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Article

Coal Spontaneous Oxidation Mechanism of Low-Molecular Compounds: Pentanol

by
Tianyi Yang
1,
Xiaobo Wang
1,*,
Wenhao Deng
1,
Sichen Liu
1,
Hanzhong Deng
2 and
Yafei Shan
3
1
College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan 030024, China
2
College of Materials Science and Engineering, Liaoning Technical University, Fuxin 123000, China
3
College of Safety Science and Engineering, Liaoning Technical University, Fuxin 123000, China
*
Author to whom correspondence should be addressed.
Fire 2026, 9(6), 253; https://doi.org/10.3390/fire9060253 (registering DOI)
Submission received: 16 April 2026 / Revised: 5 June 2026 / Accepted: 10 June 2026 / Published: 13 June 2026
(This article belongs to the Special Issue Fire Risk Management and Emergency Prevention)
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Abstract

Coal spontaneous combustion (CSC) remains a major hazard in coal mining. Research on CSC has largely focused on macromolecular structures, while the behavior of low-molecular-weight compounds remains unclear. Using B3LYP/6-311G density functional theory, this study systematically reveals thirteen microscopic reaction pathways, active sites, and the energy barrier order of pentanol during coal spontaneous combustion. The oxidation proceeds via thirteen multi-step pathways involving bond breaking and formation, with the dominant reaction being oxygen attack on the -CH2OH group to produce pentanal (CH3CH2CH2CH2CHO) and water as the main products. The priority order of thirteen reaction pathways between pentanol and oxygen was established as: Path 6 > Path 3 > Path 8 > Path 5 > Path 4 > Path 1 > Path 11 > Path 10 > Path 9 > Path 12 > Path 7 > Path 2. The results reveal the multi-step bond-breaking and formation mechanism at the molecular level, providing a fundamental theoretical framework for understanding the radical chain oxidation mechanism of low molecular weight compounds in CSC.

1. Introduction

Coal remains the cornerstone of Chinese energy security in the short term [1], while it is susceptible to spontaneous combustion during mining, transportation, and storage due to its oxidizing and exothermic properties [2,3]. Coal spontaneous combustion (CSC) is a major hazard in Chinese coal mines [4], accounting for over 60% of underground fire incidents [5]. CSC releases large amounts of toxic and harmful gases, wastes non-renewable resources and threatens the ecological environment [6], and can also trigger gas and coal dust explosions that endanger life and property [7,8].
Experimental and simulation techniques [9] have been widely used in the study of coal spontaneous combustion. Based on the widely accepted coal-oxygen complexation theory [10,11], studies on the complex oxidation kinetics of coal spontaneous combustion have shown that this process involves oxygen adsorption, surface reactions, radical transformation, and heat accumulation [12]. Extensive studies on CSC’s complex oxidative kinetics have been conducted focusing on transformation mechanisms and reaction pathways of active species and functional groups [13,14]. Chen [15] combined experiments with DFT calculations to reveal the evolution and reaction pathways of active groups during coal oxidation. Zhang [16] systematically analyzed the oxidation pathways of -OH, -CHO, -CHOHCH2-, and -COOH using quantum chemistry. Qin [17] identified the key roles of oxygen-containing groups and side chains in coal oxidation via DFT. Gao [18] revealed how aldehyde and carboxyl groups regulate coal spontaneous combustion. Huo [19] found that aliphatic -CH2- is the main contributor to exothermic reactions. Zhu [20] elucidated aldehyde oxidation and self-reaction pathways and their role in CO/CO2 formation. Zhu [21] clarified the effect of hydroxyl position on reaction activity. Previous research has examined variations in oxidation products across coal ranks, the evolution of characteristic functional groups, and radical reaction pathways. Furthermore, the role of low-molecular-weight compounds (LMCs) in CSC has gradually gained increasing attention. For example, Wang and Cui [22,23] revealed reaction pathways of low molecular ketones and hydrocarbons.
LMCs constitute only 20% of coal organics. They interact with the macromolecular framework via noncovalent bonds, exhibiting faster decomposition and lower activation energies, significantly affecting coal’s caking, liquefaction, and spontaneous combustion, including combustion rate and product composition [24,25,26]. Current research focuses on the oxidation mechanisms of hydrocarbons or ketones [22,23], confirming that highly reactive LMCs have a significant promoting effect during CSC. However, research on LMCs such as pentanols during CSC remains limited, making further investigation of their spontaneous combustion mechanisms of significant theoretical and practical value. The intrinsic chemical properties of pentanol oxidation, including transition state geometries and energy barriers, are primarily governed by the molecule’s own electronic structure and spatial configuration rather than by constraints from the macromolecular framework. Consequently, the essential difference between pure and coal-embedded pentanol oxidation lies in the reaction environment, such as oxygen concentration and diffusion rate, and these environmental factors do not affect the quantum chemical calculation results presented in this paper.

