Rapid isolation and structural characterization of alkali-soluble lignins during alkaline treatment and atmospheric refining of wheat straw

The chemical compositions of two alkali-soluble wheat straw fractions LA, isolated by two steps of precipitation from two different alkali-refining treatment regimes are reported. The physico-chemical properties of the two lignin samples were characterized by using gel permeation chromatography (GPC), ultraviolet (UV), infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy and alkaline nitrobenzene oxidation. No significant difference of the phenolic monomers composition was found in the case of the two lignin samples. The isolated lignins contain rather low amounts of polysaccharide sugars (0.7%) and have low molecular weights (M(w), 1906-1937). The results also showed that two lignin fractions LA (alkali lignin), contained roughly equal amounts of guaiacyl (G) and syringyl (S) units with relatively fewer p-hydroxyphenyl (H) units and appeared to be very closely associated to phenolic acids and glucuronic acid or 4-O-methylglucuronic acid.