CPMAS ¹³C NMR and pyrolysis-GC-MS studies of structure and liquefaction reactions of Montana subbituminous coal

This paper reports on the application of solid-state CPMAS ¹³C NMR and flash pyrolysis-GC-MS for characterization of the macromolecular network of a Montana subbituminous coal and its residues from temperature-programmed and non-programmed liquefaction (TPL and N-PL) at final temperatures ranging from 300 to 425°C in H-donor and non-donor solvents. The combined use of ¹³C NMR and Py-GC-MS revealed that this coal contains significant quantities of oxygen-bearing structures, corresponding to about 18 O-bound C per 100 C atoms and one O-bound C per every five to six aromatic C's. The oxygen-bearing components in the coal include catechol-like structures, which seem to disappear from the liquefaction residues above 300°C; carboxyl groups, which almost disappear after 350°C; and phenolic structures, which are most important in the original coal but diminish in concentration with increasing temperature. These results point to the progressive loss of oxygen functional groups and aliphatic-rich species from the macromolecular network of the coal during programmed heat-up under TPL conditions. The higher conversions in TPL runs in H-donor tetralin (relative to the conventional N-PL runs) suggest that the removal of carboxylic and catechol groups from the coal and the capping of the reactive sites by H-transfer from H-donors at low temperatures (≤350°C) have contributed to minimizing the retrogressive crosslinking at higher temperatures. Quantitative calculation of NMR data and mathematical correlation were also attempted in this work. For 24 liquefaction residues derived under significantly different conditions, linear correlations between C-distribution and reaction temperature (≥ 300°C) have been found, which can be expressed by a simple equation, C_i = αf_i + βT, where f_i and C_i, represent content of aromatic, aliphatic, or oxygen-bound carbons in the original coal and residue, respectively; T stands for the reaction temperature; α and β are constants.