Molecular recognition-induced structural flexibility in ZIF-71

Abstract

The adsorption performance of ZIF-71 towards two common volatile organic compounds, chlorobenzene and phenol, has been evaluated using a number of experimental techniques and Grand Canonical Monte Carlo (GCMC) simulations. Experimental results indicate that ZIF-71 can adsorb large quantities of chlorobenzene, while its adsorption performance for a similar molecule, such as phenol, is reduced by one order of magnitude. Synchrotron X-ray powder diffraction patterns confirm the presence of structural changes in ZIF-71 upon exposure to a phenol/water solution, i.e., a phase transition from the open-pore (op) ZIF-71 structure to the narrow-pore (np), highly dense, ZIF-72 phase. Although this phase transition does not occur in the presence of pure water, GCMC simulations suggest that phenol molecules adsorbed at the pore mouth and/or in structural defects may promote water accessibility to the inner core of the ZIF-71 microcrystals, thus promoting this thermodynamically favored phase transition. Inelastic neutron scattering (INS) and neutron diffraction (ND) experiments confirm the ZIF-71 to ZIF-72 phase transition, exclusively in samples exposed to a phenol/water solution. The presence of entrapped water molecules in the inner core of ZIF-71 crystals upon phenol adsorption, even after a drying step, suggests that the phase transition is likely initiated at the external surface of the ZIF-71 crystals.