## 3 Results and Discussion
### Influence of Thermal Evolution on FTIR spectroscopy Characteristics of Functional Groups in Coal

Table 4: Attribution of characteristic peaks in FTIR spectra of coal [53]. Figure 5: FTIR spectra of coal samples. specific bands are different for each coal sample, indicating that each coal sample contains about the same types of functional groups, but there are differences in the contents [55]. Figures 6 and 7 shows the split-peak fitting profile of the IR spectra of the test. The hydroxyl absorbance peaks of the coal samples are mainly located in the 3700-3600 cm\({}^{-1}\). range, i.e., a large amount of free -OH is present in the coal samples, possibly with alcohols, phenols, and organic acids [28]. The distribution of the fitted peaks in the wave number range of 3000-2800 cm\({}^{-1}\). is relatively uniform in the range of the aliphatic absorption peaks, which indicates that these four coal samples essentially contain various aliphatic functional groups such as CH\({}_{3}\), CH\({}_{2}\), and CH in cycloalkanes or aliphatic groups. In the range of absorption peaks of the oxygenated functional groups, DX 1# and DX 2# have similar shapes of absorption peaks, and HY 1# and HY 2# have similar shapes of absorption peaks, and both have larger and sharper absorptions within 1100-1000 cm\({}^{-1}\).. In addition, the intensities of the absorption peaks of DX 1# and HY 1# are larger than those of DX 2# and HY 2#, respectively; this indicates more S=O stretching vibrations. Meanwhile, the presence of aromatic C=C stretching vibrations and CH\({}_{3}\) asymmetric deformation vibrations in the coal samples is indicated by the presence of distinctive characteristic peaks at 1600 and 1440 cm\({}^{-1}\). in the four  
Since the area of each absorption peak is affected by the selection of the baseline, the intensity of the light, and other factors, it does not reflect well the effect of magmatic thermal evolution on the functional groups of coal. To better analyze the changing pattern of the functional groups in the coal samples, the area percentage of the absorption bands associated with each structure was selected as the study parameter (as shown in Figure 8). When compared to the normal coal samples, the thermally evolved coal samples exhibit a lower percentage of oxygen-containing functional groups, a higher percentage of aromatic structures, and increased aliphatic functional groups compared to DX 2# and HY 2#. This suggests that the thermal evolution induced by magma may have facilitated the breakdown of certain molecular compounds in coal and the removal of oxygen-containing functional groups, leading to an increase in aliphatic structures. Additionally, it results in the continuous condensation of smaller molecular compounds, thereby enhancing the degree of coal aromatization. \(F_{a}\) and (R/C)u can reflect the information of aromatic structures in the macromolecular structure of coal to some extent [43, 56]. The magmatic thermal evolution coal samples, DX 1# and HY 1#, have higher \(f_{a}\) and (R/C)u values than the normal coal samples, DX 2# and HY 2#, respectively, indicating that the thermal evolution of magma makes the smaller molecular compounds in the coal samples condensed, which promotes the degree of coal aromatization. A(CH\({}_{3}\))/A(CH\({}_{2}\)) can reflect the length of aliphatic chains in coal [42], and the magmatic thermally evolved coal samples, DX 1# and HY 1#, have higher A(CH\({}_{2}\))/A(CH\({}_{3}\)) values than the normal coal samples, DX 2# and HY 2#, respectively, reflecting that magmatic thermally evolved coal samples have longer aliphatic side chains than normal coal samples in their macromolecular structures. Figure 7: Fitting Gaussian peaks of Fourier transform infrared spectra for magmatic thermal evolution coal samples (HY 1#) and normal coal samples (HY 2#). (**(a)** 3600–3000 cm\({}^{-1}\). FTIR spectra fitting curve of HY 1#; (**b**) 3000–2700 cm\({}^{-1}\). FTIR spectra fitting curve of HY 1#; (**c**) 1800–1000 cm\({}^{-1}\). FTIR spectra fitting curve of HY 1#; (**d**) 900–700 cm\({}^{-1}\). FTIR spectra fitting curve of HY 1#; (**e**) 3600–3000 cm\({}^{-1}\).