Experimental and simulation study on hydroxyl group promoting low-temperature oxidation of active groups in coal

The influence of hydroxyl groups in coal on low-temperature oxidation of other active groups was studied by analyzing the reactivity of six small molecule models under various conditions by using the reactive force field molecular dynamics method. Fourier-transform infrared spectroscopy was performed to investigate the influence of hydroxyl on the coal self-reaction process at low temperatures. Finally, the electrostatic potential, bond length, and frontier orbital energy level of each reaction model were calculated using density functional theory. The calculation results revealed that the reactivity of the alcohol hydroxyl structure was the strongest without and with oxygen, and numerous hydroxyl radicals were generated during the reaction process. Hydroxyl radicals accelerated the reaction rate of the active group to be faster than O2. Hydroxyl radicals can affect the reaction of the active group through two routes, one route is the hydrogen abstraction reaction, and the other route is electrophilic substitution. The amount of the OH–OH structure in coal decreased by 3 % under the oxygen-free condition at 343 K compared with that under the normal temperature (298 K). This phenomenon played a crucial role in the self-reaction process of coal and promoted –COOH and –CH2– reactions. Quantum chemical calculations revealed that the energy gap value of each reaction structure decreased, and the reaction activity increased. Hydroxyl radicals affect the electron cloud density of the active group. The nucleophilic reaction sites of ethyl and methyl structures increased, the CC bond between carboxyl and aldehyde structures and benzene ring was long, and the ability of decarboxylation and dealdehyde was enhanced. Furthermore, the benzene ring of the phenolic hydroxyl group was activated, and the nucleophilic reaction ability of the alcohol hydroxyl group was enhanced.