【作者】 武新颖；

【导师】 陈晓春；

【作者基本信息】 北京化工大学 ， 化学工艺， 2009， 博士

【摘要】 丙烯腈是三大合成材料——合成纤维、合成橡胶、合成塑料的基础原料。丙烯氨氧化法合成丙烯腈是一个典型的烃类选择氧化过程,产品丙烯腈是连串反应的中间产物。该反应遵循“氧化-还原”机理。这种选择氧化反应是以复合金属氧化物为催化剂、催化剂中的晶格氧参与氧化还原过程的多相催化反应。晶格氧参与主反应生成目的产物,气相或吸附氧导致副反应生成有机副产物和COx。近年来,烃类晶格氧选择氧化新工艺备受关注,其原理实质上是人为地使催化剂的还原和氧化再生过程在两个分开的反应器中进行,以达到控制反应进程、提高选择性、节约资源和保护环境的目的。对于晶格氧氨氧化合成丙烯腈新工艺,前人已经做了一些研究。该工艺在还原阶段,催化剂中的晶格氧作为氧化剂选择性氧化丙烯和氨生成丙烯腈,此时催化剂被还原；在催化剂氧化再生阶段,使催化剂和热空气接触,发生氧化反应,补充前一阶段损失的晶格氧,从而构成完整的催化循环过程。因此,分别考察催化剂的还原和氧化再生阶段的反应特性,了解每个阶段的反应机理及其动力学特征,对于改进晶格氧氨氧化合成丙烯腈新工艺具有重要意义。本文针对目前国内工业装置中广泛使用的一种钼铋系催化剂MB98,利用X-射线衍射、激光拉曼光谱、光电子能谱和热分析等手段,研究了它的氧化／还原反应特性,计算了其动力学参数,推断了该催化剂的氧化／还原反应机理。首先,设计了一套实验装置使催化剂的还原和氧化再生过程分开进行。在微型固定床反应器中,分别采用丙烯／氨气,氢气作为还原剂,还原新鲜催化剂至不同还原程度；再应用XRD、Laser Raman和XPS的方法表征新鲜的和不同还原程度的催化剂。结果表明：在无氧的条件下还原新鲜催化剂,还原过程分4步反应序贯进行。还原过程中,Fe2Mo3O12和(Fe／Co／Ni)MoO4中的晶格氧通过体相扩散向钼酸铋迁移；催化剂的储氧能力不仅与钼酸铋有关,还与Fe2Mo3O12和(Fe／Co／Ni)MoO4有关。还原生成的FeMoO4,MoO2和Bi主要分布于催化剂体相；还原过程中,元素Mo,Bi首先在催化剂表面富集,然后向体相迁移,Fe,Co,Ni由体相向表面扩散。被氢气还原的催化剂与被丙烯／氨气还原的催化剂具有相似的晶相组成和表面组成。其次,采用热重分析法研究该催化剂的还原本征动力学。应用Achar-Sharp微分和Coats-Redfern积分对照的方法,获得了该催化剂还原反应的动力学三因子：活化能(E)为142.0kJ／mol,指前因子(A)为9.84×109min-1,动力学模型函数的积分式为g(α)=(1-α)-1-1,表明催化剂还原反应遵循“化学反应级数控制(n=2)”的机理。再次,以还原态催化剂为研究对象,结合催化剂再氧化后XRD与XPS结果,采用热重分析法原位考查催化剂的氧化反应行为,结果表明：还原态催化剂补氧过程中,在280℃、340℃、440℃时分别先后补充与Bi、Fe、Mo结合的体相晶格氧,然后在510℃时补充表面晶格氧；沉积的炭在370℃左右开始剧烈燃烧,直到460℃结束；最佳的再氧化温度大约在440℃左右。最后,采用热重分析法研究了还原态催化剂的氧化反应本征动力学。应用热分析动力学方法中的等转化率和主曲线法,获得了还原态催化剂氧化反应的动力学三因子：结果表明：活化能(E)为140.01kJ／mol,指前因子(1nA)为21.527min-1,氧化动力学模型函数积分式为g(α)=[-ln(1-α)]1／0.922,表明还原态催化剂氧化反应遵循成核生长的反应机理。更多还原

