Effects of hydroxyl on oxidation characteristics of side chain active groups in coal

Due to the extreme complexity of coal structure and the reaction between coal and oxygen,six molecule models were constructed based on hydroxyl (—OH) and side chain active groups including —OCH、—CHOHCH3 and—OCH3 by employing B3LYP-311G (d,p) method of Density Functional Theory (DFT). On the basis of frontier or- bital and quantum chemistry theories,the electrostatic potentials,frontier molecular orbital,and thermodynamic param- eters of these six constructed molecule models were simulated and calculated by using the software of Gaussian 16. Al- so,the oxidization characteristics of the side chain active groups and their responses to the hydroxyl (—OH) were in- vestigated. It was found that the electrostatic potentials near the hydrogen atoms were positive,and the negative electrostatic potentials concentrated around oxygen atoms. This phenomenon implied that active sites of the nucleophilic reac- tion lied on hydrogen atoms,whereas the oxygen atoms were active sites of the electrophilic reaction. Furthermore,it was found that the adjacent hydroxyl group (—OH) would reduce the electrophilic reactivity of the side chain active groups and increase the nucleophilic reactivity of —CHO and —CHOHCH3 . Nevertheless,the nucleophilic reactivity of —OCH3 would disappear when the hydroxyl group (—OH) located next to it. Analysis of the Highest Occupied Or- bital (HOMO) and the Lowest Unoccupied Orbital (LUMO) of each active group indicated that the stabilities of the active groups were not in agreement with the bonding abilities of their front orbitals. The propensity of active groups re- acted with oxygen mainly relied on the bonding ability of LUMO. The stronger the bonding ability was,the easier the active groups reacted with oxygen. Since hydroxyl (—OH) could change the characteristics of electrons in the front or- bital,it exhibited significant impact on the compound reactions between active groups. More specifically,the hydroxyl benefited the reactions between oxygen and —CHO as well as —CHOHCH3 . However,hydroxyl (—OH) converted the reaction between —OCH3 and oxygen from the spontaneous exothermic reaction to the nonspontaneous endothermic reaction. This study is of importance for revealing the microscopic mechanism of coal spontaneous combustion and de- veloping new efficient inhibitors of coal spontaneous combustion.