Molecular composition of water-soluble lignins separated from different non-food biomasses

•Molecular composition of water-soluble lignins from biomasses for energy•DFRC-GC-MS distinguished among phenolic residues of water-soluble lignins.•2D-HSCQ allowed identification of residual presence of dimeric structures in lignins.•31P-NMR quantified carboxyl groups in lignin molecules.•Water-soluble lignins may become composite in industry and stimulants in agriculture.

Separation of water-soluble lignins from lignocellulosic biomass provides a new and still poorly exploited feedstock to increase the sustainability of biorefineries. We applied derivatization followed by a reductive cleavage (DFRC) method, 2D-HSQC-NMR, and 31PNMR after 31P-labeling, to investigate molecular composition in water-soluble lignins obtained by alkaline oxidation from three biomass materials for energy (miscanthus, giant reed and an industrially pre-treated giant reed). Chromatographic identification of lignin products cleaved by DFRC showed a large predominance of guaiacyl (G) units in all biomasses and a lesser abundance of syringyl (S) and p-coumaryl (P) monomers. Our S/G ratios disagree with those reported in literature by other lignin separation methods. Carboxyl functions (ferulic and pcoumaric acids) were revealed by heterocorrelated 1H–13C HSQC-NMR, and confirmed by 31P-NMR spectra of 31 P-labeled lignin molecules. An understanding of molecular composition of water-soluble lignins from biomass sources for energy is essential for lignin most efficient exploitation in either industrial or agricultural applications.

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