A comparison of lignin polymer models (DHPs) and lignins by 31P NMR spectroscopy

Abstract

Fractionated guaiacyl (G) and guaiacyl/syringyl (GS) DHPs prepared by continuous (Zutropf, ZT) and discontinuous (Zulauf, ZL) dehydrogenation schemes were subjected to quantitative 31P NMR spectroscopy and their various hydroxyl groups determined. Two milled wood lignin samples from softwood and hardwood species were also examined. It was found that GS-DHPs resemble more GS milled wood lignins than G-DHPs resemble G milled wood lignins. The total phenolic-OH contents of ZT-DHPs are always lower than those of ZL-DHPs, in good agreement with the theory which predicts more β-O-4 linkages in the former. Furthermore, in agreement with the dehydrogenation theory, the bulk character of G-DHPs results from the formation of condensed units. In GS-DHPs, the S units are etherified early in the polymerization process, while the bulk character of such samples is mainly due to G-units. Regarding the secondary hydroxyl groups the results obtained for the G-DHPs are extremely low. This underlines again the principal differences of G-DHPs compared with GS-DHPs and MWLs. The erythro to threo ratio of the G-type samples was found to vary between 1 and 1.5, indicating only minor dependence on molar mass or mode of preparation. The erythro to threo ratio in GS-type samples was found to vary from 1.6 to 4.3, showing the highest value for low molar mass ZL-DHPs and lowest values for the cherry tree MWL.