Effect of hydrothermal treatment intensity on the formation of degradation products from birchwood

Hydrothermal treatments (HT) of birchwood were conducted at various intensities to extract the hemicelluloses before pulping. The amount of hemicellulose-derived sugars in the hydrolysates, including xylulose, an isomerization product of xylose, reached first a maximum and then decreased with further increasing treatment intensity. The hydrolysates also contained furanic compounds, carboxylic acids, and a large variety of aromatics, the amounts of which were dependent on HT intensity. At high treatment intensities, furfural and acetic acid were the main products quantified. Numerous nonvolatile, low molar mass carboxylic acids were also formed, with 3-deoxypentonic acid being the most abundant. Additionally, almost 40 aromatic monomers and up to 30 dimers were detected. Syringaldehyde was the main monomer and syringaresinol was the main dimer. Some aromatic compounds could not be identified. The complexity of the hydrolysates, particularly after high-intensity HT, requires selective filtration and purification methods before the hydrolysates can be utilized in downstream processes.