Chapter 6 - Reaction mechanisms in protonic zeolites

Abstract

This chapter emphasizes on the application of quantum mechanical methods to the study of microporous materials with a strong focus on mechanistic concepts related to hydrocarbon activation catalyzed by acidic zeolites. A strong emphasis is also placed on the relationship between transition-state (TS) structures and their energies as a function of the zeolite micropore structure. The well-defined microporous structure is essential to the molecular sieve capability of zeolites, and that when used as a catalyst or catalyst support, it will induce product selectivity when reactions are diffusion limited. Because of their very limited solvation power, the enthalpies of reaction catalyzed by zeolites compare more closely to gas-phase reactions than to reactions catalyzed by homogeneous high-dipolar acids. Proton-exchanged zeolites can induce alkylation, transalkylation, isomerization, and cracking reactions. Moreover, they are also applied to achieve organic fine-chemical reactions. Some of the mechanistic aspects related to the reactions induced by proton-exchanged zeolites are the carbocationic nature of the transition states and complexity of reaction mechanisms. Short-range polarization of the zeolite-framework oxygen atoms is essential for the stabilization of TS complexes. The importance of matching TS structure size and shape with the dimensions of the zeolite micropores supports the idea that improved zeolite catalysts can be developed with greater specificity. The lock and key principle also controls TS selectivity in zeolite catalysis.