构建超薄硅基分子筛薄膜及其有机异构体的高通量分离

It is widely accepted that gas permeability is practically important for zeolite membranes in gas separations. But the diffusion limitations of gases in small pores adversely affect the gas permeability across zeolite membranes. This problem can be overcome by increasing zeolite porosities and effective membrane surface areas, reducing membrane thickness, and etc. In principle, high permeability can be achieved with thin membranes. Thus, there are four basic objectives in this project: (1) to design ultra-thin zeolite membranes for gas separations, which are in contrast to previous efforts in the preparation of different thick membranes with a variety of zeolite structures (the membrane thickness is usually above 1 µm); (2) to propose a general synthesis method for the fabrications of ultra-thin zeolite membranes; (3) to elucidate the formation mechanism of zeolite membranes; (4) to explore the potential applications of ultra-thin membranes in gas separations, and to build up a separation model of gas permeation through zeolite membranes. The above goal will be achieved via the following reasonable routes: (1) the ultra-thin membranes will be prepared by secondary crystallization of seeded layers with small nanocrystals which are hydrothermally synthesized using a long-chained multi quaternary ammonium-type organic template; (2) the preparations of ultra-thin CHA、MFI、BETA-type zeolite membranes will be exemplified using this proposed strategy (assisted approach with ultra-small crystallites as seeds); (3) the crystallization behaviors of zeolite membranes (crystal overgrowth, oriented growth, etc.) will be studied by in-situ atomic force microscopy (AFM) in couple with X-ray diffraction, scanning electronic microscopy, transmission electron microscopy and other techniques; (4) the separation of organic isomers will be carried out on prepared ultra-thin membranes.

分子筛薄膜的通量是衡量膜气体分离性能的重要指标，其主要影响因素包括：分子筛自身的孔隙率、薄膜面积、薄膜厚度，其中薄膜厚度是影响膜通量至关重要的参数之一。因此合成超薄分子筛膜可以有效地提高薄膜通量。目前，大多数制备的分子筛薄膜厚度基本在微米级以上；且制备方法过于单一，很难总结出合成的规律性和方法的广谱性。本项目拟引入长链型多铵类化合物为有机模板剂用于具有单层分子筛结构单元厚度纳米晶的合成；并将此纳米晶作为晶种，采用二次合成法弥补晶种之间的晶体间隙，通过控制晶体在二次生长中的生长速度，从而获得连续致密超薄的硅基分子筛膜（CHA、MFI、BETA）。晶种在薄膜中二次生长的规律通过耦合原位原子力显微镜与X-光衍射、扫描电子显微镜、透射电子显微镜等技术手段进行研究，进而揭示晶种法合成分子筛膜的机理。合成的超薄连续致密的硅基分子筛膜应用于分离分子尺寸接近的有机异构体，从而真正实现分子筛筛分效果。

分子筛因其有序孔道结构、分子水平孔径、高孔隙率，成为优良膜材料。膜分离是一种高效、低能耗分离技术，使分子筛膜成为研究重点。本项目开展了以下工作：（1）不同分子筛纳米晶的可控制备：以四乙基氢氧化铵为模板，辅于微波，制备了CHA、AEI纳米晶。CHA和AEI纳米晶呈片状形貌，晶粒大小为450×450×50 nm3和450×300×70 nm3。调节微波条件，制备了不同长径比纳米片。以四丙基氢氧化铵及四丁基氢氧化铵为模板，在高反应物浓度下水热合成了纯硅MFI、MEL纳米晶。MFI和MEL微晶具有球形形貌，大小比较均一，大概在150-200 nm之间。（2）薄膜中晶体生长的研究：以MFI分子筛为例，系统研究了表面胶转化法中MFI二次生长规律。考察母液陈化、母胶渗透、旋涂速度、晶化时间、晶化温度五个参数。扫描电镜和X-射线衍射结果发现母液陈化时间是合成连续MFI膜的最重要参数，因为母液在经过一定时间陈化后才形成粘稠胶体。MFI晶体之间实现了良好共生，形成了亚微米(500 nm)薄膜。在此基础上，我们制备了大面积连续的CHA、AEI、MEL分子筛膜。（3）分子筛膜分离性能的研究：根据分子筛孔化学特性，针对性地开展了薄膜分离工作。海水脱盐结果显示Na+、K+、Mg2+、Ca2+的截留率分别为99.87%、99.96%、99.97%、100%，水通量为1.82-3.37 kg m-2 h-1。渗透实验与分子模拟结果阐释了高截留率归功于AEI的筛分效果，纪录级别水通量归功于高吸附量、快扩散速度及方向。针对MFI中孔性质，初步探讨了MFI分子筛膜在C6有机异构体的分离。渗透汽化结果表明MFI膜对正己烷具有选择透过性，正己烷对于其他异构的选择性在17-33之间，正己烷通量为0.66 kg m-2 h-1。高选择性主要因为MFI对正己烷的择型性。研究内容具有科学和工程价值：纳米晶合成与分子筛膜形成机理能够帮助我们更好地了解晶体生长，为精确制备提供基础；分子筛膜在海水脱盐和C6化合物分离为我们海水淡化提供素材，为化工过程提供先进分离手段。

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