[Advances in Catalysis] Volume 31 || Nonacid Catalysis with Zeolites

ADVANCES I N CATALYSIS . V O L I I M k 31

Nonacid Catalysis with Zeolites I . E . MAXWELL

Koninklijke ~Slwl l - Ltrhoratoriimni. Ani.or~rcltrrn ShcTll Rr.wrr i~h B . V .

Arn.vicrdmw, Thc Ni~tlir~rlcmdc

I .

I1 .

I l i .

IV .

V .

VI .

VII .

VIII .

Introduction . . . . . . . . . . . . . . . . . . 2 A . Scope . . . . . . . . . . . . . . . . . . 2 B . Zeolite Composition and Structure . . . . . . . . . . 3 C . Zeolite Ion Exchange and Acidity . . . . . . . . . . 5 D . Zeolite Cation Siting . . . . . . . . . . . . . . 6 E . Diffusion in Zeolites . . . . . . . . . . . . . . 9 Oxidation Reactions . . . . . . . . . . . . . . . 10 A . Carbon Monoxide Oxidation . . . . . . . . . . . . 10 B . Alkane Oxidation . . . . . . . . . . . . . . . 13 C . Alkene Oxidation . . . . . . . . . . . . . . . 14 D . Oxidation of Nitrogen-Containing Compounds . . . . . . 16

F . Miscellaneous Oxidation Reactions . . . . . . . . . . 18 Hydrogenation and Dehydrogenation . . . . . . . . . . . 18 A . Hydrogenation . . . . . . . . . . . . . . . . 19 B . Dehydrogenation . . . . . . . . . . . . . . . 21 Oligomerization Reactions . . . . . . . . . . . . . . 24 A . Ethylene . . . . . . . . . . . . . . . . . 24 B . Acetylene . . . . . . . . . . . . . . . . . 29 C . Propylene . . . . . . . . . . . . . . . . . 30 D . Cyclopropenes . . . . . . . . . . . . . . . . 31 E . Butadiene . . . . . . . . . . . . . . . . . 32 F . n-Butene . . . . . . . . . . . . . . . . . 36 G . Isobutylene . . . . . . . . . . . . . . . . . 37 Carbonylation . . . . . . . . . . . . . . . . . 39 A . Methanol . . . . . . . . . . . . . . . . . 39 B . Ethanol and Higher Alcohols . . . . . . . . . . . . 45 C . Ethylene . . . . . . . . . . . . . . . . . 46 Hydroformylation . . . . . . . . . . . . . . . . 46

Methanation . . . . . . . . . . . . . . . . . . SO A . Palladium Zeolites . . . . . . . . . . . . . . . 51 B . NickelZeolites . . . . . . . . . . . . . . . . 52 C . Ruthenium Zeolites . . . . . . . . . . . . . . 53

E . Oxidation of Sulfur-Containing Compounds . . . . . . . 17

A . Cobalt Zeolites . . . . . . . . . . . . . . . . 47 B . Rhodium Zeolites . . . . . . . . . . . . . . . 49

Conversion of SynthesisGas to Hydrocarbons . . . . . . . . 55

I Copyright s 1982 by Acadcrnic Press. Inc

All r izhiz ofrepr, ~iuct ion i n a n y form rrservcd ISBN n- I 2-no7x31-7

2 I. E. MAXWELL

IX. Miscellaneous Reactions . . . . . . . . . . . . . . 58 A. Water-GasShift . . . . . . . . . . . . . . . 58 B. Kolbel-Engelhardt Reaction . . . . . . . . . . . . 59 C. Water Splitting. . . . . . . . . . . . . . . . 59

X. General Conclusions and Future Prospects . . . . . . . . . 61 A. Activity and Selectivity . . . . . . . . . . . . . 61 B. Stability. . . . . . . . . . . . . . . . . . 62 C. Active Species and Reaction Mechanisms . . . . . . . . 64 D. Future Prospects . . . . . . . . . . . . . . . 66 References . . . . . . . . . . . . . . . . . . 68

I. Introduction

A. SCOPE

In recent years there have been considerable academic and industrial research efforts carried out in the field of zeolite catalysis. The main part of this work has, however, been related to reactions where the zeolite is used as a solid acid, e.g., isomerization, cracking, hydrocracking, etc. This is hardly surprising since this area represents the major application to date of zeolites as industrial process catalysts. Reviews on zeolite catalysis have therefore also in general tended to concentrate more on the work related to acid catalysis ( I -7a).

In principle, zeolites offer considerable scope for surface modification. The ion-exchange properties and the periodicity (crystallinity) of zeolites, for example, enable functional metal ions or complexes to be stabilized in a variety of oxidation states and coordination geometries on specific surfaces in a well-defined pore structure. This represents a valuable starting point in the design of catalytic surfaces.

Although these characteristics of zeolites have been recognized for some time, only a relatively small effort has been carried out in this area. This is probably due to the remarkable success of zeolites in solid acid catalysis. More recently, however, there has been renewed interest in the applications of zeolites in nonacid catalysis. In particular, zeolites have been successfully applied to reactions such as oxidation, reduction, olefin oligomerization, carbonylation, hydroformylation, synthesis gas conversion, and water splitting.

The purpose of this review is to discuss the above applications in some depth with the intention of comparing where possible the advantages or

NONACID CATALYSIS WITH ZEOLITES 3

disadvantages of zeolites with alternative, perhaps more conventional catalysts for these reactions. A greater emphasis is given to the most recent and more novel applications of zeolites to heterogeneous catalysis. Attention is also focused on probable reaction mechanisms and active sites. A final section is included where the unique features of the zeolites for nonacid catalysis are highlighted and future research prospects in this area are evaluated. The technical literature, scientific publications, and patent literature have all been searched-mainly covering the period from 1972 up to and including the first half of 1980.

B. ZEOLITE COMPOSITION AND STRUCTURE

A very brief discussion of the structure and chemistry of zeolites relevant to this review is now given. For a more comprehensive discussion of this subject the reader is referred to an excellent book by Breck (8).

Zeolites are crystalline aluminosilicates (synthetic and natural), with a chemical composition which corresponds to the general formula

where M"' is the cation which balances the negative charge associated with the framework aluminum ions. The framework ions, Si4+ and A13+, are each tetrahedrally coordinated to four oxygen anions. The periodic three- dimensional network, which is so characteristic of zeolites, is formed by linking the (SiO,) and (A10,)- tetrahedra through shared oxygen ions. These tetrahedra tend to form rings, containing from four to twelve tetrahedral units. Such rings normally form the entrances to channels or cages in

TABLE 1 Composificin und Pore Purumefrrs of Sonic Zeoliics

Void Pore Thermal volume diameter decomposition Si/AI

Type Unit-cell composition (mIjmI) (Ay temp.' ( 'C) ratio

Zeolite A Na,,(A102),z(Si0,),, 0.47 4.2 700 1 .0 Zeolite X Na,,(AIO,),,(SiO,),,,, 0.50 7.4 772 1.23 Zeolite Y Na,,(AIO,),,(SiO,),,, 0.48 7.4 793 2.43 Mordenite Na,(AlO2),(Si0,),,, 0.28 6.7 x 7.0 I000 5.0

~

I A = 1 O - I nm. From Ref. 2.

4 I. E. MAXWELL

A

FIG. 1. Schematic diagram of zeolite A. (Reproduced from Ref. 2 with permission from the author and Elsevier Scientific Publishing Company, Amsterdam )

zeolites and thus define the pore diameter for a particular structure. The composition pore volumes and pore diameters for a number of common zeolites relevant to this review are given in Table 1.

Zeolites A, X, and Y all consist of tetrahedra linked to form cubo- octahedra or so-called sodalite cage units. When these units are linked through four-membered rings zeolite A is formed (see Fig. l), whereas linking via the six-membered rings results in zeolites X and Y (see Fig. 2). The latter two zeolites only differ in the Si/Al ratio.

Mordenite has a channel-like pore structure in which the basic building blocks consist of five-membered rings. A view of the mordenite structure perpendicular to the main channels is shown in Fig. 3.

In general, zeolites have good thermal stability, but this is further improved by increasing the Si/Al ratio as shown in Table 1. The hydrothermal stability of zeolites also increases with decreasing aluminum content.

F

FIG. 2. Schematic diagram of zeolites X and Y . (Reproduced from Ref. 2 with permission from the author and Elsevier Scientific Publishing Company, Amsterdam.)


Frc. 3. Section through the mordenite structure (Z) perpendicular to the main channels. (Reproduced from Ref. 2 with permission from the author and Elsevier Scientific Publishing Company. Amsterdam.)

C. ZEOLITE ON EXCHANGE AND ACIDITY

The framework charge-compensating cations in a zeolite, which for synthetic zeolites are normally sodium ions, can be exchanged for other cations of different type and/or valency. However, care must be taken during ion exchange to avoid strongly acidic solutions which can lead to proton exchange with the zeolite metal cations or even structure collapse. For example, zeolites A, X, and Y decompose in 0.1 N HCI. The more silica-rich zeolites such as mordenite are, however, stable under such conditions.

Acidity can be introduced into a zeolite in a number of different ways:

( 1 ) ion exchange with NH:, followed by thermal decomposition, i.e.,

N H i Z - + H*Z- + NH,(q)

(2) hydrolysis of ion-exchanged polyvalent cations followed by partial dehydration, i.e.,

M(H,O):+Z- + M(OH)'"- "+Z- + H +Z- + (x - I)H,O

(3) direct proton exchange, i.e.,

N a + Z + H + -+ H'Z- + Na'

(4') reduction of exchanged metal ions to a lower valency state, i.e., M"+Z- + :H, + M("-')+Z- + H + Z -

The so-formed proton or Bronsted acid sites can be further dehydroxylated

6 1. E. MAXWELL

to form Lewis acid sites, i.e.,

H H

\Si/O\Al Si / \ / \

0 0 0 0 0’ ‘ 0 0’ ‘ 0 o/ ‘0

As will be seen in the ensuing discussion, for most of the catalytic reactions discussed in this review, surface acidity leads to undesirable side reactions. In these cases care must be taken during catalyst preparation to avoid the introduction of acidity via any one of the above-described routes.

D. ZEOLITE C A T I O N SITING

In most of the reactions discussed the active entity of the zeolite catalysts is introduced via ion exchange. Thus a knowledge of the possible siting of cations is a prerequisite for an understanding of the location and nature of the active sites in zeolites. In this respect the periodicity of the internal surface of the zeolites provides an almost unique opportunity to study the surface composition in considerable detail using powerful analytical methods such as X-ray diffraction.

In zeolite A the six-membered rings (eight in total) facing onto the main cavity are the preferred cation sites (see Fig. 1). I f there are more than eight cations per unit cell, the remainder generally lie in the plane of the eight- membered ring entrance to the large cavity, but are displaced off-center so that there is an asymmetrical ionic interaction with the oxygen anions.

In recent years Seff and co-workers (Y) have extensively studied cation siting in zeolite A using single-crystal X-ray diffraction techniques. In favorable cases these workers have also been able to obtain detailed information on the interactions between cations and absorbate molecules. Two examples are shown in Fig. 4, where the adsorption complexes formed when acetylene and NO are adsorbed in Co(I1)A have been resolved. In the former case it is proposed that a weak complex is formed via an induced dipole interaction with the polarizable n-orbitals of the acetylene molecule. For the NO complex there is good evidence for electron transfer resulting in a complex between CO(II1) and NO-. In both cases the organic molecules


FIG. 4. (a) Complex formation between Coz+ and acetylene in zeolite A ; (b) complex formation between NO and Co2+ in zeolite A. (Reprinted with permission from Ref. Y. Copy- right 1976 American Chemical Society.)

interact with the cations sited at the six-membered rings. The interaction results in a displacement of the cation toward the large cavity to give a more tetrahedral-like coordination with the organic fragment. Although zeolite A is less interesting than larger pore diameter zeolites from the point of view of catalysis (pore diameter is 4.2 A), these model studies do provide some insight into the types of cation adsorbate interactions, which may also occur in larger pore zeolites.

Cation siting in zeolites X and Y is more complex due to the larger number of potential sites. In general, most of the cations lie along the crystallographic threefold axes of the cubic unit cell. Various positions along these axes have been arbitrarily given Roman numbers as shown in Fig. 5 . Only the site I1 cations on these axes, which are located at the six-membered rings (cf. zeolite A), are able to interact with adsorbate molecules present in the supercage. Access to the smaller sodalite cages is severely limited by the small diameter (2.2 A) of the six-ring entrance. An additional site (so-called 111) has been identified in the supercage, near the four-membered rings which

I . E. MAXWELI

FIG. 5 . Faujasite framework and cation siting. (Roman numerals indicate cation sites; arabic numbers show oxygen-atom numbering scheme.)

link adjacent sodalite cages (see Fig. 5) . There is good evidence that ions located at these sites are the active entities for hexane dehydrocyclization over TeNaX (10) and butadiene cyclodimerization over CuNaY (11). The location of site 111 at the entrance to the supercages, together with the unsaturated coordination geometry of the cations, provides an ideal sile for interaction with adsorbate molecules.

The siting of cations in mordenite is generally less well understood than that in the zeolites described above. Smith and co-workers (12) have, however, in recent years carried out a number of single-crystal X-ray analyses on various cation-exchanged forms of mordenite. These workers correctly emphasize, however, that the cation population densities are subject to unknown errors due to pseudosymmetry. The alkali metal ions are dis- tributed over four major sites, namely:

site I , at the end of the side pocket off the main channel; site 11, in the side pocket at the center of an eight-ring; site IV, at the junction of the side pocket with the main channel; site VI, at the wall of the main channel.

Only Na' ions were found in site I due to space restrictions.


Alternative methods to ion exchange may be employed to introduce active metal components into zeolites such as pore volume impregnation and vapor-phase adsorption of volatile compounds. In these cases the siting of such species in the zeolite pore structure is generally less well defined.

E. DIFFUSION I N ZEOLITM

In view of the importance of diffusion in zeolites it would be an omission not to include a brief discussion on this subject. This is perhaps the least well understood, but it is often directly related to the unique catalytic properties of these materials. Weisz (13) has lucidly described diffusion phenomena in zeolites using a plot of diffusivity against pore size, as shown in Fig. 6. Zeolites with pore diameters in the range of 4-9 8, are shown to provide a region of diffusivity beyond the regular and Knudsen regions, which Weisz has termed the configurational regime. This is the region, where molecules

D (c m2/ sec)

I

lo-2

lob

do

I6l2

10" J I I I I

REGULAR

CONFIGURATIONAL

I 10 loo lo00 I 10 v -

(ANGSTROMS) (pm)

PORE SIZE

FIG. 6 . Diffusivity D and size of aperture (pore). (Reprinted with pcrmission from Ref. 13. Copyright 1973 American Chemical Society.)

10 1. E. MAXWELL

must diffuse through spaces of near-molecular dimensions and is thus of considerable importance in shape-selective catalysis.

As shown in Fig. 6, the configurational region spans an enormous range of diffusivities (approximately ten orders of magnitude). In this region subtle differences in configurational structure can have a large effect on diffusivity. For example cis- and trans-butene have diffusion coefficients which differ by two orders of magnitude in CaA (14). Even more subtle effects are possible such as a periodic variation in diffusivities with increasing carbon number for n-paraffins in erionite (15).

Thus it is evident that zeolites offer considerable potential for steering reaction selectivity on the basis of differences in molecular shape. These possibilities extend far beyond the more familiar molecular sieve effects where bulky molecules are simply excluded entry into the zeolite cavities due to pore diameter restrictions.

The foregoing discussion refers solely to intraparticle diffusivity (micro- pore diffusion) as distinct from interparticle effects (macropore diffusion). Since a practical zeolite catalyst will consist of composite particles, each containing a large number of individual zeolite crystals, it is important to make a clear distinction between these two types of diffusion. In some cases macropore diffusion may be important in determining the overall reaction kinetics but will obviously not introduce or affect shape selectivity in any way.

Although diffusivity is often important in zeolite catalysis, other factors may also be crucial in determining shape selectivity. Recent work by Post (15a), for example, has shown that the shape selectivity behavior observed for the relative cracking rates of hexane isomers over H-ZSM 5 zeolite (see Section VlII) could not be understood on the basis of their measured diffusivities. Spatial restrictions imposed on transition-state species formed within the zeolite pores provide a possible explanation for the observed results.

I I . Oxidation Reactions

A. CARBON MONOXIDE OXIDATION

The oxidation of CO is widely used as a test reaction for oxidation catalysts because of its simplicity. Thus, there is quite an extensive literature on CO oxidation using various zeolite catalysts. The parent (sodium forms) of zeolites show very little oxidation activity as might be expected and therefore the majority of the studies have concentrated on transition metal ion- exchanged forms.


5 -

4 -

I

It is of particular interest to compare the relative activities of transition metal ion-exchanged zeolites with, for example, the corresponding oxides in order to gain some insight into the influence of the zeolite lattice. Boreskov rt al. (16) compared the specific activities of CuY and CuO for CO oxidation calculated on the basis of surface copper concentrations. Although the specific activity of CuY increases with increasing copper level, even at 16 wt.% Cu the activity is 2.5 orders of magnitude less than that of CuO (see Fig. 7). A similar behavior has also been demonstrated (17, 18) for Fe, Ni, Co, and Cr ions exchanged into zeolite Y and their corresponding oxides (i.e., a-Fe,O,, NiO, Co,O,, and Cr,O,, respectively). In addition, the activation energies for the transition metal ion-exchanged zeolites are considerably higher than those for the corresponding oxides ( I 7, 18) (e.g., CuY, 19.5 kcal/mol'; CuO 13 kcal/mol; NiY, 26 kcal/mol; NiO, 9.5 kcal/mol ; COY, 16 kcal/mol ; Co,O,, 1 1 kcal/mol).

Iron-containing zeolites are somewhat expectional in that the specific activity of FeY is independent of the iron content (18, 1 Y ) (in the range 0.4-5 wt.%) and although less active than a-Fe,O,, the activation energies for the zeolite and oxide catalysts are the same ( 1 8 kcal/mol).

I t has also been shown (20) that for transition metal ion-exchanged zeolites X and Y, the activities for CO oxidation increase exponentially with in-

f" cuo

- 2 6 10 14 %

tu-

FIG. 7. Comparison of specific activity of CuY and CuO for CO oxidation at 450°C. (Re- produced from Ref. 16 with permission from the authors and Plenum Publishing Corporation, New York.)

' 1 kcal = 4.2 k J .

12 I . E. M A X W t L L

creasing metal ion standard oxidation potential (i.e., Cu+ > Fez+ > Cr3+ > CuZf z Ni2+).

More recently it has been shown (21-24) that the equilibrated pH of the transition metal ion-exchange solution is also critical in determining the specific activity of zeolite catalysts. The results obtained for the CuY system (21) are shown in Fig. 8.

The influence of both transition metal ion loading and ion-exchange solution pH has been attributed to the formation of catalytically active metal oxygen bridge species within the zeolite cavities (i.e., M"+-02 --M"+ where the anion corresponds to extralattice (i.e., nonzeolite framework) oxide ions. The formation of such species would be expected to be favored by high metal loadings and hydrolysis conditions during ion exchange, as is observed. In fact there is some direct evidence from ESR (21), Mossbauer spectroscopy (25) , IR spectroscopy (26) , and magnetic measurements (23) to support the existence of these oxygen-bridged species.

Boreskov (18) has proposed a model for transition metal compounds in which the rate of oxidation is assumed to be determined by the rate of electron transfer between oxygen and the transition metal ion. This process is further assumed to be facilitated with increasing degree of covalency of the metal- oxygen bond. Thus the more covalent transition metal oxides are more active than the rather ionic metal ion-exchanged zeolites. The oxygen- bridged species as described above is considered to be more covalent in character, and hence more active for oxidation catalysis than the transition

tog (SPECIFIC ACTIVITY) cm3/(sec otorn Cu)

4 6 8 PH

FIG. 8. Specific activity of CuY zeolite as a function of ion-exchange solution pH. (Rcpro- duced from Ref. 21 with permission from the authors and Plenum Publishing Corporation, New York.)

NONACID C'ATAI.YSIS WITH ZEOLITES 13

metal ion-framework oxygen interaction. However, this model has not been sufficiently developed to provide a detailed mechanistic scheme which would explain the observed reaction kinetics (16, 20,23), which are generally first order in carbon monoxide and fractional or zero order in oxygen concentration.

Beyer ct a/ . (27) carried out a detailed kinetic study on CO oxidation over CuNaY zeolite after various pretreatments. The results obtained were explained in terms of the relative concentrations of the species Cu2+, Cu', and Cuo present in each catalyst.

Paetow and Riekert (28, 2Y) in a careful study have compared the relative activities of Cu2+-exchanged zeolite T and mordenite with various copper- containing compounds. On the basis of turnover numbers per CO chemisorption site the Cu2 +-exchanged zeolites are 2-4 orders of magnitude less active than CuO, CuMn,O,, and CuCr,O,. This was considered to be consistent with the involvement of lattice oxygen as an intermediate which is easier to remove in oxides than zeolites.