2. Geometric Configuration of Pentanol in LMCs

The basic structural unit of pentanol was optimized using DFT at the B3LYP/6-311G level with Gaussian03 (Revision E.01, Gaussian, Inc., Wallingford, CT, USA). Its equilibrium geometry is shown in Figure 1, where white, gray, and red spheres represent H, C, and O atoms, respectively. Single bonds connect C-C and C-H, while double bonds connect C-O. Bond lengths (R), bond angles (A), and dihedral angles (D) are listed in Table 1, Table 2 and Table 3. R (5,6) refers to the C5-O6 bond. A (4,5,6) is the angle between the C-C and C-O bonds. D (14,4,5,6) is the dihedral angle involving H14, C4, C5, and O6.

3. Active Site and Chemical Reactions That React with Oxygen in Pentanol Molecule

According to molecular orbital theory, frontier orbitals govern molecular reactivity, including HOMO and LUMO. The HOMO and nearby occupied orbitals tend to donate electrons, while the LUMO and nearby unoccupied orbitals tend to accept electrons [24]. For pentanol, the active sites are the atoms with the highest electron cloud density in the HOMO. The frontier orbital energy also reflects the ease of electron transfer, and the charge density distribution in Table 4 identifies C1 to C5 as the atoms most susceptible to oxygen attack. Therefore, these four atoms are the primary reactive sites for oxidation and spontaneous combustion of pentanol.
The chemical reaction steps of coal oxidation and spontaneous combustion are coal fragments under external forces, oxygen diffuses and adsorbs onto the coal surface, then reacts to form intermediates and products, and finally gaseous products desorb into the air. Based on the main gases generated during coal oxidation and spontaneous combustion, atoms C1 to C5 exhibit the highest reactivity during the oxidation of coal. According to chemical reaction mechanism theory [25], the reaction equations for coal spontaneous combustion are given by Paths 1 to 13, as illustrated in Figure 2.
Oxygen initially targets the -CH2OH group of pentanol, yielding products like pentanal and H2O, or butane and HCOOH, as indicated by Paths 1 and 2. Oxygen then attacks the terminal -CH2- group in CH2OHCH2-, forming products like propane and CHOCH2OH, or CH3CH2CH2·CCH2OH and H2O, or butanal and CH3OH, denoted by Path 3 to 5. Then oxygen targets -CH2- within CH3CH2CH2-, producing CH2OHCH2CHO and CH3CH3, or CH3CH2CHO and CH3CH2OH, or CH3CH2·CCH2CH2OH and H2O, represented by Path 6 to 8. Oxygen then attacks the -CH2- group in CH3CH2-, yielding·CHCH2CH2CH2CH2OH and H2O, or CHOCH2CH2CH2OH and CH4, or propanol and CH3CHO, as shown in Path 9 to 11. Finally, oxygen interacts with the terminal CH3- group, forming butanol and HCHO, or CH2CH2CH2CH2OH and H2O, as shown in reactions Path 12 and 13.

4. Calculations of Reactions in Pentanol in Coal

4.1. Analysis of Geometrical Configurations and Chemical Reaction Processes of Each Stationary Point

Pentanol in coal reacts with oxygen through thirteen distinct paths during combustion, denoted as R (reactants), O (oxygen), TS1–TS28 (transition states), MI1–MI28 (intermediates), and P1–P13 (products). The molecular formulas of each abbreviation are given in Table 5.
R + O→TS1→MI1→TS2→MI2→TS3→MI3→P1 + H2O Path1
R + O→TS1→MI1→TS4→MI4→TS5→MI5→P2 + HCOOH Path2
R + O→TS6→MI6→TS7→MI7→TS8→MI8→P3 + CHOCH2OH Path3
R + O→TS6→MI6→TS9→MI9→P4 + H2O Path4
R + O→TS6→MI6→TS10→MI10→TS11→MI11→P5 + CH3OH Path5
R + O→TS12→MI12→TS13→MI13→TS14→MI14→P6 + CH3CH3 Path6
R + O→TS12→MI12→TS15→MI15→TS16→MI16→P7 + CH3CH2OH Path7
R + O→TS12→MI12→TS17→MI17→P8 + H2O Path8
R + O→TS18→MI18→TS19→MI19→P9 + H2O Path9
R + O→TS18→MI18→TS20→MI20→TS21→MI21→P10 + CH4 Path10
R + O→TS18→MI18→TS22→MI22→TS23→MI23→P11 + CH3CHO Path11
R + O→TS24→MI24→TS25→MI25→TS26→MI26→P12 + HCHO Path12
R + O→TS24→MI24→TS27→MI27→TS28→MI28→P13 + H2O Path13

4.1.1. Geometric Configurations and Chemical Reactions of Path 1

The optimized geometries and structural parameters for oxygen attacking the -CH2OH group of pentanol to form CH3CH2CH2CH2CHO and H2O are shown in Figure 3.
Quantum chemical calculations reveal the reaction mechanism as path R + O→TS1→MI1→TS2→MI2→TS3→MI3→P1 + H2O. Oxygen attacks the -CH2OH group in pentanol, drawing two H atoms from -CH2-. Two C-H bonds elongate from 1.09572 Å to 1.11106 Å and from 1.09698 Å to 1.11289 Å, entering unstable TS1 with a single imaginary frequency of 1287 cm−1. Upon elongation and fracture, the H atom leaves C and bonds with O, while O bonds with C, generating stable MI1. MI1 changes significantly in bond and dihedral angles, forming unstable TS2 (172 cm−1). The OH group’s angularity then inverts largely, proceeding to stable MI2. With a C-O bond in -CHOHOH extending from 1.44894 Å to 1.80665 Å and an O-H bond stretching from 0.97352 Å to 1.28957 Å, TS3 forms with a frequency of 1737 cm−1. Following the rupture of the O-H bond, the H atom moves to the -OH group and bonds with C, progressing to stable MI3, which decomposes into final products.

