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

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 (Ro,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 falH/fal* and bridge carbon ratios (XBP), along with a lower faS/fa’ 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.