Crystal Size Dependent Flexibility in ZIF-7: From Macro- to Nanoscale

Flexible metal–organic frameworks (MOFs) are highly desirable materials for gas separation, but most of them become rigid when the particle size is reduced toward nanoscale. We aim to comprehend the effect of textural properties such as crystal size, its distribution, and morphology on the gate-opening behavior stimulated by the 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 μm 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 the ZIF-7 framework in the 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.