面向离子液体分离和回收的薄层复合正渗透膜研究

Ionic liquids (ILs) have attracted growing attention as green solvents for a diverse range of applications such as synthesis, catalysis, and natural polymers processing due to their low melting temperature, low vapor pressure and high solvation ability. However, the high price of ILs is one major obstacle for their wide applications in industry, and their ecotoxicity is also recognized to have possible hazards to the environment. The separation and recovery of ILs, especially for those soluble ILs in water, therefore plays a key role in industrial applications of ILs as green solvents in a real sense. In this project, we propose to separate and recover ILs from water by forward osmosis (FO) whose driven force is the osmosis pressure variation resulting from the weak hydration of ILs relative to inorganic salts. Bidirectional freezing technique based on polymer/crystallized solvent system is used to prepare polymer membranes with vertically oriented pores which can be used as the FO membrane support. The co-deposition method directed by electric field can be applied to charged modification and hydrophilization of the surface of inner pores, endowing the FO support with a highly depressed internal concentration polarization effect. Moreover, the polycationic electrolyte is introduced to the traditional interfacial polymerization to prepare the selective skin layer. The tuning mechanism on the charge density, hydrophilicity, thickness and cross-link degree of the skin layer will be systematically studied. Then detailed investigations will be executed to clarify the role of FO membrane structure in the separation performances such as permeability, stability and anti-fouling property. The study will construct a novel strategy to prepare a novel kind of thin-film composite FO membranes with high water flux, IL rejection, anti-fouling property and stability, which is expected to facilitate the popularity of ILs in industry field and contribute to the development of membrane science.

离子液体(IL)以独特的低熔点、低挥发、高溶解性被公认是一类绿色溶剂，在合成、催化、天然高分子加工等众多领域得到广泛关注。但其高昂的价格制约了工业生产中的规模化应用，其生物难降解性会对环境造成潜在危害。对IL进行高效分离和回收再利用是使其真正成为工业绿色溶剂的关键。本课题基于水合作用的强弱所形成的渗透压差，提出利用薄层复合正渗透膜来对IL进行分离与回收。采用结晶性稀释剂取向冷冻法制备聚合物垂直通孔膜，通过电场诱导共沉积实现膜孔内壁的荷电/亲水改性，获得能有效抑制内部浓差极化效应的支撑层；引入正电性大分子胺类水相单体进行界面聚合反应，系统揭示选择性皮层荷电性、亲水性、厚度以及交联度的调控机制，阐明膜结构调控对分离性能尤其是渗透性、抗污染和稳定性的影响规律，建立高水通量、高IL截留率、高浓缩能力、抗污染以及高稳定性薄层复合正渗透膜的制备新方法，为IL的推广应用和膜科学的发展提供新思路。

高附加值化学品的高效回收再利用契合“大力发展绿色工业”这一国家重大战略需求，是全面实现“碳中和”和“碳达峰”的必由之路。离子液体作为一种新兴的绿色溶剂，具有稳定性高、溶解力强、不易挥发等优点，在有机合成、电催化、能源、新材料等多个领域的重要性正日益凸显。但离子液体的持续推广和规模化应用会产生大量含IL废水，不但危害环境、损害作为“绿色溶剂”的声誉，也降低宝贵资源的利用效率。高效分离回收废水中的水溶性离子液体，是实现其可持续规模应用的重要基础。因此，解决离子液体循环利用的瓶颈问题，发展膜法分离回收新方法，是落实绿色经济可持续发展战略的必然需求。本项目结合理论计算和实验测定，证实了离子液体具有与无机盐迥异的弱水合作用和临界聚集现象；揭示了离子液体水溶液与相同浓度的无机盐水溶液之间存在巨大的渗透压差；同时，离子液体水溶液的渗透压不会随其浓度的增加而发生显著衰减。在此基础上，建立了以膜两侧渗透压差为驱动力，利用正渗透过程来浓缩和回收水溶性离子液体的方法体系；以此为理论依据，考察了具有不同支撑层结构的薄层复合膜分离和回收离子液体的性能。结果证实，该方法可将浓度为1 wt％离子液体水溶液浓缩至58-78 wt％。理论计算证实了离子液体水溶液的渗透压低于同等浓度的无机盐水溶液，具有低的蒸汽压。基于该原理，我们通过疏水性微滤膜渗透蒸馏方法，实现了24小时内将稀离子液体水溶液稀释至84 wt%。实验结果证实，低膜厚、大孔径、高孔隙率和低孔曲率有利于水蒸气的渗透。该方法通过蒸汽传质，相较于正渗透浓缩法具有更低的返盐通量（<0.2 gm-2h-1）和极高的离子液体截留率（>99.99%）,可以保证最终回收产物的高纯度和低损失。本项目研究结果将为发展面向IL/水分离的分离膜材料提供一种新的策略，为离子液体的回收再利用提供新的思路，不但具有十分重要的应用前景，还可望丰富分离膜制备、膜分离技术、表面与界面等学科内涵，为我国吸附与分离材料设计与性能调控作出贡献。