Evolution of the chemical structure of Hat Creek resinite during oxidation: a combined FT-IR photoacoustic, NMR and optical microscopic study

Abstract

Resinite, hand picked from a subbituminous coal in the Hat Creek No. 1 deposit in south central British Columbia, Canada, was subjected to oxidation in air at temperatures between 50°C and 400°C. The initial and oxidized resinite samples were analyzed for variations in molecular structure by the use of Fourier transform infrared (FT-IR) photoacoustic spectroscopy and 1H and 13C nuclear magnetic resonance spectroscopy. The changes in molecular structure determined were correlated with differences observed in optical reflectance and fluorescence properties. The resinite in its initial state was predominantly aliphatic in nature and could be characterized as a diterpenoid, with alkene and carboxylic functionalities exocyclic to the six-membered rings composing the diterpenoid skeleton. