高通量、耐酸性CHA分子筛膜微结构的精准调控与耐酸机制研究

In recent years, dehydration of organics in acidic conditions such as dehydration of acetic acid is significantly demanded in industrial applications. At present, the most zeolite membranes showed either poor acid resistance or low permeation flux. According to the results in our former studies, we found that the grain boundaries and local aggregations of amorphous/poorly crystalline Al species in membrane surface displayed poorer acidic resistance than CHA crystal layers. In this program, we propose a novel and systematic strategy for the modifications of zeolite seeds, seed layers and membrane layers. The microstructures of hydrophilic zeolite CHA membranes such as framework Si/Al ratio, crystal intergrowth and thickness could be optimized by means of the regulation of seed layer’s quality, gel modification and the addition of crystal intergrowth supporting substances, and so on. Uniform zeolite CHA membrane with thickness of less than one micrometer will be expected after synthesis modification, and morphologies of the asymmetric substrates, seed crystals and membrane layers. The laws of the changes in microstructure along with the changes in synthesis parameters will be studied systematically. The mechanism of the interactions of crystal intergrowth supporting substances with membrane intergrowth will be discovered. The relation of the special microstructures and performance (flux and acidic resistance) will be investigated. The optimized zeolite CHA membranes should own both high fluxes and acidic resistance. The new membrane will be expected to have improved water flux (2 times higher than that of current membrane) and enhanced acidic resistance (stable at pH≤2). These ongoing findings in this program should support theoretically the application of zeolite membranes to new fields of the pervaporation hydration in acidic conditions.

近年来，酸性条件下有机物脱水如乙酸脱水在工业上有重大需求。目前，常用分子筛膜材料的耐酸性和渗透通量难以同时达到工业要求。根据我们前期研究结果发现，低硅CHA型分子筛膜易产生晶间缺陷和无定形Al在表面局部聚集，这些比晶体层更易在酸性条件下被破坏。本项目拟“晶种—晶种层—膜层”全过程构筑为研究思路，通过晶种层质量调控、合成液定向调控以及高密度电荷助剂添加等手段来精准调节膜层微结构（包括硅铝比、致密性和膜厚），从而获得兼具高通量和耐酸性的CHA型分子筛薄膜，膜层厚度控制在1μm以下；系统探讨成膜过程中微结构的变化规律，阐释添加的助剂构建共生晶体环境的作用机制；揭示膜层结构特性与性能的构效关系，确保此晶体的微结构在含酸的有机物脱水中的稳定性和透水性能，膜层水渗透速率比目前数据提高2倍以上，并在pH≤2的体系中保持长期稳定性。通过研究以期为分子筛膜在酸性渗透汽化脱水的应用提供理论依据。

与常规膜法有机物脱水相比，应用体系中含酸性则要求膜材料具备更强的亲水性兼备耐酸性。然而，由于分子筛膜层微结构控制水平有限和酸性条件对膜质量要求高，使得难以获得高通量且耐酸性的膜层。为此，项目从骨架Si/Al比值调控出发，设计合成了一系列不同骨架硅铝比、不同阳离子配对的耐酸性CHA分子筛膜。基于对膜微结构的精准调控，优化筛选出低硅CHA分子筛、高硅CHA分子筛、RHO分子筛等作为晶种，分别开发了离子热法、金属阳离子配对法和外延法等3种创新性制备方法用于ILs-CHA、Cs-CHA和RHO@CHA分子筛膜制备，这些不仅适当提高骨架硅铝比（Si/Al=3~5），还有利于减少晶间缺陷、提升成膜质量，从而实现调控膜的稳定性和膜表面微观结构（亲水性和耐酸性）来提高膜的分离性能。同时，采用吸附、SEM、XRD和UV-Raman等表征技术表征了CHA型分子筛晶体和膜，对分子筛在支撑体上非均相晶化成膜的过程与机理进行了系统研究，并通过DFT对晶体的次级结构单元组装行为进行了模拟预测，进一步对分子筛膜晶化机理进行了验证。通过4年的基础性研究，与低硅CHA膜（Si/Al<3）相比，CHA膜的渗透通量、长时间热稳定性和耐酸碱稳定性得到显著提高，特别是耐酸性，突破地在90 oC水/醋酸（40/60 wt%）溶液中7d中表现出良好的稳定性。本项目除了完成计划任务的研究之外，还将CHA膜构效关系进行拓展延伸，目前已构建出非对称复合支撑体耐强酸的CHA长膜，该成果正在整理发表中；且基于该膜层的稳定性将其发展出用于脱硝的分子筛基整体式催化材料，2022年此方向已获又一国家自然科学基金(22268022)立项资助。申请人以第一或通讯作者在J Membr Sci, Inorg Chem, J Solid State Chem等杂志上发表标注受本项目资助的SCI和EI科研论文7篇，授权中国发明专利3项，申请中国发明专利2项，培养硕士研究生9名。各项指标均达到项目预期。本项目的成功执行促进了渗透汽化行业的技术发展，同时也为分子筛膜材料的后续工业化制备和其他领域应用拓展提供了重要的理论和实践基础。