AB-INITIO CALCULATIONS ON THE MECHANISMS OF HYDROCARBON CONVERSION INZEOLITES - SKELETAL ISOMERIZATION AND OLEFIN CHEMISORPTION

Quantum chemical calculations on zeolite-catalysed hydrocarbon convers ion mechanisms have been carried out. The one-step skeletal isomerisat ion (methyl shift) and the olefin chemisorption reactions have been co nsidered. The results obtained indicate that the product distribution of both reactions ais determined by the activation energies rather tha n by the reaction heats. The shift of an existing branch is calculated to be significantly (by about 10 kcal/mol) easier than branch formati on in n-alkanes in agreement with the experimental data. Only a small difference is found between n-butane and n-pentane isomerisation, whic h contrasts with experiment ana suggests that at least one of these re actions does not proceed via a one-step methyl shift. Calculations sho w also that alkyl groups larger than methyl may be shifted. (C) 1997 E lsevier Science B.V.