An interesting application of zeolite-catalyzed CO oxidation has developed pertaining to fluid cracking catalysts (FCC). This was based on the discovery by Chen and Weisz (30) that minute amounts (0-100 ppm) of platinum-group metals incorporated into zeolites produced catalysts which were highly active for CO conversion under FCC regeneration conditions. It was further demonstrated that these oxidation promoters could be incorporated directly into the zeolite FC catalysts without any adverse effects on cracking selectivity (31). This promoter technology has now been fully developed and is currently successfully applied in commercial practice. Numerous patents (32) have appeared in recent years relating to this particular application. It would be of interest, particularly in view of the low metal loadings, to determine to what extent the zeolite matrix contributes to the exceptional oxidation activity of these catalysts. To date, the appropriate studies do not appear to have been carried out.

B. ALKANE OXIDATION

Metal ion-exchanged forms of zeolite X are active catalysts for the complete oxidation of methane to carbon dioxide and water (33,34). In general the platinum metal ion-exchanged forms (e.g., Pt, Pd, Ir) are considerably more active than the first-row transition metal ion forms (e.g., Cu, Mn, Cr, Fe, Ni). The kinetics were best described as first order in methane and zero order in oxygen. There is no general agreement on the mechanism and neither (33, 34) has the possible existence of bridged-oxygen species been considered.

14 1. E. MAXWELL

The oxidative dehydrogenatioii of cyclohexane to benzene has been studied more extensively. Transition metal ion-exchanged forms of zeolite Y have been shown (34-39) to be particularly active catalysts for this reaction. Although the platinum metal ions exhibii the highest activity, CuY was found to be the most selective for benzene formation (38,39).

Moshida et ul. (40) carried out a comparative study of cyclohexane oxidative dehydrogenation using Cu'Y (prepared via Cu+ exchange in liquid ammonia) and Cu2+Y. Both catalysts exhibited quite good benzene selectivities (> 90%), but interestingly showed different reaction kinetics. The reaction orders in oxygen were unity and one-half for Cu2'Y and Cu' Y , respectively. This was interpreted in terms of the active species being molecular and dissociatively adsorbed oxygen, respectively. Such a marked difference in mechanism for benzene formation does not seem very probable. The different kinetics may simply arise due to a change in the rate-determining step for oxygen activation. The possible role of bridged-oxygen species has also been neglected, which is likely to be particularly important for Cu'Y under oxidation conditions.

Minachev et UI. (41, 42) have recently examined alkali metal ion forms of various zeolites (A, X, Y, L, chabazite, erionite, and mordenite) for cyclohexane oxidative dehydrogenation. Not surprisingly these alkali metal ion forms are considerably less active than those containing transition metal ions (reaction temperatures of approximately 300" and 450"C, respectively). Further, cyclohexene rather than benzene is the predominant product (selectivity to cyclohexane 67-84"/,), particularly with small-pore zeolites. In fact, NaA was the most active zeolite tested (42) , which strongly suggests that the reaction is simply occurring on the outer surface of the zeolite crystallites.

C. ALKENE OXIDATION

Transition metal ion-exchanged zeolites are active catalysts for alkene oxidation but generally result in deep oxidation to carbon dioxide and water (43-45). In common with CO and alkane oxidation, the platinum metal ions are more active than the first-row transition metal ions. Mochida et al. (43) have been able to correlate the catalytic activity of ion-exchanged Y zeolites for propylene oxidation with a so-called Y parameter as shown in Fig. 9. This parameter was considered to express the tendency of the metal ion toward the formation of a dative n-bond with propylene. Further, it was shown that with increasing Y factor there was a decrease in reaction order, which was considered evidence of increased propylene adsorption. In a more recent study of CuX zeolites, Gentry et ul. (45) found some evidence

NONACID CATALYSlS WITH ZEOLITES 15

c .-

FIG. 9. Correlation between the catalytic activity and the parameter Y in propylene oxidation. Y = 10(In/In+ l ) ( r / n l ' z ) , where I , is the nth ionization potential, Y is the ionic radius of the metal ion, and n is its formal charge. (Reproduced from Ref. 43 with permission from the authors.)

for the participation of Bronsted acidity as well as Cu2+ ions in the total oxidation of propylene.

A rather interesting application of zeolite-based alkene oxidation catalysis has been demonstrated by Japanese workers (46, 47). In particular, a Pd2+, Cu2+Y zeolite was shown to be an active and stable heterogeneous oxidation catalyst which is analogous to the well-known homogeneous Wacker catalyst system containing PdC1, and CuC1, (48). Under Wacker conditions (i.e., alkene/O,/H,O) the zeolite Y catalyst was shown to convert ethylene to acetaldehyde and propylene to acetone with selectivities in excess of 90% with CO, as the major by-product.

The reaction mechanism was also shown to parallel that of the homogeneous system, where Pd2+ is reduced to Pdo and is catalytically reoxidized by Cu2., i.e.,

[Pdz+, 22-1 + C,H, + H,O + CH,COCH, + [Pd', 2(H+Z-)]

[Pd', 2(H+Z-)] + 2[CuZf, 22-1 -+ [Pdzf, 22-1 + 2[CuiZ-, H'Z-1

2[Cu+Z-, H+Z-] + to, -+ 2[Cu2+, 22-1 + H,O

The most active catalysts were obtained by preexchange with Cu2+ followed by postexchange with Pd2+. This would seem to indicate the importance of cation siting in promoting electron transfer which was assumed to occur via adsorbed water molecules which bridge PdO and Cu2+ through a hydrogen bond.

In principle, the zeolite catalyst system would offer advantages over the existing homogeneous catalyst, particularly with respect to corrosion due to the absence of HCl and chlorine-containing by-products. However, acetaldehyde and acetic-acid production via ethylene has recently become less economically attractive compared to methanol carboxylation chemistry.

16 I . E. M A X W E L L

Further, acetone via Wacker chemistry must compete with 2-propanol dehydrogenation and coproduction in the Hock phenol process (48).

I t remains, however, as a very interesting example of selective oxidation using mixed transition metal ions in a zeolite matrix. The extent to which the zeolite matrix directly or indirectly facilitates the electron transfer step between Pd' and Cu2+ has not been examined. This would seem worthy of study within the general concept of electron transfer reactions in heterogeneous catalysis.

In the absence of Pd2+ but in the presence of steam, Mochida et ul. (4Y, 50) showed that propylene could be oxidized over Cu2+Y to yield a mixture of products such as 2-propano1, acetone, and acrolein.

D. OXIDATION OF NITROGEN -CONTAINING COMPOUNDS

The oxidation of NO and NO, is of industrial importance for cleaning combustion-flue gases. Transition metal ion-exchanged zeolites have been shown (51) to be highly active catalysts for this reaction. The relative activities are shown in Fig. 10, from which it can be seen that equilibrium conversions of NO to NO, can be achieved with Cu2+X at reaction temperatures as low as 350°C. Kinetic studies showed that the reaction rates were fractional order in both NO and 0,. The following reaction mechanism was therefore proposed, for example, with Cu2+X,

This mechanism would seem to be quite plausible since there is good evidence for the formation of both nitrosyl (51-55) and oxygen-bridge complexes (16-26) with transition metal ions in zeolites. Interestingly, for a Cr3+Y catalyst (51), the activity was enhanced by H,O and there was no deactivation by SO,, indicating that such a catalyst would likely be quite robust under practical feed conditions.

Williamson et al. (56) showed that Cu2+Y was an active catalyst for the oxidation of NH, to N, and H,O. A mechanism was proposed involving the intermediate formation of an amine complex [Cu(NH,),]'+. The NH, reduction of Cu2+ and Cu' in this complex was proposed as the slow step with reoxidation via 0, being very rapid. This mechanism was consistent with the kinetic expressions which were shown to be first order in NH, and zero order in 0,.

NONACID CATALYSIS WITH Z;EOLITt!S 17

100 *.- Loaded on NaX

\. 8 0 1 .- ‘-,Equilibrium

.c

I I 1 I

600 700

Reaction temperature (K)

FIG. 10. Activities of transition metal ions loaded on zeolite X (space velocity 23,400 cm3 (g catalyst)- ’ hr- ’, NO 300 ppm, 0 , 6 . 4 vol”/,). (Reproduced from Ref. 51 with permission from the authors and the American Chemical Society.)

E. OXIDATION OF SULFUR-CONTAINING COMPOUNDS

Mars and co-workers (57-60) have systematically investigated the behavior of various porous materials including zeolites toward the catalytic oxidation of H,S with 0, to elemental sulfur. This process is attractive for H,S removal from gas streams with low H,S contents. These workers showed that the product sulfur, which is adsorbed in the pores, is the catalytically active entity, i.e., the reaction is autocatalytic. Sulfur radicals proved to be the active sites for oxygen chemisorption. The pore diameter of the support was shown to be important in determining the catalytic activity. The catalytic sulfur was found to be most active in supports with pore diameters in the range 5-10 A. Thus NaX was found to be a highly suitable support which was competitive with industrial active charcoals with regard to both activity and selectivity towards sulfur formation. More recent mechanistic studies (61) on H,S oxidation over NaX and NaY have confirmed the previous findings of Mars and co-workers (57-60). However, significant SO, formation was also observed, particularly at reaction temperatures in excess of 70 C.

Complete oxidation of H,S to SO, can be achieved by incorporating a metal function into the zeolite. For example, vanadium on mordenite has been claimed (62) to be a very effective catalyst for this reaction.

Pearce and Lunsford (63) demonstrated that Mn2+Y was an active catalyst for SO, oxidation at ambient temperature. The product SO:- was,

18 I. E. MAXWELL

however, strongly adsorbed in the zeolite pores, poisoning the catalytic activity. Interestingly, there was quite good evidence that hydrated Mn" ions sited in the supercage were the catalytic entities.

F. MISCELLANEOUS OXIDATION REACTIONS

Transition metal ion-exchanged zeolites X and Y are active catalysts for hydrogen oxidation (64, 65). There is in fact a close parallel in terms of relative activity and kinetics between hydrogen oxidation and carbon monoxide oxidation over these catalysts. The platinum metal ion-exchanged forms are the most active, followed by the first-row transition metal ions. The reaction rates are first order in hydrogen and close to zero order in oxygen (65). Further, there is evidence, particularly for Cu2+Y (65), that Cu2+-0-Cu2+-bridge species are the active sites. In this case the zeolite catalyst is also intrinsically less active than the corresponding oxide, CuO.

Various ion-exchanged forms of zeolite Y have been investigated as catalysts for the oxidation of molecules such as y-butyrolactone (66), tetralin (67), aromatic amines (68), benzene (69), methanol (70), benzyl alcohol (71), and pyrocatechol (72). In general, transition metal ions or complexes have been incorporated into the zeolite.

Space restrictions do not permit a detailed account of these studies. However, in general the selectivities to the more useful partial oxidation products are rather poor.

The patent literature claims that olefins can be partially oxidized to epoxides (73) or hydroxy epoxides (74) and alcohols may be oxidized to ketones or aldehydes (75) using various metal ion-exchanged zeolites. In the examples given, the selectivities or conversion levels to the desired products are not particularly attractive. Metal ion-exchanged zeolites do, however, appear to be quite useful catalysts for effluent treatment. For example, Cu2+X and Cu2+Y are claimed to be good catalysts for the total oxidation (incineration) of chlorinated organic compounds (76).

111. Hydrogenation and Dehydrogenation

Since hydrogenation and dehydrogenation have been extensively covered in a number of previous review articles (1-7), the discussion here is largely confined to recent developments.


A. HYDROGENATION

In the past it has been generally accepted that alkaline earth and certain trivalent metal ion-exchanged zeolites are inactive for hydrogenation reactions. This was hardly surprising since these zeolites do not normally possess conventional sites where hydrogen molecules can be activated. Soviet workers have claimed, however, that such zeolites, (A, Y, mordenite, omega, chabazite) which evidently do not contain transition metal ion or other impurities are indeed active catalysts for the hydrogenation of alkenes, aromatics (77-89), and oxygenates (90-93). The activities were shown to be dependent on both the cation and zeolites structures. The cations were proposed as the active sites, whereby hydrogen dissociation is induced by the dipole formed between the cation and oxygen of the zeolite framework, e.g.,

Na'Z- + H, ti NaH H . 2

There is, however, no direct evidence for the above equilibrium, and further, these results have not yet been confirmed by other workers. This would seem to be very worthwhile doing in order to clarify this situation.

Conventional zeolite-based hydrogenation catalysts are prepared by ion exchange with a transition metal ion followed by reduction. As previously discussed, the reduction step leads to the simultaneous formation of acid sites, i.e.,

M"+Z + ( n / 2 ) H 2 + Mo + nZH

Recently, a nickel zeolite hydrogenation catalyst has been prepared by a novel route (94) involving the adsorption and decomposition of nickel carbonyl onto NaX, which would not be expected to result in the formation of acid sites. In general, the platinum metal-containing zeolites are more active than those containing other transition metals. For example, in zeolite Y the following activity series has been found,

PtNaY > PdNaY >> NiNaY

The methods of ion exchange and subsequent reduction are also important in determining the final metal dispersion and hence the catalyst activity

Zeolite-based hydrogenation catalysts containing platinum and palladium have increased resistance toward sulfur poisoning (101- 104, and a higher activity (95, 105) than many other supports. In recent years there has been some effort devoted to attempt to explain this phenomenon. Although there is general agreement that the catalytic surface of the zeolites most probably

(77,95-100).

20 1. E. MAXWELL

exhibits weaker bonding toward the electronegative sulfur atoms, a detailed understanding does not yet exist. At least three proposals have been made, namely :

(1) The presence of an atomic dispersion of the metal (101); (2) the formation of a charge-transfer complex between the metal and

strongly acidic O H groups (102,103), e.g., M" . . . HOZ", resulting in an electron-deficient metal atom;

(3) the presence of small (< 10 A), electron-deficient metal clusters inside the zeolite cages (104).

Figueras et ul. (105) found some direct evidence for electron-deficient palladium clusters on various cation-exchanged forms of zeolite Y from C O adsorption experiments. In particular, a correlation was observed between the turnover number for benzene hydrogenation and the CO stretching frequency. The shift toward higher frequency with increasing support acidity was considered as evidence for increased electron acceptor properties of the support. Further studies will, however, be required to provide a more detailed understanding of this phenomenon.

Other recent studies have included zeolites containing copper (106), nickel (107-110), rhodium (111, 112), rhenium (113), and ruthenium (114, 115). For nickel-containing zeolites (e.g., NiY) three types of nickel were identified

(1) nickel particles, in the range 20-500 8, on the outer surfaces of the zeolite crystalli tes ;

( 2 ) small metallic clusters retained inside the zeolite cavities; (3) unreduced Ni2+ cations inside the zeolite cavities.

(1 101, e.g.,

The catalytic behavior is, not surprisingly, dependent on the relative concentration of these different types of nickel. There was evidence that type (1) nickel particles were similar in behavior to this metal on other supports. However, type ( 2 ) nickel clusters exhibited low activity toward benzene hydrogenation.

Japanese workers (112) recently carried out a comprehensive study of the catalytic activity of RhY as a function of degree of reduction. Three different oxidation states of rhodium (Rh", Rh', Rho) were distinguished by using the X-ray photoelectron spectroscopy (XPS) technique. It was shown that Rh' was active for both ethylene hydrogenation and dimerization, whereas Rho was active for hydrogenation of both ethylene and acetylene. The demonstration of hydrogenation activity for Rh' Y is particularly interesting in view of the analogy with homogeneous catalyses where complexes containing formally monovalent rhodium [e.g., RhCI(PPh,), and RhH(Co)(PPh,), (116)] are highly active catalysts for the hydrogenation of olefins.


Coughlan et al. (114,115) recently prepared a series of ruthenium catalysts on zeolites A, X, Y , L, mordenite, and compared their catalytic activities for benzene hydrogenation. Perhaps the most remarkable result was the correlation between the turnover number and the metal surface area (determined by hydrogen adsorption), as shown in Fig. 1 1. This was considered to provide evidence for an influence of metal crystallite size or activity. This is rather surprising since other workers have shown (117) that for Pt, Pd, and Ni on various supports such as silica, alumina, silica-alumina, and silica- magnesia, benzene hydrogenation activity is independent of the metal particle size and the support used. However, the interpretation given for the correlation in Fig. 1 1 is based on the assumption that all the ruthenium metal accessible to hydrogen is also accessible to benzene molecules, which was not conclusively demonstrated. In this respect it is interesting to note that ruthenium in zeolite L exhibited the highest turnover number.

There is an extensive patent literature on the use of zeolite based hydrogenation catalysts. Recent examples include aromatics hydrogenation in the presence of sulfur ( ] I d ) , pour-point reduction (1 19), and general hydro- processing (120).

B. DEHYDROGENATION

Most studies have used the dehydrogenation of cyclohexane to benzene as a test reaction. Kubo et al. (121, 122) found a good correlation between

. z

/

i

i;; Slrn'g'

(TURNOVER NUMBER)

FIG. 1 I . Dependence of the catalytic activity (turnover number) of ruthenium A, X, Y, L, and mordenite zeolites for the hydrogenation of benzene on the surface area S of the ruthenium zeolite: 0, mordenite-4; 0, A-2; A, X-I ; A, X-2; m, Y-l ; W, Y-4; 0, L-2; @, L-4. (Re- produced from Ref. 115 with permission from the authors.)

22 I. E. MAXWELL

the amount of hydrogen adsorbed and the catalytic activity of PtY zeolites for cyclohexane dehydrogenation. In addition, the catalyst precalcination temperature prior to reduction had a marked influence on activity. As shown in Fig. 12 an optimum activity was obtained for a calcination temperature of 300"C, which corresponded to platinum particles in the range of 20-50 A in diameter. These workers concluded that very small platinum particles (< 20 diam.) in zeolite Y had reduced hydrogen adsorption capacity and hence reduced dehydrogenation activity.

Dehydrogenation activity has been demonstrated for Rh, Co, and Ni forms of zeolites X and Y (123-125). Both cyclohexane and tetralin dehydrogenation to benzene and naphthalene, respectively, have been used as test reactions. For N i x zeolites, unreduced Ni2+ ions were considered (124) to be the active centers. The incorporation of Ca2+ ions into the zeolite

40 -

30 .. I

ae

I

- N r

0"

20- 0

al C

.- Y) L

0

10 -

'

100 200 300 400 500

Calcination temperature ("C)

FIG. 12. Dehydrogenation of cyclohexane over 0.24 wt.% Pt--Nay at 300°C. (Reproduced from Ref. 122 with permission from the authors and Bulletin of the Chemical Society of Japan.)


resulted in an increase in activity, which was attributed to an increase in the concentration of accessible Ni2+ sites.

Corma et al. (126) found that PtNaY was an active but rather unstable catalyst for methylcyclohexane dehydrogenation to toluene. These workers studied both the dehydrogenation and the catalyst decay kinetics. I t was concluded that the reaction occurs via a series of consecutive partial dehydrogenation steps, the first of which was rate determining. Further, catalyst deactivation was caused by coke deposition from partially unsaturated precursor molecules.

This section would be most incomplete without discussing the extensive studies of Mobil workers on the dehydrocyclization activity of TeNaX (127-131). Miale and Weisz (127) first showed that the incorporation of electronegative elements such as sulfur, selenium, and tellurium in NaX resulted in enhanced catalytic activity for n-hexane conversion and that depending on the element present, the catalytic characteristics shift from carbon-carbon cracking to dehydrocyclization. In fact, TeNaX was shown to be a very selective catalyst for parafin dehydrocyclization. Comprehen- sive studies (128-130) involving the influence of hydrogen, single-crystal X-ray diffraction, and diffuse reflectance spectroscopy produced a remarkably detailed picture of the most likely aromatization site in the zeolite. As shown in Fig. 13 this was shown to be a telluride ion (Te2-) located in the supercage and coordinated to two Na' ions at sites I1 and 111.

In a final paper (131) in this series the novel TeNaX catalyst was compared with Cr,O,/Al,O, catalyst for n-hexane dehydrocyclization activity. Both

FIG. 13. Projection ( I , T, 0) of TeNaX structure. Solid lines, silicon-aluminum framework; circles, oxygen framework. (Reproduced from Ref. 129 with permission from the authors.)

24 I . E. MAXWELL

catalysts showed the same dependence of reaction rate on hydrogen pressure. Further, the kinetic data were consistent with the stepwise dehydrogenation mechanism which had been previously established for Cr20,/A1,0, catalysts (132).

IV. 01 igomerization Reactions

A. ETHYLENE

1. Dimerizution

A number of zeolite-based catalysts are active for the dimerization of ethylene. The major products are n-butenes ( 1-butene, truns-2-butene, cis-2-butene), i.e.,

The isomer ratios are often close to the equilibrium composition at the particular reaction temperature, indicating isomerization as well as dimerization catalytic activity. The two most extensively studied zeolite catalysts contain nickel and rhodium, incorporated via ion exchange, and will be discussed separately.

