The Evolution Law of Molecular Structure of Vitrain and Durain During Low–Medium Coalification

Abstract

Molecular structural disparities between maceral components are intrinsic factors governing their reactivity and physicochemical behaviors during storage and transportation. To investigate the molecular structural differentiation between vitrain and durain in low- to medium-rank coals (R_o,max = 0.65–1.71%), this study selected samples of long-flame coal and gas coal from the Huanglong Coalfield, coking coal from the Hedong Coalfield, and fat coal from the Weibei Coalfield. The microstructural variations in macroscopic coal components during coalification were analyzed using Fourier transform infrared spectroscopy (FTIR), ¹³C nuclear magnetic resonance (¹³C-NMR), and X-ray photoelectron spectroscopy (XPS). The results indicated that the aromatic structures of vitrain are predominantly trisubstituted, with their proportion consistently exceeding that in durain. In contrast, durain exhibits a progressive transition from trisubstituted to pentasubstituted aromatics with increasing coal rank, accompanied by higher aromaticity, condensation degree, and aromatic carbon content. The d₀₀₂ size of the vitrain decreased from 3.82 to 3.47, while that of the durain decreased from 3.52 to 3.40. Both values showed a gradual decline, with the vitrain exhibiting a larger reduction than the durain. This indicates that the lateral extension of the microcrystalline structure in the durain is more developed, resulting in tighter molecular connections. ¹³C-NMR analysis further reveals that durain possesses higher f_al^H/f_al^* and bridge carbon ratios (X_BP), along with a lower f_a^S/f_a^’ ratio, reflecting a greater degree of aromatic ring condensation. XPS analysis revealed that durain generally contains a higher oxygen-functional group content but lower C-C/C-H content compared to vitrain. Collectively, these findings confirm significant structural divergence between vitrain and durain during coalification, with durain exhibiting more developed aromaticity, structural condensation, and organizational order.

1. Introduction

During the transition away from fossil fuels, China’s relatively abundant coal reserves mean that its energy structure, dominated by coal [1], will not change in the short term. The country will continue to depend heavily on coal as its primary energy source. Therefore, it is crucial to leverage coal’s role as a primary energy source and a safety net in the process of achieving the dual carbon goals [2,3,4]. Efforts should focus on promoting the clean and efficient utilization of coal, advancing its conversion technologies, and innovating coal-based energy systems to support the development of a new energy landscape. From a chemical perspective, coal is composed of two main components: inorganic and organic matter. The organic component, which makes up the majority of coal, consists of high-molecular-weight compounds primarily made of elements such as carbon (C), oxygen (O), hydrogen (H), nitrogen (N), and sulfur (S). The molecular structural variations within coal are the primary determinants of its diverse physical and chemical properties, which, in turn, significantly influence the dynamics of coalbed methane production [5,6]. A comprehensive understanding of coal at the molecular level is thus essential for optimizing the extraction and utilization of coalbed methane, advancing both theoretical and practical applications in the field [7,8].

With the advent of technological advances, a broad spectrum of modern analytical techniques has been employed to unravel the molecular structural characteristics of coal, among which are 13-carbon nuclear magnetic resonance (¹³C-NMR), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), notably including methods that provide complementary insights into both the chemical bonding environment and the microstructural organization. Li Xia et al. [9] investigated the variation in microcrystalline parameters in relation to coal rank, with a specific focus on the coalification leap transition. Their findings reveal that, in general, the microcrystalline parameters exhibit a clear trend: both L_c and L_a increase, whereas the d₀₀₂ spacing decreases. This pattern suggests that, as coalification progresses, there is a significant enhancement in both the size and degree of condensation of the aromatic layers, reflecting a more advanced state of structural ordering. These results highlight the critical role of coal rank in influencing the structural evolution of coal during the coalification process. Yu Chun Mei et al. used FTIR to analyze bituminous coals, revealing that as coal rank increased, the relative abundance of aromatic hydrogen rose while aliphatic hydrogen content decreased [10]. Additionally, the number of aromatic rings in the coal structure increased, indicating a direct relationship between coal rank and the degree of aromaticity, shedding light on the structural evolution of coal during maturation. Li Wu et al. further demonstrated that with increasing vitrinite reflectance, the abundance of C-H groups and aliphatic/oxygenated functionalities in the vitrinite fraction progressively decreases, while the CH₂/CH₃ ratio initially declines and subsequently increases [11]. Zhou He et al. employed Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction to investigate the microstructural components of coal [12]. Their findings revealed that the aromatic structural parameters (f_a, I, DOC) of the vitrinite fraction were consistently lower than those of the inertinite fraction. In contrast, the aliphatic side chain content was higher in the vitrinite fraction compared to inertinite. These observations suggest that the vitrinite fraction exhibits lower maturity and aromaticity relative to the inertinite fraction, yet it possesses a higher hydrocarbon potential. Vitrain is rich in vitrinite components, while durain is rich in inertinite components. Vitrinite/inertinite components belong to the microscopic components, while vitrain/durain belong to the macroscopic level. While much of the current research focuses on the influence of metamorphic grade on the molecular structure of coal, studies addressing the molecular characteristics of different macroscopic coal components remain relatively sparse [13,14].

