A theoretical study on the mechanism for thiophene hydrodesulfurization over zeolite L-supported sulfided CoMo catalysts: Insight into the hydrodesulfurization over zeolite-based catalysts

The mechanism for the hydrodesulfurization (HDS) of thiophene over zeolite L-supported sulfided CoMo catalysts was investigated using density functional theory (DFT) calculations. Both the direct desulfurization (DDS) pathway and hydrogenation (HYD) pathway for thiophene were studied in detail. In the DDS pathway, the CS bond of thiophene is directly broken by hydrogenolysis. In the HYD pathway, hydrogenation of one or both CC bonds occurs before CS bond scission. On the basis of the energetic analysis of the elementary steps, the favored reaction pathway was proposed: (1) Firstly thiophene is hydrogenated to 2,5-dihydrothiophene (2,5-DHT) intermediate; (2) then the CS bonds of 2,5-DHT are broken by the hydrogenolysis pathway or elimination pathway. The Mulliken charge results show that zeolite L loses electrons after the adsorption of thiophene and all the other hydrogenated intermediates, indicating that zeolite L acts as electron donor in the HDS reaction. The pore framework of zeolite L plays a key role in decreasing the energy barrier by the stabilization effect. Compared with other zeolites, the intrinsic superiority of zeolite L as a support for CoMo catalysts in thiophene HDS reaction can be attributed to its perfectly fitted pore structure and ideal stabilization effect for the reactant. This study provides an atomic-scale understanding of the HDS mechanism of thiophene over zeolite L-supported sulfided CoMo catalysts.