CO2耦合甲醇择形芳构化多功能分子筛催化剂的构筑

Direct conversion of methanol to para-Xylene (PX) over shape-selective ZSM-5 catalyst is a new synthetic route, which is in accordance with the Chinese “Rich coal, oil shortage” resource structure. However, the current shape-selective catalysts reported generally show the much low PX selectivity and the rather short lifetime. Here we would like to solve above problems through the synergy of following three aspects: 1) the improved molecule-diffusivity for the shorter channel and the enhanced pore utilization (for reaction) of the special morphology ZSM-5; 2) the multifunctional catalysis depending on the three kinds of reactive sites inside the channel; 3) the coupling dehydrogenation promoting aromatization (form PX) in the presence of CO2. For the research content, firstly, we will develop the stable & reliable in-situ synthesis method for the Zn/ZSM-5 nanoneedles with the shorter channel, which also have the Zn-Lewis acid sites for dehydrogenation and the Brönsted acid sites. Then, we will explore the rational placement of reactive sites inside the channel of the shape-selective catalyst, in order to achieve the high-efficiency methanol-to-paraxylene reaction with coupling dehydrogenation in the presence of CO2. It has three research procedures as following: 1) constructing the active sites for the high-efficiency reaction of CO2 with hydrogen, through loading NiO species on Zn/ZSM-5; 2) adjusting the acidic strength and improving the stability of framework Al, through loading rare- or alkaline-earth metal oxide; 3) eliminating the external reactive sites and adjusting the size of pore-openings, by means of silica deposition. Meanwhile, the structure-performance relationship & the deactivation reason for catalyst and the key unknown problems on the aspect of reaction mechanism (for instance, what is the C8 reactive intermediate forming PX through dehydrogenation?) will also be studied in depth. And these obtained results will be used to guide the optimization of preparation method for catalyst. Based on the above works, we finally hope to design and construct a novel multifunctional & shape-selective catalyst with the high-efficiency matching and synergy on the three aspects: reactive-sites alliance, selectively reacting, shape-selectivity & high-efficiency diffusion for molecules. It can effectively control the reaction paths with promoting the aromatization to form PX, also suppressing the further side-reactions involved in PX and the coke deposition on catalyst surface. Its target performance is both methanol conversion > 98% and PX selectivity > 70% during 100 h time-on-stream. Our work may provide the new thinking and method for developing the shape-selective zeolite catalyst with high-performance.

甲醇择形芳构化是符合我国资源结构特点的对二甲苯（PX）合成新技术，但目前催化剂存在PX选择性低和寿命短的问题，本项目拟通过异形分子筛孔道的扩散性能提升、孔内三种多功能化活性位的合理设计及其CO2耦合脱氢促进加以解决。首先研究短孔道针状纳米Zn/ZSM-5（形成Zn-Lewis酸脱氢活性位和质子酸）的原位合成方法；并进一步探索择形芳构化复合活性位的有效调控方式以实现高效CO2耦合芳构化：负载NiO修饰构建CO2活化反应（与氢）中心，稀土/碱土金属氧化物改性调节酸性位强度，SiO2沉积择形修饰消除外表面活性位并调控孔口尺寸。同时，深入研究催化剂构效关系、失活机制和C8中间体反应机理，反馈指导制备方法优化。通过“活性位组合-选择性反应-分子择形扩散”的高效匹配协同，促进芳构化生成PX，并抑制产物PX深度副反应和表面快速积碳，即有效管控反应路径。使得耦合芳构化催化性能达到以下指标：单程连续反应100 h以上，甲醇转化率>98%，烃类产物中PX选择性>70%。本项研究可为高性能择形催化剂研制提供新思路与方法。

甲醇择形芳构化是符合我国资源结构特点的对二甲苯（PX）合成新路线，本项目研制了具有高PX选择性和良好反应稳定性的CO2耦合甲醇择形芳构化多功能催化剂。即首先建立了短孔针状Zn/HZSM-5原位水热合成方法，并实现其晶体尺寸、表面锌物种化学状态分布等精细调变；而后确定了通过对其负载La2O3+MgO+NiO和三次液相硅沉积来制备多功能催化剂的修饰方法。并通过对催化剂表面锌物种化学状态、表面酸性、微孔孔口尺寸、晶内微孔扩散反应路径长度等物化性质与芳构化性能之间构效关系的认识，结合对芳构化关键C8中间体及其演变路线和催化剂失活机制的理解，优化制备方法和工艺，最终构筑制得一种具有良好择形孔道扩散性和合理配置三种内表面反应活性位（NiO活性位、弱-中强质子酸性位和Zn路易斯酸中心）的耦合芳构化催化剂，凭借其“活性位组合-选择性反应-分子择形扩散”的高效匹配协同，合理管控反应路径，大幅提高PX选择性并显著抑制积碳失活。该催化剂单程连续运行150 h，PX选择性>71%，甲醇转化率>98.5%。本项目研究为高性能择形催化剂的研制提供了新思路与方法。