【作者】 汪洋；

【导师】 陈丰秋； 詹晓力；

【作者基本信息】 浙江大学 ， 化学工程与技术， 2007， 博士

【摘要】 本文以原位DRIFTS、分子探针以及动态RWMC（Random Walk Monte Carlo）模拟为主要研究手段，对钼铋催化剂上丙烯（氨）氧化反应过程进行了详细的研究，尤其是对反应机理的研究，包括完整的丙烯氧化反应网络、丙烯腈生成路线、丙烯醛转化为丙烯腈历程、乙醛转化为乙腈等，同时利用Monte Carlo方法对钼铋催化剂表面上的丙烯氨氧化动态反应过程进行了模拟，基于以上研究结果提出了丙烯氨氧化催化剂的概念设计，获得如下的研究成果：（1）成功构建了多相催化反应体系的原位漫反射红外光谱实验平台，并实现了丙烯氧化反应体系和丙烯氨氧化反应体系催化剂表面反应机理的在线原位研究。丙烯氧化反应体系的原位漫反射红外光谱研究结果表明，由丙烯脱氢生成的烯丙基中间物种吸收峰在1454cm^-1和1427cm^-1处；基于此提出了较完整的丙烯氧化反应网络：丙烯选择氧化生成丙烯醛主要经历丙烯脱除甲基上的一个H，生成烯丙基物种，该物种嵌氧生成σ-O络合物，脱氢得到丙烯醛前驱体，脱附生成丙烯醛；完全氧化产物（CO_x）是由催化剂表面上生成甲酸盐、羧酸盐、碳酸盐等形式的中间物种转化而成；较低反应温度下丙烯氧化生成丙酮，通过烯醇式中间物种氧化生成丙酮前驱体得到。（2）丙烯氨氧化反应体系的原位漫反射红外光谱研究结果表明，氨在钼铋催化剂表面上主要以四种形式存在，分别为（ⅰ）1558cm^-1处的氨解离形成的NH₂中间物种；（ⅱ）1651cm^-1和1435cm^-1处的Bronsted酸性位上形成的NH₄⁺中间物种；（ⅲ）1246cm^-1处的Lewis酸性位上形成的配位氨中间物种；（ⅳ）1029cm^-1处的Mo=NH物种，尤其是1029cm^-1处的Mo=NH物种发现为该基团的存在提供了IR依据。丙烯腈生成路线为：丙烯在钼亚氨中间物种（Mo=NH）的作用下按照Grasselli模型直接生成丙烯腈；丙烯选择氧化的中间产物丙烯醛在表面配位氨中间物种的作用下通过加成反应生成丙烯腈。乙腈可能的生成路线为：丙烯通过降解反应生成甲醛和乙醛，后者与催化剂表面上形成的Mo配位氨物种通过加成反应，脱水氧化并最终生成乙腈。（3）确定了丙烯氨氧化催化剂表面上关键的活性相，并引入二进制数技术构建了丙烯氨氧化催化剂表面物理模型；在本文所完善的丙烯氨氧化反应机理基础上提出丙烯氨氧化反应机理模型；进而利用Matlab成功构建了模拟丙烯氧化与氨氧化的动态表面反应过程的RWMC模拟模型。RWMC模拟结果表明：适宜的晶格氧传递速率对反应过程最为有利，催化剂达到一定的还原度后丙烯转化率达到最高；催化剂表面上向Mo空位和O_A空位上传递晶格氧速率有所不同，O_A空位上的补氧速度快于Mo空位上的补氧速度；O₂离解化学吸附概率P_disO2对丙烯腈生成贡献最大。依据模拟结果提出了丙烯氨氧化催化剂进一步改进的方向：改善活性中心的反应性能，提高丙烯分子在Mo=NH上与Mo=O上脱α-H速率之比，优化催化剂体相结构，实现更多更快地将气相氧转化为晶格氧并加快晶格氧向Mo、O_A空位传递速率。（4）在活性位分离和相间协同作用设计理念指导下，结合全文的实验研究结果和模拟结果提出了丙烯氨氧化催化剂的概念设计：（ⅰ）使O₂→(O^2-)_L过程顺利进行以解决催化剂表面上Mo=O物种的来源；（ⅱ）使NH₃→NH^2-过程顺利进行，在催化剂表面上生成足够的Mo=NH物种；（ⅲ）催化剂表面上Mo=O物种与Mo=NH物种的数量与配比必须适宜，以使在催化剂表面上形成丰富的O=Mo=NH物种。由此提出了丙烯氨氧化催化剂优化设计的实现路径：（ⅰ）利用活性位分离设计理念对丙烯氨氧化催化剂结构进行优化以达到提高丙烯腈收率的目的；（ⅱ）基于相间协同作用设计理念设计单晶相催化剂，即将具有丙烯脱氢、嵌氧（氮）等催化功能的活性位集成到单晶相催化剂中；（ⅲ）设计低温下激活氨的催化剂，通过减少由裂解反应生成的副产物以及CO_x的生成，从而提高丙烯腈收率。最后对丙烯氨氧化反应器的优化设计及反应过程的优化调控提出了相应思路。更多还原

