Fig. 1

ZIF-7 structure and its CO₂ adsorption behavior. a The building unit, sodalite cage, of ZIF-7 with two types of six-member-ring pores (A, B) and one type of four-member-ring pores on its walls. Part of the framework is simplified by replacing Zn−bIm−Zn with Zn−Zn (bIm = benzimidazolate, C₇N₂H₅, Zn: gray). In our previous work¹⁸, we have demonstrated CO₂ molecules prefer to be adsorbed in six-member-ring pores, as shown here. In addition, there are more CO₂ molecules adsorbed in pore B than in pore A. CO₂: red. The symmetry of CO₂ has been disregarded for clarity. b CO₂ adsorption isotherms of ZIF-7 at 195 and 298 K, p_CO2 = 1–100 kPa, illustrated by the structural behaviors of ZIF-7. c The structural change of pore A in the ZIF-7-II to ZIF-7-I phase transition. Zn: gray; N: blue; C: green; H: white. d The structural change of pore B in the ZIF-7-II to ZIF-7-I phase transition. Zn: gray; N: blue; C: green; H: white. See the crystal structures of ZIF-7-I and ZIF-7-II in Supplementary Fig. 2 and Supplementary Table 3