Smart light-responsive hierarchical metal organic frameworks constructed mixed matrix membranes for efficient gas separation

One type of new light-responsive hierarchical metal organic framework (MOF) has been successfully prepared using Co(NO3)3·6H2O as the metal salt and 4,4′-azobenzenedicarboxylic acid as the ligand by microwave method for the first time. It is found that MOF [Co(AzDC)] exhibits a light-responsive characteristic to SO2 adsorption due to the presence of azo group from the ligand. The light-responsive hierarchical MOFs are incorporated into Matrimid® 5218 (PI) matrix to prepare mixed matrix membranes (MMMs) for gas separation application. The morphology, crystallinity, chain mobility and thermal stability of MMMs are explored. Results show that Co(AzDC) may elevate both the CO2(SO2) permeability and CO2(SO2)/N2 selectivity of the MMMs. In particular, the Co(AzDC) doped MMMs exhibit the significantly improved CO2(SO2)/N2 selectivity from 33 (123) for PI control membrane to 78 (420) for MMMs, overcoming the 2008 Robeson upper bound for CO2/N2 system. Size-sieving effect of Co(AzDC) with pore size 0.35 ​nm enhances the selectivity, while the –N=N– group from Co(AzDC) shows affinity to CO2 molecular rather than N2, also elevating selectivity of MMMs. In brief, enhanced selectivity of high-performance membrane is attributed to incorporation of Co(AzDC) particles, which displays synergistic effects both in size-sieving and CO2-philic interaction for CO2/N2 separation. Smart highly selective interface is constructed in MMMs by switching the configuration of MOFs from cis to trans. The SO2 permeability and SO2/N2 selectivity of MMMs are investigated under both visible light and ultraviolet light states, and the SO2/N2 separation performance under visible light is notably improved in comparison with that under ultraviolet light state.