Following the unusual breathing behaviour of 17O-enriched mixed-metal (Al,Ga)-MIL-53 using NMR crystallography

摘要: The breathing behaviour of 17O-enriched (Al,Ga)-MIL-53, a terephthalate-based metal-organic framework, has been investigated using combination solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction (PXRD) and first-principles calculations. These reveal that the observed for as-made, calcined, hydrated subsequently dehydrated mixed-metal MIL-53 materials differs with composition, but cannot be described as compositionally weighted average seen two end members. Although form adopted by as-made material is independent metal upon calcination, higher levels Al adopt an open pore (OP) form, found member, substitution Ga results in mixed materials, OP narrow (NP) forms co-existing. member prone to decomposition under calcination conditions used, low level starting synthesis (5%) leads MOF stable calcination. Upon hydration, all almost exclusively closed (CP) structure, strong hydrogen bonding interactions water leading distinct resonances from carboxylate oxygens 17O NMR spectra. When dehydrated, different framework structures are members, Al-MIL-53 NP Ga-MIL-53, proportion increase systematically content contrast initial spectra show increased preference clustering like cations increases. This not result small-scale dry gel conversion reactions used enrichment, similar cation distribution also (Al0.5,Ga0.5)-MIL-53 synthesised hydrothermally enriched via post-synthetic steaming.