Thermally induced in-situ growth strategy for flexible ZIF-8 composite membranes with efficient hydrogen separation

Metal-organic framework (MOF) membranes have emerged as promising candidates for hydrogen separation, but the fabrication of defect-free and flexible MOF membranes is still challenging. Here, we propose a thermally induced in-situ growth strategy to develop flexible membranes consisting of amorphous Zn-2-methylimidazole complex (a-Zn-MeIM) and crystalline zeolitic imidazolate framework-8 (c-ZIF-8) on polysulfone substrate, denoted as amorphous Zn-MeIM/crystalline ZIF-8 (a-Zn-MeIM/c-ZIF-8) membranes. During the thermal treatment of pre-sprayed mixture containing a-Zn-MeIM on the top surface of polysulfone substrate, a-Zn-MeIM is converted to c-ZIF-8. Similar coordination environments featured with Zn2+-2-methylimidazolate coordination configurations between flexible a-Zn-MeIM and rigid c-ZIF-8 provides enhanced interfacial compatibility, effectively eliminating cracks and grain boundary defects. Benefitting from both the flexibility of amorphous phase and well-defined pore structure of crystalline phase, the resulting a-Zn-MeIM/c-ZIF-8 membranes show flexibility and preferential diffusion of H2 molecules. The membrane structure can be modulated by adjusting cZIF-8 concentration of sprayed colloid and thermal treatment temperature. The optimized a-Zn-MeIM/c-ZIF-83.75 membrane with flexibility exhibits a H2/CH4 mixed separation selectivity up to 96.9 at 90 degrees C. Additionally, a-ZnMeIM/c-ZIF-83.75 membrane with an area of 644 cm2 has been prepared, promoting the practical applications of MOF membranes with good processability.