Study On Molybdenum-based Sulfur-resistant Catalysts For Methanation From Syngas

Methanation is a process converting syngas produced from coal gasification into methane.Supported Ni catalysts show high activity for this reaction.However,Ni catalysts are susceptible to be poisoned by S-containing compounds.Therefore,desulfurization is necessary before methanation reaction because of high S content in coal.For Mo-based catalysts,desulfurization is not needed before methanation due to the sulfur-resistant property.Unfortunately,usually Mo-based catalysts reveal low activity and poor stability.Hence,the research and development of Mo-based catalysts with satisfactory performance is the key for this process.In the present study,supported MoS₂/?-Al₂O₃,unsupported MoS₂,and MoS₂-Al₂O₃ catalysts are prepared and investigated respectively.The effects of preparation conditions on catalytic performance were also studied and various characterization techniques were used to analyze the relationship between the structures and the performance of catalysts.Some conclusions are obtained as following.1.For the MoO₃/?-Al₂O₃ catalysts prepared by impregnation,low activity of MoO₃/?-Al₂O₃ is resulted from the formation of Al₂?MoO₄?₃ which can not be reduced readily.As evidenced by XRD,with catalysts impregnated at 70?,the peak of Al₂?MoO₄?₃ was weakened,while MoO₃ peak was strengthened and the reduction temperature of MoO₃ was lower compared with catalysts impregnated at 50?and 90?.Moverore,with the deceasing of calcination temperature,the formation of Al₂?MoO₄?₃dropped,but MoO₃ increased and the reduction temperature became lower.2.Unsupported flower-like sulfur-resistant MoS₂ catalysts with good methanation performance have been successfully synthesized by hydrothermal method.The catalytic activity is improved by higher S/Mo mole ratio in feed.Moverover,when the MoS₂catalyst was further sulfided in 3%H₂S/H₂,activity is improved obviousely,which have the CO conversion of 92.7%and the CH₄ selectivity of 78.7%.Characterization show that structures with longer and more ordered multi-layered MoS₂ are formed,which are benefit for the activity and stability.The increasing rate of crystallite size of used catalyst with sulfdation is slower than catalyst without sulfdation,which may result in a higher stability.3.MoS₂-Al₂O₃ catalysts prepared by solid state gridding method have lower activity than the unsupported flower-like sulfur-resistant MoS₂ catalysts including lower CO conversion and CH₄ selectivity.However,the stability is improved.When MoS₂-Al₂O₃ and MoS₂-Graphite catalysts were prepared by one-pot hydrothermal method,the order of CO conversion follows pure MoS₂=MoS₂-Graphite>MoS₂-Al₂O₃,and for the initial CH₄ selectivity,MoS₂>MoS₂-Graphite>MoS₂-Al₂O₃.For the ultimate CH₄ selectivity,MoS₂=MoS₂-Graphite>MoS₂-Al₂O₃.A core-shell structure of MoS₂-Al₂O₃ have been obtained by two-step hydrothermal method and the activity is almost the same with pure MoS₂ but with much higher stability.