Abstract
We have carried out the first systematic in situ variable-temperature high-resolution 1H NMR study of coal samples between 25 and 230 °C based on the CRAMPS (combined rotation and multiple-pulse spectroscopy) technique. The coal samples studied include two high-volatile and one low-volatile bituminous coals from the Argonne premium coal bank. We found unexpectedly that there are no dramatic changes of 1H CRAMPS resolution over this temperature range. Slight deterioration of resolution at high temperature suggests interference of thermally promoted molecular motion with coherent averaging by the multiple-pulse sequence. Based on a “time-suspension” experiment using the CMG-48 pulse sequence and proton dipolar-dephasing experiments, we estimated the correlation time of the thermally promoted molecular motion as being about 10 μs, which is several orders of magnitude slower than the molecular motion induced with pyridine saturation of coal at room temperature. This result suggests that thermal treatment alone up to 230 °C is not enough to break either the covalent bonds or the noncovalent associative bonds that connect the macromolecular network of coal. With improved dipolar-dephasing experiments based on BR-24 1H CRAMPS detection, we were able to analyze quantitatively the proton dipolar-dephasing behaviors of aliphatic and aromatic protons in coal. We found that the dipolar-dephasing curve of a coal at high temperature can be described as two Gaussian dephasing components, which indicate a coexistence of molecules with two distinctly different mobilities. The promotion of molecular mobility in coal at higher temperature is attributed to an increase of the fraction of molecules in relatively mobile states. This 1H CRAMPS study provides detailed correlations of molecular dynamics with molecular structure for coals at temperatures between 25 and 250 °C.
Published Version
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