A Kinetic Model on Autocatalytic Reactions in Woody Biomass Hydrolysis

Woody biomass is heterogeneous and contains many kinds of macromolecular carbohydrates and aromatic components, including hemicellulose, lignin and cellulose. Efficient utilization of woody biomass requires selective removals/hydrolysis as means of fractionation. Besides the extractives, hemicelluloses are the most easily separable main component of woody biomass and thus form the bulk of the wood extracts obtained from the hot-water extraction. Xylan is the main polymeric molecule in hardwood hemicelluloses. Xylan is a polymer made of -xylopyranose units linked through (1→4)-glycosidic bonds, where arabinose, acetyl groups, and uronic acids are also present as lateral chains. Hydrolysis of hemicelluloses or xylan produces 5-carbon sugars (mainly xylose), 6-carbon sugars (mainly glucose), and acetic acid. Hydrolysis and extraction/dissolution are similar in that proton is an effective catalyst to the breaking of (1→4)-glycosidic bonds. A mechanistic model has been proposed to describe the extraction process. Kinetic model analysis and dynamic simulations have been generated to illustrate the various aspects of extraction such as the degree of polymerization of hemicellulose in woody biomass, oligosaccharide and acetate concentrations in extraction liquor. Under high temperatures, the acetyl group in woody biomass hydrates and dissociates, causing the extraction liquor pH to drop. The formed proton further catalyzes the de-acetylation and hydrolysis. The extraction reactions are thus autocatalytic. Water and acetic acid dissociation (proton forming) constants as functions of temperature have been developed in this study. Higher temperatures result in higher levels of water and acetic acid dissociation to form protons. The extraction liquor pH has been related to the temperature through the dissociation constants. All the (1→4)-glycosidic bonds are susceptible to break, thus leading to polydispersed polysaccharides in the extraction liquor. In the case of fextraction/dissolution, proton and water molecules need to be adsorbed onto the solid surface at the (1→4)-glycosidic bond sites for the reaction to begin. This reaction occurs on the solid (biomass) surface, whereas hydrolysis occurs in the liquid phase. The formation of methanol, acetic acid and uronic acid is similar to the formation of sugars in the hydrolysis of polysaccharides, where C—O—C links are broken by the insertion of a water molecule catalyzed by a proton. The resulting liquor contains a host of polysaccharide species, methanol, acetic acid, etc. The monosaccharide concentration increases as the residence time increases, with polysaccharides showing a maximum during the extraction/dissolution process.