Effect of hydrothermal treatment on biomass structure with evaluation of post-pyrolysis process for wood vinegar preparation

Hydrothermal treatment can arouse the comprehensive evolution of biomass structure, which broadens the horizons for the development and optimization of terminal products from biomass valorization. The dynamic evolution of the chemical structure of cotton stalk during hydrothermal treatment of 180–280 °C within 0–120 min was comprehensively studied by various ex-situ characterization techniques, as well as its basic properties. The feasibility of wood vinegar preparation from the hydrothermally treated cotton stalk (HTCS) was evaluated by pyrolysis at 350 °C. The carbon content of the HTCS samples increase from 44.68% to 65.96% with increasing hydrothermal temperature from 180 °C to 280 °C without retention, and from 53.86% to 57.95% at 230 °C with increasing residence time from 0 min to 120 min, respectively. Meanwhile, the oxygen content of the HTCS samples decrease significantly with intensifying hydrothermal treatment. The alkali metals in the HTCS samples are removed apparently with increasing hydrothermal severity. Hydrothermal temperature has a more significant effect on the chemical structure than residence time. Hemicellulose was decomposed at 180–200 °C, and lignin decomposition occurred above 200 °C, which was intensified at 260–280 °C without retention and at 230 °C within 30–60 min. Amorphous cellulose was decomposed at 200–230 °C, and the crystalline cellulose was mainly decomposed at 230–280 °C and at 230 °C within 0–30 and 60–120 min, respectively. The HTCS samples show the hydrophilic surface characteristic due to a deal of residual surface oxygen-containing groups. The growth of the aromatic system could be promoted under the hydrothermal treatment below 260 °C. The hydrothermally treated cotton stalk at 230 °C without retention could be used to prepare wood vinegar with the abundant phenols and ketones through pyrolysis at 350 °C. Overall, the study will provide insight into the preparation of diverse value-added products and guidance to the fabrication of advanced functional materials from hydrothermally treated biomass.