Interactions of organic liquids with coals:: Analysis by solid-state 13C nuclear magnetic resonance

We analyzed the solid-state C-13 cross-polarization/magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectra of three types of coal samples that had been immersed in toluene or n-hexane for 1 h. The coal samples were from the United States (Upper Freeport (UF) reserve, 86 wt % carbon, and Wyodak (WY) reserve, 75 wt % carbon), and Australia (Yallourn (YL) reserve, 65 wt % carbon). The spectra of toluene- and n-hexane-immersed UF and n-hexane-immersed WY coals were identical to the spectra of the corresponding fresh coals: sorbed molecule peaks were not observed, which indicates that almost all the molecules in these coals were in a mobile state. The spectra of toluene- and n-hexane-immersed YL coal and toluene-immersed WY coal showed peaks for toluene and n-hexane, demonstrating that some of the molecules were bound relatively strongly to the coal. The C-1 carbon of toluene was not observed in the CP/MAS NMR spectra of toluene-immersed YL and WY coals, so it is possible that the toluene molecules observed in the NMR spectra were bound to these coals at the methyl or C-4 carbon through an OH(coal)-pi(toluene) interaction. In the CP/MAS spectrum of n-hexane-immersed YL, the C-3 carbon of n-hexane was observed, but the C-1 and C-2 carbons were not. This result indicates that n-hexane molecules may bind to the coal only at C-3, whereas the C-1 and C-2 carbons remain more mobile. For YL coal immersed in toluene or n-hexane, or WY coal immersed in toluene, the observed peak intensities of toluene or n-hexane in the coals were smaller than expected, given the content. This result indicates that a proportion of the toluene or n-hexane molecules in the coal interacted relatively strongly with the coals via OH-pi bonding and that the remaining molecules were in a mobile state and interacted weakly with the coals by pi-pi bonding or some other weak interaction.