Molecular shape-selectivity, zeolite catalysis

Derouane, E.G., New aspects of molecular shape-selectivity catalysis by zeolite ZSM-5 Imelik, B. Naccache, C. ... 

Figure 4.20 MTG/MTO reaction path and aromatics distribution with different zeolites as catalysts. (Reprinted from C.D. Chang, W.H. Lang, W.K. Bell, Catalysis in Organic Reactions, Molecular Shape-Selective Catalysis in Zeolites, pp. 93-94. Copyright 1981. With permission from Marcel Dekker.)...

In principle, all the kinetic concepts of intercalation introduced for layer-structured silicates hold for zeolites as well. Swelling, of course, is not found because of the rigidity of the three dimensional frame. The practical importance of zeolites as molecular sieves, cation exchangers, and catalysts (cracking and hydrocracking in petroleum industry) is enormous. Molecular shape-selective transport (large differences in diffusivities) and micro-environmental catalysis (in cages and channels)... 

To date, no chiral zeolite or molecular sieve has been obtained. However, Newsam et al. (48) have shown that zeolite beta is an intergrowth of two distinct structures polymorph A and B. Polymorph A forms an enantiomorphic pair. Thus, synthesis of one of the enantiomorphs of polymorph A would yield the first chiral zeolite and initiate the possibility of performing intrazeolitic asymmetric catalysis. Shape selective asymmetric catalysis would be the ultimate achievement in shape selective catalysis, and would certainly be a step closer toward truly mimicking enzyme catalysis. 

The catalytic properties associated with the molecular shape-selectivity exhibited by ZSM-5 are now well known. Recent work by Martens et al. (1995) has revealed that the external surfaces of zeolite crystals have also to be considered as potential shape-selective environments. Thus, strong evidence has been obtained for a lock-and-key model, which involves a form of pore mouth catalysis with bulky long-chain molecules that cannot penetrate into the intracrystalline micropores. The proposed lock-and-key model for n-alkane isomerization over ZSM-22 zeolite (with tubular pore openings of 0.55 x 0.45 nm) seems likely to be valid for other catalytic reactions. 

In addition to practical applications, metal cluster-derived catalysts, particularly intrazeolite metal cluster compounds, may aid in the identification of catalytically important bonding and structural patterns and thereby further our molecular understanding of surface science and heterogeneous catalysis. The ship-in-bottle technique for the synthesis of bulky metal-mixed metal cluster compounds inside zeolites and/or interlayered minerals has gained growing attention for the purpose of obtaining catalytic precursors surrounded by the interior constraint, imposing molecular shape selectivity. Such approaches may pave the way to offer the molecular architecture of hybrid (multifunctional) tailored catalysts to achieve the desired selectivity and stability for industrial processes. 

The major activities in the science and application of zeolite catalysts are still observed in the field of (shape selective) acid catalysis. However, additional thrust areas can be clearly identified today, viz. zeolites in oxidation or base catalysis, applications in environmental protection, catalysis by ship-in-the-bottle complexes, to enumerate just a few. Many aspects of zeolite catalysis have been covered in a number of recent review articles [e g., 1-6] including the potential catalytic applications of ultra-large pore molecular sieves [7]. Hence there is no real need, nor would it be feasible on the limited number of pages allotted to this review, to cover every aspect fi om the huge amount of work done recently in the field. Rather, the authors restricted themselves to selected topics in catalysis by zeolites which, in their own view, deserve particular attention in the years to come. 

P.B. Weisz and V.J. Frilette, Intracrystalline And Molecular-Shape-Selective Catalysis by Zeolite Salts. J. Phys. Chem., 1960, 64, 382-383. 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

The concept of molecular shape-selective catalysis is based on the action of catalytically active sites internal to the zeolitic framework, to diffusivity resistance either to reactant molecules or to product molecules or to both and to void limitation to reaction intermediates.This implies an intimate interaction between the shape, size and configuration of the molecules and the dimension, geometry and tortuosity of the channels and cages of the zeolite. Several types of effects exist ... 

