Upgrading bio-oil by esterification under supercritical CO2 conditions

Esterification of acetic acid (AC), propanoic acid (PA) and a mixed acid (AC + PA + acrylic acid) as well as real bio-oil with ethanol under supercritical CO2 (scCO2) conditions were investigated. It was found that these carboxylic acids could afford much higher conversions under scCO2 condition in comparison with esterification at atmosphere pressure, revealing the promoting effect of scCO2 as a result of that the formed esters are prone to be phase-transferred into the scCO2 phase. Since raising pressure at constant temperature can enhance the dissolving power of scCO2, it was observed that esterification conversion increased with pressure. The results esterifying these acids separately showed significant difference in their conversions, whereas simultaneous esterification of the mixed acid mixture afforded very close conversions, suggesting that a transesterification mechanism might occur in the case of simultaneous esterification under scCO2. The appropriate operation conditions for upgrading bio-oil by scCO2 extractive esterification were determined as at 80°C and 28.0 MPa for 3.0 h. Under this condition, the total acid conversion could reach up to 86.78%. pH value for the upgraded bio-oil increased from 3.78 to 5.11. The total volatile components in the upgraded bio-oil approached up to 100% at 140°C.