INVITED REVIEW PAPER Effect of acid type in WOX clusters on the esterification of ethanol with acetic acid

Abstract−Tungsten oxide clusters supported on silica (WOX/SiO2) with different W loading levels and the effect of acid type on the esterification of acetic acid with ethanol were examined. The catalysts were characterized using various techniques (XRD, Raman spectroscopy, NH3-TPD and FT-IR) to investigate the crystallinity and the nature of the acid sites. The change in the composition of two tungsten oxide species (polytungstate and crystalline WO3) leads to the change of Lewis acid to Brønsted acid ratio. Importantly, the ratio of the two different acid types has a substantial effect on the catalytic activity. The fraction of Lewis acid to total acid sites rapidly changed from 23 % to 77 % due to the pres-ence of crystalline WO3. Where the Lewis acid sites accounted for 55 % of the total acid sites, the WOX/SiO2 catalyst showed the highest catalytic activity among the prepared catalysts