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

• Published in: Catalysis today 2008-04, Vol.133, p.106-112
• Main Authors: Braga, Valdeilson S., Barros, Ivoneide C.L., Garcia, Fillipe A.C., Dias, Sílvia C.L., Dias, José A.
• Format: Article
• Language: English
• Subjects: Esterification; Niobium pentoxide; Silica–alumina; Solid acid; Supported catalyst
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2007.12.091

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 CH 2Cl 2 or ethanol solution with NbCl 5 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 Nb 2O 5 (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 Nb 2O 5 by impregnation. The catalysts with about 10 wt.% of Nb 2O 5 prepared by aqueous impregnation reached the monolayer, while by grafting it predominates bulk Nb 2O 5, 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.% Nb 2O 5 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.