Carbon nanotubes grown on ZIF-L(Zn@Co) surface improved CO2 permeability of mixed matrix membranes

Abstract

Increasing CO2 permeability via tailoring CO2 diffusivity and selective absorption represents a critical endeavor toward achieving efficient CO2 separation. Herein, hierarchical structures composed of carbon nanotubes in-situ grown on carbonized ZIF-L (Zn@Co) (C-ZIF) were developed as nanofillers in a Pebax polymer to fabricate mixed matrix membranes (MMMs) with high CO2 permeability. The high volatility of zinc ions facilitated formation of a porous matrix in ZIF-L (Zn@Co) carbonization, while cobalt ions catalyzed in-situ growth of carbon nanotubes on C-ZIF surfaces due to their high carbon solubility at elevated temperatures (≥600 °C). This hierarchically structured C-ZIF with nanotubes increased perturbation of molecular chains of polymer membrane and thus improved its gas diffusivity toward CO2, evidenced by the decreased glassy transition temperature and crystallinity of MMMs with 10% C-ZIF-700 loadings. Meanwhile, C-ZIF facilitated transformation of saturated Me–N4 to unsaturated Me–N2 and imidazole-N to pyridine-N, thus providing abundant active sites for CO2 adsorption. Taken together, a largely increased CO2 permeability (161.3 Barrer) of MMMs with 10 wt% C-ZIF-700 was observed with a CO2/N2 selectivity of 51.4, representing a 62.1% increase compared to that of pure Pebax membrane (99.5 Barrer). This study demonstrates that carbonized-ZIF with hierarchical nanostructures may offer a great potential in constructing high-performance MMMs for various gas separations.