Fractional and structural characterization of lignins isolated by alkali and alkaline peroxide from barley straw

A sequential treatment of dewaxed barley straw with sodium hydroxide, diffe rent concentrations of hydrogen peroxide, and potassium hydroxide/sodium bo rate degraded various proportions of the original lignin and solubilized. d ifferent amounts of the original hemicelluloses. The isolated lignin fracti ons were subjected to comprehensive structural characterization by UV, FT-I R, and C-13 NMR spectroscopy, and their chemical compositions were analyzed by alkaline nitrobenzene oxidation. All of the lignin fractions were typic al of grass lignins and had weight-average molecular weights between 1750 a nd 2190. It was found that the peroxide treatment at low concentrations (le ss than or equal to2.0%) resulted in a slight increase in the amount of car boxyl groups, whereas the treatment at a relatively high concentration of a lkaline peroxide, such as at 3.0% H2O2, led to a noticeable oxidation of th e lignins, as shown by an increase of carboxyl groups. Moreover, the result s obtained indicated that the successive treatments with alkali and alkalin e peroxide under the conditions used did not significantly affect the beta -O-4 structures of lignins. Substantial amounts of etherified ferulic acids were cleaved by the sequential treatments with alkaline peroxide, as shown in the C-13 NMR spectra. The results underscore the structural differences between alkali- and alkaline peroxide-soluble lignins from barley straw.