Ethylene dimerization catalysis has, however, been more thoroughly investigated for the broader range of homogeneous catalysts. For example, active metal complexes containing titanium, nickel, iron, cobalt, rhodium, ruthenium, and palladium, are all known (133). Where possible, comparisons will be made with the relevant homogeneous catalyst systems.

a. Nickel Zeolites. Heterogeneous catalysts prepared by the impregnation of Ni2+ on amorphous aluminosilicate (followed by calcination to yield dispersed NiO) have been known for some time (134-1 37). Typically, n-butenes are obtained with selectivities of about 80% (based on ethylene reacted) at temperatures of 275-300 'C and atmospheric pressure. In a series of publications, Russian workers (135) reported on the comparative activities and selectivities of a variety of Ni2'-exchanged zeolites, together with NiO on amorphous aluminosilicate. In addition, some attempts were made to define the nature of the active site.

Catalysts were prepared (138,139) by both ion exchange and impregnation of zeolites with Ni2+. Ion exchange resulted in the most active but least selective (approximately 25% selectivity to n-butenes under conditions com-


parable to those described above) catalyst. The degree of acidity of the zeolite substrate was varied, i.e., Cay , CaX, HY, HM (mordenite), but no obvious correlation between acidity and activity or selectivity was found.

Treatments with hydrogen (at 480°C) resulted in complete and partial losses in activity (139) for impregnated and ion-exchanged catalysts, respectively. This could be correlated with the degree of reduction which was incomplete under these conditions for the ion-exhanged catalysts. It was therefore concluded that the active site involved Ni2+ ions bonded to the oxygen anions of the zeolite framework and that nickel metal was inactive in the temperature range 150-250°C.

Yashimo et al. (140) found that NiY (70% exchanged, equivalent to 9 wt. % Ni) was very active and selective for ethylene dimerization at only 20'C and 200 Torr ethylene pressure. The higher nickel loading would not seem to be sufficient to account for this very marked difference in activity as found between the Russian and Japanese workers. It may be relevant that the latter workers carried out their exchange using NiCl, [the former workers used Ni(NO,),]. Halide ions were shown to be essential to the ethylene dimerization activity of Rh(1) complexes (141a). Further work will, however, be required to clarify this aspect.

The mechanism of ethylene dimerization for heterogeneous nickel- containing catalyst systems does not appear to be at present well understood.

b. Rhodium Zeolites. Rhodium complexes are well known for their homogeneous catalytic activity for a variety of reactions with olefins (133). In general, they exhibit high activity and specificity for these reactions which include olefin oligomerization. For example, the simple hydrated chloride of rhodium (RCI,.3H20) was shown by Alderson e f af. (1416) in 1965 to catalyze the dimerization of simple olefins including ethylene with high selectivity. However, it was some ten years later before studies on a zeolite- based heterogeneous rhodium catalyst were published.

Under very mild conditions (0-20"C, 200 Torr ethylene pressure), ethylene was shown to be selectively dimerized to n-butenes over RhY (140). As shown in Fig. 14, 1 -butene was formed initially but further isomerized to an equilibrium composition of n-butenes with increasing reaction time. In a comparative experiment using HY as a typical solid-acid catalyst, no ethylene conversion was measurable up to 200'C, and at higher temperatures unselective polymerization and cracking reactions occurred. This provided good evidence that the selective dimerization over RhY did not proceed via a carbenium ion mechanism.

The same authors (140) then carried out a number of experiments where the catalyst pretreatment conditions were varied and reagents were added to the reaction mixture (i.e., ammonia, pyridine, and CO) in order to elucidate the type of active site. Rather surprisingly, it was concluded that

26 1. E. MAXWELL

0 20 40 60 80 100 120 140 i0

react ion t ime (min)

FIG. 14. Composition change of produced n-butenes with reaction time. Reaction conditions: catalyst, RhY (0.30 g) activated by evacuation at 300°C for 1 hr; temperature, 0 C ; ethylene initial pressure, 200 Torr. (Reproduced from Ref. 140 with permission from the authors.) 0, I-butene; 0, trans-2-butene; @, cis-2-butene.

the active sites in RhY were highly dispersed zero-valent rhodium. This conclusion contrasts with studies on the homogeneous rhodium system (141) which indicated that a monovalent rhodium complex was the active entity.

More recently, other Japanese workers (1 12) also investigated the active sites in RhY for ethylene dimerization. These workers found that there was a distinct optimum activation temperature with respect to catalytic activity as shown in Fig. 15. XPS studies on the same catalyst (50% exchanged RhY) showed that there were substantial shifts in the Rh 3d line intensities and binding energies as a function of activation temperature. By comparison with known compounds the XPS spectra were interpreted in terms of a composition of trivalent, monovalent, and zero-valent oxidation states of rhodium. Deconvolution of the spectra gave the relative concentrations of each oxidation state for each activation temperature, as shown in Fig. 16. Comparison of Figs. 15 and 16 shows that there is a good correlation between the ethylene dimerization activity and the Rh( I ) concentration at various activation temperatures, with a maximum at 250 C. Contrary to suggestions by the previous workers (140), it was therefore proposed that Rh(1) was the active oxidation state for the ethylene dimerization reaction.

NONACID CATALYSIS WITH ZEOLITES

0.5

- c

0.4 C .- E . &

0.3 - P

0 2 ql 2 0.2

f 0.1

z P r

K

-

-

-

-

27

0.2

0.1

0 I 0 100 200 300 400 500

Activation Temperature ("C)

FIG. 15. Ethylene dimerization activity for RhNaY catalyst as a function of activation temperature. (Reproduced from Ref. 112 with permission from the authors.)

100

80

ae .- 5 60

i?

- +-

U 0 .-

40

3

20

0 100 200 300 400 500 600

Activation Temperature ("C)

FIG. 16. Oxidation state of Rh in the RhY (50%) catalyst as a function of activation temperature: 0. Rh(I1l) (Rh 3d 5/2, 310.2 eV); 0, Rh(l) (308.3 eV); and 0, Rh metal (307.5 eV). (Reproduced from Ref. 112 with permission from the authors.)

28 I . E. MAXWELL

Clearly, with good evidence for Rh(1) as the active species in RhY, there is now a consistency between the homogeneous (141) and heterogeneous catalyst systems. A mechanism for the homogeneous dimerization reactions has been proposed by Cramer (141), which involves the formation of anionic Rh(1) complexes. The presence of such species inside the cavities of the anionic zeolite framework does not seem very probable.

Further studies will be required to elucidate the mechanism and details of the active species for the RhY catalyst.

c. Palladium Zeolites. Palladium complexes, such as tetrachlorobis- (ethylene) dipalladium (PdCI,C,H,), , are known homogeneous catalysts for the dimerization of ethylene to n-butenes (133). It is therefore not so surprising that Pd zeolites have recently been shown to exhibit activity for this reaction (142-146). Both impregnated and ion-exchanged catalysts have been prepared, the latter by exchange using aqueous solutions of [Pd(NH,),]CI,. Na mordenite, Nay , CaNaY, and CaNaX were used as substrates. Catalytic activity for ethylene oligomerization was observed in the temperature range 50-200°C [atmospheric pressure, gas hourly space velocity (GHSV) 900 hr-’1. However, in general the selectivity to n-butenes was rather poor (maximum 60%), even at relatively low ethylene conversions. Hydrogen treatments were found to deactivate the catalyst due to the reduction of Pd” to PdO. This result together with some XPS data was used to arrive at the conclusion that cationic palladium was the active species. Under milder conditions (20”C, 200 Torr ethylene) other workers (140) found PdY (also prepared via ion exchange with [Pd(NH,),]CI,) to be inactive for ethylene dimerization.

In general, the active homogeneous palladium complexes contain a halide ion (133), typically chloride, in the coordination sphere. The influence of halide ions on the Pd zeolite catalysts would therefore seem worthy of study.

In addition to the previously discussed catalysts (i.e., NiY, RhY, PdY), Yashima et al. (140) also examined Cry , RuY, COY, Fey, MnY, CdY, and ZnY for catalytic activity under mild conditions (20”C, 200 Torr ethylene). All these catalysts, with the exception of PdY, were prepared by ion exchange using the corresponding transition metal chloride salts. Chromium Y zeolite exhibited high activity, but poor selectivity to butenes. A batch experiment using benzene as a solvent showed that highly crystalline polyethylene was the major product, which will be discussed in more detail later. Ruthenium Y zeolite showed selective dimerization activity to n-butenes, but was less active than RhY and NiY. The remaining metal ion-exchanged zeolites were all inactive under the chosen reaction conditions.

d. Other Metal Zeolites.


2. Polymerization

During screening studies for oligomerization activity of various transition metal ion-exchanged Y zeolites, Yashima et al. (140) found that CrNaY (82% ion exchanged with CrCl,) was highly active for ethylene polymerization. A more detailed study (147) showed that the polyethylene produced had a high melting point, high molecular weight, high density, and a linear chain structure without branching. In fact, the properties of the polymer product were considered to be similar to those obtained using chromium oxides in the Phillips process. On the basis of spectroscopic studies and due to the influence of various calcination procedures on catalytic activity, it was proposed that Cr2+ ions in the zeolite cages were the active sites for the polymerization reaction. However, Russian workers (148) carefully prepared Crz + -exchanged zeolite Y under oxygen-free conditions and found that this material was inactive for ethylene polymerization. A similar lack of consensus as to the active species exists for the Phillips catalysts where both Cr3+ (149, 150) and CrZf (151-153) have been proposed to be the active oxidation states. Further studies will be necessary to resolve this question. More recently, Czech workers (154) have shown that the polymerization activity of chromium zeolite Y catalysts can be considerably enhanced when ion exchange is carried out using ultrastable HY. These workers propose that this is due to an improvement in the stability of the active valence state of chromium in the ultrastable form of the zeolite.

Cobalt-containing zeolites have been studied for polymerization of ethylene (155-157). The catalysts which were prepared by precipitating cobalt carbonate together with zeolites A, X, Y, and mordenite were not very selective, yielding large amounts of ethane as well as C, and C, hydrocarbons.

B. ACETYLENE

Kruerke (158) appears to have been the first to screen various metal ion- exchanged zeolites for acetylene oligomerization activity. Although detailed data have not been published, these studies showed that Ni2+- and Co2+- exchanged zeolite Y were very active for this reaction, producing benzene at near room temperature. The Mn2+-exchanged form of zeolite Y was only slightly active, whereas the Na', Ca2+, Zn2+, and Cu2+ forms were all inactive for this reaction. French workers (159, 160) more recently screened a wide range of first-row transition metal ion-exchanged Y zeolites and concluded that the active cations for acetylene trimerization were those with an even number of partially filled d orbitals, i.e., d8(Ni2+, Co'), d6(Fe2+),

30 1. E. MAXWELL

d4(Cr2+) were active, whereas d'O(Zn2+, Cu+), d9((Cu2+), d7(Co2+), and dS(Mn2+, Fe3+) were found to be inactive. This activity behavior seems to be consistent with a model suggested by Union Carbide workers (159, where the reaction path involves two acetylene molecules simultaneously coordinated to the metal cation. These studies (159,160) were unfortunately only based on infrared spectroscopic measurements rather than microreactor experiments and thus provide relatively little information with regard to catalytic activity, selectivity, and stability. The NiY system was examined in some detail and a correlation was found between catalytic activity and the number of Ni2+ ions in the supercages. This accounted for the fact that NiY was only active when the exchange level exceeded 12 Ni2+ cations per unit cell. At lower exchange levels these cations are preferentially located in the hexagonal prisms and sodalite cages, which are inaccessible to acetylene molecules.

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethyIbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages.

C. PROPYLENE

1. Nickel Zeolites

Recently, a large variety of ion-exchanged zeolites of type X and Y were examined (163) for propylene oligomerization activity. These included Lay, Lax , CeX, MgX, NiY, COY, AIY, MgX, MnY, NIX, COX, and CaX zeolites which were tested in a fixed bed reactor at 190'C. With the exception of Nix, all the zeolites tested showed rather unselective hydrodimerization activity leading to a wide variety of paraffinic products. The appearance of saturated C, , C,, C, , and C, products was indicative of cracking reactions was well as hydrogen transfer.

Nix, however, showed a high selectivity resulring in 95.5';.:, dimers under the reaction conditions. The product composition of propylene dimers was studied as a function ofcontact time in order to distinguish between primary and secondary products. 3-Methylpentenes were shown to be primary prod-


ucts and not formed via secondary isomerization of either hexenes or 2-methylpentenes. This indicated that a mechanism was operative similar to that found (164) for NiO on silica-alumina. Cyclobutane derivatives were invoked as intermediates in order to explain the primary product distribution.

In contrast to NIX, NiY showed very poor selectivity for dimerization; the major products resulted from hydrodimerization and cracking reactions. It was concluded that these unselective reactions occurred over strong acid sites which were present on all the studied catalysts with the exception of Nix .

2 . Alkali Metal Zeolites

In a patent to Asahi Electro-Chemical Co. (165), alkali metal- (i.e., Li, Na, K, Rb, or Cs) containing zeolites are claimed as active and selective catalysts for dimerizing C,-C, a-olefins. An example is given in which a catalyst is made by adding potassium metal (30 wt.%) to calcined KA. In a batch experiment at 150°C (heptane solvent) hexenes are obtained at the rate of 0.2 g (g catalyst)-' hr-'. By-product formation is claimed to be inhibited under these conditions.

D. CYCLOPROPENES

Zeolites have been shown to exhibit a rather novel type of catalysis for the cycloaddition reaction of cyclopropenes to give tricyclohexanes (166), i.e.,

For a variety of methyl-substituted cyclopropene molecules, and a variety of zeolites (e.g., NaCaA, NaA, CaA, NaX, and HY), the reaction selectivity to dimer, diene, or polymer was found to be determined by at least three factors, namely :

(1) type of active site, e.g., KA > NaA; ( 2 ) size of reactant molecule (i.e., number of methyl substituents); (3) shape and size of zeolite pores. Dimerization was found to occur only at a specific ratio of the size of the

cylopropene (or methyl-substituted) molecule to the diameter of the pores. The best results were obtained with KA and NaA; HY resulted in polym-

erization in all cases. An ionic dimerization mechanism was proposed whereby the spatial constraints within the zeolite (with the correct size ratio) impede the approach of a third olefin molecule which would lead to polymerization.

32 I . E. MAXWELL

E. BUTADIENE

Butadiene dimerization catalysts have been quite extensively studied, although the major effort has been concentrated on homogeneous catalyst systems using complexes containing nickel (167). iron (168), cobalt (169, and palladium (1 70).

The products are normally the cyclodimers, 4-vinylcyclohexene and cycloocta- 1 ,5-diene, together with some trimer, cyclododeca-l,5,9-triene and possibly a small amount of divinylcyclobutane depending on the conditions and the particular catalyst, i.e.,

Until recently when a number of these homogeneous catalysts were heterogenized (I 71), the only known heterogeneous catalysts were based on metal ion-exchanged zeolites.

1. Copper Zeolites

The first work in this area appeared in the form of two patents assigned to Union Carbide in 1969/1970 (I 72, 173). These patents described methods of preparation of monovalent copper-containing zeolites which were claimed to be active and selective catalysts for the cyclodimerization of butadiene to 4-vinylcyclohexene (VCH), i.e.,

The same catalysts were also claimed to catalyze the coupling reaction between butadiene and acetylene to produce 1,4-cyclohexadiene, i.e.,

Both reactions could be carried out under mild conditions (100-1 10°C and atmospheric pressure). The very high selectivity to VCH in the former reaction is of particular interest since the only other butadiene dimerization catalysts (167) known at that time were homogeneous and in general gave, in addition to VCH, several other dimerization products (e.g., 1,4-cyclo-


(from reducing agent)

octadiene and divinylcyclobutane). VCH is of potential commercial interest since it can be readily converted to styrene via oxidative dehydrogenation.

Two methods of preparing Cu+ zeolites have been described ( I 72-1 74) , namely,

(1) direct exchange of the zeolite with Cu’ ions (via Cul in liquid ammonia), and

(2) initial exchange with Cu2+ ions followed by mild reduction to the Cu+ form.

Catalysts prepared via the latter method were found to be more rapidly deactivated during reaction. This is almost certainly due to the formation of Bronsted or Lewis acid sites during the reduction step (175, 176), i.e.,

Ht

Hf cui J

0 0 0 0

Si A1 Si 0’ ‘ 0 0’ \o o/ \o 0’ \o

The acid sites so formed catalyze the polymerization of butadiene which leads to catalyst deactivation.

Maxwell et al. (177, 178) studied the deactivation of reduced Cu2+Y catalysts for butadiene cyclodimerization in some detail. This work showed that the catalyst stability could be markedly improved by using NH, as a reducing agent and choosing the activation conditions such that excess NH, remains selectively chemisorbed on the zeolite acidic sites. Further, the Cu2’Y-derived catalyst was thermally stable to 850°C and was therefore able to withstand a regeneration procedure which involved a polymer burn-off at 550°C. By contrast, the catalysts prepared by direct exchange with monovalent copper, i.e., Cu+Y, formed CuO irreversibly when heated above 330°C.

An X-ray structure analysis was carried out (179) on a single crystal of natural faujasite which had been exchanged with CuZc ions, dehydrated, and then exposed to butadiene. The major effect of adsorbing butadiene was to induce a migration of copper cations to site 111, located at the pore entrances to the supercages (see Fig. 17). The unsaturated coordination of

34 I . E. MAXWELL

S i , A t

FIG. 17. Perspective view showing the siting of Cu(II1) cations at the pore entrance to the supercage (Cu2+-exchanged faujasite, dehydrated at I SWC, butadiene adsorbed). The occu- pancy factors are such that there is approximately one Cu(11l) cation per two pore entrances.

these cations to the zeolite framework and their ideal location for interaction with adsorbate molecules led to the suggestion that these were most probably the sites where the butadiene cyclodimerization reaction occurs.

However, the zeolite is not a unique substrate for this reaction, as is indicated in a recent patent (180), where it is shown that a Cu+-exchanged mont- morillonite clay and synthetic amorphous aluminosilicate will also catalyze butadiene cyclodimerization with high selectivities to VCH ( 2 95:'<). Pre- exchange of these aluminosilicates with Cs+ ions was claimed to increase catalyst stability. This is most probably explained by a reduction in surface acidity resulting from the alkali metal ion exchange.

The remarkable feature of all the above-described catalysts is their ability to give very high selectivities to VCH, in contrast to the homogeneous


catalyst systems (167). These homogeneous catalyst studies showed that the presence of at least two strong ligands in the coordination sphere of the metal ion was required for the formation of the o-allyl, n-ally1 complex, a precursor to VCH, i.e.,

l+

The possibility therefore existed that for these Cu+ aluminosilicate catalysts, oxygen anions fulfill this strong ligand role. To check this proposal a Cu+ complex was prepared (179) which contained extremely weak ligands, namely, cuprous trifluoromethanesulfonate. Surprisingly, this complex under homogeneous reaction conditions was also found to catalyze butadiene cyclodimerization selectively to give VCH (90%). It was therefore concluded (179) that in this case the selectivity to VCH probably resided in the electronic structure of the cuprous ions rather than any particular ligand effect of the zeolite framework.

2 . Other Metal Zeolites

Soviet workers (181) studied the cyclodimerization of butadiene on various cationic forms of zeolite X. However, it is noteworthy that the temperature range where catalytic activity was observed (300-6OO'C) was considerably higher than that for the previously discussed Cu' zeolite catalysts (90- 1 lO'C). Interestingly, under these conditions the parent form of the zeolite, i.e., NaX, is also on active catalyst and forms VCH in high yields (99%). Ion exchange of NaX with Cu2+, Ni2+, Cr3+, and Co2+ (0.5-1%) apparently had no marked effect on activity. However, cation exchange with rhodium resulted in a 2-2.5-fold increase in the rate of butadiene cyclodimerization. The catalytic activity of NaX could possibly be due to a concentration effect of the reactant molecules in the zeolite pores. This would be expected to be particularly pronounced if the reaction was second order in butadiene. Such is the case for the thermal reaction of butadiene at 650'C (182). Furthermore, VCH is the major reaction product only at short residence times, probably because this molecule has the lowest standard heat of formation of the possible cyclodimerization products (183). The activity and selectivity of NaX for butadiene dimerization could possibly be due to rate enhancement of the thermal dimerization reaction under the prevalent conditions (i.e., relatively high temperature, short residence times).

