ZSM-5分子筛作用下MTH反应体系中烯烃生成的密度泛函理论研究

发布时间：2018-10-09 14:19

【摘要】：甲醇制碳氢化合物(MTH)反应是煤及其它非石油含碳资源到液体燃料或轻烯烃等化工生产过程中的一个重要反应。该过程的反应物(甲醇)几乎可以从任何可气化的含碳资源中获得,其产物范围从轻烯烃到汽油。该技术可以把我国的能源供应劣势转化为优势,从而在很大程度上缓解我国的石油危机。因此,MTH技术受到研究者越来越多的关注。传统的ZSM-5分子筛被认为是MTH反应的有效催化剂,但也存在严重的积碳问题,制约着ZSM-5分子筛催化剂的高效利用。关于在MTH反应体系中,ZSM-5分子筛催化剂主要有两种积碳失活机理,即覆盖活性位和堵塞孔道。分析当前有关实验研究工作发现:对于MTH反应体系中催化剂的积碳原因,基本一致的说法是反应过程中产生的副产物,即多环芳烃体积过大而堵塞催化剂孔道,而多环芳烃的生成始于第一个C-C键,所以本课题拟采用密度泛函理论广义梯度近似方法(DFT-GGA),通过34T簇模型和Al/Ga-ZSM-5周期性模型的构建及优化,对MTH反应过程中第一个C-C键及初始烯烃生成进行了系统的理论研究。通过计算得出以下结论:1、在ZSM-5催化剂上,甲醇生成二甲醚主要有两种不同的反应机理。一是协同机理,即两个甲醇分子在Br?nsted酸催化下直接生成二甲醚;二是分步机理,即通过甲氧基中间体完成。计算结果表明在ZSM-5催化剂上,二甲醚生成主要是通过协同机理进行。2、在ZSM-5催化剂作用下的MTH体系中,计算结果表明第一个C-C键物种是1,2-二甲氧基乙烷。3、对于第一个C-C键的形成,通过比较活化能垒发现:在34T簇模型上得到的活化能垒要显著高于周期性模型上得到的活化能垒,表明周期性模型更能全面地反映ZSM-5分子筛的全笼性质。4、对于在周期性模型上的机理论证,得出路径5(包括反应ZOCH3+CH3CH2OCH3→ZO-+CH3O+(CH3)CH2CH3;ZO-+CH3O+(CH3)CH2CH3→ZOCH2CH3+DME;ZOCH2CH3→ZOH+CH2=CH2)是生成初始烯烃的最佳反应路径,生成的初始烯烃为乙烯。
[Abstract]:The (MTH) reaction from methanol to hydrocarbon is an important reaction in the chemical production process from coal and other non-petroleum carbon-containing resources to liquid fuels or light olefins. The reactant (methanol) of the process can be obtained from almost any gasifiable carbon-containing resource, and its products range from light olefins to gasoline. This technology can transform our country's energy supply inferiority into superiority, thus alleviate our country's oil crisis to a great extent. Therefore, more and more researchers pay attention to MTH technology. Traditional ZSM-5 molecular sieves are considered as effective catalysts for MTH reaction, but there are serious carbon deposition problems, which restrict the efficient utilization of ZSM-5 molecular sieve catalysts. There are two mechanisms of deactivation of ZSM-5 molecular sieve catalyst in MTH reaction system, that is, covering the active site and blocking the pore channel. By analyzing the current experimental work, it is found that the main reason for the carbon deposition of catalysts in the MTH reaction system is that the byproducts produced in the reaction process, that is, polycyclic aromatic hydrocarbons (PAHs) are too large in volume to block the pore channels of the catalysts. The generation of polycyclic aromatic hydrocarbons (PAHs) begins with the first C-C bond, so we use the generalized gradient approximation (DFT-GGA) method of density functional theory to construct and optimize the 34T cluster model and Al/Ga-ZSM-5 periodic model. The first C-C bond and initial olefin formation in MTH reaction were systematically studied. The following conclusions are obtained: 1. There are two different reaction mechanisms for the formation of dimethyl ether from methanol over ZSM-5 catalyst. One is the synergistic mechanism, that is, two methanol molecules in Br? Nsted acid catalyzes the formation of dimethyl ether directly, and the second is the stepwise mechanism, which is accomplished by methoxyl intermediate. The results show that the formation of dimethyl ether on ZSM-5 catalyst is mainly carried out by synergistic mechanism. In the system of MTH under the action of ZSM-5 catalyst, the first C-C bond species is 1 ~ (2) -dimethoxy ethane. 3. For the formation of the first C-C bond, By comparing the activation energy barrier, it is found that the activation energy barrier obtained on the 34T cluster model is significantly higher than the activation energy barrier obtained by the periodic model. It is shown that the periodic model can fully reflect the whole cage property of ZSM-5 molecular sieve. For the mechanism of periodic model, it is concluded that path 5 (including ZOCH3 CH3CH2OCH3 ZO- CH3O (CH3) CH2CH3;ZO- CH3O (CH3) CH2CH3 ZOCH2CH3 DME;ZOCH2CH3 ZOH CH2=CH2) is the best route for the formation of initial olefin. The initial olefin produced is ethylene.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ221.21;O641.1

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