Two-dimensional electron paramagnetic resonance (2D-EPR) spectra of Illinois No. 6 and Upper Freeport coals have been measured. From 2D-nutation spectroscopy, it is confirmed that the main EPR signal around g = 2 is attributed to free radicals, but not to radical pairs. Both coals showed nuclear modulation effects due to1H and naturally abundant 13C nuclear spins. Spinecho correlation spectroscopy (SECSY) experiments elucidate the distribution of modulation frequencies included in a broad EPR spectrum. The observed dip in the 13C matrix spectrum obtained from SECSY is interpreted in terms of the decrease of vitrinite radicals, which are weakly interacting with 13C nuclei, in the central region of the EPR spectrum. The analysis of the broad hyperfine spectra due to 1H and 13C nuclei in the hyperfine sublevel correlation spectra suggests the existence of several kinds of radicals in coal. Most of unpaired electrons are considered to be trapped in aromatic rings. The ratio of 13C/1H of hyperfine spectra increases with coal rank, suggesting a new index of coal rank.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology