Shape-selective reactions

Organic Chemistry II Shapes, Selected Reactions and Biopolymers... 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Shape Selective Reactions of Alkylnaphthalenes in Zeolite Catalysts... 

The advantages of shape selective catalysis are alreacfy ejq)loited in a number of industrial processes [11-14]. Astonishingly, virtually all these processes rely on a single structural type of catalyst, viz. zeolite ZSM-5 in various modifications, or its titanium containing analogue TS-1 [15]. It is, moreover, noteworthy that many of these processes convert and/or produce mononuclear aromatic compounds. It is not surprising, therefore, that a vast scientific literature exists on shape selective reactions of benzene derivatives in zeolite ZSM-5. 

The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-25] on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 - unlike on H-mordenite or HY - the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 25]. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel s experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. 

Shape-selective reactions occur by differentiating reactants, products, and/or reaction intermediates according to their shape and size in sterically restricted environments of the pore structures of microporous crystals16. If all of the catalytic sites are located inside a pore that is small enough to accommodate both the reactants and products, the fate of the reactant and the probability of forming the product are determined by molecular size and configuration of the pore as well as by the characteristics of its catalytic center, i.e., only a reactant molecule whose dimension is less than a critical size can enter into the pore and react at the catalytic site. Furthermore, only product molecule that can diffuse out through the pore will appear in the product. 

Shape selective reactions are typically carried out over zeolites, molecular sieves and other porous materials. There are three major classifications of shape selectivity including (1) reactant shape selectivity where reactants of sizes less than the pore size of the support are allowed to enter the pores to react over active sites, (2) product shape selectivity where products of sizes smaller than the pore dimensions can leave the catalyst and (3) transition state shape selectivity where sizes of pores can influence the types of transition states that may form. Other materials like porphyrins, vesicles, micelles, cryptands and cage complexes have been shown to control product selectivities by shape selective processes. 

The above examples of shape selective reactions show the complexity of such systems and that several factors need to be considered before shape selective control can be realized. The use of other porous supports besides zeolites such as carbon molecular sieves, clays, pillared clays and related materials to catalyze shape selective reactions appears to be growing. Molecular modeling of the spatial constraints of various pores is also an area of increased research effort. 

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... 

One of the most in ortant properties of zeolites is their ability to cany out shqie selective reactions [5]. These can be cl sified as, firstfy, product shape selective reactions in which the only products formed are those which can diffiise out of e pores of die zeolite, second, reactant shape selective reactions which occur when some of the molecules in a reactant mbcture are too large to diffiise through the catalyst pores, and, thirdfy, restricted transition-state selective reactions in which the only reactions which occur are those in which qiace exists in the pores or cavities to allow the formation of the activated transition state con lex. In some cases where the zeolite is three dimensional the gze of the channel intersections will also be a determining ictor. This unique catalytic property is related to the pore size of the zeolite and has led to the synthesis of zeohtes with a very w e range of pore gzes. 

The combination of synthesis and modification techniques gives us a chance to rationally design or tailor zeolite structures. For example, we can increase shape selectivity by modifying or eliminating active sites on the external surface of zeolite crystals. Although this outside surface may represent only 2-5 % ot the total surface area, acid sites located there are more accessible to reacting molecules than acid sites in the pores. As these catalytic sites are not shape selective, they catalyze a disproportionate amount of non-shape selective reactions. 

S. SHAPE SELECTIVE REACTIONS IN LARGE-PORE ZEOLITES... 

Coke deposition and aging are basic catalyst constraints, constraints lifted in ceitain applications by the discovery of ZSM-5 and related zeolite compositions and pore Structures. It was recognized a number of years ago that coke formation within tiie pores of a zeoUte can be a shape selective reaction (ref. 1). This area has now been excellently reviewed by M. Guisnet and P. Magnoux (ref, 2). 

Abstract Recent progresses in molecularly imprinted metal-complex catalysts are highlighted in this chapter. Molecular imprinting is a technique to produce a cavity with a similar shape to a particular molecule (template), and the molecularly imprinted cavity acts as shape-selective reaction space for the particular reactant. The application of the molecular-imprinting technique to heterogeneous metal-complex catalysts is focused in the viewpoint of a novel approach in the design of shape-selective catalysis mimicking enzymatic catalysis. 

Surface molecular imprinting provides the following three advantages for the design of selective catalysts (1) the formation of unsaturated active metal sites, (2) the formation of a shape-selective reaction space around the active metal center, and (3) high durability (and therefore recyclability) of the unsaturated metal center due to its protection by inorganic matrix overlayers. 

P-xylene is the most valuable xylene isomer due to its importance for the production of terephthalic acid, for which there is a demand in the polymer industry. Because of the complexity of separating the close-boiling components in the Cg-aromatic fraction, it is of great interest to produce p-xylene selectively. It has been reported that over modified H-ZSM-5 catalysts p-xylene can be produced in great excess of its thermodynamic equilibrium value in various shape-selective reactions (ref. 1). We will show that enhanced para-selectivity can be achieved even over unmodified H-ZSM-5 catalysts in the methanol reaction ... 

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