AmericanChemicalSociety.com

In the past year, variations in the chemical composition of kerogens from high volatile bituminous coal near two igneous dike intrusions in the Illinois Basin were characterized by advanced 13C solid-state nuclear magnetic resonance (NMR). The techniques included quantitative direct polarization/magic angle spinning (DP/MAS), cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, 13C chemical shift anisotropy (CSA) filtering, CHn selection, 1H-13C two-dimensional heteronuclear correlation (2D HETCOR) NMR, and recoupled C-H long-range dipolar dephasing techniques. 13C NMR was able to characterize the immediate chemical environment of carbons in coal kerogens in terms of bonding and the presence of heteroatoms. With decreasing distance to dike contacts along sample sequences in a coal seam, the predominant structural changes in coal kerogens with increasing thermal maturity expressed as increasing Ro included (i) the elimination of aliphatics and an increase in aromaticity; (ii) a reduction of the relative abundance of aromatic carbons bonded to hydrogen atoms, possibly due to a replacement of aromatic hydrogen by aryl groups via cross-linking; and (iii) an increase in the aromatic cluster size in coal kerogens approaching the larger dike, as revealed by spectral analysis and recoupled C-H long-range dipolar dephasing. Our previous NMR study on a series of Type II kerogens (Ro from 0.29 to 1.27%) indicated that the progressive increase in aromaticity with increasing Ro was mainly related to the shortening of alkyl chains attached to the aromatic cores without growth of fused rings. In contrast, increasing aromaticity in Type III kerogens studied here was additionally related to conversion of alkyl components to aromatic rings, and an increase in aromatic cluster size.