Esterification of acetic acid with alcohols using supported niobium pentoxide on silica–alumina catalysts

Supported niobium pentoxide catalysts are active for a variety of acid and redox catalyzed reactions such as dehydration, hydrogenation and esterification. In this work, niobium pentoxide has been supported on silica–alumina using two methods: grafting in CH2Cl2 or ethanol solution with NbCl5 precursor, and impregnating in aqueous solution with ammonium niobium oxalate. XRD, TG/DTG/DTA, FTIR, DRIFTS, FT-Raman, and calorimetric titration with pyridine were used to characterize the materials. Calcination of the materials at 800 °C produced crystalline Nb2O5 (T-phase) for the samples prepared by grafting, while a mixture of T- and H-phases are formed by aqueous impregnation, indicating the higher stability of the former materials. Thermal analysis confirmed a stronger interaction of niobium oxide species on the silica–alumina surface for the grafting procedure. On the other hand, partial neutralization of hydroxyl groups occurred on the support surface by grafting process, and the neutralization reaction is practically complete at 25 wt.% of Nb2O5 by impregnation. The catalysts with about 10 wt.% of Nb2O5 prepared by aqueous impregnation reached the monolayer, while by grafting it predominates bulk Nb2O5, as demonstrated by FT-Raman. These catalysts also showed the highest acidity, when calcined at 300 °C. Based on these results, liquid phase esterification reactions of acetic acid were carried out with different alcohols (ethanol, n-butanol, and iso-pentanol). The yields were about the same, independent of the preparation method for 10 wt.% Nb2O5 catalysts. The conversions using molar ratio 2:1 (acid:alcohol) after 8 h showed good results (83, 87, and 91%, respectively) and 100% selectivity for all esters.