36 1. E. MAXWELL

A patent (184) describes zeolite-based catalysts for the direct production of ethylbenzene from butadiene, i.e.,

The reaction was carried out at very high temperatures (500°C) using various cation-exchanged forms of mordenite (i.e., Ni", Cu2+ , Zn2+, Co2+, Cr3+, Pb2+, H', Bi3+, Mn2+) . Comparative experiments showed that the Co2+- exchanged zeolite gave the highest selectivity to ethylbenzene (54.3%) and the lowest level of solid residue on the catalyst under the chosen reaction conditions. The reaction almost certainly involves the cyclodimerization of butadiene to VCH followed by dehydrogenation to form ethylbenzene. It is not clear from the patent whether hydrogen is a product of the reaction or whether hydrogen transfer occurs. The formation of some ethylcyclohexane suggests that the latter reaction is occurring to some extent. The rather low selectivities and relatively high extent of solids formation render these catalysts unattractive for a single-step route from butadiene to ethylbenzene.

F. II-BUTENE

n-Butene dimerization to octenes is of potential commercial interest, since the octenes are useful intermediates in the preparation of higher alcohols (e.g., nonanols) via hydroformylation. The higher alcohols are themselves intermediates for the synthesis of biodegradable detergents such as alkylsulfates. The traditional heterogeneous catalysts for the oligomerization of lower olefins are supported COO and NiO. More recently, however, nickel-exchanged zeolites have attracted attention as catalysts for these reactions.

In a patent (185), ion-exchanged zeolites containing group VIII metal ions were claimed as catalysts for the dimerization of lower olefins. In fact, only nickel-exchanged zeolite X was given in the examples. An interesting additional feature of the patent was the pretreatment of NIX (5% Ni by weight) with an organic or inorganic base in order to improve the catalyst performance.

The base pretreatment increases both activity and selectivity. The latter effect is almost certainly due to neutralization of residual zeolite acidity which would catalyze undesirable side reactions such as oligomerization, hydrooligomerization, polymerization, and cracking.

More detailed information on 1-butene dimerization over N i x is given in a publication (186) by the same authors of the above-mentioned patent


MOLAR FRACTION AT REACTOR EXIT

‘ O r

0 10 20 -I 30

r ( g hr/mol) RESIDENCE TIME

1-Butene dimerization over a NiX/Li,O catalyst at 180°C. (Reproduced from Ref. FIG. 18. 187 with permission from the authors and La Chimica c L’lndustria.)

(185). For these studies the NIX precursor catalyst has been impregnated with molten lithium acetate. The catalyst displayed remarkably good selectivity to octenes. A typical set of results is shown in Fig. 18, where the reactant and product concentrations are given as a function of contact time, 7. The results indicate that isomerization of 1-butene to an equilibrium mixture with 2-butenes (cis and trans) occurs rapidly, followed by the slower dimerization of butenes to octenes. The reaction was carried out under mixed phase conditions with the 1-butene reactant in the gas phase and the octene products in the liquid phase (trickle-bed reactor). Under the conditions the reaction rate was found to be mass-transfer controlled.

In a subsequent paper (187) the same authors studied the kinetics of propylene/ 1-butene codimerization over the same NiX/Li,O catalyst.

As might be expected, the selectivity to heptenes is not particularly high due to competitive reactions such as self-dimerization of propylene and butene.

G. ISOBUTYLENE

Isobutylene is present as 20-30% of the C , fraction from the naphtha cracking process. A number of different upgrading reactions with isobutylene have been carried out industrially (with and without prior separation from the C , fraction). One of these includes the acid-catalyzed oligomerization

38 1. E. MAXWELL

to dimers and trimers (48) which are useful intermediates for the manufacture of plasticizers. Russian workers, in particular, have been systematically studying the use of zeolites as catalysts for isobutylene oligomerization for some years. The activities and selectivities of a variety of ion-exchanged zeolites have been examined by these workers (188-1 91).

A number of general trends can be discerned, i.e.,

(1) X-type zeolites are more active than their zeolite Y analogs. (2) Polyvalent cation-exchanged forms are more active than their mono-

valent analogs. (3) The activity order for the alkali metal ion forms is inversely pro-

portional to the ionic radius, i.e., Li' > Na' > K + > Rb'. (4) The hydrogen forms of zeolite X and Y are generally more active,

but less selective, than their cation-exchanged forms. Side reactions include hydrogen transfer (resulting in the formation of coke and paraffinic products), double-bond migration and disproportionation.

In general, the catalysts are not stable due to the build-up of coke. However, regeneration can be accomplished by a coke burn-off at 480°C in a stream of air.

(6) CO, has been found to have a promoting effect (191-193) on the overall catalyst activity, but decreases oligomerization selectivity.

Although the reactions have been generally described in terms of a carbenium ion mechanism, this does not altogether explain the catalytic behavior of the alkali metal ion-exchanged zeolites or the selectivity behavior. An ionic mechanism of the type previously described for cyclopropene dimerization would seem to be more appropriate for the alkali metal ion- exchanged zeolites, where the activity does seem to correlate qualitatively with the electrostatic field ( e / v ) exerted by the cation.

The CO, effect has been explained (IYf) in terms of the formation of alkali carbonate or bicarbonate species, where the cation is replaced by a proton derived from a water molecule. This proposal does indeed account for both the observed increase in activity and decrease in selectivity which is similar to proton-exchange forms.

Rhein and Clarke (194) screened a wide variety of alkali metal ion- containing zeolites (A, X, Y, and L) for isobutylene polymerization activity under mild conditions (room temperature, in an autoclave). In general the activities were rather low, requiring some days to achieve reasonable polymer yields. The polymers obtained were characterized by a bimodal molecular weight distribution. The low and high molecular weight peaks were at approximately 250 and 3000-5000, respectively. Polymerization only oc-

( 5 )


curred in the presence o f zeolites with pore-entrance diameters in excess of 5 A (i.e., zeolites 3A and 4A were inactive), which is close to the kinetic diameter of isobutylene. This does seem to indicate that the reaction occurs at least in part within the pores of the zeolite. Polymer yields for different zeolites were on the order of 5A > X > Y > L. Ion exchange with various transition metal ions gave the following order for polymer yield: Pd > Pt x Ni > Cu.

V. Carbonylation

Carbonylation of methanol has in recent years become a commercially important route for the production of acetic acid and methyl acetate. In- dustrial catalysts are at present homogeneous, based on cobalt and more recently rhodium compounds. The cobalt catalysts are less active (195) and require more severe operating conditions (i.e., 250°C, 650-750 atm) than the rhodium-based catalysts (196) ( 1 70-25O0C, 7- 14 atm).

It is likely that synthesis gas and methanol (also made from synthesis gas) could become important basic building blocks of the petrochemical industry in the near future, particularly as coal and natural gas gain use as feedstock materials. Such a development would be expected to increase the relative importance of carbonylation reactions in industrial chemistry.

A. METHANOL

Since the discovery of highly active homogeneous rhodium catalysts for methanol carbonylation by Monsanto workers (f 96) , there has been considerable industrial and academic research effort in this area. The catalyst system consists not only of a rhodium complex, but also includes a halogen promoter, preferably containing iodine, typically methyl iodide, which is regenerated at the end of the catalytic cycle. The reaction is highly selective, giving high yields (99%) of carbonylated products (i.e., acetic acid or methyl acetate, depending on reaction conditions). The trace by-products are usually dimethyl ether and acetaldehyde. In 1970 Monsanto ( IY6) brought the first acetic acid plant on stream which was based on this new homogeneous catalyst system. The important advantages of the rhodium catalyst are the higher methanol conversion level and lower operating pressure compared to the previous cobalt catalyst system developed by BASF (195).

40 I . E. MAXWELL

The reaction mechanism proposed by Roth et ul. (196) is as follows:

CH,OH + HI # CH,I + H,O (1)

Rh(I)L, + CH-J CH,Rh(III)IL, (2)

CH,Rh(III)IL, + CO#CH,Rh(III)ICOL, (3)

CH,Rh(III)ICOL,c-LCH,CORh(III)IL, (4)

( 5 ) CH,CORh(III)IL, + H,O#Rh(I)L, + CH,COOH + HI

where L, are ligands which may be of different types. Step (1) involves the formation of methyl iodide, which then reacts with

the rhodium complex Rh(I)L, by oxidative addition in a rate-determining step (2) to form a methylrhodium(II1) complex. Carbon monoxide is incorporated into the coordination sphere in step (3) and via an insertion reaction a rhodium acyl complex is formed in step (4). The final step involves hydrolysis of the acyl complex to form acetic acid and regeneration of the original rhodium complex Rh(I)L, and HI. Typical rhodium compounds which are active precursors for this reaction include RhCl, , Rh,O, , RhCl(CO)(PPh,), , and Rh(CO),CI, .

Although the homogeneous catalyst systems have been successfully applied in commercial practice, some intrinsic problems associated with catalyst separation remain. This has led to considerable interest in the development of a suitable heterogeneous analog. Rhodium compounds have been heterogenenized on substrates such as carbon (197), alumina (198, 19Y), and synthetic polymers (200). More recently, zeolites have also attracted quite some attention as a support material for carbonylation catalysis, as is discussed later.

Scurrell (201) recently briefly reviewed the literature on heterogenized homogeneous rhodium catalysts for methanol carbonylation up to 1976.

1. Rhodium Zeolites

a. Zeolite X . Russian workers appear to have been the first to prepare a rhodium-zeolite catalyst (202) (i.e., RhNaX) and to show that this material was highly active, stable, and selective for methanol carbonylation. This catalyst was prepared by impregnating NaX with an aqueous RhC1, solution neutralized with 10% NH, to pH 4.5. At atmospheric pressure and 250% methyl acetate was obtained with a 87-90% selectivity (dimethyl ether was the sole by-product) at a methanol conversion level of about 79%. The specific activity of this catalyst (- 50 g methyl acetate (g Rh)-' hr- '), which contained 0.2 wt. % Rh, well exceeded that of 3 wt. Rh supported on activated carbon (197) (8-18 g methyl acetate (g Rh)-' hr- ' ) (197).

In a subsequent publication, Nefedov et ul. (203) examined the influence


of various catalyst and process parameters for this reaction. It is interesting that NaX was found to be inactive for dimethyl ether formation in the absence of methyl iodide but quite active in the presence of the latter (98% conversion of CH,OH, at 250"C, GHSV 0.64 hr- '). Although this was not stated in the publication, it would appear that NaX is a catalyst for the dehydrohalogenation reaction, i.e.,

CH,OH + CHJ ~t CH,0CH3 + HI

The specific activity for both methyl acetate and dimethyl ether as a function of rhodium level on the catalyst was measured and is shown in Fig. 19. Clearly, there is a marked decrease in specific activity for both products with increasing rhodium level in the range 0.5-1 wt.% Rh. The optimum in terms of catalyst efficiency was considered to occur in the range 0.25-0.5 wt.% Rh. The reaction rate was found to be zero order in both CO and CH,OH partial pressures, as has also been found for the homogeneous catalyst system (196).

Russian workers have also investigated the influence of different support materials on methanol carbonylation activity and selectivity. A summary of the results obtained is given in Table 11. It would appear from these data that the selectivity to methyl acetate decreases with increasing support acidity. These results are consistent with those of other workers (204) who also found rather poor carbonylation selectivities (< 50%) for rhodium complexes supported on y-alumina, due to ether formation. Thus NaX is shown to be the best support resulting in a catalyst with both high activity and selectivity. In a comparative study (205), RhNaX catalysts were prepared by both ion exchange and impregnation with RhCI,. The former catalyst was superior with regard to both activity and selectivity, provided that the chloride ions were thoroughly removed following ion exchange. A

0.2 0.6 1 .o Rh. %

FIG. 19. Specific activity of RhNaX for methanol carbonylation as a function of rhodium concentration. (Reproduced from Ref. 203 with permission from the authors and Plenum Publishing Corporation, New York.)

42 I . E. MAXWELL

TABLE 11 E f f i c , t of Support Type on Activity rrnd Selec.riui/v of Imprtgtiuted

Rhodium Cutulysts ,fbr Methunol Crirhotiylririon“

CH,OH Selectivity” to CH,OH Selectivity” to conversion CH,COOCH, conversion CH,COOCH,

Support (mol ‘x) (mol”/,) Support (mol%) (mol ‘i;)

NaX 78.0 74 NaM 34.6 I 1 NaY 80.5 59 A1203 57.5 17 NaA 31.6 59 Amorphous 43.6 0 CaX 60 1 alumino-silicate

~

“0.5 wt. ”/, Rh, 250”C, GHSV 0.67 hr-’, atmospheric pressure, CO/CH,OH = 1.5-1.6,

I, Selectivity, S , to CH,COOCH, was calculated using S = [ T ~ ~ ~ ~ ~ J ( ~ ~ ~ , ~ ~ + rrlher)Ir where CH,I/CH,OH = 0.25.

rEltrr = rate of formation of ester; = rate of formation of ether (sole by-product).

similar influence of chloride ions was observed for carbon-supported rhodium catalysts (198, 206,207).

Treatment of the ion-exchanged RhNaX catalyst with hydrogen (205) results in almost complete reduction of Rh3+ to Rho with a consequent much reduced carbonylation activity, whereas ether formation remains virtually unaffected. These results demonstrate that the dehydration activity is a function of the support only and that cationic rather than zero-valent rhodium is the active entity for the carbonylation reaction.

More recently, Christensen et al. (208) prepared RhNaX by immersion of NaX into an aqueous solution of RhCI, , probably resulting in some ion exchange as well as impregnation of rhodium. The results obtained for methanol carbonylation with respect to both specific activity and selectivity were very similar to those obtained by Nefedov et a/. (203).

b. Zeolite Y . Japanese workers have very recently prepared RhNaY catalysts (209) and compared their methanol carbonylation activity with rhodium supported on A1,0,, SO,-Al,O,, SiO,, and a cation-exchange resin (Amberlite 200 C ) . RhNaY was considerably more active than all the other catalysts, with rhodium or SiO, and Si0,-AI,O, being almost completely inactive for carbonylation. Three RhNaY catalysts were also prepared (209) by exchange with the chloride, nitrate, and sulfate salts of rhodium and shown to have very similar activities.

The optimum rhodium level, with respect to specific activity, for RhNaY catalysts was found (209) to be about 0.6 wt. ‘%, Rh (4% replacement of Na’ assuming Rh3’), which corresponds to only approximately one rhodium site per unit cell of zeolite Y [i.e., an approximate stoichiometry of RhNa,o(A1O,),,(SiO,)l,,l. This implies that the average distance between


rhodium sites is about 25 A. Catalysts containing higher rhodium levels (2-7 wt. % Rh) not only led to decreased specific activity (i.e., carbonylation rate per gram of Rh), but also exhibited decreased stability.

c. Kinetics and Mechanism. For both RhNaX (impregnated) (203) and RhNaY (210), the rate of carbonylation was found to be zero order in the CH,OH and CO partial pressures. At low CH,I/CH,OH ratios (< 0.2), the rate of carbonylation is first order in the CH,I partial pressure for both X (203, 211) and Y (210) zeolite catalysts, thus leading to the rate expression

r = KPCH31P:OP:H,0H

This is in fact analogous to that found for the homogeneous rhodium complexes (196,212).

At higher CH,I/CH,OH ratios (> 0.2) there is evidence that, at least for the zeolite catalysts, the reaction order in CH,I decreases dramatically (203,211). Christensen et al. (211) also examined the effect of CH,I partial pressure on CH,OCH, formation over RhNaX. A rate expression of the following form was found :

for CH,I/CH,OH, 0.05-0.23. This would seem to support earlier studies (203) on RhNaX which indicated that dimethyl ether formation was facilitated by methyl iodide. Activation energies for methanol carbonylation for RhNaX, RhNaY, polymer-supported RhCI(CO)(PPh,), , and homogeneous RhCl, are compared in Table 111. There is clearly quite good agreement between these values for both heterogeneous and homogeneous catalyst systems. Thus the kinetic data indicate that similar reaction mechanisms are occurring.

The above rate expression is also consistent with the previously discussed mechanism where the oxidative addition of CH,I to a Rh(1) complex is the

rether = Kp:2j

TABLE 111 Compurison of Acriuution Energies for Methunol Curbonylution

with Vurious Homogeneous and Heterogeneous Catalysts

Activation energy

Catalyst (kJ mol-') Reference

RhCI, 61.5 212 (homogeneous)

RhNaX 60 211 RhNaY 56.5 210 Polymer supported 55 201

RhCI(CO)(PPh, ),

44 I . E. MAXWELL

rate determining step. The subsequent insertion of CO to form an acyl Rh(II1) complex implies that CH,I rather than CH,OH is carbonylated. In order to gain direct evidence of this or otherwise, Takahashi et al. (210) studied carbonylation over RhNaY using deuterated methanol (i.e., CD,OD/CH,I). Despite some slow alkyl exchange between CD,OH and CH,I, it could be demonstrated by analysis of the CH,CO/(CH,CO + CD,DO) ratio as a function of methanol conversion that indeed there is not direct carbonylation of CH,OH and that this occurs via CH,I, which is regenerated at the end of the catalytic cycle.

There is also direct spectroscopic evidence for the formation of a rhodium acyl complex as an intermediate during methanol carbonylation over RhNaX. Scurrell (213) showed that infrared bands characteristic of the complex CH,CORh(III)ICOL, were formed at 2085 and 1710 cm- ' after exposure of RhNaX to CO and CH,I at 100°C.

With regard to specific activity, it is instructive to compare these rates for a variety of heterogeneous catalysts under similar conditions to those shown in Table IV. Clearly, RhNaY, prepared by ion exchange has the highest specific activity. This is indicative of a high degree of dispersion and good accessibility of these ions to reactant molecules. However, when this specific rate is compared with the best estimate (complicated by the strong pressure dependence in the range 0-2 atm) available from homogeneous catalysis

TABLE IV Comparison qf ' Specific A ctioities, for Mrlliutiol Curbonylcrtiun

wirh Various Hereroyrnrous Rhodium Cutulysrs"

Specific Activation Rh activity

energyh loading (g ester Catalyst (kJ/mol) (wt. %) (g Rh)-' h - ' ) CH,I/CH,OH Reference

RhCI,/A120, 58 0.43 4.3 0.1 20Y Rh-cation 58 0.37 8.0 0.1 209

RhNaX exchange resin

(impregnation 58 0.5 21 0. I 203 (immersion) 60 0.6 17 0.12 211

RhNaY 56.5 0.43 65 0.09 209

RhCI, 61.7 - - 300 -0.1 212 (ion exchange)

(homogeneous)

" 200 C and 1 atm. * The values for the activation energy have been used to correct rate constants to a common

temperature of 200'C.


(212) using RhC1, , RhNaY heterogeneous catalyst is evidently inferior. Thus it would appear that for the zeolite, either the activity per rhodium site is intrinsically lower or only a fraction of the rhodium ions is accessible to the reactant molecules in the supercages. Further work will be required to elucidate this point.

The relatively large average distance between rhodium sites (- 25 A) for maximum catalyst specific activity as found for RhNaY (210) strongly suggests that isolated rhodium species are the active entities. The first-order dependence on rhodium concentration suggests that this is also the case in the homogeneous system (196). However, by contrast, a second-order rate dependence of the Rh(1) complex concentration was found for polymer- supported RhCI(CO)(PPh,), (201). A transition state involving two Rh(1) sites was invoked to explain these kinetic observations.

d. Metal Promoters. Russian workers have recently screened a variety of transition metal oxides and chlorides for possible promoter effects on methanol carbonylation. The addition of < 1% CuCl,, FeCl,, NiCl,, CrC1, , or CoC1, to RhNaX is claimed (214) to increase the yield to methyl acetate and decrease ether formation. Iron oxide (Fe,O,) addition to RhNaX (215) was found to have an optimal promoter effect with a Rh/Fe ratio of 2 . Indeed, this catalyst gave a good selectivity to methyl acetate (94%) at quite a high activity level (126 g ester (g Rh)- hr- at 210°C). A similar promoting action was found for CuO (216) with an optimum at a Rh/Cu ratio of unity. The mechanisms of these promoter effects do not appear to be understood at present.

2. Other Metal Zeolites

Nefedov et al. (217) screened a variety of first-, second-, and third-row transition metal ions (i.e., V, Cr, Fe, Co, Ni, Cu, Mo, Rh, Pd, Ag, Ce, Hf, W, Pt, Au), impregnated as metal salts (0.5 wt. % metal) onto NaX. With the exception of rhodium none of these metal ions showed significant methanol carbonylation activity.

B. ETHANOL AND HIGHER ALCOHOLS

Carbonylation activity of ethanol using ethyl iodide as a promoter has been investigated using RhNaX as a catalyst (208,211). Some carbonylation activity does occur, with formation of ethyl propionate; however, ether and also olefin formation results in rather poor selectivities to the ester. The selectivity decreases with increasing ethanol conversion. Olefin formation was ascribed to a dehydrohalogenation reaction, i.e.,

CH,CH,I -+ CH,=CH + HI

46 1. E. MAXWELL

The observed lack of an overall consumption of ethyl iodide was attributed to reaction of HI with ethanol (21 1), i.e.,

HI + CH,CH,OH --c CH,CH,I + H,O

A comparison of the absolute rates of methanol and ethanol carbonylation (211) indicated that the poor selectivity in the latter case is due to an increase in the rates of the side reactions rather than a large decrease in the rate of carbonylation. These results contrast with the homogeneous system, where ethanol carbonylation was reported (218) to be considerably slower (18 times) than with methanol.

