甲苯烷基化合成对二甲苯(PX)过程择型催化剂制备的方法创新与过程控制机制研究

Para-xylene (PX) is one of the most important aromatics widely used as the raw materials of polyethylene terephthalate and polyester. Recently, the self-sufficient rate of PX in China is less than 50%. Therefore, it is meaningful for carrying out some research on the synthesis of PX. Commercially, the synthesis of PX by alkylation of toluene with methanol catalyzed by zeolites such as ZSM-5 and MCM-22 is typically shape-selective catalysis process.The design and preparation of shape-selective catalysts therein is the key to obtain a highly shape-selectivity for the production of PX. However, the acidic sites on the external surface of zeolite usually result in the decrease in para-selectivity. The metal oxides modefied zeolites shape-selective catalysts prepared by complexation-impregnation method have been developed, which can maintain the cover of the external acidic sites and the protection of the inner acidic sites on zeolite can be achieved simultaneously. Consequently, the catalysts prepared by complexation-impregnation method exhibit excellent para-selectivity, activity and stability in shape-selective catalysis process. This object covers the following three parts: (1) the process optimization and control for the stability of complex during the complexation-impregnation process; (2) the dispersion mechanism of the metal complexs on the external surface of zeolites and the relationship with the cover of the external surface acidic sites of zeolite; (3) the catalytic performance and the principle thereof for the synthesis of para-xylene over metal oxides modefied zeolites prepared by complexation-impregnation method. The research of this object is believed to provide a further outline for the preparation of highly effcient shape-selective catalysts.

对二甲苯（PX）是一种重要的石油化工产品，主要用于聚酯的生产。当前，我国PX自给率不足50%，积极开展PX合成方面的研究具有重要的现实意义。通过甲苯甲醇直接烷基化合成PX是典型的择形催化过程。微孔分子筛ZSM-5和MCM-22 是常用的择形催化剂，但其外表面酸性位的存在会降低其择形性能。通过络合浸渍法为PX的合成过程设计金属氧化物改性微孔分子筛择形催化剂，可以实现在覆盖分子筛外表面酸性位的同时而不影响其孔道内的酸性质，进而实现在获得高选择性的同时保持催化剂的高活性和高稳定性。本项目拟研究：(1) 络合浸渍过程中络合物稳定性的过程优化与控制；(2) 金属络合物在分子筛外表面的分散机制及其与氧化物覆盖分子筛外表面酸性位效果的相关性；(3) 络合浸渍法制备的金属氧化物改性微孔分子筛催化剂对合成PX过程的择形性能及规律。本课题的研究将为石油化工择形催化剂的制备领域提供一条便捷、高效的新途径。

对二甲苯（PX）是一种重要的石油化工产品，主要用于聚酯的生产。我国的PX的自给率仅为43%，将资源过剩的甲苯最大限度地转化成高附加值PX一直是石油化工过程中一个重要的研究方向。传统的甲苯歧化合成PX中甲苯利用率低下，而以甲苯和廉价的甲醇为原料利用微孔分子筛（以ZSM-5和MCM-22为代表）择型催化烷基化合成PX可有效提高甲苯利用率。 其中，该催化反应的反应条件和PX的选择性直接与微孔分子筛的孔道结构和表面酸性质等有着中重要关联。. 本项目以MCM-22为催化剂研究平台，围绕覆盖该材料的表面酸性位而不影响孔道内酸性位的前提下，通过MgO预沉积、TiO2改性等多种方法制备了多种改性的MCM-22催化剂。研究发现：相比传统的Mg(NO3)2等金属盐直接浸渍，MgO预沉积过程中的有机模板剂因为尺寸大于MCM-22孔口尺寸，可以有效保护分子筛的孔道结构；而TiO2改性则构筑了较大尺寸的金属盐前驱物，避免了金属氧化物沉积在MCM-22孔道内。在覆盖外面表酸性位的同时保护原孔道内部的酸性位。该两种催化剂在择型催化合成PX过程中，在反应温度为350-400°C条件下，甲苯的转化率分别为15.27%和23.35%，而PX的选择性分别达到为65.38%和58.99%。该催化剂具有良好的催化稳定性。结合多种表征技术，提出了构筑大尺寸金属前驱体物质的设计原理。此外，除了烷基化反应，本项目还制备了g‑C3N4‑HMCM‑22复合材料，研究了其在催化间二甲苯异构化合成PX反应中催化性能。研究发现：g-C3N4的引入可以改变间PX异构化反应的产物分布。结合催化剂表征发现g-C3N4能有效抑制孔口的Brönsted酸强度以及增加了外表面Lewis酸性位点的数量，从而提高了PX的选择性。. 本项目围绕PX的合成，为金属氧化物改性微孔分子筛择形催化剂的制备开辟一条便捷高效的新途径，对于积极开展PX合成方面研究工作具有重要的现实意义。