Effect of temperature on structure evolution in char from hydrothermal degradation of lignin

• Char produced at 330 °C has the largest surface area and the largest pore volume.
• Crystalline structure of produced char is highly ordered with high hydrothermal temperature.
• Char produced at higher hydrothermal temperature exhibited higher thermal-stability.
• A possible mechanism was proposed for char structure evolution during lignin hydrothermal process.

Hydrothermal degradation of lignin was carried out at 280–365 °C and the structure of solid char residue was extensively examined by means of scanning electron microscopy (SEM), nitrogen absorption/desorption, X-ray spectroscopy (XRD) and Fourier transform infrared spectroscopy (FTIR). The thermal stability of the produced char was estimated by Thermogravimetry–Fourier transform infrared spectroscopy (TG–FTIR). The char yield was 16.8% at 310 °C, which then increased with the temperature and reached the maxima value of 26.77% at 365 °C. SEM photo indicated that decomposition of lignin was enhanced with the increasing temperature, producing char with rough surface and few vesicles. The char at 330 °C had the largest surface area (2.5936 m2/g) and the largest pore volume (0.0189 cm3/g). XRD spectrum revealed that the char prepared at higher temperature produced higher ordered crystalline structure. Most of functional groups in char identified by FTIR were eliminated at 350 °C, except for the hydroxyl group. The char produced under high hydrothermal temperature exhibited high thermal-stability according to the right-shifted DTG curve against temperature. A mechanism was proposed to explain the possible steps for char structure evolution during lignin hydrothermal process in subcritical water, involving cleavage of the weak bonds at low temperature, and elimination of functional groups and carbonization at high temperature. The results would help improve the understanding of lignin degradation in subcritical water and optimize the hydrothermal process for producing value-added chemicals from lignin.