Reconsidering Catechol Contributions in Quantitative 31P NMR Analyses of Lignins

Abstract

Quantitative 31P NMR spectroscopy is widely used to determine phenolic hydroxyl group contents in lignins, yet the contribution of catechol functionalities is generally ambiguous. Using a series of catechol-containing lignin model compounds, we demonstrate that phosphitylated catechol hydroxyl groups yield signals that overlap extensively with conventional guaiacyl and syringyl regions and cannot be reliably distinguished by quantitative 31P NMR spectroscopy. Similar results were obtained by aromatic 1H–13C HSQC correlations under standard conditions. To address this limitation, we developed a complementary methodology combining quantitative 31P NMR with a semiquantitative chelometric/UV–vis determination of catechol groups. Application to technical (kraft and organosolv) and analytical (milled wood and enzymatic) grade lignins from softwoods, hardwoods, and grasses revealed measurable catechol contents in all samples examined and indicated that catechol functionalities accounted for approximately 6–27% of the phenolic hydroxyl content conventionally assigned to guaiacyl units, especially, in technical and modified lignins. Additionally, model-compound data further revealed that 5-methoxy-catechyl structures can contribute to the region conventionally assigned to syringyl phenolic hydroxyls. Taken together, our results suggest that conventional 31P NMR assignments may overestimate guaiacyl and syringyl hydroxyls in lignins containing catechol groups.