Study On PtSn/Sio₂ Catalyst For Acetic Acid Hydrogenation To Ethanol

Ethanol（EtOH）is an important feedstock for the synthesis of numerous products and could be widely used in many fields,such as chemical,pharmaceutical,military industry.EtOH is also a kind of clean fuel with excellent environmental benefits and has been commercially used as an additive or a substitute for gasoline.As an additive,EtOH could increase the oxygen content and improve combustion performance of gasoline.The worldwide method for EtOH production is based on the grain approach with corn and sugarcane as feedstocks.With the increasing demand of EtOH production,it will lead to a challenge for food supply.Considering the energy structure in China,this dissertation is about an alternative process for synthesis of EtOH by syngas.These approaches include the synthesis of acetic acid（AcOH）by syngas and then follows the hydrogenation of AcOH to EtOH.It is an environmental and efficient method,on account of the simple technics,mild reaction conditions,high atom economy.In order to understand the influence of preparation methods on catalysts structure and hydrogenation performance,three different approaches were used to synthesize PtSn/SiO₂ catalyst.The results indicated that a well fabricated PtSn/SiO₂ catalyst with high dispersion of Pt particles and large amount of Lewis acid sites was prepared by a modified two-step sol-gel（MTSG）method.The synergy between Pt and Sn species was remarkably enhanced compared with those prepared by impregnation（IM）method,consequently resulting in excellent activity and ethanol selectivity.Meanwhile,a parallel reaction paths model was proposed for the hydrogenation of AcOH based on the kinetic parameter analysis,in which ethanol and AcOEt were formed competitively through the adsorbed ethoxy,derived from the dissociated adsorption of AcOH.According to the investigations on the influence of Sn/Pt ratio,the amount of Lewis acid sites increased accompanied with a decline of Pt dispersion with the increasing Sn/Pt ratio.Excess high Sn/Pt ratio could inevitably lead to severe coverage of Pt particle surface by SnOx species.The TOF_Pt increased with the increasing molar ratio of L/Pt and then remained nearly unchanged.Therefore,a balancing effect between exposed Pt atoms and Lewis acid sites should be responsible for the superior catalytic performance of PtSn/SiO₂-MTSG catalyst.The influence of thermal treatment temperature on surface structure of PtSn/SiO₂ catalysts was also investigated.According to experiment results,Pt dispersion decreases with the increasing thermal treatment temperature.Simultaneously,the amount of Lewis acid sites was remarkably influenced by the pretreatment owning to the variation of Sn species.Interestingly,the catalytic activity showed the similar variation trend with that of Lewis acid sites,indicating the important roles of Lewis acid in AcOH hydrogenation.PtSn/SiO₂-100 catalyst exhibited the highest activity in AcOH hydrogenation,owing to its highest Pt dispersion,largest amount of Lewis acid sites and synergy between the closely contacted Pt and SnO_x（x<2）species.Meanwhile,the TOF of Pt and the amount of Lewis acid sites were also associated.It is clear that the TOF_Pt displayed an increasing trend with the increase of L/Pt and then remained nearly the same.Consequently,the balancing effect between Pt and Lewis acid sites was explained,which could account for the enhanced catalytic performance.This is in line with the results obtained in Sn/Pt mol ratio part,indicating that the balancing effect exists in AcOH hydrogenation over PtSn/SiO₂ catalysts.