CRACKING OF HYDROCARBONS ON ZEOLITE CATALYSTS - DENSITY-FUNCTIONAL AND HARTREE-FOCK CALCULATIONS ON THE MECHANISM OF THE BETA-SCISSION REACTION

Quantum chemical calculations on the mechanism of the beta-scission re action in zeolites were performed using density functional theory and Hartree-Fock methods. The results obtained indicate that the potential energy surface for this reaction is very complex. Three reaction path s were identified: path RL, one-step via the ''ringlike'' transition s tate (TS); path HBCP, via the ''hydrogen-bonded'' TS and substituted c yclopropane: and path HB, one-step via the ''hydrogen-bonded'' TS, Tra nsition states in all reaction paths represent complexes of the carboc ation-like fragment with the negatively charged cluster, whereas both initial and final states represent alkoxy species with a covalent bond between a carbon atom of the hydrocarbon portion and a zeolite oxygen . The dependence of calculated activation energy on the cluster model of zeolite and on the calculation level is discussed. The B3LYP/6-31+G*//B3LYP/6-31G* activation energies for p-scission of but-1-oxy and pent-2-oxy with the H3Si(OH)AlH2(OSiH3) cluster were found to be 57.3 and 52.3 kcal/mol, respectively.