晶种工程：高性能分子筛气体分离膜的制备

Membrane-based separation process has been considered as an emerging technology for chemical industry due to its low energy consumption, high efficiency and easy operation. In particular, zeolites and metal organic frameworks (MOFs) as representative of molecular sieve membranes, have shown a promising perspective of industry application, owing to their high flexibility in adjusting pore apertures and chemical compositions. In order to maximize both selectivity and permeability of molecular sieve membranes, herein we put forward a new concept called “seed project”. Inspired from finger-like hollow fibers possessing lower diffusion resistance due to the higher porosity, herein we propose that high performance molecular sieve membranes can be constructed from hollow molecular sieves. In comparison with solid molecular sieves, the diffusion barrier for hollow molecular sieve is expected to be substantially reduced, making them ideal candidates as seeds for growing molecular sieve membranes with high permeability. On the other hand, it has been demonstrated that under normal conditions highly oriented molecular sieve membranes exhibit higher selectivity than randomly oriented ones, which can be attributed to an effective reduction of non-selective intercrystalline diffusion of molecules through the grain boundary. In this project we plan to prepare well-intergrown, highly oriented, hollow and ultra-thin MFI zeolite membranes and MIL-125-NH2 MOF membranes, respectively. It is anticipated that the overall gas separation performance can thus be significantly enhanced through microstructural optimization.

膜分离作为近年来新兴的化工分离技术,具有能耗低、分离效率高及操作简便等特点。特别是以沸石及金属有机化合物为代表的分子筛膜由于其优良的结构与化学可调控性而具有广阔的工业应用前景。为最大限度提升分子筛膜的分离选择性与渗透通量，受指状孔结构的中空纤维膜由于较高的孔隙率而拥有更低扩散阻力的启发，我们创造性地提出“晶种工程”这一概念，尝试采用具有空心内部结构的分子筛晶体作为结构基元构筑高性能分子筛膜。相较于实心分子筛晶体，具有空心内部结构的分子筛晶体的扩散阻力仅源于其壳层部分。若将其作为晶种，将有望显著提升膜材料的渗透通量。与此同时，保持分子筛晶粒择优取向生长则有助于减少位于晶间界处的非分子筛孔道缺陷，从而提高其分离选择性。本项目中，我们拟以MFI型沸石及MIL-125-NH2型金属有机骨架化合物为研究模型，制备连生度高、高度取向且具有中空内部结构的超薄分子筛膜,以期实现气体分离性能上的显著提升。

申请人在项目执行期内，成功制备了高度b-轴取向超薄MFI沸石膜、优先(h0h)-轴取向等级结构MFI沸石膜、高度c-轴取向超薄NH2-MIL-125型MOF膜、高度(111)-轴取向UiO-66（包括UiO-66和NH2-UiO-66）型MOF膜、高度(110)-轴取向ZIF-7型MOF膜、高度取向2D MOF（包括ZrPP和Cu-TCPP）膜。上述制得的分子筛膜分别拥有优异的n-/i-C4H10、H2/CO2、CO2/N2、CO2/CH4、H2/N2和H2/CH4分离性能；其分离性能不仅整体超越相对应气体混合物2008年Robesen分离性能上限，而且绝大部分都位居同类型分子筛膜分离性能最高值。与原有项目计划书相比，相应变动之处主要为以下方面：1）对于MFI沸石膜，分离体系从p-/o-xylene混合物变更为n-/i-butane；2）对于NH2-MIL-125型MOF膜，从制备中空结构c-轴取向膜结构转变为超薄c-轴取向膜微结构；3）在项目资金支持下，额外成功制备了高度(111)-轴取向UiO-66型MOF膜、 (111)-取向NH2-UiO-66型MOF膜、(110)-取向ZIF-7型MOF膜、(00l)-取向2D ZrPP膜和c-轴取向2D Cu-TCPP膜；4）在项目资金支持下，额外尝试采用超临界流体（SCF）辅助法制备高性能ZIF-8和ZIF-67膜分别用于C3H6/C3H8和H2/CO2高效分离。上述研究成果具有以下重要科学意义：1）深化了对分子筛膜构效关系的认知，特别是进一步确认了膜材料优先取向和等级结构对分离性能的重要影响；2) 发展了有机碱刻蚀工艺制备超薄沸石纳米片的新方法，对纳米片形貌演化历程进行了深入分析，这对于简便高效制备多种类型分子筛纳米片具有借鉴意义；3) 将等级结构的理念引入多晶分子筛膜微结构，为进一步提升膜材料分离性能提供了新思路；4) 将单模微波辅助加热法引入分子筛膜制备过程，系统研究了单模微波辐射对分子筛（膜）成核和生长的影响，初步揭示了单模微波辐射和分子筛间的作用机制，为温和反应条件下简便快速制备高性能分子筛膜开辟了新途径；5）成功以SCF作为反应介质制备高性能MOF膜，同时对SCF反应介质中MOF膜成核和生长动力学有了初步认知。由于该方法所固有的绿色可持续性，对于批量制备高性能MOF膜有很好的应用前景。