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.

Bioethanol has been researched as a potential alternative to substitute liquid fossil fuels due to its eco-friendly characteristics and relatively low production cost when compared to other bio-based fuels. First generation bioethanol is produced from raw materials rich in simple sugars or starch, such as sugarcane and corn, which are food sources. To avoid the fuel versus food dilemma, second generation bioethanol aims at using non-edible raw materials, as lignocellulosic agricultural residues, as source of fermentable sugars. Hydrolysis with sub/supercritical water has demonstrated great potential to decompose the lignocellulosic complex into simple sugars with several advantages over conventional processes. This review provides an overview of the state of the art on hydrolysis with sub- and supercritical water in the context of the reuse of agricultural residues to produce suitable fermentation substrates for the production of second generation bioethanol. Recent applications and advances are put into context together, providing an insight into future research trends.