¹³C NMR Characterization of Soda and Soda/Aq Spent Liquor Fractions

Abstract

Spent liquor fractions from soda and soda/anthraquinone cooks of loblolly pine wood meal were examined by C-13 NMR spectroscopy. The material which precipitated from the liquors at pH 7 as well as that remaining in solution were acetylated and fractionated on a styrene-divinylbenzene gel column. This technique was an attempt to simplify spectral interpretation and to elucidate the distribution of anthracenyl structures within the spent liquor components. The NMR spectra of fractions of the precipitated material revealed that anthracenyl structures chemically bound to lignin increased with decreasing molecular weight (MW), whereas, polysaccharide content decreased markedly with decreasing MW. The NMR spectra of the soluble material had no anthracenyl peaks, but did contain a significant peak at 105 ppm. This is in the same location as a predominant peak observed in C-13 NMR spectra of hardwood lignins, and which has been assigned to C2 and C6 of syringyl structures. However, in the present study the 105 ppm peak has been tentatively assigned to a carbohydrate carbon since it is prominent only in carbohydrate-rich fractions. Also, its aliphatic nature was confirmed by double resonance NMR experiments.