Restricted transition-state molecular shape selectivity, zeolites

I he recent literature related to selective skeletal isomerization of -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, o-butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of (he ciitalysl that leads to poisoning of (he acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective. 

It has been shown by using a methodology combining molecular dynamics and an energy minimization technique 60) that in the PER pores (cavities and channels), the formation of Cti olefin intermediates is inhibited. These theoretical results agree well with the experimental indications of restricted transition state shape selectivity. Indeed, for materials such as zeolites and MeAPOs, most of the sites are expected to be located inside the micro-pores. Molecular sieves with large mesopores and/or large external surface... 

Shape Selectivity. One of the most important features of zeolite catalysts is their ability to act as a molecular sieve because the channels have molecular dimensions. Three types of shape selectivity can be distinguished reactant, product, and restricted transition state selectivity, depending on whether reactants can enter, products can leave, or intermediates can be formed in the zeolite catalyst, respectively. Medium-pore zeolites have been shown to have excellent restricted transition state selectivity. The high resistance toward coke formation on medium-pore zeolites has also been attributed to this type of shape selectivity. Transition state selectivity and product selectivity have been observed directly in the methanol conversion on ZSM-5 by means of magic-angle-spinning NMR. ... 

In early work on molecular shape-selectivity [1-3], three kinds of mechanisms were envisaged. Reactant selectivity occurs when some molecules of the feed are too bulky to diffuse through the zeolite pores and are prevented from reacting. Product selectivity occurs when among all the product molecules formed within the pores, only those with the proper dimensions can diffuse out and appear as products in the bulk. Restricted transition-state selectivity occurs when certain reactions are... 

Friedel Crafts type alkylations of benzene by alkenes involve the initial formation of a lattice associated carbenium ion, formed by protonation of the sorbed olefin. The chemisorbed alkene is covalently bound to the zeolite in the form of an alkoxy group and the carbenium ion formed exists only in the transition state. As would be expected fixjm conventional Friedel Crafts alkylation, the reaction rate over acidic molecular sieves also increases with the degree of substitution of the aromatic ring (tetramethyl > trimethyl > dimethyl > methyl > unsubstituted benzene). The spatial restrictions induced by the pore size and geometry frequently inhibit the formation of large multisubstituted products (see also the section on shape selectivity). 

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