The macroscopic components of coal-rock significantly affect methane adsorption and desorption behavior [15]. This influence arises from variations in coal surface properties and pore characteristics, which are fundamentally driven by the distinct molecular structures at the microscopic level [16,17]. In this study, coal samples representing different metamorphic grades were selected, and the separation of various macroscopic coal-rock components was subsequently performed to assess their individual contributions. By integrating FTIR, XRD, ¹³C NMR, and XPS analyses, this study performs a comprehensive molecular-level investigation of the elemental composition and structural heterogeneity of coal. Such an approach enables the elucidation of structure–property relationships and the characterization of associated kinetic behaviors, thereby deepening the fundamental understanding of coal molecular architecture. These insights provide a robust theoretical basis for advancing coalbed methane exploration and utilization strategies.

2. Materials and Methods

2.1. Coal Sample Preparation

The Huanglong Coalfield is situated in the Ordos Basin, characterized by a monoclinal structure inclined toward the northwest. The Liulin Shaqu No. 1 Coal Mine is located in the central section of the Hedong Coalfield and the southwestern part of the Liulin Mining District, featuring abundant resources and high-quality coal. The Hancheng area is situated in the eastern segment of the Weibei Uplift Zone at the southeastern margin of the Ordos Basin. The terrain in Hancheng is uneven, with slightly higher elevations in the northwest and slightly lower elevations in the southeast. The region exhibits diverse landforms, including mountains, plains, and river floodplains.

This study investigates long-flame coal and gas-producing coal from the Huanglong Coalfield, coking coal from the Hedong Coalfield, and fat coal from the Weibei Coalfield. Representative coal samples were collected from the Yuanzigou Mine in the Yonglong District and the Huangling No. 2 Mine in the Huangling District of the Huanglong Coalfield, the Liulin Shaqu No. 1 Mine in the Liliu District of the Hedong Coalfield, and the Xiangshan Mine in the Hancheng District of the Weibei Coalfield. The four coalfields under investigation exhibit distinct metamorphic stages corresponding to the various ranks of low- to medium-rank coals. At the working face of the coal mine in the sampling area, a geological hammer is employed to collect bulk coal samples of appropriate size. Based on the differences in color, luster, and fracture structure between vitrain and durain, the wire-cutting method is applied to the vitrain-rich bands and durain regions, labeled as YZG-VC, YZG-DC, HL-VC, HL-DC, LL-VC, LL-DC, XS-VC, and XS-DC, with their respective sampling locations depicted in Figure 1. Industrial analyses, elemental composition assessments, and vitrinite reflectance measurements were performed in accordance with the standards outlined in GB/T 212-2008 [18] (“Methods for Industrial Analysis of Coal”), GB/T 31391-2015 [19] (“Elemental Analysis of Coal”), and GB/T 6948-2008 [20] (“Microscopic Determination of Vitrinite Reflectance of Coal”).

2.2. Construction of References

2.2.1. Fourier Transform Infrared Spectroscopy Testing

The Fourier Transform Infrared spectroscopy (FTIR) analysis was conducted using a Bruker INVENIOS infrared spectrometer (Bruker Corporation, Karlsruhe, Germany), with a scanning range of 4000–400 cm⁻¹ and a spectral resolution of 4.0 cm⁻¹. Mineral-depleted semi-coke samples were dried with KBr in a vacuum oven (DZF-6050E, Wuxi Marite Technology Co., Ltd., WuXi, China) at 353 K for 8 h [21], followed by grinding in a mortar to achieve a 1:100 sample-to-KBr ratio. The resulting mixture was then compacted into circular pellets using the standard potassium bromide (KBr) pellet technique. Infrared spectra were subjected to peak fitting analysis utilizing dedicated software, with the corresponding peak positions presented in

Table 1. Functional Groups in Infrared Spectra and Relative Peak Areas.

Absorption Peak Assignment	Position (cm−1)	Absorption Peak Assignment	Position (cm−1)
di-substituted benzene ring	750 cm−1	Symmetric CH3 stretching vibration of alkanes	2860 cm−1
tri-substituted benzene ring	800 cm−1	CH stretching vibration in alkanes	2900 cm−1
tetra-substituted benzene ring	840 cm−1	Asymmetric CH2 stretching vibration in alkanes	2920 cm−1
penta-substituted benzene ring	860 cm−1	Asymmetric CH3 stretching vibration in alkanes	2940 cm−1
Ash content	1100 cm−1	Hydroxyl–nitrogen hydrogen bonding	3100~2800 cm−1
C-O in phenols, alcohols, ethers, and esters	1100~1300 cm−1	Cyclic tightly associated hydroxyl hydrogen bonding	3200 cm−1
CH3 and CH2 asymmetric deformation vibration	1440 cm−1	Hydroxyl–ether oxygen hydrogen bonding	3300 cm−1
aromatic nucleus C=C	1600 cm−1	Self-associated hydroxyl hydrogen bonding	3400 cm−1
C=O stretching vibration	1650 cm−1	Hydroxyl-π hydrogen bonding	3516 cm−1
COOH stretching vibration	1700 cm−1	Free hydroxyl group	3611 cm−1
Symmetric CH2 stretching vibration in alkanes	2840 cm−1

.

