Structural and physico-chemical characterization of hemicelluloses from ultrasound-assisted extractions of partially delignified fast-growing poplar wood through organic solvent and alkaline solutions

One organic and three alkaline hemicellulosic fractions were isolated by an ultrasound-assisted extraction which partially delignified the fast-growing poplar wood. Successive treatments were conducted with dimethyl sulfoxide under ultrasonic irradiation at 570 W, 25 °C for 30 min, 70% ethanol containing 1% NaOH, 3% NaOH and 6% NaOH at 75 °C for 3 h, respectively. The four hemicellulosic fractions obtained were comparatively studied by sugar analysis, alkaline nitrobenzene oxidation of bound lignin, GPC, FT-IR, 1D and 2D NMR spectroscopy as well as TGA and DTA. The results showed that the ultrasonic treatment and sequential extractions with three different concentrations of NaOH led to a release of 75.5% of the original hemicelluloses and 96.2% of the lignin. All four purified hemicellulose fractions contained relatively low amounts of associated lignin, ranging between 0.96 and 3.10%. In addition, the hemicellulosic fraction H4 isolated with 6% NaOH is formed by a linear backbone of four (β-1 → 4)-xylopyranosyl residues and at least one of the xylose residues is monosubstituted at C-2 by a 4-O-methylglucuronic acid, giving a typical ratio of 4-O-methyl glucuronic acid to Xyl of 1 to 4.