Covalently bonded zeolitic imidazolate frameworks and polymers with enhanced compatibility in thin film nanocomposite membranes for gas separation

Mixed matrix membranes (MMMs) for gas separation show promising applications in energy and environment related fields. However, the poor compatibility between particles and polymers in MMMs is the main problem. Zeolitic imidazolate frameworks (ZIFs) as inorganic-organic hybrid porous materials have better compatibility with polymers than other inorganic particles, but interfacial defects between ZIFs and polymers still have been observed in MMMs. In this work, the interfacial compatibility in MMMs has been enhanced by building covalent bonds between amine modified ZIF-8 (NH2-ZIF-8) and polyamide in thin film nanocomposite (TFN) membranes prepared by in situ interfacial polymerization (IP). The attendance of amino groups on the surface of NH2-ZIF-8 was confirmed by both experiments and molecular simulation. The covalent bonds formed between NH2-ZIF-8 and trimesoyl chloride (TMC), the monomer in organic phase of IP, were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Compared with the TFN membrane with ZIF-8, the TFN membrane with NH2-ZIF-8 exhibits both higher CO2 permeance and higher CO2/N2 selectivity, which is mainly attributed to the enhanced interfacial compatibility and disturbed chain segment of polyamide by covalent bonds. Moreover, at high content of NH2-ZIF-8 in membranes, no interfacial crack appears between NH2-ZIF-8 and polyamide in membranes, which also confirms the excellent compatibility between NH2-ZIF-8 and polyamide. The TFN membrane with NH2-ZIF-8 shows high separation performance for CO2/N2, CO2/NO, and CO2/He gas pairs, which surpasses the Robeson's upper bounds.