a given aluminosilicate structure the 29Si chemical shifts for each kind of Si(nA1) unit are only marginally affected by variation in Si/AI ratio. Signals corresponding to the same type of Si(nA1) unit in zeolite A and zeolite ZK-4 should therefore coincide. Thus, assignment of the signal in the spectrum of zeolite A is possible by matching it with one of the peaks in the spectrum of ZK-4. The 29Si MAS NMR spectrum of ZK-4 does indeed contain several peaks and good agreement between (Si/AI)NMR and (Si/Al)XRF is obtained when the resonance at -89.2ppm is assigned to Si(4AI) rather than Si(3Al). The original structure of zeolite A is therefore vindicated, and it is appropriate that a controversy caused by an NMR result was finally resolved by NMR. A full account of the controversy is given elsewhere (43,44). To summarize, the 29Si resonance of the Si(4Al) unit in faujasite is at ca. -83.9 ppm, but in zeolite A, which is structurally related (see Fig. 7), at ca. - 89.2 ppm. Thomas et al. (56) were the first to suggest that the unusually low chemical shift in zeolite A is due to the presence of a unique structural unit: strained double four-membered rings with Si-0-A1 angles of 129,152, 152, and 177".The presence of the nearly linear linkages modifies the bonding, which in turn affects the value of the chemical shift. This problem will be considered fully in Section II1,G. Early MAS NMR work indicated (71) that apart from zeolite A, zeolite Losod also displays Si(3 Al) ordering, and that sodalite and cancrinite may occur in either Si(4Al) or Si(3Al) ordering schemes. This is now thought to be incorrect. In Losod, cancrinite, and sodalite, Si/AI = 1.00, so that the assignment of the single-peak spectra was based on the value of the chemical shift alone. It has become apparent since (see Fig. 13) that the chemical shift

ALUMINOSILICATE CATALYSTS

233

TABLE VI 29Si M A S N M R Chemical Shifts far the Single

Signal Observed in Various Cationic Farms of Zeolite A"

*%i chemical shift (ppm from TMS)b

Cationic form Na-A Ba-A A$-A Ag-A with enclathrated AgNO, TI-A Li-A a

-88.9 -90.5 - 87.5 - 88.2 - 88.8

-85.1

Ref. 56. All values i-0.5 ppm.

ranges corresponding to Si(nAl) and Si[(n It: 1)All building units overlap considerably; futhermore, Klinowski et al. (71)'compared chemical shifts of different cationic forms of cancrinite. It is now clear (see Table VI) that 29Si chemical shifts are not insensitive to the type of cation. The origin of this effect is as yet uncertain, but it may be caused by the distortion of Si-0-T bonds (T = Si or Al) in the framework by highly polarizable cations, such as Li+. E. 29Si MAS NMR

OF

GALLOSILICATE ZEOLITES

It has been known for many years that A1 and Si in the zeolitic framework can be substituted by Ga and Ge, respectively, in the course of direct synthesis. Several (Si,Ga)- and (Ge,Ga)-zeolites (often containing Al as the third tetrahedral component) as well as (Ge,AI) materials have been reported, a d their overall structures were shown to be closely similar to their aluminosilidate counterparts. In particular, gallium analogues of sodalite and faujasite have been prepared by Selbin and Mason (72) and Kuhl (61), gallium thomsonite by Barrer et al. (7.9, and gallium analcime by Ponomareva et al. (74). A number of gallosilicate zeolites not containing aluminum have been prepared in our laboratory (75). Vaughan et al. (76) and Thomas et al. (77) measured 29Si MAS NMR spectra of the sodium forms of (Si,Ga)-sodalites and (Si,Ga)-faujasites. All preparations in ref. 76 contained some aluminum (attempts at preparing completely Al-free compounds were unsuccessful), but the amounts involved were so small (less than 5 % ) that the influence of A1 on 29Si spectra was

234

J.

M. THOMAS AND J. KLlNOWSKl

1

-70

d

-90

-1 10

ppm from TMS FIG.17. *'Si MAS NMR spectra at 39.5 MHz (76): (a) Zeolite ZK-4; (b) zeolite Na-X; (c) (Si.Ga)-sodalite.

negligible. (Si/Ga),,, ratios calculated from a formula similar to our Eq. (15) agreed closely with chemical analysis, which shows that in every case the gallosilicate equivalent of the Loewenstein rule applies (i.e., that no Ga-0-Ga linkages are present), and that the assignment of NMR resonances is correct. The composition of (Si,Ga)-sodalite was such that Si/Ga > 1, while (Si,Al)-sodalite crystallizes with Si/Al = 1.00. More than one 29Sisignal must therefore be observed (Fig. 17c). It was found that the distribution of signal intensities corresponding to the various Si(nGa) units in gallosodalite was

235

ALUMINOSILICATE CATALYSTS

TABLE VII A Comparison of Relative Intensities in the %i Spectra of ZK-4, Na-X, and (Si,Ga)-Sodalite"

ZK-4 (Si, AI>X (Si, Ga)-sodalite

(Si/AI) = 1.32 (Si/AI) = 1.27 (Si/Ga) = 1.28

1.32 1.74 1.25

3.18 2.16 1.98

' Ref. 76. T denotes A1 or Ga, as appropriate for each sample.

different from that measured in aluminosilicate zeolites ZK-4 and X of similar S i n ratio (T = A1 or Ga). Results given in Table VII indicate that the distribution of Si and Ga in gallosodalites, while always Loewensteinian, is different from the distribution of Si and A1 in aluminosilicate zeolites. The 29Sichemical shifts in gallium zeolites show interesting trends. First, as seen from Table VIII, they span a wider range 125.1 ppm in (Si,Ga)-X] than in the corresponding alluminosilicates [18.5 ppm in (Si,Al)-XI. Second, the Si(nGa) silicon atoms are deshielded (i.e., their chemical shift is less negative) in comparison with Si(nA1) silicon atoms with the same n in aluminum zeolites (Fig. 17 and 18). The difference is proportional to n: Si(0Ga) and Si(OA1) shifts are very similar, while the difference between Si(4Ga) and Si(4Al) is 6.6 ppm. This indicates that it is Ga in the first tetrahedral coordination sphere that is the main factor here. The origin of this effect is unknown, but Vaughan et al. (76) suggest that, as in aluminosilicate zeolites, it may be related to the magnitude of the T-0-T angle. The "nonbonded TABLE VIII Comparison of the Chemical Shifts for the Sodium Forms of Aluminofaujasite (Si,AI)-X and Gallofaujasite (Si, G a t X of Different Compositions"

Chemical shift (6, ppm from TMS) Sample

(Si/lhMR

Si(4T)

Si(3T)

Si(2T)

Si(1T)

Si(0T)

(Si, AI)-X (Si. AI)-X (Si, Ga)-X

1.14 1.17 1.11

- 83.9 -84.6 - 77.7 6.6

-88.1

-93.1 -94.2 -90.3 3.4

-97.9 -98.8 -96.4 2.0

- 102.4 - 103.1 - 102.8 0

AC

- 89.0 - 84.2 4.4

'From ref. (76). b T = Al or Ga. A denotes the difference

0.

- (ahi,Ai for signals with the same T.

236

J. M. THOMAS AND J. KLINOWSKI

1 . 1 . I

-70

-90 -110

-

-70

-90 -110

1 I I . 1

-70

-90

-110

ppm from TMS FIG.18. "Si MAS NMR spectra of (Si, Ga)-faujasites (76) containing small amounts of aluminum and with Si/(Ga + Al) ratios of (a) 1.11, (b) 1.26, and (c) 2.00, compared with the spectra of their (Si, Al) analogues with Si/Al ratios of (d) 1.24, (c) 1.44, and (f) 1.95.

radius" approach by O'Keefe and Hyde (78) indicates that Si-0-Ga angles would generally be smaller than Si-0Al angles, and consequently gallium substitution would tend to deshield the silicon atom. The "Ga MAS NMR spectra of gallosilicate zeolites are rather too broad [full width at half maximum (FWHM) ca. 60 ppm] for ready interpretation ( 77).

F. HIGHLY SILICEOUS ZEOLITES In naturally occurring zeolites the Si/AI ratio is always less than ca. 5, but materials with much lower A1 contents can be prepared in the laboratory (36). Some of them are potent catalysts, and have the added bonus of high thermal stability due to high silicon content. In 1972 the Mobil Oil Corporation synthesized and patented a range of highly siliceous zeolites. ZSM-5 with the unit cell formula Na,A1,Si9,~,0,,,~16H,0 with x < 27 and typically about 3 (corresponding to Si/AI = 31) is the best known member of this range (33, 79); ZSM-11 is another. The structure of ZSM-5 is based on two interlinked channel systems formed by 10-membered rings ca. 5.5A in diameter (see Fig. 19). Zig-zag

ALUMINOSILICATE CATALYSTS

237

FIG.19. The structure of ZSM-S/silicate: (a) Secondary building units (indicated by bold lines) each composed of 12 tetrahedral atoms are linked into chains, one of which is shown in the c direction; (b) The chains are interlinked to form a three-dimensional framework in which there are 10-membered ring openings (5.5 A in diameter) running in the [OlO] direction. In this portion of the ac structural projection, 0 denotes a tetrahedral site.

channels are running in the [lo01 direction, straight channels in the [OlO] direction. ZSM-5 possesses remarkable sorptive properties, stemming from the desirable aperture size and large internal volume. It is a powerful acid catalyst, capable of converting methanol into gasoline and the benzene/ ethylene mixture into ethylbenzene. A crystalline microporous material called silicalite, isostructural with ZSM-5 but containing only traces of aluminum, was described by Grose and Flanigen (80) and Flanigen et al. (81). Unlike ordinary zeolites, silicalite is organophilic and hydrophobic and can remove from water a variety of dissolved organic compounds. Both ZSM-5 and silicalite display remarkable “shape selectivity”: because of the geometry of the channels only certain reactants may enter and diffuse through the crystals, and only certain products may diffuse out of the intracrystalline space. In the light of what has been said in Sections II1,B-III,D about the appearance and interpretation of 29Si MAS NMR spectra of zeolites, one might expect the spectrum of a highly siliceous zeolite to be uncomplicated displaying a single Si(4 Si) signal, sometimes with a smaller Si(3Si) resonance,

238

J. M. THOMAS AND 1. KLINOWSKI

I

I

-80

-100

I

-120

I

-140

ppm from TMS

FIG.20. 29Si MAS NMR spectrum of zeolite ZSM-5 (Si/AI

= 33.3) at 79.80 MHz

(90).

depending on the Si/Al ratio. The spectrum of a typical sample of ZSM-5is indeed featureless (Fig. 20). The discovery by Fyfe et al. (82) that the spectrum of a sample of silicalite with a particularly low aluminum content shows considerable fine structure (Fig. 21) was therefore something of a surprise. The resolution of the spectrum is exceptional-superior to that observed in any other zeolite. The complete spread of the components is ca. 6 ppm, and the chemical shifts of all the peaks are characteristic of Si(4Si) groupings in highly siliceous materials. The observed multiplicity must arise from the crystallographically nonequivalent tetrahedral environments of the Si(4Si) sites. The spectrum may be simulated by a minimum of nine Gaussian signals, the intensities of which are approximately in the ratio 1 :3:2:3: 10: 1: 1:2: 1 (Fig. 21b). The relative intensities did not show perceptible change on a 10-fold increase in the delay time of the experiments, which indicates that they are quantitatively reliable. Using the intensities of the well-resolved lowest and highest field signals as base units of one, the total intensity of the peaks in the Si(4Si) multiplet is found to be approximately 24. This suggests that the space group of silicalite is one which contains 24 nonequivalent sites in the structural repeat unit, rather than the Pnnia space group, which had been favored earlier, but which has only 12 distinct sites. More recently an even better resolved "Si spectrum of silicalite has been obtained (471) (Fig. 22). It is premature, at this stage, to attempt to assign individual 29Sisignals to specific crystallographic sites, mainly because the details of the structure of silicalite are not sufficiently well established.

-113.2

-113.1

I

I

-112.5

-111.6

\'

'

-1163

-108

.

-110

-112

-114

-116

-118

-100

-165

-iio

-115

-120

-125

Ppm from T M S

.

-108

-108

-110

-110

-112

-114

-112 -114 ppm from T M S

-116

-118

-116

-118

FIG.21. (a) High-resolution 29Si M A S NMR spectrum of silicalite at 79.80 MHz (82); 6550 free induction decays were accumulated; repetition time 5 sec. (b) The spectrum given in (a) can be computer-simulatedusing the minimum number of nine Gaussian-shaped peaks, shown individually below the simulated spectrum. The areas of the peaks are, from left to right, in the ratio 0.98:2.70:2.19:2.63: 10.35: 1.30: 1.61: 1.87:0.82 (see text).

240

J. M. THOMAS AND J. KLINOWSKI 16.71

1

-108

-112

-110

-116

-114

-110

ppm from TMS FIG.22. *'Si MAS NMR spectrum of silicalite with Si/AI > loo0 at 99.32 MHz (471). Relative intensities of signals (normalized to 24) are indicated.

CRYSTALLOGRAPHICALLY NONEQUIVALENT TETRAHEDRAL SITES G . RESOLVING A few cases have come to light in which crystallographic nonequivalence of tetrahedral sites for silicon has been reflected in the 29Si MAS NMR spectra of zeolites. The most complex and the best resolved example is the spectrum of silicalite which was discussed in Section 111,F; another is the spectrum of the well-ordered natural zeolite scolecite (57), which contains two signals corresponding to nonequivalent Si(3AI) units (see Fig. 23). The correctness of 2

-70

-9Q

-1

io

ppm from TMS

FIG.23. "Si MAS NMR spectrum of natural scolecite (CaAI,Si,0,,~3H,O) from Poona, India, at 79.80 MHz (57). For this material (si/Al)NMR = 1.57 and (Si/AI),,, = 1.53.

24 1

ALUMINOSILICATE CATALYSTS

the assignment of the latter spectrum is supported by the agreement between and (Si/Al)xRF and also by the reference to the structure of scolecite as determined by Falth and Hansen (83). Using the number of Si and A1 sites as given in that reference, Klinowski et al. (57) assigned the two Si(3Al) signals to the nonequivalent Si,(A11AI,A12Si, ) and Si3(AllAllA12Sil) silicon atoms. However, apart from silicalite, the spectra of synthetic zeolites do not reveal signals that may be assigned to nonequivalent silicon sites. In particular, no fine detail is observed in the spectra of zeolite ZSM-5 (Si/AI ratio typically ca. SO), which is isostructural with silicalite, and of similar degree of crystallinity (see Fig. 20). However, studies of highly siliceous

-90

-10

I

-110

-100

,

,

,

-120

-80 -90

I

,

-80 -90 -100 -110 -1.70

-90

-100 -110 -120

I

I

I

-100

-110

-120

ppm from TMS FIG.24. 29Si MAS NMR spectra at 79.80MHz (above) of zeolite Y, zeolite omega (synthetic mazzite), offretite, and mordenite, and their dealuminated forms (below).

242

J. M. THOMAS AND J. KLlNOWSKl

I

\

I

-100

I

-110

-120

ppm from TMS

FIG.25. 19Si MAS NMR spectra at 79.80 MHz of zeolite ZSM-5(top) with Si/AI = 33.3 (compare Fig. 20) and of its hydrothermally dealuminated form (bottom) (471).

zeolites have revealed that when A1 is isomorphously replaced by Si in the course of treatment with silicon tetrachloride vapor or ultrastabilization (Section III,J and III,M) the resolution of the spectrum improves, and both the width and the chemical shijt of the Si(4Si) signal are affected. Since the overall zeolitic structure is unchanged, these observations strongly suggest that the resolution of the 29Si MAS NMR spectra of zeolites is ultimately governed by the amount of A1 present and/or its distribution in the framework. Fyfe et al. (8449) and Thomas et al. (90, 91) tested this hypothesis by gradually decreasing the A1 content of a number of zeolites, while monitoring the 29Sispectrum. Figures 24-26 show the 29Sispectra of zeolites before and after hydrothermal treatment (91,92).The framework Si/Al ratio in the products was of the order of several hundred. The intensities of the two signals in the spectrum of the siliceous zeolite omega are in the 2: 1 ratio. While zeolite omega is more appropriately called synthetic mazzite (94), both the previously suggested structure (95) and the structure of mazzite (96) call for two nonequivalent tetrahedral sites in a 2: 1 population ratio. Accordingly, Thomas et al. (91) were able to assign the

243

ALUMINOSILICATE CATALYSTS

-100

- 1 10

-120

ppm from TMS

FIG.26. 29Si MAS NMR spectra (471) at 79.80 MHz of the novel zeolite theta-1 (439) (top) and its hydrothermally dealuminated form (bottom).

signals to these sites (see Table IX).A similar situation obtains in dealuminated offretite. The structure of mordenite, on the other hand, is known to containfour nonequivalent types of site in the 2: 1:1 :2 population ratio, and it is clear that the largest signal at - 115 ppm in Fig. 24 must be a composite. The spectrum of dealuminated ZSM-5is essentially identical to that of silicalite, but as we already said in Section III,F, the assignment of the individual signals to specific types of site is not yet possible.

244

J. M. THOMAS AND J. KLINOWSKI

TABLE IX Structural Parameters of Some Zeolites and the Chemical Shiji for the Si(4Si) Resonances" Zeolite

SijAl ratio (parent material) ~~

4.24

Offretite

3.67

Mordenite

6.2

Y

2.61 4.82 1.66

ZK-4

29Si chemical shift (ppm from TMS)

Signal assignment

~

Omega

TMA-sodalite

T-0-T angleb 140.8 151.9 142.5 151.3 150.4 152.3 I56.W 144.8 158.0 148.0

Ref. 91. bThis value represents the mean of the four T-0-T tetrahedral site. Average value for 158.1" (Si,) and 153.9"(Si3).

- 106.0 - 114.4 - 109.7 - 115.2 - 112.2 -113.1 -115.0 - 107.1 - 116.2 -111.0

T,

a

angles which define a central

Fyfe et al. (89) were able to dealuminate hydrothermally even zeolite ZK-4, which is isostructural with zeolite A and has a relatively low Si/Al ratio (Fig. 27). In the light of the preceding discussion one might expect crystallographic nonequivalence of the various Si sites to appear not only when Al is isomorphously substituted by Si, but also when zeolites are purposely synthesized with a very high Si/AI ratio. One such example is silicalite which was fully described in Section III,F. Unfortunately, relatively few zeolites can be synthesized with a sufficiently high Si/AI ratio for such detail to become perceptible in the spectra. Higgins et al. (97) measured high-resolution 29Si NMR spectra of two samples of zeolite ZSM-39 with Si/AI ratios of 93.5 and 46.8, respectively. The crystal structure of ZSM-39 is known (98) and the tetrahedral framework is isostructural with the 17 A cubic gas hydrate and consists entirely of five- and six-membered rings. The zeolite contains only three kinds of silicon site in the T,:T,:T, = 1:4:12 population ratio. The spectra contain three distinct signals in the required intensity ratio, while the T, signal in the more siliceous sample is composed of three superimposed peaks. Higgins et al. (97) conclude that this probably indicates loss of face centering of the Fd3m unit cell, which results in the disappearance of the three-fold symmetry axis along [ 11 13, making T, sites nonequivalent. X-Ray diffraction (XRD) studies of ZSM-39 support this conclusion (98). Kokotailo et al. (99) measured the spectra of very highly siliceous samples of the zeolite

245

ALUMINOSILICATE CATALYSTS

c

Y

A

*

l

-60

a

’

*

I

-100

8

.

*

1

-120

p p m from MeLSi

.

L

L

50

.

I

u)

1

1

.

L

.

30 20 20 (degrees)

1

I

10

5

FIG.27. 29Si MAS NMR spectra at 99.3 MHz (left) of (A) zeolite Na-A, (B) zeolite ZK-4, (C) zeolite ZK-4 treated hydrothermally at 700°C for 48 h; (D)-(F) are the corresponding powder X-ray diffraction patterns. Taken from ref. 89.

and found that the T, resonance was resolved into three components of equal intensity designated as T3,Ti and TY (Fig. 28). Another NMR study of silica clathrates, termed “clathrasils,” involving 29Siand I3C, was carried out by Groenen et al. (100). A very interesting development was the discovery (101) that the 29Si spectra of silicalite are highly sensitive to even small amounts of sorbed organics such as ethanol, 1-propanol, n-decane, and especially benzene (Fig. 29). The transformation of the spectra is accompanied by a distortion of the silicate framework, as demonstrated by X-ray diffractometry (102). As the data bank of 29Sichemical shift values has grown, Thomas et al. (56) suggested that a correlation exists between the chemical shift values and the T-0-T angles in zeolitic frameworks. As we have seen in Section III,D, the

246

J. M. THOMAS A N D J. KLlNOWSKl

r

l

-lo0

l

l

-110

l

l

-120

I

1

-1 30

8 (ppm from MqSi) FIG.28. 29Si MAS NMR spectra (99) at 79.5 MHz of zeolite ZSM-39after calcination at 450°C: (a) Si/AI = 285, (b) Si/Al = 2400, (c) Si/AI = 310; (d) computer simulation of the experimental spectrum in (c) in terms of Gaussian curves.

247

ALUMINOSILICATE CATALYSTS

-105

-1 15

-125

FIG.29. 29SiMAS NMR spectra ( K J I ) at 39.76 MHz of silicalite (Si/AI) > 4400) containing sorbed organic molecules. (A), parent material; (B), containing ethanol; (C), containing 1propanol; (D), containing n-decane; (E), containing benzene.

Si(3Al) signal in zeolites X and Y is coincidental with the Si(4Al) signal in zeolites A and ZK-4, and they suggested that this related to the larger value of the Si-0-A1 angle in the latter structures. Soon after, Grimmer et al. (103) measured the unusually negative (- 128.5 ppm from TMS) chemical shift in the (nonzeolitic) mineral zunyite and commented on the apparent relationangle in zunyite, which is almost 180". ship between 6 and the Si-0-Si

J. M. THOMAS 4ND J. KLINOWSKI

0 Zeolite

R

0 Offretote A Mordenite

0

Zeolite Y

@ TMA

t

- Sodalite

ZK-4

I

FIG.30. Relationships between the mean T-0-T for the Si(4Si) signals in zeolites (56).

I

\

angle and isotropic *'Si chemical shift

Spectral information on several dealuminated zeolites ( 9 4 , as well as on ZK-4 and TMA-sodalite studied by Jarman (104), who also considered the quantitative relationship between 6 and 8, is given in Table IX. The average values of 8 for hydrothermally dealuminated zeolite Y and dealuminated acid-washed mordenite, as determined by X-ray diffraction, are very close to those in nondealuminated materials, and Thomas et a/. (56) assumed that this is also true for other dealuminated zeolites. They proposed the following linear correlation between 8 and 6 (see Fig. 30):

where 6 is given in ppm from TMS and 8 in degrees. This line also fits the exceptional 8 for zunyite as well as several naturally occurring polymorphs of silica ( 105). Higgins and Woessner (106) correlated 29Si chemical shifts in the framework silicates cristobalite, quartz, albite, and natrolite with the mean Si-0 bond distances and arrived at the following relationship: A =

1.3717 x lo3 x

d&,

- 2312

(6, ppm from TMS),

(17)

which is in good agreement with the result of semiempirical calculations (108). Smith et al. (111) proposed quantitative relationships between chemical shifts and several structural parameters in the albite-microcline series. The earliest studies of the relationship between the 29Sishielding tensor and the length of the Si-0 bond (107) involved polycrystalline silicates

249

ALUMINOSILICATE CATALYSTS

Ca, [Si,O,(OH),] and [(CH,),N],Si,O,,~69H2O.A semiempirical quantum-chemical rationalization of the correlation between 29Sichemical shift in framework silicates and the change of the s character of the oxygen orbitals in the Si-0-Si CJ bond has been proposed by Engelhardt and Radeglia (109), while Tossell (110) correlated chemical shifts and the orbital energy differences obtained from X-ray spectra. Similar relationships obtain between structure and chemical shift for other nuclei, notably 31P(112-114). Equation (16) correlates the chemical shift of the Si(4AI) signal with the magnitude of the T-0-T angle. There is evidence, however, that similar relationships exist for Si(nA1) signals with n # 0. Indeed, this line of inquiry was triggered by the large difference in the Si(4Al) chemical shifts between zeolites A and X. In an attempt to extend these arguments to allfive Si(nA1) signals, Ramdas and Klinowski (115) used the accumulated NMR data on many zeolites to derive a semiempirical relationship between the isotropic chemical shift of an Si(nA1) signal and E d T T , the total (nonbonded) Si . . T distance (T = Si or Al) calculated from the T-0-T angle, assuming constant Si-0 and A1-0 bond lengths of 1.62 and 1.76 A, respectively:

6 = 143.03 - 20.34 where 6 is given in ppm from TMS and

dTT

1

dTT,

in

A, and

(18)

C dTT= [3.37n + 3.24(4 - n)] sin 26

-

The procedure was as follows. First, 6 was plotted as a function of E d,, for Si(OA1) units. An approximately linear relationship was found (see the lowest plot in Fig. 31). Next, assuming the same slope and including an additional term, 7.954 to account for the paramagnetic contribution of n tetrahedral aluminum atoms to the chemical shift of the central silicon, the remaining four lines in Fig. 31 were drawn for Si(nA1) units with n = 1,2,3, and 4 (these lines are therefore predictions rather than fits to experimental points). It is evident that the experimental data are close to the theoretical lines, despite the empirical character of the expressions. Ramdas and Klinowski suggest that their work may serve as a basis for the reliable estimation of the average value of the T-0-T angle for each kind of silicon in an aluminosilicate of unknown structure (including amorphous materials) from 29Si MAS NMR alone. Radeglia and Engelhardt (216)explained the observed correlation between 29Sichemical shifts and the mean Si-0-T bond angles in terms of a simple quantum-mechanical model. They derived the following correlation:

6=6,+nxa+

b cos e cos 6 - 1 ’

250

J. M. THOMAS AND J. KLINOWSKI

Ed,,

th

FIG.31. Plots of isotropic 29SiMAS NMR chemical shifts (in ppm from TMS)versus C d, (in A), the calculated total Si ...T nonbonded distance, for Si atoms in the five kinds of Si(nAl) tetrahedral environments.Aluminosilicatescorrespondingto the various points can be identified by consulting ref. 115 from which the figure is taken.

where 6 is given in ppm from TMS, and do, a, and b are constants. The correlation is confirmed by an extended set of experimental data, and is consistent with models involving 8 and sin (8/2) proposed in the literature, in particular with that of Ramdas and Klinowski (115). It is now easy to see why 29Si MAS NMR signals in the spectra of certain zeolites, such as zeolite omega, lead to incorrect (Si/Al)NMRvalues when assigned to individual Si(nA1) units in the simple fashion described in Section 111,C. When Ad, the chemical shift difference between signals from nonequivalent Si(nA1) units with the same value of n (8.4 ppm in the case of Si,(OAl) and Si,(OAl) in zeolite omega), is similar to the shift difference between Si(nA1) and Si[(n f 1)All units, the effects of A1 substitution and crystallographic nonequivalence overlap. The signals are composites, and the intensities ISl(nAI) cannot be directly read off the spectrum. In the case of zeolite omega, the actual spectrum is the sum of two mutually overlapping families of signals, which we denote Si,(OAl), Si,(l Al), SiA(2Al), and Si,(OAl), Si,(l Al), and SiB(2Al).The entire "Si MAS NMR spectrum of zeolite omega has been interpreted in this way (117). When Ad is observable, but much less than the difference between Si(nA1) and [Si(n k 1)Al] chemical shifts, a broadening of the spectral linewidths is observed. For example, the "Si spectrum of chemically untreated mordenite is broad because it is a superimposition offour sets of signals.

25 1

ALUMINOSILICATE CATALYSTS

In the future, simplification of NMR information brought about by dealumination will enable spectroscopists to identify such overlaps and reach correct interpretation of the spectra. DETERMINING RESOLUTION, LINESHAPE, AND RELAXATION H. FACTORS The earliest 29Si MAS NMR spectra of zeolites, measured in a magnetic field of 2.35 T or lower, showed modest resolution. The considerable improvement in resolution on increasing the field (compare Fig. 32a and b) must therefore be due to the removal of some field-dependent effect. Resolution increases until fields of ca. 4.70Tare reached, and no further improvement is observed (as opposed to S / N ) above this value. Dipolar 29Si-29Sicoupling is unlikely to be responsible, in view of (1) the magnetic dilution of the 29Sinucleus; (2) the weakness of such coupling, which is in any case removable by MAS; and (3) the fact that dipolar interactions are field independent. Melchior (118) was first to suggest that the likely cause is the 29Si-27A1dipole-quadrupole interaction. Such interactions give rise to splitting and broadening of I3C MAS NMR spectra of a-carbon atoms in amino acids (119-123), effects which disappear when 14N (spin 1) is substiIn zeolites an analogous effect is caused by the tuted by 15N (spin deviation of the quantization axis for 27Al from the direction of the applied magnetic field caused by the quadrupolar interaction; the dipolar Hamiltonian then contains angle-dependent terms other than (3 cos' 0 - 1) and is not averaged to zero by MAS. It has been shown (124-126) that these

4).

Si( 2 A l I

-80

-100

-120

ppm from T M S

FIG.32. *'Si MAS NMR spectra of zeolite Na-Y (Si/AI = 2.61) at two magnetic fields (44): (a) 17.96 MHz spectrum; (b) 79.80 MHz spectrum.

252

J. M. THOMAS AND J. KLINOWSKI

interactions, which are inversely proportional to the cube of the internuclear distance, are also strongly dependent on the magnetic field. It seems likely that the 29Si-27A1coupling is significant mainly in Si(nA1) units with n # 0 (note that the Si(4Si) signal is narrow even at low fields) and that it is eliminated in the fields of 4.70 T or larger. Solid-state spectra of spin nuclei and their second moments due to magnetic dipolar coupling with nuclei of spin >$in static and MAS experiments have been simulated (153). It has been shown before that the removal of aluminum from the framework leads to very marked narrowing of NMR signals. When no A1 is present, the Si(4Si) lines are very narrow indeed (50.7 ppm). Fyfe et al. (84-88) studied the effect of dealumination on linewidths of Si(4Si) signals in several zeolites. They found that substantial line narrowing occurs at Si/AI > 100, which indicates that the effect must be long range in nature. They suggest (86) that it is caused by a chemical shift interaction due to distribution of A1 in the second-nearest and further coordination shells of silicon. Following Meier and Moeck (127),Klinowski (128)considered this effect in terms of “coordination sequences” N,, giving the number of T-atoms in the ith coordination sphere of the atom under observation. Thus, N , = 4 (by definition), and for the faujasite structure N, = 9, N , = 16, N , = 25, and N , = 37. Consider the average number, of aluminum atoms in each coordination sphere (see Table X). The Nf’ aluminum atoms in the ith coordination sphere may be distributed among the N i T-sites in many different magnetically nonequivalent ways, each giving rise to a different chemical shift for the central Si atom: each Si(nA1) peak is in fact a sum of many narrower signals. Intrinsic crystallographic nonequivalence of Si atoms in certain zeolites is superimposed on this effect. Klinowski and Anderson (129) considered the range of the chemical shift effect necessary to account for the experimental results. They noted that “nth coordination sphere” rarely implies distances of the order of n x dTT,where

mf’,

Average Number, &:‘,I

Na-Y Dealuminated zeolite Y a

Ref. 128.

TABLE X ofAl Atoms in the ith Coordination Sphere of a Si(4Si) Atom in Faujasites of Diflerent SiIAl Ratio“

2.61 55

0 0

2.61 0.17

4.64 0.30

7.24 0.47

10.72 0.69

25.21 1.63

ALUMINOSILICATE CATALYSTS

253

is the average distance between neighboring T atoms. For example, Si atoms facing one another across the 10-membered channels in ZSM-5, i.e., in one another’s$fth coordination sphere, are ca. 5.5 A apart, while d T T = 3.0 A. They then calculated the average distance, cTsi-Al, in various zeolites for different A1 contents. For example, in ZSM-5 with Si/AI = 33, cTsi-Al = 5.7 A. This quantity is proportional to the cube root of the Si/Al ratio and thus increases slowly with dealumination, which is why very high Si/Al ratios must be reached before the effect becomes negligible. Hays et al. (130) were the first to observe that thermal treatment of ammonium-exchanged ZSM-5 improves the resolution of the 29Si MAS NMR spectrum to the point when it becomes indistinguishable from that of silicalite; treating Na-exchanged ZSM-5 has no such effect. In order to explain this unexpected result, some of the same authors (131) monitored heat treatment of numerous samples of H + , N H f , and TPA+-exchanged ZSM-5, as well as Na+-exchanged samples. For the first category they observed a marked improvement in resolution, and argue that it is due to the removal, at high temperatures, of adjacent silanol groups present in the structure as crystallographic defects, resulting in ring closure in the silicate framework. Woessner and Trewella (132) observed indirect (electron-coupled) nuclear spin interactions in the 29Si MAS NMR spectra of low albite. Three signals were measured, corresponding to T2M, T 2 0 , and T I M silicon sites, respectively. The last two signals were split and the magnitude of the splitting was field independent. This is the first time such effects, which are not normally observed in “true solids” (3), have been found for the 29Si nucleus. The chemical shift anisotropy of the 29Si nucleus is generally small, and thus unlike in 13Csolid-state NMR at high fields, no sideband problems are encountered in MAS spectra of framework silicates. 29Si spin-lattice relaxation times of crystalline aluminosilicates are relatively short in comparison with those for 13C in solids. While until recently there was no systematic study of Tl in zeolites, it was generally assumed that it is controlled by spin diffusion from paramagnetic centers. Farlee et al. (133) measured T,’s of 5-30sec in various zeolites (using composite pulse inversion at 60 MHz) and found that, for a given zeolite, TI of a silicon atom in a Si(nAl) unit varies little with n, and is also insensitive to a change in the Si/Al ratio. This indicates that 29Si relaxation is not affected by the presence of 27Al and also explains why short recycle times used by most workers (typically ca. 5 sec, i.e., much shorter than 5T1)still give quantitatively reliable spectra. On the other hand, relaxation is strongly dependent on the state of hydration of the zeolite. For example, fully hydrated zeolite Na-A had Tl = 7 sec, but this increased to 49 sec on dehydration of the sample (133). When the dehydrated zeolite was saturated with D20, relaxation remained slow (Tl = 33 sec). This dTT

254

J. M. THOMAS AND J. KLlNOWSKl

result indicates that dipolar contribution of 29Si in zeolite Na-A, and probably other zeolites, is dominant. There is obviously a significant component of the spectral density of the proton motion at 60 MHz. The lengthening of TI after the H,O + D20exchange, and the presence of nuclear Overhauser enhancement suggest that paramagnetic impurities do not play a significant part in ,'Si spin-lattice relaxation in hydrated synthetic zeolites. It is also likely that relaxation is field dependent. Moreover, the degree of crystallinity appears to be very important: when a sample of a synthetic zeolite is made amorphous by thermal treatment, Tl increases by orders of magnitude, while the concentration of paramagnetics remains unchanged. Sometimes MAS spectra of minerals show particularly broad lines and spining sidebands. Oldfield et al. (152), who measured such spectra for 29Si, 27Al,and 'jNa in the feldspar sanidine [(K,Na)AlSi,O,], argue that these effects are due not to chemical shift anisotropy, but rather to the presence of large magnetic susceptibility broadening. "Si spin-lattice relaxation in zeolite ZSM-39 (97) and a range of zeolites ZSM-5 (134) have been studied. As-synthesized zeolite ZSM-5 with TPA as template has very long TI (up to 145.1 sec for Si/Al = 5000) in a 300 MHz magnet. When 1,6-hexanediol is used as template, Tl was as short as 2.6 sec. Calcination causes a marked reduction of Tl (to several seconds) in all samples. Further reduction of Tl was observed when cyclohexane or benzene were adsorbed. +

I. 27Al NMR STUDIES OF ZEOLITES 27A1 is a very favorable nucleus for NMR investigations: it is 100% abundant, with I = and a chemical shift range of ca. 450 ppm (135). The linewidth of the 27Al resonance signal is a sensitive function of the symmetry of the nuclear environment. Useful chemical information can be obtained from the spectra provided quadrupole-coupling and chemical-shift effects can be separated. There are three situations in which this is possible: in solution, in single crystals, and under conditions of fast specimen rotation. With quadrupolar nuclei of noninteger spin, the central (ic+ - 5 ) transition, the only one which is normally observed, is independent of the quadrupolar interaction to first order as illustrated in Fig. 3, but is affected by second order quadrupolar effects which are inversely proportional to the magnetic field (see Section 11,D).The best spectra will therefore be obtained at very high magnetic field strengths. The "Al resonance is useful in the study of ionic solutions, since the rates of exchange around the trivalent cation are so slow that separate species can

255

ALUMINOSILICATE CATALYSTS

TABLE XI Nuclear Quadrupole Coupling Tensors in Some Aluminosilicates Coordination number eQVJh of aluminum (MHz)

Mineral Spodumene (LiAISi,O,) Euclase (HBeAISiO,) Beryl (&A1,Si,O,,) Natrolite (Na,AI,Si,O,, Albite (NaAISi,O,) Microcline (KAISi,O,) Kyanite (AI,SiO,)

Sillimanite (AI,SiO,) Andalusite (AI,SiO,)

. 2H,O)

6 6 6 4 4 4 6 6 6 6 6 4 6 5

2.950 5.173 3.093 1.663 3.29 3.21 10.04 9.37 6.53 3.70 8.93 6.77 15.7 5.9

rt"

Reference

0.94 0.698 0 0.5029 0.26 0.21 0.27 0.38 0.59 0.89

Not determined 0.08 0.69

Maximum eigenvalues and asymmetry parameters.

often be observed (135). In particular, the spectra of alkaline aluminate compounds have been extensively studied (136). It was found that in aluminate anions four-coordinated aluminum (with respect to oxygen) resonates at 60-80 ppm from the six-coordinated A1 in AI(H,O):+. Thus, 27Al NMR is a sensitive tool for determining the coordination of aluminum. Conventional single-crystal studies of aluminosilicates date back to the early 1950's, and principal components of the quadrupole-coupling tensors together with asymmetry parameters have been determined for a number of minerals (see Table XI). Wide-line-NMR measurements of the dependence of the 27Al and 23Na quadrupole-coupling constants in analcime and zeolites Na-X and Na-A on the water content were carried out by Gabuda et al. (137). Alkali and alkaline-earth aluminosilicates are insoluble, and Miiller et al. (138)resorted to tetramethylammonium (TMA) aluminosilicates to measure 27A1 chemical shifts in aluminosilicate solutions. Solutions with different Si/AI ratios and the pure TMA aluminate solution were studied. The molar ratio TMA0H:Si:Al varied from 3:0:2 to 9:6:2. Theoretically there are 15 distinct Q"(mSi) units with Q = A1 (n 0-4; m 0-n). However, dimeric aluminate anions are found only in very concentrated solutions and even then in very small quantities, which led the authors to suggest that the Loewenstein rule is obeyed in aluminate and aluminosilicate anions. The exclusion of AI-0-A1 linkages limits the number of possibilities to five

256

J. M. THOMAS AND J. KLINOWSKI

Q"(nSi) structural units with n = 0, 1, 2, 3, and 4. Several resonances were identified in the spectra, and the proposed interpretation of the various lines is that in the series where Q denotes the central four-coordinated Al atom the chemical shift decreases as follows: Q"

Q'(1Si)

Q'(2Si)

Q3(3Si)

19.5

14.3

69.5

64.2

ppm from AI(H,O)i+

where the numbers below the Q symbols denote chemical shifts of the proposed groupings. No Q4(4Si) signal was observed. Muller et al. (139) were the first to carry out a systematic investigation of 27Al MAS NMR spectra of polycrystalline aluminates (at 70.4 MHz). They found that the isotropic 27Al chemical shifts depend primarily on the coordination of aluminum with respect to oxygen. For the tetrahedral coordination, chemical shifts of 55 to 80 ppm from Al(H,O);+ were observed, while octahedral Al resonated at 0-22 ppm. These results are in full agreement with the studies of Akitt et al. (150) in aqueous solutions of various aluminum species. As the structure of a number of aluminates has not been fully established by X-rays because single crystals of necessary size were not available, this result is of considerable value for structural elucidation. Thus, only six-coordinated Al was found (140) in 2Ca0.Al20,.8H,0 and C a 0 ~ A l , 0 , ~ 1 0 H 2 0both , of unknown structure. The presence of a range of 27Alchemical shifts, both for four- and six-coordinated aluminum indicates that the shift is influenced not only by the coordination number, but by other effects such as the composition of the second coordination sphere and the nature of the cation. The first 27Al MAS NMR study of zeolites was carried out by Freude and Behrens (151). They measured, first, chemical shifts and half-widths of signals from stationary samples of zeolites Na-A, TI-A, Na-Y, and TI-Y at 16 MHz. For MAS frequencies of vR such that vR > v:/vL the central line of the 27Al resonance is reduced to about f of its original value. Freude and Behrens next calculated the quadrupole frequencies V , and shifts of the center of gravity of each line due to the quadrupole interaction, i.e., V, = ( v p - v L ) / v L at 70 MHz. Then, apparent line positions and line widths, 6:; and G V : : ' ~ ~ , were measured experimentally using MAS at 70 MHz. The corrected chemical shift value at 70 MHz was then calculated from the relationship 6 , = 6,, - 6,. They were several ppm different from the apparent values (see Table XII). Line narrowing of 27Al MAS spectra has been considered in detail (14-16,141,154). In terms of quantities defined in Eq. (8) the characteristic line width, s, for a quadrupole nucleus is (141) s = Vi[I(I

+ 1) - $]/12VL.

