Quantum Chemical Calculation of the Effects of H2O on Oxygen Functional Groups during Coal Spontaneous Combustion

The effects of H2O on the low-temperature oxidation characteristics of coal have always been one of the keys in the research of coal spontaneous combustion, but most studies rely on experiments for macroscopic derivation, and theoretical researches at the microlevel are rarely mentioned. In this paper, phenylacetaldehyde, phenylethyl alcohol, phenylacetic acid, and ethylbenzene hydroperoxide were used as modeling compounds of coal molecules containing aldehyde (-CHO), alcohol hydroxyl (-OH), carboxyl (-COOH), and peroxide (-C-O-OH). The surface electrostatic potential (ESP), electron density of atoms in molecules (AIM), and reduced density gradient (RDG) of coal molecules were calculated by density functional theory (DFT), and the thermokinetic parameters of low-temperature oxidation of coal molecules with or without H2O were analyzed. The results show that the extreme positive and negative ESPs are located at the H and O atoms of oxygen functional groups (OFGs), respectively, which are the active sites for H2O adsorption. The AIM and RDG show that the phenylacetaldehyde···H2O complexes have two kinds of adsorption configurations with two and three hydrogen bonds, and that the phenylethyl alcohol···H2O complexes also have two kinds of adsorption configurations with one and three hydrogen bonds, and that both phenylacetic acid···H2O and ethylbenzene hydroperoxide···H2O only have one adsorption configuration, forming two and three hydrogen bonds, respectively. According to electron density ρ(r) and potential energy density V(r), the adsorption strength of H2O by four kinds of oxygen functional groups is ranked as -C-O-OH > -COOH > -OH > -CHO. The thermokinetic parameters show that H2O can increase the activation energy (ΔE) of the oxidation reactions of phenylacetaldehyde and phenylethyl alcohol, which can inhibit the reaction and decrease the activation energy (ΔE) of the oxidation reaction of phenylacetic acid and ethylbenzene hydroperoxide, which can promote the reactions.