Supported niobium pentoxide materials have been effective catalysts for a variety of acid and redox reactions (e.g., dehydration of alcohols, esterifications, etc.). To the best of our knowledge, there are no reports about Nb 2 O 5 supported on silica-alumina. Catalysts of Nb 2 O 5 /SiO 2 -Al 2 O 3 were prepared with 2, 5, 10, 15, 20, and 25 wt % of Nb 2 O 5 by aqueous solution impregnation using ammonium niobium oxalate on silica-alumina. The materials, after being dried at 100 °C, were calcined at 800 °C and characterized by several methods. Investigation through X-ray diffraction showed the typical patterns of crystalline Nb 2 O 5 , which were composed of mixtures of orthorhombic and monoclinic phases (T, M, and H, respectively) present in the materials with content higher than 10 wt %. DTA curves displayed an exothermic peak at 1356 °C (average) without mass loss (confirmed by TG), which may be ascribed to a phase transition (H phase formation) of Nb 2 O 5 supported on silica-alumina. Pure Nb 2 O 5 ‚nH 2 O showed a transition from amorphous to hexagonal or orthorhombic phase (TT or T, respectively) at 567 °C. FTIR and DRIFTS results confirmed the reaction of the niobium oxide with the hydroxyl functionality of silica-alumina. The formation of surface niobium pentoxide species over the support through selective and progressive consumption of hydroxyl groups from the support and the appearance of characteristic niobium hydroxylated species on the surface were demonstrated. Raman spectra attested a two-dimensional overlayer of niobium pentoxide on silica-alumina at contents below 10 wt %. At higher concentrations, the absorptions characterize the formation of phases T and H of bulk Nb 2 O 5 .

Biodiesel (FA esters) has become very attractive as an alternative diesel fuel owing to its environmental benefits. Transesterification is the most usual and important method to make biodiesel from vegetable oils. This article investigates the potential for using Raman spectroscopy to monitor and quantify the transesterification of soybean oil to yield ethyl esters. The differences observed in the Raman spectra of soybean oil after transesterification were a peak at 2932 cm −1 (ν CH 2 ), the displacement of the ν C=O band from 1748 to 1739 cm −1 , and the bands at 861 (ν R-C=O and ν C-C ) and 372 cm −1 (δ CO-O-C ). Uni-and multivariate analysis methods were used to build several analytical curves and then applied in known samples, treated as unknowns, to test their ability to predict concentrations. The best results were achieved by Raman/PLS calibration models (where PLS = partial least squares regression) using an internal normalization standard (ν =C-H band). The correlation coefficient (R 2 ) values so obtained were 0.9985 for calibration and 0.9977 for validation. Univariate regression analysis between biodiesel concentration and the increasing intensity of ν CH 2 band or ν C=O displacement showed R 2 values of 0.9983 and 0.9742, respectively. Although spectroscopic methods are less sensitive than chromatographic ones, the data obtained by spectroscopy can be correlated with other techniques, allowing biodiesel yield and quality to be quickly assessed.

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