Regulation of CO2 Pressure-Induced Flexibility of Zeolitic Imidazolate Framework-7 Using a Mixed Linker Strategy: An In Situ Positron Annihilation Spectroscopy Study

Abstract

The external stimuli-induced framework flexibility of zeolitic imidazolate frameworks (ZIFs) is a fascinating physical phenomenon. Flexible ZIFs offer varying pore aperture under gas pressures leading to inferior separation selectivity because the flexibility-induced enlargement in the aperture size allows diffusion of larger size gas molecules. Desolvated ZIF-7 crystals show high flexibility and phase transformation from the narrow pore (np) to the open pore (op) phase under CO₂ pressure. Regulation of the gas-framework-induced flexibility of ZIF-7 is of paramount importance for enhancing its selectivity for gas separation and storage. Herein, we have exchanged the benzimidazole linker of ZIF-7 with varying amount (up to 62%) of 4,5-dichloroimidazole (dcIm), maintaining the sodalite topology and intracrystalline porosity. As a result of loading of the halogenated linker, the op phase of ZIF-7 can be obtained at room temperature in a desolvated form, which is otherwise exhibited only by the solvated ZIF-7. Using positron annihilation lifetime spectroscopy (PALS), it has been established that the pore architecture is significantly varied depending on the extent of linker mixing. The framework flexibility of ZIF-7 is determined by indexing the vacant volume available at the pore sites under the increasing CO₂ pressure using in situ PALS. The flexibility of the mixed linker framework indexed through vacant volume evolution at pore sites under CO₂ pressure is observed to be drastically reduced as compared to that of ZIF-7 due to the presence of the halogenated linker in the framework. The present study confirms that the pore architecture and flexibility of ZIF-7 can be efficiently regulated by incorporating varying amounts of dcIm in ZIF-7 frameworks.