Dispersion and Acidity of Niobia on Nb2O5/γ-Al2O3

Supported niobium pentoxide materials are effective catalysts for a variety of reactions. Nb2O5/gamma-Al2O3, catalysts with different Nb2O5 loadings were prepared by aqueous solution impregnation using niobium oxalate as a precursor on gamma-Al2O3. The samples were characterized with respect to the dispersion state of the niobium oxide species on gamma-Al2O3 by X-ray power diffraction (XRD) and laser Raman spectroscopy (LRS). The nature of the surface acidity was investigated using Fourier-transform infrared spectroscopy of pyridine adsorption (Py-IR). The catalytic activity of the as-prepared catalysts was evaluated by the condensation reaction of iso-butene (IB) and iso-butyraldehyde (IBA) to form 2,5-dimethyl-2,4-hexadiene (DMHD). Results reveal that the dispersion capacity (Gamma(Nb)) of Nb on gamma-Al2O3 is about 7.6 mu mol.m(-2). This value is almost identical to the density of the octahedral vacant sites of the preferentially exposed (110) plane (7.5 mu mol.m(-2) on the surface of the gamma-Al2O3 support. Additionally, the "incorporated model" suggests that Nb5+ cations are located on the vacant sites of the (110) plane on gamma-Al2O3. These results suggest that isolated niobia (NbOx) species are present and are bound to the surface of the gamma-Al2O3 support through Nb-O-Al bonds at a loading well below that corresponding to monolayer dispersion. This is consistent with the result from LRS. The formation of isolated NbOx species, which binds to the surface of the support through Nb-O-Al bonds, causes a decrease in the amount of surface Lewis acid sites (LAS) on the Nb2O5/gamma-Al2O3 catalysts. With an increase in Nb2O5 loading, polymeric NbOx species arc formed by the Nb-O-Nb bridging of neighboring isolated NbOx species and Bronsted acid sites (BAS) are generated. We found that the catalytic activity towards the condensation reaction of IB and IBA to form DMHD increased because the amount and strength of the Bronsted acid sites increased as the number of polymeric niobia species increased. When the loading exceeds the monolayer dispersion capacity, the catalytic activity (turnover frequency (TOF) of DMHD) decreased because of the formation of the three-dimensional NbOx species. Additionally, the selectivity of DMHD decreased because of an increase in the strength of the Br6nsted acid sites. We suggest that the strength of the Bronsted acid sites are related to the state of NbOx on the surface of the Nb2O5/gamma-Al2O3 catalysts.