Upgrading of bio-oil: Effect of light aldehydes on acetic acid removal via esterification

Since fast pyrolysis derived bio-oils are not a simple hydrocarbon mixture, but rather contain a variety of oxygenated compounds including acids and aldehydes, upgrading is required in order to use bio-oils as transportation fuels. Esterification is one of the attractive routes to convert acids contained in bio-oil to more desirable esters. Although organic acid esterification is a simple reaction, no work has been reported as to whether the presence of other reactive oxygenated compounds such as aldehydes affect the reaction. In this study of bio-oil model compounds, the impact of acetaldehyde and propionaldehyde on acetic acid esterification was investigated on organic–inorganic mesoporous silica functionalized with propylsulfonic acid groups. No impact of these two aldehydes on acetic acid consumption was seen at 100 °C, regardless of the concentration of ethanol used. However, at 70 °C and 50 °C, the conversion of acetic acid was observed to be lowered during the reaction in the presence of the aldehydes. When either of the aldehydes were present, the acetic acid conversion was about 6% and 28% lower than that in its absence at 70 °C and 50 °C, respectively. It appears that the more significant aldehyde impact on acetic acid conversion seen at the lower reaction temperatures was due to the esterification rate being slow relative to the rapid competitive acetalization rate of the aldehydes with ethanol.