Acetic acid hydrogenation to ethanol over supported Pt-Sn catalyst: Effect of Bronsted acidity on product selectivity

Gas phase hydrogenation of acetic acid was investigated over a series of SiO2-Al2O3 supported platinum-tin (Pt-Sn) catalysts. The active metals were impregnated over the support using incipient wetness technique and the resulting catalyst samples were characterized by Transmission electron microscopy, Hydrogen pulse chemi-sorption, BET surface area analyzer, Powder X-Ray diffraction, NH3-Temperature programmed desorption and H-2-Temperature programmed reduction methods. Acetic acid hydrogenation reaction was carried out in an isothermal fixed bed catalyst testing unit. The results revealed that bimetallic Pt-Sn catalyst forms Pt-Sn alloy upon reduction which favors acetic acid hydrogenation to ethanol compared to competing side product CH4. The magnitude of Pt-Sn alloy formed per unit mass of catalyst depends upon the Pt/Sn molar ratio in the calcined catalyst sample. 3 wt% Pt-3 wt% Sn on SiO2-Al2O3 was found to be the optimum catalyst loading, resulting in 81% acetic acid conversion with 95% ethanol selectivity at 2 MPa and 270 degrees C. Further increase in ethanol selectivity would require prevention of esterification of acetic acid with ethanol, which leads to formation of ethyl acetate as by-product. The effect of catalyst acidity on acetic acid conversion and ethanol selectivity was studied and it was observed that proton donating capability of the support leads to the formation of ethyl acetate as by-product which, in turn, reduces ethanol selectivity. The ethanol synthesis reaction and esterification reaction over Bronsted acid sites takes place in series. The rate of esterification reaction was found to be highly dependent on the Bronsted acid density of the catalysts. Other catalyst parameters have little role on ethyl acetate yield.