Study Of Methanol Dehydrogenation To Methyl Formate Catalyzed By Copper-based Catalysts

Methyl formate is an important chemical widely used as solvent,antiseptic,preservative and gasoline additive.It is also used as raw material to produce high value-added downstream products,which was recognized as a universal intermediate.Blind expansion of production capacity led to serious production capacity excess and low price of methanol.Methyl formate can be produced by the processes,such as esterification of methanol with formic acid,methanol carbonylation,methanol dehydrogenation and syngas method.Among them,methanol dehydrogenation process has attracted great attention because of its obvious advantages including single raw material,mild reaction conditions,short process flow,and that by-product CO and H₂are the raw materials in the methanol synthesis.Copper-based catalysts are the ideal catalysts and exhibit the best performance in the reaction of methanol dehydrogenation to methyl formate.The reaearch of copper-based catalysts in methanol dehydrogenation has made some progress.Enhancing the catalytic performance of catalysts and revealing the relationship between catalyst structure and catalytic performance are worth studying.This paper focused on the effect of the composition and structure of copper-based catalysts on the reaction of the methanol dehydrogenation to methyl formate.1.The CuO/SiO₂ catalyst precursors were prepared by different precipitation procedures and calcination at different temperatures.The reduced Cu/SiO₂ catalysts catalyzed the methanol dehydrogenation to methyl formate.When the CuO/SiO₂catalyst precursors were prepared by the addition of copper nitrate aqueous solution into ammonia aqueous solution?reverse addition precipitation?and co-current flow addition of both aqueous solutions?co-current flow addition precipitation?,after reduction with gaseous hydrogen,the small-sized metallic copper nanocrystallites were formed in the reduced Cu/SiO₂ catalysts as compared to those prepared by the addition of ammonia aqueous solution into copper nitrate aqueous solution?positive addition precipitation?.The reduced Cu/SiO₂ catalysts prepared by the reverse addition precipitation method with relatively lower acidity and basicity exhibited higher catalytic activity for the formation of methyl formate than those prepared by the co-current flow and positive addition precipitation methods.The reduced Cu/SiO₂ catalysts prepared by the calcination at a lower temperature exhibited higher catalytic activity for the formation of methyl formate than those prepared at a higher calcination temperature.The surface metallic Cu⁰ and Cu⁺species catalyzed the methanol dehydrogenation to methyl formate meanwhile the surface Cu⁺cations enhanced the decomposition of the resultant methyl formate to CO and H₂.2.The Cu/ZnO/SiO₂ catalysts were prepared by addition of ammonia aqueous solution into salts and SiO₂ aerogel aqueous solution?the positive addition precipitation method?and co-current addition aqueous solutions of salts and ammonia into SiO₂aerogel aqueous solution?the co-current flow precipitation method?.The reduced Cu/ZnO/SiO₂ catalysts prepared by the positive addition precipitation method had larger metallic Cu⁰ crystallites sizes and lower acidity and basicity than those prepared by the co-current flow precipitation method.The addition of ZnO in the Cu/ZnO/SiO₂catalysts decreased their acidity and basicity.The reduced Cu/ZnO/SiO₂ catalysts with a larger metallic Cu⁰ crystallite size and a lower basicity gave a higher selectivity of methyl formate.The small-sized Cu⁰ crystallites not only enhanced the methanol dehydrogenation reaction but also improved the decomposition of intermediate methyl formate to CO and H₂.High basicity could enhance the decomposition of intermediate methyl formate to CO and H₂.The addition of ZnO can reduce the proportion of Cu⁺on the catalyst surface,inhibited the decomposition of methyl formate and improved the selectivity of methyl formate.3.A series of Cu/ZnO/Al₂O₃ catalysts were prepared by continuous co-precipitation method.Na₂CO₃,NaOH and NH₃·H₂O were used as precipitants and the calcination temperatures of the catalyst precursors were 400,550,700 and 850 ^oC,respectively.The reduced catalysts prepared with Na₂CO₃ as precipitant had the smallest metallic Cu⁰ crystallites sizes and the largest specific surface,which exhibited the highest methanol activity;The reduced catalysts prepared with NaOH as precipitant had the largerest metallic Cu⁰ crystallites sizes and the smallest specific surface,which exhibited the lowest methanol activity,however the lowest basicity improved the selectivity of methyl formate.In addition,a relatively higher calcination temperature enhanced the interaction between the Cu⁰ and the ZnAl₂O₄,increasing the methanol conversion and decreased the selectivity of methyl formate,but a higher calcination temperature?850 ^oC?greatly decreased the specific surface area and basicity of the catalyst,resulting in the great decrease in methanol conversion but great increase in methyl formate selectivity.