Quantitative and Structural Characterization of Native Lignin in Hardwood and Softwood Bark via Solid-State NMR Spectroscopy

A major factor limiting bark's industrial use is its greater recalcitrance compared to wood. While lignin is widely recognized as a significant contributor, precise characterization of lignin in bark remains sparse, presenting a crucial gap that impedes understanding of its impact. In this study, we employed advanced solid-state nuclear magnetic resonance (NMR) spectroscopy to analyze bark samples from various species, including willow, poplar, and pine. We established and verified that lignin methoxy peak at 56 ppm serves as a reliable quantitative metric to assess lignin content, with which we calculated the lignin contents in bark are significantly reduced by more than 70% compared to those in wood. Furthermore, in situ characterization revealed significant reduction of β-ether linkage in bark lignin across species, revealing a more condensed and resistant structural configuration. Our results have substantially advanced our comprehension of the composition and structure of native lignin in tree bark.