甲醇定向转化制烃催化作用机制的研究

Conversion of methanol to liquid fuels and/or chemical is an industrially important and a high value-added process. However, this process is very complex, and involves numerous reactions, resulting in a very wide product distribution. It is a great challenge to selectively produce single chemical or significantly narrow product distribution. To realize this goal, the key point is to develop highly selective and stable catalysts. Thus, it is very interesting and necessary to study catalytic mechanisms and deactivation behaviors of molecular-sieve catalysts since it is directly beneficial to the design and preparation of highly efficient catalysts. In this project, attempts will be made to investigate the effects of pore structures, crystal sizes and acidities of molecular-sieve catalysts on the formation, compositions and changes of "hydrocarbon pool" and its interaction mechanism with reactants, intermediates and products by using a series of spectroscopic techniques and GC-MS to in situ trace the reaction process in combination with a variety of model reactions, isotopic experiments and theoretic calculations. The purpose of this project is to disclose the intrinsic factors determining product selectivity and catalytic stability and illustrate the formation mechanism of first C-C bond, growth mechanism of carbon chain and coking mechanism on the external surface and in the pores of catalysts. In order to well understand the deactivation behavior of catalysts, the effects of crystalline structures and framework architectures and compositions of molecular sieves on their phase-transformation and hydrolysis behaviors, and the changes of fine structures, surface areas and acidities of catalysts during the reaction process will be also studied.

甲醇转化制液体燃料和化学品具有重要的工业应用价值。然而，甲醇转化过程极其复杂，产物分布很宽，实现甲醇定向转化具有较大挑战性，关键是制备高选择性和高稳定性催化剂。要实现这一点，必须对甲醇转化催化反应机理和催化剂失活行为进行深入研究。为此，本项目拟采用多种光谱技术和GC-MS原位追踪反应过程，并辅以探针反应、同位素实验和理论计算，深入认识不同孔道结构、晶粒尺寸和酸性特征的分子筛催化剂中"烃池"物种形成、组成和结构演变规律及其与反应物和生成物的相互作用机制，揭示影响产物选择性的本质原因，初步阐明C-C形成机制、C链增长机制和积碳形成机理。通过研究分子筛晶体结构特征和组成对骨架结构相转变行为和水解行为的影响规律、反应过程中催化剂精细结构、比表面和酸性特征的变化规律，揭示催化剂失活行为。

本项目通过调变合成原料、引入不同杂原子以及进行适当的后处理方法等制备了一系列具有不同催化性能的分子筛，并结合多种原位光谱和理论计算揭示了分子筛孔道结构和酸性影响催化性能的本质原因。系统研究调变硅源对ZSM-5分子筛微观结构和催化性能的影响；结合27Al MAS NMR和Co-UV-Vis等手段证实调变硅源明显改变了铝分布位置；催化测试结果指出以硅溶胶为硅源可显著提高丙烯选择性并延长催化寿命。改变晶化条件可以制备具有不同酸量的H-MCM-22分子筛，结合UV-Vis-DRS和Raman光谱证实MCM-22外表面口袋易积炭。采用草酸脱铝则可以选择性去除外表面酸位点，从而显著提高分子筛催化稳定性。引入适当含量的硼(B)可获取具有不同酸位分布的B-Al-MCM-22，硼的引入可以集中更多酸位点于正弦孔道，并提高低碳烯烃选择性和催化寿命。对ITQ-13分子筛合成方法进行改良，通过引入一定量的锗(Ge)，可以大幅提高反应寿命和丙烯选择性，所制备的Al-Ge-ITQ-13非常适合于甲醇制丙烯过程。此外，结合13C同位素标记实验、原位DRIFTS、核磁共振光谱和DFT计算等方法深入研究了不同分子筛甲醇转化反应机理。同位素脉冲实验指出当酸位点主要占据ZSM-5的直孔道或正弦孔道，较窄的反应空间会明显提高烯烃循环的贡献，有助于促进丙烯等长链烯烃的生成；但是酸位点集中在交叉孔道则会明显加速芳烃循环，提高乙烯和芳烃的选择性。改变分子筛酸强度同样可以调节双循环的催化角色；降低酸强度会降低芳烃循环的贡献而提高烯烃循环的比例。结合DFT计算从微观原子尺度揭示了MCM-22分子筛三套孔道系统在MTO过程中不同的催化角色。这些研究成果加深了对甲醇转化反应的认识，有效推进甲醇定向转化的研究，为制备高活性分子筛催化剂提供了重要的理论支撑。

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