Fine-Tuning of the Pore Aperture and Framework Flexibility of Mixed-Metal (Zn/Co) Zeolitic Imidazolate Framework-8: An In Situ Positron Annihilation Lifetime Spectroscopy Study under CO2 Gas Pressure

The mixed-metal (Zn/Co) strategy has been used to enhancethe gasseparation selectivity of zeolitic imidazolate framework-8 (ZIF-8)-basedmembranes. The enhancement in selectivity has been attributed to possiblemodifications in the grain boundary structure, pore architecture,and flexibility of the frameworks. In the present study, we used insitu positron annihilation lifetime spectroscopy (PALS) under varyingCO(2) pressure to investigate the tuning of the pore architectureand framework flexibility of mixed-metal (Zn/Co) ZIF-8 frameworkswith varying Co contents. The random distribution of Zn and Co metalnodes within the highly crystalline frameworks having an SOD topologywas established using electron microscopy, Fourier transform infraredspectroscopy, and Raman spectroscopy. The inherent aperture as wellas cavity size of the frameworks, and the pore interconnectivity tothe outer surface, were observed to vary with the Co content in ZIF-8due to the random distribution of Zn and Co metal nodes in the frameworks.The aperture size is reduced with the incorporation of an additionalmetal (Zn or Co) in ZIF-67 or ZIF-8, respectively. The aperture sizeremains the smallest for a lower Co content (& SIM;0.20) in ZIF-8.The framework flexibility determined by in situ PALS measurementsunder CO2 pressure continuously reduces with increasingCo content in ZIF-8. A smaller aperture size as well as low flexibilityof ZIF-8 with a low Co content is seen to be directly correlated toa higher separation selectivity of membranes prepared with this mixed-metalcomposition.