A 13C CP/MAS NMR evaluation of the structural changes in wheat straw subjected to different chemical and biological pulping conditions

Wheat straw pulps prepared by chemical (soda) and biological (enzymatic or fungal) treatments were analyzed by 13C CP/MAS NMR spectrometry under quantitative acquisition conditions. The most significant changes reflected in the spectra as a result of soda cooking correspond to: (i) decrease of methoxyl content of the residual lignin (56, 153, 147 and 135 ppm), and (ii) deacetylation of hemicellulose fractions and saponification of cinnamyl esters concomitant to the release of alkali-soluble fractions (21 and 172 ppm). Reaction time was the factor with the greatest bearing on the former process, whereas soda concentration and temperature play an additional role in the latter. The decrease of the methoxyl/aryl ratio both after chemical and biological pulping suggests preferential removal of S-type lignin units. The comparison between quality parameters of the pulps and the 13C NMR integration data suggests that the linkage breakdown between straw macromolecules has a greater influence on paperboard properties than the neat extent of the chemical and biological removal of lignin fractions.