Uptake of liquid alcohols by the flexible Fe(III) metal-organic framework MIL-53 observed by time-resolved in situ X-ray diffraction

A comprehensive, time-resolved, energy-dispersive X-ray diffraction study of the uptake of liquid alcohols (methanol, ethanol, propan-1-ol and propan-2-ol) by the flexible metal-organic framework solid MIL-53(Fe)[H(2)O] is reported. In the case of the primary alcohols, a fluorinated version of the MIL-53(Fe) host (C2/c symmetry V ca. 1000 Å(3)), in which a fraction of framework hydroxides are replaced by fluoride, shows uptake of alcohols to give initially a partially expanded phase (C2/c symmetry, V ca. 1200 Å(3)) followed by an expanded form of the material (either Imcm or Pnam symmetry, V ca. 1600 Å(3)). In the case of methanol-water mixtures, the EDXRD data show that the partially open intermediate phase undergoes volume expansion during its existence, before switching to a fully open structure if concentrated methanol is used; analogous behaviour is seen if the initial guest is propan-2-ol, which then is replaced by pyridine, where a continuous shift of Bragg peaks within C2/c symmetry is observed. In contrast to the partially fluorinated materials, the purely hydroxylated host materials show little tendency to stabilise partially open forms of MIL-53(Fe) with primary alcohols and the kinetics of guest introduction are markedly slower without the framework fluorination: this is exemplified by the exchange of water by propan-2-ol, where a partially open C2/c phase is formed in a step-wise manner. Our study defines the various possible pathways of liquid-phase uptake of molecular guests by flexible solid MIL-53(Fe).