Shape-Selective Catalysis - American Chemical Society

Chapter 4

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Organic Catalysis over Zeolites and Other Molecular Sieves: Shape-Selective Effects in Catalytic Transformations Paul B. Venuto Consultant, 1073 Princeton Drive, Yardley, PA 19067

With the discovery of the ZSM-5 family of medium pore zeolites, there evolved a remarkable capability for shape selective control of reaction selectivity in zeolite catalysis. Recently, there has been steady expansion of organic catalysis into synthesis of fine chemicals, pharmaceuticals and the environmental area. Much of this effort involves refined shape-selective applications of medium pore zeolites and exploits the potentials of the growing number of new zeolites. The focus of such activity includes regiospecific aromatic substitutions, halogenation of aromatics, nucleophilic substitution over Cu zeolites, dehydrocyclization, selective catalytic hydrogenation, heterocyclic synthesis, selective partial oxidations, base catalysis, carbonyl condensation reactions, olefin oligomerizations, diverse isomerizations and some stereoselective reactions.

Structure-reactivity-selectivity effects in "classical" organic chemistry in solution or fluid phases—in the absence of the perturbing influence of zeolite micropores—are enormously complex in themselves. Variations in molecular structure are associated with steric, inductive, field, electronic and other effects which, under diverse reaction conditions, profoundly affect reaction selectivity. And when the situation is perturbed by conducting organic transformations within the micropores of a crystalline zeolite or other molecular sieve, many new dimensions of complexity are added to the reaction system. As shown in Figure 1, these new dimensions offer great opportunity for control of reaction pathways and selectivity, but the added considerations of diffusion and mass transfer effects must be carefully assessed.

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© 2000 American Chemical Society

In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Organic Reactant Structure Catalyst Structure


Reaction System

Reactant Shape Selectivity Relative Diff usivity Shape Selectivity 1 J

Product Shape Selectivity

Restricted Transition State Shape Selectivity

Stereoselectivity

Figure 1. Interaction of organic reactant, catalyst structure and reaction system in zeolite-catalyzed transformations: the genesis of shape selectivity. From the I960's through the early 1980's, numerous commercially important applications of zeolites as petroleum, petrochemical and synthetic fuels catalysts became well established. These included fluid catalytic cracking and hydrocracking using synthetic faujasite catalysts, reforming of light straight run naphtha with Pt/KBaL and paraffin isomerization over Pt/mordenite. But it remained for the discovery of ZSM-5 and other medium pore, high silica-alumina zeolites to enable a remarkable capability for shape-selective control of reaction selectivity. As illustrated in Table 1, this has led to significant industrial applications in aromatics processing (e.g., xylene isomerization, ethylbenzene synthesis and selective toluene disproportionation), catalytic dewaxing, ZSM-5 octane-enhancing additives in cracking, the methanol-to-gasoline process and other developments. The potentials of shape-selective catalysis have also been greatly amplified by the added dimension of isomorphous substitution and the availability of a large variety of A1P0 -, SAPO- and MeAPO-type compositions. Shape selectivity by mass transport discrimination includes at its extreme boundaries, complete reactant shape selectivity and product shape selectivity, but includes many subtle variations between these boundaries that are based on relative diffusivities. Spatioselectivity or restricted transition state selectivity can occur within zeolite microchannels when the pore size is insufficient to allow formation of the required transition states or intermediates (Figure 1). And there are many other variations that may involve pore mouth or external surface effects. A number of excellent reviews on shapeselectivity can be found in the literature, (e.g., 1-3). In the last 10 to 15 years moreover, there has been steady expansion of organic catalysis using zeolites, including significant thrusts into the synthesis of fine chemicals and intermediates, pharmaceuticals , and the environmental area. This is extensively documented in a, 4



