Hydrothermal carbonization of glucose: Secondary char properties, reaction pathways, and kinetics

Hydrothermal carbonization (HTC) is a promising green route to synthesize carbon materials and valorize biomass waste. The intricacy behind the conversion is enormous, requiring investigations of the mechanisms starting from the fundamentals. Intending to contribute to the rudiments of HTC, this work provides a systematic insight into the decomposition of glucose during HTC. The interest in glucose stems from its nature, indeed it is: one of the building blocks of biomass, a substrate for producing advanced carbon materials, and a precursor of secondary char - the solid phase deriving from re-polymerization and condensation reactions from the liquid phase back onto solid. The work approaches the glucose HTC holistically, focusing on kinetics and characterization of the products. A 1.1 M glucose solution was hydrothermally carbonized over several operating conditions (180–270 °C and 0–8 h) with analysis of the solid and liquid phases. Results show that hydrochar changes significantly with time only at 180 °C. Above 180 °C, the solid yield stabilizes at 47–50 % and the carbon content at 67–70 %. The time-evolution of liquid intermediates (like HMF and small carboxylic acids) detected through NMR enabled the deduction of formation pathways and calibration of a higher-order kinetics model. Meanwhile, SEM and Raman spectra show hydrochars characterized by typical nano/microspheres, having a size distribution dependent on operating conditions and mainly an amorphous structure, with a tendency towards graphitization as the HTC severity increases. All the hydrochars show photoluminescence, a functionality that opens to several application possibilities.