Study on Chemical Bond Dissociation and the Removal of Oxygen-Containing Functional Groups of Low-Rank Coal during Hydrothermal Carbonization: DFT Calculations

The molecular structure model of lignite was constructed, and the dissociation and removal mechanism of different C-O bonds and oxygen-containing functional groups was investigated using density functional theory (DFT) calculations. First, the bond order and bond dissociation enthalpy (BDE) were analyzed to predict the strength of different chemical bonds, and differences in the BDE and bond order were related to the difference in the fragment structure and electronic effects. The first group to break during hydrothermal carbonization (HTC) is the methyl of Ph(CO)O-CH3, followed by the C-O of CH3-OC(O)OH; the hydroxyl in Ph-OH is the most thermally stable group, followed by the hydroxyl in CH3OC(O)-OH. In addition, the orbital localization analysis has also been carried out. All three chemical bonds of Ph(CO)OCH3 show the characteristics of sigma bond, while Ph(C.O)OCH3 and Ph(CO)-OCH3 with the Mayer bond order (MBO) greater than 1 also contains certain pi bond characteristics. The lignite van der Waals (vdW) surface electrostatic potential (ESP) was constructed and visualized, and the results showed that the oxygen-containing functional groups mainly contributed to the area with a large absolute ESP. Finally, weak interactions between water molecules and lignite at different sites were described by independent gradient model (IGM) analysis. Models A, B, and E formed weak interactions with the hydrogen bond as the main force; model E showed the weakest hydrogen bond, while model C showed van der Waals interaction as the dominant force. In addition, some steric effect was also observed in model D.