No 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.

Flexible metal-organic frameworks (MOFs) are highly desirable materials for gas separation but most of them become rigid when the particle size is reduced towards nanoscale. We aim to comprehend the effect of textural properties such as crystal size, its distribution and morphology on the gate-opening behaviour stimulated by adsorption of guest molecules in ZIF-7. The synthesis conditions are varied to obtain ZIF-7 batches with crystal sizes ranging between 0.05 and 15 m with various size distributions. We report for the first time a CO2-filled open pore phase of ZIF-7 at 195 K (OP2) derived from in situ powder X-ray diffraction (PXRD) data measured in parallel to CO2 physisorption. The adsorption of CO2 on ZIF-7 indicates persisting flexibility for all particle size regimes; with the crystal size, its distribution and morphology having a significant impact on both gate-opening and gate-closing pressures and slope of CO2 adsorption isotherms. 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 signifies the importance of intermolecular interactions to overcome the energy barrier for linker movement/gating of the framework.