Zn(II)-modified imidazole containing polyimide/ZIF-8 mixed matrix membranes for gas separations

Finely tailoring the interfacial interaction to minimize the defective structure in the hybrid membranes is a key to yield a mixed matrix membrane with enhanced gas separation performance. Here, a highly selective mixed matrix membrane based on imidazole containing polyimides and ZIF-8 fillers is reported. The ZIF-8 with imidazole linker offers a more compatible interface with imidazole containing polyimides. At a MOF loading of 20 wt. %, the membranes have a 2.2-fold increase in the gas permeabilities over unfilled polymers with exceptional H-2/CH4 and CO2/CH4 gas selectivities of 224 and 58, respectively. The H-2 and CO2 gas permeabilities are 78.5 and 20.3 Barrer, respectively. The Zn2+ post-modification enables the formation of metal coordination crosslinking with enhanced polymer/ZIF-8 interaction, as indicated by increased glass transition temperature, improved thermal stability and insoluble properties in organic solvents. With the proper control of Zn2+ treatment conditions, gas permeabilities of H-2 and CO2 can be increased to 110.1 and 27.4 Barrer, respectively, with the constant H-2/CH4 and CO2/CH4 gas selectivity. For membranes treated at higher Zn2+ concentration, an enhancement in H-2/CH4 gas selectivity was observed with the selectivity increasing from 223.9 to 318.3 and H-2 permeability around 72.3 Barrer. The separation performance of H-2/CH4 for all Zn2+ modified membranes exceeds the 2008 Robeson upper bound. This facile approach to tune polymer/MOF interaction via metal ion modification promotes the rational design of high-performance gas separation membranes.