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Table 1 Some Commercial Applications Utilizing Shape Selective Zeolites Description Process Fluid catalytic ZSM-5 as additive for octane-enhancement and generation of cracking light olefins Selectoform. Octane-enhancement after reforming by selective paraffin cracking (erionite) M-Forming Post reforming process for octane-enhancement and benzene reduction via selective paraffin cracking and aromatic alkylation via cracking fragments M2-Forming High temperature conversion of paraffins and other aliphatics to B T X aromatics and light gas byproducts MLDW, Dewaxing of lube base stocks or distillates via selective MDDW cracking of n- and singly-branched paraffins, with preservation of essential lube molecules MOGD Processing of C - C olefins to gasoline and distillate via oligomerization, disproportionation and aromatization MTG Conversion of methanol to gasoline-range ( C - C ) isoparaffins and aromatics MTO Conversion of methanol to C - C olefins but not aromatics MVPI, MHTI, Xylene isomerization to give high yields of /?-xylene; various M H A I , MLPI catalytic systems and process modes MTDP Disproportionation of toluene to produce benzene and xylene with long cycle life and minimal side reactions MEB Synthesis of ethylbenzene in high yield with minimal side reactions MBR Reduction of benzene in reformate with octane boost by alkylation with light olefins and cracking of low octane paraffins ISOFIN Isomerization of «-butene/w-pentene to corresponding isoolefins; useful for ether synthesis 2

1 0

4

2

10

5

number of comprehensive recent reviews (4-10). Among others, commercial processes for manufacture of substituted pyridines, terJ-butylamine, and 4methylthiazole have been implemented, and, using the remarkable titanium zeolite material TS-1, mild oxidations for synthesis of hydroquinone from phenol and ammoxidation of cyclohexanone have been reported. There is continuing aggressive exploration and scoping of the potentials for organic catalysis over zeolites in a variety of areas as shown in Figure 2, with continued progress in exploiting the potentials of the steadily increasing arsenal of new zeolites and molecular sieves (including the MCM-41 family of mesoporous materials and related compositions), and in understanding reaction mechanisms and the subtleties of structure-reactivity-selectivity relationships (11) in these complex systems.


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Many of these explorations comprise refined and finely-tuned shape-selective applications of the ZSM-5 family of medium pore zeolites and even larger pore materials, often with emphasis on conversion of molecules with polar and/or multiple functional groups or other sensitive structural configurations.

Figure 2. Scope of recent activity in organic catalysis over zeolites, from a survey of the recent literature (11); thickness of arrows denotes relative intensity of effort. A number of spécifie examples in organic catalysis over zeolites that demonstrate various aspects of shape-selective catalytic behaviour are worth noting: In the nucleophilic aromatic substitution of chlorobenzene with ammonia to form aniline over a variety of Cu-exchanged zeolites, improved selectivity for monoamination, i.e., formation of aniline rather than diphenylamine, was greater with Cu-mordenite and Cu-ZSM-5, probably from spatioselective inhibition effects (12). In the highly /?ara-selective bromination of toluene to form bromo-toluenes at 25°C. in the presence of propylene oxide as promoter, a bulky Br -oxirane complex was thought to increase steric restrictions favoring para-selectivity (13). In the dimerization of styrene over H Y and Η-beta at 72° C , a mixture of the cis- and trans- isomers of 1methyl-3-phenyl indane was formed (14). A small but meaningful degree of cisselectivity was observed with Η-beta (cis/trans ratio ~ 1.3-1.4). Possible involvement of spatioselectivity, product selectivity by relative diffusion rates, or even external surface effects may explain this observation. Restricted transition state selectivity may also underly the stereoselective reduction of 4-tert2


