Interaction of methanol with the flexible metal-organic framework MIL-53(Fe) observed by inelastic neutron scattering

- A spectroscopic signature of guest-induced swelling of a flexible MOF has been identified.
- Librations of the linker contribute to pore opening of the MOF structure.
- Complementary in situ powder XRD shows changes in long-range order during guest uptake.

In situ powder X-ray diffraction and thermogravimetry show that the iron(III) carboxylate material MIL-53(Fe) expands in two steps upon exposure to methanol vapour to give a 'half open' phase and then a fully expanded structure; these correspond to the uptake of one and two molar equivalents of methanol, respectively. This knowledge allows interpretation of inelastic neutron scattering (INS) spectra, measured using TOSCA at ISIS, of the solid loaded with various amounts of water, methanol or d3-methanol (CD3OH), aided also by DFT simulation of INS spectra of the analogous MIL-53(Al) structure. The key signature of the expansion and contraction of the structure is the shift of bands in the 400 cm−1 region: these are assigned as due to the librations of the terephthalate rings, involving motions that result in distortion of the aromatic ring and a rocking motion of the aromatic ring about the bonds to the carboxylate groups.