Molecular modelling in zeolite catalysis

Quantum-chemical studies of the alkylation of toluene by methanol in Mordenite illustrate the fundamentals of transition state selectivity. It appears that, as in the enzymes, the reorientation energy of the reactant molecular so as to make the reaction channel to a particular product accessible, is the controlling parameter. This implies that the lock and key selectivity model of shape selective zeolite catalysis is based on the relative stability of pre transition state intermediates (1). These energies are due to the dispersive Van der Waals interaction, not currently require the use of force fields to be simulated correctly. In a second part of the talk we will concentrate on Lewis acidic properties of Zn2+ and the corresponding (ZnOZn)2+ oxycations (2). It will appear that their reactivity sensitively depends on ion-exchange cation positions. Size match with cavity as well as the negative zeolite lattice charge distribution is important. Quantitative prediction of the relative energies is only possible when full consideration of lattice relation is important. Quantitative prediction of the activation is studied. Hydrogen recombination is rate limiting in the ZnOZn2+ cluster, whereas CH activation is rate limiting when catalyzed by Zn2+ (3). Finally the importance of diffusional effects will be discussed for the isomerization of hexane catalyzed by Mordenite. Single-file diffusion occurring in one-dimensional pores has a dramatic effect on the concentration dependence even of monomolecular reactions that are found to proceed through a maximum. Using theoretically and experimentally determined elementary rate parameters on quantitative prediction has been made of crystallite size dependence (4).