合成气直接合成对二甲苯（PX）的催化剂研究

Para-xylene (PX) is one of the most important raw materials in chemical industry. It provides a sustainable way to produce PX through the coal-, natural gas-, and biomass-derived syngas instead of petroleum resource. Being different from the PX synthesis via methanol intermediate from syngas, the conversion of syngas via the Fischer-Tropsch synthesis (FTS) process is not restricted by the thermodynamic equilibrium and can be easily regulated to reach a high level. In this study, it is proposed to investigate the direct and selective synthesis of PX from syngas by synergistic catalysis between iron-based FT and HZSM-5 catalysts. In spite of the merit of FTS route, two key problems still need to be solved. One is that the reaction environment of these two kinds of catalysts is different. The basicity environment is favorable for the iron-based FTS, however, the acidic HZSM-5 zeolite is beneficial for the aromatization of FT products. Another is that syngas conversion on the combined FT and zeolite catalysts becomes very complicated due to many reaction pathways to be involved in this process, which leads to the difficult controlling in the product selectivity. To solve these problems, the research contents include three aspects: 1) to investigate how the catalytic performance is related to the composition and structure of FT and zeolite catalysts, synergy of two kinds of active sites, operating conditions, etc., 2) to establish the reaction network and get insights into how to control the reaction pathways, which can be achieved by simulating the aromatization of FT products with model reactant molecules, DFT calculations, various catalyst characterization techniques, etc., 3) based on the intensive studies of 1) and 2), it is expected to develop a high-performance technology for direct and selective synthesis of PX from syngas by guiding its conversion with the controllable multi-functional synergy catalysis which could be achieved by well-designed catalyst through introducing inert nano-layer between FT and zeolite catalyst, creating the suitable pore structure and acidic environment of zeolite, modifying the exterior acidic sites of zeolite, etc.

对二甲苯（PX）是化学工业最重要的基本原料之一，以替代石油的合成气直接制PX是保障其可持续供给的重要途径。不同于合成气经甲醇路线，合成气经费托路线制PX的转化率不受热力学平衡限制。本项目研究通过铁基费托与分子筛（HZSM-5）耦合协同催化合成气直接制芳烃。针对耦合的两类催化活性中心反应环境（酸碱性）不同，及合成气在耦合催化剂上转化路径复杂且难于控制的关键问题，本项目拟通过探索具有特定组成、结构的铁基费托和分子筛催化剂、活性中心耦合方式和反应条件等对反应性能的影响；利用模型反应分子模拟费托产物在分子筛上的芳构化反应、结合DFT计算和多种催化剂表征手段等建立合成气直接制芳烃的反应网络及其控制途径；通过上述研究，设计合成费托催化剂与分子筛之间具有惰性过渡层、分子筛具有合适孔道结构和酸性环境，及分子筛外表面酸位修饰等结构特点的可控多级功能耦合的诱导合成气定向转化直接制PX的高性能催化剂。

芳烃，特别是对二甲苯是石油化学工业最重要的基础化工原料，发展以煤炭（生物质）气化的合成气或温室气体CO2/H2为原料一步制芳烃技术具有重要意义。本项目主要研究内容包括：（1）通过铁基费托催化剂的设计调控费托反应产物分布，考察其在HZSM-5上生成芳烃选择性和稳定性的影响。（2）通过采用模型反应物分子（低碳烷烃、乙烯、丙烯和1-己烯等）在各种分子筛（不同酸性、不同孔道尺寸、不同孔道结构和非择形位点靶向去除等）上的反应行为、模拟费托反应环境（H2O/CO2）等手段对影响芳烃生成的关键中间反应过程（如裂解、齐聚、异构化、氢转移和芳构化等）的关键影响因素进行了深入研究。（3）通过费托催化剂与HZSM-5分子筛的装填比例、活性中心的空间距离，以及分子筛惰性包覆层修饰等手段深入研究了两类不同活性中心的相互影响作用机制，及其对反应活性、产品选择性和催化稳定性的影响。上述研究结果表明：铁基费托催化剂耦合HZSM-5分子筛催化合成气或H2/CO2反应一步制取芳烃，应避免两类不同活性中心（碳化铁需要碱性反应环境，分子筛酸性位点）的相互影响；费托产物分布、两类活性中心的距离变化导致的中间产物扩散到分子筛上引起的局部浓度的差异、芳烃产物在HZSM-5外表面酸性位点上的异构化，以及低碳烯烃在外表面酸性位点上发生异构和加氢反应等是影响芳烃分布和目标产物选择性的关键因素。基于系统的实验、表征和理论计算对反应机理的研究，我们采用Na/Fe费托催化剂与HZSM-5@SiO2催化剂耦合，以H2/CO2为原料，获得液相产物中高达99%的芳烃（主要是容易分离的高价值甲苯、对二甲苯和对甲基乙基苯），气相产物主要为高价值的低碳烯烃，且具有很好的催化稳定性。该性能优异的CO2加氢一步制对二甲苯催化剂具有良好的应用前景，对于助力推进碳达峰和碳减排具有重要的现实意义。

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