70 butyleyclohexanone to ds^-teTt-butyleyclohexanol over zeolite beta in a MeerweinPonndorf-Verley reduction (wo-C H OH as reducing agent) as recently reported by the Delft group (15). Catalytic reaction patterns diagnostic of both 10-ring and 12ring micropores have been observed in recent studies over zeolite MCM-22; the 10ring behavior comprised tendencies toward shape selectivity (as in ZSM-5). Among a number of possible explanations for the 12-ring behavior, the proposal that reaction occurs in half-supercage "pockets" on the external surface is an interesting possibility (16). A broad spectrum of selective catalytic oxidations have been reported under mild conditions using TS-l(Ti-ZSM-5) (e.g., see References 6 and 10). Product distributions in many of these reactions are enhanced via restricted transition state selectivity, i.e., para- and/or 1,2,4-substitution are favored, as well as minimization of coke and gas formation because of the negligible acidity. A n interesting example in this area is the mild oxidation of aniline to azoxybenzene with H 0 over TS-1. In this reaction, which shows high selectivity for azoxybenzene (17), conversion was highly dependent on crystallite size. Mechanistically, however, it was thought that reactions with the mononuclear aromatics occurred inside the zeolite channels (e.g., the key catalytic step for transfer of reactive oxygen from the titanium-peroxo complex of TS-1 to aniline), while formation of the bulkier azobenzene and azoxybenzene probably occurred on the external surface. Also in the oxidation area, "ship-in-the-bottle" Fe phthalocyanine complexes occluded in NaY have been used as catalysts for the selective partial oxidation of cyclohexane to adipic acid with ter/-butyl hydroperoxide (18). In synthesizing the catalyst, the constituents of the complex are brought into the ~12Â supercage of zeolite Y , and assembled in situ; after synthesis, the complex is entrapped (much like product shape selectivity), thus allowing reasonably effective "site isolation"and selective oxidation. A question that has interested the catalytic community for some time is the reason for the highly effective light paraffin aromatization activity of Pt/KBaL zeolite. Although a number of interesting mechanistic explanations have been proposed (e.g., see Reference 10), there still remained a number of significant inconsistencies. A recent study (19) proposed that the high dehydrocyclization activity/selectivity of Pt/KBa L derives from intrinsic but structure sensitive catalytic properties inherent in large, clean (i.e., unblocked) Pt ensembles, whether these are within or outside zeolite channels. Apparently spatioselective inhibition of formation of coke precursors is provided by the protective environment of the one-dimensional L-zeolite channels, thus preserving a clean Pt surface for 1,6-diadsorption and aromatization. Also involving a Pt containing zeolite, selective catalytic hydrogénation of cinnamaldehyde can be cited as an example where interaction of the dimensions of the organic reactant, the metal particle and the zeolite pore lead to structure-sensitive reaction and controlled selectivity (20). Specifically, over Pt-containing beta catalysts prepared by different methods of activation, high selectivity for the unsaturated alcohol was obtained with beta containing large (> 20Â) Pt clusters, while both the saturated aldehyde and alcohol were obtained with compositions with small (< 10Â) Pt clusters.


3

7

2

2


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Considering some early classical work (21), the abrupt cut-off at about C with ZSM-5 and ZSM-11 in the methanol-to-gasoline reaction, with generation of essentially no A + aromatics, can be related to restrictive transition state control dictated by the 10-membered ring zeolite, as can the unique resistance of ZSM-5 to coke formation in this reaction. Also, in the classical work of Olson and Haag on selective toluene disproportionation (22), there was a unique interplay and coupling of chemical and diffusion kinetics over coke-modified ZSM-5. The coke plugged a significant fraction of pores on the ZSM-5 surface, thus increasing channel tortuosity and lengthening the intracrystalline diffusion path. This allowed the competitive diffusion advantage of p-xylene over the ortho- and meta-isomexs (D ID ~10 - 10 ) to be enhanced significantly, while the longer intracrystalline residence time allowed rapid isomerization of the ortho- and meta- isomers to the para-isomer. This is an example of shape selectivity by relative diffusion rates that is of such great magnitude that it mimics shape selectivity by product exclusion. Turning briefly to mesoporous materials, a moderately acidic MCM-41[30 À] (Si/Al=95) was effective in the synthesis of the extremely bulky 6,8-di-teri-butyl-2phenyl-2,3-dihydro[4,i/]benzopyran from two very large reactants, (2,6-di-ter/-butyl phenol and cinnamyl alcohol) (23); zeolite Y was ineffective even after introduction of some mesoporosity by controlled steaming. A pure silica MCM-41 [30Â] on which 40% heteropoly acid ( H P W O , size ~ 12 Â) was dispersed comprised a large-pore mesoporous catalyst that now had very strong protonic acid activity, but could still handle bulky organic molecules (24). This composition was effective in the synthesis of 2-(l-phenylethyl)-4-teri-butyl phenol from reaction of 4-teri-butyl phenol and styrene. However, even in this large mesoporous channel system, spatioselective constraints inhibited formation of the even bulkier di-alkylated product. 1 0


n

para

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meta

o t o r t h o

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Literature Cited 1. 2.

