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

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 C-alpha-H bond of alpha-phenethyl alcohol and the O-H 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 center dot 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 degrees C), 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-ethylphenol > alpha-phenethyl alcohol > beta-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. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Automated summary

This study investigated the reaction characteristics of hydroxyl groups in coal molecules, revealing that the reaction pathways and thermodynamic parameters are dependent on the position of the hydroxyl group. The presence of hydroxyl groups can lead to the generation of water and oxygen-containing free radicals during oxidation reactions, providing insights for assessing coal spontaneous combustion risk and developing flame retardants.