(20)

257

ALUMINOSILICATE CATALYSTS

TABLE XI1 Calculation of the Corrected Value of the 27Al Chemical Shift in Various Zeolites from the Spectra of Stationary Samples at 16 M H z and ofRotating Samples at 70 M H z " . ~

~

Na-A (hydrated) TI-A (hydrated) TI-A (dehydrated) Na-Y (hydrated) TI-Y (hydrated)

4.2 2.9 8.4 5.1

220 183 310 256 211

4.1

-2.1 -1.8 -5.3 -3.5 -2.6

~~

414 282 130 564 511

56.0 55.5 51.1 51.9 59.1

58.1 51.3 56.4 61.4 61.1

From ref. 151. For explanation of symbols see text.

For 27Al,I = $, and we have s = 2Vi/3VL

(21)

In order for a line to be narrowed by MAS the spinning frequency vR must be greater than s. It follows that the largest interaction which can be narrowed in an 11.7 T magnetic field (in which vL = 130.32 MHz for 27Al)must obey the relationship

vi/vR c 195.48 MHz

(22)

Samoson et al. (141) illustrate their arguments with the 27AlMAS NMR spectrum of sillimanite. For the signal from six-coordinated A1 in sillimanite, s = 9.1 kHz (148), which means that the signal cannot be narrowed by MAS; however, for four-coordinated Al, s = 5.3 kHz (< vR), and a clear-cut line shape which compares well with theoretical calculations is indeed found in the spectrum. Fyfe et al. (155) measured high-resolution solid-state 27Al MAS NMR spectra of a number of zeolites at 104.22 MHz. All the spectra contained one narrow peak with a chemical shift ranging, in different materials, from 51.5 to 65.0 ppm from Al(H,O);+ (see Table XIII). In dealuminated zeolites an additional signal was observed corresponding to six-coordinated A1 in the zeolitic channels. Figure 33 shows the very substantial improvement in the quality of the spectrum on increasing the magnetic field from 23.45 MHz for 27Al(proton frequency 90 MHz) to 104.22 MHz. The improvement, due to the reduction of second-order quadrupolar interaction, involves the increased intensity and symmetry as well as the reduction of the width of the peak. Fyfe et al. (155) give the maximum error in their observed chemical shifts as less than 1.5 ppm,

258

J. M. THOMAS AND J. KLINOWSKI

TABLE XI11 Parameters of”A/ M A S N M R Spectra ofzeolites at 104.22 M H f

Na-A Na-X Na-Y

NHi-Y Dealuminated Na-Y Na-cancrinite hydrate Na,Li-cancrinite hydrate Mordenite (large port) Mordenite (small port) Sodalite hydrate Natural sodalite Losod ZSM-5 Gmelinite Ferrierite Chabazite

1.OO 1.19 1.35 1.59 1.67 1.87 1.95 2.00 2.35 2.45 2.56 2.61 2.75 2.61 55 1.00 1.08 6.62 5.59 1.oo 1.00 1.00 20-40 2.40 5.0 2.22

58.6 61.3 61.5 61.3 61.0 61.2 61.3 61.2 61.4 61.4 61.4 61.2 61.4 61.6 54.8 58.9 62.7 55.1 55.1 61.0 65.0 55.8 5 1.5-54.8 59.2 54.3 58.4

390 565 67 1 672 596 647 50 1 586 550 474 538 560 537 549 1428 583 595 610 562 477 306 83 1 700 549 879 598

From ref. 155. Chemical shifts are given in ppm from AI(H,O):+.

calculated from the widths of peaks in stationary spectra and assuming that low-field broadening is entirely quadrupolar in origin. At 104.22 MHz the linewidths in all zeolite samples were substantially the same, and all chemical shifts fell within the 50-70 ppm range reported for four-cordinated A1 by Miiller et al. (139) and by Freude and Behrens (151). Chemical shifts in zeolites X and Y remains virtually constant while the A1 content is halved; on the other hand shifts in different zeolites with the same A1 content are sometimes quite different, although highly siliceous materials correspond in general to low values of chemical shift. Fyfe et al. (155) also recorded spectra of samples of zeolite Y dealuminated using silicon tetrachloride vapor. This method is discussed in detail in Section

ALUMINOSILICATE CATALYSTS

259

1 (bl la I

ppm from

AI(H,OI,S*

FIG.33. High-resolution "A1 MAS NMR spectra of zeolite Na-Y (Si/AI = 2.61) at two magnetic fields (155): (a) 104.22 MHz; (b) 23.45 MHz. The chemical shift in (a) is 61.3 ppm from AI(H,O):+; the two low-intensity signals equidistant from the main peak are spinning sidebands.

111,M; here we shall only say that (1) an additional 27Alsignal appears at 0.0 Ifi 2 ppm, due to residual six-coordinated A1 and (2) there is a large change in the chemical shift of tetrahedrally coordinated Al-from 61.3 ppm in the parent material to 54.8 ppm in the dealuminated product. The latter effect indicates that the chemical shift is related to the neighborhood of the A10:- tetrahedron. The first-order tetrahedral neighborhood of an A1 atom is Al(4Si) (the Loewenstein rule), so that the actual magnitude of the shift must be governed by geometric factors such as T-0-T angles and by the composition of further tetrahedral coordination shells. In particular, the direction of the change in chemical shift on dealumination is in agreement with the arguments of Freude et al. (157) that 27Al shift should decrease as the number of A1 atoms in the neighborhood of a central A1 atom decreases. 27Al MAS NMR spectra of chemically untreated zeolites are thus much simpler than those of their 29Sicounterparts. This is a direct consequence of the fact that while five types of Si(nA1) environments are possible for the silicon atom, only one possibility exists for the aluminum. However, while Si in zeolites is always present in four-coordination, A1 can be four- or sixcoordinated, and 27Al MAS NMR is a very sensitive quantitative probe for this. In other words, 27Al NMR is most valuable in probing the coordination, quantity, and location of A1 atoms in chemically treated zeolites, but less useful than 29SiNMR for direct structural determination. 27Al MAS NMR is sometimes able to distinguish crystallographically nonequivalent tetrahedral A1 atoms. One such example is zeolite omega, where two separate types of site are observed at 130.32 MHz (Figs. 48 and 51), in agreement with the structure of this zeolite (see Section 111,G). However, these sites cannot be separated at 52.11 MHz, which indicates that second-order quadrupolar effects are large.

260

J. M. THOMAS AND J. KLINOWSKI

Important structural information is provided by the 27A1 MAS NMR spectrum of silicalite, a porous solid isostructural with zeolite ZSM-5 (see Section 111,F). It has been argued (81) that silicalite has no aluminum and no cations in this structure and therefore no ion-exchange properties. In other words, it was claimed not to be a zeolite, and the original patent (80) describes it as a “porous form of silica,” and asserts that such aluminum as may be present is in the form of A1,0, impurity. Because it is not possible to monitor the coordination of very small amounts of A1 (of the order of hundreds of ppm) by X-ray crystallographic methods, these claims could not be tested directly. Fyfe et al. (82) have demonstrated that: 1. The aluminum in silicalite is readily detectable by 27A1 MAS NMR; 2. All the aluminum is tetrahedrally coordinated to oxygen: 3. There are at least two distinct types of tetrahedral framework sites occupied by the aluminum; 4. The Si/AI ratio in the sample can be estimated from the spectra.

Figure 34 gives the 27A1 MAS NMR spectrum of the same sample of silicalite which produced the 29Sispectrum in Fig. 22. The fairly broad signal centered at ca. 55.6 ppm from external Al(H,O)g+ is clearly characteristic of tetrahedral coordination. The signal shows fine structure due to at least two components, at 54.5 and 56.7 ppm, indicating the presence of crystallographically nonequivalent sites for tetrahedral A]. An even better resolved 27A1 spectrum has been obtained (471) (Fig. 35). It is interesting to note that when zeolite ZSM-5 is thoroughly dealuminated by a hydrothermal treatment (84,85) to reach Si/AI = 800, the same splitting of the residual tetrahedral 27A1signal is seen in the spectrum. Because of the 100% isotopoic abundance of 27A1 and its very short spin-lattice relaxation time, even traces of aluminum are often detectable by MAS NMR. For example, Thomas et al. (156) were able to show that aluminum present as an impurity in soda glass is four-coordinated. However, quantitative determination of A1 concentration in the sample is only possible when the quadrupolar effects are not so large as to affect significantly the apparent intensity of the signal. Kentgens et al. (15) used 27A1MAS NMR to study the effect of hydration/ dehydration on the electric field gradient at the 27Al nucleus in zeolites ZSM-5 and H-Y at three different magnetic fields corresponding to proton frequencies of 300, 180, and 60 MHz. Figure 36 shows their spectra of HZSM-5. The initial decrease of FWHM of the signal in the hydrated samples on lowering the magnetic field suggests that at the highest field FWHM is not controlled by quadrupolar effects; the authors suggest that a distribution of 27A1 chemical shifts is responsible, to be overtaken, at lower fields, by quadrupolar effects. It is also seen in Fig. 36 that for dehydrated samples of

26 1

ALUMINOSILICATE CATALYSTS 54.5

I 56.7

i

50

75

150

25

100 50 0 p p m f r o m AI(H20)g+

1

-50

FIG.34. High-resolution "A1 MAS NMR spectrum of silicalite at 104.22 MHz (155); 176,214 free induction decays were accumulated; repetition time was 0.1 sec.

80

40

0

ppm from AI(H,0)6m

FIG.35. High-resolution 27AIMAS NMR spectrum of silicalite (471) at 104.22 MHz.

262

J. M. THOMAS AND J. KLINOWSKI HYDRATED

D E H V DRAT E D

FIG.36. "AI MAS NMR spectra of zeolite H-ZSM-5 at three magnetic field strengths (IS). The spectra of hydrated materials represent the following number of FIDs (from top to bottom): 10,000, 130,000, and 350.000; while the spectra of the dehydrated materials needed 250,000 and 180,000 FIDs. At 15.6 MHz no signal was obtained for the dehydrated material after 750,000 FIDs.

H-ZSM-5 quadrupolar effects are always dominant. The same applies to both the hydrated and the dehydrated samples of zeolite H-Y (not shown). Figure 37 shows the spectra of Na-ZSM-5 as a function of time of hydration. In dehydrated samples quadrupole interactions are so strong that the tetrahedral A1 line disappears, to reappear on rehydration. Dehydration has no effect on octahedrally bound Al. These findings are in broad agreement with the wide-line measurements by Genser (158) who from the second moment of the 27Al line in hydrated zeolite Y calculated vQ = 390 kHz, but was not able to observe a signal in dehydrated zeolite. Gabuda et al. (137) observed increased values of vQ after dehydration of zeolites. For the hydrated and dehydrated analcime the values were 270 and 390 kHz, respectively; for Na-X, 165 and 285 kHz; and for NaA, 75 and 165 kHz.

263

ALUMINOSILICATE CATALYSTS

hydrated

I..............

0

10

FIG.37. *’A1 MAS NMR spectra of zeolite Na-ZSM-5 as a function of rehydration time at 78.2 MHz (15). Each spectrum represents 25,000 FIDs.

The authors of ref. I5 explain the observed strong quadrupolar effects by a distortion of A10:- tetrahedra on dehydration, possibly caused by the closeness of the “bare” cation. They justify the postulate of a distribution of ’’A1 chemical shifts with an observation of several aluminous species with distinct Tl values and chemical shifts. More work is needed to elucidate these important effects fully. Farlee and Corbin (437) and Veeman and Scholle (438) have reported ‘B spectra of a series of “boralites” (boron-containing analogs of ZSM-5 and ZSM-11) with Si/B ratios in the range 10-lO00. They found that the quadrupole interaction of boron in hydrated H-boralite was small (quadrupole coupling constant of less than 0.8 MHz), implying a highly symmetrical electric field gradient corresponding to tetrahedral coordination of ‘B to four oxygen atoms. The single and symmetrical NMR line was at -23 ppm from 1 M aqueous H,BO,. After ammonium exchange and calcination, however, a substantial amount of trigonal boron was found, with the “B nuclei resonating at -40 ppm. The much larger quadrupole coupling constant of 2.55 MHz for this signal indicates that on dehydration the BO,

’

’

264

J. M. THOMAS AND J. KLINOWSKI

tetrahedron becomes severely distorted, so that the boron atom is coplanar with three oxygens. When the calcination temperature was 400°C, 27% of total boron was in trigonal sites, and 38 % after calcination at 700°C. On the other hand, the fraction of trigonal boron produced in this way was independent of the Si/B ratio of the parent material.

J. DECATIONATION AND ULTRASTABILIZATION Dehydration and thermal treatment of ammonium-exchanged zeolite Y under high vacuum at temperatures up to 600°C is known as “decationation.” During this process water is removed, while the cation decomposes to give off ammonia; crystalline zeolite, nominally in “hydrogen form,” is left behind. At higher temperatures, hydroxyl groups are also eliminated from the crystal as water; this latter process is known as “dehydroxylation.” McDaniel and Maher (159-161) were the first to report that upon thermal treatment of NH,-Y, under a particular set of conditions, thermal stability of the zeolite is considerably increased. The product retains crystallinity at temperatures in excess of 1000°C,while the decomposition of the sodium form of the zeolite takes place at ca. 800°C. This process is known as “ultrastabilization.” Ultrastable zeolite Y is very well suited as a catalyst for hydrocracking reactions-much more so than the “as-prepared” zeolite, which is too acidic and has insufficient thermal stability. Ultrastable faujasitic catalysts are a cornerstone of the petroleum industry, and it is not surprising that much effort has been devoted to the study of their properties amd methods of preparation. The following summarizes the most important observations: 1. Thermally treated NH,-Y has a greatly increased thermal stability. 2. The process is facilitated by low residual sodium contents of the zeolite. 3. The atmosphere above the sample during treatment is very important, and the process proceeds more easily when carried out in the presence of water vapor. 4. The properties of the product depend on the bed geometry during treatment, and three distinct types of treatment are usually distinguished: shallow bed (SB), the zeolite layer is less than 3 mm thick and is slowly heated to the activation temperature under vacuum; normal bed (NB), the zeolite layer is thicker, but is also heated under vacuum; deep bed (DB), a thick layer of zeolite is gradually heated under atmospheric pressure. The DB process gives the most stable product. 5. The unit cell dimension of the ultrastable zeolite is smaller than in the parent material. 6. The DB zeolite has a greatly reduced ion-exchange capacity, which

265

ALUMINOSILICATE CATALYSTS

\Ji/

I

H

0

6

\ I -Si-O-Al--O-Si/ I

? I

Si

/I\

/ \

\ -Si-0-H /

/

H-0-SiH \ I 0 I Si / I \+NH,+AI(OH),

I 0 \ I -Si-0-Si-0-Si/ I

9I

/

+4H20

\

Si /I\

Slap I HZO

FIG.38. The proposed (162) course of ultrastabilization of zeolite NH,-Na-Y.

indicates that framework A1 has been removed. It can be subsequently leached out of the zeolite with acids and other reagents. 7. Framework vacancies created by the removal of A1 are subsequently reoccupied by Si. The above observations are consistent with the reaction given in Fig. 38. However, despite extensive studies employing a range of techniques, many questions remained unanswered prior to the advent of solid-state NMR. The two most important questions were the mechanism of A1 removal in stage I and the origin of the Si required in stage 11. It can be argued, for instance, that silicon which reoccupies vacancies left by aluminum must create vacancies of its own elsewhere in the framework. Ultrastabilization has been studied using solid-state NM R by several research groups (163-171). Klinowski et a1 (163) used 29Si and 2’A1 MAS NMR in tandem to examine a series of four samples subjected to different types of treatment, while determining the Si/Al ratio from spectral intensities using Eq. (15). 29SiNMR clearly shows (Fig. 39) how A1 is removed from the framework, and how the resulting vacancies are subsequently reoccupied. The starting material (sample 1) had Si/Al = 2.61. Sample 2 was prepared by calcining sample 1 in air at 400°C for 1 hr. Its 29Si spectrum is significantly = 3.37. However, chemical analysis shows no different with (Si/A1)NMR change in composition-the “missing ” A1 is now in six-coordination (Fig. 39b), and there is a consequent loss of ion-exchange capacity. Sample 3 was prepared at 700°C in the presence of steam, and its 29Si spectrum is completely transformed, giving (Si/Al)NMR = 6.89. Sample 4 was made by repeated application of the procedure, followed by prolonged leaching with acid. The product has Si/AI > 50 (by chemical analysis) and the unit cell parameter was reduced by 1.58% in comparison with sample 1. Its 29Si spectrum shows one sharp Si(OA1) peak at -106.9 ppm (compared with

266

I

J. M. THOMAS AND J. KLINOWSKI

I

-3Q

-90

t

-90

1

I

I

-100

-110

1

-100 ppm from TMS

I

-1M

J -120

1

-120

L

200

I

100 0 ppn from IAlIHIO{l3'

FIG.39. High-resolution "Si (at 79.80 MHz) and "AI (at 104.22 MHz) MAS NMR studies of the ultrastabilizationof zeolite Y (163):(a) Parent zeolite NH,-Na-Y; (b) after calcining in air for 1 hr at 400°C; (c) after heating to 700°C for 1 hr in the presence of steam; (d) after repeated ion exchange, heating, and prolonged leaching with nitric acid.

- 107.4 ppm in quartz) and a very small broad signal at ca. - 101.3 ppm, attributable to the residual Si(1Al) units. Sample 4 is very crystalline to Xrays, and its "Si spectrum clearly shows that framework vacancies have been reoccupied. If this were not the case, it would be more complex, reflecting a range of possible environments for Si atoms including one, two or three neighboring hydroxyl groups. In an independent "Si MAS NMR study, Maxwell et al. (164) reached very similar conclusions.

-100

ALUMINOSILICATE CATALYSTS

267

MAS NMR shows directly (16.3)how the occluded six-coordinated A1 builds up at the expense of the four-coordinated A1 in the framework (see the right-hand side spectra in Fig. 39). The spectrum of sample 4 contains a broad residual tetrahedral peak and an extremely sharp octahedral signal due to motionally free Al(H,O);+ in the cationic positions, not removed by leaching with acid. As shown in Section III,G, a variety of other zeolites can be ultrastabilized without structural damage. 29Si MAS NMR of the products, in which all framework A1 has been isomorphously replaced by Si, reveals important information about the number of different kinds of nonequivalent crystallographic sites and their relative populations. Three questions concerning ultrastabilization remain outstanding. They regard the precise mechanism of A1 removal, the nature of the intermediate defect structure (both are depicted schematically in Fig. 38), and the origin of the silicon needed for framework reconstruction. Gas sorption studies (1 72) indicate that materials prepared in a manner similar to that for sample 4 in ref. 163 (see above) contain a secondary mesopore system with pore radii in the range 15-19 A, suggesting that tetrahedral sites are reconstituted with silicon that, contrary to earlier speculations, does not come only from the surface or from amorphous parts of the sample, but also from its bulk, which may involve the elimination of the entire sodalite cages. Engelhardt et al. (165) used 29Si MAS NMR with cross-polarization in order to detect “surface” Si atoms attached to one or two hydroxyl groups. From spectra without CP they determined, using Eq. (19, framework Si/Al ratios, and by difference with the results of chemical analysis, the amount of nonframework Al. Si(3Si)(OH) groups were found at -100 ppm, and Si(2Si)(OH), groups at - 90.5 ppm, although the former signal coincides with that of Si(1Al,3Si) groupings. Their spectra are given in Fig. 40. Zeolite Y treated hydrothermally at 540°C for 3 hr shows the presence of Si(3Si)(OH) groups due to defect sites in the CP spectrum (Fig. 40f), although their absolute amounts could not be determined because the enhancement factor (CP efficiency) was not known. When the so-treated sample is extracted with 0.1 M HCl at 100”C, not only interstitial but also framework A1 is removed and many “hydroxyl nests” are formed (Fig. 40h). They cannot be healed, as at such a low temperature migration of siliconbearing species required by Fig. 38 must be insignificant. For repeatedly deep bed (DB)-treated and acid-extracted samples, 29Sispectra with and without CP are similar, which indicates that almost all vacancies are healed (not shown). The extent of dealumination was found (166) to be limited by the degree of ammonium exchange of the starting material, and also to depend on the temperature and water vapor pressure during treatment. Depending on the conditions, any desired composition of the product up to Si/Al z 8 could be

268

-

.

-80

!

J. M. THOMAS AND J. KLlNOWSKl

T

-100

s

q

-120

-

-80

-100

-120

I

-80

s

-100

*

-

-120

ppm f r o m

I

-80

-

I

-100

1

.

I

-120

-80

I

I

-100

I

I

-120

I

-80

1

1

-100

TMS

FIG.40. High-resolution 29SiMAS NMR study of progressive ultrastabilization of zeolite Y (Si/AI = 2.37) (165). Upper spectra without, lower spectra with cross-polarization. (a) and (b), Zeolite Na-Y (sample 1); (c) and (d), sample 1 after 50% NH; exchange (sample 2); (e) and (f), sample 2 after DB treatment at 540°C for 3 hr (sample 3); (g) and (h), sample 3 after extraction with 0.1 M HCI for 3.5 hr at 100°C (sample 4); (i) and (k), sample 3 after twofold ammonium exchange and DB treatment at 815°C for 3 hr (sample 5); (I) and (m), sample 5 after extraction with 0.1 M HC1 for 3.5 hr at 100°C (sample 6).

obtained and no preferred Si/AI ratios were found. For a given temperature the degree of dealumination increases approximately linearly with the degree of NHf exchange, but with a different slope for each temperature. The degree of dealumination is always far below the degree of exchange, which indicates that a considerable number of acidic protons (“structural OH groups” produced by the decomposition of N H f ) must be retained. At low water vapor pressures, the extent of dealumination was limited by the availability of water. In another 2’Si MAS NMR study, Engelhardt et al. (167) considered the problem of Si, A1 ordering in dealuminated zeolites Y in the range of

I

1

-12(

269

ALUMINOSILICATE CATALYSTS

compositions 2.5 I Si/AI I 5.8. They found that the relative spectral intensities were independent of the method of dealurnination [shallow bed (SB), DB, acid extraction] or conditions of thermal treatment, but depended only on the final value of the Si/AI ratio. As was pointed out in Section III,D, this does not necessarily mean that the ordering is the same, although this is a strong possibility. This is because “random” distribution and various different ordering schemes may give rise in principle to the same relative spectral intensities. BosAEek et al. (168) used wide-line 27AlNMR measurements of stationary samples to measure the EFG at the nuclear site in decationated zeolites. In zeolite Na-Y they measured a line half-width of B V , ~ ,= 61 kHz (for vL = 16 MHz) which led, via theoretical considerations (173) to vo = 840 kHz; the calculated field gradient was 2.9 V/A2. In hydrated samples this gradient was partially averaged by random reorientation of water molecules, giving C ~ V ,= / ~ 5.7 kHz and vQ = 256 kHz. When zeolite NH,-Na-Y was treated at 400°C under DB conditions a decrease in the number of observable A1 atoms was found as the degree of ammonium exchange increased from 0 to 90 %. In the latter case, only ca. $ of A1 present in the zeolite is observed by 27Al NMR (see Table XIV). The authors estimate vQ > 1.2 MHz for the unobservable Al. However, extralattice A1 can be detected by contacting the zeolite with a 38% solution of acetylacetone (Hacac) in ethanol, whereupon mobile Al(acac), complexes are formed, and a very narrow 27AlNMR line results; the solution does not affect framework aluminum. It was found that the amount of six-coordinated (i.e., extra-framework) A1 increases from 5 % in 84 De Na-Y 300 SB zeolite to 50 % in 84 De Na-Y 500 DB zeolite (in this notation the first number refers to the TABLE XIV Number oj’A1Atoms Observed by N M R in Samples ofNH,-Na-Y Decationated at 400°C Under Deep Bed Condition9 Degree of ammonium exchange (%)

Number of A1 atoms per cavityb

Number of Na+ ions per cavity‘

0 35 50 70 90

6.9 f 0.5 6.1 f 1.0 5.8 f 0.5 2.8 k 1.0

6.7 4.4 3.4

k 1.0

0.7

1.8

2.0

From ref. 168. Determined from 27AINMR signal intensity.The total Al content is 6.7 atoms per cavity. ‘Determined by chemical analysis.

TABLE XV Numbers of OH Groups per Cavity (iof Unit Cell), Framework Alwnimun A t o m ( A l Y A Extra-Framework Aluminum (Al* - A F A and Alwnimun in the Form of Mobile Hydrated Complexes ( A F a Rehydrated m l i t e (0 = 1)

Dehydrated zeolite

Na-Y 85 DeNa-Y 300 DB 85 DeNa-Y 300 SB 85 DeNa-Y 400 DB 85 DeNa-Y 400 SB 85 DeNa-Y 500 DB 85 DeNa-Y 450 SB' 85 DeNa-Y 600 DB 85 DeNa-Y 600 SB 85 DeNa-Y 700 DB 85 DeNa-Y 700 SB

5.8 f 0.3 7.0 f 0.3 4.4 f 0.3 6.7 f 0.3 2 8 f 0.2 6.5 f 0.3 1.8 f 0.3 0.7 0.3 1.4 f 0.3

6.7 f 0.0 4.7 f 0.4 5.5 f 0.4 3.7 f 0.4 5.8 f 0.4 2 7 f 0.4 5.9 & 0.4 1.1 f 0.4 1.2 f 0.4 0.8 & 0.4 0.7 f 0.4

0.0 f 0.0

2.0 f 0.4 1.1 f 0.4 2.9 f 0.4 0.9 f 0.4 3.9 f 0.4 0.8 f 0.4 5.6 f 0.4 5.5 0.4 5.9 & 0.4 6.0 f 0.4

+

6.7 f 0.0 4.9 f 0.2 4.9 f 0.2 3.6 f 0.2 4.9 f 0.2 1.7 f 0.2 4.5 f 0.2 1.1 f 0.2 26 f 0.2 1.4 f 0.2 1.5 f 0.2

0.0 f 0.0

0.0 f 0.0

0.0 f 0.05

1.8 f 0.2 1.8 f 0.2 3.1 f 0.2 1.8 f 0 2 5.0 f 0.2 2.2 f 0.2 5.6 f 0.2 4.1 f 0.2 5.3 f 0.2 5.2 f 0.2

1.4 f 0.2 1.3 f 0.2 1.1 f 0.2 1.4 & 0.2 0.4 f 0.1 1.1 f 0.2 0.1 f 0.1 0.1 f 0.1 0.0 f 0.1 0.1 f 0.1

1.5 & 0.2 1.3 f 0.2 3.4 0.3 0.4 f 0.1 3.3 & 0.3 1.0 f 0.2 5.1 f 0.4 5.3 f 0.4

From ref. (169).

'Al* = 6.7 is the number of Al atoms per cavity in the parent material as determined by chemical analysis.

Al(acac), is the number of extra-framework aluminum atoms per cavity which can be extended using an acetylacetone/ethanol solution. 'As SB treatment at temperatures above 450°C damages the mlitic framework, results for the 450 SB sample are given.

ALUMINOSILICATE CATALYSTS

27 1

degree of ammonium exchange, the second to temperature of treatment;

“De”means “decationated” and the final symbol distinguishes between the deep bed and the shallow bed process). Freude et al. (169) carried out a systematic study of the relative amounts of four- and six-coordinated A1 in thermally treated zeolite Y,using wide-line and MAS 27Al NMR at 16 and 70.34 MHz, respectively. Table XV gives the results calculated per one faujasitic supercage (i of the unit cell). It is evident that loss of 27Alline intensity takes place in treated zeolites in comparison with the parent material, evidently due to extra-framework A1 being in an environment of low symmetry. It is of interest to consider the possible status of this “invisible” aluminum. It could be present as Al(OH),, A1(OH)2+, Al,O,, or some polymeric aluminous species. Resing and Rubinstein (174) observed a loss of intensity of the 27Al signal on hydrolysis of zeolite Na-X and interpreted this as due to the formation of Al(OH), complexes. However, Freude et al. (169) think that the hydroxide is unlikely to be present in stabilized zeolites, which are strong solid acids, as it is not favored in acidic aqueous solutions. The “low symmetry environment” may be the surface of the crystallites or of the secondary pore system. Indeed, Lohse and Mildebrath (175) found A120, clusters inside the mesopore system formed as a result of the proposed “condensation of lattice defects” during thermal treatment (172), while Dwyer et al. (176,177) and Ward and Lunsford (1 78) reported an enrichment in A1 at the external surface of the particles of ultrastable zeolite Y using techniques other than NMR. It is also of interest to use MAS NMR for the study of the thermal treatment of zeolites which are nor in the ammonium-exchanged form. In an X-ray study, Pluth and Smith (179) found electron density at the center of the sodalite cages in dehydrated zeolites Ca-A and Sr-A and attributed this to a partial occupancy of these sites by a four-coordinated aluminous species. No such effect was found in zeolite A exchanged with monovalent cations. Corbin et al. (180) used 27Al MAS NMR to examine commercial samples of K-A, Na-A and (Ca,Na)-A, as received (see Fig. 41). For K-A and Na-A, only framework tetrahedral A1 species were observed, with chemical shifts of 57 and 52 ppm respectively. However, in (Ca,Na)-A an additional intense resonance at 78 ppm, typical of Al(0H); but definitely not due to framework aluminum, was also found (see Fig. 41). A much weaker signal, also at 78 ppm, was detected in zeolite Sr-A; its intensity increased greatly on heating the sample to 550°C. Freude et al. (183)came to very similar conclusions in their NMR study of heat-treated zeolite Ca-A. They found that maximum framework dealumination occurs at 500°C and corresponds to ac. 17% of total Al.

212

J. M. THOMAS AND J. KLINOWSKI

1

150

100

50

1

1

0

-50

-100

PPM

FIG.41. "AI MAS NMR spectra (180) at 78 MHz of zeolites K-A (a), Na-A (b), (Ca,Na)-A (c). and Sr-A (d) (all as prepared), and of zeolite Sr-A after heating to 550°C (e).

The most likely explanation of the production of octahedral A1 in NH: -exchanged (or rather H-exchanged) zeolites on heating, but tetrahedral A1 in Ca- and Sr-exchanged materials is as follows. Tn solutions at low pH, aluminum exists as AI(H20)2+. As the pH rises, this species becomes deprotonated; at high pH A1 is present in the form of the tetrahedral Al(0H); irrespective of concentration (150,181,182).Similar considerations apply to solids. Basicity of oxy-compounds (note that zeolites can be treated as mixed oxides) can be treated in terms of a function derived from the Pauling electronegativity of the atoms present (184-186). Dent Glasser (187) found that this scale of basicity, originally developed to describe the behavior of oxide glasses and melts, gives valid results for silicates.

K. NMR STUDIES OF ZEOLITIC ACIDITY The study of acidic surface sites capable of donating protons to or accepting electrons from adsorbed molecules is one of the most important

273

ALUMINOSILICATE CATALYSTS

areas in heterogeneous catalysis. In particular, it is important to know the concentration, strength, and accessibility of the Brensted and Lewis acid sites and details of their interaction with the adsorbed organics. In zeolites, Brensted acidity arises because of the presence of accessible hydroxyl groups associated with framework aluminum (the so-called “structural hydroxyl groups”) and can therefore be studied using ‘H and 27Al MAS NMR. Both kinds of acidity are conveniently monitored by observing the ‘H, I3C, I5N, and 3 ’ P resonances from the adsorbed molecules. NMR studies of the acidity of solid surfaces prior to 1976 have been extensively reviewed (188-192). The earliest work employed the method of moments, and Fraissard et al (192) were the first to consider in detail the second moment of the proton NMR resonance in nonspinning solids. In general, when the line shape is determined solely by the 1-1 and I-S magnetic dipole-dipole interaction, the Van Vleck second moment, M,, is given by (193)

Bj,

where Oj, is the angle between the internuclear vector r j k and the direction of the external magnetic field, and the summation involves N nonequivalent nuclei I and M nuclei S. Since in the powder all values of angle B are represented, the powder average 8 3 cos2B - 1) = f and the expression for the second moment simplifies greatly;

M , = CJrf’ + C&,

(24)

where

c, = -I(Z 3 + l)r*>’ 5

x lo6*,

1

so that, when M, is expressed in lo-’ T2, the internuclear distances r are in Angstrom units. The values of the constants C , in Eq. (24) for the nuclei

274

J. M. THOMAS AND J. KLINOWSKI

TABLE XVI Values o/ the Constant C, in Eq. (24for the Nuclei Discussed in This Review" Isotope

Cl

Isotope

CI

'H 2H

358.1 22.51 270.5 184.3 22.64 3.677 76.79 317.0 125.3 283.7

29Si

14.13 58.69 12.81 519.8 17.73 3.355 17.62 27.4 16.55 119.3

lLi IlB 13C I

sN

l1O

"F "Na

"A1

3'P

*'Ti

slv 9 5 ~ 0

"'Rh "'Cd

lZ9Xe lqsPt

20sTI

See text. Ref. 193.

discussed in this review are given in Table XVI. If C, is required, the value given in Table XVI should be multiplied by 8. When there are different kinds of nonresonant nuclei, Si, in the sample, Eq. (24) should contain further terms in Cs,/rg,. Because of the r - 6 dependence, broadline NMR is an extremely sensitive technique for measuring internuclear distances. Stevenson (194) took advantage of this in his study of the location of protons in dehydrated zeolite H-Y. He measured the average M, = (0.71 f 0.04) x lo-* T2 and found that M z is virtually independent of the sodium content which indicates that the distance between 'H and the residual 23Na in the zeolite is large. The only interactions which may be responsible for dipolar broadening are thus the homonuclear H-H and the heteronuclear H-A1 interactions (Fig. 42). In order to estimate the magnitude of the former, deuterium, which has a much smaller magnetic moment, was successively substituted for the proton. Stevenson found that the homonuclear interaction was very small and estimated rH-H > 4.5 A, indicating that the protons are located in the faujasitic supercages. From Eq. (24) he then estimated rH--Al = 2.38 f 0.03 A. Taking the X-ray values I,., =0.95 ,,- 1.03 A and rA,--O = 1.72 A he arrived at the AI-0-H angle of 116", close to the 120" required for the complete sp2 hybridization of the oxygen atom (see Fig. 43). By reference to the known fact that the C-H bond becomes more acidic as the p character of the carbon decreases, he suggested that the acidity of zeolites may be due to the decreased p character of the bridging oxygen in the bond. Pulsed NMR experiments enable one to determine separately the homonuclear, MY,and the heteronuclear, MY,components of the second moment

275

ALUMINOSILICATE CATALYSTS

10-31 4

I

I

5

6

1 7

I

B

0

INTERNUCLEAR DISTANCE

10

(A)

FIG.42. Second'moment of the broadline 'H NMR spectrum at 40 MHz of zeolite H-Y (194) versus the internuclear distance between the proton and other magnetic nuclei.

