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

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 () to the open pore () phase under CO 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 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 CO pressure using in situ PALS. The flexibility of the mixed linker framework indexed through vacant volume evolution at pore sites under CO 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.