Structural features of lignin-rich solid residues obtained from two-step acid-hydrolysis of Miscanthus biomass (Miscanthus sacchariflorus Benth.)

•Miscanthus was subjected to two-step hydrolysis under different concentrations of H2SO4.•Maximum concentration of monomeric sugars was obtained for 72% and 2% treatment.•Yields and properties of lignin-rich residues were influenced by acid concentration.•Solid residues were further thermally stabilized as a result of H2SO4 treatment.

Miscanthus was subjected to two-step hydrolysis under different concentrations of sulfuric-acid (first-step: 72–18%; second-step: 36−2%) for conversion to monomeric sugars and lignin-rich solid residues. Compared to native lignin isolated from Miscanthus, there were obvious reductions of phenolic hydroxyl(0.5–2.6% vs. 5.9%) and methoxyl groups(3.0–13.9% vs. 16.5%) in solid residues. TGA confirmed that thermal stability of solid residues was proportional to greater abundance of condensed lignin structures and inversely proportional to remaining carbohydrate amount. Py-GC/MS revealed that sum of lignin derivatives (H-,G-,S-type phenols) amounted to 23–76% of the levels of MWL, meaning that lignins in solid residues seem to be highly condensed.