面向高性能MTP催化应用的多级孔结构ZSM-5沸石的新方法制备及其构效关系研究

Resulting from the predictable reserves of fossil fuels – more coal and less oil in China, methanol-to-propene (MTP) conversion process has been considered to be an important part in our energy strategies, in which preparation of high-performance catalysts is one of the key technologies. ZSM-5 zeolites is a generally adopted catalysts for MTP reaction. However, due to its sole microporosity, the formation of carbonaceous depositions and catalyst deactivation would be easily occurred. As a result, catalyst regeneration needs to be frequently conducted. Hierarchically structured zeolites (HSZs) are the new discovery in porous materials. Benefitting from the interconnection of well-developed porous structures on different length-scales (micro-, meso- and/or macro), high activity and long-lived lifetime of HSZs have been reported in many chemical processes. Till now, although a few methodologies have been designed for preparation of HSZs, the mass use of environmentally harmful mesoporous templates is the major obstacles to their practical applications. Green and low-cost preparation of HSZs is urgently desired. Moreover, in MTP process, the effect of hierarchically porous structures need to be clarified. For these reasons, herein it is proposed to carry out the study of novel methodology for preparation of ZSM-5 HSZs without the mesoporogens and its structure-performance relationship in MTP catalytic conversion. In this project, synthesis of high-quality and single-crystalline ZSM-5 HSZs is one of the main research targets. Based on the new design of dry gel conversion (DGC) synthesis with steam-assisted crystallization, research interests would be focused on the controllable preparation of HSZs just with a small amount of microporous structure-directing agents (SDA). The detailed research content include the effect of Si/Al ratios, microporous SDAs, pH value, pre-treating conditions for dry gels and steam-assisted crystallization parameters on the product crystallinity, textural properties, hydrothermal stability and surface acidic properties. It is expected that the resultant HSZs would combine the characters of high activity and high stability of crystallized microporous zeolites and of the fast diffusion path of mesoporous structures. Moreover, it would be discussed about the formation mechanism of HSZs by the newly developed DGC process under the absence of mesoporogens. On the other hand, the synthesized HSZs would be explored for MTP catalytic applications. For higher product yield/selectivity of propene and longer-lived catalyst lifetime, study of the structure-performance relationship of HSZ catalysts would be carried out basing on the characterization results and mathematical analysis with qualified microstructural factors of HF and ACI. In summary, the project production would offer the experimental and theoretical supports for the development of synthesis science of HSZs and their practical applications in MTP conversion.

甲醇制丙烯是适应我国多煤少油能源现状的发展战略，这其中高性能催化剂是关键。ZSM-5沸石是MTP反应常用催化剂，但单一微孔结构容易产生积碳、失活，催化剂需要频繁再生。多级孔结构沸石具有较强的抗失活性，但制备过程需要较多的介孔模板剂，难以满足应用要求，且多级孔对MTP催化性能的影响有待明确。本项目基于干凝胶蒸汽辅助晶化法的新设计，提出开展无模板剂条件下的多级孔结构ZSM-5沸石的制备科学与MTP催化应用研究。项目以高质量、单晶型ZSM-5多级孔结构沸石的绿色、低成本、可控制备为目标，探讨无模板剂新方法制备多级孔结构沸石的形成机理及影响合成材料孔结构、表面酸性质的关键因素；面向高性能MTP催化应用，以提高丙烯收率、延长催化剂寿命为目标，结合微结构量化因子HF和ACI，开展多级孔沸石催化剂构效关系的量化研究。项目的实施将为多级孔结构沸石制备科学的发展及其MTP催化应用研究提供理论和实验基础。

甲醇制丙烯是适应我国多煤少油能源现状的发展战略，这其中高性能催化剂是关键。ZSM-5沸石是MTP反应常用催化剂，但单一微孔结构容易产生积碳、失活，催化剂需要频繁再生。多级孔结构沸石具有较强的抗失活性，但制备过程需要较多的介孔模板剂，难以满足应用要求，且多级孔对MTP催化性能的影响有待明确。本项目基于干凝胶蒸汽辅助晶化法的新设计，无介孔模板剂条件下首先实现了硅铝比30～400范围内的单晶型多级孔结构ZSM-5的可控制备，明确了影响合成材料表面酸性质与孔结构的关键因素，阐释了骨架表面亲/疏水性调控策略制备多级孔结构沸石的新机制；相关方法可拓展用于制备粒径均一且可调控TS-1沸石纳米晶；合成多级孔ZSM-5沸石在异丙苯（IPB）与 1,3,5-三异丙苯（TIPB）的催化裂解反应中展现出更强的抗失活与容碳能力；在邻二甲苯异构化制对二甲苯（PX）的反应中，对目标产物 PX 的收率是传统 ZSM-5 沸石的3倍；在苯甲醛与乙醇的羟醛缩合反应中，多级结构沸石则表现出更高的苯甲醛转化率与循环使用稳定性（5次循环之后活性损失< 3%）。由于微/介孔多级结构与表面酸性质的协同催化作用，项目制备的高硅比（Si/Al~180）多级孔ZSM-5沸石在甲醇制丙烯反应（MTP）中体现出优异的催化性能，在甲醇质量空速为1.0 g g-1h-1的条件下，反应初始丙烯选择性高达 54.4%，催化剂寿命（t90）达到 175 h；进一步优化材料制备工艺（如微孔结构导向剂TPAOH与Si 的摩尔比从0.07提高到 0.13时），合成材料的颗粒尺寸从1μm减小到200 nm，催化剂的初始丙烯选择性从 54.1%提高到55.9%，催化剂寿命（t90）从91 h提高到220 h。对合成多级孔沸石进行了元素改性及表面钝化处理，与同组成微孔沸石及改性前多级孔沸石相比，合成后表面改性材料在丁烯裂解、二甲苯岐化等典型反应中体现出更为优异的催化性能。