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Different ratios of phosphotungestic acid supported on silica gel were prepared by an impregnation method with PWA loadings ranging from 1 to 30% w/w and calcined at 350 and 500 • C for 4 h in a static air atmosphere. The catalysts were characterized by thermogravimety (TG), differential thermal analysis (DTA), X-ray diffraction, FT-IR spectroscopy and N 2 adsorption measurements. The surface acidity and basicity of the catalyst were investigated by the dehydration-dehydrogenation of isopropanol and the adsorption of pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The gas-phase estrification of acetic acid with ethanol was carried out at 185 • C in a conventional fixed-bed reactor at 1 atm using air as carrier gas. The results clearly revealed that the catalyst containing 10% w/w PWA/SiO 2 is the most active and delivers reaction selectively to ester with 85% yield. The Brønsted acid site resulting from hydroxylation of tungsten oxide plays the main role in the formation of ester. Figure 1 shows TG and DTA curves of the pure silica, pure PWA and PWA supported on silica. It is seen 514 Perspective Perspective 55 Sheinthauer M, Cheung TK, Jentoft RE, Grasselli RK, Gates BC and Knozinger H, Characterization of WO x /ZrO 2 by vibrational spectroscopy and n-Pentane isomerization catalysis. J Catal 180:1-13 (1998). 56 Baertsch CD, Soled SL and Iglesia E, Isotopic and chemical titration of acid sites in tungsten oxide domains supported on zirconia. J Phys Chem B 105:1320-1330 (2001). RESULTS AND DISCUSSION Thermal analysis

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