磷酸硅铝分子筛空心晶体的形成机制

Silcoaluminophosphate molecular sieve (SAPO-34) shows superior performance in the process of methanol to olefins (MTO) in term of methanol conversion and light olefins selectivity. Unfortunately, SAPO-34 has a short life time due to the rapid coke rate, which can be overcomed by reducing the diffusion distance (nano-crystals) and lowering the concentration and intensity of acidic sites. However, the synthesis of the nano-crystals with a low acidity has various disadvantages. To reduce intracrystalline diffusion distance of the olefins products, improve the removal efficiency of heat and decrease coke rate, this proposal aims at synthesizing the SAPO-34 crystals with a lower acidity, sub-micron size, thin wall thickness and hollow interior. The hollow SAPO-34 crystals are prepared by inserting phases of raw materials in the homogeneous sol, adding the multiphase interfaces in the gel which is formed in the early period of hydrothermal crystallization and controlling the dissolution-recrystallization speeds of the gel and the raw materials. Upon the analysis of the effect of the key parameters determining hollow interior formation on the crystallization process, this proposal focuses on the investigation on the formation mechanism of the hollow SAPO-34 crystals. Thereby, coordinated adjustment of the crystal size, the wall thickness and the acidity is performed, and then the correlation of the crystal size, the wall thickness and the acidity to the methanol conversion rate, light olefins yields, coke rate and life time is established. The novelty of this proposal is the design of the hollow SAPO-34 crystals, the investigation on the formation mechanism and the optimization designing of the hollow Structure and acidity, which might provide experimental and theoretical foundation for the highly efficient utilization of molecular sieve based catalysts.

硅铝磷酸盐分子筛（SAPO-34）在甲醇制烯烃过程（MTO）中表现出超高甲醇转化率和优异的低碳烯烃选择性，但结焦快、寿命短。该缺点可通过降低扩散距离（纳米晶体）以及降低酸位强度和密度克服，然而低酸性纳米晶体的制备存在诸多问题。为了降低产物烯烃在晶体内的扩散距离，提高反应热的移除效率，降低结焦速率，本项目拟通过在均一溶胶中置入原料相，在水热晶化初期的凝胶中设置多相界面，调控凝胶和原料相的溶解再结晶速度，制备低酸性、亚微米、空心、薄壁的SAPO-34晶体。通过分析空心形成的关键影响因素对结晶过程的影响，研究空心结构的形成机制；从而实现空心晶体的粒度、壁厚和酸性的协同调控；建立粒度、壁厚和酸性与甲醇转化率、低碳烯烃收率、结焦速率和寿命的对应关系。本项目的新颖性在于设计空心SAPO-34晶体，探索空心晶体的形成机制，优化空心结构和酸性设计，为分子筛催化剂的高效利用提供实验和理论依据。

甲醇制烯烃（MTO）技术的研究重点之一是催化剂的筛选、制备与性能调控。作为该过程最常用的催化剂，SAPO-34分子筛存在结焦速率大，寿命短等问题。增加物质和能量在分子筛晶格中的传递和扩散效率，是制备长寿命MTO催化剂的有效途径。本研究采用在微孔分子筛晶体中构建空心结构、大孔结构和介孔结构的方法，增加反应物和产物在分子筛晶格中的扩散效率，从而减少二次反应，降低结焦速率，延长催化寿命。.以焙烧高岭土为复合硅铝源，以吗啡啉和乙二醇或丙三醇为复合结构导向剂，制备出空心SAPO-34，表征了空心晶体的性质。空心的形成机制为：SAPO-34在高岭土颗粒周围成核，并围绕高岭土颗粒生长，而后晶体颗粒逐渐连接生长，内部高岭土中的硅和铝逐渐溶解，不断参与到周围的晶体搭建，形成空心的SAPO-34晶体。.以焙烧高岭土为复合硅铝源，以吗啡啉和β-环糊精（β-CD）为复合结构导向剂，制备出微孔-大孔多级孔结构的SAPO-34晶体，详细研究了β-CD含量对微孔-大孔SAPO-34晶体性质的影响规律，建立了大孔结构的形成机制。以硅溶胶为硅源，以吗啡啉和β-CD为复合结构导向剂，制备出微孔-介孔多级孔的SAPO-34。.分别表征了空心结构、微孔-大孔结构和微孔-介孔结构的SAPO-34的MTO催化性质。相对于微孔SAPO-34晶体，三种特殊结构的SAPO-34均表现出优异的烯烃选择性和催化寿命。.本项目的研究不仅丰富了提高微孔扩散和传质效率的手段，为延长SAPO-34 催化剂寿命提供新思路,并为高效分子筛基催化剂的设计提供有益借鉴。