Decoupled temperature and pressure strategies in hydrothermal process of cellulose: A comprehensive study

[Display omitted] •Develop a novel Decoupled Temperature-pressure Hydrothermal (DTPH) system.•Reveal hydrothermal mechanisms of cellulose during DTPH treatment.•Pressures >12 MPa largely transform crystalline structures into amorphous regions.•Decoupled pressure facilizes repolymerization of polyfuranic/aromatic compounds.•Sustainable hydrochar production achieved under low pressure and temperature. Hydrothermal treatment of cellulose has attracted extensive attention for generating carbon materials, chemicals, and combustible gases. Unlike conventional methods where the temperature and pressure were inherently coupled, a decoupled temperature–pressure hydrothermal (DTPH) system was developed in this work, which allowed for independent manipulation of temperature (150–290 °C) and pressure (4–20 MPa). Our research revealed that higher pressures, particularly beyond 12 MPa within the DTPH system, were pivotal in transforming the crystalline structure of cellulose into amorphous structure. This significantly speeded up the hydrolysis process, leading to the enhanced conversion to 57.0 %, as well as the efficient production of assorted aqueous phase species, such as 5-hydroxymethylfurfural, levulinic acid, and formic acid. Furthermore, the decoupling pressure also promoted the repolymerization of aqueous phase species into carbon microspheres, which are primarily composed of water-soluble polyfuranic and aromatic compounds. Low-temperature carbonization at 200 °C was achieved by extending the reaction durations under 8 MPa. Such an approach underscored the capacity of decoupled system to promote specific reaction pathways, including the direct formation of hydrochar from original cellulose, with the carbon content up to 69.4 %. The study suggests that through the optimization of DTPH system, it is feasible to produce high-value-added products, enhancing the sustainability of biomass conversion technologies.