【作者】 李如珍；

【导师】 张敏华；

【作者基本信息】 天津大学 ， 化学工艺， 2011， 博士

【摘要】 乙酸乙酯（EA）是醋酸的重要下游产品，也是一种重要的绿色有机溶剂。EA作为一种高档溶剂，其生产方法的研究一直是关注的热点，其中乙醇脱氢法被认为是最有应用前景的工艺路线。目前Cu-Cr系催化剂对EA的选择性较高，且已成功应用于工业生产。量子化学方法可以深入研究反应基元步骤中各物种的结构和电子性质等，从而为催化剂改性和工艺改进提供理论依据。本文以Cu和Cu/Cr₂O₃催化剂作为研究体系，试图深入研究乙醇脱氢制备乙酸乙酯的反应机理，从微观角度分析Cu和Cr₂O₃催化剂组分在催化反应中的作用，揭示催化剂构效关系。首先通过表征和实验研究分析了Cu-Cr系催化剂的结构。结果表明，Cu-Cr系催化剂中Cu和Cr元素的存在形式分别是Cu0和Cr₂O₃，催化剂表面的活性中心是Cu0物种。以上实验结果为搭建合理的催化剂模型奠定了基础，保证了模拟研究的可靠性。基于实验研究结果，本文构建了活性中心Cu的模型，用DFT方法计算了乙醇脱氢反应中各个相关物种（包括CH₃CH₂OH、CH₃CH₂O、CH₃CHO、CH₃CO、CH₃COC2H₅）在Cu（111）面上最稳定的吸附位置与构型。参照Colley机理和Kanichiro机理，对乙醇脱氢制备乙酸乙酯过程中每一步基元反应进行过渡态搜索，得到相关反应能垒数据。通过对比两个机理的计算结果，认为在Cu （111）面上乙醇更倾向于按照Colley反应机理生成乙酸乙酯，乙醇分子断开O-H键脱氢生成乙氧基的能垒最高，反应能垒为1.32eV。通过搭建Cr₂O₃和Cu/Cr₂O₃模型，研究了乙醇在其上的吸附和脱氢过程。研究结果表明，乙醇分子吸附在Cu/Cr₂O₃催化剂上时，Cu₄团簇与Cr₂O₃的交界处提供的活性位最稳定，其最稳定构型的吸附能为0.80eV；乙醇与Cu₄团簇之间形成Cu-O键，这样就使得乙醇分子更好地化学吸附在Cu上，为其下一步脱氢提供有利条件；乙醇在Cr₂O₃（001）表面、Cu（111）表面和Cu/Cr₂O₃体系脱氢生成乙氧基的能垒分别为0.42eV、1.32eV和0.59eV，Cr₂O₃本身也较纯铜有较好的脱氢能力；铜和三氧化二铬的协同作用在很大程度上增强了乙醇分子在Cu/Cr₂O₃催化剂上的吸附稳定性，有效地降低了乙醇脱氢解离为乙氧基这一步的能垒，强化了Cu的活性中心作用，提高了乙醇的转化率。结合实验和模拟方法，深入研究Cu基催化剂的酸碱性。利用原位吸附吡啶-FT-IR测定方法得出纯Cu催化剂表面具有L酸中心，没有B酸中心；Cu-Cr催化剂具有L酸中心，同时Cr的加入，引入了B酸位，但这些B酸中心的数量相比L酸来说较少。分子模拟结果表明当有缺电子的分子或基团攻击时，纯铜表面和Cr₂O₃（001）面具有亲核性，即有L碱中心；而当有富电子的分子或基团攻击时，纯铜表面和Cr₂O₃（001）面具有亲电性，即有L酸中心。在乙醇脱氢过程中会产生很多游离态的H，铜和Cr₂O₃（001）面可以接受质子，因而具有B碱中心；但Cu不具备给出质子的能力，没有B酸中心，而Cr₂O₃（001）面可以给出质子，具有B酸中心。且Cr₂O₃表面的酸性强于Cu表面，Cr₂O₃的加入增加了催化剂的Lewis碱性和B酸性，可使催化剂酸碱性质更好地匹配。本文的研究成果对设计、开发和改进乙醇脱氢法新型催化剂与工艺具有重要的指导意义。更多还原

【Abstract】 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.更多还原