MECHANISMS OF HYDROCARBON CONVERSION IN ZEOLITES - A QUANTUM-MECHANICAL STUDY

Ab initio quantum chemical methods have been used to study the mechani sms for the hydrocarbon conversion in zeolites (cracking, isomerisatio n, alkylation, etc.). It is found that the intermediates are covalent alkoxide species and indicate that there is no energy ordering of thes e alkoxides according to their primary, secondary, or tertiary nature. Transition states have been located for the most important conversion reaction steps for hydrocarbons up to C6. The reactions are concerted and the transition states are ionic and ring-like. Due to charge delo calisation in the transition state the activation energies depend on t he nature of the initial and final alkoxides. These calculations are t he foundation for a new model of the hydrocarbon conversion which can better describe the influence of the zeolite as will be illustrated wi th calculations on acid sites with varying acid strength. (C) 1997 Aca demic Press.