4.1.2. Geometric Configurations and Chemical Reactions of Path 2

Path 2 features an attack by oxygen on the -CH2OH group of pentanol, which yields CH3CH2CH2CH3 and HCOOH. Its molecular configurations and key parameters are given in Figure 4.
The reaction path R + O→TS1→MI1→TS4→MI4→TS5→MI5→P2 + HCOOH involves oxygen attacking -CH2OH in pentanol. This interaction pulls two H in -CH2- away, stretching and breaking C-H bonds from 1.09572 Å to 1.11106 Å and from 1.09698 Å to 1.11289 Å, forming TS1 with a frequency of 1287 cm−1. Following bond elongation and rupture, H detaches C and bonds with O, creating an active state MI1, which further transforms into TS4 (257 cm−1). The -OH group in TS4 inverts significantly to transition to stable MI4. Then the C-C bond stretches from 1.51751 Å to 2.03961 Å, and the O-H bond extends from 0.97543 Å to 1.31276 Å, entering TS5 (2191 cm−1). The broken O-H bond’s H bonds with C in -CH2-, progressing to MI5, subsequently decomposing to ultimate yields.

4.1.3. Geometric Configurations and Chemical Reactions of Path 3

Oxygen in Path 3 targets the -CH2- group in -CH2CH2OH of pentanol, which creates CH3CH2CH3 and CHOCH2OH. Figure 5 outlines its optimized configurations and chemical parameters.
In the reaction R + O→TS6→MI6→TS7→MI7→TS8→MI8→P3 + CHOCH2OH, oxygen engages with the -CH2- group, drawing two H atoms and stretching C-H bonds from 1.09400 Å to 1.12024 Å, forming unstable TS6 with a frequency of 1286 cm−1. H then moves from C to O atom and forms a bond, creating stable MI6. With C-C bond stretching from 1.52911 Å to 1.94908 Å and the O-H bond in -CHOH- extending from 0.97148 Å to 1.39898 Å, MI7 undergoes intricate chemical transformations, entering unstable TS8. Finally, the broken O-H bond’s H in -CHOH- forms a new bond with C in CH3CH2CH2-, proceeding to stable MI8, which decomposes into final products.

4.1.4. Geometric Configurations and Chemical Reactions of Path 4

Figure 6 shows the microscopic optimized geometries and structural parameters for oxygen attacking the -CH2- group in -CH2CH2OH of pentanol to form CH3CH2CHCH2C·CH2OH and H2O.
In the reaction R + O→TS6→MI6→TS9→MI9→P4 + H2O, oxygen targets -CH2- group in -CH2CH2OH. Two C–H bonds are stretched from 1.09400 Å to 1.12024 Å and broken due to the attraction between O and H atoms, forming unstable TS6 (1286 cm−1). H then departs from C and bonds with O, producing MI6. MI6 undergoes complex chemical changes with the C-H bond in -CHOH- stretching from 1.08916 Å to 1.43161 Å and C-O bond in C-OH extending from 1.6820 Å to 2.14943 Å, entering TS7 (790 cm−1). H from the broken C-H bond then bonds with O from the broken -OH group, creating MI9 and breaking down to yield final products.

4.1.5. Geometric Configurations and Chemical Reactions of Path 5

In Path 5, oxygen attacks the -CH2- group in -CH2CH2OH, which yields CH3CH2CHCH2C:CH2OH and H2O. Its molecular structures and parameters are presented in Figure 7.
In the process R + O→TS6→MI6→TS10→MI10→TS11→MI11→P5 + H2O, oxygen’s attraction to H in -CH2- alters C-H bonds from 1.09400 Å to 1.12024 Å, generating TS6 (1286 cm−1).Followed by the elongation and rupture of a C-H bond to 2.01774 Å, C bonds with O and creates a stable state MI6. The O-H bond in -CHOH- of MI6 rotates, generating unstable TS10 (387 cm−1). Then the O-H bond in -CHOH- stretches from 0.97270 Å to 1.34107 Å and the C-C bond in -CH2CH2OH stretches from 1.52512 Å to 2.04361 Å, generating unstable TS11 (2211 cm−1). H atom from the broken O-H bond bonds with -CH2OH, forming stable MI11.

