Aqueous Phase Hydrogenation of Acetic Acid and Its Promotional Effect on p-Cresol Hydrodeoxygenation

A systematic study of the comparative performances of various supported noble metal catalysts for the aqueous phase hydrogenation of acetic acid (as a model carboxylic acid in bio-oils) by itself and in combination with p-cresol (as a model phenolic compound in bio-oils) is presented. It was found that Ru/C catalyst shows the highest activity for acetic acid hydrogenation among the tested catalysts, followed by Ru/Al2O3, Pt/C, Pt/Al2O3, Pd/Al2O3, and Pd/C. CH4 and CO2 were observed to be the major products on all of these catalysts at typical hydroprocessing temperatures (similar to 300 degrees C). A systematic study on parametric effects with the Ru/C catalyst shows that the product distribution is dependent upon the temperature and presence of water. At low temperatures (similar to 150 degrees C), acetic acid hydrogenation is favored with higher selectivity to ethanol, while at high temperatures (similar to 300 degrees C), acetic acid decomposition and ethanol reforming/hydrogenolysis dominate with CO2 and CH4 as the major products. When water is replaced with n-heptane at otherwise similar conditions, the esterification reaction is favored over ethanol reforming/hydrogenolysis, resulting in substantial formation of ethyl acetate. With a mixed feed of acetic acid and p-cresol over the Ru/C catalyst, acetic acid hydrogenation is suppressed and p-cresol hydrodeoxygenation is favored, as inferred from the observed high selectivity to methylcyclohexane.