Moreover, thermal and hydrothermal stability can be improved via the post-synthetic functionalization step, which may facilitate membrane activation. A study on the porosity effect (windows and cavity) was reported using positron lifetime for ZIFs; ZIF-90, ZIF-93, and ZIF-94 . Data analysis revealed that the smaller pore size (micropore or ultramicropores) of ZIFs is optimal for CO2 adsorption under ultralow pressure . ZIFs crystals with the smaller pores (e.g., ZIF-7 and ZIF-94) showed higher CO2 adsorptions than ZIFs materials with larger pores (e.g., ZIF-11 and ZIF-93) at low pressures (<1 bar) . In contrast, the opposite is true at higher pressures, i.e., the larger-pore structures showed significantly higher adsorption . Hollow ZIF-8 containing meso/macro pores showed good adsorption of CO2 with an adsorption capacity of 1.05 mmol/g at 1 bar .