【Abstract】 The oxidation and ammoxidation of propylene over bismuth molybdate catalyst was investigated in detail using in situ DRIFTS, molecule probe and dynamic RWMC simulation. Attention was particularly focused on the mechanism of propylene oxidation and ammoxidation, including the complete reaction networks of propylene oxidation, the route of acrylonitrile formation, the formation route of acrylonitrile from acrolein, and the transformation to acetonitrile from acetaldehyde. Conceptual design about propylene ammoxidation catalysts was proposed associated with dynamic simulation of propylene ammoxidation over bismuth molybdate catalyst. Such results were obtained as follows:The in situ DRIFTS equipment was successfully constructed with application to heterogeneous catalytic system, and in situ study on the surface reaction mechanism of propylene oxidation and ammoxidation was carried out. The results of in situ DRIFTS on propylene oxidation show that the allyl species originated from α-H abstraction of propylene were located at the wavenumbers of 1454 cm^-1 and 1427 cm^-1. The rate-determining step is the abstraction of an α-H abstraction from a lattice oxygen linked to a bismuth ion to form a π-allyl intermediate coordinated to a molybdenum ion. The metal-oxo group then attacks the allyl intermediate forming a σ-bonded oxygen-allyl species, which is in a rapid equilibrium with the π- bonded species. The σ-bonded species then transforms to acrolein by further abstraction of α-H. Such species as formate, carboxylate, carbonate transform to deep oxidation products （CO_x）. The acetone was derived from propylene by an enolic species at lower temperatures.Based on in situ DRIFTS of propylene ammoxidation, four kinds of ammonia species on catalyst surface include: （i） ammonia dissociate adsorption with the formation of surface NH₂ species (1558 cm^-1) can take place, （ii） complete proton transfer can occur with the formation of an NH₄⁺ ion (1651 cm^-1 and 1435 cm^-1) on Bronsted acid sites, （iii） ammonia coordination (1246 cm^-1) on Lewis acid sites,which plays a crucial role in the formation of acrylonitrile from acrolein with partial oxidation of propylene, （iv） molybdenum-imido species （Mo=NH） (1029 cm^-1) are possibly proposed as the intermediates during the formation of acrylonitrile from the direct ammoxidation of propylene, which provides IR evidences of the presence of Mo=NH. The propylene molecule transforms to formaldehyde and acetaldehyde by degradation, the latter occurs to addition reaction with coordinated ammonia species, and forms acetonitrile by dehydration and oxidation.It is determined that the key active phase exists on the surface of propylene ammoxidation catalyst. The physical model of catalyst surface is built by means of binary digit technique, and the mechanism model of propylene ammoxidation is built. Thereby, the RWMC model applied to simulate the dynamic process of propylene oxidation and ammoxidation is constructed using Matlab. The simulation results show that the proper transfer rate of lattice oxygen is favorable for the reaction process, and the propylene conversion reaches the maximum after the reduction of catalyst to a certain degree. The transfer rate of lattice oxygen to the Mo vacancy differs from that to the O_A vacancy, and the latter is more rapid than the former. The probability of oxygen dissociative chemisorption contributes most favorably to the formation of acrylonitrile. Therefore, the improved way of propylene ammoxidation catalyst is proposed above the simulation results. It is inferred that catalytic performance improves greatly if the ratio of capacity for dehydrogenation from adsorbed propylene molecule on catalytically active site of molybdenum metal-imido group （Mo=NH） to that on catalytically active site of molybdenum metal-oxo group （Mo=O） becomes much higher. Another way is to optimize the bulk structure of catalyst to achieve the target of more lattice oxygen originated rapidly from gas oxygen and to speed up the transfer of lattice oxygen to the vacancies of Mo and O_A site.The conceptual design of propylene ammoxidation catalyst is proposed associated with the experimental and simulated results of this paper under the guide of site isolation and phase cooperation. Firstly, plenty of molybdenum-oxospecies on catalyst surface is obtained to make gas oxygen transform favorably into lattice oxygen. Secondly, enough molybdenum-imido species is derived from the lattice imido species transformed favorably from ammonia. Finally, it is fitting for the number and proportion of molybdenum-oxo species and molybdenum-imido species to form abundant oxo- molybdenum-imido species on catalyst surface. The actualized means of optimal design about propylene ammoxidation catalyst are suggested on the basis of this conceptual design. The yield of acrylonitrile is improved to optimize catalyst structure using site isolation. Single phase is designed based on phase cooperation, i.e., to integrate such multi-functionalities as hydrogen abstraction, oxygen-insertion （or nitrogen-insertion） into single phase. The acrylonitrile yield is enhanced to reduce the deep oxidation products and by-products originated from degradation reaction by designing catalyst for ammonia activated at low temperature. Finally, corresponding ideas of optimal design on propylene ammoxidation reactor and optimal controlling to reaction process are presented.更多还原