沸石催化噻吩脱硫的理论模拟

发布时间：2018-01-05 19:10

本文关键词：沸石催化噻吩脱硫的理论模拟　出处：《上海应用技术大学》2016年硕士论文　论文类型：学位论文

【摘要】：燃料油中的硫及其化合物经过燃烧能够生成硫氧化物(SOx)等对环境和人类有害的物质。随着环境污染日益严重,环境问题越来越受到人们的重视,因而要求燃料油中的硫含量越来越低。减少噻吩类化合物的含量是目前对燃料油进行深度脱硫,降低其中硫含量的关键。传统的催化加氢脱硫方法难以去除油品中的噻吩类硫化物,而且所需反应条件高,生产成本高。酸性沸石分子筛可单独催化噻吩裂解脱硫,所需反应条件也并不苛刻,具有广阔的应用前景。本文用两层ONIOM方法,从理论的层面上研究了H-beta, H-MCM22, H-ZSM5, H-FAU以及金属离子(Li+,Na+,K+,Cu+和Ag+)交换beta沸石催化噻吩进行裂解脱硫的反应。反应机理分为单分子噻吩裂解机理和双分子噻吩裂解机理,计算得到了反应中各个中间体和过渡态的结构与能量。结果表明,在单分子机理中,反应决速步是噻吩C-S键的断裂；而在双分子机理中,决速步是噻吩的二聚化。在这四种纯酸性沸石上,噻吩按照单分子机理进行裂解脱硫生成各过渡态所需的活化能都很高,在实验条件下难以进行；而按照双分子机理,生成各过渡态消耗的活化能都比较合理,噻吩应按照双分子机理在这四种酸性沸石上进行裂解脱硫。在金属离子交换beta沸石上,噻吩按照两种反应机理生成各过渡态所需的能量都很高,实验条件下都难以进行,因此金属离子交换beta沸石不适用于催化噻吩裂解脱硫。ONIOM总能量分为QM和MM两部分能量,将这两部分能量分开并加以分析能清楚地阐明沸石骨架如何影响了反应路径,特别是过渡态的结构和活化能。当MM能量为负值时,表示沸石骨架有利于过渡态形成,可以降低过渡态活化能,反之不利于过渡态形成。通过对四种典型沸石(beta,MCM22,ZSM5,FAU)的分析,我们发现,四种沸石骨架虽然不同,但都更支持双分子噻吩裂解机理,这可能是由于双分子中间体体积更大,与沸石之间的相互作用更显著所致。
[Abstract]:Sulfur and its compounds in fuel oil are harmful to the environment and human beings through combustion. With the increasing pollution of the environment, people pay more and more attention to the environmental problems. Therefore, the sulfur content in fuel oil is becoming lower and lower. Reducing the content of thiophene compounds is the current deep desulfurization of fuel oil. The traditional catalytic hydrodesulfurization method is difficult to remove thiophene sulfides from oil products and the reaction conditions are high. The production cost is high. Acid zeolite molecular sieve can catalyze thiophene pyrolysis desulfurization separately, and the reaction conditions are not harsh, so it has broad application prospect. In this paper, two-layer ONIOM method is used. H-beta, H-MCM22, H-ZSM5, H-FAU and the metal ion Li Li Na K have been studied theoretically. Cu and Ag) exchange beta zeolites to catalyze thiophene pyrolysis and desulphurization. The reaction mechanism can be divided into single molecule thiophene cracking mechanism and bimolecular thiophene cracking mechanism. The structure and energy of the intermediate and transition states in the reaction are calculated. The results show that the breaking of the C-S bond of thiophene is the key to the reaction in the single molecule mechanism. In the bimolecular mechanism, the fast step is the dimerization of thiophene. In these four pure acidic zeolites, the activation energy of thiophene for pyrolysis and desulfurization of thiophene is very high according to the single molecule mechanism. It is difficult to carry out under the experimental condition; According to the bimolecular mechanism, the activation energy consumed in the formation of each transition state is reasonable. Thiophene should be pyrolyzed and desulfurized on these four acidic zeolites according to the bimolecular mechanism, and on metal ion exchange beta zeolites. The energy required for the formation of each transition state of thiophene is very high according to the two reaction mechanisms, and it is difficult to carry out under the experimental conditions. Therefore, metal ion exchange beta zeolites are not suitable for catalytic thiophene pyrolysis desulfurization. The total energy of ONIOM is divided into two parts: QM and MM. The separation and analysis of these two energies can clearly explain how the zeolite skeleton affects the reaction path, especially the structure and activation energy of the transition state, when the MM energy is negative. The results show that the zeolite skeleton is favorable to the formation of transition state and can decrease the activation energy of transition state, otherwise it is not conducive to the formation of transition state. FAU), we found that although the four zeolites have different skeletons, they all support the decomposition mechanism of thiophene, which may be due to the larger volume of bimolecular intermediates. The interaction with zeolite is more obvious.
【学位授予单位】：上海应用技术大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TE624.55

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