多级孔ZSM-5催化甲醇芳构化：孔结构与外表面酸性质调控及其构效关系

ZSM-5 zeolite has good catalytic activity in the methanol to aromatics (MTA) reaction. However, the relatively small and sole micropores in ZSM-5 zeolites significantly influence the mass transfer of the reactants and products to/from the active sites, which would result in relatively easy coke formation and deactivation of catalysts. The hierarchical ZSM-5 zeolite can possibly combine the advantages of each individual pore size regime. Therefore, the hierarchical ZSM-5 exhibited long catalytic stability during the reaction of MTA due to its improved massmass transfer properties and tolerance for a larger amount of coke. At the same time, the pore structure and the external surface properties of hierarchical ZSM-5 zeolite had great influence on the catalytic performance. In this project, hierarchical ZSM-5 zeolite will be synthesized in the presence of alkyltriethoxysilane (ATES) and tetraethoxysilane (TEOS) as the silica source. The tunable pore structure will be produced under the mesopore-template function of the alkyl group in ATES. The variable external surface acidic properties will be obtained via post-synthtic dealuminization by organic acid. The inherent relationship between the catalyst’s textural parameters and the product selectivity, catalytic stability will be revealed. Combining the physico-chemical properties, catalytic data and coke analysis, we will accurately understand the influence mechanism of pore structure and the external surface properties on the catalytic stability, product selectivity and carbon deposition. The achievement of this project will provide theoretical basis and practical guide on the preparation of hierarchical ZSM-5 zeolite with high catalytic performance for the MTA reaction.

ZSM-5沸石具有良好的甲醇芳构化反应活性，然而其较小的微孔孔道不利于大分子反应产物的扩散与传输，致使催化剂易积炭失活、稳定性较差。多级孔ZSM-5沸石由于介孔结构的引入，具有良好的传质扩散能力，能有效减缓积炭从而提高催化稳定性，同时，其孔道结构和外表面酸性质对反应性能也有很大影响。本项目以烷基三乙氧基硅烷（ATES）和正硅酸乙酯为混合硅源，制备多级孔ZSM-5沸石，利用ATES中硅烷基（Si-R）的“造介孔”作用，对其孔道结构（介孔孔径、微孔/介孔匹配）进行有效构筑，采用有机酸脱铝改性，实现对外表面酸性质（酸密度、酸种类、酸点分布）的调节。探讨催化剂孔道结构、外表面酸性质与甲醇芳构化产物选择性、催化稳定性间的内在规律。结合催化剂织构信息、反应评价结果以及积炭分析，揭示孔道结构、外表面酸性质对催化稳定性、产物分布以及积炭行为的影响机制，为获得高性能的甲醇芳构化催化剂提供理论和技术依据。

甲醇在催化剂上的转化是一个非常复杂的过程，甲醇生成芳烃过程伴随大量副反应，导致产物中芳烃的含量极低，且催化剂的使用寿命短。因此一直无法实现工业化。本论文采用三种不同方法合成多级孔ZSM-5分子筛，研究了实验条件对分子筛结构和酸性的影响，并考察了催化剂结构、酸性与芳构化性能之间的关系。本论文研究内容如下：.（1）通过酸改性法对ZSM-5分子筛进行脱铝处理，并对其进行理化性质表征和性能评价。结果表明，酸改性在一定程度上调整了ZSM-5的孔结构，但显著减少了分子筛的酸量。在甲醇芳构化反应获得的产物中，芳烃的含量显著提高。.（2）通过碱改性法对ZSM-5分子筛进行脱硅处理，并对其进行理化性质表征和性能评价。结果表明，碱改性使分子筛形成介孔结构，显著延长了催化剂的使用寿命。同时，使L酸量和B酸量明显降低，可得到合适B/L值的ZSM-5分子筛。经0.5mol/L乙酸钠溶液改性的分子筛展现出最高的催化性能。与微孔ZSM-5相比，SA-ZSM-5-0.5的使用寿命由20 h提高到74h。.（3）通过添加MTES合成出具有微-介孔复合结构的ZSM-5分子筛，并对其进行理化性质表征和性能评价。结果表明，在分子筛结构中形成良好连通的介孔结构，缩短了产品在ZSM-5中的停留时间，对催化剂的寿命具有积极影响。同时，通过提高预晶化温度不仅使分子筛的总酸量降低，而且调节了B酸和L酸的比例。ZSM-5-M-90展现出极好的催化性能，与微孔ZSM-5相比，此分子筛的使用寿命由38h提高到72h。