Removal of O-containing functional groups during hydrothermal treatment dewatering: A combined experimental and theoretical theory study

•The removal mechanism of O-containing functional groups were studied.•Effect of O-containing groups on floatibility characteristics was studied.•The ESP, bond order, and BDE of lignite molecular structure were calculated by DFT.•The essential reason for decrease of water content during HTD was explained. Hydrothermal treatment dewatering (HTD) is a potential means for efficiently and cleanly utilizing lignite due to its comprehensive modification, including dehydration and deoxidation.These processes are as tightly linked to the removal of O-containing functional groups in lignite. However, the O removal mechanism during the HTD process is not clear. It is important to deeply study this problem for the efficient application of upgraded lignite. Herein, density functional theory (DFT) simulations combined with experiments have been carried out to investigate the removal mechanism of O-containing functional groups. In addition, the HTD influence on the flotation characteristics was studied. The results showed that the carbon content and the calorific value increase after the HTD process, which improve the lignite quality. The oxygen content decreased from 28.55 to 16.67%, and the O/C atomic ratio decreased from 0.33 to 0.16 at 310 °C. The FT-IR results showed that the decrease in oxygen content is due to the cleavage and decomposition of methoxy, alcohol hydroxyl, and carboxyl groups. These results are consistent with the DFT results of bond orders and bond dissociation energies (BDE). The BDE order in the HTD process is: CO in methoxy