Removal of oxygen-containing functional groups during hydrothermal carbonization of biomass: Experimental and DFT study

The main purpose of this study was to investigate the evolution of oxygen-containing functional groups in the hydrothermal carbonization (HTC) of biomass using 13C DP/MAS NMR. Meanwhile, some other physicochemical properties were also studied. After HTC at 260 °C, the peak strength of O-alk decreased by 47.64% and that of aro-C increased by 28.62%, indicating that HTC could lead to severe dehydroxylation, decarboxylation and aromatization reactions. According to the results of density functional theory (DFT), the ESP values of cellulose molecular surface mainly concentrated between −25 kcal/mol-15 kcal/mol, and the difficulty of removing oxygen-containing functional groups from difficult to easy follows the order: (C5H9O)CH2–OH > (C5H9O)–CH2OH > (C5H9O2)–OH(4) > CH3O–CH3 > (C5H9O2)–OH(3) > (C5H9O)O–CH3 > (C5H9O)–OCH3. Compared with the raw sample, hydrochars have higher aromatics and calorific value, which indicates that HTC can transform biomass into higher grade fuel, providing the possibility to alleviate the depletion of fossil resources. [Display omitted] •13C DP/MAS NMR was used to quantitatively characterize the oxygen-containing functional groups.•The physical and chemical properties of hydrochars were tested.•DFT calculations were used to simulate the hydrothermal carbonization of biomass.