Nefedov et al. (219) found that Fe,O, impregnated on RhNaX had a promoting effect on ethanol carbonylation selectivity as was the case for methanol (215).

Christensen et al. (208) were unable to carbonylate 2-propanol using RhNaX, but few details were given. Russian workers (220), however, showed that the carbonylation rate of higher alcohols could be markedly increased over RhNaX, by increasing the CO pressure.

C. ETHYLENE

Nefedov et al. (221) have reported that ethylene can be selectively hydro- carboxylated to form propyl propionate over NaX which had been exchanged with group VIII metal ions. The activity and selectivity decreased along the series Rh >> Pd > Ni > Co. In autoclave experiments they found that Rh, NaX (1% Rh) at 250°C and 60 atm CO pressure gave 100% conversion of ethylene in the presence of n-propanol and n-propyl iodide with a selectivity of 98.7% to propyl propionate, i.e.,

CH,=CH, + CO + HOCH,CH,CH,

1 c H , c H , coocH,cH,cH,

n-Propyl iodide has a promoter function as for carbonylation and is regenerated at the end of the catalytic cycle.

VI. Hydroformylation

Hydroformylation is now a well-established process in the chemical industry. The reaction involves the addition of CO and H, to an olefin yielding normal and iso-aldehydes :


RCH=CH, + CO + H, -+ RCH2CH,CH0 + CHO

I RCHCH,

Industrially the straight chain isomer is generally the most desired product and hence the normal/iso product ratio obtained for a given catalyst is of importance. Further, the hydrogenation activities of catalysts vary considerably such that alcohols can in some cases be obtained in a single step (222). The first catalysts developed for this reaction were based on cobalt carbonyl and later cobalt carbonyl phosphine complexes. However, more recently attention has been focused on the intrinsically much more active rhodium catalysts (222, 223). A simplified mechanism for (223) cobalt- and rhodium-catalyzed hydroformylation has been proposed which involves the following steps:

(1) reaction of a neutral hydride carbonyl complex with an olefin to form an alkyl complex ;

( 2 ) insertion reaction with CO to form an acyl complex ; (3) reaction with H, to form aldehyde and regenerate the hydride

carbonyl complex.

The selectivity toward straight-chain aldehydes can be increased, for example, by incorporating bulky electron-donating ligands (e.g., PR,) into the metal coordination sphere. These products are also favored by low reaction temperatures and high CO partial pressures (222).

The existing commercial hydroformylation processes are carried out using homogeneous catalysts (222, 223) with the associated disadvantages of catalyst product separation, metal deposition, and catalyst regeneration. This has prompted interest in methods of heterogenizing these homogeneous catalysts. Metal carbonyl complexes have been supported on various types of polymers, SiO,, functionalized SO, , carbon, and A1,0, (224). By comparison, there has been relatively little interest in the use of zeolites as supports. This is probably due to the obvious problems involved in incorporating a bulky metal carbonyl and/or phosphine complex into the relatively small pores of a zeolite. However, more recently there have been two publications (225,226) in which metal carbonyl clusters appear to have been formed in situ and hence encapsulated into zeolites, resulting in active hydroformylation catalysts.

A. COBALT ZEOLITES

The study carried out by Centola et al. (225) was apparently stimulated by a patent (227) in which cobalt zeolites X and Y were claimed as catalysts

48 1. E. MAXWELL

i'i

i s0 f 0

0 1 2 3 4 5 6 7 RUN TIME, hr

FIG. 20. Rate of propylene hydroformylation with CoNaCaA catalyst as a function of run time. (Reproduced from Ref. 225 with permission from the authors and Lu Chimica P L'lndzis- tria.)

for the liquid-phase hydroformylation of C, and higher olefins. Centola et al. (225) prepared three types of catalyst by exchanging zeolites NaA, NaCaA, and NaX with a 1 N aqueous solution of cobalt(I1) nitrate, resulting in cobalt levels of 1-12 wt.%. Propylene hydroformylation was carried out in a continuous-flow reactor. The reaction was carried out in the gas phase in the pressure range 100-400 atm, and care was taken to avoid propylene condensation. The catalysts were simply pretreated by heating under vacuum at 45OoC, after which the feed (propylene/CO/H,) was introduced. Typically, an induction period was observed, followed by a transition phase of apparent high activity, and finally a steady state as shown in Fig. 20. The induction period was attributed to the reduction of Coz+ to Coo and formation of carbonyl complexes. This seems very plausible but no direct evidence of carbonyl formation was given. The transition period was considered to be due to a homogeneous catalyzed reaction caused by the removal of small amounts of metal carbonyl from the zeolite.

Some typical results obtained for various Co zeolites under steady-state conditions are shown in Table V. Clearly, CoNaA and CoNaCaA display quite good propylene hydroformylation activity, whereas CoNaX is relatively inactive. In all cases the selectivity to total aldehydes was 99%. the other products being isobutyl and n-butyl alcohol.

Cobalt losses from the catalyst after 60-h operation did not exceed 3%. This is perhaps not too unexpected under gas-phase reaction conditions. The proposed in situ formation of cobalt carbonyl complexes, which are encapsulated in the zeolite cavities, does seem to be plausible.

The performance of the CoNaA and CoNaCaA zeolites in a gas-phase reaction was considered to offer a number of advantages over the existing homogeneous catalyst processes. However, it was noted that the specific activity of these heterogeneous catalysts (in moles aldehyde per moles


TABLE V Propylene Hydroformylation Data Obtuined Using Various Cobalt Zeolites"

level Temp. Pressure Aldehyde Normal/iso Catalyst fwt. %) ("C) ( a W yield aldehyde

CoNaX 8.0 220 200 1 .o 1.23 CoNaA 6.4 210 310 21.0 1.12 CoNaCaA 6.4 210 260 21.0 1.37

Conditions: propylene, 10 m o l x ; CO, 45 mol % ; H,, 45 mol x; flow rate, 20 Nl/hr. Defined as moles aldehyde per 100 moles of propylene.

cobalt per hour) was lower than for the homogeneous analogs. This fact, together with the poor activity of CoNaX, suggests that a considerable percentage of the cobalt remains in regions of the zeolite framework (e.g., sodalite cages) which are inaccessible to reactant molecules.

A recent patent (to UOP) (228) claims that active hydroformylation catalysts can be prepared by reacting an aluminated zeolite (prepared from a hydrosol) with HCo(CO), vapor. The HCo(CO), complex apparently reacts with surface hydroxyl groups releasing hydrogen and yielding a surface-bound cobalt carbonyl complex. The catalyst so formed is claimed to hydroformylate higher olefins to aldehydes and alcohols at 120°C and 240 atm pressure.

B. RHODIUM ZEOLITES

A RhNaY (1 wt.% Rh) catalyst was prepared by ion exchange using an aqueous solution of [Rh(NH,),]CI, (226, 229), followed by treatment with a CO/H, ( l / l ) mixture at 130°C and 80 atm pressure. The catalyst so formed was observed to have good activity and high total aldehyde selectivity (-95%) for the liquid-phase hydroformylation of hexene-1 . Some typical results are shown in Table VI, which indicate that the normal/iso aldehyde product ratio is similar to that obtained with homogeneous rhodium car- bony1 catalysts (223).

The catalyst was examined (226) before and after the pretreatment with CO/H, by means of infrared (IR) spectroscopy. IR bands characteristic of terminal and bridging carbonyl groups associated with rhodium carbonyl clusters were observed following this pretreatment of the catalyst. However, these spectra were apparently not identical to those of the known rhodium clusters, Rh,(CO),, or Rh,(CO),, . The differences were associated with the positions of the bands due to bridging carbonyl groups. It is conceivable that

50 I . E. MAXWELL

Hexene-1 Normal, iso Pressure conversion aldehyde

(atm) (mol yo) ratio

50 > 80 0.8 80 > 90 I .o

100 > 95 1.1

a Liquid-phase, autoclave experiments. I, Conditions: catalyst conc., 1.2 mg (at. Rh) - '

liter- I ; temperature, 80°C; run time, 3 h r ; solvent, hexane.

an alternative rhodium catalyst cluster is formed as a result of the spatial restrictions of the zeolite Y supercage (13 A diam.).

The authors also claimed that the zeolite catalyst exhibited unusually high selectivity to dialdehydes (60%) in the hydroformylation of 1,5-hexadiene. Unfortunately, comparative data obtained under similar conditions with homogeneous rhodium carbonyl catalysts were not presented.

A patent (230) to Atlantic Richfield Co. claims that hydride platinum group metal carbonyl complexes such as CIRh(PPh,), supported on zeolites, for example, N a y , are suitable catalysts for the hydroformylation of low molecular weight olefins. However, since the bulky metal complex cannot diffuse into the inner pores of the zeolite it must simply be adsorbed on the external surface of the support. This is consistent with the rather poor catalyst stability which was attributed to leaching of the active species from the support.

VII. Methanation

The synthesis of methane from synthesis gas (CO/H,) is of considerable importance in the production of substitute natural gas (SNG) i.e.,

3Hz $- CO -+ CH, + HZO

In fact, with the expected trend toward upgrading coal and the depletion of United States natural gas reserves, methanation should further increase in importance. The methanation reaction is thermodynamically favorable even at high temperatures and pressures and there are a variety of metals which catalyze this reaction. Vannice (231) compared the turnover numbers for a variety of Al,O,-supported group VIII metals and showed that the rate of


methane formation could be correlated with the heat of adsorption of CO (see Fig. 21). For these metals the rate of methanation increased with decreasing heat of CO adsorption, giving the following order of activity : Ru > Fe > Ni > Co > Rh > Pd > Pt > Ir.

Although nickel is not the most active metal, for reasons of cost and stability, commercial catalysts are based on this metal. These catalysts are in general supported on alumina and contain relatively high metal loadings. There is, however, interest in the development of more thermally stable, sulfur resistant, and possibly even regenerable catalysts in the future. It may be for these reasons that zeolite-based methanation catalysts have recently attracted more interest.

A. PALLADIUM ZEOLITES

Vannice (232) measured turnover numbers for methanation on a variety of well-characterized palladium catalysts. The supported catalysts were all more active than unsupported palladium (Pd black) and PdHY was intermediate between Pd/SiO, and Pd/Al,O, in specific activity (see Table VII). As is evident from the data there is no obvious correlation betwen particle size and turnover number. It was therefore suggested that the enhanced activity of various supported catalysts was due to a metal-support interaction. Figureas et al. (105) also found good evidence for a support effect during benzene hydrogenation studies. In this case the palladium zeolite

CO heat of adsorption (kcallrnol)

FIG. 21. Correlation between methanation activity and AHa for CO. (Reproduced from Ref. 231 with permission from the author.)

52 1. E. MAXWELL

TABLE VI1 Comparison oj'A4erhanaiion Turnovrr- Numbers

f o r Various Palladium Catalysts

Turnover Average Pd number particle

loading (sec-' x lo3, size Catalyst (wt. %) 275-C) (8)

Pd/AI,O, 2 12 48 9.5 10 I20 2 7.4 82

PdHY 0.5 5.9 31 Pd/SiO, 4.75 0.32 28

4.15 0.26 46 Pd black - 0.15 2100

catalysts were more active than palladium supported on silica and alumina. It was proposed that the palladium turnover number increased with increasing electron-acceptor properties of the support. A similar mechanism may be applicable to palladium-supported methanation catalysts (232).

B. NICKEL ZEOLITES

The methanation activity of a series of NiCaY catalysts was recently studied by Bhatia eta(. (233). These authors found that the turnover numbers increased with increasing metal loading, whereas the average particle size remained constant and they attributed this result to increased support acidity and availability of Ni'. Unfortunately, the degree of reduction of Ni2+ to Nio does not appear to have been measured, which might also explain the results obtained. Elliott and Lunsford (234) more recently measured the methanation activity of NiNaY (2 wt. % Ni) and found this to be considerably less active than Ni/AI,O,. The turnover numbers obtained by various workers are compared in Table VIII. It is apparent that the nickel/ zeolite catalysts are significantly less active than the Ni/AI,O, catalysts. As indicated in Table VIII this does not appear to be a particle-size effect. Elliott et al. (234) have proposed that the alkali metal ions (in this case Na') of the zeolite may be responsible for this decreased activity. However, it is noteworthy that the relatively large average nickel particle size (140 A) means that most of the nickel is on the external surfaces of the zeolite crystals where a relatively low concentration of alkali metal ions might normally be expected.


TABLE VIII Comparison of Methanation Activities for Zeolite and Alumina Supporied Nickel Catalysls

Average Nickel particle Reaction Turnover loading size temp. number

Catalyst (wt. %) (A) ("C) (sec-' x lo3) Reference

NiCaY 6.8 7.8 300 7.68 233 NiNaY 2 140 280 5.0 234 Ni/AI,O, 2 360 280 230 234

5 - 275 32 23 I

C. RUTHENIUM ZEOLITES

Elliott et al. (234) very recently carefully prepared and thoroughly characterized a variety of ruthenium methanation catalysts supported on zeolite Y. The purpose of the study was to prepare the catalysts in such a way that the metal remained finely dispersed within the zeolite cavities. For such a catalyst, a significant metal-support interaction might be expected to occur (6 ,23) and thereby induce a change in catalytic behavior. The methanation turnover numbers for RuNaY and RuCaY are compared with Ru/A1,0, in Table IX. The results show that the very high ruthenium dispersion achieved for the zeolite catalysts does not appear to have had very much effect on the methanation specific activity. However, the RuY and RuCaY catalysts were more stable during the methanation reaction than Ru/Al,O, . The deactivation process was attributed to the formation of excess surface carbon via dissociation of CO.

TABLE IX Comparison of Initial Methanution Activities f o r Zeolite

und Aluminu Supported Ruthenium Cutulysts

Average Turnover Ruthenium particle number

Catalyst (wt. %) (A) 280°C) loading size (sec-' x lo3,

RuNaY 0.5 9.8 9.4 2 10 31.5

RuCaY 0.5 9.5 15.9 Ru/A1,0, 0.5 43 19.8

54 1. E. MAXWELL

The same workers (234) also studied the methanation behavior of bimetallic clusters of Ru/Ni and Ru/Cu in zeolite Y. Such clusters can be formed by metals, such as ruthenium and copper which are immiscible as bulk metals (235, 236). The turnover numbers versus bimetallic cluster composition are shown in Fig. 22. Dilution of ruthenium with copper clearly causes a marked decrease in specific activity. This decrease in activity is also accompanied by a decrease in methanation selectivity. This was attributed to an inhibiting effect of copper on the ruthenium hydrogenolysis activity.

The addition of nickel to ruthenium has a less pronounced effect on the methanation activity. This is hardly surprising since nickel is also intrinsically active for methanation. However, dilution of ruthenium with nickel does result in a marked increase in catalyst stability. A catalyst of composition 0.5% Ru, 2% NiY was more stable than those prepared from the pure metals. This improved stability was attributed to an improved balance between the rates of dissociation of CO and hydrogenation of surface carbon, thereby preventing the formation of excess surface carbon. The data presented indicated that a similar improvement in stability was obtained for 0.5% Ru, 2% Ni on A1,0,, which demonstrates that this effect is not support sensi tive.

Of particular interest was the fact that the bimetallic cluster catalysts, i.e., RuNiY and RuCuY, had considerably better metal dispersions than the pure NiY and CuY catalysts. Further, the zeolite-supported bimetallic catalysts were more resistant to sintering during methanation than those supported on alumina. Particle-size measurement indicated, however, that most of the bimetallic clusters were too large to be located inside the zeolite pores.

Gupta et al. (237) studied the effect of in siru ?-irradiation on the methanation activity of RuNaX (1.8 wt.% Ru). An enhancement in activity was found on y-irradiation of the catalyst. This enhanced activity was attributed to an increase in the rate of hydrogenation of surface carbon. No comparative data were presented for other support materials.

Unfortunately, there have been no published studies on the poison resistance of zeolite-based methanation catalysts. NiY catalysts containing Cr,O, , for example, have been shown (238) to exhibit much improved sulfur- poisoning resistance under ethane hydrogenolysis conditions. This potential advantage of zeolites would seem worthy of further study. In addition, bimetallic cluster catalysts might offer improved activity without the disadvantage of high rate of deactivation due to the formation of excessive surface carbon. There is some evidence that the zeolite support enhances the stability of these small metal clusters. The recently discovered high silica zeolites (239) of the type ZSM-S/ZSM-ll might be of interest as support materials, due to their high hydrothermal resistance.


T : ’ : H p 6 L Ru/Ni

z L 4 : . Ru/Cu

E 2 8

0 100 80 60 40 20 0

Atom % Ru

FIG. 22. Turnover number versus atom percent ruthenium in (a) ruthenium-nickel zeolites and (b) ruthenium-copper zeolites. (Reproduced from Ref. 234 with permission from the authors.)

VIII. Conversion of Synthesis Gas to Hydrocarbons

It is beyond the scope of this review to cover the rapidly expanding topic of synthesis gas conversion to hydrocarbons in any great detail. However, it would be an omission not to mention a number of aspects which are at least related to nonacid catalysis. The current world shortage of cheap crude oil has stimulated intense research activity to develop commercially viable processes to convert coal-derived synthesis gas (CO/H,) into liquid fuels and petrochemical feedstocks. The synthesis of hydrocarbons from synthesis gas via the Fischer-Tropsch (FT) (240) reaction has been known for some 50 years and has been successfully applied in both Germany and South Africa. However, this catalytic reaction occurs by a well-defined chain growth mechanism which has the disadvantage of yielding a very broad product distribution. The Schulz-Flory polymerization kinetics, which accurately describe this process, show that the maximum attainable selectivity to a product in the gasoline range is only 48%. Zeolites offer enormous potential as catalysts in this area since the limitations of Schulz-Flory kinetics can be overcome by utilizing the shape-selective properties of these support materials.

Initial studies using cobalt-, nickel-, and iron-modified zeolites X and Y (241, 242) were, however, not particularly encouraging with relatively poor activities, selectivities, and stabilities. This situation has now changed dramatically with the discovery by Mobil Oil Corporation of a new series of synthetic high-silica zeolites. The so-called ZSM-5 zeolite (in the H form) is capable of converting methanol quantitatively to hydrocarbons and water (239), i.e.,

xCH,OH -+ (CH,), + xH,O

56 1. E. MAXWELL

where the hydrocarbons are in the range C, to C,, and the gasoline fraction (rich in aromatics) has a high research octane number (90-100). This novel acid catalysis is not limited to methanol as reactant, as is demonstrated by the fact that a wide variety of oxygenate, olefin, and paraffin feedstocks can also be upgraded to a product boiling in the gasoline range ( 2 4 2 ~ ) . The constrained structure of the zeolite (as shown in Fig. 23) is directly responsible for the shape-selective process which yields a narrow range of product molecular weights. Another important feature of this zeolite catalyst is the relatively low rate of coke formation. The reasons for this improved stability do not seem to be completely understood as yet, but are very likely to be related to the specific zeolite structure which probably inhibits the formation of coke precursor molecules. Derouane has recently described this behavior (242b) as an example of restricted transition-state selectivity.

Clearly, with this development in mind, i t was an obvious step to combine a CO reducing function with such a zeolite and thereby produce a single-step shape-selective catalyst for synthesis gas conversion to gasoline. This has been achieved in a number of ways, for example by impregnating the active elements of conventional FT or methanol synthesis catalysts into ZSM-5 (e.g., Fe, Ru, Tho , , HfO,, Zn, Zn/Cr, and ZrO,) (243-247). The latter oxide component is claimed (246) to result in a catalyst with much improved resistance to sulfur poisoning. In general, these bifunctional catalysts result in much improved aromatics production and a very substantial reduction in heavier (C :,) hydrocarbons than would be obtained for the CO reduction catalyst alone. This component need not necessarily be incorporated into the zeolite itself, since physical mixtures of CO-reducing catalysts and zeolites have also been shown to be effective for selective synthesis gas conversion (243-247). In addition, this physical mixture may also include a water-gas shift catalyst (248, 249).

FIG. 23. A schematic diagram showing the structure of zeolite ZSM-5. (Reprinted with permission from Ref. 239. Copyright 1976 American Chemical Society.)


Jacobs has described ( 2 4 9 ~ ) these two different approaches in terms of secondary (physical mixtures) and primary (FT function in zeolite matrix) effects. In the former case the results obtained can be quite well understood in terms of the separate behavior of each component. However, in the latter case the results may be different since the primary FT products are formed inside the spatially restricting pores of the zeolite.

