Computational Elucidation of the Transition State Shape Selectivity Phenomenon

Abstract

The most commonly cited example of a transition state shape selective reaction, m-xylene disproportionation in zeolites, is examined to determine if the local spatial environment of a reaction can significantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136 kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediated pathway. Both pathways are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trimethylbenzene), but relative selectivity to the other two isomers varies with pore geometry, mechanistic pathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different common zeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration of pore shape. Variation of the environment shape at the critical transition states is thus shown to affect the course of reaction. Barrier height shifts on the order of 10-20 kJ/mol are achievable. Observed selectivities do not agree with the transition state characteristics calculated here and, hence, are most likely due to product shape selectivity. Further examination of the pathways highlights the importance of mechanistic steps that do not result in isomer-defining bonds and leads to a more robust definition of transition state shape selectivity.

Towards a molecular understanding of shape selectivity

“…The fact that our approach is successful despite such a striking-to many readers, perhaps even alarming-simplification can be explained in part by the findings of very accurate quantum chemical calculations 30,31 : detailed investigations of how zeolitecatalysed reactions proceed show that the nature of the acid site is remarkably similar for zeolites with markedly different topologies. That is, the acid sites in, say, narrow-pore or large-pore zeolites differ little in their reactivity.…”

“…The basic structural unit of all zeolite frameworks consists of a silicon or aluminium atom tetrahedrally coordinated to four oxygen atoms. Any zeolite built of silica and oxygen only is neutral, but replacing Si 41 by Al 31 creates a negative charge on the framework. All such framework charges are neutralized by cations that reside inside the zeolite pores, where they can move freely and be exchanged against other cations.…”

“…We also note that zeolites obviously catalyse many chemical reactions other than hydroconversion, and it is well known that different reaction classes are often efficiently catalysed by zeolites that differ in their basic chemical composition. For example, whereas hydroconversion requires aluminosilicates, shape selective oxidation reactions are in general catalysed by titanosilicates (zeolites in which the framework Si 41 is replaced by Ti 41 rather than Al 31 ). The chemical characteristics of zeolites thus clearly play an important role in achieving the desired catalytic activity.…”

Combined Density Functional Theory and Monte Carlo Analysis of Monomolecular Cracking of Light Alkanes Over H-ZSM-5

“…45 To create an acidic site, one of the 96 Si atoms in the unit cell was replaced by an Al atom at the T12 site 46 and the resulting negative charge was compensated by a proton bonded to one of the neighboring framework oxygens. The siting of Al in the ZSM-5 framework is neither random nor controlled by the stabilization energy of the Al atoms in the framework but depends mainly on the conditions of the zeolite synthesis.…”

A Link between Reactivity and Local Structure in Acid Catalysis on Zeolites

“…These demonstrations have remained elusive for alkoxidemediated hydrocarbon catalysis because the complexity of the relevant reaction networks, often comprising hundreds of chemical reactions and elementary steps, makes the isolation of primary pathways a formidable task. Also, as discussed by Clark et al, 37 the inability to explicitly account for spatial constraints imposed by the zeolite framework in terms of entropic and enthalpic contributions to the stability of inter-mediates and transition states, combined with the ensembleaveraging required because of the structural and chemical nonuniformity of zeolite protons, precludes the unambiguous assignment of reactivity differences among zeolites to perturbations induced by the simple encapsulation of reactants or transition states within zeolite cavities.…”