Fig. 1: Structural characteristics of hydrothermal sulfuric acid lignin (HSAL).

a Synthesis of HSAL. During acid hydrolysis of lignocellulosic biomass, sulfuric acid lignin (SAL, yield was 35.2 wt% based on lignocellulosic biomass) was converted into a water-soluble lignin material HSAL (yield was 46.7 wt% based on SAL) with a mild hydrothermal reaction. b, Fourier transform infrared spectroscopy (FT-IR) spectra showed the difference of methoxy groups and hydroxy groups between SAL and HSAL. The red star indicates absorption of methoxy groups at 1465 cm⁻¹; the blue area indicates absorption of hydroxy groups between 3200 and 3500 cm⁻¹. c Methoxy group content in SAL and HSAL. d Measurement of phenolic hydroxy groups and alcoholic hydroxy groups of HSAL using nuclear magnetic resonance (NMR) spectra. e Quantification of hydroxy group content of HSAL in d. f Quantification of hydroxy group content of SAL. P-OH, phenolic hydroxyl group; A-OH, alcoholic hydroxy group. g Model diagram of lignin monomer structural units during HSAL synthesis with the change of methoxy groups and phenolic hydroxy groups. The graphs depict mean and standard deviation (error bars) and individual data points. Statistically significant differences between the means were analyzed with Student’s t-test (n = 3 independent experiments) in (c, e, f).