2.2.2. X-Ray Diffraction Testing

The X-ray diffraction (XRD) analysis was conducted using a Bruker D8 Advance X-ray diffractometer (Bruker Corporation, Karlsruhe, Germany), equipped with a copper (Cu) target generating K radiation. The instrument was operated at a voltage of 40 KV and a current of 40 mA. Data acquisition was performed with a step size of 0.025°, covering a 2θ range from 5° to 80°. The scanning rate was set to 5°/min (ω), and intensity was recorded in counts per second (CPS). Peak fitting was applied to the diffraction patterns for precise analysis [22]. Key parameters, including the diffraction angles of the 002 (2θ₀₀₂) and 100 (2θ₁₀₀) peaks, along with the corresponding band areas (A₀₀₂ and A_γ), were extracted. From these data, the interlayer spacing (d₀₀₂), the layer stacking order (L_c), the in-plane crystallite size (L_a), and the number of layers (N_ave) were calculated based on the full-width at half-maximum (FWHM) of the peaks.

$$d_{002}={\displaystyle \frac{\lambda }{{2sin \theta }_{002}}}$$

(1)

$$L_c={\displaystyle \frac{0.94\mathsf{\lambda }}{{\mathsf{\beta }}_{002}{cos \theta }_{002}}}$$

(2)

$$L_a={\displaystyle \frac{1.84\mathsf{\lambda }}{{\mathsf{\beta }}_{100}{cos \theta }_{100}}}$$

(3)

$$N_{ave}={\displaystyle \frac{L_c}{d_{002}}}$$

(4)

In this formula, λ ≈ 1.54 nm denotes the X-ray wavelength; θ₀₀₂ and θ₁₀₀ correspond to the diffraction angles of the (002) and (100) planes, respectively, expressed in degrees; β₀₀₂ and β₁₀₀ refer to the full width at half maximum (FWHM) of the (002) and (100) reflections, also in degrees; and the constants 0.94 and 1.84 are employed as shape factors in the microcrystalline size estimation [23,24].

2.2.3. Nuclear Magnetic Resonance Carbon-13 Spectroscopy Testing

The ¹³C CP/MS NMR experiment was conducted using a Bruker AVANCE III 600 NMR spectrometer (Bruker Corporation, Karlsruhe, Germany). A high-resolution 3.2 mm dual-resonance MAS probe was employed, with a rotor frequency of 4100 Hz. The ¹³C resonance was detected at 25.152 MHz, utilizing a 4 μs pulse width and a 1 s pulse delay. Peak fitting was carried out with structural assignments based on the chemical shifts of carbon nuclei, as detailed in

Table 2. Assignment Table of ¹³C-NMR Chemical Shifts for Coal Samples.

Chemical Shift (ppm)	Primary Affiliation	Symbol
0~16	aliphatic methyl, terminal methyl	fal*
16~23	methyl connected to aromatic ring
23~36	methylene, methylidene, methylene group in saturated cycloalkane	falH
36~50	the α-position carbon on aliphatic and aromatic carbons
50~75	methoxy group, phenoxy group, oxygen-connected methylene carbon, oxygen-connected secondary methylene carbon	falO
75~90	ring-in oxygen bonded to fatty carbon
100~129	protonated aromatic carbon	faH
129~137	inter-ring bridging aromatic carbon	faB
137~148	alkyl-substituted aryl carbon (side-chain aryl carbon)	faS
148~165	oxygen-substituted aromatic carbon	faP
165~188	carboxyl carbon	faC
188~220	carbonyl carbon	faN
129~165	deprotonated carbon	fa’
100~165	sp2 hybridization in aromatic rings	fa
100~220	aromatic carbon ratio	fal
0~90	fat-to-carbon ratio	faH

It should be noted that all parameters reported in the table are expressed as percentages. The individual components are defined as follows: f_a^N = f_a^B + f_a^S + f_a^P;f_a’ = f_a^H + f_a^N; f_a = f_a’ + f_a^C; f_al = f_al^* + f_al^H + f_al^O; f_a + f_al = 100%.

[25,26].

2.2.4. X-Ray Photoelectron Spectroscopy Testing

X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250 (Thermo Fisher Scientific, Wilmington, DE, USA) spectrometer. The base pressure during analysis was maintained at 10⁻⁴ kPa. High-resolution spectra were acquired with a pass energy of 20 eV, while survey scans were recorded at 150 eV, using an energy step size of 0.05 eV. All binding energies were calibrated against the C 1s peak at 284.6 eV.

3. Results and Discussion

With increasing coal rank, the ash yield of the four samples exhibits a progressive increase, whereas the volatile matter content shows a corresponding decline. For a given coal sample, the vitreous (specular) lithotype consistently contains higher levels of moisture and volatile matter than the dull lithotype, while the latter is characterized by a greater ash yield (

Table 3. Results of Coal Sample Quality Analysis.