In a recent communication (250), deviations from Schulz-Flory kinetics were observed for a RuNaY synthesis gas conversion catalyst (see Fig. 24). A comparative catalyst, prepared by impregnating silica with ruthenium, i.e., Ru/SiO,, and tested under the same conditions, yielded a product distribution which gave a good fit to Schulz-Flory kinetics. The sharp decrease in chain growth probability for C:, products over RuNaY is perhaps surprising for such a relatively large-pore zeolite. Further studies (251-253) on this system indicated that there was a correlation between the ruthenium particle size in the zeolite and the product distribution.

FIG. 24. Coke yield versus shape selectivity of paraffin conversion for various zeolite catalysts. (Reproduced from Ref. 266 with permission from the authors.)

58 1. E. MAXWELL

This work is of particular interest if indeed an alternative mechanism exists for zeolite catalysts, other than pore-restricted shape selectivity, which may be employed to avoid broad product distributions in synthesis gas conversion reactions. In this respect it is of interest to note that in a recent communication (254), non-Schulz-Flory kinetics were reported for a series of cobalt hydrocarbon synthesis catalysts. The catalysts were prepared by depositing cobalt carbonyl on aluminas of different pore diameters and surface areas. A correlation was found between pore diameter and product distribution. Shape-selective effects for supports with pore radii in the range 65-3000 (i.e., outside the configurational range of diffusivities) do not seem to be very probable. However, a metal particle-size effect of the type proposed for RuNaY (250-253) may be a possibility. It should, however, be mentioned that in a study carried out by King ( 2 5 4 ~ ) where ruthenium FT catalysts were prepared on a variety of supports, including zeolites X and Y, no correlation was observed between product chain length and metal particle size. Further work in this area will be necessary to confirm these proposed particle-size effects. The resistance of such small metal particles to sintering is, of course, of crucial importance to any practical application of such catalysts. Other recent examples of shape-selective FT catalysts include Cd-vapor reduced CoA zeolite (2546) and ion-carbonyl complexes incorporated into zeolite Y ( 2 5 4 ~ ) .

Clearly, this field of research is in its infancy and the high level of current activity should lead to considerable further development.

IX. Miscellaneous Reactions

A. WATER-GAS SHIFT

The water-gas shift reaction, i.e.,

H,O + CO Z? C O , + H,

is of importance in adjusting a coal-derived synthesis gas to a product of more preferable composition. More active and more stable catalysts would enable increased productivities per reactor. In this respect a recent communication (255), which presented data indicating that RuX and RuY were more active for water-gas shift than conventional copper-based catalysts, is of interest. Infrared spectroscopy and temperature programmed reduction studies indicated that the active species was a complex within the zeolite pores of the form [Ru(NH,),(OH),(CO),]"~ (n < 3) where the ruthenium is in the oxidation state Ru(1) or Ru(I1) or both. An interesting aspect was that


there is apparently no known homogeneous equivalent of this complex which is stable up to 250°C. However, it seems doubtful as to whether this heterogeneous catalyst would be sufficiently stable under the more severe commercial operating conditions.

B. KOLBEL-ENGELHARDT REACTION

The less familiar Kolbel-Engelhardt reaction, i.e.,

4CO + 2H20 F? CH, + 3C0,

is considered to be a two-step reaction involving a water-gas shift followed by hydrogenation of CO to produce methane and/or higher hydrocarbons, i.e.,

CO + H 2 0 + CO, + H,, CO + 3H2 @ CH4 + H 2 0

Conventional catalysts are based on iron/iron oxide mixtures (256). Recently, Lunsford and co-workers (257) have shown that reduced RhNaY is also an active catalyst for this reaction at 329°C. A high selectivity to methane was observed and there was evidence of a two-step reaction scheme as shown above. Unfortunately, no comparative data were presented under similar conditions for a conventional catalyst.

C . WATER SPLITTING

There is considerable interest in developing an economical thermochemical reaction cycle that would effect the decomposition of water into hydrogen and oxygen (i.e., water splitting). Such a cycle could in principle provide a method of manufacturing hydrogen, a nonpolluting fuel, from an unlimited source. Most processes proposed to date, however, involve a large number of reaction steps and require the use of highly corrosive chemicals (258).

A few papers have appeared recently wherein water splitting using various cation-exchanged forms of zeolites has been demonstrated.

The reaction schemes, in general, can be written as follows:

2M(H20):' + 220- & 2M'"- ')+ + 2ZO-H" + )O,

M("-')+ + ZO-H+ 9 M(H,o):+ + zo- + +H,

(1)

(2)

It appears that quite a number of metal ions, M"', can effect reaction (l), which Jacobs et al. (259) have termed zeolite autoreduction. Kasai et al. (260) have proposed that for reaction (2) to proceed, the reduction potential

60 1. E. MAXWELL

for the half-cell reaction, i.e., M"+ + e- + M("-l)+

must be more negative than the proton reduction half-cell reaction, i.e.,

H + + e - + f H z (E" = -0.414V)

In this way these authors were able to explain the fact that although reaction (1) involving Cu2+/Cu+ (Eo = +0.158 V) proceeded for Cu2+-exchanged mordenite, reaction (2) could apparently not be accomplished. Further, for the systems Cr3+/Cr2+ (Eo = -0.41 V) and In3+/ In2+ (Eo = -0.49 V) both reactions (1 ) and (2) were shown to occur. To facilitate reaction (2), ion exchange was carried out with H mordenite, thus increasing the concentration of 2 0 - H + . Repeatable two-step water thermolysis cycles were apparently achieved in both cases.

Jacobs et af. (261) have shown that photolysis of water can be achieved using AgY zeolite. Irradiation using sunlight on water vapor-saturated AgY yields oxygen, and thermal treatment (600 C ) of the reduced silver zeolite restores the Ag+ cations with the evolution of hydrogen, i.e.,

2Ag' + 2ZO' + HzO ---+ 2Ag0 + 2ZO-H+ + $0,

Ago + ZO-H+ S A g ' + ZO- + i H ,

The system was, however, unstable due to silver metal sintering and de- hydroxylation of the zeolite. Both these reactions are of course favored by the relatively high temperature required for the oxidative desorption of hydrogen. Interestingly, the above reaction is nonallowed according to the half-cell potential criteria of Kasai el a/. (260) (i.e., EoAg+(aq)/Ago = +0.7996 V). Leutwyler and Schumacher (262) have, however, pointed out that the standard redox potential where both species are hydrated is negative [i.e., EoAg+(aq)/Ago(aq) = - 1.8 V]. The real situation in the zeolite is probably somewhere intermediate between these two extremes and could thus satisfy the previously described criteria. The above workers also calculated that the photolytic oxygen-producing step in silver zeolites must involve two photochemical steps since the photon energy available from sunlight is insufficient for a realistic single-step process. Similar problems with regard to reversibility of the water-splitting cycle, such as silver agglomeration were also found by these workers.

Kuznicki et uf. (263) have shown that zeolite 3A exchanged with Ti3+ is also capable of photolytic splitting of water. The ion-exchanged zeolite was shown to yield hydrogen in water under illumination with visible light. ESR studies indicated that the active entity for the hydrogen production was a higher valency state tetrahedral titanium(1V) oxygen complex with an


unpaired electron shared equally by the oxygen atoms. This is rather surprising since in all the previously discussed systems the reduced form of the metal ion is the active entity in the hydrogen production step. The second step, required to complete the water-splitting cycle, normally accomplished by heating with the evolution of oxygen, could not be demonstrated for the titanium-exchanged zeolite A system. Further, Ti3-+ -exchanged zeolites X and Y were found to be inactive for photolytic hydrogen production from water. This is perhaps somewhat unexpected since these zeolites are also known to form titanium oxygen complexes (264) containing an unpaired electron. Kuznicki et al., however, infer in their paper (263) that the photo- formed radical in TiA contains dissociatively adsorbed oxygen, whereas for TiY there is good evidence that the radical is a nondissociatively adsorbed superoxide species (264). Such a major difference in the modes of oxygen adsorption could possibly explain the observed results.

In conclusion, the chromium and indium water thermolysis systems described (260) and patented (265) by Union Carbide workers do appear to be the most promising zeolite-based schemes in terms of providing a stable process cycle. The simplification of a two-step process provided by the zeolites is clearly advantageous compared to the alternative many-step processes. As explained by Kasai et d. (260) this simplification is achieved with zeolites as a result of the large entropy change (> 120 e.u.) associated with the endothermic first step. Such a large entropy change is attributed to the large number of water molecules involved in reaction (1). Alternative water- splitting schemes in general require many more steps in order to achieve the net increase in entropy (40 e.u.) associated with this reaction, at reasonable temperatures (258). Further research on this catalytic application of zeolites would seem worthwhile in view of the promising results so far obtained and the projected economics. Thermochemical production of hydrogen has been projected to compete with coal gasification and electrolytic processes in the latter part of this decade (258).

X. General Conclusions and Future Prospects

A. ACTIVITY AND SELECTIVITY

The often quite remarkable activities and selectivitites achieved using various cation-exchanged zeolites can be best illustrated by using highlights from the foregoing discussion. Selective oxidation of ethylene to acetaldehyde can be achieved using I’d2’, Cu2+-Y (46, 47) and as yet no other heterogeneous Wacker-type catalyst has been reported. A novel n-hexane

62 I . E. MAXWELL

dehydrocyclization catalyst can be prepared by reducing tellurium which has been incorporated into NaX (127). RhNaY catalyzes the dimerization of ethylene under extremely mild conditions to selectively yield n-butene (dimer) products (140). CrNaY is a very active catalyst (140) for the polymerization of ethylene yielding a high-density unbranched polyethylene product. NiNaY selectively catalyzes the trimerization of acetylene to benzene (159, 160) rather than the tetramer, as occurs when simple nickel salts are used in homogeneous catalysis. Small-pore alkali metal ion zeolites such as KA selectively dimerize cyclopropene to tricyclohexane (166), whereas larger pore zeolites (e.g., NaX, N a y ) and solid acid catalysts result exclusively in polymerization. Transition metal ion-exchanged zeolites catalyze the dimerization of butadiene (1 72-18]) over a broad temperature range, but in constrast to homogeneous catalysts 4-vinylcyclohexene is the sole product over zeolites. NiNaX dimerizes n-butene (185, 186) with high selectivity to octane products. Carbonylation of methanol to methyl acetate can be achieved with very high specific activity and selectivity using RhNaY (209,210) in the presence of an iodide promoter. The specific activity of this zeolite catalyst exceeds that of any other heterogeneous carbonylation catalyst to date. RuNaY is an active catalyst for synthesis gas conversion (250) and yields a product distribution which markedly deviates from Schulz-Flory kinetics as is normally observed for the more conventional Fischer-Tropsch catalysts.

The relatively high activities of these catalysts can in most cases be attributed to the high dispersions of the active species. These are normally incorporated as cations via an ion-exchange process and thus remain bound onto the extensive inner surface of the zeolites by electrostatic forces. The selectivities observed, for example, in oligomerization reactions where, in general, dimers are formed in preference to higher oligomers, may be a direct consequence of the spatial limitations imposed on transition-state complexes within the small zeolite cavities.

B. STABILITY

It is perhaps not sufficiently widely recognized that for the successful development of a new industrial catalyst, good stability may often be just as critical as activity and selectivity. Further, in order to achieve economically acceptable space time yields, catalysts are generally run at high work rates (gram product per gram catalyst per hour). For many processes this places a heavy heat/mass transfer load on the catalyst which often accelerates deactivation processes.

Although there is, in general, only very limited information on zeolite


catalyst stability in the literature, a number of quite different decline mechanisms have been observed for the systems discussed. For example, the stability of Pd2+-Y for partial oxidation of alkenes is rather poor. This can be considerably improved (46, 47) by incorporating Cu2+ into the zeolite, which increases the rate of reoxidation of PdO back to the catalytically active Pd2+ ion. Although this is, in fact, analogous to the homogeneous Wacker catalyst system, it is nevertheless indicative of facile electron transfer reactions within the cavities of a zeolite.

The deactivation of the Cu' -Y butadiene cyclodimerization catalyst was attributed to polymerization reactions catalyzed by the zeolitic acid sites. The marked improvement in stability achieved (177) by the chemisorption of NH, on these sites is consistent with this deactivation mechanism. A major problem to be anticipated with heterogeneous cobalt and rhodium hydroformylation catalysts is leaching of the active metal from the catalyst surface, particularly during liquid-phase operation, with a consequent decline in activity. The quite good stability of CoNaA and RhNaY for olefin hydroformylation (226) may be due to the encapsulation of metal carbonyl clusters in the zeolite cavities. For the RhNaY catalyst a comparative experiment using a Rh/carbon catalyst, under the same reaction conditions, confirmed the improved stability of the zeolite for this reaction. A RuNiY methanation catalyst (234) was found to have much improved stability over that obtained for NiY and RuY. This was attributed to the formation of small bimetallic clusters which provided a better balance between the rates of CO dissociation and hydrogenation of surface carbon, thereby substantially reducing the rate of formation of deactivating surface carbon. There was also evidence that improved dispersions of the bimetallic catalysts were obtained on the zeolite support compared with, for example, alumina. Moreover, the zeolite-supported bimetallic clusters did appear to be more resistant to sintering. The much-improved stability of catalysts based on ZSM-5 zeolites for synthesis gas, and methanol conversion reactions is due to markedly reduced rates of coke formation compared to many other zeolites.

Rollmann and Walsh (266) have recently shown that for a wide variety of zeolites there is a good correlation between shape-selective behavior, as measured by the relative rates of conversion of n-hexane and 3-methyl- pentane, and the rate of coke formation (see Fig. 24). This correlation was considered to provide good evidence that intracrystalline coking is itself a shape-selective reaction. Thus, the rather constrained ZSM-5 pore structure exhibits high shape selectivity, probably via a restricted transition-state mechanism (242b), and therefore has a low rate of coke formation. Zeolite composition and crystal size, although influencing coke formation, were found to be of secondary importance. This type of information is clearly

64 I . E. MAXWELL

invaluable in the choice of zeolites for reactions where coke formation may occur. This will normally only be the case if acid sites are also present in the zeolite to catalyze the formation of coke precursors (267). There is a very definite need for more fundamental work of this type to provide a better understanding of decline behavior, which is so essential to designing zeolite catalysts with sufficient stability.

C. ACTIVE SPECIES AND REACTION MECHANISMS

The crystallinity of zeolites has the advantage that they lend themselves rather more than conventional heterogeneous catalysts to the study of active sites. This is exemplified in the literature where the active species in zeolites for a wide variety of reactions have been identified. There are fewer examples where the reaction mechanisms have been unambiguously defined, although they can often be inferred from studies on analogous homogeneous or heterogeneous catalysts.

For example, oxygen-bridged species have been shown (18,21-26,51,65) to play an important role in transition metal ion-exchanged zeolite oxidation catalysts. However, a detailed mechanistic scheme for most of these oxidation reactions is not yet available. By contrast, a very detailed picture has been obtained (128-130) of the active site in a TeNaX dehydrocyclization catalyst which involves a telluride ion coordinated to two Na' ions inside the supercage. Further, the mechanism was shown (131) to be similar to conventional dehydrocyclization catalysts and thus involved a stepwise dehydrogenation scheme. The active sites for transition metal ion-containing zeolite Y ethylene dimerization catalysts are very likely ds metal ions, i.e., Rh' (112), Ni2' (1.39, and PdZ+ (140). In the case of RhNaY three different rhodium oxidation states could be distinguished using XPS (112), and the dimerization activity was found to correlate well with the concentration of Rh' species. This is a particularly interesting demonstration of the potential that this technique would seem to offer in defining active oxidation states in zeolites.

A number of transition metal ion-exchange zeolites are active for acetylene trimerization (159, 160), and the criterion for activity appears to be an even, partially filled d-orbital, i.e., dH (Ni", Co'), d" (Fez'), d4 (Cr"). This has led to the suggestion that the mechanism must involve a complex in which there is simultaneous coordination of two acetylene molecules to the transition metal ion. The active oxidation state for CuNaY butadiene cyclodimerization catalysts has been unambiguously defined as monovalent copper (172-180). The d" electronic configuration of Cu' is consistent with the fact that isoelectronic complexes of Nio and Pdo are active homogeneous catalysts for this reaction. The almost quantitative cyclodimerization selec-


tivity to 4-vinylcyclohexene exhibited by copper zeolite catalysts is indicative of a reaction mechanism which proceeds via a o-allyl, n-allylbutadiene complex. Single-crystal X-ray diffraction studies ( I 79) have shown that copper ions migrate toward sites I1 and I11 in the supercages on adsorption of butadine into faujasite. The favorable location and unsaturated coordination geometry of the cations at site 111 led to the proposal that these were the active sites.

The active species for rhodium zeolite carbonylation catalysts (203, 210, 211) is very likely a four-coordinate d8 complex of the type [Rh'L,]+Z- where the ligands, L, could be oxygen anions of the zeolite framework and/or reactant molecules (e.g., methanol). Significantly, maximum specific activities are obtained for low metal loadings (- 0.6 wt.% Rh) (210), suggesting that these sites are on average relatively far apart ( -25 A). There is good evidence that the reaction mechanism on the zeolite catalysts is very similar (203,210,211,213) to that found for homogeneous catalysts.

For hydroformylation over cobalt and rhodium zeolites the active species have not been defined. However, in the case of RhNaY the in sizu formation of a rhodium carbonyl cluster has been identified (226) by infrared spectroscopy. Interestingly, this cluster appears to be different from known compounds such as Rh,(C0),2 and Rh,(CO),,. This does suggest that alternative carbonyl clusters may possibly be formed in zeolites due to the spatial restrictions of the intracrystalline cavities. The mechanism of hydroformylation in these zeolites is probably similar to that known for homogeneous catalysis.

A complex of the type [Ru(NH,),(OH),(CO),]"+ (n < 3) has been proposed (255) as the active species for catalysis of the water-gas shift reaction by RuX and RuY zeolites. Interestingly, there does not appear to be a homogeneous analog to this complex.

The active sites for catalyzed water splitting over zeolites involve both metal cations and protons sites in the zeolite pores. It has been proposed (260) that suitable zeolites must contain metal ions with a standard reduction potential (i.e., M"+/M("- ' I ) which is more negative than the proton reduction potential (i.e., - 0.414 V). Furthermore, the redox cycle involving this metal ion must be reversible in the zeolite. Redox couples involving Cr3+/Cr2+ and In3+/In2+ satisfy this criterion and accomplish water thermolysis with good cycle repeatability. The two-step mechanism first involves the dehydration and reduction of the metal ions with water splitting to form protons on the surface of the zeolite and the evolution of oxygen. The second step involves the reoxidation and rehydration of the metal ions with simultaneous deprotonation of the zeolite surface and the evolution of hydrogen.

From the foregoing discussion it is evident that in terms of active species and reaction mechanism there is often a close parallel between ion-exchanged

66 I . E. MAXWELL

zeolites and homogeneous catalysts. This is particularly true for reactions such as alkene partial oxidation, alkene oligomerization, acetylene and butadiene coupling reactions, carbonylation, and hydroformylation. In a sense, the zeolite can be regarded as a large, rigid anion, balancing the positive charge of the metal ion or complex, where the oxide anions of the zeolite framework may or may not participate in the metal ion coordination sphere. There is some evidence to suggest that the zeolite framework stabilizes such metal ions in unusual coordination geometries and oxidation states and in addition results in a very high degree of metal ion dispersion. The thermal stability of most zeolites is such that high-temperature exothermic regeneration procedures (e.g., polymer or coke burn-off) are often quite feasible.

D. FUTURE PROSPECTS

To gain some insight as to how the subject of nonacid zeolite catalysis has advanced in recent years, it is instructive to read, for example, the sections on olefin oligomerization and carbonylation in the 1968 zeolite catalysis review by Venuto and Landis (268). The activities, selectivities, and stabilities of the zeolite catalysts (usually in the acid form) used for these reactions were extremely poor. By comparison, as has been discussed, a number of metal ion-exchanged zeolites are now known to catalyze these reactions under mild conditions with high selectivities and good stabilities. There is every reason to believe that this trend will continue, particularly if cross-fertiliza- tion is maintained between the disciplines of zeolite and homogeneous catalysis. A large number of publications on zeolite catalysis are unfortunately of only limited value in that the catalysts are often not well characterized, or the reaction conditions are not well defined. In addition, there is often no indication of catalyst stability. Studies in which zeolite catalysts are compared, under similar conditions, with conventional catalysts (where these exist) are invaluable in ascertaining the role of the zeolite support itself. I t is encouraging to observe that there does seem to be an increase toward this comparative approach.

To the author’s knowledge, there are at present no major industrial processes which could be strictly defined as nonacid catalysis that make use of zeolite-based catalysts. This is in contrast to acid catalysis where zeolites continue to make an impact. Technically, a number of zeolite-based catalysts for reactions, such as Wacker chemistry and olefin or diolefin oligomerization reactions, appear to be quite attractive, and it is almost certainly economic factors that have limited further development.

