Effects of Linker and Metal Exchange on Gate Opening of Zeolitic Imidazolate Framework-8: A Combined Gas Adsorption and Positron Annihilation Spectroscopy Study.

Zeolitic imidazolate framework-8 (ZIF-8) exhibits gate opening under N2 pressure at 77 K leading to adsorption of a larger amount of N2 gas as compared to its inherent pore volume. Gate opening in ZIF-8 is attributed to the twisting of the linker at a certain gas pressure leading to a reversible phase transformation from the ambient-pressure phase to the high-pressure phase. Controlling gate opening in ZIF-8 is essential because it leads to diffusion of larger gas molecules through the framework, resulting in poor gas separation selectivity. Metal and linker exchange in the ZIF-8 framework has been shown to affect the gas separation selectivity, which could be attributed to plausible modifications in the gate opening mechanism. We have investigated the gate opening mechanism of Co- and benzimidazole (bIm)-exchanged ZIF-8 using N2 adsorption and in situ positron annihilation lifetime spectroscopy (PALS) under N2 pressure complemented with theoretical calculations. N2 adsorption measurements show that the characteristic double step in the isotherm, a signature of gate opening, occurs at a relatively higher pressure for Co-exchanged ZIF-8 and remains absent in bIm-exchanged ZIF-8. These observations are ascribed to the higher energy barrier imposed on linker twisting by the stronger Co-linker bond and bulkier bIm linker. Theoretically optimized structures of the hybrid frameworks under N2 pressure support these explanations. In situ PALS measurements under N2 pressure at room temperature showed that the gas pressure-dependent pore size variations are fully consistent with the gate opening characteristics of the hybrid ZIF-8. The present study suggests that the changes in the gas separation selectivity of metal- or linker-exchanged ZIF-8 are directly linked to the modifications in the gate opening characteristics. The present study provides experimental proof that these strategies can be utilized to further enhance the gas separation selectivity of the ZIF-8-based membranes.