Xylan Depolymerisation under Subcritical Water Conditions

Abstract

Xylan, the principal constituent of hemicellulose is the one and only source for the commercial production of furfural and value-added furans, and the potential exists for much greater utilisation of such products as fuel additives and chemical feedstocks. Under hydrothermal conditions, xylan breaks down into oligomers, sugars, furans and acids, but the detailed product distribution is not well understood, especially considering the effects of temperature, pH, and residence time. The objective of this study is to understand xylan depolymerization under hydrothermal conditions (200-300˚C, 120-150bar). Xylan from Beechwood has been used as the primary reactants and its decomposition products have been characterised as oligomers of different molecular weight (123,000-1000 g/mol), xylotetrose, xylobiose, xylose, organic acids, and furans. The effects of temperature and reaction time on xylan decomposition and product distribution have been established. A detailed kinetic model based on parallel and consecutive reactions has been developed, to understand the disappearance of xylan, providing insight into the factors determining selectivity to oligomer production, xylose monomer formation from oligomers and direct oligomer degradation to organic acids. Further the detailed reaction mechanism of xylose decomposition under the same conditions has been studied and incorporated in the kinetic model. Kinetic modelling reveals that all the oligomers disappear at a higher rate compared to xylose; with increasing temperature, xylotetrose and xylobiose directly from organic acids, thus reducing the xylose and furanic product yield. The proposed model provides a basis for optimising operational conditions for xylan depolymerisation.