Sample	Ro,max (%)	Mad (%)	Aad (%)	Vad (%)	FCad (%)	Cdaf (%)	Hdaf (%)	Odaf (%)	Ndaf (%)	Vitrinite (%)	Inertinite (%)	Exinite (%)	Mineral (%)
YZG-VC	0.65	5.21	0.36	35.57	58.86	76.60	4.80	17.47	1.12	68.3	27.8	2.2	1.7
YZG-DC		4.31	1.68	24.97	69.05	76.67	4.29	18.03	1.02	39.4	54.1	2.1	4.4
HL-VC	0.75	3.93	3.45	30.95	61.67	76.85	4.98	16.85	1.32	72.8	22.8	1.1	3.3
HL-DC		3.87	7.24	24.70	64.19	77.41	4.12	17.45	1.02	13.3	74.6	7.3	4.8
LL-VC	1.41	0.75	18.29	13.60	67.36	83.46	4.53	9.25	1.76	73.0	24.0	0.0	3.0
LL-DC		0.27	21.33	12.35	66.05	85.57	4.02	9.09	1.32	26.7	67.3	2.2	3.8
XS-VC	1.71	0.84	10.45	22.72	65.99	89.38	3.98	5.27	1.37	73.9	19.2	0.0	6.9
XS-DC		0.79	26.65	17.63	54.94	91.36	3.91	3.59	1.14	18.2	73.5	2.6	5.7

) [27]. With increasing carbon content, heteroatoms such as oxygen and hydrogen, together with thermally labile structural moieties, are progressively eliminated [28,29]. Concomitantly, the organic macromolecular network of coal undergoes densification and develops a more ordered orientation. Compared with dull coal, vitreous coal exhibits lower concentrations of carbon and oxygen, whereas its hydrogen and nitrogen contents are relatively enriched. Vitrain is rich in vitrinite, while durain is rich in inertinite, with durain containing a higher proportion of inorganic minerals [30].

3.1. FTIR Analysis

Fourier Transform Infrared Spectroscopy is widely employed to identify the presence and elucidate the structural characteristics of functional groups within the molecular framework of coal [31]. As illustrated in Figure 2, the baseline-corrected infrared spectrum reveals that the absorption peak corresponding to the aromatic hydrocarbon structures (900–700 cm⁻¹) becomes progressively more distinct with advancing coalification, evolving from a broad and diffuse profile into a sharper and well-defined band, accompanied by a concomitant enhancement in absorption intensity. The intensity of absorption peaks corresponding to oxygen functional groups (1000–1800 cm⁻¹) diminishes with increasing coal rank, although these functional groups remain a significant component in all coal samples, albeit at reduced concentrations. Similarly, the intensity of the absorption peak associated with the aliphatic hydrocarbon structure (3000–2800 cm⁻¹) decreases as the degree of coal alteration increases, indicating a progressive loss of aliphatic C-H bonds [32].

Notably, the absorption peak for aliphatic hydrocarbons is more pronounced in vitrain compared to durain, with low- to medium-rank light coal exhibiting a higher proportion of aliphatic structures. The absorption peak of coal in the 3600–3000 cm⁻¹ region progressively diminishes with increasing Ro, where vitrain consistently exhibits greater peak intensity than durain. These observations suggest that, during coalification, the abundance of hydroxyl groups progressively declines owing to a concomitant loss of oxygen-bearing functional groups, thereby reducing the inherent moisture content of the samples [33].

3.1.1. Aromatic Structural Changes

By analyzing peak splitting, distinct substitution patterns on the benzene ring can be resolved [34]. The identification of these substitution patterns offers valuable structural insights, aiding in the reconstruction of the primary aromatic framework. In coal, four primary substitution patterns are observed on aromatic rings: di-substituted benzene (characterized by peaks in the 750–730 cm⁻¹ range), tri-substituted benzene (peaks between 810 and 750 cm⁻¹), tetra-substituted benzene (peaks in the 850–810 cm⁻¹ range), and penta-substituted benzene (peaks from 900 to 850 cm⁻¹). In coal, the trisubstituted aromatic configuration predominates (Figure 3a,b) [35,36], with the proportion of trisubstituted aromatics in vitrain consistently surpassing that in durain. The proportion of di-substituted benzene in vitrain gradually increased before declining, while that in durain showed a fluctuating upward trend. The proportion of tetra-substituted benzene gradually decreased until it disappeared, with both vitrain and durain exhibiting consistent trends. During the low-rank coal stage, the concentration of pentasubstituted aromatics in vitrain exceeds that in durain. Conversely, at the medium-rank coal stage, durain exhibits a higher proportion, indicating a more pronounced cyclization of aliphatic side chains and dehydrogenation–aromatization processes of naphthenic hydrocarbons in durain.

To quantitatively characterize the aromatic structures, the parameters f_a, I, and DOC were employed, and a correlation analysis was conducted to elucidate the evolutionary patterns of the aromatic structural parameters (Figure 4) [37].The aromaticity (I) and degree of condensation (DOC) represent the ratio of the out-of-plane deformation vibrations (900–700 cm⁻¹) of aromatic ring CH to the peak areas of aliphatic hydrocarbons (3000–2800 cm⁻¹) and aromatic C=C (1600 cm⁻¹). The aromatic carbon ratio (f_a) refers to the proportion of aromatic carbon relative to the total carbon content (Figure 4a). During coal evolution, f_a, I, and DOC display fluctuating yet overall increasing trends. Notably, at equivalent R_o,max values, the aromatic indices of durain are consistently higher than those of vitrain. This pattern suggests that the progressive cleavage of aliphatic side chains promotes an enrichment of aromatic structures, thereby enhancing the degree of aromatization in durain relative to vitrain.

