Preparation And Properties Of Covalent Organic Framework Nanofiltration Membrane

Nanofiltration is a pressure-driven membrane separation technology that offers a cost-effective and energy-efficient way for purifying and recycling wastewater and waste organic solvents.To enable wider deployment of nanofiltration technology,high-permeability,high-selectivity,and high-stability membranes are required to enhance the separation efficiency.Two-dimensional covalent organic framework（COF）is an emerging crystalline organic porous material with high porosity,uniform pore size,and excellent solvent-resistant properties that hold great potential to fabricate high-performance nanofiltration membrane.However,COF materials are usually synthesized as insoluble and infusible microcrystalline powders,bearing poor processability and membrane-forming properties.It remains a critical issue and a grand challenge to fabricate defect-free COF membranes.In this study,the three fabrication steps of COF membrane involving reaction,crystallization,and membrane formation were analyzed.By decoupling or synergistically optimizing the three steps,four approaches were proposed to fabricate COF membrane without defects,which exhibited a remarkable improvement in nanofiltration performance.The details are summarized as follows:“Reaction-crystallization”first and“membrane formation”after:（i）For the step of membrane-forming,a mixed nanosheet assembly approach was proposed based on the vacuum-assisted self-assembly method.Graphene oxide（GO）nanosheets with a high aspect ratio were used to assist the up-down stacking of COF nanosheets,leading to the formation of defect-free GO-CTF membrane with an ultrathin thickness of only～32 nm,which can provide ultra-short channels for molecule transport.The resulting membranes were applied in separating organic dyes from water,which exhibit a remarkable improvement in the permeance of water and rejection of various dyes.Particularly,the water permeance and Eriochrome black T rejection was improved to226 L m^-2h^-1 bar^-1and 99%,respectively.（ii）For the step of reaction-crystallization,a phase-transfer polymerization process was developed to synthesize COF nanosheets with high crystallinity and high aspect ratio morphology of up to 2000.Through vacuum-assisted self-assembly method,the obtained COF nanosheets were assembled into a defect-free COF membrane with high crystallinity and crystallographic orientation,which can provide well-defined one-dimensional channels for molecule transport.The resulting membranes were applied in separating organic dyes from organic solvent,which exhibit a remarkable improvement in the rejection of various dyes and permeance of organic solvents including apolar,polar protic,and polar aprotic ones.Particularly,the ethanol permeance and Evans blue rejection was improved to 58L m^-2h^-1 bar^-1and 100%,respectively.“Reaction-crystallization-membrane formation”:（i）A photo-tailored crystallinity strategy of COF membrane was proposed based on the reaction of photo-induced isomerization of Schiff base.The crystallinity of the whole COF membrane,fabricated by interfacial polymerization method,was adjusted by adjusting the intensity of photo irradiation.By well coordinating membrane crystallinity and membrane-forming properties,a defect-free COF membrane with sufficient crystallinity and ultrathin thickness of～47 nm was fabricated,providing ultra-short channels for molecule transport.（ii）A concept of heterocrystalline COF membrane was proposed,which can be fabricated based on the photo-tailored crystallinity approach by a two-step procedure,i.e.,dark reaction for the construction of high-crystalline regions with crystalline porous structure followed by photo reaction for the construction of low-crystalline regions,thus linking the high-crystalline regions tightly and flexibly.A defect-free membrane with uniform pore size and ultrahigh porosity of up to 1443 m² g^-1 was fabricated,which is the most porous membrane among the reported nanofiltration membranes,offering high-density channels for molecule transport.The resulting membranes were applied in separating organic dyes from organic solvent,which exhibit a remarkable improvement in the rejection of various dyes and permeance of organic solvents including apolar,polar protic,and polar aprotic ones.Particularly,the ethanol permeance and Evans blue rejection was improved to 81 L m^-2h^-1 bar^-1and 99%,respectively.