Preparation and Characterization of Co-Mo Model Sulfide Catalysts for Hydrodesulfurization

Abstract

Development of highly active hydrodesulfurization (HDS) catalysts is one of the most urgent problems in the petroleum industry. Better characterization and understanding of the nature of HDS catalysts on the molecular scale are of great importance for the rational design of highly active HDS catalysts. Our approaches for this purpose involve the fabrication of model catalysts using metal carbonyls to overcome the difficulties caused by the heterogeneity of practical catalysts. One of our new approaches uses the synthesis of intrazeolite homogeneous Mo, Co and Co-Mo sulfide clusters with well defined structures. The local structure of intrazeolite Mo sulfide dimer clusters, Mo₂S₄, depends on the composition of the host zeolite. Thermally stabilized Co₂Mo₂S₆ binary sulfide clusters, which show catalytic synergies between Co and Mo for thiophene HDS, with a thiocubane type structure are formed in zeolite. The intrinsic HDS activity of Co sulfide clusters depends on the host zeolite. The hydrode-selenium reaction of selenophene, combined with an extended X-ray absorption fine structure (EXAFS) study, over Mo₂S₄ and MoS₂ clusters suggests a microscopic HDS reaction mechanism. More practical modeling of Co-Mo sulfide catalysts using a selective preparation method of CoMoS phases supported on refractory oxides is established to understand the nature of practical HDS catalysts. No effects of the support are found with the CoMoS phase supported on Al₂O₃, TiO₂ and ZrO₂, whereas the CoMoS phase supported on SiO₂ shows a higher intrinsic activity. The fraction of the CoMoS phase accessible to NO adsorption is elucidated, based on the number of the CoMoS phase in the model catalyst, suggesting a new model of the CoMoS structure. The maximum potential HDS activity of Co(Ni)-Mo(W) catalysts is evaluated by using Co(CO)₃NO as a probe molecule. Such model catalysts provide important information about the effects of the catalyst preparation and additives, the catalyst structure and the fundamental aspects of HDS catalysts such as microscopic reaction mechanisms and structure-reactivity relationship.