[Eq. (23)]. Freude et al. (171) used those to confirm that My-*' b My-H and arrived at numerical results very close to those given by Stevenson (194). By measuring T2 relaxation times of protons in SiOH and AlOH and measuring the second moment of the 'H resonance in silica-alumina, Schreiber and Vaughan (195) were able to divide the NMR signal into SiOH and AlOH components. Where the sample undergoes fast rotation, the second moment of the NMR resonance is reduced. For molecules rotating freely inside a zeolitic cavity of

276

J. M. T H O M A S A N D J. KLINOWSKI #-,Hydrogen

FIG.43. Local environment of the hydrogen atom in zeolite H-Y (194).

known geometry and composition, the component of the second moment resulting from the dipolar interaction with the framework aluminum can be calculated. For example, for the cubic zeolite Na-A with Si/AI = 1.00, and thus containing 96 Al atoms per /3-cage, Freude (193) calculated that the component of second moment of any freely rotating nuclear species ('H, "N, 13C, lZ9Xe, etc.) due to interaction with the two kinds of A1 is M; = 2.05 x lo-'' T 2 and M $ = 9.29 x lo-'' T2. Thus, as expected, the second moment is increased to a much greater extent in the /3-cages which are smaller. The first measurements of the 'H NMR signal from the hydroxyl groups in solids were carried out by O'Reilly et al. (196). Because of strong dipolar interactions the lines were very broad and, although information on internuclear distances could, as explained above, be obtained in principle, 'H chemical shgt could not be measured with sufficient accuracy, nor could signals originating from structurally distinct hydroxyl groups be resolved. This became possible only with the advent of magic-angle spinning. High-resolution 'H MAS NMR is the most advanced tool for the measurement of zeolitic acidity, which is essential for the understanding of the mechanisms underlying many zeolitic-mediated catalytic reactions. While, as explained above, proton-proton distances in dehydrated zeolites are relatively large and therefore the homonuclear interactions, whose removal requires the use of multiple-pulse line-narrowing techniques such as WAHUHA ( Wuugh, Huber, and Hueberlen, its discoverers), are not generally significant, high-resolution proton work in the solid state is beset by difficulties. The main causes of the difficulties are the strong heteronuclear interactions, the narrow range of the chemical shifts for the proton (as a result of which only a limited number of lines can be resolved), and, in the case of zeolites which are hygroscopic, the need to dehydrate the sample thoroughly to avoid the signal from adsorbed water and the need to use spinners which do not contain protons. A further complication in 'H MAS NMR studies of aluminosilicate catalysts is the heteronuclear dipole-dipole interactions with

ALUMINOSILICATE CATALYSTS

277

quadrupolar S spins when :v is not insignificant in comparison with vf, so that the 'H NMR line shape is modified by the second-order quadrupole interaction, Bohm et al. (153) calculated the second moments of lines for stationary and spinning samples taking this effect into account. The expression (MyAS/MYV)''*,where MYAS is the second moment of the MASnarrowed line (without sidebands) and MYV the second moment calculated according to Van Vleck [Eq. (23)], is a measure of the attainable line narrowing. For a spin $ nucleus such as 27Al, < vf, and Bohm et al. (153) give

vi

(MyAS/M;V)1/2= 0.90 vSQ l vSL.

vg

Taking = 0.84 MHz for A1 in dehydrated H-Y (obtained by measuring the 27Al NMR linewidth) and vf = 15.6 MHz (in a 60 MHz magnet) Freude (193) calculated that lines should be narrowed by a factor of 20.63. Clearly, a greater line narrowing will be achieved at higher magnetic fields, where ':v is larger. Despite these problems, the Leipzig group in particular have used the technique to obtain very important information on the chemistry of hydroxyls and gels. Freude et al. (171) measured 'H MAS NMR spectra of DB-treated zeolite NH,-Y at 60 MHz and considered the magnitude of the proton chemical shift as a measure of Brransted acidity. They point out that the calculated chemical shift for the bare proton is 30.94 ppm from TMS and that for methanol, phenol, m-cresol and m-nitrophenol dissolved in CCl,, O H protons resonate at 0.52, 4.28, 5.67, and 10.58 ppm, respectively, which agrees with the sequence of the pK, values of these compounds in water (17,9.9, 10.1, and 7.2, respectively). Noting that hydroxyl groups in the nonacidic silica gel resonate at ca. 1.6 ppm from TMS (see Table XVII), and that in the spectra of dehydrated H-zeolites a number of lower field resonances are resolved, they argue that 'H MAS NMR spectra can be treated as distribution functions of acidity. Dehydrated zeolite samples were spun in sealed glass ampoules. By comparing the spectra of their zeolite samples with those of various inorganic materials, Freude et al. conclude that terminal Si-0 hydroxyls and hydroxyls attached to extra-framework aluminum resonate at ca. 2 ppm, and the acidic hydroxyls at 6-10 ppm. Table XVII gives the details of the spectra of the various zeolites with and without sorbed deuterated pyridine. The shift of spectral lines to lower field in pyridine-treated samples is attributed to the formation of hydrogen bonds between O H groups and the organic base. In another paper, Freude et al. (170) used 'H MAS NMR at 270 MHz with the WHH-4 pulse sequence to obtain high-resolution spectra of thermally

278

J. M. THOMAS AND J. KLINOWSKI

TABLE XVII ‘H MAS N M R Chemical Shifis and Linewidths of Hydroxyl Groups in Various Dehydrated Sorbents‘

Adsorbent Silica gel Y-AW, Amorphous a1umi nosilicate DeNaY

Loading solvent

-

Pyridine

CaNaY H-mordenite

( %)

Chemical shift (ppm from TMS)

Linewidth (Hz)

100 100 22

1.6 2.0 1.8

170 290 210

78 15 85 68 32

6.0 1.7 9.9 1.6 6.0 1.5 9.1 1.6 1.8 8.6 1.7 8.4

510 220 800 260 5 50 230

60 40 Pyridine Pyridine

a

Relative intensity

100 50 50 50 50

440 220 280 830 290 850

From ref. 171.

treated zeolites (see Fig. 44). Three distinct lines are present, and their chemical shifts with assignments suggested by the authors are as follows; 1. 2.0 ppm-due to terminal OH groups and hydroxyl groups attached to extra-frameworkAl. This line is significant only in sample 500 SB,where it amounts to 40% of total spectral intensity. 2. 4.2-5.0 ppm, due to structural hydroxyl groups. 3. 6.8-8.0 ppm, due to structural hydroxyl groups.

In addition, some samples also gave a signal at 7.1 ppm from the residual NHd cations; the amount of the latter was determined by themodesorption and subtracted from the intensity of line (3). Thus the sum of intensities of (2) and (3) gave the true total content of acidic hydroxyl groups. They had T2of 60-75 psec, while sample 500 SB contained an additional free induction decay (FID) component due to extra-framework hydroxyls. Scholle et al. (297, j98) used ‘HMAS NMR to study the acidity of the hydroxyl groups in zeolite H-ZSM-5 and its borosilicate “equivalent,” known as H-boralite, at various water contents. They were able to distinguish terminal and water hydroxyls from acidic hydroxyl groups in the framework, although the resolution of their spectra was lower than that achieved by the

279

ALUMINOSILICATE CATALYSTS 4.2 4.4

n

500 DB 4.1

FIG.44. High-resolution 'H NMR spectra with MAS and WHH-4 sequence of 88% NHf-exchanged zeolite NH,-Na-Y at various activation conditions quoted as temperature (40,450, and 500°C) and deep bed (DB) or shallow bed (SB) (170).

Leipzig/Jena workers (170,171). The H-ZSM-5 was found to be more acidic than boralite. There is at present only a handful of publications involving 15N NMR of molecules sorbed on zeolites, but they establish beyond doubt the power of this technique for the study of zeolitic acidity and other surface phenomena. The nitrogen atom in molecules such as ammonia and pyridine has a lone pair of electrons and binds directly to the surface site. One is therefore observing large effects on a nucleus with a wide (ca. 900 ppm) range of chemical shifts, rather than more indirect influence as in the case of 13C. Michel et al. (199,200) and Junger et al. (201) measured the spectra of isotopically enriched ammonia, trimethylamine, pyridine, and acetonitrile on various zeolites at 9.12 MHz and found that resonance shifts depended strongly on the interactions of sorbate molecules with cations and Brransted and Lewis acid sites. The 15N chemical shift changes by 18.5 ppm as the pore-filling factor 8 of I5NH, on zeolites Na-X, Na-A, Na-mordenite, and Na-Y varies between 0 and 1. In sodium forms, the resonance shift is mainly due to intermolecular interactions. For ultrastable zeolite Y, the "N

280

J. M. THOMAS AND J. KLINOWSKI

resonance of ammonia does not change between 8 = 0.2 and 0.72, and is approximately equal to that measured for liquid ammonia (18 ppm from nitromethane). Michel et al. conclude that at higher 8 the ammonia molecules are packed so closely that their resonance shifts become liquid-like. In ultrastabilized samples strong association of ammonia molecules occurs even at low coverages leading to constant chemical shift. At low coverages, the resonance shift of 15NH3on zeolite H-Y remains constant and is close to that for NH: solutions, which shows that all ammonia molecules are converted into ammonium cations as a consequence of interaction with structural hydroxyl groups. Consideration of the equilibrium between the surface sites and the sorbate allows the resonance shifts for the surface complexes to be obtained and to eliminate the influence of the exchange process. The

-646

-1.5

-60.5

-w.6\

(d 1

(b)

-4.2

-32

1

125

1

1

75

.

1

25

'

'

-25

PPm

.

1

-75

.

1

1

-125

'

1

.

75

1

'

25

1

'

-25

1

.

-75

1

-125

PPm

FIG.45. Effect of calcination on the I I P MAS NMR spectrum at 80.96 MHz of P(CH,), adsorbed on zeolite H-Y(202). Samples were degassed at 80°C for 1 hr prior to measurements. Samples calcined at (a) 400°C; (b) 500°C; (c) 600°C; (d) 700°C. The resonance at ca. -3 pprn is assigned to [(CH,),PH]' complexes formed on the Brensted acid sites; resonances in the region of ca. - 32 to - 58 ppm in samples calcined at 500°C correspond to the phosphine on the Lewis acid sites; and the signal at -58 ppm in samples calcined at high temperatures are due to the phosphine on AI,O, clusters in the zeolitic cavities. Chemical shifts are in ppm from 8 5 % aqueous H,PO,.

28 1

ALUMINOSILICATE CATALYSTS

formation of pyridinium ions in decationated zeolites has been followed, leading to direct determination of the number of interacting hydroxyl groups. 15N is far superior to 13C for this purpose. Acetonitrile can be conveniently used for characterization of interactions with the exchangeable cations and Lewis acid sites (199-201).Electron acceptor strength of decationated zeolites increases with the increased temperature of activation, the rise being particularly drastic in the region 300-400 K. NMR shows unambiguously that the exchangeable cations in zeolite X act as adsortion centers. Lunsford et al. (202) used trimethylphosphine as a probe molecule in their "P MAS NMR study of the acidity of zeolite H-Y. When a sample is activated at 4OO0C, the spectrum is dominated by the resonance due to (CH,),PH complexes formed by chemisorption of the probe molecule on Brmsted acid sites. At least two types of such complexes were detected: an immobilized complex coordinated to hydroxyl protons and a highly mobile one, which is desorbed at 300°C. (see Fig. 45) +

L. THEMECHANISM OF DEHYDROXYLATION OF ZEOLITES When zeolite H-Y obtained by decationation of NH,-Y is heated further, water is irreversibly lost from the framework. The dehydroxylated zeolite Y displays Brransted and Lewis acid properties. The mechanism for this process H

H

I

I

/O\

SCHEME1.

/O\-/O\+ ,A\

/S\

lA\

/O\

/O\

+ HZO

Suggested mechanism for decationation of NH,-Y.

(Scheme 1) suggested by Uytterhoeven et al. (203) has become well established in the literature. Scheme 1 requires the following: 1. A defect structure involving three-coordinated Si and A1 is formed, with Lewis acidity being due to the latter. 2. The amount of four-coordinated A1 decreases with increasing degree of dehydroxylation. After complete dehydroxylation half of the A1 atoms remain four-coordinated.

282

J. M. THOMAS AND J. KLINOWSKI

3. The number of four-coordinated A1 atoms is always greater than or equal to the number of structural hydroxyl groups (because two OH groups disappear per each framework A1 atom).

The scheme has been repeatedly questioned in the light of X-ray and IR spectroscopic results which do not support it. Based on their IR measurements, Jacobs and Beyer (204) proposed an extra-framework (A1,O) species acting as a Lewis acid, in place of the hypothetical three-coordinated framework Al. However, the strongest arguments against the scheme of Uytterhoeven et al. (203) come from NMR work, which is an excellent example of how ‘H and 27Al MAS NMR can be used in cooperation (168,170). The amounts of “terminal” and “structural” hydroxyls were separately measured using ‘H NMR, while the amount of four-coordinated A1 was readily obtained from 27Al MAS NMR. No three-coordinated Si or A1 was observed; the amounts of structural hydroxyls and four-coordinated A1 were always equal; and much less than half of four-coordinated A1 was found after complete dehydroxylation. It is therefore clear that dehydroxylation is always accompanied by the release of A1 from the framework. It seems that when the A1 atom in the vicinity of a structural OH group is lost from the framework, the group is simultaneously dehydroxylated. Deep-bed treatment produces four times as much extra-framework A1 as shallow-bed treatment at the same temperature.

M. ISOMORPHOUS SUBSTITUTION IN THE ZEOLITIC FRAMEWORK Catalytic and other properties of zeolites are strongly dependent on their aluminum content, and it is therefore desirable to be able to vary the latter while retaining the topology and crystallinity of the parent structure. The aluminum content of zeolites can be decreased by acid washing, and by using a number of reagents such as ethylenediaminetetraacetic acid (EDTA), chlorine gas, acetylacetone, or phosgene, but all these treatments produce framework vacancies. As was described in the preceding sections, aluminum in the zeolitic framework is isomorphously substituted by silicon in the course of ultrastabilization, which consists of heat treatment of ammonium-exchanged zeolites. A new method of achieving such substitution in synthetic faujasites has been reported by Beyer and Belenykaja (205). It involves passing silicon tetrachloride vapor through a bed of dehydrated zeolite at elevated temperatures. Klinowski et al. (206-209) and Hays et al. (210) applied ”Si and 27Al MAS NMR to the study of this remarkable reaction in a number of zeolites.

ALUMINOSILICATE CATALYSTS

283

Dehydrated zeolite Na-Y (Si/Al ratio 2.61) was treated (206) at 560°C with dry argon saturated (at room temperature) with SiCl, for 3 hr. Aluminum was successively substituted in the zeolite framework by silicon and removed in part from the crystals in the form of volatile AlCI, observed as white vapor. The zeolite was then flushed, also at 560’32, with dry argon, and the temperature was gradually reduced. The product was then repeatedly washed with water. Each step in the substitution reaction, taking zeolite Na-Y with an Si/Al ratio of y / x as starting material can be written Na,(AIO,),(SiO,),

+ Sic&

-

Na,- ,(AIOz)x- t(Si02)y+,+ AICI,

+ NaCl

As indicated by NMR (see below) part of the aluminum remains in the solid as NaAlCl, formed from the high temperature reaction of NaCl with AlCl,, which gives Al(H,O);+ on contact with water. This, together with the sodium chloride, can be successively removed by washing the product repeatedly with aqueous acid. The aluminum content of the highly crystalline product is similar to that of the ZSM zeolites. X-Ray powder diffraction, IR spectroscopy, and high-resolution electron microscopy all show that the crystal structure of the product is the same as that of the parent material, although the unit cell parameter decreases by 1.5 % as a consequence of the tetrahedra. difference in size between the A10:- and the The 29SiMAS NMR spectra given in Fig. 46 undergo a dramatic change in the course of the reaction. The single peak in the spectrum of the dealuminated material arises from Si(OAl), i.e., from Si(4Si) groupings: essentially all other groupings having been eliminated. The hump on the base line comes from amorphous material (probably silica) in this particular sample, The progress of the same reaction can also be monitored using ”A1 MAS NMR. The spectrum of the parent zeolite Na-Y (Fig. 47a) shows a single relatively narrow signal with a chemical shift of 61.3 ppm from AI(H20)2+, corresponding to four-coordinated A1 (see Section II1,I). After treatment with SiCl, but before washing with water, apart from the signal from the residual framework A1 and the peak from the emergent six-coordinated A1 occluded in the intracrystalline space, there is an additional signal due to NaAlCl, also occluded in the zeolite (Fig. 47b). The chemical shift of the latter peak, at 100.8 ppm, is close to the value of 95.9 ppm measured (207) for crystalline LiAlCl,. Upon washing the sample with water, chloroaluminate is largely removed or converted to the hydrated cation, Al(H,O);+, which may be considered as the “exchangeable” cation. It is significant that the amount of six-coordinated A1 (at 54.8 f 0.2 ppm) is ca. $ of the residual framework A1 as required by charge balance (Fig. 47c). The FWHM of the tetrahedral signal is much greater than in the parent material (1428 Hz as compared with 560 Hz), which indicates a distribution of immediate environments for the

284

J. M. THOMAS AND J. KLINOWSKI

I -80

1

-90

I

I

-100

-110

I -120

Silo All

1

-80

-90

-100

ppm f r o m

-110

-120

TMS

FIG.46. Dealumination of zeolite Na-Y using SiCI, vapor studied by 29Si MAS NMR spectroscopy at 79.80 MHz (206). (a) Parent material (Si/AI = 2.61); (b) after “complete” dealumination (corresponding to the *’A1 MAS NMR spectrum in Fig. 47d) (Si/AI = 55).

aluminum atoms remaining in the framework. A comparison of Fig. 47 (c and d) shows that most, but not all, of the six-coordinated A1 can be removed by washing. This is probably due to ion-exchange equilibrium between Na’ (and H,O+ if the sample is acid washed) and Al(H,O)g+ competing for the cationic sites. The poorer signal-to-noise ratios in Fig. 47(b-d) in comparison with that in Fig. 47a are due to the very much lower concentration of aluminum in these samples. Silicon tetrachloride treatment can produce faujasites with very high Si/Al ratios in a single step, but works less well with other zeolites. The reasons for this are not clear. However, 27Al MAS NMR shows unambiguously that other zeolites, notably mordenite (208,209),zeolites omega (209)and ZSM-5 (59, are also dealuminated in this way. This is often not detectable by other methods since, after being removed from the framework, the aluminum may

285

ALUMINOSILICATE CATALYSTS

-

I non framework (tetrahedral)

framework (tetrahedral)

\ (b)

(0)

framework (tetrahedral)

I framework (octahedral)

nonframework framework (tetrahedral) (octahedral)

(tetrahedral)

nonframework

FIG.47. Dealumination of zeolite Na-Y using SiCI, vapor studied by "AI MAS NMR spectroscopy at 104.22 MHz (207). (a) Parent zeolite Na-Y; (b) dealuminated material before washing; (c) after washing with dilute acid; (d) after repeated washing. Note that the aluminum jettisoned from the zeolitic framework is first bound tetrahedrally as NaAICI, (see text), but after washing adopts octahedral coordination.

remain in six-coordination in the zeolite channels and cavities. Chemical analysis then detects no change in the Si/Al ratio. This is the case in SiC1,-treated mordenite; other examples include zeolite omega (Fig. 48) and ZSM-5 (Fig. 49). In zeolite omega the six-coordinated A1 is evidently extremely mobile (FWHM of the peak is only 0.2 ppm) which is understandable given that this material possesses the widest channel system (channels more than 7.4 A in free diameter) of any known zeolite. In the case of ZSM-5 the motion of octahedral aluminous species is restricted by the narrowness of the channels (ca. 5.5 A in diameter). Accordingly, the FWHM of the octahedral peak, measured at the same magnetic field strength as for zeolite omega, is 1.1 ppm. Figure 49 shows a lower field spectrum. It is interesting to note that the high-field spectrum in Fig. 48 contains two signals coming from crystallographically nonequivalent A1 atoms-in agreement with the crystal structure of zeolite omega, providing for two distinct

286

I. M. THOMAS AND J. KLINOWSKI

100

60

80

40

P. -20

0

20

-40

ppm from AI(H,O$ FIG.48. 27Al MAS NMR spectrum at 130.32MHz of zeolite omega dealuminated with silicon tetrachloride vapor (209).

tetrahedral sites (Figs. 50, 51). When the spectrum is measured at the lower magnetic field of 4.70 T ("A1 frequency of 52.11 MHz) only one tetrahedral signal is observed. It is evident that the line-broadening influence of the second-order quadrupolar interaction, inversely proportional to the magnetic field strength, is reduced at 11.74 T (Fig. 48). This demonstrates the advantages of high-magnetic field solid-state NMR spectroscopy of quadrupolar nuclei. The 29SiMAS NMR spectrum of zeolite ZSM-5 extensively dealuminated with SiCI, approaches that of silicalite (84).

I Octahedral

100

50

0

-50

h

ppm from AI(H20).

a*

FIG.49. *'AI MAS NMR spectrum at 52.11 MHz of zeolite ZSM-5 dealuminated with silicon tetrachloride vapor (57).

ALUMINOSILICATE CATALYSTS

287

FIG.50. Projection drawing, viewed along [Ool] of the structure of zeolite omega (synthetic mazzite). There are two distinct tetrahedral sites, one more (A) and one less (B) accessible via large channels. The unit cell is enclosed within the dashed lines.

While the aluminum content of zeolites can be decreased with relative ease, until very recently it has not been possible to increase framework A1 content after completion of synthesis. Anderson et al. (211) reported a successful reversal of the reaction of zeolites with SiCl,. They used *'A1 MAS NMR to show that when highly siliceous zeolite ZSM-5 (with %/A1 > 400) is treated with AlCl, vapor at 400°C for 12 hr, aluminum is isomorphously substituted for silicon in the zeolitic framework and also enters six-coordinated (octahedral) intrazeolitic positions. Figure 52a gives the spectrum of the parent material. Very small amounts of four-coordinated A1 are present, but the signal (at ca 55 ppm) can be observed only when a very large number of scans are accumulated; there are also small amounts of six-coordinated A1 in the

ppm from AI(H,O)P

FIG.51. 27AI MAS NMR at 130.32 MHz of zeolite omega (synthetic mazzite) (93).

288

J. M. THOMAS AND J. KLINOWSKI

t

1

,

1

I

1

1

,

1

,

I

I

I

I

40

60

20

.

,

,

,

1

0

ppm from A I ( E , O ) ~

FIG.52. 27AI MAS N M R study at 130.32 MHz of the alumination of ZSM-S/silicalite with AICI, vapor (211). (a) Parent material with Si/AI > 400; (b) treated zeolite with Si/AI N 50; 500 scans were acquired in each spectrum.

sample. In the aluminated silicalite the situation is dramatically changed (see Fig. 52b). First, the tetrahedral signal becomes significant after a very few scans. Clearly the amount of tetrahedral Al has been considerably increased. Second, there is an octahedral signal with approximately 5 of the intensity of the tetrahedral one. The narrowness of the signal at 0 ppm indicates that the six-coordinated Al is highly mobile, such as in the hydrated A13+ cation. Moreover, the intensity of this signal does not decrease on prolonged washing of the sample with water, which indicates that six-coordinated Al, albeit mobile, neutralizes the framework charge created in the course of alumination and cannot therefore leave the zeolite crystals. These observations are consistent with the following reaction: (SiO,),

+ 4AIC13

-

A13' [(A102)3(Si02),- J

+ 3SiC14

where (SiO,), symbolizes the highly siliceous parent material. The Si/AI ratio of the product, as determined by energy-dispersiveX-ray analysis, is ca. 50. It

ALUMINOSILICATE CATALYSTS

289

is also possible that AICI, vapor reacts with the surface hydroxyl groups in the parent zeolite and with structural faults involving nests of four hydroxyl groups, although four-coordinated A1 is created only in the latter case. Chang et al. (481) and Dessau and Ker (482) have also described the insertion of aluminum into the framework of highly siliceous ZSM-S/silicalite and ZSM-11 (with %/A1 ranging from 300 to 25,000) by reaction with AICI, and AIBr, vapors and aqueous solutions of (NH,),AIF,. They support their conclusions, which are similar to those reached by Anderson et al. ( 2 1 4 , with *’A1 MAS NMR and infrared spectra and the measurements of temperatureprogrammed desorption of ammonia. N. PRECURSORS IN ZEOLITE SYNTHESIS

Zeolitic aluminosilicates are prepared in the laboratory by hydrothermal synthesis at moderate temperatures. The reaction mixture must contain silicon (as soluble silicate or colloidal silica), aluminum (in the form of aluminate, aluminum hydroxide or alkoxide), and must be strongly basic. The nature of the cation (i.e., of the base) has strong structure-directing influence. For example, zeolite A is formed in NaOH solutions; however, if KOH is used instead, zeolite L, with a completely different structure, crystallizes. Some highly siliceous zeolites are synthesized in the presence of quarternary amines, and a large number of organic bases have been tried with a view to preparing novel zeolitic structures. The role of the base is thought to be twofold: (1) it alters the gel chemistry; (2) it serves as a “template” controlling the geometry of the tetrahedral units, thus providing the initial building block for a particular type of structure (30). Crystallization of a zeolite is preceded by the formation of an aluminosilicate gel and involves an “induction period.” The mechanism by which zeolites form from such gels is among the least well understood aspects of zeolite chemistry. The main reason is the complexity and heterogeneity of the synthesis mixture composed of the amorphous gel, the supernatant solution, and the emergent zeolite crystals. The three phases have in the past been studied separately using diverse techniques such as the molybdate method, chromatography, trimethylsilylation, NMR, hydrogen electrode measurements, and equilibrium centrifugation (36,212).The most important questions to be answered are as follows: 1. What is the nature of the Al- and Si-bearing species in the mixture? Are aluminosilicate ions present? Are the secondary building units found in zeolitic frameworks already present in solution? 2. What is the structure-directing role of the base and the status of the “template theory”? How can one base give rise to so many different

290

J. M. THOMAS AND J. KLINOWSKI

structures? Why is the same structure sometimes obtained under completely different conditions? The chemical state of dissolved silica (if SiO, is supplied as silica sol) must influence the nucleation and growth of crystalline silicates. Cary et al. (213) dissolved isotopically enriched silica in H,0/D20 and, using NMR, concluded that tetrahedral dimers corresponding to pyrosilicic acid, H,Si,O,, built of two Q' units were present in addition to monomers (QO). The former species resonated at -9.26 ppm from TMS and accounted for up to 6 % of total spectral intensity. While silicate and aluminate solutions have been extensively studied using "Si and 27AlNMR (see Sections III,B and III,I, respectively), there is only a handful of publications that investigate mixed (Si,Al)-bearing solutions by NMR spectroscopy with a view to elucidating the mechanism of zeolite synthesis. There is thus a considerable scope for further work in this important area. chemical shifts in The work of Miiller et al. (138), who measured TMA-aluminosilicate solutions, has been discussed in Section III,I. Briefly, they identified four kinds of Al- centered units, i.e., Qo, Q'(1 Si), Q2(2Si), and Q3(3Si), and suggested that no Al-0-A1 linkages are present in aluminosilicate anions. De Jong and Dibble (214) and Dibble et al. (215) carried out a series of experiments with mixtures containing Na' and aluminate and silicate anionic species. The solutions, 3M in Si, 0-0.4 M in Al, and with Na/Si = 3, were clear and no gels formed until the onset of zeolite nucleation. While no incontrovertible proof of the actual existence of aluminosilicate anions was given, the spectra were strongly affectedby the addition of Al. When the solution is 0.4 M in A1 the FWHM of the Qo signal doubled in comparison with pure silicate; the FWHM of the Q' signal increased by 10% when aluminate was added. "Si and spectra did not change during the induction period, but when crystallization came to completion no A1 was detected in the supernatant solution, which gave a typical silicate spectrum. Derouane et al. (216) studied the influence of pH and the addition of Na', Cs', and tetrapropylammonium ion (TPA') on 27Al NMR spectra of sodium silicate/sodium aluminate solutions. At pH <4 only octahedral Al(H,O)z+ species were formed and the spectra were not affected by the addition of either cation. At pH7 a very broad resonance was observed at 10-25 ppm, presumably attributable to precipitated six-coordinated polymeric aluminous species. At pH 10.5 a resonance at ca. 80 ppm appeared in the absence of silicate, corresponding to Al(0H);; it broadened on adding TPA+, but narrowed again when Na' was also added, suggesting that Na' interacts with aluminate species in preference to TPA' , When sodium silicate was added to the solution, a broad line appeared at ca. 55 ppm corresponding

ALUMINOSILICATE CATALYSTS

29 1

to four-coordinated Al. In another experiment, aluminosilicate gels were first prepared at pH4, then the pH was increased to 10.5. The resulting 27Al lines were twice as broad as when the reagents were mixed at pH 10.5. Derouane et al. propose molecular schemes to describe the formation of aluminosilicate gels under various conditions, and conclude that the role played by TPA' as counterion depends on the availability of the alkali cations. Boxhoorn et al. (217) studied silicate species present in the ZSM-5 synthesis mixture of composition 288 SiO, :A1,0, :9Na20:42(TPA),0: 5400H20. 29SiNMR revealed the presence of many different species in the mixture, from monomers to branched silicate anions. When DMSO, MeOH, or EtOH were added, however, the spectrum simplified to a single sharp signal at -98.0 ppm suggesting the presence of a double four-, five- or six-membered ring species. NMR alone could not distinguish which of the three was present, as their chemical shifts differ only by ca. 1 ppm. The authors claim that attenuated total reflection Fourier transform IR spectroscopy and mass spectroscopy point to the assignment of the NMR signal to the double fivemembered ring, which would then serve as a precursor species for ZSM-5. Engelhardt et al. (218) used 29Si and 27Al MAS NMR to study solid aluminosilicate gel precursors in the synthesis of zeolite A. Sodium silicate and sodium aluminate solutions were mixed to give reactant mixture of composition 2Si0,: A1,0,: 3.3 Na,O: 170H,O. In one case the precipitate formed was separated after 5 min; other solid samples were prepared by heating the reaction mixture to 80°C for 4, 6, or 17 hr prior to filtration. As shown in Fig. 53a the initial gel gives a broad (FWMH = 420 Hz)29Sisignal at -85 ppm from TMS, which is typical of Si(4Al) units in an amorphous or highly disordered environment. The spectrum did not change when C P was applied, which means that Si(3Al)(OH) units were not present in significant amounts. Partial transformation of the spectrum was observed when the gel was separated after 4 hr (Fig. 53b), but in the sample from the mixture treated for 17 hr a narrow peak at - 89.4 ppm was measured, which is characteristic of zeolite A (see Section 111,D). The 27Al MAS NMR spectra always featured a single peak at 59 ppm from A1(H20)2+,corresponding to four-coordinated Al. With progressing crystallization the 27Al signal narrowed considerably, probably due to the removal of residual quadrupolar interactions caused by an asymmetric environment around the A1 nucleus in the incompletely formed crystals. In an attempt to throw further light on the role of intermediates in the synthesis of zeolite A, the same authors (219) considered two different starting sodium silicate solutions: solution A, the same as that used in ref. 218 above, had Csio2= 1M and Na/Si = 2, while solution B had Csiol = 1.65M

292

J. M. THOMAS AND J. KLINOWSKI

1

1

- 74

,

,

- 82

1

.

-90

,

1

-98

1

1

-106

,

,

1

-114

ppm f r o m T M S

FIG.53. 29Si MAS NMR spectra at 39.74 MHz of solid intermediates in the synthesis of zeolite Na-A (218).Solid separated from the reaction mixture kept at room temperature (a) after 5 min (see text), (b) after 4 hr at 80"C,(c) after 6 hr at 80"C,(d) after 17 hr at 80°C.

and Na/Si = 0.6. The "Si NMR spectra of the solutions are given in Fig. 54 in which the various Q" groups have been defined in Section III,B, and QZ,. denotes cyclotrisilicate anions. They estimated the quantitative distribution of the various silicate units from the integrated signal intensities in the spectrum (see Table XVIII). As could be expected, solution A contains mainly low molecular weight silicate anions, whereas in solution B polymeric silicate anions containing branching and cross-linking units predominate. The mean degree of condensation of the silicate ions, defined as the inverse of the mean number of terminal SiOH groups, was 0.37 and 0.71 for solutions A and B, respectively. Engelhardt et al. (218) showed that solution A gives rise to an aluminosilicate gel with Si/AI = 1 ;in contrast, when solution B is mixed with the aluminate solution in such a way as to form reaction mixture of the same compostion, the initial aluminosilicate gel has Si/AI = 2.4, i.e., a very different composition. Crystallization of that gel has been subsequently

293

ALUMINOSILICATE CATALYSTS Q0

Q3

Q2

-70

- 90

-80

- 100

-110

6 (ppd

FIG.54. 29Si NMR spectra at 19.87 MHz of sodium silicate solutions (218,219). (a) Solution A with Csiol = 1 M and Na/Si = 2; (b) solution B with Csio, = 1.65 M and Na/Si = 0.6. For the meaning of Q" symbols see text.

studied using 29Si NMR. (See Table XIX.) The spectra of samples 1 and 2 (withdrawn from the reaction mixture at room temperature after 1 and 5 min, respectively) indicate silica-rich amorphous aluminosilicate composed mainly of Si(2Si,2Al) and Si(1Si,3Al) building units in a disordered arrangement (Fig. 55). The 29SiNMR lines, broadened by a distribution of chemical shifts, are centered at - 90 and - 95 ppm. For samples 3-6, withdrawn after TABLE XVIII The Distribution of the Various Q" Silicate Building Units" in Sodium Silicate Solutionsb _____

_____

~~

c,,,, Solution

(M)

NaPi ratio

A B

1 1.65

2 0.6

"In mol % Si. Ref. 219.

Qo

Q'

Q$.,

28 3

23 7

15 0

Q2

Q3

28 32

46

6

Q4 0 12

294

J. M. THOMAS AND I. KLINOWSKI

TABLE XIX Parameters of the ”Si and ”A1 MAS N M R Spectra and the Chemical Composition of Solid Gel Intermediates and Zeolite Na-A Preparedfiom Solution B”

Sample number

Reaction conditions 1 min at 20°C 5 min at 20°C 30 min at 80°C 1 hr at 80°C 2 hr at 80°C 4 hr at 80°C 22 hr at 80°C

-93.2 -92.0 - 86.5 -84.7 -85.3 -86.4 - 89.4

FWHM (Hz)

A1 (PPm)

600 650 410 370 390 290 70

55.7 -

58.5 58.2 58.2

FWHM (Hz)

Si/Al ratio

2.4 2.0 1.2 1.1 1.1 1.1 1.o

From ref. 219. See text.

times varying from 30 min to 4 hr at 8WC, a gradually sharpening signal assignable mainly to Si(4Al) units is observed. Again, as in the earlier study, the 29Sispectrum of the final sample (after 22 hr at 80°C) is typical of highly crystalline zeolite A. The increasing A1 concentration of the gel phase with the increasing reaction time is further reflected by the chemical shift for the single 27A1 signals listed in Table XIX (55.7 ppm for sample 1, but 58.2 ppm for sample 7). Thus, solutions A and B, when mixed with sodium aluminate so as to give reaction mixtures with the same overall composition, give rise to very different intermediate solid samples. The 29Sichemical shift typical of zeolite A is reached from the “aluminous” direction when a solution A is used, but from the “siliceous” direction when solution B is employed (compare Figs. 53 and 55). This shows that the nature of the reaction intermediate is greatly dependent on the nature of the initial silicate solution. The most plausible interpretation of the above results is as follows: A1 in aluminate solutions is present in the form of Al(0H); anions irrespective of the A1 concentration or the Na/Al ratio (150,181,182). When solution A, containing low molecular weight silicates and thus a high proportion of reactive SiOH groups, is mixed with the aluminate solution, a predominantly aluminous intermediate results. When, by contrast, solution B, richer in silicon and with fewer silanol groups, is used, a silca-rich intermediate is formed. But the composition of zeolite A (with Si/Al = 1.00) is achieved via structural rearrangements in the amorphous precursors and is dictated by the final structure itself. Scholle et al. (220) used 27Al MAS NMR at 130.3 MHz to study the intermediates of zeolite ZSM-5containing tetrapropylammonium hydroxide (TPAOH) as template as a function of crystallization time. For the samples

ALUMINOSILICATE CATALYSTS

J

'

"

"

'

295

"

-60 -80 -100 -120

6 (ppm) FIG.55. 29Si MAS NMR spectra at 39.74 MHz of solid intermediates in the synthesis of zeolite Na-A using solution B (219). Solid separated from the reaction mixture kept at room temperature; (a) after 1 min (see text); (b) after 30 min at 80°C; (c) after 2 hr at 80°C; (d) after 4 hr at 80°C;(e) after 22 hr at 80°C. Chemical shifts are in ppm from TMS.

with crystallization times of 12hr, 1 day and 3 days, two distinct 27Al resonances, both corresponding to tetrahedrally coordinated aluminum, were observed; for the sample crystallized for 8 days, there was an additional shoulder indicating a third distinct tetrahedral site (compare Figs. 34 and 35 in Section IIIJ). The latter sample was the most crystalline and had Si/Al = 12.5 The spectrum of zeolites H-ZSM-5 and H-ZSM-11 showed no fine structure. Neither did iow-field spectra (at 78.2 MHz) of TPA-ZSM-5,

296

J. M. THOMAS AND J. KLlNOWSKl

but this is evidently due to the increased second-order quadrupolar interactions. More work is necessary to explain how TPA' cations induce the nonequivalence of tetrahedral Al sites.

