Effect of Water on CP/MAS sup 13 C NMR Spectra of Low-Rank Coals

摘要

From the investigation reported here, it appears that under the ''best'' conditions fairly representative, reproducible solid sup 13 C spectra of low-rank coal can be obtained from vacuum dried samples. The contact time that yields maximum intensity is near 1000 mu s for the aromatic portion of both lignites and slightly shorter for the aliphatic portion. Thus, the selection of 1000 mu s as the optimum contact time slightly favors the aromatic part of the spectrum. This positive effect is offset by some aromatic signal loss, estimated at <3%, during spinning sideband suppression. We are currently investigating the relationship between signal intensity and the amount of dry coal carbon in the rotor both by absolute intensity measurements and by internal standard spin-counting experiments. It appears that the dried coal samples give much better signal intensity relationships than coals containing small amounts of water. Performing CP/MAS sup 13 C experiments with complex solid samples such as coal and peat is becoming more common. This technique has opened exciting new areas of structural investigation in coal science and will certainly continue to grow rapidly in popularity. The coal scientist will have to become thoroughly conversant with such concepts as the decoupler power requirements, 90 and 180 exp 0 flip angles, T sub 1 rho values for his samples, optimal cross polarization times, correct delay, spinning speed, presence of radicals, effect of coal ash and many other factors. As each new experiment is designed, new variables may be introduced. Each variable must be carefully scrutinized, particularly for obvious effects such as the one reported here. When a series of coal samples are compared, or when low-rank coal spectra are being acquired, the samples must be dry enough to allow tuning within the range of the NMR probe or erroneous intensities and fa values will result. 11 refs. (ERA citation 11:019013)