Quantitative ¹³C NMR study of structural variations within the vitrinite and inertinite maceral groups for a semifusinite-rich bituminous coal

To determine the structural variation within vitrinite and inertinite maceral groups, fractions with purities over 90% in vitrinite and semifusinite were obtained by density gradient centrifugation from a medium volatile Australian bituminous coal and the bulk structural compositions of the maceral concentrates were determined by the quantitatively reliable single pulse excitation (SPE) solid state ¹³C NMR technique. As previously reported for coals and chars, the aromaticities determined by cross polarisation are often lower than those by SPE, due to the unfavourable spin dynamics. As expected, the aromaticities of the vitrinite fractions are significantly lower than those of the semifusinite ones, but the aromaticity, the fraction of non-protonated aromatic carbon and the number of rings per cluster all increase with density within both the maceral groups. The vitrinite and semifusinite fractions contain 3-6 and 9 to over 15 aromatic rings, respectively. Methyl groups account for greater proportions of the aliphatic carbon with increasing density. These structural trends are consistent with the variations evident in random reflectance.