The activity of solid niobic acid treated with sulfuric acid and with phosphoric acid was tested in the catalytic esterification
of oleic acid and in transesterification of soybean oil with methanol. X-ray diffraction studies revealed niobic acid treated
with sulfuric acid to be an amorphous solid, while niobic acid treated with phosphoric acid presented a crystalline phase,
probably due to the formation of hydrogen niobium phosphate. Among the characterization methods employed here, XPS was particularly
instrumental in showing that niobium dihydrogen phosphate and niobium hydrogen sulfate moieties are the acidic active sites
responsible for the catalytic effect. The latter is more active according to ammonia TPD studies. Yields as high as 78% in
the esterification of oleic acid with methanol and 40% in the transesterification of soybean oil catalyzed by NbSO4 were recorded.