0. NMR STUDIES OF EXCHANGEABLE CATIONS The sorptive, catalytic, and ion-exchange properties of zeolites depend strongly on the kind, position, and mobility of the charge-balancing cations. Since chemical shifts and multiplicities of lines are related to site occupancy and their widths to cationic mobility, NMR can in principle provide important information on the nature of the intracrystalline environment. Unfortunately however, the cations most often found in natural and synthetic zeolites are not ideal for NMR studies. The 23Nanucleus has a large quadrupolar moment, while cations of most spin nuclei (such as Y,Rh, Ag, Cd, Pb, Hg, Os, and Pt) are of relatively little interest to the zeolite chemist. For these reasons much attention has been paid to 7Li, which has a small quadrupolar moment, and to 205Tl,an attractive spin Q nucleus. O n the other hand, the 'jNa resonance can be used as a sensitive probe for the electric field gradient in zeolites and the effects brought about by sorbed molecules. The early wide-line NMR experiments on quadrupolar nuclei in zeolites have been performed by Lechert and co-workers (221-229) and are discussed fully in a review by Lechert (190). Briefly, the 'jNa spectra of zeolite X with Si/AI close to unity consist of a single featureless line ca. 12 kHz wide. For synthetic faujasites with Si/AI > 1.29 the line becomes asymmetric and its intensity decreases rapidly. Such spectra are very difficult to interpret, but certain conclusions on the occupancy of the cationic sites at low Si/Al ratios can be drawn. A point charge-multipole model led Lechert (225) to the conclusion that not all 23Na nuclei in zeolite X are observable by NMR. Calculated linewidths for S; and S, sites are too large for these cations to be observable, and according to the model the entire signal intensity comes from 23Na in the S, cationic sites in the double six-membered rings joining the sodalite cages. Lechert believes that the main contribution to the EFG comes from induced dipoles at the sites of oxygen atoms. Line shapes depended on the amount of intracrystalline water, and sharp narrow lines were observed at high water contents, which might be due to the averaging of the EFG by the motion of the sorbate. The effect of sorbed polar molecules on the spectra can be explained in terms of their influence on the induced dipoles. 23Na spectra of zeolites A and Y could not be satisfactorily interpreted by such model calculations. Until recently only broad-line NMR has been used for observing the 'jNa resonance. The technique is not well suited to obtaining absolute intensities,

ALUMINOSILICATE CATALYSTS

297

as this requires double integration of the measured derivative spectrum. Basler (230) used the pulsed resonance technique at 16 MHz in which the initial intensity of the FID is proportional to the number of 23Nanuclei and can be calibrated against the 'H signal in the same sample. Zeolite Na-Y with Si/AI = 2.36 contains seven N a + per supercage, of which three are located in S, and S; sites and four in S, sites, i.e., in the middle of six-membered rings lining the cage. The total measured 23Na intensity corresponds to four Na+ per cage, and Basler concludes therefore that the NMR signal comes from S2 sites. When these sites were selectively exchanged by Ca2+,no 23Na signal was observed, which would tend to confirm this conclusion, in contrast to earlier work (190,225). West (231) observed no 23Naresonance in dehydrated synthetic faujasites, suggesting that the EFG at the cationic site is larger than in hydrated samples because of the displacement of the cations away from their high-symmetry positions. The signal appeared when seven H,O molecules per cage were present. Fully hydrated Na-X and Na-Y had Tf of 100 and 140 psec, respectively, while in dehydrated samples much faster transverse relaxation was observed. The 7Li resonance in zeolites is also difficult to interpret, even though the quadrupole moment is much lower. Lechert et al. (227) believe that the 'Li linewidth is controlled by the dipole-dipole interaction with 27Al nuclei in the aluminosilicate framework. According to Herden et al. (232) the increase of 7Li frequency from 9 to 21 MHz does not affect the second moment of the spectra in zeolites Li-X and Li-Y, which means that the quadrupolar interaction is small. The second moment was also independent of the Si/AI ratio. The mean Li-A1 distance calculated from the van Vleck formula was 2.35A. Small amounts of divalent cations reduce the movement of Li+ considerably, with the activation energy for this process increasing from 30 to 60 kJ/mol. ,''TI is a very favorable nucleus for solid-state NMR studies: it has 1 = i, high natural abundance and high sensitivity. Its large chemical shift range makes it possible to observe individual environments of the nucleus. Thallium can be easily introduced into zeolites by cation exchange. Freude et al. (233,234) measured "'TI NMR spectra of Tl-exchanged zeolites X, Y, and A as a function of water content and temperature. They interpreted the spectra of zeolite A as superimpositions of three lines, and ascribed the observed changes in line shape to thermal motion of TI3 cations located near the center of the eight-membered rings. With increasing water content and/or temperature the frequency of jumps between the four cationic sites in the plane of the ring increases; it is 10' sec-' at 100°C in dehydrated crystals or at room temperature with 4 H,O molecules per large cage. Jumps out of the plane (lo3 sec-' at 200°C in dehydrated crystals or at room

298

J. M.

THOMAS AND J. KLINOWSKI ?Po0ppm

(h)

n -It--

LtR

"0

FIG.56. 20sT1NMR spectra (231) at 51.92 MHz of dehydrated zeolites (TI, Na)-A. The reference R marks the position of the signal in hydrated (12TI)-A. (a) (12TI)-A;(b) (10TL2Na)A; (c) (ETl,4Na)-A;(d) (6T1,6Na)-A; (e) (STl,'INa)-A;(f) (4Tl,8Na)-A; (g) (3Tl,9Na)-A; (h) (2TI,IONa)-A.

temperature with 10 H,O molecules per cage) lead to translational diffusion of T1' cations through the crystal. This motion is dominant at higher water contents. West (231) carried out a series of 'OsTl NMR measurements in zeolites (Na,Tl)-A. The fine structure of the resonance line was associated with at least two specific types of cation site. Figure 56 shows that for low Ti contents one site group is preferentially exchanged by Tl'. The lines from hydrated samples were quite narrow with FWHM varying linearly from 180 Hz in (2TI,lONa)-A to 500 Hz in (12TI)-A. Figure 57 shows the spectra of samples (12Tl)-A and (3T1,9Na)-A as a function of water content. West concludes that there is rapid self-diffusion of T1' cations in the latter sample when dehydrated, and that this motion is completely quenched with partial hydration, In general, the effect of sorbed water is to increase the mobility of the T1+ cations, but not that of Na' cations, which remained stationary in all samples. It is known that the 29SiNMR chemical shift in zeolites is sensitive to the type of the exchangeable cation (53, which indicates the presence of interactions between cations and the framework. In particular, the substitution of Na' by Li' in zeolite A and in synthetic faujasite moves the "Si resonances ca. 4 ppm downfield in both cases. Melchior et al. (235) have used this effect to study the location of cations in a series of partially exchanged zeolites (Li,Na)-A. They found that the average "Si chemical shift is not a

ALUMINOSILICATE CATALYSTS

299

FIG.57. 205T1NMR power spectra (231) at 10.3 MHz as a function of water content for zeolites (a) (3ll,9Na)-A, and (b) (12Tl)A. The water content is given in parentheses as number of H,O molecules per a-cage.

linear function of fractional Li' exchange, and must be interpreted in terms of preferential occupation of particular cation sites by Li +.This preferential site occupancy is, in turn, sensitive to the degree of hydration of the zeolite. In anhydrous and partially hydrated (Li,Na)-A, both 29Si and 'Li NMR provide evidence for local and possibly long-range order of Li' and Na+ cations.

300

J. M. THOMAS AND J. KLINOWSKI BETWEEN ZEOLITES AND P. INTERACTIONS

THE

ADSORBEDSPECIES

1. General Considerations

Section III,K has revealed how the study of adsorbed molecules such as pyridine, ammonia, trimethylamine, or acetonitrile can reveal important information concerning zeolitic acidity. In general, NMR of adsorbed species is a powerful way of probing the nature of the solid. Relaxation times 7'' and T, depend on the motion of molecules which contain the nuclei (236) and their measurement often leads to the various kinetic parameters for the adsorbed molecules, the knowledge of which is essential for the understanding of the mechanism of many zeolite-mediated processes. The diffusion coefficient of the reactants and products in a catalytic reaction, which can be determined from NMR, is often rate limiting. Relaxation studies can also determine surface coverage by the sorbed species and provide information about the distribution of adsorption energy between the different sites on the surface of a catalyst. For these reasons a great deal of NMR work has been done with adsorbed species in zeolites in the course of the last twenty years. From the applied viewpoint the emphasis is on water and hydrocarbons as guest molecules; from the fundamental viewpoint species such as Xe, SF6, H,, CH,, and NH, are of special interest. It is outside the scope of this review, which is primarily concerned with solid-state NMR, to survey this extensive field fully, especially since several good reviews are already in existence (188,190,237-239, 440). Instead, a general discussion followed by a summary of most important results will be given, and part'icular stress will be laid on recent work in the field.

2. Sorption and Mobility of Water Much attention has been given to the NMR behaviour of water adsorbed on zeolites (237-271). In all of these systems some of the adsorbed molecules are restricted by intermolecular forces from undergoing random reorientation, and consequently single crystals may give proton NMR spectra exhibiting fine structure due to imperfect motional averaging of the nuclear dipole-dipole interactions. Whether or not fine structure is observed depends on the width of the line, which is in turn governed by the nature of the intracrystalline motion. Early NMR studies on water in single crystals of natural zeolites (240-246), have been summarized by Buckingham and McLauchlan (279).Analcite, natrolite and thomsonite yield 'H NMR spectra with linewidths of the order of several tens of kHz, while faujasite gives a sharp resonance due to the absence of orientation effects in the supercages. In chabazite, gmelinite, edingtonite, and stilbite the 'H spectrum exhibits sharper lines with a doublet structure caused by dipolar interactions between

301

ALUMINOSILICATE CATALYSTS

T (K) 250

300

500 400

on

HtO

C Y -I

2.0

3.0 10YT

4.0

5.0

(K-')

FIG.58. Proton relaxation times T, and T2 for water adsorbed in zeolite Na-X at 12 MHz (248); 0 denotes the results of Kvlividze et al. (271).

the protons in the partially oriented water molecule. Careful studies have enabled orientational parameters S,, as defined by Saupe (280) to be determined. These were found to be temperature dependent in the expected manner. In gmelinite two doublets are observed (243)resulting from two sets of water molecules, one diffusing along the channels and the other through eight-membered rings in the gmelinite cages. The proton resonance of HDO in the chabazite framework (243)consists of a triplet due to J-coupling to the spin 1 deuteron. Relaxation times TI and T, have been determined as a function of temperature and surface coverage in various zeolites, particularly of the faujasite type. The early experiments have been troubled by the very strong dependence of relaxation rates on the concentration of paramagnetic impurities. In order for the relaxation values to be meaningful, such impurities expressed as Fe content must be below ca. 6ppm. Figure 58 shows the variation of TI and T, for water adsorbed in a particularly pure sample of zeolite Na-X (248). The authors (248) account for the experimental results using a model of the intracrystalline fluid, which is about 30 times as viscous as bulk water at room temperature. It shows a broad distribution of molecular mobilities (the ratio T,/T, at the minimum in TI is much larger

302

J. M. THOMAS AND J. KLINOWSKI

than expected for a single correlation time) and is about as dense as liquid water. The median correlation time is T* = 2.8 x

lO-’’exp[417/(T - 189)] sec.

By observing the free induction decay following an rf pulse, two distinct exponential components of T, could be distinguished in faujasitic zeolites. The fast decay immediately following the pulse has been attributed (264-266) to water inside the sodalite cages (T2= 50 psec at room temperature) and the slower decay (T, = 40 msec) to water in faujasitic supercages. Signal intensity measurements indicate that over a wide range of coverages there are four H,O molecules per sodalite cage. The application of the theory of Zimmermann and Brittin (281) to those relaxation times leads to the conclusion that no exchange of water between the two reservoirs occurs on a time scale of several seconds. For the fast-decaying component of the FID, TI and T,are unaffected by the concentration of paramagnetic impurities, confirming that the motion of the molecules to these relaxation centers is restricted. On the other hand, the relaxation of water in supercages is very strongly affected by the paramagnetics (239,248-251,263). Pfeifer et al. (263) conclude from their measurements of TIand T’ versus temperature in samples with controlled water contents that the lifetime of sorption complexes of water is 3.5 x lO-’sec at 50°C with nonlocalized cations and at -10°C with localized ones. Water was found to be bound more strongly in faujasites with higher Si/Al ratios, which agrees with model calculations by Dempsey (282) of the electrostatic fields around cations. At higher coverages the mobility of H,O is independent of the Si/Al ratio and is two orders of magnitude lower than in bulk water. The T, relaxation times of 50psec and 40msec given in the preceding discussion correspond to line half-widths of 6.4 kHz and 8 Hz, respectively. Whipple et al. (265) concluded that the line widths of several hundred Hz which are obtained in practice must be due to bulk magnetic susceptibility effects. This type of line broadening is removable by MAS (273) and they were the first to obtain high resolution spectra with linewidths similar to those expected from the T, values. Kasai and Jones (270) applied MAS to the study of water in zeolites A, X, Y, mordenite, ZSM-5, and silicalite. They found that although the signals were sometimes quite broad, their chemical shifts were characteristic of the zeolite (Fig. 59). They interpret this as the effect of the disruption of hydrogen bonding of bulk water by the zeolitic framework and of the interaction of water molecules with framework oxygens. An inverse relationship was found between the chemical shift and the Si/Al ratio. The chemical shift of water in silicalite is quite different from that in ZSM-5 and does not fit this

303

ALUMINOSILICATE CATALYSTS

Y

-I

7

I

6

I

5

I

4

I

3

I

2

I 1

I

0

ppm f r o m dimethylsiloxane FIG.59. ‘H MAS NMR spectra at 100 MHz from fully hydrated zeolites Na-A, Na-Y, Na-mordenite (Na-M), and Na-ZSM-5 (270).

relationship. It is, on the other hand, similar to that in silica gel, which indicates that water molecules in silicalite are located only at the external surface of the crystallites, presumably hydrogen bonded to the hydroxyl groups. The same authors measured the chemical shift of water in Ba-, Ca-, and Na-mordenites. Considering the effect of the cation on the proton spectra of aqueous solutions of electrolytes (274),one would expect larger dependence of 6 on the type of cation than is usually found. Kasai and Jones (270)suggest therefore that in zeolites water molecules interact primarily with the framework oxygen. Chemical shifts in zeolites exchanged with divalent cations are larger than for monovalent cations because more oxygens are “exposed” in the former case. Support for this explanation comes from the relative magnitudes of heats of immersion of various zeolites measured by Barrer and Cram (283). Relative linewidths indicate that the mobility of water is inversely related to the size and number of cations. Investigations of self-diffusion coefficients have been carried out by Parravano et al. (252) and Karger et al. (259, 275-278) using pulsed-fieldgradient NMR techniques. The residence time t of a molecule in a particular

304

J. M. THOMAS AND J. KLINOWSKI

state and the self-diffusion coefficient, D, define the mean square jump length for isotropic diffusion in three dimensions: ( d 2 > = 6Dr.

Karger (259) measured D = 2.5 x lo-' cm2 sec-' at -35°C for zeolite X with 12 H,O molecules per supercage, and 1.4 x lo-' cm2 sec-' for 30 molecules. This leads to jump lengths of 8 and 6 A, respectively, compared with ca. 2 A in bulk water. Residence times and jump distances in zeolites are greater than in the liquid. Riedel et al. (260) concluded from pulse-gradient experiments that the mean diffusion distance of water in Na-Y is larger than the average size of the particle. This was confirmed by the measurements of temperature dependence of linewidth conducted by Whipple et d.(265). Kasai and Jones (270) show the coalescence of the proton signal in the intimate mixture of zeolites with very different chemical shifts for water, which confirms this finding further. Figure 60 shows that a single line at an average value of chemical shift is obtained when zeolites Na-X and Na-Y are mixed.

I

7

I 6

I

5

I

4

I 3

I 2

I

1

I

0

ppm from dimethyleiloxane FIG.60. 'HMAS NMR spectra of the 50:50 mixture (by weight) of zeolites Na-X and Na-Y compared with the spectra of the individual zeolites (270).

ALUMINOSILICATE CATALYSTS

305

It is well known that high-silica zeolites such as silicalite are hydrophobic. Addition of hexane to ZSM-5 does not affect the NMR signal from water, but addition of butanol has a very marked influence. This indicates that butanol displaces water from the intracrystalline space to the outer surface of the zeolite particles.

3. Multinuclear Studies of Sorbed Species The early NMR studies of molecules sorbed on zeolites used 'H and 19F resonances. Truly multinuclear work involving 13C, 15N, and lz9Xe in particular began in the early 1970s with the advent of modern Fourier transform spectrometers. The low natural abundance of 13C (1.1 %) can be compensated for by working at high fields, using cross-polarization, and by resorting to isotopically enriched compounds. O n the other hand, 13C has the advantage of a large range of chemical shifts (ca. 250 ppm as opposed to ca. 10 ppm for 'H) and gives much narrower resonance lines as a result of the absence of homonuclear dipolar interactions. "C- H interactions are removed by high-power decoupling and the chemical shift anisotropy (which can be considerable. particularly for carbonyl and aromatic carbons) by magicangle spinning. The resulting 13C spectra often contain much valuable information. The same applies to 15N, although the use of isotopically enriched species is normally unavoidable when working with this nucleus which has the natural abundance of only 0.365 % Diffusional behavior of sorbed species is studied by NMR using one of three approaches; the van Vleck method of moments, relaxation measurements, and the pulsed-field-gradient method. An example of the use of the method of moments is the work of Stevenson (194) on 'H resonances in zeolite H-Y (see Section 111,K).Another is the study by Lechert and Wittern (284) of C6H6 and C,H,D, adsorbed on zeolite Na-X. Analysis of second moments of 'H resonances allowed the intra- and intermolecular contributions to the spectra to be extracted. Similarly, second moments of 'H and "F spectra of cyclohexane, benzene, fluorobenzene, and dioxane on Na-X provided information about orientation of molecules within zeolitic cavities (284-287). The measurement of relaxation times Tl and T2 and the subsequent application of the theory formulated by Bloembergen et al. (236), and extended by Kubo and Tomita (272) and Torrey (288), leads to the determination of motional and thermodynamic parameters such as mean times between molecular jumps, diffusion coefficients, and activation enthalpies for translation. For example, Resing and Thompson (289,290) used this

306

J. M. THOMAS AND J. KLINOWSKI

approach in their study of diffusional behavior of SF6 in zeolite Na-X by monitoring the ”F resonance as a function of temperature. Pulsed-field-gradient NMR (291-292), in which spin echoes are measured in the presence of a time-dependent magnetic field gradient, has been used to determine effective diffusion coefficients, D,, , in beds of zeolite powder. Barrer (35) quotes the expression for the spin-echo amplitude given by Karger (259) in the form:

where u and g are, respectively, the width and amplitude of the gradient pulses at intervals At, p is the fraction of the sorbate molecules in the intracrystalline space, and D* and D: are, respectively, the intra- and intercrystalline self-diffusion coefficients. The quantity in the square bracket, Deff, is approximately equal to ( d 2 ) / 6 A t where (d’) is the mean square displacement of the molecule over the interval At. There are two limiting situations (35): 1.

t

2.

t

B At, i.e., (d’)’’’ 4 crystal radius. In this case Dcff = D*. 4 At, i.e., (d’)’/’ >> crystal radius. In this case Deff = pD:.

Pulsed-field-gradient studies of methane sorbed on zeolite (Ca,Na)-A and n-butane and n-heptane on zeolite Na-X (259,293-294) under the conditions of case 1 above showed that Dcffdecreases with increasing hydrocarbon chain length and with the fractional saturation of crystals, 8.At 20°C and 0 = 0.8, cm’ sec-’ for n-butane and Deff ( = D*) is 1.4 x lo-’ and 6.3 x n-heptane, respectively, which is similar to the values measured in bulk liquids. An intriguing aspect of these measurements is that the values of D* determined from NMR and from sorption kinetics differ by several orders of magnitude, For example, for methane on (Ca,Na)-A the value of the diffusion coefficient determined by NMR is 2 x lo-’ cm’ sec-’, and the value determined for sorption rates only 5 x lo-’’ cm2 sec- The values from NMR are always larger and are similar to those measured in bulk liquids. The discrepancy, which is, of course, far greater than the uncertainty of either method, remained unexplained for several years, until careful studies (267,295,296)showed that the actual sorption rates are not determined by intracrystalline diffusiot), but by diffusion outside the zeolite particles, by surface barriers, and/or by the rate of dissipation of the heat of sorption. NMR-derived results are therefore vindicated. Large diffusion coefficients (of the order of cm2 sec-’) can be reliably measured by sorption kinetics

’.

ALUMINOSILICATE CATALYSTS

307

only in large crystals of natural zeolites. One NMR study (297) has indicated that surface barriers are indeed present in zeolite powders. The usefulness of 13C NMR to the study of various hydrocarbons adsorbed in zeolites has been demonstrated by Deininger and Michel et al. (298-300, 306). Changes of ”C chemical shift with respect to bulk liquid were observed in (Na,Mg)- and (Na,Ag)-faujasites and depended on the degree of pore filling, the nature of cations, and the structure of hydrocarbons. For (Na,Ag)-zeolites with high silver content, where each adsorbed molecule comes into contact with an Ag’ cation, the shifts with respect to those measured in Na-zeolites are nearly the same as in solutions of silver salts, indicating that chemical shift is determined by bonding to silver. For lower Ag contents line shifts are smaller because of the rapid exchange of molecules between Na’ and Ag’ sites. The strength of the Ag’-olefin bond does not change significantly as the temperature is increased. The shift of carbons forming double bonds is not affected by the type of cation (301). Large low-field shifts have been observed in CO sorbed on decationated zeolites (302).This is believed to be caused by interactions of the hydrocarbon with the Lewis acid sites, The motionally narrowed spectrum of p-xylene adsorbed on zeolite ZSM-5 (303) is consistent with translational diffusion and rotation of the molecule, while the spectrum of o-xylene shows essentially a “rigid lattice” anisotropy pattern, indicating that translational diffusion is very slow. Meiler and Pfeifer (493) measured ”C and ‘H NMR spectra of carbon monoxide, carbon dioxide, and benzene adsorbed on ZSM-5 and silicalite. The 13Csignal from benzene was a superimposition of two lines corresponding to relatively mobile molecules (narrow Lorentzian line) and strongly adsorbed molecules (broad asymmetric line similar to that in polycrystalline benzene). Quantitative interpretation of the spectrum was possible via the measurement of the transverse proton relaxation times, T’, as a function of temperature and coverage. Recent work involving 13C NMR studies of sorbed species is summarized in Table XX. Quadrupolar interactions can offer direct information on the dynamics of organics within zeolite crystals. Eckman and Vega (304) studied the 2H quadrupolar echo decay in perdeuterated p-xylene adsorbed on zeolite ZSM5. The deuterium quadrupolar interaction usually dominates the spin Hamiltonian, so that the powder pattern can be used as a test for models of molecular motion. At - 75°C and 25°C typical rigid-lattice spectra were obtained. At 100°C however, the resonance arising from the aromatic deuterons was motionally narrowed, while the methyl resonance was not, The authors conclude that p-xylene molecules reorient about an axis which passes through the C3axes of the methyl groups.

308

J. M. THOMAS AND J. KLINOWSKI

TABLE XX Recent I3C Studies of Sorbed Species in Zeolites Sorbate Butene isomers Propene 1-Butene trans-But-2-ene CO, enriched in "C CO and CO,

Butanol enriched in I'C 1-Butene

Ethane Carbonaceous residues from catalysis Ethene, propene, isobutene, 2-methyl1-butene o-Xylene, p-xylene Methanol Pyridine Formic acid

Zeolite

Study

Reference

TI, T2, chemical shifts TI, chemical shifts. NOE" enhancements, effects of paramagnetics TI, chemical shifts, interactions (Ge, AI)-X with cations MAS spectra, TI,physi- and Various chemisorption Chemical shifts versus coverage Na-A, Na-X, Na-Y, and temperature (180 to H-Y, Na-mordenite 400 K). Interaction with Lewis sites Na-Y TI and T' versus temperature (220 to 420 K), NOE effects Chemical shifts; calculations of Na-X, (A& Na)-Y NMR parameters from electron densities (Ca, Na)-A Tracer desorption kinetics H-ZSM-5, H-mordenite CP/MAS; identification of species

Na-X and Na-Y Alkali-metal exchanged zeolites X and Y

H-ZSM-5

Oligomerization studied at 300 and 313 K

ZSM-5 H-Y

Molecular mobilities at 310 K Reaction intermediates in the production of alkanes Brensted acidity Unidentate and bidentate formate species

H-Y NH,-Y, H-Y

NOE, nuclear Overhauser effect.

The use of lZ9Xeas a molecular probe will be discussed separately in Sections III,Q and III,R. THE NATURE OF ZEOLITIC MICROSTRUCTURE BY Q. PROBING HIGH-RESOLUTION NMR

When studying the properties of a particular zeolitic catalyst it is not sufficient to consider the overall crystal structure in terms of a space group and pore dimensions. It is also essential to know factors such as the degree of

ALUMINOSILICATE CATALYSTS

309

crystallinity, crystal habit and size, and the nature of structural defects, if any. In particular, the importance of detecting, identifying, and characterizing the defective nature of zeolites cannot be overemphasized since structural faults, with their oft attendant changes in channel size, internal surface area, and accessibility, can give rise to substantial modification of sorptive behavior and catalytic performance, such as activity and shape selectivity. For example, catalytic studies have shown (314) that the 12-membered ring channels in offretite are effectively blocked by a relatively low concentration of stacking faults-not more than 3 % of such faults are adequate to induce shape selectivity to n-hexane over other, larger hydrocarbons in these systems. The unusual reactant and product selectivities of TMA-offretite have also been attributed (315) to the presence of random stacking faults. Likewise, product distributions in the conversion of methanol to petrol (gasoline) are governed, in part, by the amount of ZSM-11 intergrown with the ZSM-5 catalyst. One method that has been shown (316-318) to be capable of characterizing-at the subunit-cell level-intergrowths inside catalytically significant zeolites such as ZSM-5, offretite and zeolite L, is high-resolution electron microscopy (HREM). The method works because direct, real-space images (recorded at point-to-point resolution of ca. 2.4 A) can be taken (319-321) of zeolites and in such a manner as to reveal directly the size and disposition (in projection down an appropriate zone axis) of the cavities or channels. Figure 61 shows a small section, viewed along [lo01 of the offretite structure (dealuminated so as to remove the exchangeable cations and thereby render the channels more visible in the image). The six-membered (s) and eightmembered (e) cavities are clearly seen. Using a strategy developed in this laboratory (321) it is possible to use HREM to pinpoint the presence of, for example, a single sheet of the sodalite structure inside an erionite crystal (Fig. 62). This approach is extremely powerful and endowed with the ability to reach near-atomic resolution. But it is a difficult one to apply in routine analyses because it demands electron microscopic expertise of a high order. It is also a destructive procedure, and not easily adapted for rapid detection and characterization of intergrowths in as-prepared zeolites in general. As has been mentioned in Section III,G, West (101) and Fyfe et al. (102) found that traces of adsorbed species (aromatic hydrocarbons and alkanols) radically change the 29Si MAS NMR spectrum and the XRD pattern of silicalite. It is too early to predict the potential of this fascinating discovery for the structural elucidation of zeolites, but one can speculate about the possible consequential pitfalls. One of them is the extreme sensitivity of the effect, requiring less than one molecule of sorbate per unit cell of the sorbent. Quantitative measurements will therefore have to be carried out under very

310

J. M. THOMAS AND 1. KLINOWSKI

FIG.61. (a) High-resolution electron micrograph of offretite viewed along [lo01 direction (477); (b) schematic drawing; (c) computer simulation, e and s refer to eight- and six-membered rings, respectively.

carefully controlled conditions. Second, it is known (332) that certain zeolites easily undergo structural modification. For example, the structure of silicalite changes from monoclinic to orthorhombic in the temperature interval 24-8OoC, and this transition, which is reversible, is clearly reflected in the NMR spectra (333). On the face of it, the structural sensitivity of high-silica zeolites to the adsorbed species is not altogether surprising. The fact that physical adsorption can cause structural changes in an adsorbent is well known. Bangham et

ALUMINOSILICATE CATALYSTS

31 1

FIG.62. High-resolutionelectron micrograph of erionite viewed along [lOO] direction (476). The stacking defect marked by arrows consists of a single sheet of the sodalite structure.

al., in a series of papers published in the period 1934 to 1944 (322-325) showed that coal and charcoal undergo expansion as a result of the lowering of surface free energy consequent upon the adsorption of gases and vapors. Yates (326-328) demonstrated that when argon, nitrogen, or oxygen are physically adsorbed on porous silica, dramatic changes in the volume of the adsorbent occur. Expansions of 2.5 x lo-’ % for submonolayer adsorption are common, and the behavior is represented by: (aF/dV), = - i K , where avis the volumetric expansion at constant temperature resulting from aF, the lowering of the surface free energy, with K being the bulk modulus of the adsorbent. Another promising approach to the study of microporosity of zeolites involves the measurement of the isotropic 3C NMR chemical shift which, as has been shown in the studies of the tacticity of polymers, is highly sensitive to the environment of the nucleus. In the first study of this kind, Boxhoorn et al. (329) observed that the C-3 carbon resonance from the tetrapropylammonium cation enclathrated in the framework of zeolite ZSM-5 in the course of synthesis is split into two components of equal intensity. The reason for this is that the cation is located at the cross-section of the two nonequivalent

312

J. M. THOMAS AND 1. KLINOWSKI

c2 +

2

I

I

3

N (CH,- CH,- CH, l4

c3

h CI I

n

FIG.63. I3C MAS NMR spectrum with cross-polarization of tetrapropylarnrnoniurn (TPA+) cation in zeolite ZSM-5at 50.29 MHz (330).

channels with two propyl groups lodged in each channel. Nagy et al. (330), who used thermogravimetry in combination with 13C MAS NMR to study TPA+ as well as tetrabutylammonium (TBA ') and tetrabutylphosphonium (TBP') species in ZSM-5 and ZSM-11, have also observed the split signal in the TPA'/ZSM-5 system (see Fig. 63). Thermal analysis showed that in its as-prepared state ZSM-5 contains 3.3-3.8 TPA+ cations per unit cell, which are known (329) to be chemically intact; these are located at each channel intersection. On the other hand, the unit cell of ZSM-11 contains only 2.6-3.0 TBA' or 2.5-2.6 TBP' cations. The reason for this is that, unlike ZSM-5, ZSM-11 contains two types of channel intersections of unequal size. Nagy et al. (313, 330) believe that for steric reasons TBA' and TBP' preferentially occupy the larger intersections, which explains their lower numbers in comparison with TPA' in ZSM-5. Another example of the sensitivity of 13CMAS NMR to zeolite structure is the work of Jarman and Melchior (331) who could distinguish between TMA' cations trapped in the a and p(soda1ite) cages in zeolite A structure in the course of crystallization from a precursor gel. If, therefore, the zeolite which is to be analyzed by 13C NMR, so as to evaluate the extent of intergrowth or variable cage environments is synthesized using TMA' cations as templates, then this method, as we show below, seems viable. We further illustrate the approach by reference to studies (331) of two related zeolite structures: zeolite ZK-4 (isostructural with Linde A) and the highly siliceous analogue of sodalite known as TMA-sodalite. As was shown earlier (Sections III,A and III,D), the structure of zeolite A consists of a cubic array of p-cages linked through double four-membered rings so as to form larger polyhedral a-cages. The sodalite structure (Fig. 7) consists of a dense,

313

ALUMINOSILICATE CATALYSTS

I

ZK4

C

ZK4 C (80.1.41

r

TMA

ATL7

- sodalite

65

60

55

50

6 (ppm from TMS)

FIG.64. I3C CP/MAS NMR spectra at 50.2 MHz of zeolite ZK-4 and TMA-sodalite containing TMA' cations (331). (A) Si/AI = 1.16; (B) Si/AI = 1.62; (C) Si/AI = 2.71; for TMA-sodalite SiiAl = 4.7.

space-filling array of p-cages fused at all six-membered and four-membered faces. The 13Cspectra of three samples of ZK-4, prepared with different %/A1 ratios, and of TMA-sodalite are shown in Fig. 64, while the chemical shifts are given in Table XXI.The chemical shift for absorption I is identical for the four spectra and is 2.4 ppm greater than that for the aqueous TMA' cation. Since signal I in the spectrum of TMA-sodalite must correspond to TMA' contained in the b-cage (this is the only kind of cage present) the signal with the same chemical shift in the spectra of ZK-4 is also assigned to 8-cages. Therefore, signal I1 corresponds to TMA' in the larger a-cages. The 13C chemical shifts (58.8 ppm from TMS for the 8-cage and 56.9 for the a-cage) are insensitive to the Si/Al ratio. Comparative thermogravimetric measurements indicate that NMR spectral intensities obtained using crosspolarization are approximately quantitative in this case, and that there

3 14

J. M. THOMAS AND J. KLINOWSKI

TABLE XXI 13C N M R Chemical Shifts for Zeolite ZK-4 and TMA-Sodalite" Sample ZK-4 A ZK-4 B ZK-4 C Sodalite TMA+ (as.) (I

Si/AI ratio 1.16 1.62 2.71 4.7 -

Composition of sodalite cage

Peak I (ppm from TMS)

CAI, lSi,a04,] I[A1g,2Sil,,,048]9~2[AI,.SSiI,.s04a]6.s[A14.2Silg,,04a]4~2-

52.92 58.84 58.84 58.84 -

Peak I1 (ppm from TMS) 56.96 56.97 -

56.4

Ref. 331.

is near-complete occupancy of the /?-cage over the complete composition range. In another study of this kind, Melchior et al. (345)have explored the nature of the internal structure of offretite and mazzite (zeolite omega) using material synthesized in the presence of TMA + cations. The difference between the 13C CP/MAS NMR spectrum of offretite and blocked offretite (material containing stacking faults) (Fig. 65)indicates that the method may have potential. An improved procedure, using the same overall strategy, would be to use a small probe molecule which could sense the environment of the zeolitic channels and cavities in the post mortem sense. A candidate probe molecule is CH, (13C enriched in order to avoid having to resort to cross-polarization), but it would have to be used at inconveniently low temperatures or high pressures to secure adequate uptake for NMR detection. Other possible probe species are CF, and SF,, bristling with nuclei that are very sensitive to detection and 100% abundant ("F). Finally, there is xenon, especially since ltgXe can be readily detected using straightforward NMR procedures. Xenon is a very useful molecular probe for adsorption studies. 12'Xe is a spin 3 nucleus of 26.44% natural abundance and a very wide range of chemical shifts (334). The shielding of the xenon atom with respect to the bare nucleus has been estimated to be 5642 ppm (339, and the 12'Xe chemical shift is extremely sensitive to physical environment as shown by its strong dependence on density in the pure phases: the liquid at 224K resonates 161 ppm downfield from the gas at zero density, whereas the solid at 161 K has its resonance at -274 ppm. The atomic diameter of xenon is 4.6A, i.e., comparable to the size of zeolitic channels. Fraissard and co-workers were the first to take advantage of these properties of 12'Xe for the study of xenon adsorbed in zeolites (336344). They have demonstrated that the '*'Xe chemical shift is then a sum of several

ALUMINOSILICATE CATALYSTS

315

OFFRETITE

A 0 F F R E T IT E

I

I

I

I

I

I

I

61

60

59

50

57

56

55

ppm from TMS

FIG.65. I3C CP/MAS NMR spectra of offretite and blocked offretite containing TMA' cations (345).

terms corresponding to the different interactions involving the adsorbed molecule:

6 = 60

+ 6s + 6 , + 6,,

(27) where 6 , is the reference chemical shift, corresponds to collisions between xenon and the walls of the zeolite cage, 6, is due to the intrazeolitic electric field, and 6, corresponds to collisions between the xenon atoms themselves. Clearly, 6, and 6, must be pressure dependent. In particular, 6s = C Z P S ,

Where C, characterizes Xe-surface collisions and ps is the probability factor depending only on the particular structure under consideration. The importance of Eq. (27) lies in the fact that its various terms can be determined separately. For example, dS + 6, is obtained by extrapolation of the plot of chemical shift to zero pressure. It is found that 6 , is negligible in zeolites Na-Y and H-Y, possibly because of motional averaging of electric field effects on the large faujasite supercages, but it becomes important in alkaline earthexchanged forms. Zeolites Ca-A, Na-X, Na-Y, H-Y Ca-Y, L, mordenite, and ZSM-5 have been studied using the method; because of the aperture size, xenon is not sorbed on zeolite Na-A. In synthetic faujasites, the chemical shift is a linear function of pressure and varies between 58 to 110 ppm (from Xe gas

316

J.

M. THOMAS

AND J. KLlNOWSKl

PPm

FIG.66. lZ9Xespectrum of xenon adsorbed on a mixture of zeolites Ca-A and Na-Y (3.39). Equilibrium pressure 400 tom. Chemical shifts are given in ppm from xenon gas at zero pressure.

at zero pressure) as the number of Xe atoms per supercage increases form 0.1 to 3. For other zeolites, different plots apply. If the sample consists of a mixture of various solids (for example, of two different zeolites, or a partially amorphous phase) the 12'Xe spectrum will include as many components as there are different structures, and the intensities of the components will be proportional to the number of cages of each type. This is illustrated in Fig. 66 for the mixture of zeolites Ca-A and Na-Y. Fraissard et al. have shown how to use these findings for determining the degree of crystallinity of zeolites and the strength of electric fields inside the cages. Their work concerning zeolitesupported metal particles with or without prechemisorbed hydrogen, oxygen, carbon monoxide, and other gases will be discussed in the Section III,R. Ripmeester (346) used MAS to study xenon adsorbed on zeolites Na-X and H-mordenite. In the case of faujasite containing excess sorbate, separate lines from liquid, solid, gaseous, and sorbed xenon could be distinguished (see Fig. 67). The presence of a line from adsorbed xenon at 160 K shows that sorbed xenon does not freeze at the bulk xenon melting point. The line from liquid xenon measured at 170 K shifts to high field (Fig. 67b), suggesting that sorbed xenon is more dense than bulk liquid. For H-mordenite at 240 K, two broad 129Xeresonances are observed ca. 62 ppm apart. The low-field line is attributed to xenon in the main channels. It seems that there is no exchange of xenon between the two at this

ALUMINOSILICATE CATALYSTS

317

FIG.67. Iz9Xe MAS NMR spectra of excess xenon on zeolite Na-X (346), at (a) 160 K and (b) 170 K. Lines due to solid, liquid, gaseous, and adsorbed xenon are marked s, I, g, and a, respectively;40 FIDs with 40 sec repetition rate were obtained. Also shown is the lZ9XeMAS NMR of xenon in H-mordenite at (c) 240 K and (d) 302 K; 400 FIDs at 4 sec repetition rate were obtained.

temperature, while large linewidths suggest a further distribution of molecular environments. At 302 K only a single line is observed at an intermediate frequency, showing that xenon now undergoes rapid exchange. In a subsequent paper (347), Ripmeester determined the distribution of xenon between main channels and side pockets in Na+-, K+-, NH:-, and Cs+-exchanged mordenites. For steric reasons, the side pockets in Cs'mordenite are not available for xenon sorption. It appears that '"Xe NMR also can determine directly the interior surface area of zeolite samples, which may be of considerable practical importance.

