Reexamining the Molecular/Macromolecular Model of Coal from Comparative in Situ Variable-Temperature 1H NMR Studies of Argonne Premium Coal 601 and Its Pyridine Extraction Residue

Abstract

We have carried out the first systematic in situ variable-temperature 1H NMR study, using the CRAMPS (combined rotation and multiple pulse spectroscopy) technique, of the residue from pyridine extraction of Argonne premium coal 601. The change of molecular mobility in the residue under various thermal treatments and pyridine swelling conditions were monitored quantitatively through a proton dipolar-dephasing experiment based on CRAMPS detection. Correlations among molecular structure, molecular mobility and solvent-extraction components were established through comparative variable-temperature 1H CRAMPS studies on the C5D5N-saturated original coal and its extraction residue. A critical review of the molecular/macromolecular (M/M) structural model of coal has been made on the basis of the new results, showing that the relationships between the M/M model and molecular dynamics is much more complicated than previously thought. Both the molecular phase and the macromolecular phase can show molecular dynamics behavior ranging from a lack of substantial motion to extremely mobile behavior with correlation times as short as 30 ns. Without systematic variable temperature studies, correlation between molecular mobility and the M/M model cannot be established correctly by NMR.