A catalyst-free
organosolv pulping process was applied to cup plant
(Silphium perfoliatum, S), Miscanthus grass (Miscanthus x giganteus, M), and the Paulownia tree (Paulownia tomentosa, P), resulting in high-purity lignins
with no signals for cellulose, hemicellulose, or other impurities
in two-dimensional heteronuclear single quantum coherence (HSQC) nuclear
magnetic resonance (NMR) spectra. Different biomass particle sizes
used for the organosolv pulping (1.6–2.0 mm (1); 0.5–1.0
mm (2); <0.25 mm (3)) influenced the molecular weight and chemical
structure of the isolated lignins. Principal component analysis (PCA)
of 1H NMR data revealed a high intergroup variance of Miscanthus and Paulownia lignins, separating
the small particle fraction from the larger ones. Furthermore, monolignol
ratios identified via HSQC NMR differ significantly: Miscanthus lignins were composed of all three monolignols (guaiacyl (G), p-hydroxyphenyl (H), syringyl (S)), while for Paulownia and Silphium lignins only G and S units were observed
(except for P3).