R. STUDIES OF SUPPORTED FINELY DISPERSED METALPARTICLES USING'"Xe NMR We have seen in Section II1,Q that the '"Xe chemical shift of xenon adsorbed on zeolites depends on the zeolite structure, the kind of cation, and the pressure of xenon gas. When particulate metal is present in the zeolitic cavities, an extra term must be added to Eq. (27) (341-344). Curve (a) in Fig. 68 shows how the chemical shift of xenon adsorbed on zeolite Na-Y, containing Pt particles deposited using the procedure of Gallezot et al. (401),

318

J. M. THOMAS AND J. KLINOWSKI

500

4 OC

E-a 30c

a

20c

100

0

.,

1 0

2 3 4 5 6 7 8 numbor of X r atoms por g X 10-20 1 2 numbor of Xo atoms prr cavlty

FIG.68. "'Xe NMR chemical shifts (in ppm from xenon gas at zero pressure) as a function of concentrationof xenon adsorbed on +,zeolite Na-Y; 0,Pt-Na-Y; A,Pt-Na-Y + 2 H per Pt particle; Pt-Na-Y + 4H per Pt particle (342).

depends on the number of Xe atoms per supercage. The shift is always greater than that for zeolite Na-Y without platinum. The explanation of this is as follows. Some Xe atoms collide only with the zeolite framework and therefore resonate at the frequency 8Ns-Y,while others collide also with the Pt particles. However, the exchange between the two is very fast, and consequently only one line is detected at the average value of chemical shift. In general, the '*'Xe NMR spectrum of xenon adsorbed on the M,-Na-Y system (where M, denotes x atoms of particulate metal M) depends on the nature and concentration of the metal and on the average number of atoms per particle of the metal. This effect allowed Fraissard and colleagues (341-344) to measure the average number of eight Pt atoms per particle, while electron

ALUMINOSILICATE CATALYSTS

319

microscopy showed that the average particle diameter was ca. 10 A, i.e., much larger. It appears therefore that very small metal particles are not detected by electron microscopy. Similar conclusions apply to Pd and Ir particles on zeolite Y. The spectrum of xenon on a sample of Pt-Na-Y containing a very small amount of prechemisorbed hydrogen (Fig. 68) consists of two lines; the first corresponds to curve (a) for the appropriate xenon pressure, while the position of the second line is intermediate between and 6, for the same Xe pressure. For an average of one Xe atom per supercage, a, = 163 ppm; when the amount of prechemisorbed H, increases, the first line in the 12'Xe spectrum decreases in intensity and finally disappears, the chemical shift changes being given by curves (b) and (c), respectively. Each of the two '"Xe lines after chemisorption is due to the coalescence of two components, one corresponding to Xe atoms striking the wall and other Xe atoms, while the other is due to Xe colliding with bare Pt particles, or with Pt particles with prechemisorbed hydrogen. In general the spectrum depends on several factors: (1) the nature and concentration of the prechemisorbed gas; (2) the distribution of gas between the metal particles; and (3) the distribution of metal particles inside the zeolite crystal. Fraissard et al. discussed a number of possible situations in detail, and also measured '"Xe chemical shifts of

6

(ppm)

FIG.69. '"Xe NMR spectra at 24.9 MHz of xenon adsorbed on zeolite Na-Y containing finely dispersed metal particles with and without preadsorbed ethylene (341). Spectrum I, Pt,-Na-Y; 2, Pt,-Na-Y C,H, at 25°C; 3, Pt,-Na-Y + C,H, at 60°C. Chemical shifts are in ppm from xenon gas at zero pressure.

+

320

J. M. THOMAS AND 1. KLINOWSKI

xenon on metallic particles with prechemisorbed hydrogen, oxygen, carbon monoxide, and ethylene. An example of how the technique can be applied for direct studies of catalytic reactions is given in Fig. 69 for dehydrogenation of adsorbed ethylene. The line in spectrum 1 corresponds to bare Pt particles. After adsorption of ethylene, two extra signals appear (spectrum 2) corresponding to two types of “covered” metallic complex and hydrogen atoms, respectively. The value of the ‘29Xechemical shift suggests that the dehydrogenated complex occupies a large fraction of the surface of the particles, i.e., that both carbon atoms are attached to the metal. The spectrum obtained after heating the sample to 60°C in a sealed tube (spectrum 3) suggests, on the basis of the changes in the chemical shift, that the surface coverage of metal particles by the dehydrogenated complex is lower, i.e., that only one carbon atom is attached to the platinum. These results are in full accord with the conclusions reached from low energy electron diffraction.

IV. Silica-Alumina Gels

Silica, alumina, and silica-alumina surfaces are of great importance for catalysis and chromatography. Reactivity of these materials is determined by the structure of the surface and its relative acidity, and considerable effort is being expended to characterize it. Of particular interest are the surface hydroxyl groups. Among the methods used for their study the most powerful are IR spectroscopy and titration with acid-base indicators. Conventional NMR can cope with the observation of adsorbed species, where a considerable amount of motional averaging is present; MAS NMR must be used to study the surface directly. The earliest NMR studies of oxide surfaces (362-364) involved wide-line proton NMR of adsorbed organic species. For example, Petrakis and Kiviat (363), who studied the adsorption of pyridine and thiophene on molybdenamodified alumina, found that chemisorbed and physisorbed species can be readily distinguished. When physically adsorbed, both compounds exhibited liquid-like NMR behavior with high molecular mobility even at low temperatures. Chemisorbed pyridine was much more rigidly held with essentially only a rotation about the C, molecular axis persisting to - 130°C. Pyridine was sorbed both physically and chemically, and pretreatment of the surface was not particularly significant in this respect. By contrast, thiophene was physisorbed only on surfaces previously reduced with hydrogen, and underwent a reaction on calcined but unreduced surfaces.

ALUMINOSILICATE CATALYSTS

32 1

The work of Gay and Liang (6) involved the measurement of the 13C chemical shifts of adsorbed amines. A series of amines, including pyridine, aniline, and their derivatives, was adsorbed on SiO,, A l , 0 3 , and silica-ahmina. With the exception of N,N-diethylaniline, only weak interaction with surface hydroxyls was found for amines adsorbed on silica. NMR lines were fairly narrow, indicating rapid molucular motion with correlation times of less than low6sec. For alumina surfaces, NMR lines are much broader, indicating that the adsorbed species are more tightly bound with motional correlation times at least an order of magnitude greater than on silica. Steric effects are also present, and it seems that acid sites on alumina are relatively inaccessible, so that the ethyl groups in N,N-diethylaniline hinder the access of nitrogen atoms to these sites. On mixed silica-alumina surfaces substantial upfield chemical shifts were observed in the aniline C-1 carbon, indicating protonation of the base. Gay and Liang also found large changes in I3C chemical shifts when amines interacted with Brernsted acid sites, and smaller ones when the interaction involved Lewis acid sites. They suggested therefore that fractional coverage of the surface of the amine could be calculated from the magnitude of the 13C chemical shift, based on fwo assumptions: (1) that chemical shifts are average values from protonated and unprotonated amine molecules; and (2) that the shift for a protonated molecule is the same as for the acid solution of the amine. Experiments showed that both assumptions were well founded. It follows that the fraction,f,, of amine molecules bonded to the Brernsted sites can be derived from the equation =fB613

+ (l

-fB)6n,

where dobsrdB, and 6, are the observed chemical shift, the chemical shift of the amine on the Brernsted site, and the chemical shift of the amine on nonacidic sites, respectively. If both Brernsted and Lewis sites are involved and the subscript L refers to the latter, the equation becomes

dabs = f B h

+fL&

+ (1 -fi - f L ) B n .

Both unknowns, fe and f,, can be found provided rwo different resonance lines are observed and a separate equation written for each. Liang and Gay measured 6, in an amine/BF, complex and 6 , in an amine/HCl system for 4-ethylpyridine as the probe. The low precision with which the various I3C chemical shifts were determined resulted in poor accuracy in the final calculation of f, and f,, but the method does have potential provided chemical shifts can be measured accurately. Dawson et al. used I3C CP/MAS NMR to study n-butylamine (365) and pyridine (366) adsorbed on y-alumina. Six lines are present in the spectrum of the former compound (see Fig. 70). The presence offour resonances for the

322

J. M. THOMAS A N D J. KLINOWSKI

do

50

io

io

io

rb

;

PPm FIG.70. 13C CP/MAS NMR spectra of n-butylamine (365). (A) Adsorbed on the surface of y-alumina (38,000 scans). (B) Solid HCI adduct (46 scans); (C) solid BCI, adduct (212 scans). Vertical bars indicate I 3C chemical shifts of liquid n-butylamine. two (a and /?)carbon atoms of the alkyl groups shows that Brnrnsted and

Lewis acid sites are separately observed. Measurements on amine/BCl, and amine/HCl adducts confirm this conclusion. The linewidth of the a carbon is partly due to magnetic dipolar interactions with the quadrupolar 14N nucleus, which is not completely removed by MAS. When pyridine is chemisorbed on y-alumina at 0.5 % Brunauer, Emmett, Teller (BET) monolayer surface coverage, the 13C spectrum is completely resolved with three and y carbons instead of carbon signals in the 5 : 5 : 1 intensity ratio for the a, /?, the expected 2:2: 1 ratio. The anomaly is due to the difference in the efficiency of cross-polarization, which in turn reflects the complex motion of adsorbed pyridine on the surface. The authors conclude that the molecule, bound to the surface via the nitrogen atom, is rotating about its C2 axis, which is itself

ALUMINOSILICATE CATALYSTS

323

precessing (or “wagging”). On exposure to H,O and CO, the spectrum becomes liquid-like, indicating that pyridine is now less firmly bound, or possibly competes with water for the acid sites, the ratio of line intensities being equal to the population ratio of the three kinds of carbon atoms. Ripmeester (368) took advantage of the very wide range of 15N chemical shifts to study the adsorption of isotopically enriched pyridine on y-alumina using 15N CP/MAS NMR. The spectrum contained three relatively broad lines, at 64,110, and 138 ppm relative to the nitric nitrogen in solid NH,NO,, and these were respectively attributed to:

1. Physisorbed pyridine, which on exposure to air slowly picks up water with the resulting change of the 15Nchemical shift from 64 to 78 ppm; 2. Pyridine on Lewis acid site I, the chemical shift of which does not change on exposure to air; 3. Pyridine on Lewis acid site 11, which is unstable in the presence of water vapor. No protonated pyridine (which resonates at 174 ppm) was found on initial measurement. The author attributes the subsequent appearance of this signal (on exposure to air) to the formation of carbonate or bicarbonate species which serve as counterions to protonated pyridine. Maciel et al. (370,371,374) combined 13C and 15N CP/MAS NMR to study the adsorption of pyridine on silica-alumina. Hydrogen bonding was found to be the dominant interaction at high loading levels (0.5 to 1 monolayer). At lower coverages, a Lewis acid-base complex dominates and the pyridine is significantly less mobile. Brransted complexes are found when the surface has been pretreated with HCl gas. Figure 71 shows the 13Cspectra of pyridine at different loading levels and after a pretreatment with HCl. It is seen that the resonance of the y carbon of pyridine (middle peak) is very sensitive to its chemical state, and Maciel et al. developed a model of pyridine exchange between Brmsted, Lewis, and hydrogen-bonding acid sites as well as the physisorbed state. The same authors used the lSNsignal of a fixed quantity of adsorbed pyridine as a kind of “urface indicator.” Pyridine is displaced by n-butylamine according to the sequence Brensted

- Lewis

hydrogen-bonding

physisorbed

and 15N NMR has been used to monitor the “titration” of ”N-enriched pyridine with n-butylamine that has not been so enriched. Figure 72 gives the spectra interpreted in terms of the distribution of pyridine between the various sites in the course of the experiment. Experiments analogous to the 15N study described above have been carried out using the 31Presonance of phosphine bases (372,373).Spectra of

324

J. M. THOMAS AND J. KLINOWSKI

4 1I

l " " l " " l " " l " " l " " l '

250

200

150

100

50

0

PPm FIG.71. ''C CP/MAS NMR spectra of pyridine adsorbed on silica-alumina at 50.3 MHz (370). Amounts of pyridine adsorbed per 10 g of sorbate are (a) 1.02 g/10 g. (b) 0.38 g/10 g, and (c) 1.2 g/10 g 0.31 g HCI.

+

triethylphosphine on silica-alumina (373) show resonances of physisorbed phosphine (at - 20 ppm from 85 % aqueous H3PO4) and of R3PH+ species at Brensted sites (19 ppm from 85 % aqueous H3PO4). The appearance of the spectra obtained without 'H decoupling together with the presence of 'H-j'P J coupling (ca. 490 Hz)suggest that the acidic protons are isolated from one another. Ji and Maciel(375) used triphenylchloromethane as a probe for Lewis acid sites on surfaces. The technique relies on detecting the 13C resonance from

325

ALUMINOSILICATE CATALYSTS

0.10PY on HB

0.06 Py on L

gl7NBA on B a06 NBA on L

alOPy on L QOOPy on HB

417NBA on B QO2NBA on L

0.1OPy on HB

(kl

(SI

A

0.1 7 NBA on B 412NBA on L la NBA on HB

Q17NBA on B 412NBA on L 1.0 NBA on HB

&-

0.17NBA on 0 412 NBA on L

002Py on B Ql2Py on L 005Py on HB

0.lSNBA on 0

OD6Py on B 0.12Py on L QO1 Py on HB

Q11NBA on

Q17NBA on 0

(CJ

OD0 Py on 0 410 Py on L

Q08NBA on B

(bl

W 2 P y on L 0.17Py on HB

a17 Py on B

0.17NBA on B QO9NBA on L

(81

400

200 ppm

0

400

200

0

PPm

FIG.72. "N CP/MAS NMR spectra at 20.3 MHz spectra of "N-enriched pyridine (Py) adsorbed on silica-alumina in the presence of varying amounts (in grams) of n-butylamine (NBA). B, L, and HB denote Brensted, Lewis, and hydrogen-bonding sites, respectively. Chemical shifts are given in ppm from liquid ammonia (371).

the labeled carbon in the triphenylcarbinyl cation formed in the reaction

+ C18(C6H,), - 4 c l -

+(C~HS)~C'

A strong signal due to the cation was indeed observed at 208ppm, while physisorbed triphenylchloromethane resonated at 86 ppm from TMS. Lippmaa et al. (376), Maciel and Sindorf (377), and Grimmer et al. (378) investigated the active sites on the silica gel using 'H/"Si cross-polarization MAS technique in which only the silicon atoms directly attached to hydroxyl groups, or very close to them, are observed. The spectrum in Fig. 73 shows three signals which have been assigned, by analogy to spectra of water glass

326

J. M. THOMAS AND J. KLINOWSKI

r'"'1""1""1""1 50

0

-50

-100

-150

ppm

FIG.73. 29SiCP/MAS NMR spectrum of SG-2 silica gel at 11.88 MHz (377); 1014 scans, 20 msec contact times, and 1 sec repetition. Chemical shifts in ppm from TMS.

solutions, to Si*(OSi), (at - 109.3 ppm from TMS), Si*(OSi),(OH) (at - 99.8 ppm), and Si*(OSi),(OH), (at - 90.6 ppm) groupings on the surface (asterisks denote silicon atoms being observed for each signal). Maciel and Sindorf (377) explored the dependence of cross-polarization amplitudes versus the Hartmann-Hahn contact time, and were able to arrive at semi quantitative estimates of the populations of the various groups. In a series of subsequent papers (349,379-383) they examined various silicas and derivatized silicas. 29SiCP/MAS NMR of silica samples prepared at various stages of rehydration led (382) to the conclusion that the heterogeneous silica surface consists of separate regions resembling the [lo03 and [11 13 faces of /3cristobalite. Fyfe et al. (384)obtained quantitatively reliable ,'Si NMR spectra of silica gel and high-surface area glass beads and of derivatized surfaces without resorting to cross-polarization. Spin-lattice relaxation times of these systems are of the order of 10-30 sec and spectra of adequate quality may be obtained at high fields. There is just a handful of papers in which hydroxyl groups of silica gel are studied directly by high-resolution H NMR techniques (385-387). In particular, Hunger et al. (387) were able to observe two spectral lines in the 'H MAS NMR spectrum of amorphous silica-alumina gels of different composition. The line at 2 ppm from TMS was attributed to nonacidic hydroxyls, since it also occurs in silica and alumina; the line at 7 ppm, the

327

ALUMINOSILICATE CATALYSTS

intensity of which is at a maximum for 20-30 wt % alumina, must be due to acidic hydroxyl groups. These conclusions are confirmed by the measurements of the rate of cumene cracking. The potential of other nuclei for the study of surfaces is yet to be explored. Gottlieb and Luz (388) measured 'H spectra of a number of perdeuterated molecules adsorbed on active alumina and interpreted the results in terms of quadrupolar tensors. Yesinowski and Mobley (369) have shown that 19F MAS NMR can provide useful information about fluorinated surfaces of calcium hydroxyapatite, Ca,(OH)(PO,), . In particular, 19F, 'H, and 'H MAS NMR may become powerful techniques for the study of interface systems in general.

V. Derivatized Surfaces and "Immobilized" Homogeneous Catalysts

A universally recognized way of combining the best features of homogeneous catalysts on the one hand and heterogeneous catalysts on the other is to immobilize the former using an appropriate adsorbent such that the resulting surface complex rivals or surpasses the performance of an analogous heterogeneous catalyst. Certainly this expedient is designed to take advantage of the normally high selectivity of homogeneous catalysts while, at the same time, ensuring that one of the key advantages of heterogeneous catalysts-ease of separation of products from reactants-is safeguarded. There are various ways available for derivatizing surfaces, and in particular for depositing highly dispersed metal atoms or ions, in an immobilized fashion, onto silica-rich surfaces (348-351,361). Although several physical methods of analysis [X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and the range of spectroscopies from IR to UV] have proved helpful in characterizing the nature of the immobilized catalyst, none of them, singly or in tandem, is as powerful as solid-state NMR. Quite early on, cross-polarization was used by Chang et al. (352) to improve the quality of the "CNMR spectrum (Fig. 74) of the surface of chrysotile asbestos (Ch) derivatized with vinylmethyldichlorosilane (Scheme 2). When high-power proton decoupling, magic-angle spinning, and OH

+ OH

CI, /CH=CH, /Si\ CI CH,

O\

/CH=CH2

0'

'CH,

+ 2HCI SCHEME2

328

J. M . THOMAS AND J. KLINOWSKI

c':

(b)

A

3

Si-O-SilCH2CH:CH I

CH 3

5.6%

I

1

-200

I'

'1 w w u I

'

200

Iv) 0 2

m e

x

V

CHEMICAL SHIFT (PPM)

FIG.74. "C CP/NMR spectra of organic derivatives of chrysotile asbestos (352): (a) Trimethylsilyl derivative at room temperature; (b) ally1derivative at room temperature; (c) vinyl derivative at room temperature; (d) vinyl derivative at 100 K. The spectra are referred to methanol; the chemical shifts of benzene and TMS are indicated. The proposed structures of the modifying functional groups and the weight percent of carbon are shown beside the spectra.

ALUMINOSILICATE CATALYSTS

329

cross-polarization are used, the resulting high-resolution spectra (Fig. 74a) provide quantitative information about the concentration of derivatized groupings at the surface. Similar quality spectra are obtained for the 31P nucleus in cases where silica gel, silica-alumina, or polymer surfaces are covered with transition metal (homogeneous) catalysts containing triphenylphosphine groups. Clark et al. (353) have, for example, reported the 31P CP/MAS NMR spectrum of the metal catalyst cis-[PtCl,(PPh, -C,H,-CH=CH,),] immobilized on a copolymer (Pol) of styrene and divinylbenzene, shown in Scheme 3.

ph2p\

Pt / p p h =

/ \

c1

CI

SCHEME 3

Fyfe et al. (354) have combined 31Pand 13C CP/MAS NMR studies first to identify the polymer-immobilized catalyst (Scheme 4, compound ii) formed from the precursor i by treatment with Pd(PPh,), and, second, to monitor the carbonyl insertion reaction using 'jc-enriched CO to yield iii. The use of isotopically enriched C O was required so as to record meaningful signals above those emerging from the carbon-rich polymer background.

330

J. M. THOMAS

AND J. KLlNOWSKl

Clearly, much scope exists here for the execution of many elegant experiments to clarify the mechanism of heterogeneous reactions. Thus, one could readily design model systems involving "N-enriched N,, NO, or NH, as reactants. Moreover, the high sensitivity and discriminating power of 'H solid-state NMR could be capitalized upon by using deuterated reactants either alone or in association with "C-enriched species (as, for example, in the use of 13CO/D, mixtures in experiments intended to trace the pathways of methanol synthesis and Fischer-Tropsch conversions to hydrocarbons or alcohols). One particular metallic catalyst which has attracted much attention in the study of the nature of the carbon laid down during the conversion of CO/H, mixtures is ruthenium, usually supported on a high-area silica such as Cab041 HS-5. Duncan er al. (472-474) have made elegant use of 13CNMR spin-echo techniques, and in so doing they employed 3C-enriched reactants (13CO) and suspected products (' 3C-enriched turbostratic graphite) to good advantage. They examined both Ru/SiO, and Ru powder catalyst samples without exposing the active surface to interfering impurities at several stages of the catalytic conversion of 13CO/D2 mixtures, i.e., prior to or after switching the reactant mixture of "CO/D, to He. As always with NMR studies, the amount of material at the surface (in this case "elemental" carbon) is directly proportional to spectral intensity -in contrast to the situation involving FT IR as a tool. Duncan et al. showed that four forms of nonoxygenated carbon build up on the Ru. These were designated C,, C,,, C,,, and an unreactive carbon (Scheme 5). Correlation of the isotropic shifts,

'

c,

_ _ - - --+

Cunreactive

+"IIH c,

I

SCHEME 5

nuclear dipole interactions, and the anisotropy of chemical shielding enpble C, to be identified as a carbidic carbon and C,, as alkyl groups attached to the Ru surface. But the exact nature of C,, could not be established. The unreactive carbon is very reminiscent of turbostratic graphite as judged by the respective principal chemical shielding parameters: unreactive carbon (on Ru): oII = -245 k 10 ppm, oL = -16 f 20 ppm; graphite (pure): oIl = - 251 ppm, oL = - 20 ppm.

33 1

ALUMINOSILICATE CATALYSTS

VI. Probing Supported Metal Catalysts by NMR without Utilizing High-Resolution Techniques

It would be a mistake to conclude that only by the use of MAS or CP/MAS can we ever hope to glean detailed information about the state of adsorbed reactants, the generation of intermediates, or the pathways of catalytic conversions. Very important information can emerge both from the straightforward application of conventional Fourier transform methods (provided the surface species are sufficiently mobile to yield relatively sharp resonance lines as a result of averaging out most of the broadening influences exerted by dipolar and other interactions) and from a judicious combination of conventional NMR pulse techniques. The work of DeCanio et al. (357, 358) on silica-supported Rh catalysts serves as a good example in the first category, and that of Po-Kang Wang et al. (359)on the Pt/C,H, system in the second. Catalyst precursors prepared according to the reaction: D

O

H + M(allyl), OH

~[o\M(allyl)x 0'

-2

+ 2CH3CH=CH2

have been extensively investigated by Yermakov et al. (348) and by Ward et al. (360).Here, the metal M can be Zr, Hf, Nb, Cr, Mo, W, Re, Ni, Pd, Pt, and Rh. There is considerable interest in the case of some of these catalysts for the catalyzed conversions of syngas. The 'H NMR spectrum of the Si0,-supported complex designated D O - R h ( a l l y l ) , , has just one signal at 6 = 1.2 ppm from TMS (see curve A in Fig. 75). This arises from the protons of the allyl ligands bound to Rh. The width (FWHM = 1.7 ppm) of the resonance suggests that all the allyl protons experience essentially the same average environment, and corroborates the view that all the Rh nuclei in the surface allyl complexes are present in discrete molecular entities. When the surface complex is exposed to the H, at 198 K for prolonged periods, the NMR signals from the allyl groups are seen to decrease progressively (curve B in Fig. 75) and, ultimately, to disappear. Accompanying the decreasing peak is a gradually increasing one at 6 = 4.7 ppm. This latter peak is rapidly removed on evacuation of the sample at 298 K, and the gaseous products liberated are propane and propene. A new resonance appears during the reaction with H,: it extends from 15 to - 5 ppm (curve C of Fig. 75) and is assigned to protons of the surface hydroxyls. Since propene itself has a characteristic 'H NMR chemical shift of 4.6 ppm, the shoulder at the left in curve B is attributed to adsorbed propene. When, however, the )O-Rh(ally), is first exposed to CO and then to H, at 298 K, the

332

J. M. T H O M A S A N D J. KLINOWSKI

I

15

(0

1

1

1

5

0

-5

1

-10

1

-15

5

FIG.75. 'H NMR spectra (357) of Si-O-Rh(ailyl), at 298 K (A); after exposure to H, at 298 K for 30 min (B); and after exposure to H, for 5 hr followed by evacuation (C).

reaction leading to propene and propane does not occur. The inhibition by CO was also investigated by IR spectroscopy, which showed that the allyl complex was converted to a dicarbonyl rhodium species, Rh'(CO), . DeCanio et al. (357)inferred from these results that when H2attacks the bound allyl complex, spillover of protons onto the support takes place. These protons are thought to be responsible for further protolysis of ~ O - R h ( a l l y l ) , species, thereby generating Rh"' ions and propene. After extended reaction with H,,the Rh was reduced to metal, the presence of which was suggested by the XPS studies (and the blackening of the sample): the Rh 3d,,, binding energies change from 308.9 to 307.6eV during this exposure to H,. It is reasonable to suppose that the propane forms as a result of the catalyzed hydrogenation of the propene on the Rh metal. It was shown by DeCanio et al. (357, 358) that the highly dispersed, supported Rh metal is catalytically active for toluene hydrogenation.

ALUMINOSILICATE CATALYSTS

333

VII. Bond Lengths, Structure, and Mechanism in Heterogeneous Catalysis and in Chemisorbed States

Earlier sections of this review have drawn attention to some of the ways in which information about the positions of protons and other nuclei in powdered solids may be retrieved from NMR spectra. One method, known as separated local fields (SLF) uses two-dimensional Fourier transform NMR and is particularly adaptable to the resonances of relatively dilute spin species possessing a reasonably large range of chemical shifts (e.g., 13C)coupled to a small number of nearby spins of a different nucleus (e.g., 'H). Another method, termed nutation NMR spectroscopy, involves the forced precession of nuclear magnetization. A nutation is excited by the sudden and continuous application of a resonant rf field. In 13Cnutation NMR spectroscopy a train of closely spaced rf pulses is applied and the carbon signal is sampled in the windows between the pulses. In the resultant spectrum the chemical shift is supressed, revealing the pattern arising from the 3C-'3C dipolar coupling. The resulting so-called Pake pattern has a splitting which is directly related to the C-C bond length. There is, however, one problem about recording nutation 13C spectra: in unlabeled materials the probability of finding a 13C-13C bond is very small. But with 13Cenrichment this deficiency can be overcome, and Yannoni, Clarke et al. were able to retrieve C-C bond distances with an accuracy of 1 % from noncrystalline samples of polyacetylene using this method. Of greater relevance catalytically is that the combined use of 13C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC*EC*H) is incorporated as shown in Scheme 6. It is to be noted that the 3C-1 3C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. Turning to the case of acetylene chemisorbed on supported Pt, the work of Po-Kang Wang et al. (359), using principally the SEDOR technique described earlier (Section II,F), constitutes an elegant example of how the structure of the chemisorbed species can be retrieved from NMR studies. It is worth emphasizing that polycrystalline, high-area Pt particles, typical of those employed in catalytic hydrocarbon reforming reactions, were used.

334

J. M. THOMAS AND J. KLlNOWSKl

Neither low-energy electron diffraction (LEED), nor probably extended Xray absorption fine structure (EXAFS) is capable of yielding the information pertaining to the unusual structure of the adsorbed C,H, that the methods used by Po-Kang Wang et al. (359) succeeded in doing. They found that the surface of the Pt particles (average diameter in the range 10-30 A) covered in the 1 l-SO% monolayer range at 77 K (after cleaning and outgassing the catalyst at 300°C) was 77 It 7 % covered with a grouping CCH, and 23 7 % covered with HCCH. The C-C bond length of the CCH, species is 1.44 It 0.02 A, midway between the lengths of a single and double carboncarbon bond (1.54 and 1.34 A, respectively). This suggests that there is a H

M

M-C

*/--

*/

H

,c=c* \ H

H SCHEME 7

H

ALUMINOSILICATE CATALYSTS

335

surface structure analogous to that found for the CCH, group in the triosmium hydridocarbonyl, H,Os,(CO),CCH, : H

Details of the method used by Po-Kang Wang et al. (359) are given in the original paper. Suffice it is to say that 13C-13Cdipolar couplings with 13C spin echoes, 13C-lH dipolar coupling with spin echo double resonance, and H-'H dipolar coupling with 'H multiple quantum spectroscopies were employed, and that the 13C2D, species was also used to simplify the interpretations. This work constitutes a significant achievement if only because it illustrates how the nature of hydrocarbon groupings can be deduced de nooo, for states of chemisorption much more nearly like those involved in practical heterogeneous catalysis.

VIII.

Sheet Silicates and Their Pillared Variants A.

GENERAL COMMENTS

Clays are chiefly composed of layered silicates and among the many interesting properties they possess are large capacities for cation exchange and the ability to take into their interlamellar regions water and a wide range of other molecules (alkanols, amines, aminoacids, nitriles, ketones, and many types of hydrocarbons). In the early days of oil refining, aluminosilicate clays, notably those based on montmorillonite, were used for the catalytic cracking of large hydrocarbon molecules, but in due course they were supplanted first by silica-alumina gels, and later by zeolitic cracking catalysts. Of late, however, there has been renewed interest in clays as viable broad-spectrum catalysts for organic synthesis (446-451). They are particularly good at

336

J. M. THOMAS AND J. KLINOWSKI

producing organic materials of the kind now in heavy demand, e.g., methyl-tbutyl ether (MTBE) (452,453), which is a good blending agent for gasoline in view of its high octane number, and acetyl acetate (454, 459, which is extensively used as a multipurpose solvent. Their potential, when suitably tailored and modified, for isomerization and cracking is also considerable and there is abundant scope for catalyst design with these sheet silicates. Their significance biochemically and in prebiotic processes is also substantial. To date, a number of specific catalytic performances have been identified. The majority of these take advantage of the Brensted acidity of the clays. This acidity can be boosted by appropriate cation substitution in the interlamellar region (see below); the replacement of Na+ by A13+ ions, for example, giving rise to a dramatic increase in acidity and hence in catalytic activity (450). Typical reactions catalyzed by A13 and H +-exchanged sheet silicates are the addition of either water, alcohols, or carboxylic acids to alkenes to yield alcohols, ethers, or esters, respectively. There are several other reactions which are effectively catalyzed by layered silicates possessing adequate acidity, e.g., the intermolecular elimination of ammonia from primary amines and the dimerization and oligomerization of unsaturated aliphatic acids +

-

(456).

B. STRUCTURAL CHARACTERISTICS The clays of premier catalytic interest are those known as montmorillonite and hectorite. Their structures are best discussed along with those of pyrophyllite and talc on the one hand, and with beidellite and saponite on the other. These six clay minerals and many others are composed of two distinct types of connected layers, one consisting of corner-linked SiOi- tetrahedra, the other edge-linked A(O,OH), octahedra, where A, in the case of montmorillonite, pyrophyllite and beidellite, is predominantly Al, and in the case of hectorite, talc, and saponite is predominantly Mg (see Table XXII and Fig. 76). In both pyrophyllite and talc the so-called TOT layers (i.e., tetrahedraloctahedral-tetrahedral) are neutral, so that no exchangeable cations (M"') can be accommodated in the interlamellar region. In pyrophyllite, electrical neutrality of the layers is achieved by the presence of Si4+ in all eight tetrahedral sites in the repeat unit A,Si,O,,(OH), of the oxygen framework and of A13+ in two-thirds of the available octahedral (A) sites: in talc, all the tetrahedral and octahedral sites are occupied by Si4+and MgZ+,respectively. Separate TOT layers are, therefore, rather loosely bound via the agency of weak dipolar and van der Waals forces. The four other sheet silicates (collectively known as smectites) listed in Table XXII bear a net negative charge on the TOT layers. This arises because of isomorphous substitution.

ALUMINOSILICATE CATALYSTS

337

TABLE XXII Idealized Formulae for Some Selected Clays Clay

Idealized formula

Pyrophyllite" Montmorillonite" Beidellite" Talcb Hectoriteb Saponiteb

(AI4)0et(Si8)tet02 ,(O H)4

M$, .aH,0(A14~,Mg,~'(Si8)tet020(OH)4 M!$. aH,O(AI4)"'(Si8 -xAlx)1c'020(OH)4 (Mg,r"@ir~)L"Om(O% M$. aH,O(Mg, -xLi,)"'~(Si8)t"02,(OH)4 M$ aH,O(Mg,)Oel(Si, - x A I x ~ 0 2 0 ( O H ) 4

a In these clays, two-thirds of the available octahedral sites are occupied. These are termed dioctahedral. In these clays, all (three-thirds) of the octahedral sites are occupied, and they are termed trioctahedral.

In montmorillonite some of the A13+ in the octahedral sublattice are replaced by Mg2+ions, and in hectorite some of the Mg2+ in the octahedral sublattice are replaced by Li+ ions. With beidellite and saponite, however, the isomorphous substitution takes place in the tetrahedral sublattice with A13+ replacing some of the Si4+ ions. The residual negative charges in the layers on montmorillonite, hectorite, beidellite, and saponite are counterbalanced in the natural state by coexisting interlamellar, hydrated cations, usually N a + ,

LAI, Mg

40,ZOH 4Si

60 Solvated exchangeable cations

60

451

40, ZOH LAI, Mg 40,ZOH 4Si

60

2

0 l

4

l

4 I

I

8

6 I

I

I

I

lOA I

I

FIG.76. Schematic illustration of the structure of montmorillonite (470).

338

1.

M. THOMAS AND J. KLINOWSKI

FIG.77. The arrangement of layers of water molecules (dashed lines) in hydrated montmorillonites and the corresponding interlayer spacings in (a) a one-layer clay; (b) a two-layer clay; (c) a three-layer clay. The spacing adopted by a particular montmorillonite depends on the nature of the silicate layer and on the exchangeable cation as well as on certain other conditions of intercalation (e.g., relative humidity and temperature) (470).

Ca2+,Mg2+,etc. The precise degree of layer charge and its distribution, as well as the particular nature of the interlamellar cation, are functions of the geological habitat and prior history of the clay in question. Typically, the charge deficiency, which is the origin of the cation-exchange capacity, ranges from 0.4 to 1.2 units of electronic charge per Si,O,,. These figures are to be compared with a value of 2.0 units for muscovite and phlogopite mica and zero for pyrophyllite and talc. Put differently, the charge density in these smectites is such that unit charge occupies a basal area of 45-100 A2. The swelling of clays in water results from the extra hydration of the interlamellar cations (Fig. 77). This is the best known example of the important phenomenon of intercalation, which is simply the insertion of guest species into an accommodating host, usually, but not exclusively, a layered solid. The degree of swelling, however, is governed by the nature of the interlamellar cation and the sorption isotherm often exhibits steps, as so often occurs with clathrates. The meaning of “one-, two-, and three-layer clays” is best illustrated diagrammatically (see Fig. 77). “Layers” in this context refer to the interlamellar water, though the precise chemical nature of this entrained water is not easily established and is, in any case, a function of the parent silicate. In some sheet silicates the water is believed to take up an ice-like monolayer. Recent studies reveal that the interlamellar ion and associated water are rather mobile above room temperature. Such water is readily, but not always

339

ALUMINOSILICATE CATALYSTS

completely, displaced by various organic species (amines, lactones, acids, carbohydrates, etc.). As a consequence, the interlamellar microenvironment is converted from a predominantly hydrophilic to a so-called oleophilic state, making it more conducive for the further insertion of organic species which would otherwise be difficult to intercalate into the original, water-rich smectite. It is not surprising that one-, two-, and three-layer clays of many organic intercalates can be formed with montmorillonite as host. The basal plane repeat distances (dool) for Sr2+-exchanged montmorillonite/y-butyrolactone intercalates are 13.2 (one-layer), 18.3 (two-layer) and 23.1 A (threelayer) (compare Fig. 77). The key structural features of smectite clays of relevance to discussions of their catalytic activity and selectivity are the following: 1. A wide range of organic intercalates can be formed. 2. The original interlamellar, charge-neutralizing cation can be readily replaced, as desired, by one (or possibly more) of a whole range of inorganic or organic cations. 3. They exhibit strong acidity, which is usually of the Brransted type, partly because of the influence of the strong internal electrostatic fields (ca. lo6 V cm - ') exerted on the interlamellar water (which generates protons by dissociation) or, because of the additional influence of certain hydrated interlamellar cations, notably A13 +.Cation hydrolysis, just as with strongly polarizing cations in zeolites, yields free protons, thus: [M(HzO),]~"-''+

-

[M(H,O),_,OH]("-')+

+ H+

It follows that proton-catalyzed organic reactions will be facilitated by employing layered silicates into which strongly polarizing inorganic (e.g., A13 +)cations have been inserted into the interlamellar regions, because they are the very ions which undergo cation hydrolysis, thereby generating interlamellar protons. C. ELUCIDATING THE NATURE OF THE CATALYTIC PERFORMANCE OF LAYERED SILICATES BY NMR NMR spectroscopy has advanced our knowledge of the way in which claybased catalysts function in two respects. First by revealing information, derived largely from 13Cand 'H NMR, about the nature and dynamics of the organic species present in the interlamellar regions. Second, by greatly enlarging our ability to characterize clay catalysts via 27Al and 29SiNMR. Whereas the 13CNMR has, to date, been chiefly of a conventional kind (Section VII1,D) the 27Aland 29Siwork has involved magic-angle spinning. It transpires that organic reactants are quite mobile in the intercalated states,

340

J. M. THOMAS AND J. KLINOWSKI

at room temperature and above, so that quite sharp lines are obtained in the 13Cspectra. 27AlMAS NMR has proved particularly helpful in discriminating unequivocally between tetrahedrally and octahedrally coordinated Al, while intensities of the 29Si NMR lines, just as in zeolites, enable us to determine %/A1 ratios in the tetrahedral sublattices.

D. "C

AND

'H STUDIES

Typical spectra of the p-xylenelsynthetic hectorite intercalate are shown in Fig. 78. Although the proton peaks are rather broad, they are much narrower than would be obtained from solid p-xylene, indicating that the intercalated species has considerable freedom of motion. The two proton peaks for the xylene intercalate have relative intensities of 2: 3 and the chemical shifts are very similar to the aromatic and methyl resonances in the high-resolution solution spectrum. Tennakoon et al. (465) have studied, by conventional "C NMR, the catalyzed conversion of 2-methylpropene (isobutene) to t-butanol (R = H) or to methyl-t-butyl ether i.e., 2-methyl-2-methoxy-propane (R = CH,) by

I

CH,

.-.

FIG.78. (A) 'H NMR spectrum of p-xylene-hectoriteintercalate at 80 MHz (50 scans). The vertical arrow indicates the position of a small, sharp peak arising from HDO impurity in the D,O lock reference, which has been deleted for clarity. (B) ''C NMR spectrum of the same intercalate at 20 MHz (4OOO scans) (457).

34 1

ALUMINOSILICATE CATALYSTS

addition of water or methanol, respectively: H

A13+-exchanged synthetic hectorite is a good catalyst for these conversions, and the 3CNMR spectrum obtained in the interlamellar, proton-catalyzed addition of water to 2-methylpropene is indistinguishable (Fig. 79) from that of t-butanol. Doubtless studies of this kind, where natural-abundance "C NMR signals are used to probe the chemical identity and motional freedom of reactant and product species situated in the interlamellar spaces of clays or pillared clays (see below), will become increasingly popular. Using 13CNMR linewidths and spin-lattice relaxation studies, Matsumoto et al. (466) have succeeded in discriminating between the internal and external surfaces of pillared montmorillonites. Sanz et al. (485), who used 'H NMR at low magnetic fields to study the localization of vacancies in the octahedral sheets of aluminous biotites, were able to establish unambiguously that the vacancies are located in the pseudosymmetric M1 sites. They also determined the orientation of the 0-H and Fe-H bonds in these materials.

