Mechanistic study on the influence of hydrogen bonds on the removal paths of lignite ether bonds and phenolic hydroxyl groups in hydrothermal dewatering.

To investigate the impact of hydrogen bonds on the removal mechanism of ether bonds and phenolic hydroxyl groups during the hydrothermal dewatering (HTD) process of lignite, we employed density functional theory at the B3LYP-D3/6–311 G (d, p) level to calculate the removal paths of these functional groups. The results revealed that the ether bonds in lignite underwent conversion to phenol hydroxyl and carbonyl groups through isomerization and rearrangement reactions, respectively. Specifically, the isomerization reaction (IR) exhibited a lower minimum energy barrier (339.09 kJ/mol) compared with the rearrangement reaction (RR) (376.24 kJ/mol). Regarding phenolic hydroxyl groups, paths involving H radicals facilitated removal more effectively than direct dehydroxylation. Upon the formation of hydrogen bonds with lignite and water molecules, the energy barrier of PHR1 decreased by approximately 27.5 kJ/mol. In contrast, the IR energy barrier experienced a slight decrease, whereas the RR increased marginally. Simultaneously, the rearrangement of ether bonds and direct dehydroxylation of phenolic hydroxyl groups became more challenging, indicating that hydrogen bonds render these removal processes more favorable through isomerization and H radical-assisted mechanisms. [ABSTRACT FROM AUTHOR]