Hydrogen-bonding ability induces structural flexibility in a zeolitic-imidazolate framework (ZIF-71)

Abstract

The structural flexibility of ZIF-71 has been evaluated using a series of probe molecules with different chemical characteristics. Liquid-phase adsorption processes in aqueous media confirm that the studied analytes (aniline, phenol, benzoic acid, benzaldehyde, 4-fluoroaniline, toluene, fluorobenzene and chlorobenzene) can be classified into two groups, i.e., those with high accessibility (uptake above 200 mg g⁻¹) and those with limited accessibility (approximately 10–20 mg g⁻¹, i.e., one order of magnitude lower). Synchrotron X-ray powder diffraction measurements reveal that the limited uptake for certain probes is attributed to a phase transition, from the open-pore (op), ZIF-71 phase, to the narrow-pore (np), ZIF-72 phase. Interestingly, this irreversible phase transition is exclusively promoted by probes containing highly polar functional groups. Monte Carlo simulations suggest that the observed behavior cannot be fully justified by intermolecular interactions and/or water–probe interactions, but is instead driven by the hydrogen-bonding ability of the analytes. In other words, the phase transition occurs exclusively in the presence of analytes possessing strong hydrogen-bonding ability. These hydrogen-bonding interactions appear to facilitate water penetration into the otherwise hydrophobic inner cavities of ZIF-71, thereby accelerating the kinetically controlled transition from ZIF-71 (open-pore) to ZIF-72 (narrow-pore).