The discovery of a new family of shape selective zeolites by Mobil(239) has now extended the range of pore sizes and thus the accessible range of configurational diffusion. To date the majority of studies have been carried out


using the more familiar large-pore zeolites such as X and Y. Comparative studies using zeolites with different pore structures and dimensions should provide a better insight into shape-selective and stability behavior. Measure- ments of both diffusion and reaction rates for these types of catalysts would be invaluable in elucidating the mechanisms by which shape selectivity occurs in such systems.

Zeolites also lend themselves particularly well to reactions where the catalytically active species are cationic. Under these circumstances there is a strong electrostatic interaction between the active entity and the support which minimizes activity loss via leaching processes.

The following areas would, in the author’s opinion, seem worthy of further study in the field of zeolite nonacid catalysis. A systematic investigation of shape selectivity in olefin and diolefin oligomerization reactions, by incorporating a common active component such a s rhodium into a variety of zeolite structures would provide useful additional information. There are preliminary indications (226) that unusual metal carbonyl cluster formation may occur in zeolite cages. This would also seem to be particularly interesting and has obvious relevance not only to hydroformylation but also to synthesis gas conversion reactions in general. The zeolite-based heterogeneous methanol carbonylation catalysts exhibit exceptional specific activity (209, 210), but comparison with the homogeneous system indicates that this might be further improved. The encapsulation of an effective halide promoter in the zeolite cavities of such a catalyst would remove the need to recycle these corrosive species in existing processes.

There does seem to be some quite good evidence for the existence of small electron-deficient metal clusters in zeolites (101-105), which may be related to their increased resistance to sulfur poisoning. Further studies are, however, necessary in order to provide a more detailed understanding in this area.

Zeolites also appear to be suitable supports for the formation of small bimetallic clusters between components which are immiscible as bulk metals. The Ru/Ni zeolite Y methanation catalyst is an interesting example (234) where the properties of a single active metal component could be advan- tageously modified.

The initial results obtained (258-263) using trivalent ion-exchanged zeolites as catalysts for thermolytic water splitting are quite encouraging. The simple two-step cycle, the good stability, and noncorrosive properties of the zeolite are all positive aspects. Further research toward a zeolite-based system which could operate at even lower temperatures would seem worth pursuing.

The rapidly growing world concern over future oil supplies has led to considerable research activity into the use of synthesis gas or synthesis gas-derived molecules, such as methanol, as future feedstock materials for

68 I. E. MAXWELL

both the oil and chemical industries. The unique shape-selective properties of zeolites will be utilized to constrain the chain-growth-controlled (Schulz- Flory) kinetics of conventional Fischer-Tropsch catalysts, which leads to undesirable broad product distributions. Results of initial studies in this area using FT catalyst/ZSM-5 physical mixtures, modified ZSM-5 (243-249), and even larger pore zeolites such as RuNaY (250) are quite promising. Physical mixtures of zeolites with conventional catalysts will compete with single tailor-made zeolite catalysts. A variety of catalysts will probably emerge, to meet the different product needs of the oil and chemical industries. However, it now seems certain that the zeolites will play a central role in the catalysis of these new processes, which should give an enormous stimulus to zeolite research in general.

ACKNOWLEDGMENTS

I would like to express my sincere gratitude to the following colleagues who have read the initial manuscript and offered valuable suggestions: Dr. R. S. Downing, Dr. A. L. Farragher, Mr. A. G. T. G . Kortbeek, Dr. J. C. Platteeuw, Dr. M. F. M . Post and Dr. G . T. Pott. In addition I am very grateful to Miss E. Breekland who provided considerable assistance with the literature and patent searches. The support for the concept of this review from Professor W. M. H. Sachtier is also greatly appreciated.

REFERENCES

I . Rabo, J. A. (ed.), Am. Chem. Soc. Monogr. 171 (1976). 2. Jacobs, P. A,, “Carboniogenic Activity of Zeolites.” Elsevier, Amsterdam, 1977. 3. Venuto, P. B., Catal. Ory. Synth. Con( , 6th p. 67 (1977). 4. Uytterhoeven, J. B., Puog. Colloid. Polym. Sci. 65, 223 (1978). 5. Haynes, H. W., Caul. Reu. Sci. Eng. 17, 273 (1978). 6. Gallezot, P., C u d . Rev. Sci. Eny. 20, 121 (1979). 7. Rudham, R., and Stockwell, A., Spec. Period. Rep. Ctrtul. I , 87 (1977).

8. Breck, D. W., “Zeolite Molecular Sieves.” Wiley, New York, 1974. 9. Seff, K.. Acc. C h ~ m . Rrs. 9, 121 (1976).

( 1 972).

7u. Naccache, C., Proc. Int. Con$ Ze/J/ifP,S, 5th. London p. 592 (1980).

10. Olson, D. H., Mikovsky, R. J. , Shipman, G. F., and Dempsey, E., J . Cutol. 24, 161

11. Maxwell, I . E., de Boer, J. J., and Downing, R. S., J . Cutti/. 61, 493 (1980). 12. Smith, J . V., Puoc. Int. Conf: Zeolites. 5th. Naples p. 194 (1980). 13. Weisz, P. B., Chenitech August, 498 (1973). 14. Chen, N . Y., and Weisz, P. B., C h m . En$. Prog. S.ynip. Srr. 63, 86 (1967). 15. Gorring, R. L., J . Catrrl. 31, 13 (1973).

/&i. Post, M . F . M., results to be published. 16. Boreskov, G. K., Bobrov, N. N., Maksimov. N . G., Anufrienko, V. F., Ione, K. G., and

17. lone, K. G . , Bobrov, N. N., Boreskov, K. G.. and Vostrikova, L. A,. Dokl. Aktirl. Nuzrk

18. Boreskov, G. K., Proc. I n f . Congr. Cutal., 5th 2,981 (1973).

Shestakova, N . A,, Dokl. Aknrl. Nuuk SSSR 201,887 (1971).

SSSR 210, 388 (1973).


19. Bobrov, N . N . , Boreskov, G. K., lone, K. G., Terletskikh, A,, and Shestakova, N. A,,

20. Kubo, T., Tominaga, H., and Kunugi, T., Bull. Chem. Soc. Jpn. 46,3549 (1973). 21. Maksimov, N . G. , Ione, K. G., Anufrienko, V. F., Kuznetsov, P. N., Bobrov, N. N.,

22. Bobrov, N . N . , Boreskov, G. K., Davydov, A. A,, and lone, K. G., Izv. Akad. Nauk

23. lone, K . G., Bobrov, N. N., and Davydov, A. A., Kinet. Katal. 16, 1234 (1975). 24. Schoonheydt, R. A,, Vandamme, L. J., Jacobs, P. A,, and Uytterhoeven, J. B., J . Catal.

25. Garten, R. L., Delgass, W. N., and Boudart, M., J . Carol. 18,90 (1970). 26. Dalla Betta, R. A,, Garten, R. L., and Boudart. M., J. Cutal. 41,40 (1976). 27. Beyer, H., Jacobs, P. A., Uytterhoeven, J. B., and Vandamme, L. J., Proc. Int. Congr.

28. Paetow, H., and Riekert, L., Ber. Bunsenyes. Phys. Chem. 83, 807 (1979). 29. Paetow, H., and Riekert, L.. Actu Phys. Chern. 24,245 (1978). 30. Chen, N . Y., and Weisz, P. B., Chem. Eny. Prog. Symp. Ser. 63, 81 (1967). 31. Hartzell, F . D., and Chester, A. W., Oil Gas J . 77, 83 (1979). 32. US Patent 4,064,039 (1977);

US Patent 4,064,037 (1977); US Patent 4,107,032 (1978); US Patent 4,097,410 (1978); US Patent 4,118,339 (1978); US Patent 4,151,121 (1979); US Patent 4,164,465 (1979).

Kinet. Katul. 15,413 (1972).

and Boreskov, G. K., Dokl. Akad. Nauk SSSR. 217, 135 (1974).

SSSR, Ser. Khim. 24 (1975).

43, 292 (1976).

Cuiul., 6 th I, 273 (1977).

33. Firth, J. G., and Holland, H. B., Trans. Faraday SOC. 65, 1891 (1969). 34. Rudham, R., and Sanders, M. K., J. Cutal. 27, 287 (1972). 35. Altshuler, 0. V . , and Tsitovskaya, I . L.. Izu. Akad. Nauk SSSR, Ser. Khirn. 825 (1974). 36. Tsitovskaya, 1. L., Altshuler, 0. V., and Krylov, 0. V., Dok. Akad. Nauk SSSR 212,1400

37. Alsdorf, E . , Burkkhardt, I . , Schnabel, K. Kh., Zelenina, M., Krylov, 0. V . , Altshuler,

38. Mochida, I . , Hayata, S., Kato, A,, and Seiyama, T., J . Catal. 15, 314 (1969). 39. Mochida, I . , Jitsumatsu, T., Kato, A,, and Seiyama, T., Bull. Chem. SOC. Jpn. 44, 2595

40. Mochida, I . , Ikeda, Y., Fujitsu, H., and Takeshita, K., Ind. Eng. Chem. Prod. Res. Deu.

41. Minachev, Kh. M., Tagiyev, D. B., and Karhlamov, V. V., Izu. Akad. Nauk SSSR, Ser.

42. Minachev, Kh. M . , Tagiyev, D. B., Zulfugarov, Z. G., Kharlamov, V. V., and Zelinksky,

43. Mochida, I . , Hayata, S., Kato, A,, and Seiyama, T., J . Cutal. 23, 31 (1971). 44. Altshuler, 0. V., Tsitovskaya, I . L., Vinogradova, 0. M., and Seleznev, V. A,, Izu. Akad.

45. Gentry, S. J., Rudham, R., and Sanders, M. K., J . Catul. 35, 376 (1974). 46. Kubota. T.. Kumada, F., Tominaga, H., and Kunugi. T., rnt. Chem. Eng. 13, 539 (1973). 47. Arai, H. , Yamashiro, T., Kubo, T., and Tominaga, H., Bull. Jpn. Pet. Inst. 18, 39 (1976). 48. Weissermel, K., and Arpe, H. J., ”Industrial Organic Chemistry.” Verlag Chemie,

49. Mochida, I., Hayata, S., Kato, A,, and Seiyama, T., Bull. Chem. Soc. Jpn. 44,2282 (1971).

(1973).

0. V., and Tsitovskaya, I. L., Kinet. Katal. 16, 653 (1975).

(1971).

15, 160 (1976).

Khim. 1931 (1977).

N . D.. Proc. Int. Cot$ Zeolites, Sth, London p. 625 (1980).

Nauk SSSR, Ser. Khim. 10, 2145 (1972).

Weinheim, 1978.

70 I . E. MAXWELL

50. Mochida, I . , Hayata, S., Kato, A,, and Seiyama, T., J . Catal. 19,405 (1970). 51. Arai, H., Tominaga, H., and Tsuchiya, J., Proc. Int. Congr. Card., 6th 2, 997 (1977). 52. Hennebert, P., Hemidy, J. F., and Cornet, D., J . Chem. Soc. Faraday Trans. 176, 952

53. Grabowski, E., Primet, M., and Mathieu, M. V., React. Kinet. Catal. Lett. 8,515 (1978). 54. Windhorst, K. A,, and Lunsford, J. H., J. Am. Chem. Soc. 97, 1407 (1975). 55. Windhorst, K. A., and Lunsford, J. H., J . Chem. Soc. Chem. Commun. 852 (1975). 56. Williamson, W. B., Flentge, D. R., and Lunsford, J. H., J . Curd. 37, 258 (1975). 57. Steijns. M., and Mars, P., J . C o d . 35, 11 (1974). 58. Steijns, M., Derks, F., Verloop, A., and Mars, P.. J . Catal. 42, 87 (1976). 59. Steijns, M., Koopman, P., Nieuwenhuijse, B., and Mars, P., J . C a r d 42, 96 (1976). 60. Steijns, M., and Mars, P., Ind. Eng. Chem., Proc. Res. Deu. 16, 35 (1977). 61. Dudzik, Z., and Ziolek, M., J . Catal. 51, 345 (1978). 62. US Patent 4,088,743 (1978). 63. Pearce, J. R., and Lunsford, J. H. , J . CON. Znt. SLY. 66, 33 (1978). 64. Davydova, L. P., Boreskov, G. K., lone, K. G., and Popovskii, V. V.. Kinrt. Kuttrl. 16,

65. Mahoney, F., Rudham, R., and Summers, J. V., Chem. Soc. Farridq Trat7s. I. 75, 314

66. Hatada, K., Ono, Y., and Keii. T., Chem. Lert. 439 (1974). 67. Kamneva, A. I . , Zakharova, V. I., Piterskii, L. N. , Seleznev. V . A,, and Artemov, A. V.,

68. Flockhart, B. D., Mollan, P. A. F.. and Pink, R. C., .I . Chem. Soc. Faraday Trans. 171,

69. Romannikov, V. N., Klueva, N. V., Bobrov, N. N., and lone, K. G., React. Kincf. Cutul.

70. Malashevich, L. N., Komarov, V. S., and Pismennaya, A. V., Kinct. KuuI. 19,815 (1978). 71. Tsuruya, S., Okamoto, Y., and Kuwada, T., J . Cutal. 56, 52 (1979). 72. Howe, R. F., and Lunsford, J. H., Pruc. In!. Congr. Catul.. 6th 1 , 540 (1977). 73. US Patent 3,957,690 (1976). 74. US Patent 4,021,369 (1977). 75. US Patent 3,692,840 (1972). 76. US Patent 3,989,806 (1976). 77. Minachev, Kh. M., Garanin, V. I . , Kharlamov, V. V.. and Isakova, T. A., Kine/. Katrrl.

78. Minachev, Kh. M., Kharlamov, V. V., Garanin, V. I., and Isakova, T. A,, Izo. Akud.

7Y. Minachev, Kh. M., Acta. Phys. Chem. 24, 5 (1978). 80. Topchieva, K. V., Shakhnovskaya, 0. L., Rosolovskaya, E. N.. Zhdanov, S. P., and

Samulevich, N. N., Kinet. Kutal. 13, 1453 (1972). 81. Kharlamov, V. V., Garanin, V. I., Tagiev, D. B., Minachev, Kh. M., and Goryachev,

A. A., Izu. Akad. Nauk SSSR, Ser. Khim. 673 (1975). 82. Kharlamov, V. V., Garanin, V. I . , Tagiev, D. B.. Minachev, Kh. M., and Goryachev,

A. A., Izv. Akad. Nauk SSSR, Ser. Khim. 845 (1975). 83. Minachev, Kh. M., Khodakov. Yu. S., Savchenko, B. M., and Nesterov, V. K., Izu. Aknd.

Nauk SSSR, Ser. Khim. 1722 (1975). 84. Minachev, Kh. M., Kharlamov, V. V.. Garanin, V. I . , and Tagiev, D. B., 1x1. Akod.

Nuuk S S S R , Ser. Khim. 2410 (1975). 85. Gryaznov, 2. V., Tsitsishvili, G. V., and Naskidashvili, N. N., React. Kiner. C m l . Lrtr.

7, 59 (1977).

(1980).

117 (1975).

(1979).

NrJiekhimiya 15,403 (1 975).

I192 (1975).

Lett. 5, 217 (1976).

13, I101 (1972).

Nauk SSSR, Ser. Khim 294 ( 1 976).


86. Kharlamov, V. V., Garanin, V. I., Tagiev, D. B., and Minachev, Kh. M., Izu. Akad.

87. Minachev, Kh. M., Kharlamov, V. V., Garanin, V. l. , and Tagiev, D. B., Izu. Akad.

88. Kharlamov, V. V., and Minachev, Kh. M., Izu. Akad. NaukSSSR, Ser. Khim. 280(1977). 89. Minachev, Kh. M., Kharlamov, V. V., and Kharatishvili, N. G., Izu. Akad. Nauk SSSR,

90. Minachev, Kh. M., Garanin, V. I., Kharlamov, V. V., and Kapustin, M. A,, Izu. Akad.

91. Karakhanov, R. A., Garanin, V. I., Kharlamov, V. V., Kapustin, M. A., Blinov, B. B.,

92. Minachev, Kh. M., Garanin, V. I . , Kharlamov, V. V., and Kapustin, M. A,, Izu. Akad.

93. Minachev, Kh. M., Garanin, V. I., and Kapustin, M. A., Izu. Akad. Nauk SSSR, Ser.

94. Galich, P. N., Gutyrya, V. S., and Galinski, A. A,, Proc. Int. Conf: Zeolites, 5th, Naples

95. Dalla Betta, R. A., and Boudart, M., Proc. h i . Carol. Conyr., 5th 2, 1329 (1973). 96. Minachev, Kh. M., Garanin, V. I., snd Novruzov, T. A., Izv. Akad. Nauk SSSR, Ser.

97. Sokolskii, D. V., Gogol, N. A., and Shliomenzon, N. L., Kinet. Katal. 982 (1972). 98. Penchev, V . , Davidova, N., Kanazirev, V., Minachev, H., and Neinska, Y . , Mol. Sieves

99. Mine, R. S. , Ione, K. G.. Namba, S., and Turkevitch, J., J . Phys. Chem. 82,214 (1978). 100. Dini, P., Doneo, D., Montelatici, S., and Giordano, N., J . Catal. 30, 1 (1973). 101. Rabo, J. A., Shomaker, V., and Pickert, P. E., Proc. Int. Con$ Catal., 3rd, Amsterdam

2, 1264 (1965). 102. Landau, M. V., Kruglikov, V. Ya, Goncharova, N . V., Konovalchikov, 0. P., Chukin,

G. D., Smirov, B. V., and Malevich, V. I . , Kinet. Katal. 17, 1281 (1976). 103. Chukin, G. D., Landau, M. V., Kruglikov, V. Ya, Agievskii, D. A., Smirnov, B. V.,

Belozerov, A. L., Asrieva, V. D., Goncharova, N. V., Radchenko, E. D., Konovalchikov, 0. D., and Agafonov, A. V., Proc. Int. Congr. C a d . , 6th 2, 668 (1977).

104. Gallezot, P., Datka, J. , Massardier, J., Primet, M., and Imelik, B., Proc. Int. Congr. Catal., 61h 2, 696 (1977).

105. Figueras, F., Gomez, R., and Primet, M., Adu. Chem. Ser. Am. Chem. Soc. 121, 480 (1973).

106. Clarke, A., Llyodlangston, J., and Thomas, W. J . , Trans. Inst. Chem. Eng., 55,93 (1977). 107. Tungler, A,, Petro, J.. Mathe, T., Besenjei. G., and Csuros, Z., Acta Chim. Acad. Sci.

108. Paranosenkov, V. P., and Gryaznova, 2. V., Nefiekhirniya 14,576 (1974). 109. Romannikov, V. N., lone, K. G., and Repina, V. V., Izv. Akad. Nauk SSSR, Ser. Khim.

110. Ione, K . G., Romannikov, V. N., Davydov. A. A,, and Orlova, L. B., J . Catal. 57, 126

I l l . Gryaznova, Z. V., Kolodieva, Y. V., Paranosenkov, V. P., Tsitsishvili, G. V., and

112. Okamoto, Y., Ishida, N., Imanaka, T., and Teranishi, S., J . Cutul. 58, 82 (1979). 113. Minachev, Kh. M., Ryashentseva, M. A,, and Avaev, V. I., Izv. Akad. Nauk SSSR, Ser.

114. Coughlan, B., Narayanan, S., McCann, W., and Carroll, W. M., Chem. Ind. 125 (1977).

Nauk SSSR, Ser. Khim. 2406 (1975).

Nauk SSSR , Ser. Khim. 1709 (1976).

Ser. Khim. 97 (1979).


and Minachev, Kh. M., Izv. Akud. Nauk SSSR, Ser. Khim. 445 (1975).


Khim. 1847 (1978).

p. 661 (1980).

Khim. 330 (1973).

Int. Conf., 3rd, Ado. Chem. Ser. 121,461 (1973).

Hung. 82, 183 (1974).

13 (1979).

(1979).

Krupennikova, A. Yu., Neftekhimiyo 13, 374 (1973).

Khim. 1181 (1976).

72 I . E. MAXWELL

/ IS . Coughlan, B., Narayanan, S., McCann, W., and Carroll, W. M., J . Cutul. 49, 97 (1977). 116. Harman, R. E., Gupta, S. K., and Brown, D. J., Chem. Rev . 73, 21 (1973). 117. Aben, P. C., Platteeuw, J . C., and Stouthamer, B., Proc. Int. Congr. Curul., 4th, Moscoiv

Paper 31 (1968). 118. US Patent 3,917,565 (1975);

US Patent 3,876,529 (1975); US Patent 3,912,620 (1975).

