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 −2 h −1 with a separation factor of 3876 for n-butanol dehydration, and high permeance of 42.8 L m −2 h −1 bar −1 with a rejection of 96.8% for Na 2 SO 4 removal. Our mixed-dimensional design may inspire the fabrication and application of COF membranes.

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Ultrathin heterostructured covalent organic framework membranes were fabricated through vapor–liquid interfacial synthesis for efficient liquid separations.