The potential of covalent organic frameworks (COFs) for molecular separations remains unrealized because of challenges transforming nanoscale COF materials into large-area functional COF membranes. Herein, we report the synthesis of large-area (64 cm2), ultrathin (24 nm), β-ketoenamine-linked 2D COFs using a facile interfacial polymerization technique. Angstrom-level control over single-digit nanopore size (1.4-2.0 nm) is achieved by direct integration of variable-length monomers. We apply these techniques to fabricate a series of large-area 2D COF membranes with variable thicknesses, pore sizes, and supporting materials. Tunable 2D COF properties enable control over COF membrane mass transport, resulting in high solvent fluxes and sharp molecular weight cutoffs. For organic solvent nanofiltration, the 2D COF membranes demonstrate an order-of-magnitude greater permeance than the state-of-the-art commercial polymeric membrane. We apply continuum models to quantify the dominance of pore passage resistance to mass transport over pore entrance resistance. A strong linear correlation between single-digit nanopore tortuosity and 2D COF thickness enables solvent fluxes to be predicted directly from solvent viscosity and COF membrane properties. Solvent-nanopore interactions characterized by the membrane critical interfacial tension also appear to influence mass transport. The pore flow transport model is validated by predicting the flux of a 52 nm thick COF membrane.
Keywords: covalent organic frameworks; mass transport; membranes; nanopores; pore flow model.