'

Al" ion-exchangcd synthetic hectorite exposed to isobutene

/CH3 CHj-C \CH,

2

AP+ ionixchangcd synthetic hectorite exposed to f-butanol 2

i

1

1

200

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

150 100 50 IU. x chemical shift from TMS

1

1

1

1

1

0

FIG.79. I3C NMR spectra showing that when 2-methylpropene (isobutene) is intercalated in a synthetic hectorite t-butanol is formed when the guest species reacts with the interlamellar water. The peaks labeled 1 and 2 refer to the two distinct types of carbon atom in t-butanol(453).

342

J. M. THOMAS AND J. KLINOWSKI

E. 27Al AND "Si STUDIES The principal merit of "Si and 27AlMAS NMR spectroscopy as a tool for characterizing clays and their pillared variants is that it focuses upon the short-range order or local structure of these important materials. By definition, clays lack good crystalline order, so that X-ray crystallography alone, which in any case is not good at distinguishing neighboring elements (like A1 and Si) in the Periodic Table, is not sufficiently powerful to be used as a convenient, rapid, and routine method for unambiguous structural characterization. It is a matter of some significance catalytically to be able to ascertain whether the aluminum present in a natural clay or its synthetic analogue is in a state of octahedral substitution (as in montmonllonite) or whether there is some tetrahedral substitution (as in beidellite). 27Al MAS NMR readily provides the necessary answers. For example, Diddams et al. (462) in a study of the synthesis, characterization, and catalytic performance of synthetic beidellites and their pillared analogues, monitored the fate of A1 from the gel precursor to the sheet silicate and to its pillared state by 'A1 MAS NMR (see

(tot.) 100

(oct.)

b.

' . ' -

-so

.

-Id0

.-

PPY fromTY8

FIG.80. *'A1 (at 104.22 MHz) and z9Si(at 79.8 MHz) MAS NMR spectra of (a) dried gel precursor used for synthesis;(b) Na -exchangedsynthetic beidellite;(c) Al" exchanged variant of(b); (d) Al-pillared variant of(b). Peaks labeled SS are spinning sidebands.The precursor gel is seen to contain its silicon predominantly in the Si(4Si) environment (Q4), but some Si(3Si,OH) cannot be ruled out (462). +

ALUMINOSILICATE CATALYSTS

343

Fig. 80). Clearly, some tetrahedrally as well as octahedrally coordinated A1 exists in the gel precursor, and the presence of the tetrahedrally coordinated A1 in the synthesized beidellite is established beyond dispute [see signals at ca. 70 ppm in Fig. 80(b and c)]. The extra shoulder in the octahedral A1 signal obtained from the A13 +-exchanged beidellite (Fig. 80c) is attributed to the mobile interlamellar cations, an assignment confirmed by NMR studies of static samples. There are at least two types of tetrahedrally bonded A1 sites in the pillared form (see the region from 55 to 75 ppm in Fig. 80d). Sanz and Serratosa (459) and Serratosa et al. (460) have measured 27Al MAS NMR spectra for a range of pyrophyllites, muscovites, and phlogopites. As expected (see Table XXII) only “octahedral” signals were obtained with pyrophyllites. In addition, and again in line with known crystallographic and structural principles (463, the “tetrahedral” signal is dominant in phlogopite and the “octahedral” A1 signal in muscovite. The value of 29Si MAS NMR in the structural characterization of clay-based catalysts is that the immediate environment (i.e., short-range order) of the silicon in the tetrahedral framework may be identified, and the Si/A1 ratio in this framework quantitatively determined. Both these goals can be achieved for much the same reasons as identical goals are achievable in the study of zeolites (see Section II1,C) because there are well defined, and conveniently separated (though partially overlapping) ranges of 29Si MAS NMR chemical shifts. Thus, the Si(OAI),, Si(OAI),(OSi), and Si(OAI)(OSi), groupings in the tetrahedral manifold-the fourth bond linked the silicon via oxygen, both in the octahedral manifold, generally to Al-have chemical shifts (from 29Siin TMS) at ca. - 76, - 84 to - 86, and - 85 to - 90 ppm, respectively. When silicon has no aluminums linked to it in the tetrahedral manifold the chemical shift in a sheet silicate is ca. -93 to -99 ppm (458, 461, 462, 464). Pyrophyllite has a 29Sishift of -95.1 ppm (53, 458). Armed with this information, and using an adaptation of Eq. (15) (see Section II1,C) we may write for the (Si/Al)tetratio:

where 1 is the NMR intensity for a particular 29Si peak, attributable to Si(nAl), n = 0-3. As with Eq. (15), this equation is based on the Loewenstein links exist in the tetrahedral manifold. rule, which states that no AI-0-A1 The analysis of 29SiMAS NMR spectra of layer silicates with a wide range of tetrahedral compositions (Si/Al ratio 2.7-7.7) indicates (483, 486) that the distribution of Si and A1 in these materials is indeed determined by (i) the local balance of charges, and (ii) the Loewenstein rule. In muscovite, phlogopite, and vermiculite, aluminum is randomly distributed in

344

J. M. THOMAS AND J. KLlNOWSKl

six-membered ring sites, but always in compliance with the above requirements. In margarite, however, a strict alternation of Si and Al is observed in the rings, in agreement with the results of X-ray diffraction studies. Using the above equation Diddams et al. (462) estimated from the deconvoluted (Gaussian) peaks in Fig. 80b and c, that (Si/Al)te,was 11.5 L- 1.0, so that the A 1 0 composition of the tetrahedral manifold is S i ~ ~ 3 ~ ~ ~ ~ ~ 0 6a ~range that was in good agreement with that found by wet chemical methods. Barron et al. (489) used 29SiMAS NMR for the detection in soils of the clay mineral imogilite, a tubular hydrated aluminosilicate in which Sittetrahedra are isolated by coordination, through oxygen, with three a h minum atoms and one proton. The 29Si chemical shifts found (-72 to - 76 ppm from TMS) were in good agreement with the proposed structure of imogilite as Q’(3Al) shifts reported by Lippmaa et al. (52).That imogilite is a hydroxy aluminosilicate has been established by cross-polarization experiments as follows. Maximum signal enhancements occurred at 0.75 msec contact time in comparison with 5 msec found for kaolin. Thus, in agreement with the proposed structure, the internuclear 29Si-‘H separation in imogilite is clearly smaller than in kaolin, which is known to contain hydroxyl groups bonded to silicon indirectly via aluminum. Barron et al. (490) also examined 29SiMAS NMR spectra of a range of kaolins, A14[Si4010](OH)6.All have a Q3(OAl) dioctahedral layer structure consisting of an octahedral aluminum hydroxide (gibbsite) sheet and a tetrahedral silicate sheet. Three forms of chemically identical kaolin occur: kaolinite, dickite, and nacrite, with differences being in the stacking of layers in different regular sequences. In all cases, the 29Si resonance was at -91 ppm, but a careful inspection of spectra reveals a splitting of the resonance into two signals of equal intensity. Barron et al. conclude that there are two different but equally populated silicon sites in the kaolin structure. This is probably due to the distortion within the layer and perhaps partly as a result of the need to accommodate interlayer hydrogen bonding. Good 29Si and 27Al spectra of thermally treated kaolinite (487), pyrophyllite ( 4 9 4 , and of several phyllosilicates (494) have also been obtained. Barron et al. (463) found considerable variation in Tl relaxation times between different clay minerals, with an upper limit of 1.3 hr in the case of nacrite. There was little correlation between the total Fe” content of four samples and their respective Tl values for 29Sior ‘H. Watanabe et al. (492)investigated the influence of paramagnetic impurities on 29SiMAS NMR spectra of 12 clay minerals. They found Tl to be always less than 1 sec, i.e. orders of magnitude less than reported by Barron et al. (463). There was a marked tendency for the linewidth to increase with Fe” concentration, demonstrating that dipolar interactions between 29Siand the electron spin of the Fe” ion are largely responsible for line broadening. Tl

-

~ ~ ~ ~ ~

ALUMINOSILICATE CATALYSTS

345

was found to be inversely proportional to the concentration of paramagnetic species. More work is clearly needed to resolve the controversy.

F. PILLARED CLAYS Even though naturally occurring clays, which are usually rich in exchangeable Na', K+,or Ca2+cations can be converted into viable acidic catalysts by directly or indirectly inserting protons into the interlamellar regions, such catalysts still suffer from the disadvantage of physical collapse at high temperatures. In a word, at ca. 2 W C , the interlamellar solvent species (water or a reacting organic layer) tends to be expelled and the sheets cohere, with the consequent loss of catalytic activity. One way of preventing this collapse, and, therefore, of sustaining the catalytic activity of the clay at higher temperatures is by inserting pillars, preferably of an inorganic character, which serve to keep the individual layers apart. But the generation of pillared clays, symbolized in Scheme 8, has other advantages, not the least among

-

-

-

solution

them being the merit of incorporating extra "pores" into the catalyst (461, 462, 468470). Were it possible to space the pillars evenly, and in a controllable fashion, a new type of shape-selective catalyst would be produced. Indeed the resulting high-area solid may be regarded as a twodimensional zeolite, with acidic properties comparable with those of Y-type acidic zeolites (461). One way in which pillaring can be achieved is to use solutions rich in the Aluminum hydroxymultinuclear cations [Al1304(OH)24(H20)12]7+. polymers of this kind are readily prepared in the dispersed state from many solutions containing A13 ions by appropriate adjustment of pH so that, typically, the OH/AI ratio is less than about 2.3. Both montmorillonoid and beidellitic clays can be effectively pillared in this way, and some reports detailing the catalytic performance of such pillared clays have appeared (461, 462, 468). Encouraging progress in the task of elucidating the nature of the pillars (e.g., the relative amounts of four- to six-coordinated aluminum) has been registered in independent studies by Plee et al. (461)and Tennakoon +

346

J. M. THOMAS AND J. KLINOWSKI

Progress has been made possible by the use of 27Al MAS NMR, but 29SiMAS NMR (along with IR and XRD) has also proved invaluable in this context, there being significant changes in the magnitudes of the chemical shifts experienced by Si atoms in the tetrahedral manifold as a result of pillaring. When pillared smectites without tetrahedral substitution are calcined, there is no reaction between the pillars and the smectite layers. By contrast, a considerable structural transformation occurs when pillared beidellite is calcined, which has been interpreted as the growth of a three-dimensional quasi-zeolitic framework between the two-dimensional clay layers. The acidic properties of the product are comparable with those of zeolite Y and much more pronounced than those of calcined pillared smectites without tetrahedral substitution. et af. (458, 475).

IX.

Recent Trends in the Study of Catalytic Solids by NMR

A. GENERAL COMMENTS

The great strength of multinuclear high-resolution NMR spectroscopy of solids is that it makes all atomic components of aluminosilicate catalytic systems liable to direct investigation. While this review understandably pays much attention to 29Si and 27AlNMR,many other nuclei can be readily observed in the solid state. Oxygen, the remaining major constituent of mixed oxides, can also be monitored. Klinowski et al. (57) measured 1 7 0 MAS NMR spectra of zeolite A enriched in 1 7 0 . The spectrum of a rapidly spun sample contains a single signal (with a small quadrupolar splitting), which signifies that there is only one kind of oxygen site in zeolite A, thus confirming the absence of Al-0-A1 linkages. These measurements revealed furthermore that the oxygen atoms in the linked Si0;- and A10:structural units are much more labile than was previously thought. The '0NMR of inorganic salts and aluminosilicateshas since been investigated in some detail (434-436). Further progress in solid-state NMR is likely to depend on three closely linked factors. The first is its multinuclear capability; the second its ability to study new systems, such as carbonium ions, which were not heretofore accessible to direct observation. The third factor is the existence of novel NMR techniques, which constantly expand the amount of chemical information available from the spectra. These topics are discussed in the following sections, and we need to make only some general comments here. The syntheses of novel molecular sieves such as aluminophosphates, silicoaluminophosphates (SAPO), gallosilicates, aluminogermanates, ferrosilicates, borosilicates, and chromosilicates, clearly open new vistas for the

ALUMINOSILICATE CATALYSTS

341

technique. Much scope exists for the study of oxides. For example, Zamaraev and Mastikhin (432) obtained 'lV, "0, and 23Na NMR spectra of vanadium catalysts for the oxidation of SO, and were able to identify some of the compounds present in the active component of these catalysts as well as to throw light on the interaction between the active component and the support; Oldfield et al. (432) obtained 'lV MAS NMR spectra of sodium and ammonium metavanadates; Shatlock and Maciel(433) used 13Cd NMR to examine the Cd-CdO solid solution. We now proceed to examine some recent trends in the study of catalytic solids in more detail.

B. NMR STUDIES OF CARBONIUM IONS A central feature of the mechanism that accounts for the catalytic cracking of hydrocarbons by appropriately cation exchanged zeolites is the formation of carbonium ions (also designated carbocations and alkylcarbenium ions) as intermediates. Many other reactions for which aluminosilicates,be they clayor zeolite-based, also predicate (320) the existence of carbonium ion intermediates, formed usually by proton donation from Brensted acid sites, have been discussed earlier (Section 111,K). That carbonium ions are indeed formed at the surfaces of solid acid catalysts when certain reactants (e.g., alkenes or arylalkanes) are introduced is beyond dispute. The classic work of Leftin et al. (38%392) using UV-visible spectroscopy clearly established that species such as the triphenylcarbonium, methyldiphenylcarbonium, and dimethylphenylcarbonium ions are readily produced in the absorbed phase on silica-alumina catalysts. Carbonium ions in general, especially when they are produced in solution, are amenable to characterization by 'H and 13CNMR, as the work of Olah (392, 393) demonstrates. What is of special importance so far as heterogeneous catalysis is concerned is that elegant work has already been done using 13C CP/MAS NMR on solids which consist of large proportions of stabilized carbonium ions. Lyerla et al. (394,395,399, for example, have investigated the 13C spectra of a range of carbonium ions (see Table XXIII) and have shown that the chemical shifts of the carbon atoms in the cations so formed are very similar to those found in solution by conventional high-resolution NMR methods. These workers also studied the role of the counterions (species such as SbCl;, AlCl;, and C10;) on the quality of the 13C NMR spectra. With heptamethylbenzonium tetrachloraluminate (394)the 13Clines are broadened because of interaction between the quadrupolar chlorine and the 13C nucleus; and this broadening is more pronounced at 93 K than at 300 K because there is self-decoupling owing to the thermal motion of the AICl; species at room temperature. This motion is frozen out at low temperatures.

348

1.

M. THCUAS AND J. KLlNOWSKl

TABLE XXIII A Comparison ofthe "C N M R Chemical Shijh Exhibited by Certain Carbonium Ions in the Solid State and in Solution" d (ppm from TMS) Carbonium ion

2

3

2

3

c-1

OCHzCH,

c-2

c-3

c-4

c -5

21 1.6 (210.9)

139.8 (139.9)

145.6 (143.3)

129.5 (130.3)

141.8 (143.1)

176.7 (177.1)

82.6(81.8) 76.8(75.8)

13.2(13.9) (12.6)

"The values in parentheses are for the carbonium ions in solution. Ref. 351.

Fyfe et al. (355) were able to produce a very informative 13C CP/MAS NMR spectrum of the triphenylmethyl carbonium ion by using the tetrafluoroborate counterion and by employing simultaneous "F and 'H decoupling during spectral acquisition (see Fig. 81). The nonequivalence of the ortho and meta carbons is readily seen in the spectrum. Other noteworthy achievements of solid-state "C CP/MAS NMR in the context of carbonium ions are that: (1) the sec-butylcarbonium ion can be identified at low temperatures in a sec-butyl chloride/antimony pentafluoride matrix in the temperature range 80-190 K; (2) the norbornyl carbonium ion has been characterized (356,396)at temperatures down to 5 K, there being a strong (but not yet incontrovertible) indication that the controversial "nonclassical" ion (398)exists; and ( 3 ) the homotropylium ion is best represented (399) by the completely delocalized (homoaromatic) seven-membered state (a below) rather than the incompletely delocalized state (b).

50 3

2

b

4(&-

3

2

349

ALUMINOSILICATE CATALYSTS

I

I

I

I

I

I

1

110

110

100

160

140

130

110

Chemical Shift (PPM From TMS) FIG.81. I3C CP/MAS NMR spectra (395)at 15 MHz of triphenylmethyl tetrafluoroborate at - 160°C. (A) 'Hdecoupling only; (B) 'H and "F dedoupling.

Recognizing that much has been learned about ways of stabilizing carbonium ions and about their characteristic 13C NMR features, renewed efforts should be made to identify or isolate these reactive intermediates in conversions over aluminosilicate catalysts. Due consideration will need to be given to the time scale of the NMR method and the lifetime or exchange rates of the carbonium ions. Myhre et al. (400) carried out the first experiments of that kind. They measured variable temperature 13C CP/MAS NMR spectra of carbocations which exhibit average spectra in solution owing to rapid rearrangements. By contrast, solid-state spectra at low temperatures indicate the presence of static classical ions; at higher temperatures (ca. 200 K) these ions undergo rapid degenerate rearrangements. Myhre et al. found that the 2,3-dimethyl-2-butyl cation in particular exhibits both static and rapidly equilibrating classical behavior at temperatures between 193 and 128 K; there is no broadening and coalescence of the lines accompanying a passage from the slow to the fast exchange regime, as was expected. The relative amount of the static ion decreases with rising temperature, and the relative population of the two forms is reproducible on cycling the sample temperature. These populations can be altered on changing the solid matrix. The lowtemperature spectra are a clear demonstration that these rapidly equilibrating ions have a classical structure.

C. TWO-DIMENSIONAL NMR Two-dimensional NMR was originally proposed by Jeener (402) and developed by Ernst and co-workers (403405) and Freeman and co-workers

350

1. M. THOMAS AND J. KLINOWSKI Preporotton

I

Evolution

Detection

tl

(b)

I

fl

t2 Storage of K F I D in a two-dimensional matrix

51 ( t l . t 2 ) 5, (‘1,tp)

-

1

sk

(tlat2)

‘21

I Fourier transformation of the rows

-6

FIG.82. Data flow in 2D NMR spectroscopy (418). (406-409). Waugh and his group (41&414), Stoll et al. ( 4 1 3 , and Alla and Lippmaa (416) were the first to apply the technique to solid-state problems.

Good reviews of the technique have been prepared by Freeman and Morris (417), Benn and Gunther (418) and Mehring (lo),the latter being particularly

relevant to the topics discussed in this review. In an ordinary Fourier transform NMR experiment the time-domain signal (the FID) is converted into a frequency-domain representation (the spectrum); thus a function of time, S(t,), is converted into a function of frequency, S(f2). The very simple basic idea of 2D NMR is to treat the period preceding the recording of the FID (known as the “evolution period”) as the second time variable. During this period, t,, the nuclear spins are manipulated in various ways. In the 2D experiment a series of S(t,) FIDs are recorded, each for a different t,, and the result is considered a function of both time variables, S ( t , , t,). A twofold application of the Fourier transformation (see Fig. 82) then yields a 2D spectrum, S ( f , , f , ) , which has two frequency

351

ALUMINOSILICATE CATALYSTS

axes. Chemical shift effects in the 2D spectrum are separated from the effects of the interactions present during the evolution period. The actual significance of fi and f2 depends on the particular experiment, or rather on what happens during t , . In general, there are two classes of 2D NMR spectra: (1) resolved spectra, which spread out the lines of an ordinary spectrum into two dimensions, each corresponding to a different NMR parameter; and (2) correlated spectra. Most 2D experiments to date concern NMR of solutions, but the potential of the technique for the solid state is considerable. Mehring (10) gives an admirable summary of the various experiments possible with solids; here we shall give an outline of some selected ones. Bax et al. (429) proposed a pulse sequence which allows the chemical shift anisotropy, averaged under MAS, to be recovered for the 13C nucleus. A projection of the resulting 2D spectrum onto the f2 axis gives the ordinary CP/MAS spectrum, while a projection onto f, gives the total chemical shift anisotropy pattern for all the carbon nuclei in the molecule, thus providing important information on the molecular structure. In another 2D experiment from the same group (430) the sample is spinning at the angle of 90” to the magnetic field during the evolution period, but at the magic angle during the observation period. Separation of chemical shifts from CSA patterns is also accomplished using 2D magic-angle hopping (429). In the “separated local field” technique, dipolar I-S interactions are separated from chemical shifts of nucelus S. As dipolar interactions are highly sensitive to internuclear distances, the obvious use of the method is for the determination of molecular structure in the solid state. An example is provided by the work of Hester et al. (411) and Rybaczewski et al. (414) on

J

ii

X J

X P

F

, I

I......

SPIN

j

DECOUPLING

TIMES I

+TI,

I

5-1t

-I

FIG.83. Pulse sequence for the 2D SLF spectrum of 13C-’H interactions (414). During evolution period, t l . mainly heteronuclear spin interactions are effective, whereas chemical shift interactions govern the time evolution during the detection period, t , (see text).

352

k*

J. M. T H O M A S A N D J. KLINOWSKI

-1.67

-I+

1

1.621

,.

I

CHEMCAL W F T FREQUENCY w2

FIG.84. Two-dimensional 13C NMR spectrum of ammonium hydrogen malonate (NH4HC,H204) (414) using the pulse sequence shown in Fig. 83. Heteronuclear dipolar interactions, wl, are plotted versus chemical shift frequency, w 2 . The projection of the 2D spectrum onto the w 2 axis, shown at the top, represents the ordinary ''C MAS NMR spectrum.

'

I3C NMR of organic crystals. The experiment consists of 3C-'H crosspolarization followed by an evolution period I,, during which 'H-'H dipolar interactions are removed via the WAHUHA multiple pulse sequence. During the observation period, t 2 , high-power proton decoupling is applied (Fig. 83). An SLF spectrum of a single crystal of ammonium hydrogen malonate, NH4HC3H204,shows the local fields at magnetically nonequivalent carbons (Fig. 84). The two nonequivalent carbon atoms in the CH2 group, which

353

ALUMINOSILICATE CATALYSTS

I

I

40

35

30

PPm

I

FIG.85. Two-dimensional I3C spin diffusion spectra of mixtures of adamantane and 2,2,3,3-tetramethylbutaneat 75.4 MHz (424):(a) Mixture of powders; (b) mixture by melt. Note the absence of cross-peaks between signals belonging to different species in the heterogeneous sample in (a).

ippear as a single peak in the ordinary one-dimensional ( 1 D) spectrum, are :learly distinguished. The technique may also be applied to quadrupolarjipolar spectra (419, 420) and to polycrystalline samples. Caravatti et al. (424) described a 2D NMR experiment capable of monitoring the homogeneity of solids on a molecular level. The experiment is based on the phenomenon of nuclear spin diffusion. Because of the very

354

I. M. THOMAS AND J. KLINOWSKI

strong dependence of spin diffusion on internuclear distance, it is almost exclusively confined to neighboring molecules: if spin diffusion occurs between two different species, then they must be mixed on a microscopic level. Figure 85 shows 1D and 2D spectra of heterogeneous and homogeneous powdered mixtures of adamantane and 2,2,3,34etramethylbutane.The 1D spectrum contains just four signals: A, and A, come from the carbons in adamantane, H and H, from the carbons in 2,2,3,3-tetramethylbutane.The 2D spectrum in Fig. 85a consists of four diagonal peaks and four cross peaks connecting signals belonging to the same species: clearly there is no spin diffusion between the two components, which must form separate crystallites. On the other hand, all twelve possible peaks are present in the spectrum in Fig. 85b, which indicates that the two compounds form a mixed crystal. The technique holds considerable promise for the study of homogeneity of a variety of materials, including catalysts, and can be applied to other nuclei, such as 29Siand 31P. It can also be combined with MAS, which will greatly enhance spectral resolution.

D. NMR IMAGING NMR imaging is a rapidly developing field, and its applications to date are mostly in medicine and the biological sciences, where almost invariably ‘H spins are imaged. The method relies on the presence of a linear gradient of the magnetic field in addition to the homogeneous field B,. As a result of this, identical nuclei in different parts of the sample experience different external magnetic fields, and resonate at different frequencies. The intensity of the signal is still proportional to the number of nuclei in each part of the sample. As a consequence, the spectrum is a projection of the shape of the sample onto the direction of the field gradient. Szeverenyi and Maciel (425) used a variant of the technique to image magnetically dilute nuclei (such as ” C ) in the solid state. Their experiment involves the application of standard 2D pulse sequences with a linear field gradient (6500 ”C Hz/cm) applied along the z-direction. Samples containing components with small chemical shift anisotropy give spectra with spatial and chemical shift information. For large anisotropies another pulse technique can be applied that is highly sensitive at the expense of losing chemical shift information. As an example of the method, Szeverenyi and Maciel imaged a Delrin phantom filled with adamantane, camphor, and hexamethylethane arranged in layers. The usefulness of NMR imaging of nonmedical objects will be ultimately determined by the achievable resolution. It has potential for the study of other “dilute” nuclei in the solid state, such as 29Si,31P, and I5N in catalytic systems.

355

ALUMINOSILICATE CATALYSTS

The most recent developments in 2D NMR of solids are the heteronuclear chemical shift correlation spectroscopy (421), 2D exchange NMR, which enables very slow molecular reorientations to be monitored (422), and heteronuclear J-resolved 2D NMR (423). E. ZERO-FIELD NMR As a result of the presence of strong ‘H-’H dipolar interactions, proton spectra from powdered samples are often broad: the distribution of internuclear vectors with respect to the direction of the magnetic field causes the loss of most information of interest to the chemist. Weitekamp et al. (426) have pointed out that without an externally imposed direction in space all molecular orientations are equivalent, so that equivalent nuclei in all chemically equivalent molecules should resonate at the same NMR frequency. They implemented this idea in the novel “field cycling” experiment (Fig. 86). The sample is polarized in the large magnetic field Bo and then very quickly (ca. 100 msec) adiabatically removed from the bore of the superconducting magnet to a coil below, where the fringe field due to Bo is precisely cancelled by field B,. At time t , = 0, coil B, (0.01 T) is quickly switched off and the spins evolve under the zero-field Hamiltonian. The evolution is terminated by reversing the procedure and sampling the magnetization inside the large magnet. The entire procedure is repeated for incremented values of t Fourier transformation of the resulting time-domain signal gives the zero-field spectrum. Figure 87c shows the so-obtained spectrum of barium

’;

1

B = 80 +

01

B (TESLAI

I

82

FIG.86. Schematic diagram of the field cycling apparatus and the time-dependent effective field at the sample in the zero-field NMR experiment (426).

356

J. M. THOMAS AND J. KLINOWSKI (0)

High-Field

2L

( c ) Zero-Field Powder

A

n I

I

chlorate hydrate. The observed splitting is a direct, orientation-independent measure of the internuclear distance. The proton-proton distance calculated from the splitting is 1.60 A, identical to that obtained by NMR from a single crystal and very close to that measured by neutron diffraction. The same group (427, 428) applied the zero-field principle to pure quadrupole resonance with the aim of overcoming the quadrupolar broadening of conventional spectra for nuclei with I > $. The procedure is identical to that delineated above; during time t , the nuclear spins, 27Al for example, develop under the quadrupolar Hamiltonian. Figure 88 gives the so-obtained spectra of ammonium and potassium alums, NH,Al(SO,), . 12H,O and KAI(SO,), - 12H,O, respectively. While 27Al MAS NMR is unable to distinguish between the two compounds even at very high magnetic fields, zero-field NQR clearly differentiates between the 27Al nuclei in each compound, despite their nearly identical crystal structures. The technique is

ALUMINOSILICATE CATALYSTS

I

Potassium alum

I

Ammonium alum

I

357

Mixed alums

Frequency (kHz) FIG.88. 27A1Fourier transform pure nuclear quadrupole resonance spectra of potassium, ammonium, and mixed alums (428). The pair of high-frequency lines in the mixed sample (56 mol% potassium alum, 44 mol% ammonium alum) clearly indicates the existence of two distinct *'A1 lines.

particularly suitable for nuclei with low quadrupolar frequencies (such as 'D, 'Li, "B, I4N, 23Na,and 27Al) and apart from the 27Al example described above it has already been applied to 'D and 7Li (427). The authors expect that even more sensitive differentiation between aluminum sites will be possible in systems such as zeolites.

NMR AS A MEANS OF ELUCIDATING F. SOLID-STATE ENZYMATIC REACTIONS The mode of action of enzymes, be they allosteric or covalently modulated ( 4 4 4 , is a topic of major importance and no mean complexity. It is not our intention here to cite other than a few principles and examples in relation to the way in which solid-state NMR promises to elucidate the vast, ramifying

358

J. M. THOMAS AND J. KLlNOWSKl

field of biological catalysis. We do so, however, because it is obvious that there are lessons and strategies, already recognized by biologists and organic chemists, that can be adopted and adapted in the study of heterogeneous catalysis generally. Until a few years ago biosynthetic pathways were clarified by a skillful combination of organic synthesis, enzymology, and the techniques of solution NMR. The veracity of this statement is exemplified by the progress accomplished in the study of the vitamin B12pathway. Nowadays, however, thanks to the fact that high-resolution NMR spectra of solids may be recorded relatively routinely at high fields, NMR studies may be carried out in uitro and in uiuo (442). And it is now an attainable goal to study enzyme structure and function in terms of recognizable intermediate species bound to the enzyme at its reactive site in the receptor pocket. Moreover, the results obtainable from high-resolution NMR complement, and often greatly extend, those derived from X-ray diffraction analysis on enzyme-substrate complexes. It increasingly happens in enzymology that crystalline complexes of adequate quality cannot be obtained to permit X-ray analysis, a situation which is almost invariably encountered in heterogeneous catalysis involving inorganic surfaces. The example we focus upon here concerns the use, by Scott and co-workers (443) of "C and "N NMR to determine the extent of cleavage of the scissile amide bond in glycyltyrosine, a substrate which undergoes slow hydrolysis (in the crystalline state) in the complex formed between the glycyltyrosine and the enzyme carboxypeptidase Aa (443). The presence of a peptide link, involving as it does the C-N bond, is readily detected by "C CP/MAS NMR. This is because the 14N nucleus is quadrupolar (I = 1) and the 13C has a spin of 4, so that dipolar coupling between these two nuclei is not entirely removed by magic-angle spinning (see Section 111,H).As a result, a

V

FIG.89. "C CP/MAS NMR spectrum of glycine at 22.6 MHz (122). The carbon atom bonded to nitrogen displays a characteristic doublet arising from "N quadrupole interactions.

ALUMINOSILICATE CATALYSTS

359

FIG.90. (A) Glycine-tyrosine bound to carboxypeptidase (443).Indirect attack of Glu-270 promotes the attack of a water molecule on the amido carbonyl group polarized by interaction with zinc. (B) Direct attack of Glu-270 on the amido carbonyl with formation of an anhydride.

characteristic doublet is present in the high-resolution l 3C spectrum whenever there is an intact peptide link, or indeed any C-N link, in the sample under investigation. A typical example is shown in Fig. 89 for the amino acid glycine ( +NH3-CH,-COO-). The relatively strong quadrupolar interaction of 14N with the electric field gradient at its site competes with the Zeeman interaction for the alignment of the 14N spin and modifies the angular dependence of its dipolar coupling with other spins. With 5N,which has a spin of 4,this source of doubling (or broadening) is absent in the 13C spectra, thus confirming the correct identification of its origin, The crystal structure of the complex formed between carboxypeptidase Aa (abbreviated CPA) and glycyltyrosine (Gly-Tyr) has been refined to 2.0 A by Lipscomb et al. (444, 445) and it reveals (Fig. 90) interactions between the amide carbonyl oxygen and the catalytically essential Zn, and between the amide nitrogen and the hydroxyl of tyrosine-248 (Tyr-248). Scott et al. (443) synthesized both the [13C]amido (90 % enriched) and amido[ 13C, "N]amido (90 % and 99 % enriched, respectively) isotopomers of Gly-Tyr. They then proceeded to probe the hydrolysis by a series of 13C and 15N high-resolution solid-state NMR spectra. CP/MAS NMR spectrum of the 13C-enriched As expected, the Gly-Tyr crystals displays the characteristic doublet (Fig. 9 1A). However, the 13C CP/MAS difference spectrum between the CPA (which is also rich in C-N bonds, but which is not 13C-enriched)and the CPA/Gly-Tyr complex (Fig. 91B) displays a single resonance. This signifies that, under the experimental conditions, the peptide bond of the Gly-Tyr has been cleaved. To confirm this result, the authors took the 15NCP/MAS NMR spectrum of the

360

J. M. THOMAS A N D J. KLINOWSKI

1

170

70

PPm

FIG.91. (A) 13C CP/MAS NMR spectra (443) at 25.15 MHz of [13C]amido Gly-Tyr; 0 denotes "C. (B) The difference spectrum of CPA and CPA/[13C]amido Gly-Tyr complex.

(A)

600

400

200

0

200

ppm

FIG.92. "N CP/MAS NMR spectra (443) at 20.28 MHZ of (A) [13C,15N]amidoGly-Tyr: and A denote I3C and I'N, respectively; (B) CPA + [13C,1sN]amidoGly-Tyr.

ALUMINOSILICATE CATALYSTS

36 1

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Author Index Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not cited in the text. Numbers in italics show the page on which the complete reference is listed.

