Study On The Cu-based Catalyst For Ethyl Acetate Synthesis From Ethanol Dehydrogenation

The ethyl acetate （EA） is an important downstream product of the acetic acid andalso an important green organic solvent. As a high grade solvent, the research on itsproduction process has received widely attention. Among all processes for thesynthesis of EA, the direct dehydrogenation of ethyl acetate from ethanol isconsidered as the technological route with fine application prospect. At present, theCu-Cr catalyst system shows very high EA selectivity, and has been already applied inthe industrial production successfully. The quantum chemical calculation couldprovide the information on the geometric configuration and electronic structure of theconcerned reactants, intermediates as well as products, and thus give us usefullinstructions to modify the performance of catalysts and to improve the productiontechnology.In this work, the pure Cu and Cu/Cr₂O₃catalysts were chosen as the researchsystems to reveal the possible reaction mechanism of the EA synthesis from ethanoldehydrogenation as well as the role played by Cu and Cr₂O₃components. The resultswould be helpful to understand the relationship between structure and catalystic effectfrom the microscopic point of view.First, the strcture of Cu-Cr catalyst system was analysied by characterization andexperimental study. It was found that the components Cu and Cr existed as Cu0andCr₂O₃, and then the active sites were Cu0based on the TPSR study. According to theexperimental results, the reasonable simplified catalyst model could be established,which could ensure the reliability of simulation.Based on the experimental results, the model of Cu as active sites wasconstructed, using DFT methods, the most stable configuration of all concernedspecies during ethanol dehydrogenation （including CH₃CH₂OH, CH₃CH₂O, CH₃CHO,CH₃CO and CH₃COOC2H₅） on the Cu （111） surface were obtained. Then, twopossible reaction mechanisms proposed by Colley and Kanichiro were referenced,respectively, and the transition states for the elementary reaction steps in this processwere searched. After the comparision between calculation results, it was consideredthat the possible reaction mechanism on Cu（111）surface might be the mechanismroute proposed by Colly et al. The energy barrier of C2H₅OH dehydrogenation toC2H₅OE1awas1.32eV, which should be highest of the whole reaction. Then, the models of Cr₂O₃and Cu/Cr₂O₃were built up, the adsorption anddehydrogenation of ethanol were studied. It was demonstrated that the active sitesprovided by the interface between Cu₄cluster and Cr₂O₃were most stable, when theethanol molecule was adsorbed on the Cu/Cr₂O₃. The adsorption energy of the moststable configuration was0.80eV, and the Cu-O bond was formed between ethanol andCu₄cluster. In this way, the ethanol molecule could be better chemisorbed on the Cu（111） surface, which would benefit the next ethanol dehydrogenation.The barriers of C2H5OH dissociation on the Cr₂O₃（001） surface, Cu （111） surfaceand Cu/Cr₂O₃system were0.42eV,1.32eV and0.59eV, respectively. It was found thatCr₂O₃showed also better dehydrogenation ability than pure Cu. The synergistic effectprovided by the interface between Cu₄cluster and Cr₂O₃would enhance the stabilityof the ethanol molecule adsorbed on Cu/Cr₂O₃system to a great extent. Theintroduction of Cr₂O₃could effectively reduce the energy barrier of C2H5OHdehydrogenation to C2H5O, consequently strengthen the role of Cu as active sites thusincrease the conversion of ethanol.The acidity and basicity of the Cu based catalyst were also studied by means ofexperimental determination and molecule simulation. Using in situ adsorbedPyridine-FTIR method, it was found that pure Cu surface showed Lewis acidic sites,but no Br nsted acidic sites. Cu-Cr system not only has Lewis acidic sites but also hasBr nsted acidic sites because of the introduction of Cr, while quantity of Br nstedacidity was small. The results of molecule simulation demonstrated that pure Cu andCr₂O₃surface have Lewis basic sites while attacked by molecules or groups who werepoored in electron; while the two surfaces have Lewis acidic sites while attacked bymolecules or groups who were riched in electron. There were many dissociated H inthe process of ethanol dehydrogenation, pure Cu and Cr₂O₃surface can accepted H, sothey have Br nsted basic sites; but Cu can not present H, so it doesn’t has Br nstedacidic sites, while Cr₂O₃surface can present H, so it has Br nsted acidic sites. Cr₂O₃can strengthen the Lewis basicity and Br nsted acidity of the Cu-Cr catalyst and makethe acidity and basicity of catslyst match better.The results in this work would provide some useful instructions to develop novelpractical catalysts and to improve the process technology of EA synthesis fromethanol dehydrogenation.