3. 4.

5. 6. 7. 8. 9.

Weisz, P.B. Pure Appl. Chem., 1980, 52, 2091. Chen, N.Y.; Garwood, W.E.; Dwyer, F.G. Shape Selective Catalysis in Industrial Applications, Marcel Dekker: New York, N.Y., Basel, 1989, pp.1303. Weitkamp, J.; Ernst, S.; Dauns, H . ; Gallei, E. Chem. Ing. Tech., 1986, 58, 623. Chang, C.D.; Lang, W.H.; Bell, W.K. in Catalysis of Organic Reactions, Moser, W . R . , Ed.; Marcel Dekker: New York, N.Y., Basel, 1981, pp. 7394. van Bekkum, H . ; Kouwenhoven, H.W. Stud. Surf. Sci. Catal., 1988, 41, 45. Hoelderich, W.F.; Hesse, M ; Naumann, F. Angew. Chem. Int. Ed. Engl., 1988, 27, 226 Hoelderich, W.F.; van Bekkum, H . Stud. Surf.Sci.Catal., 1991, 58, 631. Parton, R.F.; Jacobs, J.M.; Huybrechts, D.R.; Jacobs, P.A. Stud. Surf. Sci. Catal., 1988, 46, 163. Dartt, C.B.; Davis, M . E . Catalysis Today, 1994, 19, 151-186.


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Venuto, P.B. Microporous Mater., 1994, 2, 297-411. Venuto, P.B. Stud. Surf. Sci. Catal., 1997, 105, 811-852. Burgers, M.H. W.; van Bekkum, H . Stud.Surf.Sci. Catal., 1994, 84, 1981. de la Vega, F.; Sasson, Y . ; Huddersman, K . Zeolites, 1993, 13, 341. Benito, Α.; Corma, Α.; Garcia, H.; Primo, J. Appl. Catal. A: General, 1994, 116, 127-135. Creyghton, E. Thesis: New Applications of Zeolite Beta in Selective Catalytic Hydrogenations, (with Prof. H. van Bekkum), Delft Univ. Press: Delft, 1996. Ravishankar, R.; Bhattacharya, D.; Jacobs, N.E.; Sivasanker, S. Microporous Mater., 1995, 4, 83. Selvam, T.; Ramaswamy, A . V . Catal. Letters, 1995, 31, 104. Parton, R.F.; Huybrechts, D.R.C.; Buskens, Ph.; Jacobs, P.A. Stud. Surf. Sci., Catal., 1991, 65, 47. Iglesia, E; Baumgartner, J.E. Proc. 9th Int. Zeol. Conf., von Ballmoos. R.. Ed.; Butterworth-Heinemann: Boston, M A , 1993, pp.421-431. Gallezot, P.; Blanc, B.; Barthomeuf, D.; Pais da Silva, M.I. Stud. Surf. Sci. Catal., 1994, 84, 1433. Chang, C. D. Hydrocarbons From Methanol, Marcel Dekker: New York. N.Y., Basel, 1983, pp. 1-129. Olson, D.H.; Haag, W.O. ACS Symp. Ser., 1984, 248, 275. Armengol, E.; Cano, M.L.; Corma, Α.; Garcia, H . ; Navarro, M.T. J.Chem. Soc., Chem. Commun., 1995, 519. Kozhevnikov, I.V.; Sinnema, Α.; Jansen, R.J.J.; Pamin, K; van Bekkum, H. Catal. Letters, 1995, 30, 241.


Shape-Selective Catalysis - American Chemical Society

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