Study On The Hydrogenation Of Acetic Acid To Ethanol Over Copper-based Catalysts

Ethanol is one of the important chemical feedstocks and clean energies.At present,ethanol is mainly produced from fermentation of biomass,and the production cost is relatively high.It is urgent to realize the production of ethanol from diversified raw materials.China is rich in coal resources,in recent years,the production of acetic acid through carbonylation route using coal as raw material has been developed rapidly,and the development of new downstream chemical products of acetic acid has become an urgent problem for Chinese enterprises.Therefore,ethanol synthesis through acetic acid hydrogenation has received extensive attention.The noble metal catalysts used in industry own excellent catalytic performances for acetic acid hydrogenation,but the high cost and limited resources are not conducive to their large-scale application.The development of cheap and efficient Cu-based catalysts to replace precious metal catalysts for ethanol synthesis from acetic acid hydrogenation has great theoretical significance and practical value.In this study,the structure-activity relationship between the active sites of copper based catalysts and their catalytic performances,as well as the interaction between active metals and carriers,are systematically investigated by means of experiments and DFT theoretical calculations,which helps to understand the mechanism of catalytic hydrogenation of acetic acid over Cu-based catalysts from the macroscopic and microscopic levels.For copper-catalyzed hydrogenation of acetic acid,it is found that the copper is mainly in the form of metallic phase,which serves as the main active sites for the dissociation of acetic acid and H₂.The amount of these metallic copper sites is closely related to the type of supports,metal loadings and the preparation methods.In situ DRIFTs shows that acetic acid molecule is dissociated on the surface of metallic Cu to form acetate species,which is a key intermediate species in the process of acetic acid conversion.And the concentration of surface acetate species is positively correlated with the conversion of acetic acid.The increase of Cu dispersion is conducive to provide more active sites for H₂ dissociation,thus promoting the transformation of acetaldehyde to ethanol.The high specific surface area of SBA-15facilitates the dispersion of Cu,and its acid and alkalinity are relatively mild compared with Al₂O₃ and Ti O₂,which inhibits the side reactions,such as ketonization of acetic acid or ethanol dehydration.Therefore,SBA-15 is chose as the carrier of Cu catalysts.In order to investigate the impact of non-precious metal promoters on the acetic acid hydrogenation performance of Cu/SBA-15,a series of bimetallic Cu-M/SBA-15catalyst samples are prepared by the deposition-precipitation method.The results show that,the catalytic activities of Cu-M/SBA-15 are significantly improved when another metal is introduced to Cu/SBA-15.The ethanol yield on Cu-M/SBA-15 is decreased in the following order:In?Mn>Zn>Ce>Zr>Mo>W,and the exact roles of In and Mn will be further discussed.Based on the above research results,Cu-In/SBA-15 catalysts are prepared for acetic acid hydrogenation.It is found that both Cu-In alloy and metallic copper species are observed on the surface of catalyst samples,and the electrons are transferred from indium to copper in the Cu-In alloy.In situ DRIFTs and H₂chemisorption are applied to investigate the dissociation behaviors of acetic acid and H₂,as well as the hydrogenation of acetic acid.The results show that the electron interaction within Cu-In alloy can significantly promote the dissociation of acetic acid to form acetate,and the conversion of acetate into acetaldehyde,thus increasing the conversion of acetic acid.Meanwhile,the chemisorption capacity of H₂ is positively correlated with the Cu surface area.The larger the active Cu surface area is,the higher the concentration of dissociated H species on the catalyst surface will be,so as to promote the formation of ethanol from acetaldehyde.It is proposed that the catalytic activity of Cu-In/SBA-15 is significantly promoted due to the synergistic effect between the Cu-In alloy and the metallic Cu.The dissociation capacity of acetic acid is in equilibrium with the chemical adsorption capacity of H₂ when the weight ratio of Cu/In is 9:1,where the acetic acid conversion and the ethanol selectivity reached the maximum of 99%and 90%,respectively.The dissociation of acetic acid,as well as the evolution of the surface species on Cu-Mn/SBA-15 catalyst are investigated by combination of experimental characterization and DFT theoretical calculations,which helps to understand how manganese improves the catalytic performance of copper based catalysts.The active metals of Cu-Mn/SBA-15 are mainly in the form of metallic copper and Mn O.The electronic interaction between Cu and Mn O promotes the polarization of C=O bonds,which is favor to the dissociation of acetic acid and the transformation from acetate to acetaldehyde,leading to higher conversion of acetic acid.In addition,the formation of ethyl acetate through acetaldehyde condensation can also be inhibited at the interface between Cu and Mn O,which favors the ethanol formation.A relatively higher Cu/Mn ratio is necessary to achieve the balance between the amount of active sites for the dissociation of acetic acid and H₂.The highest acetic acid conversion and ethanol selectivity of Cu-Mn/SBA-15 reach 99%and 90%,respectively,which is close to the activity of noble metal catalysts.In addition,due to the interaction between Mn O and Cu species,which can effectively alleviate the agglomeration of copper crystallites,the stability of Cu-Mn/SBA-15 sample is greatly improved compared with that of Cu-In/SBA-15.