A Theoretical Investigation Into Thiophenic Desulfurization Catalyzed By Zeolites

Sulfur and sulfides contained in fuel oil through combustion can generate a lot of harmful substances, such as SOx, to the environment and human. With the increasingly serious environmental pollution, people attach more and more importance to environmental issues, and sulfur content of fuel oil is restricted lower and lower. The key of reducing sulfur content in fuel oil and of deep desulfurization is to decrease the thiophene derivatives content. But traditional hydrodesulfurization (HDS) is difficult to remove thiophene derivatives and it require severe reaction conditions and high production costs. Zeolites alone can catalyze thiophene desulfurization, and the required reaction condition is moderate. So it is a promising method to thiophene desulfurization.A two-layer ONIOM study has been carried out to investigate the cracking desulfurization reaction of thiophene catalyzed by H-beta, H-MCM22, H-ZSM5, H-FAU and metal cation-exchanged (Li+, Na+, K+, Cu+, and Ag+) beta zeolites. Two mechanisms, the unimolecular and bimolecular cracking mechanisms, of desulfurization reaction has been considered. We get the structure and energy date of each intermediate and transient state of this reactions. The results indicates that the rate-determining steps of unimolecular and bimolecular mechanisms are the C-S bond cracking of thiophene, and the dimerization of thiophene, respectively. The activation energy to generate each transient state in unimolecular cracking reactions over the four acidic zeolites is too high to process the practical reaction. However, the activation energies of the transient states produced according to bimolecular mechanism are reasonable. Therefore, thiophene should perform cracking desulfurization reaction over H-beta, H-MCM22, H-ZSM5 and H-FAU according to bimolecular mechanism. Moreover, the activation energies of the transient states produced according to no matter unimolecular or bimolecular mechanism are all too high to occur under experimental condition, so metal cation-exchanged (Li+, Na+, K+, Cu+, and Ag+) beta zeolites do not fit to catalyze thiophene cracking desulfurization.The total ONIOM energy can be decomposed into QM and MM contributions. These two contributions may be used to analyze the effect of zeolite framework on the reaction pathway, especially on structures and activation barriers of transition states. The negative MM energies are favorable for the formation of transition sates and can reduce the activation barriers and vice visa. Through the analysis of four typical zeolites, Beta, MCM22, ZSM5, and FAU, we conclude that despite the different topological frameworks, bimolecular cracking mechanism of thiophene is favorable on four zeolites, probably due to larger sizes of dimerized molecules and thus stronger interactions between intermediates and zeolite frameworks.