Ab initio calculations on the mechanisms of hydrocarbon conversion in zeolites: Skeletal isomerisation and olefin chemisorption

Abstract

Quantum chemical calculations on zeolite-catalysed hydrocarbon conversion mechanisms have been carried out. The one-step skeletal isomerisation (methyl shift) and the olefin chemisorption reactions have been considered. The results obtained indicate that the product distribution of both reactions are determined by the activation energies rather than by the reaction heats. The shift of an existing branch is calculated to be significantly (by about 10 kcal/mol) easier than branch formation in n-alkanes in agreement with the experimental data. Only a small difference is found between n-butane and n-pentane isomerisation, which contrasts with experiment and suggests that at least one of these reactions does not proceed via a one-step methyl shift. Calculations show also that alkyl groups larger than methyl may be shifted.