In situ variable-temperature proton solid-state NMR studies of molecular structure and dynamics in coals.

摘要

This work is focused on how non-covalent associative bonds in coal are affected at the molecular level by thermal treatment and solvent saturation. We have carried out the first systematic in situ variable-temperature proton nuclear magnetic resonance (NMR) study (via the CRAMPS technique) of untreated and pyridine-saturated coal samples between 25{dollar}spcirc{dollar}C and 230{dollar}spcirc{dollar}C. Major advances of previously existing proton CRAMPS experimental methods have been made to improve the reliability, robustness and efficiency of these techniques. New time-domain and two-dimensional NMR experiments based on CRAMPS techniques have been developed and applied to in situ variable-temperature studies of coal. These experiments have not only provided molecular dynamical information on coal over a large range of time scales, but have also revealed the large structural heterogeneity existing in coal over a broad range of spatial dimensions from 4 A to 350 A.; We have found that pyridine saturation can dramatically promote molecular motion, even at room temperature, while thermal treatment alone is much less effective in promoting molecular motion even at 230{dollar}spcirc{dollar}C. However, a moderate increase in temperature can significantly enhance the molecular mobility in pyridine saturated coal. Correlations among molecular structure, molecular mobility and solvent-extraction components have been established through comparative studies on pyridine-saturated Argonne premium coal 601 and its pyridine-extraction residue. We have developed a coherent view of the molecular structure and dynamics of coal on the basis of the new experimental results. Many discrepancies in the literature on these issues are resolved by this view, and the resolution enhancement due to solvent saturation is satisfactorily explained. We have also made a critical review of the molecular/macromolecular structural model of coal.; The structural heterogeneity in coal has also been addressed in this work directly by the first one and two-dimensional proton spin-exchange studies of coal based on proton CRAMPS detection. In particular, a new two-dimensional proton spin-exchange technique has been developed for probing complicated spin-exchange pathways among protons with different mobilities and chemical shifts. Information on the spatial distribution of different types of structural units in coal in terms of chemical shifts and molecular mobilities was obtained.