Structural recognition of lignin isolated from bioenergy crops by subcritical water: ethanol extraction

Lignin was isolated with subcritical water:ethanol:CO2 (Sub-CW) from biorefinery biomasses, such as giant reed (AD) and miscanthus (MG), with recovery yields about 30% on Klason lignin. Their structural composition assessed by IR and NMR techniques, as well as Derivatization Followed by Reductive Cleavage (DFRC/GC-MS). The 2D HSQC-NMR spectra elucidated that the Sub-CW extracts contained different lignin dimers and co-extracted carbohydrates. The DFRC/GC-MS revealed that syringyl molecules were more abundant in AD, while guaiacyl monomers were predominant in MG. Lignin residues were derivatized with phospholane to quantitatively estimate the amount of OH groups by 31P NMR, showing a marked predominance of aliphatic units for both lignins, due to the presence of either hydroxyls in lignin side-chain or residual carbohydrates. Lignin residues derivatized with phospholane allowed to record 31P-DOSY NMR spectra. AD-lignin showed a smaller diffusivity constant than for MG-substrate, possibly because of the larger content of disaccharides in MG lignin. We showed that the molecular composition of lignin isolated by the Sub-CW technique may differ depending on the type of biomass used for the extraction, suggesting a different industrial application of lignin from various biomass.