Restricted transition-state molecular

In general the selectivity in toluene methylation found with MFI molecular sieve catalysts is proposed to be caused either by a restricted transition state to form m- and o-xylene [10,11,12] and/or diffusional constraints of the bulkier isomers, o- and m-xylene, in the pores of zeolite ZSM5 [3,4], Recent results on the methylation of toluene based on in situ analysis of the working catalyst showed that all three isomers were primary products in toluene methylation [13]. The high p-selectivity was explained to be due to transport constraints of the bulkier... 

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... 

The most important consequence of restricted transition state selectivity is that ZSM-5 and many other medium-pore zeolites deactivate much slower than most other crystalline and amorphous catalysts. The difference is not trivial. In most acid catalyzed reactions large-pore zeolites deactivate within minutes or in hours, whereas the activity of ZSM-5 ranges from weeks to years. Most of the coke in large-pore zeolites is formed within the pores. In ZSM-5 most of the coke is deposited on the outer surface of the crystals like an eggshell over an egg [23] because coke precursors cannot form in the pores of pentasil molecular sieves. The resistance of ZSM-5 to coking makes a number of industrial processes economical. 

As most of the acid sites are located in pores of molecular size the rate and the selectivity of catalytic reactions depend not only on the intrinsic properties of the sites but also on the pore structure. A zeolite catalyst selects the reactant or the product by their ability to diffuse to and from the active sites (reactant and product selectivity). Steric constraints in the environment of the sites limit or inhibit the formation of intermediates or transition states (restricted transition state selectivity) [24,25]. The strong polarizing interaction between zeolite crystallites and adsorbed molecules leads to an unusually high concentration of the reactants in the pores. This concentration effect causes an enhancement of the rates of bimolecular reaction steps over monomolecular reaction steps [26]. 

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... 

However, so far all the applications of the JT effect theory were realized only for chemically bonded systems in their high-symmetry configuration and transition states of chemical reactions. We show here that this is an unnecessary restriction the JT type instability is inherent to all the cases of degeneracy or pseudodegeneracy in molecular systems and condensed matter including nonbonded states in molecule formation from atoms, intermolecular interaction, and chemical reactions. 

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