4.1.6. Geometric Configurations and Chemical Reactions of Path 6

Path 6 is characterized by the terminal -CH2- group in CH3CH2CH2- engaged with oxygen, yielding CHOCH2CH2OH and CH3CH3. Figure 8 delineates its molecular mechanisms and structural parameters.
The reaction R + O→TS12→MI12→TS13→MI13→TS14→MI14→P6 + CH3CH3 involves C-H bonds in -CH2- stretching from 1.09232 Å to 1.11906 Å due to oxygen’s attraction, generating unstable TS12 (1247 cm−1). C-H bond then extends from 1.11906 Å to 2.03186 Å and ruptures, with O bonding with C and detached H to form MI12. The -OH group inverts to form TS13, proceeding to MI13, an isomer of TS13. With C-C and O-H bonds stretching to 2.00171 Å and 1.36816 Å, the molecule enters unstable TS14 (2020 icm−1). Then the O–H bond breaks, and the released H atom bonds to CH3CH2-, producing CH3CH3 and stable MI14.

4.1.7. Geometric Configurations and Chemical Reactions of Path 7

In Path 7, oxygen assails -CH2- group in CH3CH2CH2-, generating CH3CH2CHO and CH3CH2OH. Figure 9 shows Path 7’s theoretical molecular configurations and chemical parameters.
During R + O→TS12→MI12→TS15→MI15→TS16→MI16→P7 + CH3CH2OH, oxygen attacks the terminal -CH2-, stretching C-H bonds from 1.09232 Å to 1.11906 Å and forming TS12 (1247 cm−1). With a C-H bond extending to 2.03186 Å, O bonds with C and is separated from H, forming MI2. After significant inversion of the -OH group in MI12, TS15 forms with a frequency of 333 cm−1 and rearranges to its stable isomer MI15. Then C-C and O-H bonds stretch to 2.03984 Å and 1.36033 Å, creating TS16 (2117 cm−1). The broken O-H bond’s H bonds with -CH2CH2OH, producing CH3CH2OH and stable MI16.

4.1.8. Geometric Configurations and Chemical Reactions of Path 8

Path 8 involves oxygen assaulting the terminal -CH2- group in pentanols’ CH3CH2CH2-, producing CH3CH2C.CH2CH2OH and H2O. Its structural mechanisms and molecular parameters are detailed in Figure 10.
During the process R + O→TS14→MI14→TS15→MI15→P8 + H2O, two C-H bonds in the -CH2- group stretch from 1.09569 Å to 1.12005 Å due to oxygen’s attraction to H atoms, creating TS14 (1311 cm−1). With the C-H bond extending to 2.01757Å and breaking, the O atom bonds with C and separates H to form stable MI14. Then C-O bond in -CHOH- group stretches from 1.47119Å to 2.27494Å and C-H bond extends from 1.0907 Å to 1.45531Å, creating unstable TS15n(517 icm−1). The H detaching from the C-H bond connects with the -OH group in the -CHOH- group, producing H2O and stable MI15.

4.1.9. Geometric Configurations and Chemical Reactions of Path 9

Path 9 involves oxygen targeting -CH2- group in CH3CH2-, yielding CH3·CCH2CH2CH2OH and H2O. Its molecular mechanisms and chemical parameters derived from theoretical calculations are detailed in Figure 11.
During the reaction R + O→TS18→MI18→TS19→MI19→P9 + H2O, oxygen attracts two H atoms from the -CH2- group in CH3CH2-, stretching and breaking C-H bonds to form TS18 (1315 cm−1). After the C-H bond extends to 2.01590 Å and breaks, O bonds with C and the detached H to form MI18. Then, the C-O bond in CH3CHOH- stretches to 2.29995 Å and the C-H bond in -CHOH- extends to 1.38611 Å, forming unstable TS19 (839 cm−1). H from the broken C-H bonds with the -OH group formed by C-O cleavage, producing H2O and stable MI19.

4.1.10. Geometric Configurations and Chemical Reactions of Path 10

Path 10 features oxygen-assailing -CH2- group in CH3CH2-, which produces CHOCH2CH2CH2OH and CH4. Figure 12 presents its reaction structures and molecular parameters.
In reaction R + O→TS18→MI18→TS20→MI20→TS21→MI21→P10 + CH4, oxygen attacks -CH2- group in CH3CH2-. C-H bonds stretch from 1.09552 Å to 1.11863 Å, forming TS18. One C-H bond extends to 2.01590 Å and O bonds with C and H atoms, forming MI18. The inversion of the -OH group leads to TS20 with a frequency of 313 cm−1, which rearranges to MI20. Then C-C and O-H bonds in CH3CHOH- stretch to 1.99668 Å and 1.36612 Å, respectively, forming TS21. The H from the broken O-H bond bonds with the CH3- group, producing CH4 and stable MI21.

