An investigation of coal by means of e.s.r., 1H n.m.r., 13C n.m.r. and dynamic nuclear polarization

Sixty coal samples of different rank and origin have been investigated by means of e.s.r., ¹H n.m.r. and ¹³C n.m.r., the last two in combination with dynamic nuclear polarization (DNP). The following parameters have been determined: the number of free radicals, the e.s.r. linewidth, the ¹H Zeeman relaxation rate, the ¹H relaxation rate in the rotating frame, the ¹H DNP enhancement, the ¹³C DNP enhancement, the ¹³C Zeeman relaxation rate and the ¹³C aromaticity, observed via ¹H¹³C cross-polarization (CP), both with and without magic-angle spinning (MAS). The relations between these parameters and coal rank have been investigated. Moreover, with DNP special experiments have been performed which provide information about the localization and the mobility of the unpaired electrons present in these coals. Finally, DNP has been used to investigate various features of the quantitative analysis of coal via ¹³C n.m.r. MAS was found to reduce the measured ¹³C aromaticity, and for three coals it was shown that even without MAS only ≈ 50% of the aromatic ¹³C nuclei are detected by the CP technique.