Covalent organic frameworks (COFs) hold great promise in molecular separations owing to their robust, ordered and tunable porous network structures. Currently, the pore size of COFs is usually much larger than most small molecules. Meanwhile, the weak interlamellar interaction between COF nanosheets impedes the preparation of defect-free membranes. Herein, we report a series of COF membranes through a mixed-dimensional assembly of 2D COF nanosheets and 1D cellulose nanofibers (CNFs). The pore size of 0.45–1.0 nm is acquired from the sheltering effect of CNFs, rendering membranes precise molecular sieving ability, besides the multiple interactions between COFs and CNFs elevate membrane stability. Accordingly, the membranes exhibit a flux of 8.53 kg m⁻²h⁻¹with a separation factor of 3876 for n-butanol dehydration, and high permeance of 42.8 L m⁻²h⁻¹bar⁻¹with a rejection of 96.8% for Na₂SO₄removal. Our mixed-dimensional design may inspire the fabrication and application of COF membranes.

Transition metal-based homogeneous photocatalysts offer a wealth of opportunities for organic synthesis. The most versatile ruthenium(II) and iridium(III) polypyridyl complexes, however, are among the rarest metal complexes. Moreover, immobilizing these precious catalysts for recycling is challenging as their opacity may obstruct light transmission. Recovery of homogeneous catalysts by conventional polymeric membranes is promising but limited, as the modulation of their pore structure and tolerance of polar organic solvents are challenging. Here, we report the effective recovery of homogeneous photocatalysts using covalent organic framework (COF) membranes. An array of COF membranes with tunable pore sizes and superior organic solvent resistance were prepared. Ruthenium and iridium photoredox catalysts were recycled for 10 cycles in various types of photochemical reactions, constantly achieving high catalytical performance, high recovery rates, and high permeance. We successfully recovered the photocatalysts at gram-scale. Furthermore, we demonstrated a cascade isolation of an iridium photocatalyst and purification of a small organic molecule product with COF membranes possessing different pore sizes. Our results indicate an intriguing potential to shift the paradigm of the pharmaceutical and fine chemical synthesis campaign.

Membrane-based nanofiltration has the potential to revolutionize the large-scale treatment of organic solvents in various applications. However, the widely used commercial membranes suffer from low permeability, narrow structural tunability, and limited chemical resistance. Here, we report a strategy for fabricating covalent organic framework (COF) membranes with solvent-responsive structural flexibility. The interlayer shifting of these COF membranes in polar organic solvents results in sub-nanopores with high selectivity. High rejection rates (>99%), high permeance (>15 kilogram meter ⁻² hour ⁻¹ bar ⁻¹ ), and excellent organic solvent resistance of these smart COF membranes are achieved for a diverse array of nanofiltration applications.

Facilitated transport membranes, with a uniform distribution of facilitated transport carriers, are highly desirable to reduce the separation energy barrier. Nevertheless, the disordered and random assembly of membrane building materials/blocks makes it hard to obtain the controlled uniform distribution of carriers within membranes. Covalent organic frameworks (COFs), equipped with a regular structure and tailored functionality, provide a favorable platform for the uniform distribution of carriers. Herein, we report an ion-exchange strategy to achieve a uniform distribution of facilitated transport carriers within a cationic COF membrane for efficient carbon capture. With the assistance of a small amount of flexible graphene oxide (GO), the potential defects between the junctions of cationic COF nanosheets were eliminated, and ultrathin (20–60 nm), defect-free COF membranes were obtained. By synergistically intensifying the diffusion selective and facilitated transport mechanisms within membranes, the optimized COF membrane exhibits a CO₂ permeance of 164.2 GPU with a CO₂/CH₄ selectivity of 27, both of which exceed those of pure COF membranes in most literature studies. Our findings may stimulate further research on functionalized COF membranes and other kinds of organic framework membranes with uniformly distributed carriers for molecule separation.

Covalent organic frameworks (COFs) with designable architectures and superior stability have afforded intriguing opportunities to develop advanced separation membranes. Currently, COF membranes are rarely used in molecular separations due to their relatively large pore sizes. Herein, we report a vapor–liquid interfacial synthesis approach to in situ generate an azine-linked COF layer onto a pre-assembled carboxyl-functionalized COF layer at room temperature, acquiring an ultrathin heterostructured COF membrane with narrowly distributed interfacial sub-nanopores. Attributed to the covalent linkages between the two COF layers, the narrowly distributed interfacial sub-nanopores of ∼0.39 nm within the heterostructured COF membranes were formed, which endow the membranes with distinct interfacial molecular sieving capacity. The resulting COF membrane with a thickness of 28 nm exhibits a superior separation factor of 4464 and a high permeation flux of 14.35 kg m⁻² h⁻¹ for water/n-butanol separations. The design of ultrathin heterostructured COF membranes with interfacial molecular sieving capacity may facilitate the development of framework nanomaterial based membranes.