4.1.11. Geometric Configurations and Chemical Reactions of Path 11

Path 11 involves oxygen attacking the -CH2- group in the CH3CH2- group, producing CH3CH2CH2OH and CH3CHO. Figure 13 presents its molecular structures and chemical parameters.
During the reaction R + O→TS18→MI18→TS22→MI22→TS23→MI23→P11 + CH3CHO, the interaction between O and H atoms stretches C-H bonds in -CH2- from 1.09552 Å to 1.11863 Å, generating TS18. Following a C-H bond’s extension and rupture, O bonds with C and separates H to form stable MI18. The -OH group alters significantly to form unstable TS22, entering stable isomer MI22. Then the C-C bond in CH3CHOHCH2- stretches from 1.53267 Å to 1.99110 Å and the O-H bond in -CHOH- extends from 0.97260 Å to 1.36946 Å, creating TS23 2053 cm−1. The H broken from the O-H bond connects with the -CH2CH2CH2OH group formed by C-C cleavage, yielding stable MI23, which decomposes into the final products.

4.1.12. Geometric Configurations and Chemical Reactions of Path 12

The CH3– group in pentanol is hit by oxygen in Path 12, creating CH3CH2CH2CH2OH and HCHO. Figure 14 presents its reaction mechanisms and structural parameters.
In the reaction R + O→TS24→MI24→TS25→MI25→TS26→MI26→P12 + HCHO, oxygen attacks the CH3- group in pentanol. Three C–H bonds elongate: from 1.09283 Å to 1.09343 Å, from 1.09314 Å to 1.11940 Å, and from 1.09215 Å to 1.11853 Å, forming TS24. Then one C-H bond extends to 2.00885 Å and breaks; O bonds with C and H, forming MI24. The OH group inversion leads to TS25, which rearranges to MI25. Then the C-C bond in -CH2CH2OH stretches to 1.97480 Å and the O-H bond in -CH2OH extends to 1.40124 Å, forming TS26. The H from the broken O-H bond bonds with the terminal -CH2- in -CH2CH2CH2CH2OH, yielding stable MI26.

4.1.13. Geometric Configurations and Chemical Reactions of Path 13

The CH3- group in pentanol is attacked by oxygen in Path 13, creating: CHCH2CH2CH2CH2OH and H2O. Figure 15 presents its reaction mechanisms and structural parameters.
In the reaction R + O → TS24 → MI24 → TS27 → MI27 → TS28 → MI28 → P13 + H2O, oxygen attacks the CH3 group of pentanol, elongating three C-H bonds: 1.09283 Å to 1.09343 Å, 1.09314 Å to 1.11940 Å, and 1.09215 Å to 1.11853 Å, forming TS24 with a frequency of 1339 cm−1. One C-H bond further extends to 2.00885 Å and breaks; O bonds with C and H atoms, forming MI24. The inversion of the -OH group forms TS27, which rearranges to MI27, an isomer of TS27. Then the C-H bond in CH2OH stretches to 1.45319 Å and breaks, while the C-O bond stretches to 2.33290 Å and breaks, forming TS28 with an imaginary frequency of 512 cm−1. The H from the broken C-H bond migrates to the -OH group formed by C-O cleavage and bonds with it, producing MI28, which decomposes to the final product.

4.2. IRC Reaction Path Analysis

Intrinsic reaction coordinate (IRC) analysis was performed on the reaction pathways at the saddle point with a 0.05 Bohr step, searching 50 points forward and backward. The results show that structural changes, including bond lengths, angles, dihedral angles, and bond breaking and formation in CH2, CH3, and CH2OH groups during pentanol oxidation, agree with theoretical analysis. Energy analysis and full optimization confirm the intermediates, transition states, and all reaction pathways. As shown in Figure 16, the potential energy changes along IRC reveal that the system energy peaks at TS1 to TS28, confirming the authenticity of these transition states.

