SINGLE-PULSE EXCITATION C-13 NMR MEASUREMENTS ON THE ARGONNE PREMIUM COAL SAMPLES

In view of the results of this study and the well-documented inherent problems with cross-polarization (CP) C-13 NMR concerning quantification for coals, it is now clear that the more time-consuming single-pulse excitation (SPE) or Bloch decay measurements are essential for obtaining aromaticities and other carbon skeletal parameters for coals. SPE C-13 NMR has been carried out on the Argonne Premium Coal Samples at both a low and a high field strength (25 and 75 MHz, respectively), high-speed magic angle spinning (13 kHz) being used to suppress spinning sidebands at the higher field. Aromaticity values measured by SPE at low and high field were generally in excellent agreement and were consistently higher than those from CP, the greatest differences being found for the two low-rank coals in the Argonne suite. The use of tetrakis(trimethylsilyl)silane as an internal standard in the low-field measurements indicated that, in general, over 75% of the carbon in the coals is typically observed by SPE.