Preparation, Structure And Performance Of Clay-supported Mo-based Catalysts For Higher Alcohol Synthesis From Syngas

Higher alcohol (C1-C6) can be used as a gasoline additive, clean fuels and chemical raw material. Among several catalyst systems for higher alcohol synthesis from syngas, molybdenum based catalysts have attracted widespread attention because of its excellent sulfur resistance. On the basis of our previous work, in this thesis, a series of clay supported K-Co-Mo catalysts were prepared by a sol-gel method combined with incipient wetness impregnation. The catalyst structure was characterized by X-ray diffraction (XRD), H2temperature-programmed reduction (H2-TPR), N2adsorption-desorption isotherm and X-ray photoelectron spectroscopy (XPS), and the catalytic performance for synthesis of higher alcohols from syngas was investigated.This thesis consists of three chapters:the first chapter is the literature review; the second chapter studied the effect of Mo loading on the structure and performance of catalyst; the third chapter mainly focused on the effect of reduction temperature on the catalyst structure and activity and the catalyst stability was also tested.The chapter one introduced the related background knowledge for higher alcohol synthesis from syngas and summarized the several catalyst systems for the reaction. Among them, the Mo-based catalysts were emphatically introduced, including the promoters and supports effects, the preparation methods and reaction mechanism etc.In the second chapter, the effect of Mo loading on the structure and performance of K-Co-Mo/clay catalysts was investigated. XRD results indicated that active component had a high dispersion on the surface of clay support. Incorporation of active components decreased the interlamellar distance of clay but had no significant influence on the crystalline structure of clay. After reduction at773K, a newCoMoO3species appeared, indicating that the reduction process enhanced the interaction between Co and Mo species.H2-TPR results indicated that increasing the Mo loading promoted the reduction of Mo6+, but had no significant influence on the reduction of Mo4+and Co2+species.XPS results revealed that for the reduced catalysts, a low valence state Moδ+(0<δ<4)species was detected, which was responsible for the non-dissociate adsorption of CO and favored the higher alcohol synthesis. From the nitrogen adsorption-desorption isotherms, the clay support exhibited a type IV isotherms with a hysteresis loop of type H3. The incorporation of active metals did not change the mesoporous structure of the clay support. The activity testing results indicated that the supported catalysts with Mo/clay weight ratio of50%showed the best performance for higher alcohol synthesis. The STY of total alcohol reached98.6g·kg-1·h-1, more than50%higher as compared to the unsupported sample. Especially the C1OH/C2+OH ratio decreased significantly, from0.88to0.50.The reason can be attributed that the mesoporous structure of the supported catalyst is suggested to prolong the residence time of intermediates in the pore to some extent, thus promoting the formation of higher alcohols. Besides, the supported catalyst was suggested to have a high active surface area, which is also conductive to the improvement of catalytic activity.The third chapter focused on the effect of reduction temperature on the structure and catalytic performance of catalysts. Increasing the reduction temperature enhanced the crystallization degree of catalysts. The supported catalyst reduced at773K showed the best activity for higher alcohol synthesis, which may be attributed to the high content of Moδ+species on the surface. The stability testing results indicated that the CO conversion and alcohol selectivity of the clay supported catalyst remained unchanged after reaction for100h, suggesting that the catalyst has a good stability.