PHYSICO-CHEMICAL CHARACTERIZATION OF DIFFERENT ALCOHOL-SOLUBLE LIGNINS FROM RICE STRAW

The treatment of dewaxed rice straw with 60% methanol, 60% ethanol, 60% n-propanol, 60% n-butanol, 60% ethanol containing 0.01 M HCl and 60% ethanol containing 0.25 M NaOH at 75 degrees C for 3 h resulted in the solubilization of 14.6, 13.0, 16.3, 12.2, 13.0 and 75.6% of the original lignin, respectively. All alcohol-soluble lignin fractions showed spectral features of GSH-type lignin. The yield and purity of the acid-insoluble lignin extracted with 60% ethanol containing 0.25 M NaOH was much higher than of those prepared with other alcohols. The purity of lignin prepared with different alcohols followed the order: methanol > n-propanol > n-butanol > ethanol. Furthermore, the treatment of 60% ethanol under alkali catalyst produced purer lignin than the 60% ethanol treatments with or without acid catalyst. The six alcohol-soluble lignin fractions were comparatively characterized by both destructive methods, such as alkaline nitrobenzene oxidation, and non-destructive techniques, such as ultraviolet (UV), Fourier Transform Infrared (FTIR), C-13 and two-dimensional Heteronuclear Single Quantum Correlation Nuclear Magnetic Resonance (C-13 and 2D-HSQC NMR) Spectroscopy, and Gel Permeation Chromatography (GPC). The 2D-HSQC NMR spectrum showed that the prominent linkage of the lignin fraction obtained with 60% ethanol containing 0.25 M NaOH was between the beta-O-4 ether substructure and the beta-5' and beta-beta' ones.