3.1.2. Aliphatic Structural Changes

The infrared spectral wavenumbers within the range of 3000 to 2800 cm⁻¹ are predominantly associated with the symmetric and asymmetric stretching vibrations of aliphatic hydrocarbon groups (-CHₓ) embedded within the coal matrix [38]. Notably, two distinct peaks are observed at approximately 2860 cm⁻¹ and 2920 cm⁻¹, which correspond to the symmetric stretching mode of CH₃ groups and the asymmetric stretching mode of CH₂ groups, respectively. The peak intensity at 2920 cm⁻¹ is notably more pronounced than that at 2860 cm⁻¹, suggesting the presence of longer aliphatic chains within the sample. The symmetric CH₂ stretch appears at 2840 cm⁻¹, while the CH stretch is observed near 2900 cm⁻¹. Additionally, the asymmetric CH₃ stretch is discernible around 2940 cm⁻¹, as depicted in Figure 2.

As the R_o,max value of coal samples gradually increases, the proportion of symmetric CH₂ in both vitrain and durain decreases, while the proportion of asymmetric CH₂ gradually increases [39]. Moreover, the growth trend of asymmetric CH₂ exceeds that of symmetric CH₂, indicating that CH₂ remains the most abundant aliphatic hydrocarbon in coal samples (Figure 5a). The proportion of symmetric CH₃ groups progressively increases, while the proportion of asymmetric CH₃ groups exhibits a gradual decline with minimal variation (Figure 5b). In vitrain, the content of CH species initially decreases before undergoing an increase, whereas in durain, the CH content consistently decreases over time.

The aliphatic structural parameter CH₂/CH₃, which denotes the ratio of methylene to methyl groups in the coal molecular structure, serves as a key indicator of the length and evolution of the aliphatic side chains [40,41]. Variations in this ratio provide insights into the degree of maturation of the coal’s aliphatic components. Notably, the CH₂/CH₃ ratio exhibits a characteristic trend: it initially increases with the degree of metamorphism before subsequently decreasing at higher ranks of coal maturation (Figure 5). This phenomenon can be principally attributed to the substantial degradation of aliphatic structures during the lower coal rank stages, which subsequently results in an increase in associated aliphatic species, such as methylene groups. In contrast, as coal samples progress into higher metamorphic stages within the medium-rank coal category, the continued loss of aliphatic components becomes pronounced, leading to the progressive polymerization of aromatic structures into increasingly complex polyaromatic systems. The CH₂/CH₃ ratio in vitrain consistently surpasses that of durain throughout the coalification process. This discrepancy is primarily influenced by the coalification environment, wherein aliphatic hydrocarbons in vitrain exhibit enhanced preservation, resulting in relatively longer aliphatic chain lengths with minimal degradation. In contrast, durain undergoes substantial fragmentation and loss of aliphatic chain structures, leading to the retention of shorter residual chains.

3.1.3. Oxygen Functional Group Changes

The spectral region between 1000 and 1800 cm⁻¹ predominantly encompasses oxygenated functional groups. This range also captures contributions from inorganic ash components, alongside deformation vibrations of aliphatic CH₂ and CH₃ groups, as well as the C=C stretching modes characteristic of aromatic hydrocarbons [42]. In lower maturity stages, the spectrum exhibits broad, well-defined peaks with relatively smooth profiles, indicative of substantial molecular connectivity. As maturation progresses, a discernible trend emerges, marked by a reduction in peak areas and the development of isolated spectral features, suggesting the breakdown of molecular integrity. This trend is particularly evident in the diminishing intensities of aromatic C-O, aromatic C=C, and aliphatic CH₂ and CH₃ vibrations. Notably, the C-O stretching bands (associated with phenols, alcohols, ethers, and esters) become increasingly sharper and more distinct within the 1000–1300 cm⁻¹ range as the material undergoes metamorphism. Furthermore, the spectral bands corresponding to durain consistently surpass those of vitrain, reflecting the greater fibrous carbon content and the heightened compositional complexity inherent in durain.

The proportion of oxygen functional groups is higher in lower-rank coals, with samples from Yuanzigou and Huangling showing significantly greater oxygen content than those from Liulin and Xiangshan. Among the three oxygen functional groups C-O, C=O, and COOH, C-O is dominant, while both C=O and COOH contribute less than 10%. In highly mature coals, the COOH group is absent. Additionally, a comparison of vitrain and durain across coal ranks (Figure 6) reveals that ash content increases progressively with higher metamorphic grade.

