A multinuclear approach to characterising disordered (Al,Sc)-MIL-53 using solid-state NMR spectroscopy

A series of new mixed-metal (Al,Sc)-MIL-53 MOFs has been synthesised solvothermally using DMF. The materials have been characterised using a combination of solid-state NMR spectroscopy, X-ray diffraction, electron microscopy, and energy dispersive X-ray spectroscopy. The breathing behaviour was followed using ¹³C MAS NMR spectroscopy and showed that the mixed-metal MOFs exhibited different pore forms to the two end members. The cation distribution in the framework was investigated using ¹⁷O NMR spectroscopy. The low natural abundance of ¹⁷O (0.038 %) requires isotopic enrichment for spectra to be acquired on a reasonable timescale, but the high cost of enriched reagents necessitates a cost-effective and atom-efficient approach. Materials were enriched using post-synthetic steaming and the cation distribution in the enriched frameworks was shown to be close to random. Evidence for framework degradation and the formation of impurities (an alumina-based material and the small-pore MOF Sc₂(BDC)₃) upon any heating (i.e., during calcination, dehydration or enrichment) was provided from ²⁷Al and ⁴⁵Sc NMR spectroscopy, leading to the enriched mixed-metal frameworks having very similar compositions (with 70–77 % Al present). Further work would be required to overcome some of the synthetic challenges encountered, but the possibility of tailoring the breathing response of a mixed-metal MOF in the presence of a guest molecule or with changes in external conditions offers interesting possibilities for the future.