Insights into molecular structure of low- to high-rank Indian coals via XRD, Raman, and solid state 13 C NMR spectroscopy: implications for coalification

A detailed understanding of coal molecular structure is crucial for optimizing its utilization and modeling coalification processes. In this study, we investigate coals spanning lignite to anthracite ranks from Indian deposits using a combination of X-ray diffraction (XRD), Raman micro-spectroscopy, solid-state 13C CP/MASNMR, and petrographic analysis. XRD reveals that all samples exhibit turbostratic carbon with larger interlayer spacing than graphite, and a clear rank trend: the (002) spacing (d002) steadily contracts while stacking height and graphitic order increase with coal rank. Raman spectroscopy (D/G band analysis) shows systematically decreasing disorder and increasing graphitic cluster size as vitrinite reflectance rises. In parallel, 13C NMR quantifies the chemical transformations: aromatic carbon fractions rise markedly while aliphatic and oxygenated carbons diminish. Notably, the aromatic bridgehead carbon fraction and related cluster-condensation indices increase with rank, indicating progressive fusion of aromatic rings into larger polyaromatic networks. These spectral trends are consistent with classical coalification paths: volatile side chains and oxygen functional groups are cleaved (decarboxylation), and remaining aromatic structures condense into ordered, graphene-like sheets. The findings provide a coherent molecular-scale picture across analytical methods, with broader implications for coal utilization (combustion and gasification behavior), design of coal-derived carbon materials, and geochemical modeling of coal maturation.