Determination of nonprotonated aromatic carbon concentrations in coals by single pulse excitation carbon-13 NMR

It is now generally accepted that single pulse excitation (SPE) or Bloch decay measurements are essential for obtaining reliable aromaticity values for coals by solid-state C-13 NMR although such measurements are considerably more time consuming than cross-polarization (CP). Just as with normal CP, SPE can be combined with dipolar dephasing (DD) to derive nonprotonated aromatic carbon concentrations. This approach has been applied to four of the Argonne premium coal samples (APCS, N. Dakota lignite, Wyodak, Illinois no. 6, and Pocahontas), two vitrinite concentrates from UK bituminous coals, and an anthracite. The results have been compared with those obtained from CP and, for the APCS, nonprotonated aromatic carbon concentrations derived indirectly from FTIR and H-1 CRAMPS. As with the overall aromaticity, SPE invariably gives higher values than CP for nonprotonated aromatic carbon concentrations with differences of up to 10 mol % carbon being found, the greatest being found for the two low-rank coals. However, in some instances, reasonable agreement has been obtained with CP data when long contact times are employed to minimize the discrimination against aromatic carbons. The aliphatic H/C ratios in the range 2.0-2.5 derived from the SPE-DD measurements are much more consistent with other available structural information on coals than the much lower values generally derived from CP, FTIR, and H-1 CRAMPS.