Quantum chemical calculation of reaction characteristics of hydroxyl at different positions during coal spontaneous combustion

Abstract

Hydroxyl groups are one of the key factors for the development of coal spontaneous combustion. Although the reaction mechanism has been studied by many scholars, the effects of their positions in the molecule on the reaction characteristics have not been considered. In this paper, Ph−CH2−CH3 was selected as the basic unit to construct small coal molecule models with one hydroxyl at different positions. The microscopic parameters of each model were calculated by density functional theory (DFT), and the elementary reaction pathways and thermodynamic parameters of hydroxyl groups were explored. It was found that the hydrogen of OH is the active site of nucleophilic reaction, the CαH bond of α-phenethyl alcohol and the OH bonds of the other molecules are most vulnerable to oxygen. All the models can generate H2O in the process of oxidation, other products of alcoholic hydroxyl groups are highly active oxygen-containing free radicals, while that of phenolic hydroxyl groups are quinones or ketones. The results of intrinsic reaction coordinate (IRC) indicated that hydrogen captured by oxygen is endothermic, while the ·OH free radical capturing hydrogen is exothermic. The activation energy of oxygen capturing hydrogen is 98–182 kJ/mol, which shows that the reaction can occur in the middle stage of coal spontaneous combustion (70–120 ℃), and the reaction rate gradually accelerate with the increase of temperature. In the same conditions, the order of the oxidation reaction rates is 2-ethyl phenol > 3-ethyl phenol > 4-ethyl phenol > α-phenethyl alcohol > β-phenethyl alcohol, which is the same as the order of reaction activities but opposite to that of activation energies. The research is helpful to strengthen the judgment of coal spontaneous combustion risk and the development of flame retardant.