4.3. Calculation of Reaction Barriers

The reaction potential energetic profiles after zero-point energy correction are shown in Figure 17, with values relative to the total energy of reactants C5H12O and O2. The reaction initially absorbs energy to form transition states TS1, TS6, TS12, TS18 or TS24, with corresponding energies of 256.82, 249.58, 231.54, 249.26 and 255.57 kJ/mol, respectively. The intermediates MI1, MI6, MI12, MI18 and MI24 are thermodynamically stable but kinetically reactive. They absorb energy to form unstable transition states, propelling the reaction toward its final products. Path 4, Path 9 and Path 13 are endothermic reactions, while the others are exothermic. The ease of reactions follows the order: Path1 > Path9 > Path4 > Path8 > Path13 > Path2 > Path3 > Path11 > Path10 > Path6 > Path7 > Path5 > Path12. The primary reaction in coal oxidation and spontaneous combustion is oxygen attacking the -CH2OH group of pentanol, yielding CH3CH2CH2CH2CHO and H2O as the main products. Notably, these energies are elementary reaction barriers, representing intrinsic molecular reactivity, which differs from that of experimentally apparent activation energies.
In Path 1, the process requires 14.2 kJ/mol to transit from MI1 to TS2, and TS2 releases 11.25 kJ/mol to form MI2. MI2 then absorbs 157.28 kJ/mol to reach TS3, which releases energy to form MI3. Finally, MI3 absorbs energy to produce P1 and H2O. Path 2’s transition from MI1 to TS4 requires only 14.19 kJ/mol, making it easy to proceed. However, the reaction from MI3 to TS4 demands a much higher barrier of 344.7 kJ/mol, rendering it difficult. In Path 3, MI6 absorbs only 1.62 kJ/mol to form TS7. MI7 then absorbs 362.78 kJ/mol to form TS8, which releases a large amount of energy to form MI8. Finally, MI8 absorbs 3.17 kJ/mol to decompose into products. MI6 in Path 4 needs to overcome a high barrier of 299.69 kJ/mol to form TS9, and MI9 also absorbs 46.78 kJ/mol to decompose, making this path difficult. In Path5, the highest barrier is 382.96 kJ/mol. Although MI6 absorbs only 9.35 kJ/mol to reach TS10 and MI11 absorbs 15.18 kJ/mol to decompose, the high maximum barrier makes Path 5 very difficult. In Path 6, MI12 absorbs 22.93 kJ/mol to form TS13, which releases energy to form MI13. MI13 then overcomes 366.31 kJ/mol to reach TS14, which releases a large amount of heat to form MI14. Finally, MI14 absorbs energy to decompose into products. In Path7, the maximum barrier is 369.41 kJ/mol, higher than that of Path 6, making Path 7 require more severe external conditions. In Path 8, MI12 absorbs 315.17 kJ/mol to form TS17, which releases a small amount of energy to form MI17, and MI17 then absorbs 45.22 kJ/mol to decompose. In Path 9, MI18 overcomes a barrier of 295.67 kJ/mol to form TS19, and MI19 absorbs 43.89 kJ/mol to decompose. The barriers of Paths 10 and 11 are too high, making the reactions very difficult. In Paths 12 and 13, the maximum barriers are 383.9 kJ/mol and 324.35 kJ/mol, respectively. Based on barrier heights, Path13 is easier than Path12.

5. Conclusions

Using DFT calculations, this study reveals thirteen reaction pathways, active sites, and energy barriers of pentanol oxidation in coal, confirming a multi-step bond breaking/formation mechanism and the correct connectivity of reactants, intermediates, and products along the minimum energy path. The main findings are summarized as follows:
(1)
The interaction between oxygen and pentanol molecules in coal spontaneous combustion can be described by the following thirteen reaction pathways:
R + O→TS1→MI1→TS2→MI2→TS3→MI3→P1 + H2O
R + O→TS1→MI1→TS4→MI4→TS5→MI5→P2 + HCOOH
R + O→TS6→MI6→TS7→MI7→TS8→MI8→P3 + CHOCH2OH
R + O→TS6→MI6→TS9→MI9→P4 + H2O
R + O→TS6→MI6→TS10→MI10→TS11→MI11→P5 + CH3OH
R + O→TS12→MI12→TS13→MI13→TS14→MI14→P6 + CH3CH3
R + O→TS12→MI12→TS15→MI15→TS16→MI16→P7 + CH3CH2OH
R + O→TS12→MI12→TS17→MI17→P8 + H2O
R + O→TS18→MI18→TS19→MI19→P9 + H2O
R + O→TS18→MI18→TS20→MI20→TS21→MI21→P10 + CH4
R + O→TS18→MI18→TS22→MI22→TS23→MI23→P11 + CH3CHO
R + O→TS24→MI24→TS25→MI25→TS26→MI26→P12 + HCHO
R + O→TS24→MI24→TS27→MI27→TS28→MI28→P13 + H2O
(2)
The active sites of pentanol in coal are from C1 to C5, with rate-controlling transition states TS1–TS28.
(3)
The reactivity order of the reaction paths is Path1 > Path9 > Path4 > Path8 > Path13 > Path2 > Path3 > Path11 > Path10 > Path6 > Path7 > Path5 > Path12. The dominant reaction in coal spontaneous combustion involves oxygen atoms attacking the -CH2OH group of pentanol, yielding CH3CH2CH2CH2CHO and H2O as the major products.