Coal at higher metamorphic grades exhibits intensified interactions with the external environment, thereby incorporating additional ash. With increasing maturity, most oxygen-containing functional groups diminish or vanish; however, ether oxygen bonds persist at relatively high levels owing to their structural linking role, particularly in vitrain and durain [43,44]. As maturation proceeds, ash content in durain surpasses that in vitrain, reflecting its formation in a weakly oxidizing environment enriched in fibrous carbonaceous components, which facilitates impurity assimilation and yields a more complex depositional setting. In vitrain, C-O bonds progressively decline, while C=C bonds become dominant. In contrast, durain displays a non-monotonic decrease in C-O bonds, accompanied by an initial rise and subsequent fall in C=C content. The transient increase likely stems from molecular weight reduction through the loss of unstable moieties and concurrent condensation reactions, whereas the later decrease is attributable to high-temperature pyrolysis at advanced coal maturity.

3.1.4. Hydroxyl Functional Group Changes

The following bonds are observed at approximately 3170 cm⁻¹, 3200 cm⁻¹, 3300 cm⁻¹, 3400 cm⁻¹, and 3516 cm⁻¹: hydroxyl–nitrogen hydrogen bonds (OH…N), cyclic hydroxyl tetramers (Cyclic OH tetramers), hydroxyl–ether bonds (OH…ether O), self-associated hydroxyl bonds (Self-associated OH), and hydroxyl–π (OH…π) interactions. Among these, self–associated hydroxyl bonds are the most prevalent, followed by hydroxyl–ether interactions, while hydroxyl–π and hydroxyl–nitrogen hydrogen bonds are less abundant. In the low-rank coal stage, hydroxyl–ether bonds are more abundant in vitrain than in durain, whereas in higher-rank coal, durain exhibits a higher proportion of these bonds compared to vitrain (Figure 7). The proportions of hydroxyl cyclic tetramers and hydroxyl–π bonds are inversely related between vitrain and durain. Vitrain contains a greater proportion of hydroxyl cyclic tetramers and fewer hydroxyl–π bonds than durain. The abundance of hydroxyl–nitrogen bonds generally increases with coal rank in both vitrain and durain, except for XS-DC, which exhibits fluctuations. Hydroxyl cyclic polymers and ether oxygen bonds exhibit a declining trend in both vitrain and durain, which is attributed to the high oxygen functional group content in low-rank coals. As metamorphism progresses, these groups, particularly hydroxyls, are progressively lost. In contrast, the proportion of self-assembled hydroxyl bonds in vitrain increases gradually, reflecting the enhanced aromatic ring condensation and tighter molecular packing. This spatial reorganization fosters the formation of self-assembled hydroxyl bonds, underscoring their central role in vitrain. As durain matures, the hydroxyl group in the hydroxyl–π bond undergoes charge transfer with the aromatic ring, thereby amplifying the influence of the hydroxyl–π interaction.

3.2. XRD Analysis

X-ray diffraction (XRD) analysis is widely employed to determine the size, stacking arrangement, and microcrystalline features of aromatic domains. The diffraction peak at approximately 24° 2θ (002) originates from the condensation of aromatic layers, whereas the peak near 44° 2θ (100) is associated with the lateral dimensions of the aromatic carbon layers [45]. As shown in the XRD patterns (Figure 8), increasing R_o,max leads to a progressive sharpening and enhanced symmetry of the 002 reflection, accompanied by a rightward shift in its peak position, while the 100 reflection becomes increasingly distinct. Compared with vitrain, which exhibits broader and more diffuse diffraction features, durain displays relatively sharper and less well-resolved peaks. This contrast arises from the distinct depositional environments: durain is primarily formed under oxidizing conditions that are mineralogically more heterogeneous, thereby imparting greater structural disorder to the carbon matrix.

The d₀₀₂ size of the vitrain decreased from 3.82 to 3.47, while the d₀₀₂ size of the durain decreased from 3.52 to 3.40 (Figure 9a). The vitrain is larger than the durain. The decrease in d₀₀₂ represents a reduction in the interlayer spacing of the coal molecular structure. As the degree of coal metamorphism increases, the polymerization of the coal molecular structure intensifies, causing the aromatic layers to become more tightly packed [46]. The relationship between L_c, L_a, N_ave, and metamorphic grade is opposite to that of d₀₀₂, showing an increasing trend. The values of L_c, L_a, and N_ave are all larger for durain than for vitrain (Figure 9b–d). The observed increase in these parameters suggests that the intensification of condensation reactions promotes the formation of more compact aromatic ring structures and intricate stacking arrangements, which correlates with the observed rise in the average layer number, N_ave (Figure 9d). These findings underscore a higher degree of aromatic ring condensation in durain relative to vitrain. Furthermore, the aromatic layer arrangement in durain is markedly more orderly and tightly packed compared to vitrain, with a more pronounced orientation of aromatic stripes. This suggests that durain exhibits a higher level of molecular ordering and structural integrity within its aromatic framework than vitrain.

3.3. ¹³C-NMR Analysis

The NMR carbon spectrum typically exhibits three distinct regions: the aliphatic carbon (0–90 ppm), aromatic carbon (100–160 ppm), and carbonyl/carboxyl carbon (160–220 ppm) (Figure 10) [47]. The carbon distribution patterns of vitrain and durain within a single coal sample are found to be essentially consistent. At lower coal ranks, the peak intensities and areas associated with the aliphatic structures are notably high, often comparable to those of the aromatic components. As coal undergoes increasing metamorphism, the intensity of the aliphatic region diminishes sharply. During the Middle Coal Stage, the aliphatic peak almost completely vanishes, signifying a substantial reduction in the content of aliphatic structures within the coal sample. Concurrently, as the vitrinite reflectance (R_o,max) value rises, there is a progressive increase in both the peak area and intensity of the aromatic region. Additionally, the aromatic peak narrows, with the full width at half maximum (FWHM) decreasing from an initial range of 100–160 ppm to 110–140 ppm.

