Fractional isolation, physico-chemical characterization and homogeneous esterification of hemicelluloses from fast-growing poplar wood

Abstract

Extraction of the dewaxed and partially delignified fast-growing poplar wood with 1.5, 3.0, 5.0, 7.5 and 8.5% NaOH at 20°C for 16 h solubilized 65.6, 71.6, 73.8, 85.6 and 89.3% of the original hemicelluloses, respectively. Xylans were found to be the predominant hemicellulosic components in the cell walls of fast-growing poplar wood. The isolated five hemicellulosic preparations were further characterized by Fourier transform infrared (FT-IR), and carbon-13 magnetic resonance spectroscopy ( 13C NMR) as well as gel permeation chromatography (GPC). The results showed that the hemicelluloses contain a main chain formed by d-xylopyranose residues linked by β-1,4 linkages. The hemicellulosic preparation, isolated with 8.5% NaOH at 20°C for 16 h, was esterified with propionyl, hexanoyl, lauroyl, and palmitoyl chlorides, respectively, in homogeneous N, N-dimethylformamide/lithium chloride system using 4-dimethylaminopyridine as a catalyst and triethylamine as a neutralizer. The effects of molar ratio of acyl chloride/anhydroxylose unit, triethylamine concentration, and reaction time and temperature on the yield and degree of substitution of the hemicellulose derivatives have been primarily examined. The degree of substitution was controlled between 0.34 and 1.44 under the reaction conditions given. Under an optimum condition (sample 8, molar ratio 3:1), over 70% of the total hydroxyl groups in native hemicelluloses were palmitoylated at 75°C for 40 min. The results obtained from the molecular weight measurements (48 000–127 200 g mol −1) revealed only a minimal degradation of the macromolecular hemicelluloses during the rapid reactions at 60–75°C for 25–40 min.