Metal cluster hydrodesulfurization catalysts based on ternary molybdenum sulfides
Catalysts based on metal sulfide cluster compounds such as Chevrel phases (MxMo6S8) have been shown to possess high activity and selectivity for hydroprocessing reactions. Previous methods of preparing crystalline Chevrel phases have involved high temperature, solid-state synthetic routes which produce low surface area materials (0.1--1.0m2/gm). In our recent research, lower temperature syntheses via solution precursors have been explored as routes to these materials. The discovery of a new class of compounds, M2x/n n+(Mo6S8)Sx (n = 1--3) has resulted. These new reduced ternary molybdenum sulfides have been shown to have both high activity and selectivity in HDS reactions with thiophene, while also exhibiting initial surface areas near 200m2/gm. The stability and activity of these materials is dependent upon the pretreatment procedure. Surface area and porosity data have revealed a decrease in surface area and an increase in average pore size as the pretreatment temperature is increased. Temperature-programmed analysis studies were performed in conjunction with XPS to examine how the oxidation states change as a function of pretreatment temperature. The mechanism through which these new reduced ternary molybdenum sulfides convert to the crystalline Chevrel phase was also determined;The low temperature synthesis route of these new reduced molybdenum sulfides has allowed the synthesis of new cluster compounds, such as Pt(Mo6S8 )S. X-ray photoelectron spectroscopy, laser Raman spectroscopy, infrared spectroscopy and microprobe analysis indicate that this new material is structurally similar to other cluster compounds we have synthesized. Pretreatment of PtMoS in hydrogen at 950°C results in its conversion to the previously unknown crystalline Chevrel phase.