Author Contributions

T.Y.: Writing—review and editing, Methodology. X.W.: Formal analysis, Investigation, Resources. W.D.: Writing—review and editing, Data curation. S.L.: Validation, Methodology. H.D.: Software, Supervision. Y.S.: Software, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Geometry of amyl pentanol.
Figure 1. Geometry of amyl pentanol.
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Figure 2. Reaction formula of pentanol with oxygen in the process of coal spontaneous combustion.
Figure 2. Reaction formula of pentanol with oxygen in the process of coal spontaneous combustion.
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Figure 3. (a) Optimized geometric structures of Path 1, (b) Configuration changes in Path 1.
Figure 3. (a) Optimized geometric structures of Path 1, (b) Configuration changes in Path 1.
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Figure 4. (a) Optimized geometric structures of Path 2, (b) Configuration changes in Path 2.
Figure 4. (a) Optimized geometric structures of Path 2, (b) Configuration changes in Path 2.
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Figure 5. (a) Optimized geometric structures of Path 3, (b) Configuration changes in Path 3.
Figure 5. (a) Optimized geometric structures of Path 3, (b) Configuration changes in Path 3.
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Figure 6. (a) Optimized geometric structures of Path 4, (b) Configuration changes in Path 4.
Figure 6. (a) Optimized geometric structures of Path 4, (b) Configuration changes in Path 4.
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Figure 7. (a) Optimized geometric structures of Path 5, (b) Configuration changes in Path 5.
Figure 7. (a) Optimized geometric structures of Path 5, (b) Configuration changes in Path 5.
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Figure 8. (a) Optimized geometric structures of Path 6, (b) Configuration changes in Path 6.
Figure 8. (a) Optimized geometric structures of Path 6, (b) Configuration changes in Path 6.
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Figure 9. (a) Optimized geometric structures of Path 7, (b) Configuration changes in Path 7.
Figure 9. (a) Optimized geometric structures of Path 7, (b) Configuration changes in Path 7.
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Figure 10. (a) Optimized geometric structures of Path 8, (b) Configuration changes in Path 8.
Figure 10. (a) Optimized geometric structures of Path 8, (b) Configuration changes in Path 8.
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Figure 11. (a) Optimized geometric structures of Path 9, (b) Configuration changes in Path 9.
Figure 11. (a) Optimized geometric structures of Path 9, (b) Configuration changes in Path 9.
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Figure 12. (a) Optimized geometric structures of Path 10, (b) Configuration changes in Path 10.
Figure 12. (a) Optimized geometric structures of Path 10, (b) Configuration changes in Path 10.
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Figure 13. (a) Optimized geometric structures of Path 11, (b) Configuration changes in Path 11.
Figure 13. (a) Optimized geometric structures of Path 11, (b) Configuration changes in Path 11.
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Figure 14. (a) Optimized geometric structures of Path 12, (b) Configuration changes in Path 12.
Figure 14. (a) Optimized geometric structures of Path 12, (b) Configuration changes in Path 12.
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Figure 15. (a) Optimized geometric structures of Path 13, (b) Configuration changes in Path 13.
Figure 15. (a) Optimized geometric structures of Path 13, (b) Configuration changes in Path 13.
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Figure 16. (a) Potential energy profiles along IRC 1, 3, 5, 6, 8,11, 12, 14, 16, 18, 21, 23, 24, 26; (b) Potential energy profiles along IRC 2, 4, 7, 9, 10, 13, 15, 17, 19, 20, 22, 25, 27, 28.
Figure 16. (a) Potential energy profiles along IRC 1, 3, 5, 6, 8,11, 12, 14, 16, 18, 21, 23, 24, 26; (b) Potential energy profiles along IRC 2, 4, 7, 9, 10, 13, 15, 17, 19, 20, 22, 25, 27, 28.
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Figure 17. (a) Potential energetic profile for Path 1–8; (b) Potential energetic profile for Path 9–13.