【Abstract】 Acrylonitrile (AN) is a versatile petrochemical intermediate used extensively in the production of acrylic fibers, resins, rubbers and specialty products. Propylene ammoxidation into AN is a typical selective catalytic oxidation process of hydrocarbons, and the AN is the intermediate product of the consecutive reaction, which obeys "Redox" mechanism, i.e., the lattice oxygen in the muticomponent metal oxide catalyst reacts with propylene/ammonia to generate AN, and the reduced catalyst is continuously reoxidized by gaseous oxygen, while the gaseous or adsorbed oxygen lead into COx and the other organic byproducts.In recent years, selective oxidation of hydrocarbons using lattice oxygen of catalyst has been attempted, the theory of which is that reduction and reoxidation of the catalyst are performed in the two reactors, respectively. There are many advantages in the new technology, such as controlling the reaction degree, increasing the selectivity of reaction, saving resource and protecting environment. The new technology of propylene ammoxidation using lattice oxygen of catalyst has been studied, where lattice oxygen of catalyst and propylene/ammonia react in a reactor, and catalyst reoxidation is performed in another separate regenerator, the whole catalytic process is completed. Therefore, the study on the performance of the reduction and reoxidation process respectively is very important for improving the new technology of propylene ammoxidation using lattice oxygen of catalyst.In this work, the kinetics and mechanism of reoxidation/reduction of MB-98 catalyst, which is used widely in industry in China, was studied by XRD, Laser Raman, XPS and thermal analysis methods.Firstly, an experimental setup was built for separating reduction and reoxidation process of the catalyst. The fresh catalyst was reduced to different degrees in a fixed bed reactor by propylene/ammonia or hydrogen; and then the fresh and reduced catalysts were characterizated by using X-ray powder diffraction, Laser Raman spectroscopy and X-ray photoelectron spectroscopy. The results showed that there are four reactions happened sequentially during the reduction processes and the lattice oxygen migrates from iron/cobalt/nickel molybdates to bismuth molybdate through bulk diffusion. The oxygen storage capacity of catalyst is not only related to the content of bismuth molybdate, but also to the content of Fe2Mo3O12 and (Fe/Co/Ni)MoO4. FeMoO4. MoO2 and Bi are mainly existed in the bulk phase in the catalyst. Mo and Bi are concentrated on the surface and then migrate from the surface to the bulk phase. Fe, Co and Ni diffuse from the bulk phase to the surface during the ammoxidation reaction. The catalyst partially reduced by hydrogen has the similar lattice and surface structure as the catalyst partially reduced by propylene/ammonia.Secondly, intrinsic kinetics of reduction of the catalyst was studied by thermogravimetric analysis method. The kinetic triplet of this reaction is obtained by comparison method of Achar-Sharp differential and Coats-Redfern integral equation. The activation energy and pre-exponential factor (A) are 142.0kJ/mol and 9.84×109min-1, respectively. The kinetic model is best expressed by Avrami-Erofeyev equation, i.e., g(a)=(1-α)-1-1, suggesting Chemical Reaction mechanism (n=2).Thirdly, along with XRD and XPS used to detect the structure of the catalyst after reoxidation, the results of the thermogravimetric analysis showed that the reoxidation of the catalyst is attributed to the replenishment of the bulk lattice oxygen in the lower temperature which is respectively combined with Bi (280℃), Fe (340℃) and Mo (440℃), and the replenishment of the surface lattice oxygen in the higher temperature (510℃). Combustion of the deposited carbon started tempestuously around 370℃and finished before 460℃. The favorable reoxidation temperature is around 440℃. Finally, intrinsic kinetics of reoxidation of the catalyst was obtained by Kissinger-Akahira-Sunose isoconversional and master-plot method. The activation energy and pre-exponential factor (1nA) are 140.01kJ/mol and 21.527 min-1, respectively. The kinetic model is best expressed by Avrami-Erofeyev equation, i.e., g(α)=[-ln(1-α)]1/(0.922), suggesting a nucleation and growth mechanism.更多还原