Comparative study of fractions from alkaline extraction of wheat straw through chemical degradation, analytical pyrolysis, and spectroscopic techniques

A comparative assessment of spectroscopic and degradative techniques was carried out to characterize fractions from the sodium hydroxide extraction of wheat straw. CPMAS 13C NMR and IR spectra revealed lignin, lignin-carbohydrate complex, and hemicellulose in laboratory and industrial fractions. Preferential removal by alkali of syringyl lignin was observed, and lignin alteration during biological treatment of pulping effluents was indicated by enhanced carbonyl and alkyl signals. Relatively high syringyl/guaiacyl ratio values were obtained by cupric oxide degradation when compared with those from NMR and PY-GC-MS, the latter including ferulic acid. Additional information on straw polymer association and the role of cinnamic acids was obtained. The high guaiacyl content in lignin associated to the hemicellulose suggested bonds between the latter and guaiacyl lignin. Most linked p-coumaric acid was merely esterified, whereas ferulic acid was preferentially associated to lignin by nonsaponifiable bonds