We report a new, simple and versatile method to obtain highly active MOF structures by carefully controlled postsynthesis thermal annealing. The active ZIF-8 structure shows highly enhanced CO 2 /N 2 selectivity and stable cyclic CO 2 uptake of ≥1.5 mmol g -1 at 1 bar and 25 °C with a heat of adsorption of ≥30 kJ mol -1 , which is over 100% greater than the ≈0.7 mmol g -1 and ≈17 kJ mol -1 , respectively in the ZIF-8.Adsorption and separation of gaseous molecules by porous solids is a critical issue in catalysis, sensing, energy storage, CO 2 capture and pollutant removal. 1 In particular, nanoporous solids such as zeolites, carbons and metal-organic frameworks (MOFs) are extensively investigated for clean energy (H 2 and CH 4 ) storage, and separation and capture of pollutant gases (CO 2 , H 2 S, NO x , SO x , I 2 , etc.). 1,2 ZIFs (zeolitic imidazolate frameworks) are a sub-family of MOFs with zeolite-like framework topology, formed from transition metals and imidazolate linkers, and well-known for their remarkable physical, chemical and thermal stability. 3,4 Among those, a prototypical ZIF-8 (Zn[MeIM] 2 = ZnC 8 H 10 N 4 ; MeIM = methylimidazolate) is one of the most studied. Its high BET specific surface area (SSA) (≈ 2000 m 2 g -1 ) and permanent porosity from its uniformly sized pore cavities (of ≈ 1.16 nm and pore volume of ≈ 0.60 cm 3 g -1 ) are particularly desirable for many potential applications, such as molecular gas storage, 3,5 separation by membrane sieving 6 or kinetic diffusion, 7 caging, 8 templating, 9 catalysis, 8-10 and shape-selective distillation 11 and sensing. 12 A large quantity of ZIF-8 can be readily synthesized by simple mixing of precursors of zinc nitrate/acetate and 2methylimidazole in a water or methanol solvent at room temperature. 13 It is also commercially available from Sigma-Aldrich.