Adsorption of CO2 can be improved using the hard structure of ZIF-90 via sequential reduction using NaBH4 and lithiation reaction (i.e., formation of ZIF-91-OLi) . ZIFs materials with high rigid structures ensure that the pore size will be preserved without change during the fabrication process . The rigid membranes can be achieved through post-synthetic modification and functionalization with imine and APTES . However, it is essential to note that there is no diffusive motion for CO2 and CH4 in rigid ZIF-8 . MMM with tiny defects in ZIF-8 enhanced the CO2/N2 separation .