A large amount of information has been published on catalytic cracking with molecular shape-selective catalysts, such as natural or synthetic crystalline aluminosilicates. Several excellent reviews have appeared on the chemistry of catalysis with zeolites (52, 53). Literature on hydrocracking of pure hydrocarbons and simple mixtures of hydrocarbons with zeolite-containing catalysts is limited. However, numerous patents on the use of zeolites in hydrocracking catalysts and published... 

E.G. Derouane, "Molecular Shape-Selective Catalysis in Zeolites - Selected Topics", Catalysis on the Energy Scene, Elsevier, p. 1-17 (1984). 

Some aspects of molecular shape-selective catalysis with hydrocarbons in zeolites J.A. Rabo... 

SOME ASPECTS OF MOLECULAR SHAPE-SELECTIVE CATALYSIS WITH HYDROCARBONS IN ZEOLITES... 

ABSTRACT. The amount of published work on molecular shape-selective catalysis with zeolites is vast. In this paper, a brief overview of the general principles involved in molecular shape-selectivity is provided. The recently proposed distinction between primary and secondary shape-selectivity is discussed. Whereas primary shape-selectivity is the result of the interaction of a reactant with a micropore system, secondary shape-selectivity is caused by mutual interactions of reactant molecules in micropores. The potential of diffusion/reaction kinetic analysis and molecular graphics for rationalizing molecular shape-selectivity is illustrated, and an alternative explanation for the cage and window effect in cracking and hydrocracking is proposed. Pore mouth catalysis is a speculative mechanism advanced for some systems (a combination of a specific zeolite and a reactant), which exhibit peculiar selectivities and for which the intracrystalline diffusion rates of reactants are very low. 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

For the acid catalysed conversion of hydrocarbons, the reaction mechanisms in absence of sterical hinderance are rather well understood, so that molecular shape-selective effects exerted by constrained environments can be isolated [8,9]. Shape-selective catalysis is also possible when other than acid functions are confined to the intracrystalline void volumes of zeolite crystals, e.g. metal [10,11], bifunctional [12] and basic functions [13]. Nowadays, catalysis on zeolites with organic substrates containing heteroatoms receives much attention. Molecular shape-selectivity seems to be superimposed on electronic factors determining the selectivities [14,15]. 

For the classic types of molecular shape-selectivity in zeolites, the reader is referred to the excellent review papers in literature [18-25]. In this paper we elaborate on the recently proposed distinction between primary and secondary shape-selectivity [26], and on the more or less abused concept of cage and window effects in cracl g and hydrocracking. In addition, some evidence available in literature for the speculative mechanism of pore mouth catalysis is presented. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Zeolite catalysed alkylation of polynuclear aromatics is considered to be simultaneously governed by several mechanisms. To achieve highly shape-selective catalysis, it is essential that the pore size precisely corresponds to the molecular dimensions of reactants and products, and to the transition state of the reaction intermediates. 

As in previous conferences, the section on catalysis contains the most papers. A general review of the different reactions which can be catalyzed by zeolites is presented by Kh. M. Minachev. H. W. Kouwenhoven discusses the isomerization of paraffins on zeolites. Cracking, isomerization, and electron transfer reactions are discussed in several papers. Correlations between particular activities and physicochemical properties are covered. Selectivities related to crystal size and molecular shapes are also studied. Most of the work is still done on modified Y zeolites, but mor-denite and erionite also receive attention. 

Activated diffusion of the adsorbate is of interest in many cases. As the size of the diffusing molecule approaches that of the zeolite channels, the interaction energy becomes increasingly important. If the aperture is small relative to the molecular size, then the repulsive interaction is dominant and the diffusing species needs a specilic activation energy to pass through the aperture Similar shape-selective effects are shown in both catalysis and ion-exchange, two important applications of these materials. 

Shape-Selective Catalysis with Zeolites and Molecular Sieves... 

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

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