A Abe, Y . , 29(65). 46 Abragam, A,, 205(12), 361 Ackerman, J.. 208(18), 350(410, 411), 351(411), 361, 372 Adams, C . R., 179(28, 29). 197 Adams, J. M., 336(452), 373 Ai, M., 162(49, SO), 184, 198 Akasaka, K., 251(121), 364 Akitt, J. W . , 254(135), 255(135, 136). 256. 272(150), 294(150), 365 Alarcon-Dim, A,, 317(401), 372 Alcock, N. W . , 18(52), 20(52), 46 Alder, B., 303(274),368 Aliev, F. V . , 180(32, 33). 182(32, 33), 197 Alla, M.,219(51), 350, 362, 372 Alma, N. C . M., 245(100), 253(130, 131), 256(141), 257(141), 281(210), 282(210), 364, 365, 367, 374 Amerikov, V. G., 177(23), 197 Amirbekov, E. N., 180(32, 33), 182(32, 33). I97 Andell, 0. S., 13(55), 20, 25(62), 46 Anderson, J. H . , 219(48), 362 Anderson, M. W . , 233(77), 236(77), 238(90), 242(90), 250(117), 252, 281(208, 209), 282(208, 209). 284(208. 209), 286(209), 287(93), 288(21 I ) , 289, 363, 364, 365, 367 Andrew, E. R., 201(2, 3), 203(3, 8). 204(3, 1 I), 208(3, 8), 253(3), 361 Aomura, K., 100, 121(17), 147(17), 157 Argauer, R. J., 217(33), 236(33), 362 Arieti, A., 175(17), 197 Ariyaritne, J. K. P., 32(70), 46 Armistead, C. G . , 52(20), 53(20), 96 Arthur, P., Jr., 3, 45

Audier, M., 201(5), 281(206), 282(206), 283(206), 284(206), 361, 367 Aue, W . P., 349(405),372 Averbuch, P., 300(241), 367 Aveston, J., 272(181), 294(181), 366

B Backvall, J.-E., 13(55), 20, 25, 46 Bailey, G. C . , 68(71), 97 Baizer, M., 3(2), 45 Baker, L. M., 55, 60(32), 96 Baldeschwieler, J. D., 300(252), 303(252), 368 Balimann, G. E., 251(119), 364 Ballantine, J. A,, 335(448, 450, 451), 336(450, 454), 342(462), 343(462), 344(462), 345(462),373 Ballard, D. G. H., 58(58, 61), 92(58), 97 Baltusis, L., 323(373), 324(373), 371 Bangham, D. H.,311(322-325), 318(324). 369, 370 Banks, R. L., 48, 96 Barrer, R. M., 217(35, 36). 233, 236(36), 242(95), 289(36, 212), 303, 306, 335(449), 362, 363, 364. 367, 369, 373 Barri, S. A. I., 243(439), 373 Barron, P. F., 344(491), 374 Barron, P. L., 344, 373 Bartholdi, E., 349(405), 372 Bartuska, V. J., 326(349), 327(349), 370 Basler, W. D., 296(227), 297(227), 300(266, 268), 302(266), 367, 368 Basolo, F., 7(23), 9(23), 18(51), 20(51), 45, 46 Bastein, A. G . T. M., 2 4 3 loo), 364 Batist, Ph., 162(26), 178(26), 182(40), 183(40), 197 375

376

AUTHOR INDEX

Bax, A., 351(430). 372 Baynham, J. W . , 233(73), 363 Beck, D. D., 57(46), 97 Behm, R. J., 155(64), 158 Behr, A., 20, 46 Behrens. H.-J., 256(139, 140). 257(151), 258(139), 365 Belenykaja, I., 281(205), 282, 367 Bell, A. T.,330(472, 473, 474), 374 Bemi, L., 348(399), 371 Benn. R., 350, 372 Bennett, J. M., 237(81), 260(81), 363 Benson. S . W . , 125(28), 157 Ben Taarit, Y.,217(40). 362 Berger, A. S., 233(74), 363 Bergman, R. G . , 156(61), 158 Bentein. T., 326(385), 371 Betghe, P. H.,359(444), 373 Beyer, H. K., 281(204, 205). 282,366, 367 Bielecki. A., 355(426), 356(426, 427, 428), 357(427, 428), 372 Biloen, P., 102(4), 157 Binsma, J., 307(301), 308(301), 369 Blackwell, C. S.,248(105, I l l ) , 364 Bloembergen, N., 300(236), 305, 367 Blom, N., 290(216), 367 Blyholder, G., 127(30), 157 Bodenhaukn, G., 212(27), 350(406, 407, 408). 353(419, 420, 424), 362, 372 Bohm, J., 252(153), 277, 365 Bonner. F., 138(39), 157 Borello, E., 49(12), 50(12), % Boreskov, G. K., 160(3), 177(23), 182(42), 183(42, 45, 47), I%, 197 Borg, F., 343(461), 345(461), 373 Bositek, V., 265(168), 269, 282(168), 366 Boudart, M., 300(252), 303(252), 368 Boxhoom, G., 253(130, 131), 291, 311, 312(329), 365, 367, 370 Bradbury, A., 203(8), 208(8). 361 Braurnan, J. 1.. 125(27), 157 Breck, D. W . , 217(37), 362 Brei, V. V., 325(376), 371 Bremer, H., 326(387), 371 Brennemann, H., 122(20), 157 Brevard, C., 314(334), 370 Bridger, R. F., 289(481), 374 Brittin, W. E., 302. 368 Bmakov, Yu. I., 49(14), 96 Brouwer, D. M., 152(58), 158

Brown, E. S., 5 . 45 Brown, G. L., 58(53), 92(53), 97 Brown, H.C., 348(398), 371 Brown, I. W . M., 344(487), 374 Brown, L. C., 255(144), 365 Bruck, D., 347(394), 371 Buckingham, A. D., 300, 368 Buist, R. J., 9(41). 46 Bujalski, R. L., 300(265), 302(265), 304(265), 368 Bulani, W., 160 Blilow. M.,306(295, 296), 308(296), 369 Bultitude, F. W . , 233(73), 363 Bunington, J. D., 183(48), 197 Bursill, L. A., 232(69), 363 Bunvell, R. L., 57(48, 49), 87(49), 122(18), 139(45), 97, 157, 158 Busca, G . . 180(34), 197 Butts, S. B., 18(52), 20(52), 46 Byliaa, A., 336(452), 373

C Caillat, R., 273( 192). 366 Calvert, R. B., 289(481), 374 Campbell, A. S., 344(489). 374 C ~ MT., R . , 216(30, 31, 65), 289(30), 362, 363 Caravatti, P.,213(27), 353(424), 362, 372 Cardew, M., 139(45), 158 Cares, W. R.. 162(25), 178(25), 178(25), 197 Caro, J., 300(267), 306(267, 294, 295). 368, 369 Canick, W. L., 5 5 , 58(54), 60(32), 92(54), 96 Carter, J. L., 102(3), 157 Cary, L. W . , 290(215), 367 Chang, C. C., 140(50), 146(50), 158, 182(43), 197 Chang, C. D., 289,374 Chang, J. J., 327, 328(352). 370 Chao, K. J., 331(358), 332(358), 370 Cheetham, A. K.,232(69), 363 Chen, N. Y.,309(314, 315), 369 Cheng, W. H., 162(5), 164(5), 165(5), 169(8), 178(5), 181(8), 1% Childs, R., 348(399), 371 Childs, R. F., 347(395), 348(355), 349(395), 370, 371 Christner, L. G.,281(203), 282(203), 366

AUTHOR INDEX Chu, C. T.-W., 289(481), 374 Chuang, I.-S., 323(370, 37 I), 324(370), 325(371), 371 Claque, A. D. H., 253(130), 265(164), 266(164), 307(301), 308(301, 312). 311(329), 312(329), 327(350), 365, 369, 3 70 Clark, A , , 54(29), 68(71). 96, 97 Clark, H. C . , 329, 348(399), 370, 371 Coey, J. M. D., 186(57), 192(57), 198 Cohen, J. P., 237(81), 260(81), 363 Cohen, M. H., 269(173), 366 Cohen-Addad, J. P., 300(255), 368 Coluccia. S . , 55(38, 39, a), 61(66), 69(40), 70(66), 74(66), 97 Conner, W. C., 139(46), 158, 182 (43). 197 Corbin, D. R.. 263, 271(180), 272(180), 366, 3 73 Cordischi, D., 175(17), 197 Comet, D., 57(49), 87(49), 97 Cossee, P., 54(25), 96 Costanzo, P. M..343(464), 373 Cotton, F. A., 30(69), 46, 134(38), 157 Couperus, P. A., 281(210). 282(210), 367 C O U ~ ~ NT.,S ,265(164), 266(164), 365 Cram, P. J., 303, 369 Cranna, N. G . , 255(142), 365 Cronstedt, A,, 213, 362 Cross, T. A., 251(120). 364 Cross, V. R., 350(410), 372 Curtin, D., 348(399), 371 Czenkusch, E. L., 58(51), 97

D Dadybuqor, D. B., 160, 180(37), 197 Dautzenberg, F. M., 102(4), 157 Davies, J., 329(353, 354). 348(399), 370, 371 Davies, M., 336(454). 373 Davis, B. R., 12(49), 46 Davis, R. E., 156(62), 158 Davison, J. M., 58(53), 92(53), 97 Davydov, A. A., 182(42), 183(42, 44,45). 197 Dawson. W. H., 321(365, 366), 322(365), 3 71 DeAtley, W. W., 3(3), 45 deBoer, N. H., 179(30), 197 Debras, G., 309(313), 369 DeCanio, S . J., 331, 332(357, 358), 370

317

Deeming, A. J., 21(57), 46 deHaan, J. W., 254(134), 365 Deininger, D., 300(249-251), 302(249-251). 307, 368, 369, 371 de Jong, A. F., 355(422), 372 de Jong, B. H. W. S . , 290(213, 215), 367 de la Calle, C., 341(485), 374 Deli, J. A., 353(424), 372 Della Gatta, E., 55(43), 67(43), 97 Delmau, J., 308(309, 311), 369 Delmon, J. A,, 317(401), 372 de Menorval, I. C., 314(340-344). 316(344), 3 17(341-344), 3 18(34 1-344), 3 19(341), 3 70 Dempsey, E., 229(60), 302, 363, 368 Dent, A. L., 122, 140(50), 146(50), 157, 158, 183(46), 197 Dent Glasser, L. S . , 272, 366 Derouane, E. G . , 273(189), 290, 307(303), 308(303, 307), 312(330), 366. 367, 369, 3 70 DeSchutter, C. T., 242(87, 89), 244(89), 245(89, 102), 246(99), 252(87), 309(102), 363, 364 Dessau, R. M., 289, 374 Dibble, W. E., Jr., 290(213), 367 Diddams, P. A., 342, 343(458, 462). 344, 345(462), 346(458), 373 Dixon, L. T., 140(50), 146(50), 158 Domaille, P. J., 10(47), 12(47), 25(47), 27(47), 30(47). 38(47), 46 Doskocilovii, D., 302(273), 368 Dow, A. W., 58(54), 92(54), 97 Dreiling, M. J., 76(75), 98 Drexler, D., 348(399), 371 Drinkard, W. C., 3(7), 4(8, 9), 6(7), 9(39), 14(7), 45. 46 Druliner, J. D., 10(47), 11(46), 12(47, 48), 14(48), 25(47). 27(47), 30(47), 38(47, 48). 45 Dubinin, M. M., 267(172), 271(172), 366 Ducros, P., 300(240, 241, 242, 246, 247). 367. 368 Dudley, R. L., 348(399), 371 Duffy, J. A,, 272(184-186). 366 Dumas, T., 160 Dumesic, J. A,, 160(2), 185(55), 189(55, 64), 192(64), 196, 198 Duncan, T. M., 308(480), 330, 374 DuPreez, A. L . , 12(50), 46

378

AUTHOR INDEX

Dutz, H., 219(47), 362 Dwyer, F. G., 244(98), 364 Dwyer, J., 271. 366 Dybowski, C., 331(357, 358). 332(357, 3581, 3 70

E Eades, R. G., 203(8), 208(8), 255(143), 361. 365 Early, T. A., 323(370), Eaton, D.R.,9(39, 40), 46 Eckman, R., 307, 369 Eishens, R. P., 139, 158 Efremov, A. A., 182(42), 183(42, 44),197 Eley, D. D., 54(28). 96 Ellis, P. D., 321(365, 366), 322(365), 371 Elston, J., 273(192), 366 Engelhardt, G., 201(4), 219(51, 52), 220(52, 53), 221(52, 54), 226(55), 231(55), 232(52, 68). 249, 265(165-167), 267( 166), 268( 165), 291(2 19), 272(218). 293(2 18, 2 19). 294(219), 295(219), 343(53), 344(52), 361, 362, 363, 364, 366, 367 England, D. C., 3(6), 45 English, A. D., 8(31), 10(46), 11(46), 45 Ernst, H., 300(269), 326(385, 386). 368, 371 Ernst, R. R., 213(27), 349(403-405). 353(424),362, 372 Ertl, G., 155(64), 158 Evans, J. C.. 292(182), 294(182), 366

F Fagerness, P. E.. 358(443), 358(443), 360(443), 361(443), 373 Fahlke, B., 291(218, 219). 292(218), 293(218, 219). 294(219), 295(219), 367 Faller, J., 43(73), 46 Falth, L., 241, 363 Faragher, A. L., 1 lO(1 I), 117, 157 Farges, J. P., 300(255, 256). 368 Farlee, R. D., 253, 263, 271(180), 272(180), 365, 366, 373 Favero, G . , 29. 46 Fechner, E., 219(50), 248(107), 249(114), 362. 364 Fenzke, D., 252(153), 277(153). 365 Feser, R., 30(68), 46 Finger, G., 300(251), 368 Fitch, F. R.,271(176, 177). 366

Flanigen, E. M.,217(31, 6 3 , 237, 260(80, 81). 362, 363 Foley, H. C., 331(357, 358), 332(357, 358), 3 70 Forissier, M.,191(73), 198 Fraissard, J., 273(189), 314(336-344). 3 16(339), 3 l7(341-344). 3 18(341-344). 319(341),366, 370 Freeman, R., 350(406, 407, 408, 409). 350, 372 Frenken, P., 278(197), 294(220), 366, 367 Freude, D., 256, 257(151), 258, 259, 265(168-171), 269(168), 270(169), 271, 273(193), 274(193), 275, 276, 277, 278(171), 278(170). 282(168, 170). 297, 326,(385, 387), 365, 366. 367, 371, 374 Frey, M. H..251(120, 123, 124), 364 Frilette, V. J., 217(32), 362 Fripiat, J. J., 273(189). 327(352), 328(352), 343(461), 345(461), 366. 370, 373 Froese, C . , 314(335), 370 Frohlich, T., 259(157), 265(168-170). 269(168), 270(169), 271(169). 277( 170). 279(170), 282(168, 170), 365, 366 Frost, R. L., 344(463, 489, 490, 491), 373, 374 Frye, J. S . , 323(373), 324(373), 371 Fubini, B., 49(16), 55(16, 43), 65(16), 67(16, 43), 69(16), 96, 97 Fyfe, C. A., 201(5), 203(7), 208(19), 2 18(43), 222(58), 226(56-58), 227(58), 228(58), 230(58), 231(58), 232(43, 56, 71), 233(56, 71, 77), 236(77), 238, 239(82). 240(57), 241(57), 242(84-89), 244(99), 245(56, 89, 98, 102), 246(99), 248(56), 252(84-88). 257, 258( 155). 259(155), 260(84, 85), 261(155), 262(163), 266(163), 267(163), 281(206208). 282(206-208), 283(206, 207), 284(57. 206, 208), 285(207), 286(57, 84), 298(56), 309, 326, 327(351), 329(353), 340(457), 346(57), 347(394, 399, 348(351, 399). 349(395), 361, 362, 363, 364, 365, 367, 370, 371. 373

G Gabelica, Z., 290(216), 309(313), 312(330), 367, 369, 370 Gabuda, S . P., 255, 262, 365 Gaddi, M.,29(66), 46

319

AUTHOR INDEX

Gajek, R. T., 216(65), 363 Gale, R. J., 101(2), I57 Gallezot, P., 317, 372 Ganapathy, S . , 208(17), 251(121, 126), 361, 364 Garonne, E., 49(12, 16). 50(12), 55(16, 38, 39, 40, 43), 61(66), 65(16), 67(16, 43), 69(16, 40). 70(66), 74(66), 96, 97 Garroway, A. N., 300(440), 323 Garwood, W. E., 309(315), 369 Gates, B. C . , 331(357, 358). 332(357, 358), 3 70 Gatineau, L., 343(461), 345(461), 373 Gay, I. D., 202(6), 321, 361 Censer, E. E., 262, 365 Germain, J. E., 191(69, 70, 71). 192(69, 70, 71), 196, I98 Germanus, A.. 279(199, 200). 366 Gemtzen, R., 55(41), 97 Geschke, D., 303(276), 368, 371 Gessner, W . , 255(136, 138), 256(139, 140), 258(139), 290(138), 365 Ghiotti, G., 49(12, 16). 50(12), 55(16, 38, 40, 43), 61(66), 65(16), 67(16, 43), 69(16. 40). 70(66), 74(66), 96, 97 Chose, S . , 255(149), 365 Giamello, E., 55(43), 67(43), 97 Gibby, M. G . , 209(21), 219, 362 Gibson, M. A., 162(15), 172(15), 175(15), 177(15), 178(15), 179(15), 180(15), I97 Giese, R. F. Jr., 343(464), 373 Gilson, J.-P., 308(31 I ) , 369 Girelli, A., 180(34), I97 Gobbi, C. G . , 208(19), 361, 367 Gobbi. G . C . , 218(43), 226(56, 57), 232(43, 56), 233(56, 77). 236(77). 238(82), 239(82), 240(57). 241(57), 242(84-88), 244(99), 245(56), 246(99), 248(56), 252(84-88). 257(155), 258(155), 259(155), 260(82, 84, 8 3 , 261(155), 264(163), 266(163), 267(163), 281(207, 208). 282(207, 208). 283(207), 284(57, 208), 285(207), 286(57, 84), 298(56), 326(384), 346(57), 348(399), 362, 363, 364, 365, 371 Golden, D. M., 125(28), I57 Gordon, H., 131(38), I57 Gorges, U.,49(15), 55(15), 56(15), 67(15), 96 Gorlov, Yu. I . , 325(376), 371 Gosser, L. W . , 6(17, 18). 7(17, 22). 9(22), 45

Gottlieb, H. E., 327(388), 371 Graham, S . H., 336(452), 373 Gramlich, V., 232(66), 363 Granger, P., 314(334), 370 Grasselli, R. K . , 180(36), 183(36, 48). I97 Green, M. L. H.. 9(38), 32(70), 46, 61(65), 70(65), 74(65), 97 Green, P. J., 300(265), 302(265), 304(265), 368 Greenwood, N. N.. 256(150), 272(150), 294( I SO), 365 Gregory, R., 336(455), 373 Griffin, R. G., 353(419, 420). 372 Grimmer, A.-R., 201(4), 219(52), 220(52, 53), 221(52), 232(52), 247, 248(107, 108), 249(112, 113). 325, 343(53), 344(52), 361. 362, 364. 371 Grinchenko, 1. V . . 255(137), 262(137), 365 Groenen, E. J. J., 245, 364 Groeneveld, C., 49(5-9). 61(9), 62(9), 67(8, 9), 96 Groombridge, C. J., 251(119, 122), 358(122), 364 Grose, R . W., 237(81), 260(80, 81). 363 Guelton, M., 308(307), 369 Guemer, E., 314(340), 370 Guggenberger, L., 8(30), 45 Gunsser, W., 296(221, 223), 367 Gunther, H., 350, 372 Gutsze, A., 300(251, 263). 302(251, 263). 368 Guyot, A., 49(10), 54(10), 96

H Haag, W. 0..122, I57 Haase, J., 271(183), 366 Haber, J., 180(35), 191(35), I97 Haberkorn, R. A., 353(420), 372 Hafner, S . S . , 254(147, 148, 149), 257(148), 365 Hageveen, H., 152(58), I58 Hahn, E. L., 210, 212(24), 362 Hall, K. W . , 106, I57 Hall, W . H., 276(196), 366 Hall, W. K . , 281(203), 282(203), 347(390), 366, 371 Haller, G . L., 57(48), 97, 122(18), I57 Halpern, B., 196 Halpern, J., 32(71), 46 Hambleton, F. H., 52(20, 21). 53(20), 96

380

AUTHOR INDEX

Hamer, G . , 120(15). 157 Hammond, B. L., 349(400). 372 Haneda, H., 124(24). 157 Hansen, S., 241, 363 Harris, C. R., 3(3), 45 Harris, R. K . , 218(46), 238(46), 251(119, 122). 331(360), 358(122), 362, 364 Harris, T. V., 370 Harrod, J . F., 120(15), 157 Hart, A. J . , 124(25), 157 Hartman, J. S . . 201(5). 222(58), 226(58), 227(58), 228(58), 230(58), 231(58), 232(71), 233(71), 257(155), 258(155), 259( 1 5 3 , 26 I ( 155). 28 I (206, 207). 282(206, 207). 283(206, 207), 284(206), 348(399),361. 363, 365, 367, 371 Hartmann, J . , 124, 157 Hartmann, P., 255(146), 365 Hartmann, S. R . , 210, 362 Hartsuck, J. A . . 359(444), 373 Hattori, H . , 128(32), 139(32), 152(57), 157, 158 Haubenreisser, U . , 249(113, 114). 364 Hauser, A . . 297(233),367 Haw, J. F., 323(370, 371). 324(370), 325(371), 371 Hawkins, B. L., 323(370, 371). 324(370), 325(371), 371 Hay, D. G . . 310(332), 312(332), 313(332), 3 74 Hayes, P., 348(399), 371 Hayes, P. J . , 329(353, 354). 370 Hays. G . R., 244(99), 245(10(3), 253(131), 265(164), 266(164), 281(210), 282, 308(312), 31 l(329, 312(329), 327(350), 364. 365, 367, 369, 370, 374 Heink, W.. 306(297). 369 Henis, J. M. S . , 308(308), 369 Hennecke, H. W.. 296(227, 229), 297(227), 367 Henrichs, P. M., 251(125),364 Herden, H., 297.367 Hermann, G . , 8(32, 34), 45 Herren C. P., 343(483, 486), 374 Hester, R. K . , 350(410, 411). 351, 372 Hexem, J. G . , 251(120, 124). 364 Hierl, G., 51(19), 55(33). 96 Higgins, J. B . , 245, 248, 254(97), 364 Hightower, J. H., 106, 157 Hightower, J . W . , 162(15, 25), 172(15),

175(15), 177(15), 178(15, 2 9 , 179(15, 25). l80(15). 197 Hill, R. W., 89(79), 98 Hockey, J. A . , 52(20, 21, 22). 53(20), 96 Hoebbel. D., 255(138), 290(138), 365 Hoff, R. E., 76(74), 98 Hoffman, J., 307(297), 369 Hogan, J. P., 48, 50(17), 55(17), 57(17), 61(68), 64(70), 68(17), 69(17), 96 Holloway, J. H., 314(334), 370 Holt, E. M . , 18(52),20(52), 46 Holzman, G. R., 219, 362 Hong, F., 185(54), 186(56), 189(54), 190(54), 192(56), 198 Hoppach, D . , 307(306), 369 Horvath, B., 5342). 97 Horvath, E. G . , 5342). 97 Hovis, G. L., 248(1 I I), 364 Hrynkiewicz, A . . 186(59), 192(59), 198 Hsieh, E. T . , 62(69), 64(69), 69(69), 97 Huang, T. H., 353(420), 372 Hubert, A. J . , 5, 45 Hucknall, D. J . , 160 Hughes, R. P., 21, 46 Huis, R., 245(100), 253(130), 263164). 266(I64), 28K2 10). 282(210), 308(312). 31 1(329),312(329), 327(350). 364, 365. 367, 369. 370 Hums, E., 61(63), 97 Hunger, M . , 265(170, 171). 275(171), 277(170, 171). 278(171), 279(170), 282(170), 326(387), 366, 371 Hunter, B. K . , 242(91), 244(91), 248(91), 364 Hyde, B . G . , 236, 363

I Ibers, J. A., 12(49), 46 Iizuka, T., 184(53), 198 Ikeda, S . . 29(64), 46 Imelik, B . , 273(192), 317(401), 366, 372 Imizu. Y . , 152(57), 158 Indovina, V., 175(17), 197 Ingram, M. D., 272(184-186), 366 h e r s , R. R., 321(365, 366). 322(365). 371 Innes, R. A . , 139(46), 157 Inomata, M . , 190(65), 192(65), 198 h a , A. P., 138(39). 157 Isirikjan, A . A., 267(172), 271(172), 366 110, T.,314(336-344), 316(339, 344),

AUTHOR INDEX

3 17(341-34). 3 I8(341-344). 3 19(341), 3 70 Ittel, S. D., 8(35), 45 Ivin, K. J., 61(65), 70(65), 74(65), 97 Iwamoto, M.,169(9), 170(9), 1%

J Jackson, W. R., 21, 46 Jacobs, P. A.. 217(41), 281(204), 282, 362, 366 Jacobson, A. J., 226(59), 229(59), 231(59), 232(70), 233(76), 234(76), 235(76), 298(235), 314(345), 3 15(345),363, 367. 370 Jaeger. H . , 310(332), 312(332), 313(332), 370 James. B. R., 5 , 45 lank. M., 386(387), 371 Janssens, J. H . G. J., 49(6), 96 Jannan, R. H., 232(70), 248, 312, 313(331), 314(345), 315(345), 363, 364, 370 Jeener, J., 349, 372 Jenkins, E. E., 244(98), 364 Jennings, T. J., 179(28, 29), I97 Jesson, J. P., 9(39), lO(43, 44,46), 11(46), 46 Jewur, S . S . , 160, 180(37), I97 Ji, X, H., 324, 371 Johnson, B. F. G., 21(57), 46 Johnson, 1. D., 343(464),373 Johnson, M.M.,89(80), 98 Johnson, R. N., 58(52, 54), 92(52, 54), 97 Jolly, P. W . , 5 , 8(33), 45 Jones, E., 58(61), 97 Jones, K., 8(33), 45 Jones, P., 52(22), 302, 303(270), 304, 96. 368 Jones. W., 335(450), 336(450), 340(465), 342(462), 343(458, 462). 344(462), 345(462), 346(458), 373 Jonson, B., 49(3), 55(3), 57(3), 96 Juckman, L. M., 134(38), I57 Junger, I., 279, 326(385, 386), 366, 371 Juranic, N. 0.. 369

K Kacirek, H., 300(266), 302(266), 368 Kaiser, S . W., 321(365, 366). 322(365), 371 Kakuta, N., 155(60), 156(60), I58 Kaplan, D. E., 212(24), 362

38 1

Karakchiev, I. G., 49(14), 96 Karapinka, G. L., 58(54), 92(54), 97 Karaulic, D. B., 369 Kirger, J., 300(259, 266, 267), 303, 304(260), 306(267, 294-296). 307(297), 308(296), 368, 369 Karol, F. J., 58(52-55), 92(52-55). 97 Kasai, P. H . , 302, 303(270), 304, 368 Kasatkina, L. A., 177(23),I97 Kassal, R. J., 4(9), 45 Katsumata, H.,184(53), I98 Kazansky, V. B.,54(25), 57(45), 96, 97 Kazusaka, A., 155(60), 156(60), I58 Kehl, W. L., 89(79), 98, 162(19), 177(19), 178(19), 180(19), 182(19). I97 Keim, W., 20,46 Kennedy, G. J., 242(84, 85, 87, 89). 244(89, 99), 245(89, 102), 246(99), 252(84, 85, 87), 260(84, 85). 286(84), 309(102), 326(384), 363, 364, 371 Kennedy, J. D., 218(46), 238(46), 362 Kentgens, A. P. M., 207. 208(15), 256(15), 260, 262(15), 263(15), 278(197), 355(422, 423). 361, 366, 372 Kerr, G . T . , 265(162), 289, 365, 374 Kersbergen, A. M., 49(5), 96 Keulks, G. W., 179(31), 180(31), 184(52), 197, I98 Khalafalla, D., 186(57), 192(57), I98 Khandelwal, B. L., 256(150), 272(150), 294( 150), 365 King, C . M.,8(29), 45 Kinsey, R. A., 208(18), 254(152), 347(432), 361, 364, 372 Kirchner, R. M.,237(81), 260(81), 363 Kirkbride, F. W., 3(4), 45 Kirkpatrick, R. J., 254(152), 364(434), 365, 372 Kiselev, V. F., 301(271), 368 Kiviat, F. E.. 320(362, 363). 371 Klinowski, J., 201(5), 208(19), 218(43), 222(58), 225(44), 226(56-58). 227(58), 228(58), 230(58), 231(58), 232(43, 44, 56, 71). 233(56, 75, 77). 236(77), 238(82, 90),239(82), 240(56, 471). 241, 242(90-92, 471). 243(471), 244(91), 245(56), 248(56. 91). 249, 250(115, 117). 251(44), 252, 257(155), 258(155), 259(155), 260(82, 156, 471), 261(155, 471), 265(163), 266(163), 267(163),

382

AUTHOR INDEX

281(206-209). 282, 283(206, 207). 284(57, 206, 208, 209). 285(207), 286(57, 209). 287(93, 21 I), 288(21 I), 289(21I). 298(56), 340(465). 346. 348(399),361. 362, 363, 364. 365, 367, 371, 373, 374 Knappwost, A., 295(221, 223), 305(285), 367. 369 Kokes, R. J . , 122, 139(46), 140(50), 146(50), 157, 158, 182(43), 183(46), 197 Kokotailo, G. T., 236(79), 242(89), 244(89. 98). 245(89, 102). 246(99), 309(102, 315). 363, 364, 369 Kolovertnov, G. D., 49(14), 96 Komiya, S., 29(67). 46 Kondo, T . , 128(35), 144(53, 54), 148(53, 56). 149(56), 157, 158 Koppi, A. J . , 344(490). 374 Korner, G., 297(232), 367 Kortbeek, A. G . T. G . , 245(100), 253(130, 131), 364. 365 Kotdnigawa, T., 308(478), 374 Koth, W., 219(48), 362 Krauss, H. L.. 49(13, 15), 50(13), 51(19), 53(24). 54, 55(15, 33-37, 41. 42). 56(15), 57(36), 61(63, 64),66(36), 67(15), 96, 97 Krenzke, L. D., 179(31), 180(31), 197 Kmeger, G., 12(50),46 Kruger, c., 8(33), 45 Kruger, .I. D., 349(400), 372 Krylov, O., 184(52), 198 Kubo. R., 305,368 Kubsh, J . E., 160(2), 196 Kiihl, G. H., 229(60, 61). 233, 363 Kumar, A., 349(403, 404),372 Kunath, D., 232(68), 363 Kunawicz, I . , 52(22), 96 Kundalkar, B., 169(8), 181(8), 1% Kundla, E., 207(14). 256(14), 361 Kung, H. H., 162(5), 164(5), 165(5), 167(6), 169(8, 111, 170(6, I I ) , 173(6), 174(6), 175(6, 18). 176(6). 177(19), 178(5, 24). 181(8, 181, 183(24), 185(54), 186(56), 189(54), 190(54), 192(56), 1%. 197, 198 Kung, M. C., 163(5), 164(5), 165(5), 169(8), 175(18), 177(19), 178(5, 24), 181(8, IS), 183(24), 196, 197 Kushnareva, E. G . , 69(73), 98 Kuznetsov, B. N., 327(348), 331(348). 370

Kzlividze, V. I . , 300(257, 258), 301, 368 Kwiatek, J . , 126(29), 157

L Lafer, L. I . , 180(32, 33), 182(32, 33), 197 Lahav, N., 345(469),374 Laing, M., 12(50),46 Landolf, G . R . , 217(33). 236(33), 362 LaProde, M. D.. 30(69), 46 Larsson, R., 49(2, 3). 55(2, 3). 57(2, 3). 93(2), 96 Lauterbun, P. C . , 219(48), 362 Lavrijsen, J . P. M . , 49(8), 67(8), 96 Lawton, S . L., 236(79), 363 Lechert, H., 296, 297( 190, 223, 300(190. 266, 268). 302(266), 305(285), 366, 367, 368, 369 Leftin, H. P., 276( 196). 347, 366, 371 Lehninger, A. L., 357(441), 373 Lenkinski, R. E., 348(399),371 Lester, G. D., 256(150), 272(150), 294(150), 365 Lewis, J . . 21(57),46 Liang, S. H. C., 202(6), 321, 361 Limbach, H. H., 349(400),372 Lin, M. J . , 175(16), 197 Lindenau, D., 55(42), 97 Lindsey, R . V . , Jr., 3(7), 6(7), 9(39), 14(7), 45, 46 Linn, W. J . . 8(29), 45, 162(27), 178(27), 184(51),197, 198 Lippmaa, E., 201(4). 207(13), 208, 219(51, 52). 220(52, 53), 221(54), 226(55), 231(55), 232(52, 68). 247(103), 249(16), 256(14, 16, 141), 257(141), 265(165167), 267(165, 166). 268(165, 167). 291(218, 219), 292(218), 293(218, 219). 294(2l9), 29x21 9), 325(376), 342(53), 344(52), 350, 361, 362, 363, 364, 365, 366, 367, 371. 372, 374 Lipscomb, W. N., 359(444, 445). 373 Lipsicas, M., 343(464), 373 Littlewood, A. B., 139(45), 158 Liu, X . , 233(75), 242(92), 287(21I ) , 288(21I), 289(21I), 363, 364. 367 Lo, C., 186(58), 192(58), I98 Lock, C. J . L., 348(399), 371 Lodge, E. A., 232(69), 363 hewenstein, W . , 215, 362

AUTHOR INDEX

Lohse. U., 226(55), 231(55), 265(165-167), 267(165, 166, 172), 268(165, 167), 271(172), 297(234), 362, 366, 367 Lok, B. M., 216(30. 31, 65), 289(30), 362, 363 Lok, S . M., 349(400), 372 Lokhov. Yu. A., 183(44), 197 Lorenzelli, V., 180(34), 197 Lovel, C. G . , 21, 46 Low, M. S . D., 139(49), I58 Lowe, I. J., 203(9), 208(9), 361 Luckevich, L., 347(395), 348(355), 349(395), 370, 371 L U ~H.-O., , 20(54), 46 Lund, C., 185(55), 189(55, 64),192(64), 198 Lundsford, J. H.,271, 280(202), 281(202), 323(372), 366, 371 Lunsford, J. H.,57(47, 50). 97. 175(16), 181(39),197 Lussier, R. J., 345(468), 374 Luz. Z . , 327(388), 371 Lyerla, J. R., 340(457), 347, 348(355), 349(395), 370, 371, 373 Lynch, T. J., 89(79), 98

M McDaniel, C. V., 264, 365 McDaniel, M. P., 50(18), 51(18), 52(18, 23), 53(18, 44). 54(30), 55(30), 57(30), 58(62), 61(67), 62(69), 64(69), 69(69), 75(67), 76(75), 81(76), 84(77), 85(44), 89(62, 78, 80), 92(81), 96, 97, 98 McDowell, C . A., 251(121, 126), 364 MacEwan, D. M. C., 343(467), 373 McFarlane, W., 218(46), 238(46),362 McGlinchey, M. J., 9(41), 46, 93(82), 98 McGovern, I. T., 118, 157 McKay, D. R., 323(370), 324(370), 371 McKeever, L. D., 272(182), 294(182), 366 MacKenzie, K. J. D., 344(487), 374 MacKenzie, N. E., 358(442, 443), 359(443), 360(443), 361(443), 373 McKinney, R. J., 23, 25(63), 27(63), 46 McLauchlan, K. A,, 300(243), 301(243), 368 Machado, F., 271(176), 366 Macho, V., 348(356), 370, 371, 372 Maciel, G . E., 323(373), 324(370, 373). 325(371), 326(349, 377, 379-383). 327(349), 347, 351(429. 430). 354,370, 371, 372

383

Macura, S. I., 369 Magee, J. S . , 345(468), 374 Maggs, F. A. P., 31 1(325), 370 Magi, M., 201(4), 219(52), 220(52, 53). 221(52, 54), 226(55), 231(55), 232(52), 265(165-167), 267(166), 268(165, 167), 291(218, 219). 292(218), 293(218, 219), 294(219), 295(219), 325(378), 343(53), 344(52), 361. 362, 366. 367, 371 Mahan, J. A,, 3(5), 45 Maher, P. K., 264, 365 Mahtab, R., 61(65). 70(65), 74(65), 97 Maiwald, W., 300(268), 368 Makowka, C. D., 212(26), 362 Malathi, N., 188(60), 192(60), 198 Malli, G., 314(335), 370 Malthouse, J. P. G., 358(442),373 Manzer, L. E., 8(31), 45 Maraschin, N. J., 58(55), 92(55), 97 Marchetti, L., 180(34), 197 Margolis, L. Ya.. 184(52), 198 Maricq, M. M., 208(20), 361 Mars, P., 160(1), 196 Marsmann, H.,218(45), 362 Martir, W., 181(39), 197 Mason, R. B., 233, 363 Massoth, F. E., 169(12), 184(12), 197 Mastikhin, V. M., 347, 372 Masuda, A., 344(492), 374 Matsumoto, M., 341(466), 373 Matsuura, I., 162(26, 41), 178(26), 182(41), 184(41), 297 Maximov, Yu.,184(52), 198 Maxwell, I. E., 265(164), 266, 307(301), 308(301),365, 369 Meadows, M. D., 208(18), 361 Meakin, P., lO(43, 44,46), 11(46), 46 Mehring, M., 204(10), 350, 351, 361 Meier, M.,7(23), 9(23), 45 Meier, W. M., 217(34), 232(66), 233(73), 236(79), 244(98), 252, 362, 363. 364, 365 Meiler, W., 279(201), 307(302, 306). 308(302),366, 369, 374 Meinhold, R. H.,344(487), 374 Melchior, M. T., 226(59), 229(59), 231, 232, 233(76), 234(76), 235(76), 251, 298, 312, 313(331), 314, 315(345),363, 364, 367, 370 Merryfield, R., 54(30), 55(30), 57(30), 96

384

AUTHOR INDEX

Messina, C. A,, 216(30, 31, 65), 289(30), 362, 363 Mestrom, P. L. M., 49(5), 96 Miale. 1. N., 289(481), 374 Michael, A., 307(302), 308(302), 369 Michel, D., 279, 281(199, 200), 307(298, 302. 306), 308 (300, 302, 309), 366, 369, 371 Miesserov, K. G., 54(27), 96 Mikhaltchenko, V. G., 183(45), 197 Mikovsky, R. J., 231, 363 Mildebrath, M., 27 1, 366 Miller, G. R.. 307(303), 308(303). 369 Millward, G. R., 309(316-318, 321). 310(477), 369, 374 Misono, M., 162(14), 172, 177(14, 20. 21). 178(14, 20), 179(14), 181(14), 182(20), 183(20), 197 Mitchell, S. A., 52(20), 53(20), 96 Miura, M., 190(66), 198 Miyahara, K., 116(9), 121(17), 123(23), 128(23), 138(43), 139(48), 144(55). 146(48), 147(17). 148(53), 153(43), 154(43), 155(60), 156(60), 157, 158 Miyamoto, A., 190(65-68), 192(65-68). 198 Mobley, M. J., 327, 371 Moeck, H.J., 252, 364 Moeseler, R., 55(42), 97 Moffat, K. A., 9(41), 46 Molgedey, G., 219(50), 248(107), 362, 364 Mollbach, R., 6(21), 45 Montez, B., 347(432), 372 Moolenar, R. J., 272(182), 294(182), 366 Morgenstern, H.J. B., 49(7), 96 Mori, K.,190(65-68). 192(65-68), 198 Morterra, C., 49(12, 16). 50(12), 55(16), 61(66), 65(16), 67(16), 69(16), 70(66), 7466). 96 Moms, G. A., 350(409), 372 Morvillo, A., 29(66), 46 Morys, P., 49(15), 55(15, 41), 56(15), 67(15), 96,97 Mosel, 6.D., 219(47), 362 Muetterties, E. L., lO(43. 44). 46 Mulay, L. N., 186(58), 192(58), 198 Mliller, D., 249(114), 255, 256(140, 154), 258, 290,364. 365 Mliller, L., 349(404), 372 Muller-Warmuth, W., 219(47). 362 Munowitz, M., 353(420). 372

Murakami, Y.,190(65-68), 192(65-68), 198 Murata, Y., 117, 118(12), 123(12), 157 Murday, J. S . . 300(261), 305(286), 368, 369 Murdoch, J. B . , 356(427), 357(427), 372 Murphy, W. J . , 242(86-88). 252(86, 87, 88), 363 Myers, D. L., 57(4?), 97 Myhre, P. C . , 348(356, 396). 349, 370. 371, 372 Mynott, R., 6(21), 45

N Naccache, D., 217(40), 362 Nagy, J. B., 307(303), 308(303, 307). 308(311), 309(313), 312, 367, 369, 370 Naito, A., 251(121, 126). 364 Naito, S., 139(47), 158 Nakamura, A., 29(67), 46 Neel, L., 188(61), 192(61), 198 Neff, B. L., 350(411, 414), 3 5 ~ 4 I1, 414). 352(414), 372 Nichols, J. A,, 254(152), 365 Niedermeyer, R., 350(406, 408). 372 Nihira, H.,138(42), 139(42), 157 Nininger, R. C . , 188(62), 192(62), 198 Norton, P. R., 155(64), 158 Nowood, D. D., 58(62), 89(62), 97 Notermann, T . , 179(31), 180(31), 184(52), 197, 198 Nozawa, Y.. 162(14), 172(14), 177(14, 20), 178(20), 181(14), 182(20), 183(20), 197 Nuijten, C. E., 49(5), 96

0 Obermeyer, R. T., 186(58), 192(58), 198 O’Brien, D. H.,124(25), 157 Occhiuzzi, M., 175(17), 197 O’Keefe, M., 236, 363 Okuhara, T.,106(6), 107(36), 108(6), 109(7, 44). 111(8), 112(8, 37, 44), 115(14, 37). 116(9), 119(6, 14, 37), 121(17), 128(14, 31, 36, 37). 129(36), 130(6), 131(6), 133(36), 135(14, 37), 136(8, 14), 138(44), 139(31, 48). 144(51-54), 145(51, 52). 146(48), 147(17), 148(53, 56), 149(56), 151(31). 157, 158 Olah, G. A., 158, 347, 371 Oldfield, E., 208(17, 18), 254, 346(434, 433, 347, 361, 364, 372, 373

385

AUTHOR INDEX

Olson, D. H., 217(34), 229(60), 236(79), 362, 363 O'Neil, R. M., 336(454). 373 O'Neill, T. G . , 3(4), 45 Onishi, T., 139(47), 158 Onuferko, J. H., 331(358), 332(358), 370 Opella, S. J., 251(120, 123, 124), 350(413), 364, 372 O'Reilly, D. E., 276, 366 Osborn, J. A., 120, 139(16), 142(16), 144(16), 146(16), 147(16), 157 Osipova, Z . G., 181(38), 197 Oskovie-Tabrizi. M., 186(58), 192(58), 198 Osredkar, R., 353(420), 372 Ozaki, A., 138(42), 139(42), 157 Ozubko, R. S., 242(86-88). 252(86, 88), 363

P Packer, K. J., 251(119, 122), 300(238), 358(122), 364, 367 Pankau, H., 297(233), 367 Pankratiev, Yu. D., 182(42), 183(42), 197 Pad, X.,300(241, 242, 244-246), 367, 368 Park, A. W., 118(13), 157 Parks, G., 54(30), 55(30), 57(30), 96 Parravano, C., 300(252), 303, 368 Parshal, C . W., 156(63), 158 Parshall, G. W.. 2(1), 45 Pass, G.. 139(45), 158 Patel. I., 336(454), 373 Patterson, R. L., 300(261), 368 Patton, R. L., 216(65), 237(81), 260(81), 363 Patzelovh, V., 265(166, 167), 267(166), 268( 167), 366 Paul, I. C . , 348(399), 372 Payne, N. C . , 12(49).46 Pearson, R. G . , 7(23), 9(23), 45 Pecherskaya, Y. I., 54(25), 96 Peglar, R. J., 52(21), 96 Pehk, T. J., 219(51), 362 Pennington, K. S . , 255(145), 365 Petch. H. E., 255(142, 145). 365 Peter, R., 219(50), 248(107), 362, 364 Peters, A. W., 231, 363 Petrakis, L., 320(362, 363), 323(370), 324(370), 371 Pettit, R., 156(62), 158 Heifer, H., 252(153), 259(157), 265(168171). 269(168), 270(169), 271(169, 183). 273(188, 191), 275(171), 277(153, 170,

