高CO2选择性有序碳纳米管复合膜的构建与微结构特性研究

Carbon dioxide (CO2) is considered as the most important greenhouse gas with the largest impact on climate change. Membrane separation has often been advocated as a suitable method for CO2 separation from gas mixtures due to the relatively low energy consumption involved. MFI zeolite membranes, a kind of microporous inorganic membranes, have been considered as alternative candidates for CO2 removal applications, in view of their high thermal, chemical and mechanical stabilities..In this study，the bottlenecks of MFI zeolite membranes in CO2 removal applications would be investigated innovatively. The single-walled carbon nanotubes (SWCNs) would be made in the zeolitic pores of b-oriented MFI zeolite membranes, which could have an estimated pore opening of 0.3-0.4 nm and high adsorption capacity of CO2; thus it could be fabricated creatively that the vertically aligned SWCNs/zeolite nanocomposite membrane with high selectivity and high flux in the separation of CO2. We intend to reveal the formation mechanism of SWCNs in the zeolitic pores by intensive studies on the catalytic cracking mechanism of organic templates in the transition metal hetero-atom MFI zeolite crystals at low temperature.Using secondary growth method, the best organization of zeolite microcrystals would be explored in order to prepare b-oriented MFI zeolite seed layers on the macroporous alumina substrates; then the film formation mechanism of zeolite seeds would be investigated systematically in the super dilute synthesis solution with transition metal hetero-atoms; finally, continuous b-oriented transition metal hetero-atom MFI zeolite membranes could be prepared by optimization synthesis conditions. Based on this, it would be deeply studied that the intrinsic links between the micro-structural characteristics of vertically aligned SWCNs/zeolite nanocomposite membranes and their separation performance of CO2, in order to achieve the preparation of SWCNs/zeolite nanocomposite membranes with high repeatability, CO2 selectivity and stability. The study could provide a new research idea and theories supports to the development of membrane separation technology for CO2 removal from natural gas, biogas and industrial blast furnace gas.

本项目拟在b-轴取向MFI型分子筛膜的分子筛孔道内构筑一种具有0.3-0.4 nm有效孔径、高CO2吸附性能的单壁碳纳米管结构，进而构建出高CO2选择性且垂直有序排列的单壁碳纳米管分子筛复合膜材料。通过深入研究有机模板剂在含过渡金属杂原子MFI型分子筛孔道内的低温催化裂解机理，揭示分子筛孔道内单壁碳纳米管的形成机理。采用二次生长法，探索最佳的分子筛微晶自组装工艺，在大孔氧化铝载体上有序组装b-轴取向分子筛晶种层，系统考察晶种在含过渡金属杂原子超稀合成液体系中的成膜机理，进而通过优化合成条件制备出致密无缺陷的含过渡金属杂原子的b-轴取向MFI型分子筛膜。在此基础上，深入研究垂直有序单壁碳纳米管复合膜的微结构特性与其宏观CO2膜分离性能的内在联系，实现膜制备的重复性，提高膜的分离性能和稳定性。本项研究可为天然气、沼气和工业高炉气中CO2膜分离脱除技术的发展，提供新的研究思路和理论支持。

二氧化碳（CO2）被认为是影响全球气候变化的主要“温室气体”，近几年随着低碳经济的兴起，如何实现CO2 的高效分离与回收利用已成为目前国际上最具挑战的研究课题之一。与传统变压吸附和化学吸收等分离方法相比，CO2气体膜分离法因其分离效率高、体积小、能耗和投资较低等优点，成为目前最具潜力的脱碳技术。MFI型分子筛膜是一种特殊的微孔无机膜材料，因其气体渗透通量大以及热稳定性和化学稳定性高等特点，在CO2膜分离方面具有良好的应用前景。然而，现有MFI 型分子筛膜的制备工艺尚不完善且成本较高；特别是，传统高温煅烧脱除模板剂工艺易导致分子筛膜产生较大的二次缺陷，严重影响和制约了MFI型分子筛膜在气体分离领域的应用。本项发明了一种低温加氢两步法脱除模板剂工艺，并深入研究了MFI型分子筛晶体内有机模板剂的加氢分解机理；进而，将该工艺应用于高硅MFI型分子筛膜内有机模板剂的脱除，以避免高温煅烧造成的膜内缺陷，并系统考察了高硅MFI分子筛膜的CO2膜分离性能。. 首先，考察了高硅MFI分子筛晶体内有机模板剂（四丙基氢氧化铵，TPAOH）在H2气氛下的低温裂解脱除规律。采用低温加氢裂解工艺，在300°C以下可有效脱除分子筛晶体孔道内的有机模板剂。相比于空气和氮气气氛，氢气还原性气氛更有利于模板剂的低温脱除，低温加氢裂解后的分子筛表面相对洁净，NH3-TPD结果表明低温加氢裂解后的分子筛具有更多的酸性位。采用旋涂法在粗糙的α-Al2O3载体片上制备出较完备的分子筛晶种层，以TPAOH为有机模板剂, 通过调控合成液的H2O/Si比例，实现了对分子筛晶体面内优先生长的调控; 经过三次水热合成得到致密交联的h0h-轴取向高硅MFI分子筛膜, 膜厚约为8μm。最后，采用先低温加氢裂解后低温空气氧化的两步法脱除工艺, 有效脱除了分子筛膜内的有机模板剂。相比于传统高温煅烧法。该法可以避免分子筛膜因脱除模板剂而形成的较大晶间缺陷. 因而采用低温两步法脱除模板剂的分子筛膜片在30℃时具有较好的CO2分离效果, 其CO2/N2分离因子达到5.2, CO2渗透通量高达5.8×10-7 mol•m-2•s-1•Pa-1。本项研究为天然气、沼气和工业高炉气中CO2 膜分离脱除技术的发展，提供了新的研究思路和理论支持。