119. US Patent 4,057,488 (1977); US Patent 4,124,650 (1978).

120. US Patent 3,865,716 (1975); US Patent 3,928,233 (1975); US Patent 3,979,277 (1976); US Patent 4,013,545 (1977).

121. Kubo, T., Arai, H., Tominaga, H., and Kunugi, T. , Bull. Chem. Soc. Jpn. 45,607 (1972). 122. Kubo, T., Arai, H., Tominaga, H., and Kunugi, T., Bull. Chrm. So(,. Jpn. 45,613 (1972). 123. Gryaznova, Z. V., Epishina, Z. V., and Mikhaleva, I. M., Dokl. Akud. Nuuk SSSR 203,

124. Epishina, G. P., Gryaznova, 2. V., Smirnov, V. S. , Krymova, V. V., and Burdshanadze,

125. Dimitrova, R . P., and Dimitrov, C., React. Kinet. Cutal. Lett. 10, 143 (1979). 126. Corma, A., Cid, R., and Lopez Agudo, A,, Cun. J . Chem. Eny. 57,638 (1979). 127. Miale. J. N., and Weisz, P. B., J . Cutul. 20, 288 (1971). 128. Lang, W. H.. Mikovsky, R. J., and Silvestri, A. J., J . Cattrl. 20, 293 (1971). 129. Mikovsky, R. J . , Silvestri, A. J., Dempsey, E., and Olson, D. H., 1. Curul. 22,371 (1971). 130. Olson, D . H., Mikovsky, R. J . , Shipman, G. F., and Dempsey, E., J . Cutul. 24, 161 (1972). 131. Silvestri, A. J . , and Smith, R . L., J . Cutul. 29, 316 (1973). 132. Kazansky, B. A., Isagulyants, G. V., Rozengart, M. I . , Dubinsky, Yu. G., and Kova-

lenko, L. I., Proc. Int. Congr. Ctrtal., Srh, Miutni Paper 92, p. 1277 (1972). 133. Lefebvre, G., and Chauvin, Y., A.sp1w.s Harnogeneous Cntul. 1, 108 (1970). 134. Eidus, Ya. T.. Ershov, N. I . , Puzitskii, K. V., and Kazanksii, 9. A,, Izo. Aktrd. Nouk

135. Eidus, Ya. T., Ershov, N . I . , Puzitskii, K . V., and Kazanksii, B. A,, Izo. Akud. Nauk

136. Eidus, Ya. T., Ershov, N. I., Puzitskii, K. V., and Kazanksii. B. A,. Iza. Aknri. Nuuk

137. Eidus, Ya. T., Ershov, N. I., Puzitskii, K. V., and Kazanksii, B. A,. 1x1. Akud. Nrierk

138. Eidus, Ya. T., Avetisyan, R. V., Lapidus, A. L., Isakov, Ya. I., and Minachev, Kh. M.,

139. Lapidus, A. L.. Isakov, Ya. I . . Stinkin. A. A., Avetisyan, R. V., Minachev, Kh. M., and

f40. Yashima, T.. Ushida, Y., Ebisawa. M . . and Hara, N.. .I . Cord. 36, 320 (1975). 141. Cramer. R., J . Am. Chem. Soc. 87,4717 (1965); Alderson, T., Jenner, E. L., and Lindey,

142. US Patent 3,644,565 (1972). 143. US Patent 3,738,977 (1973). 144. Lapidus. A. L.. Mal’tser, V. V., Garanin, V. I., Minachev, Kh. M., and Eidus, Ya. T.,

145. Lapidus, A. L., Mal’tser, V. V., Shpiro, E. S., Antoshin. G. V., Garanin, V. I., and

1339 (1972).

M. N., Izu. Akud. Nauk SSSR, Ser. Khim. 997 (1977).

SSSR, Ser. Khim. 703 (1960).

S S S R , Ser . Kkim. 920 (1960).

SSSR, S2r. Ktiim. 1 I I4 ( I 960).

SSSR, Ser. Khim. 1291 (1960).

Ix. Akud. Nuuk SSSR, Srr. Khim. 2496 (1968).

Eidus, Ya. T., Izc. Aktrd. Nuuk SSSR, Scr. Khirn. 1904 ( I97 I ).

R. V., Jr., J . Am. Chenz. SOC. 87, 5638 (1965).

1 ~ . Akod. Ncruk S S S R , Ser. Khim. 2819 (1975).

Minachev, Kh. M.. 1:o. Aktrd. Nuuk SSSR, Ser.. Khitn. 2454 (1977).


146. Lapidus, A. L., Mal’tser, V. V., Maganya, M. I., Garanin, V. I., and Minachev, Kh. M.,

147. Yashima, T., Nagata, J., Shimazaki, Y., and Hara, N., Proc. h i . Zeolite Conf:, 4th,

148. Atanasova, V. D., Shuets, V. A,, and Kazanksii, V. B., Kinet. Katal. 18, 1033 (1977). 149. Przhevalskaya, L. K., Shuets, V. A., and Kazanksii, V. B., Kinet. Katal. 11, 1310 (1970). 150. Przhevalskaya, L. K., Shuets, V. A., and Kazanksii, V. B., J. Cutal. 39, 363 (1975). 151. Krauss, H. L., and Stach, H., lnorg. Nucl. Chem. Lett. 4, 393 (1968). 152. Krauss. H. L., and Schmidt, H., 2. Anorg. Allg. Chem. 392,258 (1972). 153. Druzhkov, V. N., Zakharov, V. A., and Ermakov, Yu. I., Kinet. Katal. 14,998 (1973). 154. Tvaruzkova, Z . , Bosacek, V., and Patzelova, V., React. Kinet. Catal. Lett. I I , 71 (1979). 155. Eidus, Ya. T., Ershov, N. I., and Yem, H. C., Izv. Akad. Nauk S S S R , Ser. Khim. 1919

156. Eidus, Ya. T., Ershov, N. I . , and Hoang, C. Y.. Izv. Akad. Nauk S S S R , Ser. Khim. 1876

157. Ershov, N. I., Yem, H. C., and Eidus, Ya. T., Izo. Akad. Nauk SSSR, Ser. Khim. 894

158. Kruerke, U.. as cited by Rabo, J. A,, and Poustma, M. L., Adv. Chem. Ser. 102, 297

159. Besoukhanova, T., Pichat, P., Mathieu, M., and Imelik, B., J . Chim. Phys. 71,751 (1974). 160. Pichat, P., Vedrine, J . C., Gallezot, P., and Imelik, B., J . Catal. 32, 190 (1974). 161. Bird, C. W., “Transition Metal Intermediates in Organic Synthesis,” Ch. I . Logos,

162. Vollhardt, K. P. C., Acc. Chem. Res. 10, 1 (1977). 163. Hassan. S. M., Panchenkov, G. M., and Kuznetsov, 0. I., Bull. Chem. Sor. Jpn. 50,

164. Imai, H., Hasegawa, T., and Uchida, H., Bull. Chm?. Soc. Jpn. 41,45 (1968). 165. Japanese Patent 7,318,201 (1973). 166. Schipperijn, A. J., and Lukas, J., Rec. Trao. Chim. Pays-Bas92, 572 (1973). 167. Heimbach, P., Jolly, P. W., and Wilke, G., Adu. Organornet. Chem. 8, 29 (1970). 168. British Patent 1,085,875 (1967). 169. Candlin, J. P., and James, W. H., J. Ckem. Soc. C I856 (1968). 170. Wilke, G., Bogdanovic, B., Hardt, P., Heimbach, P., Keim, W., Kroner, W., Oberkirch,

W., Tanaka, K., Steinriicke, E., Walter, D., and Zimmerrnann, H., Angew. Chem. Int. Ed. 5, 151 (1966).

Acta Phys. Chem. 24, 195 (1978).

Am. Chem. Sor. Symp. Ser. 40,626 (1977).

(1973).

(1 974).

(1 974).

(1971).

London, 1967.

2597 (1977).

171. Scurrell, M. S., Spec. Period. Rep. Cattrl. 2, 240 (1978) and references therein. 172. US Patent 3,444,253 (1969). 173. US Patent 3,497,462 (1970). 174. Reimlinger, H., Kruerke, U., and de Ruiter, E., Chem. Ber. 103,2317 (1970). 175. Maxwell, I. E., and Drent, E., J . Catal. 41, 412 (1976). 176. Uytterhoeven, J. B., A m Phys Chern. 24, 53 (1978). 177. Maxwell, I. E., Downing. R. S., and van Langen, S. A. J., Acta Phys. Chem. 24,215 (1 978). 178. Maxwell, I. E., Downing, R. S., and van Langen. S. A. J., J. Cntal. 61,485 (1980). 179. Maxwell, I. E., de Boer, J. J., and Downing, R. S. , J . Cutnl. 61, 493 (1980). 180. US Patent 4,125,483 (1978). 181. Nefedov, B. K., and Sergeyeva, N. S., Neftekhimi!a 17, 516 (1977). 182. Sakai, T., Soma, K., Sasaki, Y., Tominaga, H., and Kunugi. T., Am. Chem. Sot.. Diu.

183. Doering, W., von E., Franck-Neumann, M., Hasselmann, D., and Kaye. R. L., J . Am. Pet. Chem. Reprints 14, paper D 40 (1969).

Chem. Soc. 94,3833 (1972).

74 I . E. MAXWELL

l84. British Patent 1,488,521 (1977). 185. US Patent 4,029,719 (1977). 186. Forni, L., Invernizzi, R., and van Mao. L., Chim. Ind. 57, 577 (1975). 187. Forni, L., van Mao, L., and Invernizzi, R.. Chim. Ind. 57, 669 (1975). 188. Kuznetsov, 0. I., Panchenkov, G. M., Guseinov, A. D., and Kiryushkin, S. G.

189. Lapidus, A. L., Rudakova, L. N., Isakov, Ya. I . , Minachev, Kh. M., and Eidus. Ya. T.,

190. Lapidus, A. L., Rudakova, L. N., Chemagina, V. P., Isakov, Ya. I . , Minachev, Kh. M.,

191. Lapidus, A. L., Isakov, Ya. I., Rudakova. L. N., Minachev, Kh. M., and Eidus, Ya. T..

192. Lapidus, A. L., Isakov, Ya. I . , Rudakova, L. N., Minachev. Kh. M., and Eidus, Ya. T.,

193. Lapidus, A. L., Isakov, Ya. I . , Mal'tsev, V. V., Rudakova, L. N., Minachev, Kh. M.,

194. Rhein, R. A,, and Clarke, J . S., Poljmwr 14, 333 (1973). 195. Hohenschutz, H., von Kutepow, N., and Himmele. W. , Hydrocurbon Process. 45, 141

(1 966). 196. Roth, J . F., Craddock, J. H., Hershman, A,, and Paulik, F. E., Chrm. Tcch. 600 (1971). 197. Schultz, R. G., and Montgomery, P. D., J . Catul. 13, 105 (1969). 198. Krzywichi, A,, and Pannetier, G., Bull. Soc. Chim. Fr. 1093 (1975). 199. Nefedov, B. K., Sergeeva, N. S.. and Eidus, Ya. T.. Izu. Akud. Nuuk SSSR, Sw. Khim.

200. Jarrell, M. S. , and Gates, B. C.. J . Cutcrl. 40, 255 (1975). 201. Scurrell, M. S.. Ploi. A4c.t. Rev. 21, 92 (1977). 202. Nefedov, B. K. , Shutkina, E. M., and Eidus, Ya. T.. Izv . Nuuk SSSR, Ser. Khim. 726

203. Nefedov, B. K., Sergeeva, N. S., Zueva, T. V., Shutkina, E. M., and Eidus, Ya. T., I:v.

204. Krzywicki, A,, and Pannetier, G., Bull. S O ( . Chim. Fr. 1093 (1975). 205. Nefedov, B. K., Sergeeva, N. S., and Eidus, Ya. T., 1:v. Nnuk SSSR, Siv. Khinz. 2271

206. Schultz, R . G., and Montgomery, P. D., A m . Clwm. Soc,., Div. Pd . Chrm. Pri.prints

207. Robinson, K. K., Hershman, A,, Craddock, J. H., and Roth, J . F.. J . Cntrrl. 27, 389

208. Christensen, B., and Scurrell, M. S., J . Chem. Soc. F m & r Trans. I, 73, 2036 (1977). 209. Yashima. T., Orikasa, Y., Takahashi, N. , and Hara, N.. J . Cutd . 59, 53 (1979). 210. Takahashi, N. , Orikasa, Y., and Yashima, T., J . Cutul. 59, 61 (1979). 211. Christensen, B.. and Scurrell, M. S., J . Chem. Soc. Fcrrtrduy Trrms. I , 74, 2313 (1978). 212. Hjortkjaer, T., and Jensen, V. W., h d . Eny. Chem. Prod. Rt*.s. Der!. 15,46 (1976). 213. Scurrell, M. S., J . RPS. Inst. Cuful. 25, 189 (1978). 214. Nefedov, B. K., and Dzhaparidze, R. V., 1 3 ~ ' . Aktrri. Nrrirk Gru-. SSR, Srr . Khim. 3, 129

215. Nefedov, B. K., Dzhaparidze, R . V., and Sorokina. A. K.. 1zo. Akud. Nuuk Gw:. S S S R ,

216. Shostakovskii, M. F., Nefedov, B. K., Dzhaparidze, R. V., and Zasorina, N. M.. Izv.

Nqfiekhimiya 12, 176 (1972).

Izv. Akud. Nuuk SSSR 1637 (1972).

and Eidus, Ya. T., Izo. Akud. Nuuk SSSR 1261 (1973).

Proc. Symp. Mech. Hydrocurb. Rmct. Hung. p. 60 (June 1973).

I:[ , . Aknd. Nuuk SSSR I896 ( I 972).

and Eidus, Ya. T., Neftekhimiyu 15, 107 (1975).

2271 (1976).

(1975).

Nuuk SSSR, S~Y. Khim. 582 (1976).

(1975).

17, B13 (1972).

(1972).

( 1 977).

Ser. Khim. 3, 235 (1977).

Aknd. Nauk Grui. SSSR, Srr. Khim. 89,93 (1978).


217. Nefedov, B. K., Dzhaparidze, R. V., and Eidus, Ya. T., Izv. Akad. Nauk SSSR, Ser.

218. Hortkjaer, J., and Jsrgensen, J. C., J . Mol. Catal. 4, 199 (1978). 219. Nefedov, B. K., Dzhaparidze, R. V., and Marnaev, 0. G., Izv. Akad. Nauk SSSR, Ser.

220. Nefedov, B . K., Sergeeva, N. S., and Krasnova, L. L., Izv. Akad. Nauk SSSR, Ser. Khim.

221. Nefedov, B. K., and Kozhukhareva, V. N., Izv. Akad. Nauk SSSR, Ser. Khim. 2279

222. Gates, B. C., Katzer, J. R., and Schuit, G . C. A., “Chemistry of Catalytic Processes,”

223. Marko, L. , Aspects Homogeneous Catal. 2, 3 (1974). 224. Scurrell, M. S . , Spec. Period. Rep. Catal. 2, 215 (1978). 225. Centola, P., Terzaghi, G., Del Rosso, R., and Pasquon, I., Chim. Ind. (Milan) 54, 775

226. Mantovani, E. , Palladino, N., and Zanobi, A,, J . M u / . Catal. 3, 285 (1977/1978). 227. US Patent 3,352,924 (1967). 228. US Patent 4,070,403 (1978). 229. German Offenlegungsschrift 2,804,307 (1978). 230. US Patent 3,940,447 (1976). 231. Vannice, M. A,, J . Catal. 37,462 (1975). 232. Vannice, M. A,, J . Cutal. 40, 129 (1975). 233. Bhatia, S . , Mathews, J. F., and Bakhshi, N. N., Acta Phys. Chem. 24, 83 (1978). 234. Elliott, D. J., and Lunsford, J. H., J . Catal. 57, 11 (1979). 235. Sinfelt, J. H . , Arc. Chem. Res. 10, 15 (1977). 236. Sinfelt, J. H . , J . Catal. 29, 308 (1973). 237. Gupta, N. M., Kamble, V. S., and Iyer, R. M., Radiat. Phys. Chem. 12, 143 (1978). 238. Lawson, J. D., and Rase, H. F., Ind. Eng. Chem. Prod. Res. Dev. 9, 317 (1970). 239. Meisel, S . L., McCullough, J. P., Lenchthaler, C. H., and Weisz, P. B., Chem. Tech. 2,

240. Denny, P. J . , and Whan, D. A.. Spec. Period. Rep. Catal. 2, 46 (1978). 241. Abdulahad. I . , and Relek, M., Erdol Kohle, Erdgas, Petrochem. Brennstofl-Chem. 25,

242. Lapidus, A. L., Isakov, Ya. I . , Guseva, I . V., Minachev, Kh. M., and Eidus, Ya. T.,

Khim. 1422 (1977).

Khim. 1657 (1978).

614 (1977).

(1977).

p. 140. McGraw-Hill, New York, 1979.

(1972).

86 (1976).

187 (1972).

Izv. Akad. Nauk SSSR, Ser. Khim. 1441 (1974). 242a. US Patents 3,827,968; 3,756,942; 3,894,103-7; 3,907,915; 4,046,825; 3,960,978. 242b. Derouane, E. G., in “Catalysis by Zeolites” (B. lmelik et a/., eds.), p. 5. Elsevier, Amster-

dam, 1980. 243. Chang, C. D., Lang, W. H., and Silvestri, A. J., J. Catal. 56, 268 (1979). 244. Caesar, P . D., Brennan, J . A., Ganvood, W. E., and Ciric, J . , J . Cural. 56, 274 (1979). 245. British Patent 1,495,794 (1975). 246. British Patent 1,489,357 (1976). 247. US Patent 4,086,262 (1978). 248. Netherlands Patent Application, 7,711,350 (1977). 249. Netherlands Patent Application, 7,804,899 (1978).

249a. Jacobs, P. A,, in “Catalysis by Zeolites” (B. Imelik et a/., eds.), p. 293. Elsevier, Amster-

250. Nijs, H . H . , Jacobs, P. A., and Uytterhoeven, J. B., J . Chem. Soc. Chem. Commun. dam, 1980.

in press (1980).

76 I . E. MAXWELL

2.51. Nijs. H. H., Jacobs, P. A,, and Uytterhoeven. J. B., J . C/wm. Sot,. C'iwm. Coninrun. 1095 (1979).

252. Verdonck, J. J., Jacobs, P. A,, Genet, M., and Poncelet, G. , J . Chern. SOC. Furuday Trun.s. I . 76, 403 ( 1 980).

253. Nijs. H . H., Jacobs, P. A,, Verdonck, J . J . , and Uyttcrhoeven, J . B., Pro(,. h r . COH/. Zcolrrcs, 5th, Nuplrs p. 633 (1980).

254. Vanhove, D., Makambo, P., and Blanchard, M., J . C ' h c w . SOC. Circm. Cornmutr. 605 (1979).

2 5 4 ~ . King, D. L., J . Caul . 51, 386 ( 1 978). 254h. Fraenkel, D., and Gates, B. C., J . A m . Ckrrn. Sor,. 102, 2478 (1980). 2 5 k . Ballivet-Tkatchenko, D.. Condurier. G. , and Mozzanega, H., in "Catalysis by Zcolites"

255. Verdonck, J . J . . Jacobs, P. A,, and Uytterhoeven, J. B., J . Chc,m. Soc.. Chcm. Cbmmwr.

256. Kiilbel, H.. and Hammer, H., Z . Elekfrockrrn. 64, 224 (1960). 257. Niwa, M., lizuka, T. , and Lunsford, J . H., J . Chrrn. Soc. C ' h ~ r n . Conrmun. 684 (1979). 258. Chao, R. E., Ind. Eny. Chern. Prod. R1.s. DCT. 13, 94 (1974). 259. Jacobs, P. A., de Wilde, W.. Schoonheydt, R. A., Uytterhoeven, J . B., and Beyer, H.,

260. Kasai, P. H., and Bishop, R. J., J . P / ~ . Y . Chem. 81, 1527 (1977). 261. Jacobs, P. A., Uytterhoeven, J . B., and Beyer, H. K., J . C'hom. SOL.. Clr~rn. Commun. 128

262. Leutwyler, S., and Schumacher, E., Chirniu 13, 475 (1977). 263. Kurnicki, S. M., and Eyring, E. M . , J. A m . Chem. Soc. 100, 6790 (1978). 264. Ono, Y . , Suzuki, K., and Keii, T., J. Phys. Ciirm. 78, 218 (1974). 265. US Patent 3,963,830 (1976). 266. Rollmann, L. D., and Walsh, D. E., J . Curd. 56, 139 (1979). 267. Walsh, D. E., and Rollmann, L. D., J . Card. 56, 195 (1979). 26N. Venuto, P. B.. and Landis, P. S., A&. CuruI. 18, 259 (1968).

(B. lmelik r~ [ I / . % eds.), p. 309. Elsevier, Amsterdam, 1980.

181 (1979).

J . ClicJm. So(,. Furuclrry Trans. 172, 1221 (1976).

(1977).

Documents

[Advances in Catalysis] Volume 31 || Nonacid Catalysis with Zeolites