Homogeneous Polymerization of Self-standing Covalent Organic Framework Films with High Performance in Molecular Separation

Abstract

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.

Polycrystalline Covalent Organic Framework Films Act as Adsorbents, Not Membranes

“…Organic dyes reported to be rejected by COF-based nanofiltration membranes adsorb onto polycrystalline COF powders with high affinities, whereas dyes reported to pass through COF membranes do not adsorb strongly. For example, an unfunctionalized, imine-linked COF powder (TAPB-PDA, Scheme ) was introduced into separate aqueous solutions of six organic dyes (Scheme ) and mixed using an orbital shaker for 72 h. The dyes were selected because they were used in prior reports of COF membrane separations and span broad ranges of size, charge, and chemical structure. ,,,− , The set of dyes included larger molecules that did not pass through the COF membranes (Rhodamine B [ RB ], Brilliant Blue G [ BB ]) , and smaller molecules that were passed by the membranes (4-nitrophenol [ NP ], 4-nitroaniline [ NA ]). ,, Despite being similar in size to those of the rejected dyes, Methylene Blue ( MB ) and Congo Red ( CR ) have demonstrated variable transport through different COF films and were also selected. ,,,− , Visual inspection of the dye solutions after being in contact with the COF powder for 72 h indicated that the dyes adsorb to varying degrees (Figure S1): RB and BB adsorb strongly to the COF powder, leaving nearly colorless aqueous solutions above the solid COF. In contrast, almost no color change was observed for NP and NA -containing solutions under identical conditions.…”

“…Similar trends in capacity and affinity were also found for BND-TFB and BND-TFP COFs. To date, many so-called “membrane” experiments have been conducted by passing dilute dye solutions (≤100 mg L –1 ) through thick COF films. ,,, At these low concentrations, dyes with high association constants will almost completely adsorb to the COF, while those with low association constants will largely remain in solution, assuming that the COF is not near saturation. Given that polycrystalline COFs have uniformly high uptake capacities, and rejection performance is often characterized using small (<15 mL) volumes of dye solution, ,,, it is reasonable to expect that adsorption could play a dominant role in controlling the observed separation performance of these systems.…”

“…Covalent organic frameworks (COFs) are permanently porous, crystalline polymers with predictable structures obtained by reacting geometrically constrained nodes and linkers to form networks of well-defined pores. − Changes in the monomer structure enable tuning of the pore size, topology, dimensionality, and linkage chemistry, , offering a rational means to impart emergent properties of interest for energy storage, − catalysis, , electronics, − sensing, , gas storage, and molecular separations. − Of the many proposed applications for COFs, rationally designed membranes with uniform porosity are among the most promising. , So far, tested COF separation membranes vary in material quality and can be divided into two categories. The first are thin films (<500 nm) with little or no apparent crystallinity, which are obtained by polymerizing monomers at liquid–liquid, liquid–solid, or liquid–air interfaces. ,− These films exhibit low water permeability (typically <50 L m –1 bar –1 h –1 ) and have shown separations of dyes ,,,− and salt solutions. ,− , The second category of COF membranes are obtained as thick films or solids using typical solvothermal or interfacial polymerization methods and are effectively polycrystalline powders fused into arbitrary shapes. ,− These forms often exhibit higher water permeability (>50 L m –1 bar –1 h –1 ) and have been used to separate organic dyes, ,− , proteins, bacteria, salts, and nanoparticles . Th...…”

“…The first are thin films (<500 nm) with little or no apparent crystallinity, which are obtained by polymerizing monomers at liquid–liquid, liquid–solid, or liquid–air interfaces. ,− These films exhibit low water permeability (typically <50 L m –1 bar –1 h –1 ) and have shown separations of dyes ,,,− and salt solutions. ,− , The second category of COF membranes are obtained as thick films or solids using typical solvothermal or interfacial polymerization methods and are effectively polycrystalline powders fused into arbitrary shapes. ,− These forms often exhibit higher water permeability (>50 L m –1 bar –1 h –1 ) and have been used to separate organic dyes, ,− , proteins, bacteria, salts, and nanoparticles . The separation capabilities of both categories of COF membranes have been attributed to a size-based sieving mechanism through the micropores or mesopores defined by the COF lattice (see Tables S1 and S2 for an overview of reported COF membranes and transport properties). ,,− …”

“…In the case of amorphous thin films, ,− the expected periodicity is ambiguous or not present. For the thicker polycrystalline solids, ,− which display some degree of crystalline ordering, claims of size-based sieving are similarly difficult to rationalize. These materials are comprised of unoriented nanoscale crystalline domains.…”