171). 278(171), 279(171, 200, 201). 281(200, 201), 282(168, 170), 297(234), 300(188, 249-251, 253, 263, 264, 269). 302(249-25 I , 263, 264), 303(276-278), 306(294, 296). 307(297, 302), 308(296, 302, 309, 479), 326(387), 365, 366, 367, 368, 369, 371, 374 Pictroski, C. F., 298(235), 367 Pines, A., 209(21), 219(49), 327(352), 328(352), 355(426), 356(426, 427, 428), 357(427, 428), 362, 370, 372 Pines, H., 122, 157 Pinnavaia, T. J., 300(440). 335(447), 343(464), 373 Pioli, A. J. P., 58(61), 97 Plee, D., 343(461), 373 Pliskin, W. A., 139(49), 158 Ploss, W., 308(479), 374 Pluth, J. J., 232(67), 242(96), 271, 363, 364, 366 Ponomareva, T. M.,233, 363 Pople, J. A,, 131(38), 157 Popovskii, V. V., 183(47), 197 Post, J. G . , 254(134), 278(198), 366 Pound, R. V., 300(236), 305(236), 367 Powell, J., 21, 46 Prager, D., 271(183), 366 Pratt, B. C . , 3(6), 45 Pribilov, A. A., 296(228), 367 hhevalskaya, L. K., 57(45), 97 Przyborowski, F . , 300(249, 250, 253). 302(249, 250), 368 Puentes, E., 5, 45 Pullukat, T. J., 76(74), 98 Purcell, E. M., 300(236), 305(236), 367 Puri, S., 188(60), 192(60), 198 Purnell, J. H., 335(448, 450, 451), 336(450, 454), 342(462), 343(462), 344(462). 345(462), 373 Pustowka, A,, 186(59), 192(59), 198 Putnis, A.. 208(19), 361

Q Qin, G., 271(176, 177), 366 Quiocho, F. A., 359(444). 373

R Rabo, J. A,, 217(42), 362 Radeglia, R., 248(108), 249, 364

386

AUTHOR INDEX

Rhlek, M., 305(285), 369 Ralison, M.C., Jr., 347(391), 371 Ramdas, S., 222(58), 226(56-58). 227(58), 228(58), 230(58), 231(58), 232(56), 233(56, 77), 236(77), 238(82), 239(82), 240(57), 241(57), 242(91), 244(91), 245(56), 248(56, 91), 249, 250(1IS), 260(82), 284(57), 286(57), 298(56), 309(321). 346(57), 363, 364, 369 Randall, 1. C., 62(69), 64(69), 69(69), 97 Rao, V., 186(58), 192(58), 198 Rauscher, H. H., 371 Rauscher, M.,306(296), 307(297), 308(296), 369 Rausell-Calom, J. A,, 343(460), 373 Ray, T., 347(432), 372 Rayanakorn, M.,336(454),373 Rayment, T., 340(465), 373 Raymond, M.,255(347-149), 257(148),365 Raythatha, R. H., 343(464), 373 Razouk. R. I . , 311(324), 318(324), 370 Read, J. F.. 57(48), 97 Rebenstorf, B.,49(2-4). 55(2-4, 34, 36). 57(2, 3, 36), 66(36), 93(2), 96, 97 Reddy, G. S., 156(63), 158 Reeke, G. N., Jr., 359(444), 373 Regent, N. I., 267(172), 271(172), 366 Reichle, W. J., 58(55), 92(55), 97 Reif, F.. 269(173), 366 Rennard, R. J., 162(19), 177(19), 178(19), 180(19), 182(19), 197 Renouprez, A. J., 317(401), 372 Resing, H. A., 300(237, 239, 248, 254, 261, 262. 440),301(248), 302(239, 248), 305(286), 307(303), 308(303), 327(352), 366, 367. 368, 369, 370. 373, 328(352) Reutter, D. W., 6(19), 45 Revillon, A., 49(10), 54(10), 96 Richmond, T. G . . 18(51), 20(51), 46 Riedel, E., 303(278), 368 Riedel, J., 300(260), 304, 368 Riggs, W. M.,8(29), 45 Riley, P. E., 156(62), 158 Rinaldi. R . , 242(96). 364 Ripmeester, J. A., 316, 317(346, 347), 323, 370, 371 Road, J . F., 122(18), 157 Roberts, 1. E., 355(421), 372 Roben, J.-L., 343(464), 373 Robinson, P. A., 58(61), 97

Rochester, C. H.,54(28), 96 Rodenas, E., 184(53), 198 Roe, D. C . , 25(63), 27(63), 46 Rogers, A. S . , 125(28), 157 Rohrbaugh, W. I . , 244(98), 364 Rooney, J. J., 61(65), 70(65), 74(65), 97 Rosenberger, H., 326(386), 371 Rothwell, W. P.,280(202), 281(202), 323(372),366, 371 Roy, R., 260(156), 365 Ruben, D. J., 353(419, 420). 372 Rubin, A., 348(399), 372 Rubinshtein, A. M.,180(32, 33). 182(32, 33), 197 Rubinstein, M.,366 Ruckenstein, E., 160, 180(37), 197 Russell, C. R., 124(25), 157 Riistig, J., 155(64). 158 Ruta, M.,300(265), 302(265), 304(265), 368 Rybaczewski, E. F., 350(414), 351, 352(414), 372

S Sachatler, W. M.H., 102(4), 157, 179(30), 197 Saito, H.. 344(492), 374 Saito, S., 128(35), 157 Saito, T., 9(38), 46 Saito, Y . , 373 Sakata, K., 162(14), 172(14), 177(14, 21). 178(14), 179(14). 181(14), 197 Sakurai, Y.,139(47), 158 Salmeron, W.. 101(2), 157 Salzer, K.,308(310), 369 Samoson. A., 201(4), 207(13, 14). 208, 219(52), 220(52, 53). 221(52, 54), 232(52, 68), 249(16), 256(14, 16. 141). 257, 265(165), 267(165), 268(65), 342(53), 344(52). 361, 363, 365, 366, 374 Samoson, A. V., 325(376), 371 Sanulevit, N. M.,306(296), 308(296). 369 Sanz,J., 341, 343(460, 483, 486). 373, 374 Sato, S . , 116(9), 157 Satterfield, C. N.. 160 Sauer, J., 326(385), 371 Saupe, A., 301, 368 Sawicka, B., 186(59), 192(59), 198 Sawicki, J . , 186(59), 192(59), 198

AUTHOR INDEX Say, 9. J., 2511119, 122), 358(122), 364 Sazonov, 9. A,, 183(47), 197 Scarpiello, D. A., 169(12), 184(12), 197 Schaefer, J., 21 1, 308(308),362, 369 Scheherbakov, V . N . , 365 Scheler, G., 256(139, 140). 258(139), 259(157), 265(169, 170). 270(169). 271(169), 277(170), 279(170), 326(386). 365, 366, 371 Scheller, D., 279(199), 281(199), 366 Schirmer, W., 267(172), 271(172), 297(234), 300(251, 253). 302(251), 303(277),366, 367, 368 Schissler, D. 0.. 138(39), 157 Schlenka, J. L., 244(98), 309(315), 369 Schlenker, J. L., 244(97. 98). 254(97), 364 Schlogl, R., 340(465), 343(458), 346(458), 373 Schlosser, M., 124, 157 Schlusberg, R. H . , 158 Schmiedel, H . , 265(168), 269(168), 282(168, 170), 296(228), 297(233, 234). 366, 367 Schneider. 9 . . 302(273), 368 Scholle, K. F. M. G. J . , 207( IS), 208( IS), 256(15), 260(15), 262(15), 263(15, 438), 278, 294, 361, 366, 367, 373 Schollner, R . , 297(232), 306(294), 367, 369 Schoolery, J . N., 303(274), 368 Schramm, S . , 208(17, 18). 361, 372, 373 Schreiber, L. B.. 275. 366 Schrobilgen, G. J . , 314(334), 370 Schroeber, D., 188(62). 192(62),198 Schuit, G. C . A., 490-9), 61(9), 62(9), 67(8, 9). 96, 162(26), 178(26), 182(40), 183(40), 197 Schulze, W. A., 3(5), 45 Schunn. R. A,, 9(37), 46 Schwartz, J. I.. 331(360), 370, 371 Schwartz, L.. 186(56), 192(56), 198 Scott, A. I . , 358(442, 443). 359(443), 360(443), 361(443), 373 Scurrell, M. S., 54(28), 96 Seidel, W. C., 5(10), 6(18, 19). 7(22), 8(26, 27), 9(22), 10(47), 12(47), 25(47), 27(47), 30(47), 38(47), 45, 46 Seisho. M.,123(22), 128(22), 157 Seiyarna, T., 169(9), 170(9), 196 Selbin, J., 233, 363 Sergeeva, A. S . , 160(3),196 Serpinsi, V. V . , 301(271), 368

387

Serratosa, J. M.,343(483, 486), 373, 374 Seyd, W . , 303(276), 368 Seyler, J. K . , 126(29), 157 Shabtai, J., 345(469),374 Shani, U., 345(469), 374 Shatlock, M. P., 346(434-436). 347, 372 Shcherbakov, V. N.. 255(137), 262(137),365 Shen, J., 158 Shen, W . , 280(202), 281(202), 366 Shen, W. X., 323(372), 371 Shepelin, A. P., 181(38), 197 Shefinski, J. S ., 350(414), 351(414), 352(414), 372 Shida, M.. 76(74), 98 Shimizu, H., 344(492), 374 Shimokawa, K., 308(478), 374 Shinoda, S., 341(466), 373 Shriver, D. F.. 18(51, 52). 20(51, 52), 46 Shrock, R. R., 120. 139(16), 142(16), 144(16), 146(16), 147(16), 157 Shvets, V. A . , 57(45), 97 Siegel, S., 117, 138(10), 157 Simizu, H., 139(47), I58 Simons, Th.,182(40), 183(40), 197 Sindorf, D. W., 325, 326(349, 377, 379383). 327(349), 370, 371 Sinfelt, J . H., 102(3), 157, 212(26), 331(359), 333(359), 334(359), 335(359),362, 370 Sinton, S . W., 356(428), 357(428), 372 Skjemstad, J. 0..344(463, 490). 373, 374 Skliarov, A , , 184(52), 198 Slack, D. A . , 242(86-88), 252(86, 87, 88). 363 Slaugh, L. H., 128(33, 34). 155(33), 157 Sleight, A . W., 162(27), 178(27), 184(51), 197, 198 Slichter, C. P.,212(25, 26). 331(359). 333(359), 334(359), 335(359), 362, 370 Slotfeldt-Ellingsen, D., 300(440), 373 Smith, D. J. H., 336(455), 373 Smith, G. W., 243(439), 373 Smith, J. V., 232(67), 237(81), 242(96), 248(105), 260(81), 271, 363, 364, 366 Smith, K. A., 208(18), 254(152), 347(432), 361, 365, 372 Smith, P. D., 58(56, 57, 62). 89(62), 92(81), 93(83), 95(56), 97, 98 Smyth, S . M.,271(176), 366 Sokolovskii, V. D., 181(38), 183(45),197 Solomon, I., 273(192), 366

388

AUTHOR INDEX

Somorjai, G. A., 101(2), 157 Sonnenberger, R.,326(386), 371 Sorlino. M., 180(34). 197 Spitz, R.,49(10, I I ) , 54(10, I I ) , 96 Springuel-Huet, M. A., 314,(339, 341). 316(339). 317(34I ), 318(34I), 3 l9(341), 3 70 Stach, H., 54, 96, 267(172), 271(172), 297(234), 300(249, 250, 251, 253). 302(249, 250, 251), 366, 367, 368 Stark, R. E., 353(419. 420). 372 Starke, P., 325(378), 371 Staudte, B., 308(479), 326(385), 371, 374 Stejskal, E. O . , 21 I , 306(291, 292). 308(308), 362. 369 Stevenson, R. L., 274, 275(194), 296(194). 305, 366 Stewart, C. D., 61(65), 70(65), 74(65), 97 Stimson, R. E.. 18(52), 20(52), 46 Stoddart, C. T. H., 139(45), 158 Stoll, M. E., 350, 372 Stone, W. E. E.. 341(485), 366. 374 Strause, S. H.,18(52),20(52), 46 Stradella, L., 49(16), 55(16), 65(16). 67(16), 69(16), 96 Stucky, G. D., 271(180). 272(180), 366 Sudmeiyer, O., 291(217), 367 Summer, C. E., 156(62), 158 Swift, H. E., 21, 34(72), 46 Swinnen, H. P. M.,49(9), 61(9), 62(9), 96 Szeverenyi, M., 351(429, 430). 372 Szeverenyi, N. M., 354, 372

T Tabony, J., 320(364), 371 Takahashi, H.. 341(466), 373 Takeuchi, A.. 138(43), 153(43), 154(43). 158 Tamaru, K., 128(35), 139(47), 157, 158 Tanabe, K., 109(7). 128(32), 139(32), 152(57). 157, 184(53),198 Tanaka, K., 106(6), 107(36), 108(6), 109(7, 44). I l l @ ) , 112(8. 37, 44),115(14, 37), I W ) , 119(6, 14, 37). 121(17), 123(22, 23), 124(24), 127(30), 128(14. 22, 23, 31, 36, 37), 129(36), 130(6), 131(6), 133(36), 135(14. 37). 136(8, 14). 138(41, 42, 43, 44), 139(31, 42, 48), 144(5155). 145(51, 521, 146(48), 147(17), 148(53, 56). 149(56), 151(31), 153(43), 154(43). 157, 158

Tanaka, Y., 128(32), 157 Tanner, J. E., 306(291),369 Tanner, S . F., 251(119, 122), 358(122),364 Tarmak, M., 221(54). 226(55), 231(55), 236(68), 247(103), 263165). 267( 165), 268(165),362, 363, 364. 366 Tashim, J . , 123(23), 128(23), 157 Tatibouet, J. M., 191(69, 70, 71), 192(69, 70, 71). 198 Tatsumi, K., 29, 46 Tau, K. D., 331(358), 331(358), 370 Taylor, B. W . , 34(72), 46 Taylor, K. C . , 57(48), 97, 122(18), 157 Tchir, W., 348(399),372 Tebbe, F. N., 10(43), 46, 156(63), 158 Teichener, S . J . , 169(13), 197 Tennakoon, D. T. B., 242(91), 244(91), 248(91), 335(450). 336(450), 340(465), 343(458), 346(458, 47% 364, 373, 374 Terasaki. 0.. 309(318), 369, 374 Thamm. H.,267(172), 271(172), 366 Theopold, K. M., 156(61),158 Thiel, C. P. A,, 155(64), 158 Thomas B., 279(199), 366 Thomas, J. M., 201(1, 51, 208(19), 218(43, 44). 222(58), 226(56-58). 227(58), 228(58), 230(58), 231(58), 232(43, 56, 69, 71). 233(56, 71, 75). 236(77). 238(82, 90),239(82), 240(57. 471). 241(57). 242(71, 92), 243(471). 244(91), 245, 248(56, 91), 250(117). 257(155), 258(155), 259(155), 260(82, 471). 261(155. 471), 265(163), 266(163), 267(163), 272(150), 281(199, 206-209), 282(206-209), 283(206, 207). 284(57, 206). 285(207), 286(57, 209), 287(93). 298(56), 309(316-321). 310(477), 335(446, 448, 450, 451). 336(450, 452454), 337(470), 338(470). 340(457, 465). 341(453, 466). 342(462), 343(458, 462). 344(462), 345(462. 470). 346(57, 458). 347(320), 348(399), 361, 362, 363, 364, 365, 367, 369, 372, 373, 374 Thompson, J. K.,300(248, 254, 261). 301(248), 302(248), 305, 368, 369 Tolman, C . A., 5(10), 6, 7(17, 22, 24, 25). 8(25-29, 31), 9(22, 36, 40, 42). lO(45, 46, 471, Il(46). 12(14), 13(45), 16(42), 17(42), 20(53), 22(42, 53, 601,25(47). 27(47), 36(47), 38(47). 43(73), 45. 46

389

AUTHOR INDEX Tomilov, N. P.. 233(74), 363 Tomita, K.. 305, 368 Torrey, H. C . , 305, 369 Tossell, J. A,, 249, 364 Toyoshima, I . , 123(23), 128(23), 157 Trewella, I. C . , 244(97), 253, 254(97). 364, 365 Tripodi, M. K., 308(308), 369 Tsay, Y.-H., 8(33), 45 Turco, A,, 29(66), 46 Turkevich, J., 138(39), 157 Turner, D. L., 350(406-409). 372 Turner, N. H., 300(261),368 Twigg. G . H.. 138(40), 157 Tyler, A. I . . 52(20), 53(20), %

U Ueda, F., 162(14), 172(14), 177(14, 21), 178(14), 179(14), 181(14), 197 Unger, K., 300(440), 373 Uytterhoeven, J. B., 281(203), 282, 366

V Van de Maesdiyk, C. G . M., 162(26), 178(26), 197 Van den Berg, J. P., 308(312), 369 Van de Ven, L. J. M., 254(134). 364 Van Der Velden. G. P. M., 278(197), 294(220), 327(350), 366, 367. 370 Van Dongen, J. P. C. M.. 307(301), 308(301),369 Van Erp, W. A., 265(164), 266(164), 281(210), 282(210), 31 1(329), 312(329), 365, 367, 376 Van Heugten, A. H., 49(7), 96 Van Heumen, C. J. M., 49(7), 96 Van Hooff, J. H. C., 254(134), 278(198), 308(312), 365, 366, 369 Van Kasteren, P. H. G., 291(217), 367 Van Krevelen, D. W., 160(1), 1% Van Reijen, L. L.. 54(26), 96 Van Santen, R. A.. 311(329), 312(329), 370 Van Willigen, H., 353(420), 372 Vasudevan, S . , 201(5), 281(206), 282(206), 283(206), 284(206), 361, 367 Vaughan, D. E. W., 226(59), 229(59), 231(59), 232(70), 233(76), 234(76), 235(76), 298(235), 314(345),3 13345). 345(468), 363, 367, 370, 374

Vaughan, R. W., 275, 308(480). 350(415), 366, 374 Veeman, W. S., 207(15), 208(15), 256(15), 260(15), 262(15), 263(15), 278(197, 198), 294(220), 355(422. 423), 361, 366, 367, 372, 373 Vega, A. J., 231, 307, 350(415),363, 369, 3 72 Vega, S., 355(421), 372 Verheijen, E., 182(40), 183(40), 197 Vickerman, J. C . , 271(176, 177), 366 Vieth, H. M., 349(400), 372 Villiger, H., 242(95), 364 Volkoff, G. M., 255(142),365 Volta. J. C . , 191(69, 73). 198 Von Lampe, F., 247(103), 364 Von Schleyer, P., 348(398), 371 VufeliC. D. R., 369

W Walker, D. L., 58(51), 97 Walter, A., 306(293), 369 Wang, P.-K., 331, 333, 334, 335, 370 Ward, M. B., 175(16), 197. 271. 366 Ward, M. D., 331(360), 370, 371 Wasylishen, R. E., 203(7), 329(353), 348(399), 361, 370 Watanabe, T., 344, 374 Waugh, J. S . , 208(20), 209(21), 219(49). 350(410-414). 351(41I , 414), 352(414). 361, 362. 372 Weber, D. C., 300(440),373 Weiss, A., 336(456), 373 Weisser. J., 20(54), 46 Weisser, B., 55(35), 97 Weisz, P. B., 217(32), 362 Weitekamp, D. P., 355, 356(426, 427). 357(427), 372 Welch, M.B., 55(44), 76(75), 81(76), 85(44) 97, 98 Welti, D.. 349(403), 372 Wendtlandt, K.-P., 326(387), 371 Werner, H., 30(68), 46 Wernsen, A,, 49(9), 61(9), 62(9), 67(9), 96 West, G. W., 245(101), 247(101), 297, 298(231), 299(231), 309, 310(333), 364, 367, 370 West, R-C., 8(29), 45 Westlake, D., 336(455), 373 Westphal, U., 53(24), 55(36, 37). 57(36), 66(36), 96,97

390

AUTHOR INDEX

Wetzels, M. L. J. A., 49(6), 96 Whipple, E. B., 300(265), 302(265), 304. 368 White, D., 243(439), 373 Whitman, G. M., 3(6), 45 Wieker. W., 325(378), 371 Wilke, G.,5 , 6(21), 8(32), 45 Williams, D., 255(144), 365 Williams, K. J., 336(454), 373 Williams, R. H., 118(13), 157 Williamson, L. J., 335(450), 336(450), 373 Williams-Smith, D. L., 93(82), 98 Wilson, A. E., 281(210), 282(210), 367 Wilson, M. A., 344(489), 374 Wilson, S. T . , 216(31). 362 Winkler. H., 300(260), 303(278), 304(260), 368 Winslow, P.. 330(472, 473, 474), 374 Winter, E. R. S., 177(22), 197 Wise, H., 100, 101(1), 157 Wittgen, P. P. M. M.. 49(5-9). 61(9). 62(9), 67(8, 9), 96 Wittern, K.-P., 305, 369 Woessner, D. E., 244(97), 248, 253, 254(97), 364, 365 Woessner, D. W., 305(287). 369 Wolthuizen, 1. P . , 308(312), 369 Wong, L-Y., 32(71), 46 Wood, B. J., 100, 101(1), 157 Wright, P. A., 242(94), 260(156), 364, 365 Wu, C . . 58(54. 5 3 , 92(54, 55), 97 Wu, C . Y., 21, 46 Wyatt, R. J., 58(61), 97 Wynn, V. T . , 204(11), 361

Y Yakerson, V. I., 180(32, 33). 182(32, 33). 197 Yamamoto. A., 29(64, 65, 67). 46 Yamamoto, N., 188(63), 192(63), 198 Yamamoto, T., 29(64, 65, 67), 46 Yamazoe, N., 169(9), 170(9), 196 Yang, B. L., 167(6), 169(7, 11). 170(6, It), 173(6), 174(6), 175(6, 18), 176(6), 178(24), 179(7), 181(18). 183(24), I85(54). I86(56), 188(7), 189(54),

190(54), 192(56), 193(7), 195(7). 196, 197, 198 Yannoni, C . S . , 347(394, 395). 348(355, 356, 396). 349(395, 400), 370. 371, 372 Yates, D. J. C., 102(3), 311, 157. 370 Yermakov. Y ., 58(59, 60),69(72, 73), 92(59, 60),95(59), 327(348), 331, 97, 98, 370 Yesinowski, J. P.. 327, 371 Yoda, Y . , 169(9). 170(9), I96 Yokoyama, S., 123(22, 23). 124(24), 128(22, 23), 157 Yoneda, Y., 162(14), 172(14), 177(14,20, 211, 178(14, 20). 179(14), 181(14), 182(20), 183(20), 197 Yorke, W., 120(15), 157 Yoshida, T., 308(478), 374 Yoshids, K . , 123(23), 128(23), 157 Young, D., 243(439), 373 Young, R. H., 251(125), 364 Yur’eva, T.M., 160(3), 162(7), 196

Z Zakharov, V., 58(59), 69(72, 73), 92(59), 93.59). 327(348), 331(348), 97, 98, 370 Zamaraev, K. 1.. 347, 372 Z ~ XD.. , 355(426), 356(426, 427, 428), 357(427, 428), 372 Zecchina, A., 49(12). 50(12), 55(38, 39, 40). 61(66), 69(40), 70(66), 74(66). 96, 97 Zerlica, T . , 180(34), 197 Zeya, M.,347(395), 348(355), 349(395),370, 371 Zhdan, P. A., 181(38), 197 Zhdanov, S. P . , 300(263. 269), 302(263), 303(277), 306(293, 296), 308(296), 368, 369 Zilm, K . , 355(426), 356(426, 427). 357(427), 3 72 Zimmermann. A. Z., 125(27), 157 Zimmermann, H., 356(427), 357(427), 372 Zimmermann, J. R., 302, 368 Ziolkowski, 1.. 192(72), 198 Zumbalyadis, N., 251(125), 364 Zwaans, P. J. C. I. M.. 49(7), 96

Subject Index A Acetylene, 13C nutation-NMR spectroscopy of, 333-335 Acids, reaction with nickel phosphite complexes, 9-10 Adiponitrile, synthesis, 2-3, 4 ADN, see Adiponitrile Albite, 255 Alkenes, unpromoted hydrocyanation, 25-31 Alkylallyl complexes, as catalysts, 122- 128 a-Allylic nickel intermediates, 15-18, 22 Alumina, in polymerization catalyst, 88-89 Aluminosilicates framework composition, 226-228 resolving crystallographically nonequivalent tetrahedral sites, 240-25 1 29Si MAS NMR studies of, 226-228 29Si NMR studies of, 218-225 Aluminum isotope NMR properties, 213, 274 in NMR studies of zeolites, 254-264 in sheet silicate studies, 342-345 Aluminum phosphate, in polymerization catalyst, 89-92 Andalusite, 255 Andrews-Beams turbine spinner, 209 Associative mechanism deuterium distribution and, 125-126 in hydrogen exchange reaction, 132 in isomerization reaction, 104-106

B Beidellite, idealized formula, 337 Beryl, 255 Bond length, determination, 333-335 Boron isotope, NMR properties, 213, 274 Butadiene, reaction with hydrogen cyanide, 14-18

Butenes, 22, 104-128, 131, 135 selective oxidation dehydrogenation on ferrite catalysts, 159-196 n-Butylamine, adsorption to silica-alumina gels, 320-327

C Cadmium isotope, NMR properties, 213, 274 Cancrinite, 224, 258 Carbon isotope NMR properties, 213, 274 in nutation-NMR spectroscopy, 333 in sheet silicate studies, 340-341 Carbonium ion, NMR studies of, 347-349 Catalyst, hydrogenation cooperative active sites, 101 specific activities of alloys, 101-103 theory, 99-101 Catalytic reaction active-site control in hydrogenation step, 137-156 in hydrogen exchange step, 128- I37 in isomerization step, 104-128 theory, 99-104 definition, 99 Chabazite, 215, 224, 258 Chemical shift anisotropy, 204-205 Chromate, in Phillips catalysts, 48-49 Chromium, hydroxyl replacement, 49-5 1 Chromium (II), in chromium/silica catalyst, 54-57 Chromium (III), in chromium/silica catalyst, 57-58 Chromium (V), in chromium/silica catalyst, 54 Chromium oxide on alumina, 88-89 on aluminum phosphate, 89-92 on silica, see Chromium/silica catalyst

391

392

SUBJECT INDEX

Chromium/silica catalyst anhydrous impregnation. 8 1-82 composition, 48-52 dehydration, 82-85 hexavalent, preparation of, 48-53 chromate versus dichmmate, 48-49 chromyl chloride, 51-52 hydroxyl replacement, 49-5 I reaction with HCI, 52-53 saturation coverage, 53 kinetic profile, 75 modifications, 76-87 polymerization over, 59-69 reduced, 54-58 reduction/reoxidation,85-87 support properties, 70-76 Chromyl chloride, 51-52 Chrysotile asbestos, 327-330 Clays, see Sheet silicates Combustion reaction, sites for, 161-166 Constant C,. values for, 273. 274 Continuous reactor, 4-5 Cross-polarization, 209-212 Cyanoolefins, hydrocyanation, 33-34 Cyclohexadiene. reaction with HCN, 19, 20 Cyclohexane, 101, 102, 103 Cyclooctadiene, reaction with HCN, 19. 20 Cyclopentadiene, reaction with HCN, 19, 20

D Decationation, 264-272 Dehydrogenation, 101-103 Dehydroxylation, mechanism, 281-282 Deuterium distribution in hydrogen exchange intermediates, 128137, 138-140

in isomerization reaction, I 1 1-1 16, 120121, 126

Dichromate, in Phillips catalyst, 48-49 Dienes. reaction with HCN, 14-21 Dipolar interactions, 203-204 Dissociative mechanism deuterium distribution and, 125-126 in hydrogen exchange reaction, 13I - 132 in isomerization reaction, 104- 106

E Enzymatic reactions, NMR studies, 357-361 Erionite, 215, 225, 309, 31 I

Ethylene, unpromoted hydrocyanation, 25-30 Ethylene polymerization active site concentration, 68-69 branching, 63-65 mechanism of, 60-62 modes of, 59-60 molecular weight control, 62-63 molecular weight distribution, 67-68 Euclase, 255

F Femerite, 215, 225, 258 Femte catalysts butene oxidation intermediates and, 161, 162

in commercial processes, 159, 160 crystallite size, effect of, 185-189 promoter effects, 184 transition metal ions, effects of, 181-183 Fluorine isotope, NMR properties, 213, 274

G Gallosilicate zeolite, Z9Si MAS NMR studies of. 233-236 Gmelinite, 215, 224, 258 Gold. in hydrogenation, 100-101

H Hartmann-Hahn condition, 210 Hectorite, 336 idealized formula, 337-341 Heterogeneous catalysts, 120- 123 bond length, structure, and mechanism, 333-335

Hexadiene, reaction with HCN, 19-20 Homogeneous catalyst, see also Nickel phosphite complexes immobilization studies, 327-330 Hydrocyanation of olefins early attempts, 3-4 unpromoted, 25-34 Hydrogenation as model catalytic reaction. 99-10] intermediates, 137-156 isotope effect, 145-149 Hydrogen cyanide, reaction with nickel phosphite complexes, 10-14 Hydrogen exchange reaction associative and dissociative mechanisms, 131-132

393

SUBJKT INDEX intermediates, 128-137 on molybdenum disulfide catalyst, 105-1 16 restricted rotation in, 131- I37 Hydrogen isotopes NMR properties, 213, 274 in sheet silicate studies, 340-341 for zeolite acidity studies, 275-279 Hydrogen semipermeable reactor, 100 Hydroxyl replacement, by chromium, 49-51

I Iron hydroxide, catalytic activity, 193- 196 Iron oxide, see Ferrite catalysts Isomerization reaction active-site control of, 104-128 associative and dissociative mechanisms, 104-106

hydrogen exchange and, 104-116 ionic and nonionic types, 122 monohydride and dihydride sites, 120-122 Isomorphous substitution, 282-289 Isotopes, NMR properties, 213, 274

K Kyanite, 255

L Lewis acid, hydrocyanation and, 11-14, 2324, 34-40 Lineshape, 251-254 Lithium isotope, NMR properties, 213, 274 Losod, 215, 224, 258

M Mazzite, 215, 242, 287, 314, see also Zeolite omega Microcline, 255 Molybdenum disulfide catalyst, 106-1 16, 128-137. 144, 147, 148 Molybdenum isotope, NMR properties, 213, 274 Molybdenum oxide catalysts, 189- 192 Montmorillonite, idealized formula, 337 Mordenite, 215, 224, 241, 243, 244, 248, 258, 302, 303, 308, 315, 317

N Natrolite, 215, 224, 255 Nickel-copper alloys, in dehydrogenation reactions, 101-103 Nickel phosphite complexes dissociation, 6-7 nitriles and, 7-8 olefins and, 8-9 reaction with acids, 9-10 reaction with HCN, 10-14 Nitrile hydrogenation mechanism, 40-44 nickel phosphite complexes and, 7-8 Nitrogen isotope NMR properties. 213, 274 in zeolite acidity studies, 279-281 NMR, see Nuclear magnetic resonance spectroscopy, high-resolution, solid-state Nuclear magnetic resonance spectroscopy, high-resolution, solid-state advantages, 346-347 application to aluminosilicate catalysts, 199-374 multinuclear, 212-21 3 to enzymatic reactions, 357-361 to zeolite acidity studies, 272-281 to zeolites, 213-320 of carbonium ion, 347-349 imaging, 354-355 low-resolution studies of metal catalysts, 331-332 magic-angle spinning method, 201 resolution, lineshape, and relaxation, 251254 solid-state interactions, 202-203 theoretical considerations, 202-21 3 chemical shift anisotropy, 204-205 cross-polarization, 209-212 dipolar interactions, 203-204 quadrupolar interactions, 205-209 two-dimensional, 349-354 zeolite microstructure studies, 308-317 zero-field, 355-357 Nutation-NMR spectroscopy, 333-335

0 Offretite, 215, 225, 241, 243, 244, 248, 309, 310, 315

394

SUBJECT INDEX

Olefins, hydrocyanation, 2-44 current process, 4-5 early attempts, 3-4 heteroatomic, 3 1-33 isomerization of intermediates, 21-24 isotropic labeling experiments, 37-38 monoatomic, 25-31 promoted with Lewis acids, 34-40 Olefins, hydrogenation, 99- 104 catalytic isomerization reactions, 103 isomerization reaction, 104, 105 nickel phosphite complexes and, 8-9 Organochromium catalysis attachment to support, 92-93 kinetics of polymerization, 93 support effects, 94-95 termination mechanism, 93-94 Organochromium catalysts, 58, 92-95 Oxidation reaction, structure-sensitive, 189192 Oxidative dehydrogenation, see Selective oxidative dehydrogenation Oxide catalysts, 117, 139-156, see also Ferrite catalysts Oxygen, role in selective oxidative dehydrogenation adsorbed, 169- 177 lattice, 177 Oxygen isotope, NMR properties, 213, 274

P Pake pattern, 333 Palladium-silver thimble, 100- 101 Pentenenitriles, hydrocyanation, mechanism of, 40-44 Phillips chromium/silica polymerization catalyst, 48, see also Chromium/silica catalyst Phosphorus isotope, NMR properties, 213, 274 Pillared clays, 345-346 Platinum, in hydrogenation reaction, 101 Platinum-gold alloys, in dehydrogenation reaction, 102-103 Platinum isotope, NMR properties, 213, 274 Potassium carbonate catalyst, 123, 124 Propane, 102 Propene, 128-134 Pulse reactor, 4-5

Pyridine, adsorption to silica-alumina gels, 320-327 Pyrophyllite, idealized formula, 337

Q Quadrupolar interactions, 205-209

R Relaxation, 251-254 Resolution, 251-254 Rhodium isotope, NMR properties, 213, 274 S

Saponite, idealized formula, 337 Scolecite, 215, 224. 240-242 Second moment, 273 Selective oxidative dehydrogenation of butenes, 159-196 adsorbed oxygen, role of, 169-177 crystallite size and, 185-189 crystal structure, role of, 180-181 kinetics, 177- 180 lattice oxygen, role of, 177 mechanism, 159- 163, 177- 180 promoter effects, 184 transition metal ions and, 181-183 Selective oxidation sites densities of, 166-169 detection, 163-166 Semibatch reactor, 4-5 Separated local fields, 333 Sheet silicates, 335-346 27Al and 29Si studies, 342-345 I3C and 'Hstudies, 340-341 mechanism of catalysis, 339-340 pillared variants, 345-346 structural characteristics, 336-339 Silica, in polymerization catalyst activity versus pore volume, 70-72 critical pore diameter, 72-74 fragmentation, 74 kinetic profile, 75 molecular weight versus porosity, 75-76 Silica-alumina gels, NMR studies of, 320327 Silicalite, 215, 225, 260, 261, 288, 307, 310 structure, 237-240 Silicates, sheet, see Sheet silicates

SUBJECT INDEX Silicon isotope NMR properties, 213, 218-254, 274 in sheet silicate studies, 342-345 in silicates and aluminosilicates, 21 8-254 Sillimanite, 255 SLF, see Separated local fields Sodalite, 224, 244, 248, 258, 309, 311, 312, 313. 314 Sodium isotope, NMR properties, 213, 274 Solid-state interactions, 202-203 Spodumene, 255 Structure, determination by nutation-NMR spectroscopy, 333-335 Styrene, unpromoted hydrocyanation, 25-3 1 SUPport for chromium catalysts, 70-76 critical pore diameter, 72-74 derivatized surface studies, 327-330 fragmentation, 74 kinetic profile, 75 of metal catalysts, 331-332 molecular weight versus porosity, 75-76 particulate metal studies in, 317-320

T Talc, idealized formula, 337 Thallium isotope, NMR properties, 213, 274 Thiophene, adsorption to silica-alumina gels, 320 Titania, promotion of Crlsilica modifications by, 76-81 Titanium isotope, NMR properties, 213, 274

U Ultrastabilization, 264-272

v Vanadium isotope, NMR properties, 213, 274 Vanadium oxide catalysts, 189- 192 Van Vleck second moment, 273

W Water, sorption to zeolites, 300-305 A

Xenon isotope in adsorption studies, 314-320

395

NMR properties, 213, 274 in NMR studies of finely dispersed metal particles, 317-320 Z

Zeolites acidity, 272-28 I *'AI NMR studies, 254-264 classification according to secondary building units, 215 decationation and ultrastabilization, 264272 dehydroxylation mechanism, 28 1-282 exchangeable cations, 296-300 gallosilicate, framework studies, 233-236 highly siliceous, 236-239 interactions with adsorbed species, 300-308 isomorphous substitution, 282-289 microstructure, 308-317 properties, 216-218 resolution, lineshape, and relaxation, 251254 resolving crystallographically nonequivalent tetrahedral sites, 240-25 1 sorption studies, 300-308 structure, 213-2 I6 synthesis, precursors in, 289-296 synthetic, silicon-aluminum ordering in, 228-233 Zeolite A, 214, 215, 221-223, 224, 245, 247, 256, 257, 258, 262, 271, 272, 302, 303, 308, 312, 315, 316 exchangeable cation studies, 296-300 silicon-aluminum ordering in, 228-233 synthesis, 291-296 Zeolite L, 215, 225, 309, 315 Zeolite omega, 2 15, 2 16, 225, 24 I , 242, 244, 248, 286, 287, see also Mazzite Zeolite theta-1, 243 Zeolite X, 214, 215, 221, 222, 247, 262, 271. 315, 317 exchangeable cation studies, 296-300 silicon-aluminum ordering in, 228-233 sorption studies, 301, 304, 305-308 Zeolite Y,214, 215, 221, 222, 241, 244, 247, 248, 251, 256, 257, 258, 259, 260, 262, 264, 275, 276, 284, 285, 308 decationation and ultrastabilization, 264-272 exchangeable cation studies, 296-300

396

SUBJECT INDEX

with finely dispersed metal particles, 317-

320 silicon-aluminum ordering in, 228-233 Zeolite ZK-4, 215. 244. 245, 247, 248, 313, 314 Zeolite ZK-5,215. 224

Zeolite ZSMS, 215, 217, 224, 242, 243, 258, 260, 262, 263, 286, 288, 302, 303. 305, 307, 309, 311, 312, 315, 317 stlllcm~e,236-238 Zeolite ZSM-I I . 309, 312 Zeolite ZSM-39, 244, 246