聚乙烯醇/氮化碳渗透汽化透水膜的微结构调控与分离机制

Aiming at the problem of "upper-bound" line in the application of pervaporation membranes, namely how to solve the “Trade-off” effect between membrane selection and permeation? In this work, a novel kind of two-dimensional soft nanomaterial, graphitic carbon nitride (g-C3N4) with selective passages for water molecules, is selected to be chemically modified with different functional groups. This modified g-C3N4 will be embedded into the crosslinked poly(vinyl alcohol) (PVA) solution to prepare organic-inorganic hybrid pervaporation membranes. Then, the organic/inorganic interface morphology can be adjusted controllably through using hydrogen bond, chemical bond and coordination bond, thereby constructing an organic/inorganic hybrid membrane with multilayer microstructures. Using water/ethanol as evaluation system, the effect of grafting group structures, grafting density, dispersibility and compatibility of the modified g-C3N4 on the membrane separation properties will be studied in depth, thereby achieving the design and controllable preparation of PVA/g-C3N4 membranes with low cost, high stability, high separation performance. Also, molecular dynamics simulation will be used to study the diffusion kinetic behaviors of water and ethanol molecules in the hierarchical domain structures of PVA/g-C3N4 hybrid membranes. The influences of embedding of two-dimensional nanomaterials on the membrane hydrophobic, swelling behavior, free volume and interface morphology will be investigated, and the key factors affecting the mass transfer process of the hybrid membrane will be discussed. Through above studies, we hope to provide some theoretical basis for the controllable preparation of the anti “Trade-off" organic-inorganic hybrid membranes.

针对渗透汽化膜应用过程中“上限平衡”问题，即如何克服膜选择性与渗透性的“Trade-off”效应，以拥有水分子选择透过通道的二维纳米石墨相氮化碳(g-C3N4)为骨架，通过化学修饰赋予其功能基团后均匀分散到聚乙烯醇（PVA）溶液中交联涂膜，通过氢键、化学键、配位键等相互作用调控分离层中有机/无机过渡界面形态，构建具有多层次微结构的有机/无机杂化透水膜；以水/乙醇为评价体系，着重探究影响膜性能的接枝基团结构和密度、颗粒在PVA中分散性、相容性等关键因素，以期实现高稳定性、高分离性能PVA/g-C3N4膜的制备；采用分子模拟技术研究水和乙醇分子在PVA/g-C3N4膜多层次限域结构中的选择性传递与分离行为，分析二维纳米材料的嵌入对膜的亲疏水性、溶胀行为、自由体积、界面形态等的影响，从而阐明杂化膜限域传质过程中的关键影响因素，为调控制备反“Trade-off”效应的有机无机杂化膜奠定一定的基础

针对渗透汽化膜应用过程中“上限平衡”问题，将拥有水分子选择透过通道的二维纳米石墨相氮化碳(g-C3N4)、活化的g-C3N4(O- g-C3N4)和聚多巴胺修饰的g-C3N4 (PDA@O- g-C3N4)纳米片引入聚乙烯醇(PVA)基体中，制备了具有优良水/乙醇分离性能和良好水通道的高选择性杂化渗透汽化膜。通过调节聚乙烯醇（PVA）与g-C3N4之间的界面相互作用力，从氢键、刚性化学键到柔性化学键，制备了系列渗透汽化透水杂化膜，杂化膜的总通量从4634 g/(m2h)降到2328 g/(m2h)；分离因子从32.4提高到57.9，最高达到202.2；该新型杂化膜可以有效地打破“权衡效应”。杂化膜连续运行120 h后，仍具有初始的渗透通量和分离因子，具有优异稳定性。具体创新点为：（1）以三聚氰胺为前驱体，制备出了具有类石墨的片层结构g-C3N4、O-g-C3N4和PDA@O-g-C3N4纳米片，三种纳米片表现出一种独特的耐热稳定性，分别高达710 ℃、763 ℃和725 ℃。（2）将3种纳米颗粒(g-C3N4，O- g-C3N4，PDA@O- g-C3N4)依次添加到PVA基质中，制备渗透汽化杂化复合膜，从而形成聚合物/无机界面的不同粘结力，从氢键(CPVA-g-C3N4)、刚性 (CPVA-O- g-C3N4)到柔性(CPVA-PDA@O- g-C3N4)无选择性化学键。对于纯CPVA膜，PVA侧链上的OH官能团与交联剂Sa中含有的COOH官能团之间的酯化反应是主要的化学反应；对于CPVA-g-C3N4膜，PVA和g-C3N4纳米片的界面相互作用主要是通过弱氢键；对于CPVA-O- g-C3N4膜，PVA和g-C3N4纳米薄片间的界面相互作用包括氢键和刚性化学键；在进一步接枝亲水性聚合物PDA后，PVA与g-C3N4之间的无选择性聚合物/无机界面孔道由氢键、刚性和柔性化学键所组成，杂化膜中的无选择性界面孔的数量明显的较少。（3）采用MD模拟预测聚乙烯醇/氮化碳杂化膜的结构与性能。g-C3N4增强膜的透水分离机理主要是g-C3N4层间的分子扩散差异，周期性晶格空穴筛分机理存在一定的缺陷。