Assembling covalent organic framework membranes via phase switching for ultrafast molecular transport

Fabrication of covalent organic framework (COF) membranes for molecular transport has excited highly pragmatic interest as a low energy and cost-effective route for molecular separations. However, currently, most COF membranes are assembled via a one-step procedure in liquid phase(s) by concurrent polymerization and crystallization, which are often accompanied by a loosely packed and less ordered structure. Herein, we propose a two-step procedure via a phase switching strategy, which decouples the polymerization process and the crystallization process to assemble compact and highly crystalline COF membranes. In the pre-assembly step, the mixed monomer solution is casted into a pristine membrane in the liquid phase, along with the completion of polymerization process. In the assembly step, the pristine membrane is transformed into a COF membrane in the vapour phase of solvent and catalyst, along with the completion of crystallization process. Owing to the compact and highly crystalline structure, the resultant COF membranes exhibit an unprecedented permeance (water approximate to 403 L m(-2) bar(-1) h(-1) and acetonitrile approximate to 519 L m(-2) bar(-1) h(-1)). Our two-step procedure via phase switching strategy can open up a new avenue to the fabrication of advanced organic crystalline microporous membranes. Covalent organic framework (COF) membranes are important in low energy and cost-effective molecular separations but most COF membranes are assembled via a one-step procedure by concurrent polymerization and crystallization which compromises the ordered structure. Here, the authors propose a two-step phase switching strategy for assembling compact and highly crystalline COF membranes decoupling the polymerization and the crystallization process.