Cracking of hydrocarbons on zeolite catalysts: Density functional and hartree-fock calculations on the mechanism of the β-scission reaction

Quantum chemical calculations on the mechanism of the β-scission reaction 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 paths were identified: path RL, one-step via the "ringlike" transition state (TS); path HBCP, via the "hydrogen-bonded" TS and substituted cyclopropane; and path HB, one-step via the "hydrogen-bonded" TS. Transition states in all reaction paths represent complexes of the carbocation-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 β-scission of but-1-oxy and pent-2-oxy with the H₃Si(OH)AlH₂(OSiH₃) cluster were found to be 57.4 and 52.4 kcal/ mol, respectively.