STRUCTURAL FEATURES OF A BITUMINOUS COAL AND THEIR CHANGES AFTER LOW-TEMPERATURE OXIDATION AND LOSS OF VOLATILES INVESTIGATED BY ADVANCED SOLID-STATE NMR SPECTROSCOPY

Quantitative and advanced ¹³C solid-state NMR techniques were employed to investigate the chemical structure of a high volatile bituminous coal and chemical structural changes of this coal after evacuation of adsorbed gases, during oxidative air exposure at room temperature, and after oxidative heating in air at 75ºC. The solid-state NMR techniques employed in this study included quantitative direct polarization/magic angle spinning at a high spinning speed of 14 kHz, cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH, CH₂, and CH_n selection, ¹³C chemical shift anisotropy (CSA) filter, 2D ¹H-¹³C heteronuclear correlation NMR (HETCOR), and 2D HETCOR with spin diffusion. With spectral editing techniques, we identified methyl CCH₃, rigid and mobile CCH₂C, CCH, NCH, aromatic CH, aromatics bonded to alkyls, small sizes of condensed aromatics, and aromatic C-O. With direct polarization combined with spectral-editing techniques, we quantified 11 different kinds of functional groups. ¹H-¹³C 2D HETCOR NMR experiments indicated spatial proximity of aromatic and alkyl moieties in cross-linked structures, confirming the macromolecular nature of coal. The proton spin diffusion experiments indicated that the magnetization was not equilibrated at a ¹H spin diffusion time of 5 ms. Therefore, the heterogeneity of different functional groups should be above 2 nm. Recoupled C-H long-range dipolar dephasing showed that the fraction of large charcoal-like clusters of polycondensated aromatic rings was relatively small. The exposure of this coal to atmospheric oxygen at room temperature for 6 months did not result in obvious chemical structural changes of the coal but heating at 75ºC in air led to oxidation of coal, generating a small signal of COO. Evacuation removed most volatiles and caused a significant reduction in aliphatic signals in its DP/MAS spectrum. These results demonstrate the applicability of advanced solid-state NMR techniques in characterizing coal.