新型高效甲醇制丙烯催化剂的结构设计、制备及其催化机理研究

Propylene is one of the most important chemical materials, which is produced from the oils. Because of the enrichment of coal while shortage of oil in China, the production of methanol from coal and its conversion to propylene become important and significant for China. Based on the mechanism of dual-cycle hydrocarbon pool mechanism in methanol-to-olefins reactions, the present project focus on the design and preparation of novel highly efficient catalytic materials with high propylene selectivity and stability for methanol-to-propylene (MTP),using one-dimensional pore structured EUO-type zeolite (10-Member Ring, 10-MR), such as EU-1 and ZSM-50, as starting materials. The EU-1 and ZSM-50 with different acid site distribution are prepared and studied in the MTP reaction, in order to study the effect of 10-MR or in 12-MR structure on the distribution of MTP products, and then clarify the mechanism of dual-cycle hydrocarbon pool mechanism of MTP in EUO-type zeolites. The alkene hydrocarbon pool mechanism via the alkene cracking and methylation pathway yields a high selectivity of propylene in 10-MR channels. The aromatics hydrocarbon pool mechanism, which produces sizable amounts of light alkenes especially propylene is promoted by the presence of 12-MR side-pockets in EUO-type zeolites, because the reaction over the acid sites in side-pockets produce the arene with high number of methyl branches on benzene rings, which has a stronger influence on the reaction forming propylene versus ethylene via aromatic hydrocarbon pool mechanism. The ZSM-5/EU-1 and ZSM-5/ZSM-50 zeolites synthesize by the inducing hydrothermal crystallization of EUO type zeolites using ZSM-5 crystals as seeds, were design to promote methanol conversion and selectivity of propylene via synchronizing the arene hydrocarbon pool and the alkene cracking and methylation pathway. Simultaneously, the interconnected channels in ZSM-5 and the presence of side-pocket structure of EUO-type zeolite promotes the diffusion of products over catalyst avoiding carbon coking when catalyst deactivation occurs, and then improve the stability of the catalyst. The project addresses the possibility of controlling product selectivity based on intimate knowledge about the MTP reaction mechanisms (dual-cycle routes) over zeolite materials, and provides theoretical foundation and scientific basis for the preparation and application of MTP catalysts.

丙烯是最重要化工原料之一，其生产依赖石油。基于我国富煤缺油的情况，煤基甲醇制丙烯（MTP）的新技术路线，具有重要的战略意义。本项目基于甲醇制烯烃的双周期碳池循环机理，以含EUO骨架结构分子筛为MTP催化剂结构设计基础，突破解决催化剂面临的选择性低和稳定性差的难题。研究分子筛中十元环和十二元环结构内发生芳烃碳池和烯烃碳池循环路线的机制，利用十元环一维孔道结构促进烯烃碳池循环路线制丙烯；利用十二元环的支袋结构得到多取代基苯，推动芳烃碳池循环路线高产丙烯。利用ZSM-5为诱导剂水热合成EUO骨架分子筛技术路线，制备ZSM-5/EU-1和ZSM-5/ZSM-50复合分子筛，协同芳烃碳池和烯烃碳池双循环路线高产丙烯；ZSM-5交叉孔道结构结合十二元环支袋结构促进产物扩散，避免积碳失活。本项目旨在从新的角度深入研究利用反应机理设计新型高效MTP催化剂，为MTP催化剂的制备和应用提供理论基础和科学依据。

基于我国富煤少油的国情，甲醇制丙烯（MTP）工艺是后油气时代丙烯生产的重要途径之一而备受关注。在影响MTP工艺效率的诸多因素中，催化剂是最关键部分。其中，催化剂丙烯选择性、丙烯/乙烯（P/E）比以及使用寿命是决定MTP工艺经济性的重要指标。本项目致力通过MTP反应历程的研究设计和开发具有工业应用价值的高性能MTP催化剂。.首先，通过研究不同拓扑结构的分子筛的MTP反应性能，发现十元环分子筛结构利于双循环机理中烯烃路线生成丙烯，且三维交叉孔道结构（ZSM-5）能促进积炭前驱物种扩散，提高催化剂稳定；十二元环分子筛结构利于双循环机理中芳烃路线生成丙烯，P/E比高；十元环和十二元环耦合能两者兼顾高产丙烯，但催化剂稳定性较差。因此，十元环ZSM-5分子筛适合于固定床MTP反应，而十元环和十二元环耦合的EU-1适合于流化床的MTP反应。.其次，项目探讨了ZSM-5晶体形貌、尺寸与MTP的构效关系。研究发现，制备纳米尺寸的ZSM-5分子筛或薄层分子筛，缩短反应物及产物分子的晶内扩散路径，能够降低二次反应发生的几率，提高低碳烯烃选择性并抑制积炭失活速率。而ZSM-5的改性处理能够改变孔结构，提高扩散性能，同时降低酸密度，提高丙烯选择性。.再次，针对纳米ZSM-5分子筛在实际生产中分离问题，开发了一种硅源解聚与原位晶化同步制备纳米晶堆积的多级孔ZSM-5分子筛新方法。在高固含量水热合成体系下，不加入介孔模板和有机添加剂的情况下，高效的合成了纳米晶堆积多级结构ZSM-5分子筛。通过对合成体系的调节，实现多级结构ZSM-5分子筛介孔的调控。在此基础上合成了纳米晶堆积的Beta和EU-1分子筛。MTP反应结果表明，多级孔ZSM-5分子筛具有良好的丙烯选择性和活性稳定性，具有极高的应用价值。.最后，对比研究EU-1和ZSM-50分子筛MTP反应性能，发现酸中心位置与孔结构共同决定双循环路线中烃池物种的生成。EU-1分子筛具有更高的丙烯选择性和P/E，且其可通过提高分子筛的Si/Al比或提高反应空速、水醇比实现。EU-1和ZSM-5的复合，扩散性能提高，催化剂使用寿命延长；同时复合后产生的孔道变化抑制大分子扩散，提高了丙烯和乙烯的选择性。研究表明，EU-1和ZSM-5/EU-1复合分子筛用于流化床MTP反应具有广阔的应用前景。