Conversion of cellulose to highly aromatic hydrochar by catalytic hydrothermal carbonization: The role of lanthanide(III) ions

Hydrothermal carbonization (HTC) is an effective resource recycling technology for lignocellulosic biomass with negative carbon emissions. However, the potential evolution process of the main coalification paths during resource recycling is not very clear, which hinders the optimization of the hydrothermal process and the design of catalysts. In this study, cellulose was chosen as a model compound to study the evolution behavior and mechanism of the main coalification paths as the HTC reaction temperature increased. Our findings revealed that cellulose was preferentially carbonized below 220 degrees C by pyrolysis reaction under high-pressure inhomogeneous conditions. At 220-250 degrees C, the main chemical composition in hydrochar was continuously converted from fiber structure to furanoic compounds, while the higher heating value (HHV) significantly increased. This change was accompanied by a continuous decrease of the insoluble cellulose, leading to complete hydrolysis and an intensification of the polymerization reaction of aqueous organics. Between 250 and 280 degrees C, the conversion of furans into phenols occurred, followed by polycondensation of phenolic components and even the nucleophilic addition of carbonyl compounds with phenol, the proportion of arene compounds in the hydrochar increased, while the proportion of furans decreased. Additionally, violent esterification reaction led to an increase in ester bonds. On this basis, we also selected acetic acid and LaCl3 for further validation and catalysis experiments. In the low-temperature hydrothermal carbonization process of cellulose, lanthanide(III) ions exhibit better catalytic effects than acid catalysts. At 200 degrees C, they can effectively accelerate the conversion of part of the cellulose into tar microparticles and promote the complete hydrolysis of cellulose into tar microparticles rich in furan structures at 220 degrees C. The conversion of hexose sugars to HMF is achieved while inhibiting the conversion of HMF to LA(levulinic acid), thus increasing the absolute content of HMF in the hydrothermal system and promoting the polymerization of HMF into coke microparticles. This study provides a theoretical basis for the production of recyclable and sustainable solid fuel resources using low-carbon hydrothermal technology.