双功能催化CO2加氢直接高选择性合成对二甲苯的研究

Direct production of para-xylene (PX) from CO2 hydrogenation offers an alternative way to solve fossil fuels and environmental issues due to its energy- and cost-efficient. However, it remains a great challenge to overcome the extreme inertness of CO2 to synthesis of PX directly from CO2. How to maximize the catalytic activity, product selectivity and catalyst lifetime in CO2 conversion over bifunctional catalysts is the main objective in this project. Based on the new strategy of reaction coupling, indium-based oxide/zeolite bifunctional catalysts composed by two types of active sites for CO2 activation and C−C coupling will be used, in which the hydrocarbon distribution can be easily turned through the shape-selective zeolites. In order to design and fabricate highly efficient bifunctional catalysts for conversion CO2 to PX, the precise control of the surface structure and the property of oxygen vacancies of metal oxides for effective activation of C-O will be carried out. Other metals such Zr and Mn will be used to stabilize those active sites responsible for the formation of intermediates. In addition, the introduction of fractal mesopores into microporous ZSM-5 and the passivation of external surface of hierarchical ZSM-5 will be executed to improve the catalyst lifetime and PX selectivity. The integration manner of the active components will also be investigated to improve the synergistic effect of bifunctional catalysts. The structure-performance relationship, the influence of the catalyst structure, the integration manner, the formation and transportation of reaction intermediates as well as rules for C−C coupling will be investigated in detail by various in-situ and ex-situ characterization methods. Moreover, the nature of active sites favors the formation of PX from CO2 hydrogenation will be clarified. This project will provide a new route and valuable information for CO2 hydrogenation to value-added PX.

将化学惰性的CO2直接转化为高附加值的对二甲苯（PX）仍是巨大的挑战。基于反应耦合的思想，本课题采用双功能催化策略，使C−O活化与C−C偶联分别在铟基氧化物与分子筛两种位点上进行，以最大限度地实现CO2的高效活化、提高PX的选择性及延长催化剂寿命。通过精准调控金属氧化物表面原子结构及氧缺陷位的性质和数量，来高效活化CO2；通过筛选、加入第二组分来稳定生成反应中间体的活性位点，提高中间体甲醇的稳定性。通过向微孔ZSM-5分子筛中引入“分形”的介孔并钝化其外表面来提高PX选择性和反应稳定性。最后，系统研究金属氧化物与改性多级孔分子筛的匹配组装和耦合方式，以强化双功能组分的协同效应。综合利用多种原位表征手段深化对构效关系和失活机理的认识，深入研究CO2活化机制、中间体形成、C–C偶联规律及协同效应机制，为构建高效CO2直接合成PX反应平台提供一条新的探索途径及理论指导。

目前，CO2加氢合成混合芳烃研究取得了一系列重要进展，但产物多为重质芳烃，同时由于分子筛的酸分布和扩散性能在PX合成中扮演的角色非常复杂，基于分子筛控制的CO2加氢精准合成高附加值的PX极具挑战性。传统的硅烷基化或Silicalite-1包覆通常会损失分子筛的反应稳定性和催化活性。本项目通过合成（i）纳米聚集的、沿b轴直孔道一维自组装、长度范围从0.16到1.41 μm的纤维状多级孔HZSM-5；（ii）具有不同壳核比的、联通良好且有效覆盖分子筛外表面酸位的核壳结构HZSM-5@S-1分子筛；（iii）由对应之字形孔道的（100）面孪晶最大程度地替代对应直孔道的（010）晶面的挛晶HZSM-5分子筛，并分别与筛选出的高效尖晶石氧化物（甲醇中间体路线）或CO2-FTS催化剂（烯烃中间体路线）组成双功能催化剂，在保持较高催化活性和稳定性的前提下，显著促进了PX的选择性合成。详细表征了催化剂的结构及酸性，并研究了这些性质与表界面的扩散行为的关系；研究指导了高选择性合成PX催化剂的开发，获得了多种高效反应的方法和途径，相关技术已进入100 mL小试阶段。