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.