用于气体分离的多孔骨架-流体分离介质复合膜的理论设计与模拟

Gas separation process plays an essential role on chemical industry and many other fields. Due to the low energy cost and compact equipment, the membrane-based separation technology has been widely investigated and attracted increasing attention in past decades. Exploring novel membrane material is at the core of developing advanced membrane technology. Membrane separation using traditional materials is governed by solution-diffusion mechanism, which results in an upper bound of performance, also known as Robeson upper limit. It indicates that high permeability and good selectivity cannot be achieved simultaneously. The thin membrane with unique nanoscale pore is considered as a candidate to circumvent this limit. It can separate various gases based on size effect as molecular sieve. Currently, numerous porous frameworks, including metal organic framework (MOF) and covalent organic framework (COF), have been synthesized that provide vast platform for construction of microporous membrane. However, the pore sizes of most frameworks are much larger than common gases. Thus these materials cannot effectively distinguish gas molecules according to their sizes or shapes. We suppose that impregnation of fluid media may adjust the channel dynamically, enhance the interaction between media and specific gas and improve the mobility in the porous structure. As a result, the permeability of required component would be significantly promoted. In this project, we will theoretically design a series of composite materials composed of porous framework and fluid media (such as oligomer and ionic liquid). By performing multiscale simulation, including quantum mechanism calculation, molecular dynamics simulation, and Monte Carlo simulation, we will study the interaction between framework and fluid phase, develop efficient and accurate approach to predict gas permeability in composite system, and take pre-/post- combustion carbon capture as typical systems to show the performance of our designed composite. By describing the gas separation process in composite, factors that affect separation performance will be discussed. The simulations from this project are expected to give insight into the mechanism of gas permeation in complicated system, and further offer a guidance to discover novel material for gas separation.

气体分离在环境能源领域有重要应用，基于膜的气体分离技术因能耗低、设备紧凑等特点近年被广泛研究。发展膜分离技术的前沿是寻找先进膜材料。传统膜材料受限于吸附-扩散机理无法同时提高透过率和选择性，均匀微孔膜被认为可能突破现有的分离效能极限。以金属有机骨架、共价有机骨架为代表的有序多孔结构为构造均匀微孔膜提供了结构基础，但其孔径往往大于气体分子，无法通过尺寸效应有效分离气体。在孔道中引入离子液体、寡聚物等液态分离介质可动态调控有效孔径，增强特定气体扩散速率和与膜的相互作用，选择性提高特定气体透过率。本项目拟通过多尺度模拟，设计多孔骨架-流体分离介质复合膜，研究多孔固相与流体之间相互作用，探索准确快速模拟复杂体系气体透过率的理论方法，以燃烧前/后二氧化碳分离为典型体系预测材料分离效能，描述气体在复合体系中的分离过程，考察影响分离性能的要素，为制备高效气体分离膜提供理论指导。

开发高性能气体分离膜有助于显著降低分离过程的能耗，实现节能减排目标。传统膜材料的分离过程受限于吸附-扩散机理，无法同时实现高透过率和高选择性，而均匀微孔膜则被认为可以突破现有膜材料分离性能上限。以金属有机骨架、共价有机骨架为代表的多孔材料为构造均匀微孔结构提供了平台，但其相对固定的孔径难以恰好满足分离不同分子的尺寸需求。在孔道中引入流动相分离介质可动态调控有效孔径，增强特定气体扩散速率和与膜的相互作用，从而提高特定气体透过率。基于多孔芳香骨架化合物，设计了一种骨架带电的结构，并在其中引入离子液体。结合分子动力学和第一性原理计算表明，离子液体与带电骨架之间产生强烈作用，引起结构收缩，显著降低了孔径。通过动力学模拟可以估算二氧化碳和甲烷在复合材料中的扩散速度与溶解度，进而推导出气体透过率。模拟结果显示，复合膜材料能兼具高二氧化碳透过率和高二氧化碳/甲烷选择性，综合性能可望超越现有材料上限。针对另一类多孔材料金属有机框架化合物，我们设计了光响应功能分子。分子模拟结果显示，将这类分子接枝在具有开放金属位点的孔道中后，通过改变带有氨基的尾链长度，可以实现光响应的二氧化碳输运能力调控。孔道内部几何结构的变化和吸附位点的开闭对气体输运过程都具有重要影响。针对另一类重要的多孔材料无定形碳，我们基于逆蒙特卡洛算法开发了程序，能够快速生成纳米尺度的氮掺杂无定形碳结构，为进一步地构建复合材料模型打下了基础。此外，我们还延伸了研究链条，通过第一性原理计算考察了用于二氧化碳和氮气等分离产品电催化转化的材料体系，全面丰富了对碳捕获和封存技术相关材料的理论认识。