Low-Pressure CO₂ Capture in Zeolite Imidazole Frameworks with Ultramicropores Studied by Positron Annihilation

In this paper, three kinds of zeolite imidazole frameworks (ZIF-90, ZIF-93, and ZIF-94) were synthesized through hydrothermal reactions. Positron lifetime results reveal two kinds of pores corresponding to the channel in the windows and central cavity of ZIFs. The average diameters of the cavities are 1.12, 1.54, and 0.80 nm in ZIF-90, ZIF-93, and ZIF-94, respectively, while the average diameters of channels are between 0.55 and 0.60 nm. CO2 adsorption results show that ZIF-90 has the highest CO2 adsorption capacities (64.4 cm3 g–1 at 298 K and 104.7 cm3 g–1 at 273 K) at 1 bar, followed by ZIF-94 (57.9 cm3 g–1 at 298 K and 79.4 cm3 g–1 at 273 K) and ZIF-93 (41.5 cm3 g–1 at 298 K and 62.5 cm3 g–1 at 273 K). This shows inconsistency with the total pore volume but is in good agreement with relative volumes of pores smaller than 1.2 nm, which are 6.55, 2.06, and 5.60% for ZIF-90, ZIF-93, and ZIF-94, respectively. Further investigation reveals that an even smaller pore size is required for CO2 adsorption under ultralow pressure. At a pressure of 0.1 bar, ZIF-94 exhibits an ultrahigh CO2 adsorption capacity (21.2 cm3 g–1 at 298 K and 32.1 cm3 g–1 at 273 K) compared with ZIF-93 (10.1 cm3 g–1 at 298 K and 14.9 cm3 g–1 at 273 K) and ZIF-90 (5.9 cm3 g–1 at 298 K and 11.6 cm3 g–1 at 273 K). These quantities coincide well with the relative volumes of pores smaller than 0.8 nm, which are 5.60, 2.06, and 1.18% for ZIF-94, ZIF-93, and ZIF-90, respectively. Our results demonstrate that at atmospheric pressure, CO2 molecules are more effectively captured by micropores, while at extremely low pressures, CO2 molecules are mainly enriched around ultramicropores such as the channels and small cavities in ZIFs.