Study On Esterification Of Bio-oil In Supercritical Methanol

Fast pyrolysis of biomass into bio-oil is one of the most promising technologies for the renewable energy resource exploration and exploitation. However, the deleterious properties of bio-oil such as thermal instability, low heating value, high moisture content, and high acidity, which might leads to corrosiveness issues, severely hinder its direct application in current internal combustion engines. Upgrading of bio-oil by esterification so as to lower its acidity would be of significance for its practical application. The effects of various components on the conversion of organic acids and upgrading of bio-oil by esterification over solid acid catalyst are studied in this paper.Using acetic acid (AC), acrylic acid (AR), acetyl acetone (AA), furfural,2-methyl phenol (MP) and water as representative compounds of different categories of compounds and p-toluene sulfonic acid as catalyst, the esterification upgrading was examined both in supercritical methanol and under atmosphere pressure. Our emphasis is focused on the effects of various components on the conversion of organic acids. The comparison of alone esterification and simultaneous esterification of AC and AR as well as the effect of water were investigated both in supercritical methanol and in liquid methanol. The experimental results show that supercritical esterification is superior to liquid-phase esterification in view of acid removal. Furthermore, a transesterification mechanism could take place when simultaneous esterifying mixed acids in supercritical methanol, leading to relatively higher conversion for the acid like AR which hardly reacts with methanol in case of alone esterification though. 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 is not only allowed to carry out at higher reaction temperature and consequently accelerates the reaction rate, but also provides a weaker hydrogen bond circumstance. Although AA and furfural had 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 could occur when furfural was present in the esterification. It is found that phenols such as MP could obviously promote the esterification of AR with methanol and inhibit its polymerization. as a result. enhancing the conversion and selectivity of esterification reaction.A solid superacidic catalyst, SO42-/γ-Al2O3, was prepared by impregnation overγ-Al2O3. The effects of H2SO4 loading amount and the calcination temperature were surveyed and the determined optimum concentration of H2SO4 and appropriate calcination temperature were 1.0 mol/L and 500℃respectively. The measurement of the prepared catalyst by Hammett titration suggests that the catalyst was super-acid. Under the prepared catalyst and using AC solution with molar ratio of 10 as the feeding stream, the AC conversion at 300℃and 15.0 MPa with a residence time of 10 min could reach up to 95%. After continuously running for 10 hours, the AC conversion was not noticeably decreased, suggesting the good stability of the catalsyt. It was also found that SO42-/γ-Al2O3 had excellent water resistance, but could be deactivated by the coking formation of furfural on the surface. Supercritical could suppress, to some extent, the coking since it can dissolve and remove the coke from the catalyst surface and thus regenerate the catalyst.Esterification of a real bio-oil over SO42-/γ-Al2O3 shows that the properties of bio-oil were evidently improved after upgrading. The heating value increased by 54.21%, and the density and viscosity decreased by 20.33% and 82.59%, respectively. The GC-MS analysis shows that most of the acids in the bio-oil could be converted into the corresponding esters and consequently the thermal stability was enhanced.