Effect of Impregnation on the Structure of Niobium Oxide/Alumina Catalysts Studied by Multinuclear Solid-State NMR, FTIR, and Quantum Chemical Calculations

Multinuclear solid-state ¹H, ²⁷Al, and ⁹³Nb NMR experiments and DFT calculations were carried out for structural characterization of alumina-supported niobium oxide catalysts with high niobium content following an every stage in the catalyst preparation. It was found that the first stage of the impregnation procedure plays a key role in determining the catalyst structure and acidity. In order to monitor the presence in catalysts of aluminum niobate phase, AlNbO₄, a series of ²⁷Al and ⁹³Nb NMR experiments was performed for several different individual AlNbO₄ samples. Aluminum and niobium NMR parameters were determined for AlNbO₄, which crystal structure contains two different crystallographic sites for each element. The compound was investigated through a combination of experimental ⁹³Nb and ²⁷Al NMR spectroscopy methods at several magnetic field strengths (9.4, 11.7, 19.4, and 21.1 T) and complemented by ab initio quantum chemical calculations of NMR parameters for these nuclei. The chemical shielding and the quadrupole coupling tensor parameters were determined for both ⁹³Nb and ²⁷Al