新型低共熔溶剂中假象转变法构筑整体式多级孔磷酸硅铝分子筛及其催化性能研究

In recent years, hierarchical zeolite monoliths have attracted considerable attention due to their unique properties such as low bed pressure drop, high mass transfer efficiency, high catalytic activity and good selectivity. However, there exist many problems in current hierarchical zeolite monoliths such as high synthesis cost, low porosity and broad mesoporous distribution. In this research project, we aim to develop a new strategy for the synthesis of high quality hierarchical zeolite monoliths by psedomorphic transforming of the monolithic mesoporous-macroporous silicoaluminophosphate gel in a novel eutectic solvent. Moreover, the formation mechanism of the hierarchical SAPO monoliths will be investigated by means of ex situ and in situ characterizations. In addition, the catalytic performance of the hierarchical SAPO monoliths in long chain alkane isomerization and methanol to olefins reactions will be investigated. Based on the above results, the structure-activity relationship between the physical, chemical properties and the catalytic performance of the hierarchical SAPO monoliths will be determined, which will provide a theoretic basis for the synthesis of target oriented hierarchical SAPO molecular sieves. It is expected that this project will not only extend the synthesis of hierarchical zeolite monoliths to ionothermal system, but also promote the development of zeolite formation mechanism and their catalytic applications.

整体式多级孔分子筛具有床层压降低、扩散阻力小、催化活性高、选择性好以及无需后续成型等特点，近年来受到研究者的广泛关注。然而，目前整体式多级孔分子筛存在合成成本高、孔隙率低、介孔分布不均一等问题。本项目拟发展一种新的高质量整体式多级孔分子筛的合成策略：以新型低共熔溶剂为反应介质，以整体式介孔-大孔磷酸硅铝凝胶为前驱体，通过假象转变的方式进行定域晶化，在引入分子筛微孔的同时保持前驱体的介孔和大孔。采用离位、原位表征手段，跟踪整体式多级孔SAPO分子筛的形成过程，阐明其形成机理。考察其在长链烷烃异构、甲醇制烯烃反应中的催化性能。构建整体式多级孔SAPO分子筛的物化性质与催化性能之间的构效关系，为合成目标导向的多级孔SAPO分子筛提供理论依据。预期本项目的研究不仅可以将整体式多级孔分子筛的合成拓展到离子热体系，而且还能推动分子筛合成机理研究和催化应用的发展。

构筑纳米和多级孔分子筛，可以缩短扩散路径、暴露更多的活性位，是改善微孔分子筛扩散和传质问题，提高催化性能的有效途径。本项目制备出了具有多种纳米和多级孔结构的磷酸铝基分子筛，并对其晶化机理和催化性能进行了深入研究。主要研究内容，代表性成果和研究意义如下：.（1）多级孔纳米f分子筛单晶的通用合成策略。采用高度均一的超浓凝胶通过非经典生长路径制备具有多级孔纳米分子筛单晶，通过调控前驱体的组成，成功制备了LTA（8-rings）、AEL（10-rings）、AFI（12-rings）、-CLO（20-rings）结构，覆盖了从小孔到超大孔类型分子筛。通过先进的冷冻透射电镜对其进行了重构，证明了介孔的连通性。该方法无需介孔模板剂，产物具有优异的催化性能。代表性成果发表在Angew. Chem. Int. Ed., 59 (2020), 3455-3459。.（2）限域空间合成多级孔MgAlPO-11。通过在介孔磷酸铝凝胶前驱体的内表面构筑碳膜，来提供限域空间，抑制晶体的过渡生长，构筑了由纳米晶交互共生而成的多级孔MgAlPO-11分子筛，其具有优异的正十二烷异构化性能。代表性成果发表在Microporous Mesoporous Mater., 262 (2018), 182-190。.（3）干胶转化结合软模板法制备高度介孔化的多级孔SAPO-11。在前驱体凝胶中浸渍有机硅烷，通过干胶转化法制备多级孔SAPO-11，通过控制晶化过程的水量实现了酸性的精细调控。得益于良好的扩散性能和适当的酸性，所制备的多级孔SAPO-11在长链烷烃异构化反应中具有优异的性能。代表性成果发表在Catal. Sci. Technol., 7 (2017), 5775-5784。.上述研究工作为理解分子筛晶化机理，构筑高效催化性能的磷酸铝基分子筛提供了理论指导意义。