Uptake of Liquid Alcohols by the Flexible FeIII Metal-Organic Framework MIL-53 Observed by Time-Resolved In Situ X-ray Diffraction

• Published in: Chemistry : a European journal Vol. 17; no. 25; pp. 7069 - 7079
• Main Authors: Walton, Richard I., Munn, Alexis S., Guillou, Nathalie, Millange, Franck
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 14-06-2011; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chemistry; metal-organic frameworks; microporous materials; solid-state structures; X-ray diffraction; zeolite analogues
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201003634

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)[H2O] is reported. In the case of the primary alcohols, a fluorinated version of the MIL‐53(Fe) host (C2/c symmetry V ca. 1000 Å3), 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 Å3) followed by an expanded form of the material (either Imcm or Pnam symmetry, V ca. 1600 Å3). 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).
Breathing under control: The expansion of flexible metal–organic frameworks in the presence of simple aliphatic alcohols is observed in real time by in situ powder X‐ray diffraction (see figure); this shows how metastable partially expanded phases may be isolated by fluorination of the metal–organic framework.