The structural parameters derived from ¹³C -NMR provide insight into the molecular structural characteristics and evolutionary trajectories of coal [48,49]. These parameters, determined through peak fitting analysis, are summarized in

Table 4. ¹³C-NMR Structural Parameters of Coal Samples.

Sample	faH	faN	faC	faB	faS	faP	fa’	fa	fal*	falH	falO	fal	XBP
YZG-VC	39.19	17.86	3.58	8.05	5.64	4.16	57.04	60.62	8.92	20.12	10.35	39.38	0.164
YZG-DC	41.56	16.38	5.18	9.24	4.36	2.77	57.94	63.12	8.24	18.79	9.86	36.88	0.190
HL-VC	40.07	17.62	2.43	8.74	5.24	3.64	57.69	60.12	9.74	21.35	8.78	39.88	0.178
HL-DC	43.55	16.13	3.76	9.8	3.57	2.76	59.67	63.43	8.72	19.82	8.03	36.57	0.196
LL-VC	39.62	17.44	19.46	9.79	3.44	4.20	57.06	76.51	5.96	5.81	11.72	23.49	0.207
LL-DC	42.88	17.72	15.97	10.59	4.56	2.58	60.60	76.57	3.75	7.72	11.95	23.43	0.212
XS-VC	32.28	22.32	19.01	12.57	7.45	2.30	57.60	76.60	6.80	8.29	11.31	23.40	0.299
XS-DC	38.37	18.60	21.21	13.54	2.61	2.44	56.97	78.18	4.78	5.31	11.73	21.82	0.311

. Within the molecular framework of coal, carbon functional groups predominantly manifest as aromatic and aliphatic carbons. Notably, the aromatic carbon ratio (f_a) exhibits a positive correlation with the degree of metamorphism (Figure 11a). Specifically, the aromatic carbon ratio in vitrinite is consistently lower than that observed in inertinite. The aliphatic structure parameter, defined as the aliphatic ratio (f_a), exhibits an inverse correlation with the aromatic carbon ratio, progressively decreasing in tandem. Among aromatic carbon species, the contribution of protonated aromatic carbon (f_a^H) is predominant, while the proportion of bridged aromatic carbon (f_a^B) demonstrates a rising trend, suggesting a more compact molecular connectivity. Notably, the relative proportion of carbonyl and carboxyl carbons (f_a^C) in durain exceeds that observed in vitrain, implying that during geological maturation, aliphatic acid side chains are progressively eliminated.

The ratio of methylene groups to the sum of aliphatic and aromatic methyl groups (f_al^H/f_al^*) serves as an indicator of the length of aliphatic side chains and the degree of branching in the molecular structure [50]. Higher values of this ratio suggest longer aliphatic side chains and reduced branching. In contrast, the durain fraction typically exhibits higher values compared to vitrain, implying that vitrain possesses a greater degree of structural branching. The ratio of side-chain aromatic carbon to total aromatic carbon (f_a^S/f_a^’) reflects the proportion of aromatic carbon in the side chains. A higher value of this ratio is indicative of weaker bonds within the aromatic structure, signifying a lower degree of structural stability in the coal matrix. Vitrain generally displays higher values than durain, although durain is characterized by enhanced stability. The bridge carbon ratio (X_BP) (Figure 11b), which quantifies the proportion of bridge carbon to peripheral carbon, increases progressively with rising reflectance (R_o,max). Throughout the metamorphic process, the condensation of aromatic rings intensifies, accompanied by a rise in the number of aromatic rings. Notably, the bridge carbon ratio of durain consistently surpasses that of vitrain, suggesting a slightly greater degree of metamorphism in durain compared to vitrain.

3.4. XPS Analysis

X-ray photoelectron spectroscopy (XPS) is a widely employed surface-sensitive technique that enables quantitative analysis of elemental composition and chemical states [51]. The predominant forms of carbon bonding are C–C and C–H, followed by C–O, with carboxyl groups occurring in the lowest abundance (

Table 5. Distribution of Carbon Elements in Samples.

Sample	C-C,C-H	C-O	C=O	COOH
YZG-VC	65.00	23.08	9.67	2.25
YZG-DC	60.46	27.29	10.05	2.20
HL-VC	59.15	29.62	10.08	1.15
HL-DC	58.37	32.81	8.83	0.00
LL-VC	69.00	18.23	11.51	1.26
LL-DC	67.12	21.63	10.05	1.20
XS-VC	72.47	17.96	8.80	0.77
XS-DC	69.24	13.58	11.10	0.61

) [52]. Deconvolution of the C 1s spectra reveals that both C–O functionalities and carboxyl groups progressively decline during coalification. In contrast, the C–C/C–H ratio exhibits a non-monotonic trend, decreasing initially and then increasing with advancing coal rank (Figure 11). The C-C/C-H ratios are predominantly influenced by the presence of aromatic carbon and alkyl side chains (Figure 12a). As coalification progresses, the shedding of aliphatic side chains during the early stages results in an increase in carbon content, accompanied by a heightened degree of aromatization in the later stages. Durain, characterized by a lower carbon content than vitrain, also exhibits a reduced abundance of alkyl side chains, a finding that aligns with previous infrared spectroscopy observations. Oxygen functional groups are the principal manifestations of oxygen in coal (

Table 6. Distribution of Oxygen Elements in Samples.

