Effect of Dehydration on the Local Structure of Framework Aluminum Atoms in Mixed Linker MIL-53(Al) Materials Studied by Solid-State NMR Spectroscopy

The present study features H-1 and Al-27 MAS NMR spectroscopic investigations on mixed ligand metal-organic frameworks (MOFs) of MIL-53(Al) type with benzene-1,4-dicarboxylate (BDC) and 2-aminobenzene-1,4-dicarboxylate (ABDC) linkers. The excellent resolution of the H-1 spectra allowed an elegant and facile quantitative analysis of the organic linkers using solid-state NMR. The actual molar fraction of ABDC in the dehydrated mixed linker MOFs was determined by evaluating the intensity of the -NH2 single at 5.6 ppm. The incorporation of amine groups led to higher field shifts of the corner-sharing AlOH signals and a more homogeneous charge distribution in the local structure of framework aluminum atoms corresponding to a decrease of the quadrupole coupling constant by similar to 1 MHz compared to that of aluminum coordinated to BDC. Upon rehydration, the local structures of the framework aluminum atoms exhibited a much lower symmetry, as indicated by an increase of the Al-27 quadrupole coupling constant by up to 3 MHz.