Development of novel ionic covalent organic frameworks composite nanofiltration membranes for dye/salt separation.

Covalent organic frameworks (COF) have desirable properties such as high porosity, low mass density, excellent heat resistance and regulatable structure, making them an ideal candidate for membrane material. Traditional methods for preparing covalent organic framework composite membranes, such as interfacial polymerization, vacuum filtration, and covalent organic framework abrasive coating. Stand-alone COF membranes produced by the above methods usually suffer from problems such as poor mechanical properties. Here, we fabricated high performance COF composite membranes by modified casting-precipitation-evaporation method. The designed composite membranes consisted of the ionic COF (iCOF) selective layer and the support layer are applied in dye/salt separation. The high permeability (∼ 68 L h-1 m-2 bar-1), high dyes rejection (97% for Rose Bengal), and low salts rejection (∼ 2.86% for NaCl) are achieved by the iCOF functional layer. The as-prepared composite membranes have a hydrophilic and highly smooth surface, making them have good anti-fouling performance. In addition, the rigid pore structure of iCOF selective layer endows the composite membranes with excellent stability, the composite membranes maintain original structure under high pressure (6 bar) and ultrasonic treatment (16 kHz for 60 min). This work may open up a novel path to fabricate iCOF composite membranes, which exhibit great potential in dye/salt separation.