Covalent organic frameworks based membranes for separation of azeotropic solvent mixtures by pervaporation

Abstract

Pervaporation of organic-organic mixtures has shown great potential as a replacement for conventional distillation processes, particularly when dealing with azeotropes and liquids with similar boiling points that are difficult to separate. However, the lack of high-performing membranes that can consistently provide reliable selectivity and high production rates has been a major hurdle in this field. Here, we used an in-situ growth method to prepare two prototypes of imine-linked COFs, i.e., COF-LZU1 and COF-300 membranes for separating azeotropic mixtures of polar/non-polar solvent, i.e., acetone/n-hexane, acetone/cyclohexane and methanol/benzene. The composite membranes comprising a continuous and defect-free COF layer on a ceramic hollow fibre substrate demonstrate high solvent fluxes of acetone and methanol, up to 20.0 and 4.9 kg m−2h−1, respectively. The preferential permeation of polar solvents through polar imine-linked nanochannels leads to a high separation factor of the studied mixtures: 30 for acetone/n-hexane, 13.4 for acetone/cyclohexane and 4.3 for methanol/benzene systems. Combined with good stability in solvent systems, our results have proved a technical feasibility of using COF-based membranes for separation of azeotropic solvent mixtures.