DFT Study On The Cu-based Catalysts For Methyl Acetate Hydrogenation To Ethanol

The process of ethanol production from hydrogenation of methyl acetate is very competitive and promising. Cu-based catalysts are the only commercial ones used in ethanol production from methyl acetate, but the main defect of the catalysts is that it is easy for the catalyst to be sintered at high temperature. How to improve the activity of catalysts and service life has become the core research object. Consequently, the Density Functional Theory(DFT) method was used to investigate the properties of Cu-based catalysts and the migration of Cu clusters on SiO2 and ZnO carriers, and calculated the adsorption and co-adsorption behavior of main species in the reaction. The influences of size and structure effects on the rate-limiting step of the hydrogenation of methyl acetate were investigated. All the researches are aimed at putting forward theoretical guidance for how to improve the performance of catalysts.Firstly, Cu clusters(n=3, 4, 5 and 13) were built and their structures and properties were studied systematically. The stable configurations of Cu clusters(n=3, 4 and 5) were close to the regular triangle, diamond, and isosceles trapezoid(all have a two-dimensional structure). The most stable configuration of Cu13 cluster is bilayer structure. It is found that the average bond length and the bond energy increase with the increase of the cluster size, resulting in a more stable structure. Comparison of the migration and agglomeration of Cu clusters(n=3, 4, 5 and 13) loaded on SiO2 and ZnO carriers shows that with the increase of Cu cluster size, ZnO supported Cu clusters exhibit better catalytic performance than SiO2 supported Cu clusters, and the interaction between Cu-Cu in Cu cluster itself strongly competes with the interaction between Cu cluster and carrier.Based on the previous results, Cu4/ZnO catalyst model was selected as the research object and the adsorption and co-adsorption behaviors of main species involved in methyl acetate hydrogenation were calculated. Analysis of the adsorption properties showed that methyl acetate, methoxyl and acetyl species are chemically adsorbed on Cu4/ZnO catalyst, while hydrogen is physically adsorbed. When methyl acetate and hydrogen are co-adsorbed on Cu4/ZnO catalyst, the interaction between methyl acetate and hydrogen is conductive to the further hydrogenation reaction. When methoxyl and acetyl are co-adsorbed on Cu4/ZnO catalyst, both of them tend to migrate and interact with Cu4/ZnO, respectively.The hydrogenation of acetyl group(the rate-determining step of methyl acetate hydrogenation) on Cu13 cluster, Cu(111) and Cu(211) surface was further studied. The results show that the reaction is exothermic on Cu13 cluster and Cu(111) surface, whereas it turns out to be endothermic on the Cu(211) surface. The order of activation energy is Cu(111)>Cu13>Cu(211). It means that the reaction is more likely to occur on the defective sites(steps, kinks, or vacancies) under real conditions. It also highlights the importance of the size and structure of catalysts in heterogeneous catalysis. All the findings in this work would provide valuable guidance for catalyst design and optimization in industrial practice.