低碳烷烃定向转化制芳烃催化剂优化设计及反应机理研究

Light alkanes can be converted to aromatics including benzene, toluene and xylene (BTX), on catalyst by cracking, dehydrogenation, cyclization and aromatization reactions. Aromatization of light alkanes has significant contribution for relieving the crisis of aromatics in China since the cheap and abundant light alkanes can produce the high value aromatics by this process. However, this process is very complicated and there is a called “backward effect” between BTX selectivity and light alkane conversion。Moreover, during the process of light alkane aromatization, the activity and stability of catalyst is not very good, and the catalytic activity center and reaction mechanism are still not very clear till now. In this work, to acquire such desired active site distribution and catalytic activity, a novel core-shell bifunctional zeolite catalyst is designed, the perfect separation of two active centers for alkane dehydrogenation and intermediate aromatization can be achieved on this kind of catalyst. In the meantime, by the theory calculation and characterization techniques combined with the results of Al distribution and siting, it can be deeply investigated to the effect of pore structure and shape-selectivity for catalyst on the cationic intermediates of aromatization process. The results of this project will help us to understand the reaction mechanism of light alkane aromatization, and also will provide good methods and guideline for the development of novel catalyst.

低碳烷烃芳构化反应是将低碳烷烃在催化剂作用下，通过裂解、脱氢、环化和芳构化等反应过程转化为BTX等轻质芳烃的过程。此工艺将廉价、来源广泛的低碳烷烃转化为高值芳烃，对缓解我国芳烃短缺问题具有重要意义。然而，该反应过程复杂，在产物选择性和转化率之间存在“逆向效应”，催化剂活性和稳定性不高，目前对催化活性中心的状态和作用机制也缺乏清晰的认识。为此，本课题拟设计合成新型的核壳结构双功能分子筛催化剂，实现烷烃脱氢活化和中间体耦联芳构化两种催化活性中心的有效分离，达到低碳烷烃高效转化制备芳烃的目的；同时采用理论计算与实验表征相结合的方法，辅以Al分布/落位的研究结果，揭示分子筛孔道限域效应对碳正离子中间体的影响规律。项目实施将深化对烷烃芳构化反应机理的认识，为低碳烷烃定向转化开发新型高效的催化剂及与之匹配的反应工艺提供理论依据与指导。

轻质芳烃苯、甲苯和二甲苯作为生产各种工业化学品的基本原料，近些年来需求量大幅增加。丙烷芳构化由于其重要的经济和战略意义引起了广泛关注。然而丙烷芳构化过程非常复杂，涉及到脱氢、裂解、齐聚、环化、氢转移和芳构化等多种反应。因此，有必要对丙烷芳构化反应机制进行深入研究，设计并研制高芳烃选择性催化剂。本论文从调控丙烷芳构化催化性能的角度出发，研究了Ga/ZSM-5型催化剂ZSM-5分子筛载体及负载Ga物种对丙烷芳构化反应催化性能的影响。主要内容和结论包括：.1. 通过改变合成母液中的Na/Al比调控ZSM-5分子筛中的骨架“铝对”含量；所合成的系列H-ZSM-5分子筛具有相近的Si/AlF比和酸性质以及相似的织构性质和形貌，但其骨架“铝对”含量不同。当母液中Na/Al比为0.8时，分子筛中骨架“铝对”含量最高，催化剂展现出较高的丙烷转化率（38.2%）和芳烃选择性（19.7%）。表征结果表明，ZSM-5中较多的骨架“铝对”促进了低碳烯烃的齐聚和环化过程，并最终生成了更多的芳烃。.2. 通过重结晶–碱处理两步法成功合成了具有微孔−介孔−大孔结构的新型多级孔ZSM-5分子筛。具有多级孔特性的ZSM-5载体可以有效地提高Ga物种的分散度。研究结果表明，高分散的Ga物种产生了更多的脱氢活性位点，促进了丙烷的转化，提高了芳烃选择性。与传统ZSM-5分子筛相比，多级孔ZSM-5材料可以有效地缩短芳烃产物的扩散路径，降低积碳速率，延长催化剂寿命。.3. 采用硝酸镓水溶液离子交换法将Ga负载到多级孔ZSM-5载体上，制备的Ga/ZSM-5催化剂在丙烷芳构化反应中展现出高催化活性、高选择性，较长的使用寿命和再循环使用性能。脉冲实验和DFT理论计算结果表明，低碳烯烃齐聚生成的高碳数烯烃（C6-C8）更倾向于优先环化生成环烷烃，随后脱氢生成芳烃；而不是优先脱氢生成二烯烃或三烯烃，再进行环化。