Surface Active Species And Catalytic Mechanism Of Cu-based Catalysts In The Methyl Acetate Hydrogenation Reaction

Ethanol,as a versatile feedstock for chemical synthesis and a promising fuel alternative or additive,has shown big commercial potential in the market.The route of ethanol synthesis from syngas via methyl acetate?MA?hydrogenation is applicable for the structure of fossil energy reserves and could promote a clean and diversified energy development in China.Cu-based catalysts have been widely used in vapor-phase ester hydrogenation reactions due to their excellent activity and selectivity for C=O bonds hydrogenation.Significant advancements on Cu-based catalysts have been achieved in the past decade.However,concerning the nature of active species,no consensus has been reached on the precise roles and the proper distribution of Cu⁰ and Cu⁺species for hydrogenation reactions.This dissertation concentrates on discussion of the copper active sites and their contributions in hydrogenation catalysis,and development of new catalyst for MA hydrogenation,in hope of solving the foresaid issues.To fabricate the Cu/SiO₂ catalysts with controllable surface copper species,the copper content,aging time and the pretreatment way of support in ammonia evaporation method were modified.Given the results of multiple and mutually corroborated characterizations,fourteen Cu/SiO₂ catalysts with full-range distribution of Cu species and similar general morphologies were obtained.The precise contributions of Cu⁰ and Cu⁺active sites was also illuminated by both in situ FTIR experiments and DFT calculations:the Cu⁺sites adsorbed the methoxy and acyl species,while the Cu⁰facilitated the H₂ decomposition.Furthermore,correlated the catalytic performance of the samples with the amount of surface Cu⁰ and Cu⁺species,the balancing effect of these two active sites was demonstrated:when the accessible metallic Cu surface area was below a certain value,the catalytic activity of hydrogenation linearly increased with increasing Cu⁰ surface area,whereas it was primarily affected by the amount of Cu⁺surface area.In order to investigate the impact of oxygen vacancies on copper electronic state and activity of Cu-based catalysts for MA hydrogenation,four catalysts using different shaped CeO₂ nanocrystals as supports were prepared.The catalytic activities significantly changed by the morphology of supports in order of rod>cube>spindle>octahedron,which agreed with the variation of the formation energy of oxygen vacancies among the corresponding exposed lattice planes of supports.Conbined the results of chemisorption and in situ FTIR experiments,it was demonstrated that the oxygen vacancies could significantly affect the distribution of copper species.In the reductive conditions,the mobile oxygens could be released from the lattice and form lots of oxygen vacancies.And the Cu⁺species were generated and stabilized by the interaction between Cu particles and the oxygen vacancies of CeO₂ support.Thus,increasing the oxygen mobility of supports could effectively increase the amount of surface Cu⁺species,which enhanced the catalytic activity for MA hydrogenation.A new Cu@CeO₂ core-shell catalyst was fabricated using a one-step sol-gel method.The Cu@CeO₂ catalysts exhibited an excellent catalytic activity and stability in MA hydrogenation.Both of he MA conversion and ethanol selectivity were above99%at liquid hour space velocity of 1.0 h^-1,and after 160 hours there was no obvious decrease in the catalytic performance.It can be inferred that the core-shell structure could not only effectively prevent the transmigration and aggregation of metallic Cu nanoparticles,but also increase the amount of Cu⁺species owing to the enlarged contact areas of copper and CeO₂.Moreover,the Cu⁰ and Cu⁺species were proved to distribute on the interface between the Cu core and CeO₂ shell,resulting in a closer relative position of the two active sites,which enhanced their synergetic effect in the catalysis for MA hydrogenation.