Construction Of Active Phase Of Transition Metal Sulfides And Mechanism Of Hydrodesulfurization

The environmental pollution caused by automobile exhaust emissions has become a major issue that must be solved in the national economy and social sustainable development.Transition metal sulfide catalysts are widely used in the hydrogenation process to produce ultra-low sulfur clean oil products.The structure of the transition metal active phase and the structure-activity relationship of hydrodesulfurization performance are the key to design and prepare high-performance catalysts.In this paper,molybdenum-based mesoporous composite metal oxides were prepared and used as precursors to study the mechanism of controlling active phase microstructure in presulfurization process.The activity and selectivity of catalysts were evaluated by DBT to reveal the structure-activity relationship between microstructure of active phase and hydrodesulfurization activity and selectivity.Ammonium nickel molybdate was synthesized by chemical precipitation method and hydrothermal reaction method.The effects of NH₄⁺/Ni²⁺molar ratio on the crystalline phase and the hydrodesulfurization performance of the catalyst were investigated.The results showed that with the increase of NH₄⁺/Ni²⁺molar ratio,the crystal phase of precursor transformed from?NH₄?₄[Ni?OH?₆Mo₆O₁₈]to?NH₄?HNi₂?OH?₂?MoO₄?₂.And the results of DBT hydrodesulfurization showed that when NH₄⁺/Ni²⁺molar ratio was 10.5,the catalyst prepared by hydrothermal reaction method had higher catalytic activity and selectivity.As can be seen from the characterization results,the catalyst has longer slab length of MoS₂nanoparticles which could expose more edge sites,thus having higher hydrogenolysis selectivity.When molar ratio of NH₄⁺/Ni²⁺was 10.5,nickel ammonium molybdate synthesized by chemical precipitation was used as precursor to study the rules of sulfiding temperature and sulfiding methods on the active phase construction of catalyst.The results showed that when ammonium sulfide was used as the sulfiding agent,nickel ammonium molybdate was first converted into Ni?NH₃?₆MoS₄ and NiMo₂S₄.With the increasing of sulfidation temperature,the sulfidation degree increased,and finally the nickel,molybdenum and sulfur elements were highly dispersed to form sulfided nanoparticles.At this time,the smallest S/Mo ratio had an adverse effect on the catalytic activity.When hydrogen sulfide was used as sulfiding agent,the Ni species was first sulfided to become NiS,and when sulfidation temperature was higher than 300°C,?Ni₃S₂?Ni₇S₆ and MoS₂ generated.With the increasing of sulfidation temperature,the average slab length and stacking number of MoS₂ nanoparticles increased.Besides,the increased curvature was not conducive to the hydrogenolysis reaction of DBT.Moreover,it was found that the use of ammonium sulfide can effectively control the bending degree of MoS₂ nanoparticles and obtain a more selective unsupported hydrodesulfurization catalyst.Finally,hydrothermal reaction method was used to synthesize Ammonium cobalt molybdate as catalyst precursor,and the active phase was prepared by ex-situ presulfurization process.The effects of Co/Mo ratios on their catalytic performance were investigated.The results showed that Co/Mo ratio could change crystal morphology and pore structure of the CoMo precursor.When Co/Mo ratio was 1/2,the conversion and selectivity of the catalyst are good.The excess Co₉S₈ produced by Co prevented the DBT molecules from reacting on the active site of CoMoS,and reduced catalyst activity.The evaluation results showed that the catalyst with Co/Mo ratio of 1/2 could form MoS₂ nanoparticles with longer average slab length,which could promote hydrogenolysis reaction of DBT.