Hydrothermal Processing of a Bio-Refinery Lignin Waste-Stream: Effects on Morphology, Physical and Chemical Properties of Resulting Carbonaceous Materials

“Green carbons” are defined as “sustainable non-graphitic, porous carbonaceous materials synthesized from highly abundant and sustainable biomass and organic materials using processes that are environmentally compatible with minimum energy footprint. In this study, carbonaceous materials (CMs) were synthesized from HP-L^TM lignin, an organosolv lignin waste-stream, using hydrothermal carbonization (HTC) at different treatment temperatures. Hydrothermal CMs showed both open and closed vesicles and were more thermally stable, with O/C and H/C ratios between 0.1- 0.4 and 0.3-1.1 respectively. There was a gradual decrease in amounts of oxygenated aromatics, methoxyl and aliphatic substructures when HTC temperature increased. The evolution of an enhanced aromatic structure at 350 °C was detailed by quantitative peak analysis of deconvoluted direct polarization (DP) nuclear magnetic resonance (NMR) spectra. At 350 °C, 77% of the sample was aromatic. The specific surface area of L350 increased from 2.7 to 1341.1 m²/g after KOH activation. Results of this study is important since it identified 350 °C as the target temperature after which significant structural modifications occur in the hydrothermal conversion of HP-L^TM lignin into carbonaceous materials.