Fractional Characterization of Wheat Straw Lignin Components by Alkaline Nitrobenzene Oxidation and FT-IR Spectroscopy

A method was developed for the isolation and fractional characterization of phenolic monomers in wheat straw lignin using (a) methanol/toluene, ethanol/toluene, or chloroform extraction for isolation of free phenolic monomers; (b) treatments with various alkalis and hydrogen peroxide for different lengths of time to extract loosely bound phenolic acids and aldehydes; and (c) alkaline nitrobenzene oxidation of lignin in the residue of alkali-treated wheat straw, extracted hemicellulose, and cellulose for determination of tightly bound phenolics. The yield of free phenolic monomers ranged from 0.007 to 0.031% of dry straw. The predominant components of the alkali-labile free monomers were found to be ferulic and p-coumaric acid, which together comprised 65−82% of the total. The results of alkaline nitrobenzene oxidation indicated a high guaiacyl content in the original wheat straw and a high relative syringyl content in treated wheat straw. A comparison of FT-IR spectroscopic data of untreated and treated wheat straw illustrated that the most obvious features in the spectra of alkaline treated wheat straw were the disappearance of the ester linkage absorption and reduced lignin content in treated wheat straw.