Characterization of chemical functional groups in macerals across different coal ranks via micro-FTIR spectroscopy

Fourier transform infrared spectra of a suite of coals of varying rank from peat to anthracite were obtained via reflectance micro-FTIR and traditional KBr pellet techniques. With increasing rank of coal samples, KBr-FTIR spectra exhibit rising aromaticity (ratio of CHar at 3000–3100 or 700–900 cm− 1 versus CHal at 2800–3000 cm− 1) and enhanced condensation of aromatic rings (ratio of CHar versus C=C at ~ 1600 cm− 1), whereas the aliphatic chain length (ratio of CH2/CH3 at 2800–3000 cm− 1) and the ‘C’ factor (ratio of C=O at ~ 1710 cm− 1 versus (C=O + C=C)) decrease. The ratio of CHal /(CHal + C=C) (i.e., the ‘A’ factor), which reflects the hydrocarbon-generating potential, initially rises at low rank and later decreases in higher-rank coal= (Ro > 0.98%). However, the trends of these semi-quantitative FTIR ratios are difficult to trace in high-rank coals (Ro > 1.50%), probably attributable to limited peak areas and enhanced uncertainties in ratio calculations. The overall evolutionary trends of functional group abundances in bulk coals and their individual macerals are similar; distinct differences in chemical properties, however, exist among maceral groups, and these variations depend on rank. Within the three maceral groups, liptinite generally exhibits the lowest aromaticity, the longest aliphatic chains having the least amount of branching, and the highest ‘A’ factor testifying to the highest hydrocarbon-generating potential. In contrast, inertinite shows the highest aromaticity and degree of condensation of aromatic domains and the lowest hydrocarbon-generating potential. Vitrinite generally exhibits intermediate characteristics between liptinite and inertinite. Compared to KBr spectra, micro-FTIR spectra present better detection capability and stronger signals in the 700–900 cm− 1 region, which allows for close investigation of aromatic CHx out-of-plane deformation modes. The rapid decrease in the peak area ratio at ~ 870 cm− 1/~ 750 cm− 1 in vitrinite from peat to low-volatile bituminous coal is followed by a significant increase after Ro > 1.50%, which indicates the dominance of highly substituted aromatic rings in immature coal's structure, while condensed aromatic domains become prevalent in higher-rank coals (semi-anthracite and anthracite). The increase in coal aromaticity during coalification is attributed largely to the accumulation of condensed aromatic structures in more mature coals. Novel reflectance micro-FTIR mapping is a promising and powerful tool providing high-resolution information on chemical properties of coal macerals relating to parent material and coalification.