Characteristics and evolution of chemical structural differences between vitrain and durain during anaerobic microbial degradation

To explore variations in chemical composition and evolutionary trends between vitrain and durain during anaerobic microbial degradation, we performed simulation experiments on hydrocarbon generation through anaerobic microbial degradation using samples from No.2 coal seam (raw coal, vitrain, and durain) sourced from the Jurassic Yan'an Formation in the Huangling Mining area. Fourier transform infrared spectroscopy and X-ray diffraction techniques were utilized for analyzing changes in functional group structures as well as microcrystalline structural evolution at different stages (I, II, III) of degraded coals. The results indicate that the cumulative methane production from mirror coal and dark coal was 343.58 μmol/g and 281.13 μmol/g, respectively. The peak gas production time for vitrain occurs 14 days earlier compared to that of durain. Notably, the peak gas production time for vitrain occurred two week earlier compared to durain. Furthermore, vitrain exhibits a substantially higher proportion of highly substituted aromatic hydrocarbons in comparison to durain. Additionally, the aliphatic structure exhibits a greater abundance of side chains in vitrain. On the other hand, durain is characterized by an enrichment of oxygen-containing functional groups and aromatic structures, along with a more stable microcrystalline structure. Microbial degradation primarily affected the unstable side chains of the aliphatic structure. Under microbial degradation, the CH2/CH3 ratio of the aliphatic structure in vitrain and durain increased from 2.20 to 2.36 and from 1.77 to 2.17, respectively. The aromatic carbon content remained virtually unchanged, with variations not exceeding 0.05. Additionally, other functional groups did not exhibit a consistent trend. Among the microcrystalline structural parameters, both the stacking degree and the number of stacking layers for vitrain and durain exhibited a decrease. Specifically, the Lc values decreased from 1.28 nm and 1.38 nm to 1.22 nm and 1.27 nm, respectively, while the average number of stacking layers (Nave) decreased from 3.51 and 3.82 to 3.34 and 3.50, respectively. Additionally, the interlayer spacing between carbon atoms increased, indicating a weakening of bridge bonds within the aromatic structures. This phenomenon resulted in the shedding of irregular low molecular weight aromatic lamellae at the edges of the aromatic nuclei. Changes in microcrystal structure were characterized by weakened bridge bond fractures, irregular edges of basic structural unit, shedding of small molecular weight aromatic layer sheets. The coefficient of variation indicates that the degree of variation in aromatic substituents is highest in vitrain and durain, while the degree of variation in microcrystalline structure is significantly greater in vitrain compared to durain. The structural evolution of vitrain and durain differs during microbial degradation. In the initial stage of vitrain, shedding of organic matter between aromatic layers results in a reduction in carbon atom spacing, damage to the aromatic structure, and an increase in aliphatic structures. In the early stage of durain, shedding occurs for both aliphatic structures and oxygen-containing functional groups at the edge of the basic structural unit. Throughout the entire process of microbial degradation for both vitrain and durain, aliphatic structures are destroyed and gradual breakdown of aromatic bridge bonds leads to disintegration of the edge structure of the basic structural unit.