Insights into Crystal Size and Synthesis Dependent Guest-Induced Flexibility of ZIF-7 Pore Structure

Abstract

Nano-sized and flexible metal-organic frameworks (MOFs) are highly desirable materials for gas separation. However, the impact of synthesis conditions and crystal size on guest induced structural flexibility in MOFs is not well-understood. We aim to comprehend the effect of crystal size and synthesis conditions on the gate-opening behaviour of ZIF-7 stimulated by incorporation of guest molecules. The synthesis conditions are varied to obtain crystal sizes ranging between 0.05 and 15 m with varying homogeneity. The adsorption of carbon dioxide on ZIF-7 indicates the influence of crystal size and distribution, as well as synthesis conditions on the gate-opening pressure and slope of CO2 adsorption isotherm. We report for the first time an intermediate CO2 filled open pore arrangement of ZIF-7 at 195 K (OP-2) using an in-house in-situ PXRD in parallel with adsorption. In-situ PXRD measurement indicated further expansion of ZIF-7 framework in presence of methanol as guest species. The capability of ZIF-7 to accommodate molecules larger than its 0.3 nm window diameter indicates the importance of intermolecular interactions to overcome the energy barrier for linker movement/breathing of the framework.