乙醇脱氢法制备乙酸乙酯铜基催化剂研究

发布时间：2018-06-18 00:59

本文选题：Cu（111） + Cu/Cr_2O_3催化剂　；参考：《天津大学》2011年博士论文

【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/Cr_2O_3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Cr_2O_3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Cr_2O_3，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CH_3CH_2OH、CH_3CH_2O、CH_3CHO、CH_3CO、CH_3COC2H_5）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Cr_2O_3和Cu/Cr_2O_3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/Cr_2O_3催化剂上时，Cu_4团簇与Cr_2O_3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Cu_4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Cr_2O_3(001)表面、Cu(111)表面和Cu/Cr_2O_3体系脱氢生成乙氧基的能垒分别为0.42eV、1.32eV和0.59eV，Cr_2O_3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/Cr_2O_3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Cr_2O_3(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Cr_2O_3(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Cr_2O_3(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Cr_2O_3(001)面可以给出质子，具有B酸中心。且Cr_2O_3表面的酸性强于Cu表面，Cr_2O_3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。
[Abstract]:Ethyl acetate ( EA ) is an important downstream product of acetic acid , and it is an important green organic solvent . As a kind of high - grade solvent , the research of its production method has been a hot spot , in which ethanol dehydrogenation is considered to be the most promising process route . The present Cu - Cr series catalyst has high selectivity to EA and has been successfully applied to industrial production . The quantum chemistry method can further study the structure and electronic properties of each species in the reaction elementary step , thus providing theoretical basis for catalyst modification and process improvement .

In this paper , Cu and Cu / Cr _ 2O _ 3 catalysts were used as the research system to study the reaction mechanism of ethanol dehydrogenation to prepare ethyl acetate . The effects of Cu and Cr _ 2O _ 3 catalyst components in catalytic reaction were analyzed from the micro - angle , and the structure - effect relationship of catalyst was revealed .

First , the structure of Cu - Cr series catalyst was analyzed by characterization and experimental study . The results show that the Cu and Cr elements in Cu - Cr system are Cu0 and Cr2O3 respectively , and the active center of the catalyst surface is Cu0 species . The above experimental results provide the foundation for setting up a reasonable catalyst model , and the reliability of the simulation study is ensured .

Based on the experimental results , this paper constructed a model of active center Cu . The most stable adsorption sites and configurations of various related species ( including CH _ 3CH _ 2OH , CH _ 3CH _ 2O , CH _ 3CHO , CH _ 3CO , CH _ 3CO2H _ 5 ) on Cu ( 111 ) surface were calculated by DFT method .

The adsorption and dehydrogenation process of ethanol on the Cu / Cr _ 2O _ 3 catalyst was investigated by means of establishing the model of Cr _ 2O _ 3 and Cu / Cr _ 2O _ 3 . The results showed that when the ethanol molecules were adsorbed on the Cu / Cr _ 2O _ 3 catalyst , the most stable activity of the Cu _ 4 cluster and the interface of Cr _ 2O _ 3 was 0.80eV .
the Cu - O bond is formed between the ethanol and the Cu _ 4 cluster , so that the ethanol molecule is better chemically adsorbed on the Cu , thereby providing favorable conditions for the next dehydrogenation ;
The surface of Cr _ 2O _ 3 ( 001 ) , Cu ( 111 ) surface and Cu / Cr _ 2O _ 3 system dehydrogenated to form an ethoxy energy barrier of 0.42 eV , 1.32 eV and 0.59 eV , respectively , and the Cr _ 2O _ 3 itself has a better dehydrogenation capability than pure copper .
The synergistic effect of copper and chromium sesquioxide greatly enhances the adsorption stability of ethanol molecules on Cu / Cr _ 2O _ 3 catalyst , effectively reduces the energy barrier of ethanol dehydrogenation and dissociation to ethoxy , strengthens the active center of Cu , and improves the conversion rate of ethanol .

In this paper , the acid - alkali of Cu - based catalyst was studied by using the method of experiment and simulation . Using in situ adsorption of pyridine - FT - IR , it was found that the pure Cu catalyst had an L - acid center and no B - acid center .
The results of molecular simulation show that pure copper surface and Cr _ 2O _ 3 ( 001 ) surface have nucleophilicity , i.e . L - base center , when electron - deficient molecules or groups are attacked .
When an electron - rich molecule or group is attacked , the pure copper surface and the Cr _ 2O _ 3 ( 001 ) surface have an electrophilicity , that is , an L - acid center . In the process of dehydrogenation of ethanol , many free H , Cu and Cr _ 2O _ 3 ( 001 ) planes can receive protons and thus have a B - base center ;
But Cu does not possess the ability to give proton , there is no B acid center , and the surface of Cr _ 2O _ 3 ( 001 ) can give proton , which has the center of B acid . The acidity of Cr _ 2O _ 3 surface is stronger than that of Cu surface . The addition of Cr _ 2O _ 3 increases the Lewis basicity and B acidity of the catalyst , which can better match the properties of catalyst acid and base .

The research results have important guiding significance for designing , developing and improving the new catalyst and process of ethanol dehydrogenation process .
【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2011
【分类号】：TQ225.241;TQ426

【参考文献】