Fractionation and Characterization of Water-soluble Hemicelluloses and Lignin from Steam-exploded Birchwood

Abstract

Two crude hemicellulosic fractions, obtained by extraction of steam-exploded birchwood with hot water, were treated with 8% NaOH at 20[ddot]C for 1 and 4h, and subsequently sub-fractionated into four lignin and four hemicellulosic fractions. Acid hydrolysis, alkaline nitrobenzene oxidation, ultraviolet (UV), gel-permeation chromatography (GPC), Fourier transform infrared (FT-IR), and carbon-13 nuclear magnetic resonance (NMR) spectroscopies were used to investigate the chemical compositions and structural features of the fractionated hemicelluloses and lignins. The sugar analyses indicated that xylose was the predominant sugar component in the four hemicellulosic fractions. Due to the autohydrolysis at elevated temperature and lower in acidity during the steam treatment processes, all the four hemicellulosic fractions showed a low degree of polymerization (DP, 38–41), with molecular-average weights between 5620 and 6160. Assignments of all the signals in the NMR spectrum led to the conclusion that the four lignin fractions, which differ from the organosolv lignins obtained from steam-exploded aspen wood meals, are still mainly composed of β—O—4 ether bonds, together with small amounts of less common β—β and β—1 carbon-carbon linkages between the lignin structural units. The weight-average molecular weights were found to be 2250–2620 with the polydispersity of 1.5.