Flexibility of Mixed Ligand Zeolitic Imidazolate Frameworks (ZIF-7-8) under CO2 Pressure: An Investigation Using Positron Annihilation Lifetime Spectroscopy

Fine tuning of the pore architecture and flexibility of zeolitic imidazolate frameworks (ZIFs) is highly crucial for realizing their applications in molecular gas separation. Mixed ligand frameworks (ZIF-7-8) synthesized by mixing 2-methylimidazole (2meIm) and benzimidazole (bIm) ligands show enhanced gas separation performance, attributable to pore and flexibility tuning. In the present study, positron annihilation lifetime spectroscopy (PALS) measurements under CO2 pressure have been used to experimentally investigate the tuning of the pore architecture and flexibility of mixed ligand frameworks ZIF-7-8 having a ZIF-8 structure and similar morphology with varying bIm content up to 18.2%. The aperture and cavity of frameworks begin to open up with an increasing bIm ligand content followed by a decrease at a higher content. On the contrary, flexibility of the frameworks indexed from PALS measurements carried out under CO2 pressure shows a decreasing trend followed by an increase. The present study shows that mixed ligand frameworks having a larger aperture size are less flexible as a result of inherent open configurations of ligands in the framework lattice. On the other hand, frameworks having a comparatively smaller aperture size show higher flexibility as a result of a possibility of twisting of the ligands under CO2 pressure, resulting in aperture opening. The pore-opening phenomenon as a result of lattice flexibility under CO2 pressure is observed to be fully reversible for ZIF-7-8.

Automated summary

Fine tuning the pore architecture and flexibility of zeolitic imidazolate frameworks (ZIFs) is crucial for their applications in gas separation. This study investigates the tuning of pore architecture and flexibility of ZIF-7-8 frameworks with different benzimidazole (bIm) contents using positron annihilation lifetime spectroscopy (PALS) measurements under CO2 pressure. The results show that the frameworks' aperture and cavity open up with increasing bIm content but decrease at higher content. Flexibility, measured using PALS under CO2 pressure, initially decreases and then increases. The study reveals that frameworks with larger apertures are less flexible due to the open configurations of ligands in the lattice, while frameworks with smaller apertures exhibit higher flexibility due to ligand twisting under CO2 pressure. The pore-opening phenomenon induced by lattice flexibility under CO2 pressure is reversible for ZIF-7-8.