g-C₃N₄ nanosheets with tunable affinity and sieving effect endowing polymeric membranes with enhanced CO₂ capture property

Two-dimensional (2D) materials have emerged as excellent nano-building blocks for high performance membranes. Among them, graphitic carbon nitride (g-C₃N₄) nanosheets possess intrinsic nanopores in plane and CO₂-philic property from the rich amine groups, which are in favor of selective transport of CO₂ molecules. Herein, for the first time, g-C₃N₄ nanosheets with tunable CO₂ adsorption properties and nanostructures were synthesized and incorporated into polyether block amide (Pebax) membrane for CO₂ separation. The g-C₃N₄ nanosheets with variable adsorption properties were produced from two monomers, dicyandiamide and melamine, and the variation of nanostructures was controlled by thermal oxidation etching process. The effects of CO₂-philic and molecular sieving properties of g-C₃N₄ nanosheets on solubility and diffusivity of gas molecules in the as-prepared membranes were systematically investigated. The results demonstrated that the g-C₃N₄ nanosheets produced from dicyandiamide and undergoing 4 h thermal etching (DCN-4 nanosheets) showed the optimal CO₂ sorption and sieving property. The membrane with 0.25 wt% DCN-4 nanosheets exhibited simultaneous enhancement in CO₂ permeance and CO₂/N₂ selectivity compared with pure Pebax membrane. Moreover, the membrane maintained its separation performance during long-term operation test, showing great potential for CO₂ capture.