Covalent organic frameworks (COFs) are typically isolated as microcrystalline powders. It remains fundamentally challenging to fabricate COFs into high-quality self-standing films to take full advantage of their ordered pore channels for molecular separation. Here, we report a new strategy for fabricating self-standing imine-linked COF films via homogeneous polymerization where films emerge from clear solutions without forming amorphous precipitates. The abundant basic nitrogen atoms of the monomers acted as a reaction controller to realize the homogeneous polymerization and also promoted the tight self-aggregation of COF crystallites to form compact films via H-bonding. The chemically supported self-standing COF films on nylon membranes were also developed via an in situ growth method. The resulting films showed an unprecedentedly ultrafast permeance of 2822 L m–2 h–1 MPa–1 with a high rejection rate (99.8%) in the filtration of a congo red (CR) solution, demonstrating the advantage of this new strategy in fabricating high-quality self-standing COF films.

No abstract

Covalent organic frameworks (COFs) have emerged as highly promising membrane materials owing to their intrinsic, tunable, and uniform pores. Due to their poor processing ability, the grand challenge for fabricating a COF film is to assemble COF crystallites into a membrane form. In addition, low crystallinity remains one of the major concerns for COF films. Herein, we report a “homogeneous–floating–concentrating” strategy for synthesizing highly crystalline self-standing COF films under mild conditions. This strategy takes advantage of a homogeneous synthesis and utilizes the slow concentration of a homogeneous reaction solution by solvent extraction and osmotic pressure to motivate the condensation reaction and push the assembly of COF crystallites to form compact films. Highly crystalline COFs were formed from the beginning of the reaction, overcoming the time mismatch between polymerization and crystallization. This strategy produced a series of self-standing COF films with superior crystallinity and porosity, better than corresponding COF films or powders reported in the literature. We synthesized a COF film with an unprecedentedly high surface area (3251 m2 g–1). These COF films exhibited outstanding permeance both for organic solvent and dye solutions along with excellent selectivity. A prominent instance is the TBDH film, which displays superior rejection (>99.9%) to Congo red solution, and unprecedented acetone and hexane permeances of 8939 and 9889 L m–2 h–1 MPa–1, respectively.

No abstract

No abstract