Abstract
Eight Argonne Premium coals and three weathered Argonne coal samples were investigated by using {sup 13}C cross-polarization-magic-angle spinning (CP-MAS) NMR spectroscopy. Proton and carbon spin-lattice relaxation measurements were performed on each coal. The carbon spin-lattice relaxation time, T{sub 1}{sup C}; the carbon spin-lattice relaxation time in the rotating frame, T{sub 1{rho}}{sup C}; and the proton spin-lattice relaxation time in the rotating frame, T{sub 1{rho}}{sup H}, are reported for aromatic and aliphatic carbons of the 11 coal samples. In general, the proton and carbon spin-lattice relaxation data can be evaluated as the sum of two exponential decays. The longer components of carbon relaxation times in the laboratory and rotating reference frames vary in a systematic way with coal rank as expressed by percent carbon. The trends can be explained in terms of motional properties of the coals and the presence of paramagnetic species. Marked changes in the relaxation parameters have been observed between pristine and weathered coals. Reduction in proton T{sub 1{rho}} values upon weathering is shown to have an adverse effect on quantitation with CP. 38 refs., 7 figs., 5 tabs.
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