A Two-Layer ONIOM Study on Initial Reactions of Catalytic Cracking of 1-Butene To Produce Propene and Ethene over HZSM-5 and HFAU Zeolites

Two-layer ONIOM calculations were carried out to study initial reactions of catalytic cracking of 1-butene to produce propene and ethene on HZSM-5 and HFAU zeolites. Direct cracking and dimerization cracking mechanisms were evaluated. The calculated data indicate that the dimerization cracking is more favorable than the direct cracking based on both kinetic and thermodynamic considerations. HZSM-5 is catalytically more effective than HFAU for the cracking reactions. ONIOM energy analysis shows that the effectiveness of zeolite is due to long-range van der Waals interaction energies that stabilize all reaction intermediates, and short-range interaction between the zeolite and the reacting species that reduce the activation energies. For the dimerization process, stepwise and concerted mechanisms are similar in energy changes. Generally speaking, dimerizations appear to be exothermic reactions with modest activation energies. The activation energies for isomerization, beta-scission, and deprotonation are lower for larger molecular fragments than for smaller ones.