Dissecting the effects of water guest adsorption and framework breathing on the AlO4(OH)2 centres of metal-organic framework MIL-53 (Al) by solid state NMR and structural analysis

The relationship between the adsorption of water on MIL-53 (Al) MOF, the structural phase of MIL-53 (Al), and the quadrupole coupling constant of Al-27 framework aluminium atom (QCC) of the MOF AlO4(OH)(2) centres (Al-sites) has been investigated by combining solid-state Al-27 MAS NMR spectroscopy with XRD analysis and DFT calculations. It is established that Al-27 QCC is primarily sensitive to water adsorption to the Al-sites and by a minor extent to the framework contraction/expansion interconversions. We thus conclude that the Al-27 MAS NMR method is sensitive enough to differentiate the effects of pore contractions and water adsorption to Al-sites basing on the changes of the QCC value.