Effect of the reactive compounds in bio-oils on esterification of the contained carboxylic acids in supercritical methanol

Using acetic acid (AC), acrylic acid (AR), acetyl acetone (AA), furfural, 2-methoxy phenol (MP) and water as representative compounds and p-toluene sulfonic acid as catalyst, the esterification upgrading was examined in supercritical methanol and under atmosphere pressure. The comparison results of esterification and simultaneous esterification of AC and AR as well as the effect of water show that supercritical esterification is superior to normal liquid-phase esterification in view of acid removal. Furthermore, transesterification reaction could take place when simultaneously esterifying mixed acids in supercritical methanol, leading to relatively higher conversion for the acid like AR which hardly reacts with methanol at normal condition. The reason for low acid conversion in the presence of water is attributed to the retarding effect of water on esterification, rather than a result of equilibrium shifting, since water tends to weaken the electrophilicity of positive carbonyl ion. However, such effect in supercritical methanol is much insignificant because supercritical reaction not only carries out at higher reaction temperature and consequently accelerates the reaction rate, but also provides a weaker hydrogen bond circumstance. Although AA and furfural have unnoticeable effect on the esterification of organic acids, AA itself can be converted into acetone and methyl acetate via a pyrohydrolysis and esterification mechanism. Acetalization reaction of aldehyde with methanol can occur when furfural is present in the esterification. It is found that phenols such as MP can obviously promote the esterification of AR with methanol and inhibit its polymerization, as a result, enhancing the conversion and selectivity of esterification reaction.