(749e) Study of Acidic Intermediates from Xylose Decomposition in Sub- and Supercritical Water

Biomass has attracted researchers as it is a good candidate for energy resource to produce renewable energy in terms of substitution of depleting fossil fuel due to its carbon neutral characteristic, in the other word, carbon dioxide produced in combustion process is consumed by biomass in photosynthesis. However, biomass always comes in a wet form, thus drying process in biomass treatment adds production cost and might not be economically feasible. One conversion technique to transform biomass into energy without necessity of drying process is so called supercritical water gasification (SCWG). Water in supercritical state (temperature and pressure above 374 ^oC and 22.1 MPa, respectively) has no boundary between liquid and gas phase due to that the water density of these two phases becomes closed, hence mass transfer between the phases is improved. This property makes supercritical water behaves as organic solvent, therefore, biomass can be fully dissolved in the water results in enhancement of high reaction rate. Among many kinds of biomass, lignocellulosic biomass or plant species are being focused since it is abundantly available on earth. It consists mainly of cellulose, hemicellulose, which contain in cell wall being merged by glue-liked lignin. The key to understand behavior of lignocellulosic biomass in supercritical water for gas production and eliminate the complexity of biomass structure is to investigate the model compound of each composition. Among these three composition, hemicellulose is less paid by attention, thus motivated us to study its mechanism in supercritical water to fulfill the missing knowledge aiming for completely gasify lignocellulosic biomass. Moreover, acid is known as good catalyst for biomass treatment and it could be produced during the process of biomass decomposition. In this study, xylose was used as a model compound for hemicellulose and its decomposition in sub- and supercritical water was elucidated. During the pathway of xylose to form gaseous products, organic acids were released and supposed to have some positive effect to the desirable reactions. In the experimental section, 7.5 wt.% of xylose solution was treated in water under sub- and supercritical region in continuous reactor varying the temperature range of 300 to 450^oC at the fixed pressure of 25 MPa. Regarding to that xylose decomposition rate is extremely high, intermediate products of xylose decomposition were observed at short residence time of 0.5 to 5 s. Main organic acidic intermediates that were observed are formic acid, especially in subcritical region, lactic acid, partially in supercritical region and the other acids, such as, acetic acid and propionic acid. The other intermediate products were xylulose from isomerization of xylose, furfural from dehydration of xylose and retro-aldol products, which are glyceraldehyde, glycolaldehyde, dihydroxyactone and formaldehyde. Taking these intermediate products into account, reaction network of xylose decomposition in sub- and supercritical water was proposed and this reaction network was a key to determine kinetics of reactions. Finally, the effect of organic acids on the other reactions and their characteristic in supercritical water were taken into consideration.