Subcritical water and ethanol extraction of hydrolysis lignin: Solvent-driven fractionation behavior and product distribution

Pressurized liquid extraction (PLE) was applied to fractionation of enzymatic hydrolysis lignin (EHL), a residual biomass derived from fully integrated cellulosic ethanol production processes. PLE extraction with water (W) and ethanol (EtOH) was performed at a constant pressure of 10 MPa to preserve acid-insoluble lignin and glucan contents in the residual solids, while recovering extracts rich in phenolics and non-polymeric carbohydrates. Water and ethanol presented 11.8 wt.% and 18.4 wt.% of overall extraction yields, respectively, while the sequential extraction (W-EtOH) reached 19.4 wt.%. After a single extraction with water or ethanol, the residual solids showed similar or lower Higher Heating Value (HHV) values compared to raw EHL. By contrast, the W-EtOH sequence at 150 °C increased HHV to 22.0 ± 0.2 MJ/kg, producing residual solids with a higher total lignin content (up to 59.0 wt.%). However, higher total phenolics (45 ± 1 g GAE 100 g -1 ) and higher carbohydrate contents (25 ± 5 wt.% extract) were obtained at 90 °C and 120 °C, for W-EtOH and water extraction, respectively. The presented approach enables the sustainable valorization of hydrolysis lignin by recovering residual carbohydrates and phenolic compounds for their subsequent integration in second-generation production processes while increasing the energy value of the residual solids.