Flexibility of mixed metal (Zn/Co) zeolitic imidazolate framework-8: an investigation using positron annihilation lifetime spectroscopy

Zeolitic Imidazolate Framework (ZIF) are porous crystalline materials with tetrahedral networks that resemble those of zeolites. Due to high thermo-chemical stability and porosity, ZIFs have many applications in gas storage and separation. The gas separation efficiency is limited due to the flexibility of their frameworks leading to diffusion of larger size molecules under applied pressure through the pore network. This phenomenon is termed as 'gate-opening'. Understanding the flexibility of ZIFs at microscopic level is of significant interest because it can explain the various novel properties of ZIFs. Positron annihilation lifetime spectroscopy (PALS) has been shown to be highly efficient in probing defects, pore architecture during gas sorption in microporous coordination polymers (MCPs) and CO₂ exposed zeolites. In the present study, in-situ PALS measurements under different gas pressures have been carried out to investigate the gas pressure induced modifications in pore architecture and flexibility of mixed metal (Co/Zn) ZIF-8. Mixed metal ZIF(Zn_0.4Co_0.6-ZIF-8) has been synthesized using metal precursor and respective ligands at room temperature. The phase purity, chemical bonding and morphology of the framework have been characterized using XRD, FTIR and SEM. Upon applying CO₂, N₂ and Ar pressures, differences in PALS spectra of Zn_0.4Co_0.6-ZIF-8 w.r.t. its vacuum state show the changes in its pore architecture. Gas pressure dependent ortho-positronium (o-Ps) annihilation parameters confirm that flexibility as well as pore architecture of the studied ZIF depends on the gas-framework interaction. The present study also confirms that o-Ps is an efficient probe to investigate the pore site specific open volume evolution during gas adsorptions in ZIFs having non-uniform pore network, which is required for complete understanding of 'gate-opening' phenomenon in flexible ZIFs at microscopic level. (author)