Sample	C-O	C=O	COO-
YZG-VC	74.45	15.77	6.49
YZG-DC	74.59	14.07	9.68
HL-VC	74.17	7.68	17.62
HL-DC	70.12	16.42	7.79
LL-VC	58.25	10.79	22.12
LL-DC	61.43	11.70	19.56
XS-VC	78.79	11.14	4.34
XS-DC	73.17	7.57	10.19

), with C-O bonds comprising the majority, followed by C=O and COO- functionalities. During the initial stages of coal maturation, the proportion of C-O bonds in vitrain and durain progressively decreases. However, as aromatization begins to dominate, an increase in C-O content is observed (Figure 12b). Notably, the COO- groups initially rise in concentration before subsequently declining (Figure 12c). This trend suggests that pyrolysis is more likely to occur in the later stages of coal evolution, as the structural instability of the COO- functionality leads to a gradual reduction in its relative abundance.

3.5. Evolution of the Coalification Stage

The coalification process is characterized by a progressive increase in carbon content, accompanied by a decrease in hydrogen, oxygen, nitrogen, and sulfur. Structurally, it transitions from peat, which contains diverse functional groups, toward anthracite, dominated by condensed aromatic rings, ultimately evolving into a graphite-like structure. This evolution involves the loss of aliphatic side chains and oxygen functional groups, coupled with enhanced aromatization and polycondensation. Coalification occurs in distinct stages: In the initial stage, aliphatic side chains and oxygen-containing functional groups are gradually removed (Figure 13). A decrease in the peak intensity at the ¹³C-NMR spectrum position of 0–90 ppm, and a reduction in the peak intensity at 2800–3000 cm⁻¹ in the infrared spectrum. During the second stage, further shedding of aliphatic structures takes place, while aromatic structures undergo aromatization (Figure 14) and condensation (Figure 15), primarily forming three-ring aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene. The increased bridge carbon ratio in ¹³C NMR and the gradual increase in structural parameters such as condensation degree and aromaticity in Fourier transform infrared spectroscopy both corroborate this finding. In the final stage, aromatic condensation intensifies, with higher-maturity medium-rank coals exhibiting tetracyclic aromatic hydrocarbons. While the overall evolutionary trends in vitrain and durain are similar, durain shows greater microstructural complexity and a slightly higher degree of coalification than vitrain within the same sample.

4. Conclusions

This study examines long-flamed coal and gas coal from the Huanglong Coalfield, coking coal from the Hedong Coalfield, and fat coal from the Weibei Coalfield, focusing on both vitrain and durain coal samples. Through material characteristic and molecular structure testing experiments, it analyzes the evolutionary patterns of microstructures in different macroscopic coal components during the coalification process. The research yields the following primary conclusions:

(1)

Vitrain predominantly exhibits trisubstitution, with trisubstituted aromatics comprising a larger proportion than in durain. In contrast, durain transitions from trisubstitution to pentasubstitution. Among oxygen functional groups, C–O bonds are the most abundant, while C=O and COOH groups are less common. Durain generally contains a higher concentration of oxygen functional groups compared to vitrain. The CH₂/CH₃ ratio in vitrain is consistently higher than in durain. Durain also displays greater aromaticity, higher condensation, and increased aromatic carbon content, suggesting that vitrain contains longer alkyl side chains, while durain is characterized by more extensively developed aromatic structures.

(2)

The d₀₀₂ size of the vitrain decreased from 3.82 to 3.47, while that of the durain reduced from 3.52 to 3.40. Both exhibited a progressive decline, with the vitrain showing a more pronounced reduction compared to the durain. These findings suggest that the lateral extension of the microcrystalline structure in durain is more developed, indicative of a denser network of molecular interactions and tighter interconnections.

(3)

During the structural evolution of the carbon skeleton, progressive coal metamorphism leads to a marked reduction in the aliphatic peak area, accompanied by a gradual increase in both the area and intensity of the aromatic peak. Concurrently, the bridging carbon ratio (X_BP) exhibits a systematic rise with increasing metamorphic rank. As a result, the proportion of aromatic carbon in the coal matrix steadily increases, whereas the contribution of aliphatic carbon diminishes as coal maturity advances.

(4)

The X-ray photoelectron spectroscopy (XPS) analysis of carbon (C), nitrogen (N), and oxygen (O) revealed that carbon predominantly exists in C-C/C-H bonding configurations, followed by C-O groups, with carboxyl groups (–COOH) being the least abundant. Notably, the content of C-C/C-H bonds in durain is typically lower than that in vitrain.