Gate Opening Induced High Pore Volume Expansion in Flexible Zeolitic Imidazole Frameworks during CO₂ Adsorption: A Direct Observation Using Positron Annihilation Spectroscopy

Although Zeolitic Imidazolate Frameworks (ZIFs) show a great promise in the field of molecular separation owing to their crystalline morphology of pores, the sieving performance of ZIFs remains limited due to their flexible frameworks leading to "gate opening " phenomenon under gas pressure. However, the pore size expansion due to gate opening may ensure the use of flexible ZIFs as an efficient gas storage material at high pressure. In the present study, we have investigated the pore volume expansion in three flexible ZIFs (ZIF-8, ZIF-67, and ZIF-7) during CO2 adsorption up to high pressure similar to 60 kg cm(-2) using positron annihilation lifetime spectroscopy. Our result provides first direct in situ observation of pore volume expansion due to the gate opening in ZIF frameworks under high CO2 gas pressure. The enhancement in pore volume at all the available gas adsorption sites in these frameworks is caused due to the linker rotation and phase transformations. ZIF-8 framework flexibility was observed to be much higher as compared to its cobalt-based analogue, i.e., ZIF-67. ZIF-7 is observed to show two step pore volume expansions first in low pressure range and second in high pressure range. The low pressure transition corresponds to well-known ZIF-7(np) to ZIF-7(lp) phase transformation. The second step is observed to be associated with much higher expansion of pore volume and must be associated with a crystal structural transformation in ZIF-7. All the phase transformations observed under CO2 pressure up to 60 kg cm(-2) at 298 K in ZIF-8, ZIF-67, and ZIF-7 are observed to be fully reversible. The present study suggests that positronium is a powerful probe for the in situ investigation of the changes in pore architecture of ZIFs during gas adsorption, which plays an important role for the application of these frameworks in high level gas storage.