The porosity of ZIFs is a key parameter in gas adsorption. The substrates shouldn't block the porosity of ZIFs crystals. The pore aperture size of ZIFs crystals can be modified using a postsynthetic modification to increase the gas separation performance. Post-synthetic procedures can constrict the pore aperture size, reduce intercrystalline defects, increase the crystal's homogenous distribution inside the membrane, or create active adsorption sites. It is crucial to balance reducing the pore aperture size and the permeability of the gases. Small pore size apertures significantly decrease the gas permeability. On the other side, selectivity is usually enhanced by the pore aperture size reduction and the reduction of grain boundary defects. Consequently, non-selective transport pathways and the ''gate opening effect'' can be prevented.