Comprehensive structural analysis of biorefinery lignins with a quantitative 13C NMR approach

The advance in analytical methodology is critical for the progress in the biorefinery and lignin commercialization. This paper reports a comprehensive approach (more than 30 common structural characteristics along with moieties specific for various lignin types) for the analysis of biorefinery lignins with quantitative C-13 NMR spectroscopy, which has been demonstrated to be significantly different from the analysis of native lignins. Experimentals required for high precision NMR spectra are highlighted. The statistic data allowed for evaluating the accuracy in the quantification of different lignin units for the first time. The analysis of various lignins originated from the key biorefinery processes of different types of biomass clearly demonstrated that, in general, lignin degradation was always accompanied with decreases in aliphatic OH (primary and especially secondary ones), oxygenated aliphatic moieties, specifically beta-O-4 units, and increasing amounts of phenolic OH, COOR, saturated aliphatic moieties and the degree of condensation. However, the differences in the quantity of different functionalities between the lignins investigated were very significant. Hardwood steam explosion lignins were the less degraded ones whereas aspen kraft lignin underwent the most severe structural modification. Finally, this report presents a comprehensive database on the structure of reference biorefinery lignins that is of primary importance for their commercialization.