Quantum chemistry calculation of reaction pathways of carboxyl groups during coal self-heating

Self Cite

In order to further understand the mechanism of coal self-heating in the initial stage, the aldehyde group was analyzed by using the quantum chemistry methods. The charge distribution, structural parameters, and molecular orbital were analyzed to determine the active sites existing in the structure of aldehyde group. Then, a chemical reaction model including five elementary reaction sequences was established. In elementary reaction E1, the hydrogen of the aldehyde group is captured by hydroxyl to form the aldehyde radical, which provides the reactant and accumulates heat for the subsequent reaction. In elementary reaction E2, the aldehyde radical further reacts to form a carbon-free radical (R · ) and CO, which is the main source for CO generation during coal spontaneous combustion. In elementary reaction E3, the aldehyde radical is oxidized to a carboxyl radical, providing the reactant for elementary reaction E4, which is directly related to CO 2 generation during coal spontaneous combustion. The thermodynamic parameters of the elementary reactions were further analyzed and confirmed by quantum chemistry methods. The results are helpful for further understanding the pathways of CO generation in the initial stage of coal spontaneous combustion, which provides theoretical support for prediction of coal spontaneous combustion.

Section: Resultsmentioning

confidence: 99%

Reaction Mechanism of Aldehyde Groups during Coal Self-Heating

Chen

et al. 2020

Self Cite

“…Alkyl side chains, , bridge bonds, and OFGs ,− are typical active groups on the coal surface. Alkyl side chains and bridge bonds are hydrophobic groups whose interactions with H 2 O are weak.…”

Section: Computational Detailsmentioning

confidence: 99%

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

Huo

Zhu

et al. 2021

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.

“…Many scholars have explored the reaction sequences of coal spontaneous combustion at low temperatures on the basis of studying the active sites. Qi et al brought forward the reaction processes of aliphatic hydrocarbons, carboxyls, , hydroxyls, and sulfur-containing functional groups, which deeply expounded the micromechanism of coal spontaneous combustion. Zhu et al , explored the pathways of the oxidation reactions when the aldehyde and hydroxyl in coal molecules are located at different sites by applying the quantum chemical calculation method.…”

Section: Introductionmentioning

confidence: 99%

“…Aliphatic functional groups mainly include −CH, −CH 2 , and −CH 3 , while oxygen-containing functional groups include −OH, −CO, −COOH, −CHO, and so on. 21−25 Not only do the original functional groups have a significant impact on the process of coal spontaneous combustion, but also a large number of radicals 26 27 carboxyls, 28,29 hydroxyls, 30 and sulfur-containing functional groups, 31 which deeply expounded the micromechanism of coal spontaneous combustion. Zhu et al 9,10 explored the pathways of the oxidation reactions when the aldehyde and hydroxyl in coal molecules are located at different sites by applying the quantum chemical calculation method.…”

Section: Introductionmentioning

confidence: 99%

Quantum Chemistry Calculation Study on Chain Reaction Mechanisms and Thermodynamic Characteristics of Coal Spontaneous Combustion at Low Temperatures

Huo

Zhu

et al. 2021

The coal spontaneous combustion phenomenon seriously affects the safety production of coal mines. Aiming at the problem of complex coal molecular structure and incomplete reaction sequences at present, the mechanisms and thermodynamic parameters of coal spontaneous combustion chain reactions were explored by combining experimental detections and molecular simulations. First, the active groups on the surface of coal were obtained by Fourier transform infrared spectroscopy (FTIR), mainly including methyl (−CH3), methylene (−CH2), methyne (−CH), phenolic hydroxyl (−ArOH), alcohol hydroxyl (−ROH), carboxyl (−COOH), aldehyde (−CHO), and ether (−O−), and the coal molecular models containing functional groups and radicals were established. According to the charge density, electrostatic potential, and frontier orbital theories, the active sites and active bonds were obtained, and a series of reactions were given. The thermodynamic and structural parameters of each reaction were explored. In the chain initiation reaction stage, O2 chemisorption and the self-reaction of radicals play a leading role. In this stage, heat gradually accumulates and various radicals begin to generate, where the intramolecular hydrogen transfer reaction of a peroxide radical (−C–O–O·) can produce the key hydroxyl radical (−O·). In the chain propagation reaction stage, O2 and −O· continuously consume active sites to accelerate the reaction sequences and increase the temperature of coal, and index gases such as CO and CO2 generate, causing the chain cycle reactions to gradually form. The chain termination reaction stage is the formation of stable compounds such as ethers, esters, and quinones, which can inhibit the development of chain reactions. The results can further explain the reaction mechanism of coal spontaneous combustion and provide references for the development and utilization of chemical inhibitors.