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 <sup>1</sup>H, <sup>27</sup>Al, and <sup>93</sup>Nb 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, AlNbO<sub>4</sub>, a series of <sup>27</sup>Al and <sup>93</sup>Nb NMR experiments was performed for several different individual
AlNbO<sub>4</sub> samples. Aluminum and niobium NMR parameters were
determined for AlNbO<sub>4</sub>, which crystal structure contains
two different crystallographic sites for each element. The compound
was investigated through a combination of experimental <sup>93</sup>Nb and <sup>27</sup>Al 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 <sup>93</sup>Nb and <sup>27</sup>Al