Figure 17. (a) Potential energetic profile for Path 1–8; (b) Potential energetic profile for Path 9–13.
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Table 1. Molecular bond length of pentanol.
Table 1. Molecular bond length of pentanol.
NumberAtomic RelationBond Length
(Å)		NumberAtomic RelationBond Length
(Å)
R1R (1,7)1.09R10R (3,4)1.54
R2R (1,8)1.09R11R (4,14)1.09
R3R (1,9)1.09R12R (4,15)1.10
R4R (1,2)1.54R13R (4,5)1.52
R5R (2,10)1.10R14R (5,16)1.10
R6R (2,11)1.10R15R (5,17)1.10
R7R (2,3)1.54R16R (5,6)1.46
R8R (3,12)1.10R17R (6,18)0.97
R9R (3,13)1.09
Table 2. Molecular bond angle of pentanol.
Table 2. Molecular bond angle of pentanol.
NumberAtomic RelationBond Angle
(°)		NumberAtomic
Relation		Bond Angle
(°)
A1A (7,1,9)107.6A17A (12,3,4)109.2
A2A (7,1,2)107.8A18A (13,3,4)111.0
A3A (8,1,9)111.0A19A (3,4,14)107.7
A4A (8,1,2)107.7A20A (3,4,15)111.1
A5A (9,1,2)111.1A21A (3,4,5)111.4
A6A (1,2,10)111.4A22A (14,4,15)109.4
A7A (1,2,11)109.4A23A (14,4,5)109.4
A8A (1,2,3)109.4A24A (15,4,5)113.1
A9A (10,2,11)113.1A25A (4,5,16)106.2
A10A (10,2,3)106.2A26A (4,5,17)109.4
A11A (11,2,3)109.4A27A (4,5,6)109.0
A12A (2,3,12)109.0A28A (16,5,17)109.2
A13A (2,3,13)109.2A29A (16,5,6)109.8
A14A (2,3,4)109.8A30A (17,5,6)113.2
A15A (12,3,13)113.2A31A (5,6,18)106.6
A16A (7,1,9)106.6
Table 3. Molecular dihedral angle of pentanol.
Table 3. Molecular dihedral angle of pentanol.
NumberAtomic RelationDihedral Angle
(°)		NumberAtomic RelationDihedral Angle
(°)
D1D (7,1,2,10)−178.6D21D (2,3,4,5)177.2
D2D (7,1,2,11)−62.6D22D (12,3,4,14)177.9
D3D (7,1,2,3)59.2D23D (12,3,4,15)60.6
D4D (8,1,2,10)61.7D24D (12,3,4,5)−60.9
D5D (8,1,2,11)177.7D25D (13,3,4,14)−66.3
D6D (8,1,2,3)−60.5D26D (13,3,4,15)176.5
D7D (9,1,2,10)−58.4D27D (13,3,4,5)55.0
D8D (9,1,2,11)57.6D28D (3,4,5,16)56.2
D9D (9,1,2,3)179.3D29D (3,4,5,17)175.9
D10D (1,2,3,12)57.7D30D (3,4,5,6)−64.1
D11D (1,2,3,13)−58.8D31D (14,4,5,16)178.4
D12D (1,2,3,4)179.6D32D (14,4,5,17)−62.0
D13D (10,2,3,12)−64.6D33D (14,4,5,6)58.0
D14D (10,2,3,13)178.9D34D (15,4,5,16)−66.1
D15D (10,2,3,4)57.3D35D (15,4,5,17)53.5
D16D (11,2,3,12)179.7D36D (15,4,5,6)173.5
D17D (11,2,3,13)63.2D37D (4,5,6,18)176.7
D18D (11,2,3,4)−58.5D38D (16,5,6,18)56.4
D19D (2,3,4,14)56.0D39D (17,5,6,18)−63.4
D20D (2,3,4,15)−61.2
Table 4. Front orbit analysis of pentanol.
Table 4. Front orbit analysis of pentanol.
AtomHOMOC1C2C3C4C5
Charge density
(Hartree)		−0.25−0.52−0.34−0.31−0.35−0.09
Table 5. Chemical formulas of each symbol.
Table 5. Chemical formulas of each symbol.
SymbolFormulaSymbolFormula
R/TS1CH3CH2CH2CH2CH2OHTS15/MI15CH3CH2CH(OH)CH2CH2OH
MI1/TS2/MI2CH3CH2CH2CH(OH)CH2OHTS16/MI16/P7CH3CH2CHO
TS3/MI3/P1CH3CH2CH2CH2CHOTS17/MI17/P8CH3CH2C:CH2CH2OH
MI3/P1CH3CH2CH2CH2CHOTS18CH3CH2CH2CH2CH2OH
TS4/MI4CH3CH2CH2CH2CH(OH)2MI18CH3CH(OH)CH2CH2CH2OH
TS5/MI5/P2CH3CH2CH2CH3TS19/MI19/P9CH3C:CH2CH2CH2OH
TS6CH3CH2CH2C:CH2OHTS20/MI20CH3CH(OH)CH2CH2CH2OH
MI6/TS7/MI7CH3CH2CH2CH(OH)CH2OHTS21/MI21/P10CHOCH2CH2CH2OH
TS8/P3CH3CH2CH3TS22/MI22CH3CH(OH)CH2CH2CH2OH
TS9/MI9/P4CH3CH2CH2C:CH2OHTS23/MI23/P11CH3CH2CH2OH
TS10/MI6/MI10CH2(OH)CH2OHTS24CH3CH2CH2CH2CH2OH
TS11/MI11/P5CH3CH2CH2CH3MI24/TS25/MI25CH2(OH)CH2CH2CH2CH2OH
TS12CH3CH2CH2CH2CH2OHTS26/MI26/P12CH3CH2CH2CH2OH
MI12/TS13/MI13CH3CH2CH(OH)CH2CH2OHTS27/MI27CH2(OH)CH2CH2CH2CH2OH
TS14/MI14/P6CHOCH2CH2OHTS28/MI28/P13HC:CH2CH2CH2CH2OH
MI14/P6CH(OH)CH2CH2OH
Compounds with the same chemical formula that may originate from different reaction pathways or be isomers are labeled with different letters in the table.
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Yang, T.; Wang, X.; Deng, W.; Liu, S.; Deng, H.; Shan, Y. Coal Spontaneous Oxidation Mechanism of Low-Molecular Compounds: Pentanol. Fire 2026, 9, 253. https://doi.org/10.3390/fire9060253

AMA Style

Yang T, Wang X, Deng W, Liu S, Deng H, Shan Y. Coal Spontaneous Oxidation Mechanism of Low-Molecular Compounds: Pentanol. Fire. 2026; 9(6):253. https://doi.org/10.3390/fire9060253

Chicago/Turabian Style

Yang, Tianyi, Xiaobo Wang, Wenhao Deng, Sichen Liu, Hanzhong Deng, and Yafei Shan. 2026. "Coal Spontaneous Oxidation Mechanism of Low-Molecular Compounds: Pentanol" Fire 9, no. 6: 253. https://doi.org/10.3390/fire9060253

APA Style

Yang, T., Wang, X., Deng, W., Liu, S., Deng, H., & Shan, Y. (2026). Coal Spontaneous Oxidation Mechanism of Low-Molecular Compounds: Pentanol. Fire, 9(6), 253. https://doi.org/10.3390/fire9060253

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