Gas-phase dehydration of glycerol to produce acrolein was investigated at 315 degrees C over Nb2O5 catalysts calcined in the temperature range of 350-700 degrees C. The catalysts were characterized by nitrogen physisorption, TG-DTA, XRD, and n-butylamine titration using Hammett indicators to gain insight into the effect of calcination temperature on catalyst texture, crystal structure, and acidity. Calcination at 350 and 400 degrees C produced amorphous Nb2O5 catalysts that exhibit significantly higher fractions of strong acid sites at -8.2 <= Ho <= -3.0 (Ho being the Hammett acidity function) than the crystallized Nb2O5 samples obtained by calcination at or above 500 degrees C. Glycerol conversion and acrolein selectivity of the Nb2O5 catalysts were dependent of the fraction of strong acid sites (-8.2 <= Ho <= -3.0). The amorphous catalyst prepared by the calcination at 400 degrees C, having the highest fraction of acid sites at -8.2 <= Ho <= -3.0, showed the highest mass specific activity and acrolein selectivity (51 mol%). The other samples, having a higher fraction of either stronger (Ho <= -8.2) or weaker acid sites (-3.0 <= Ho <= 6.8), were less effective for glycerol dehydration and formation of the desired acrolein. (c) 2007 Elsevier Inc. All rights reserved.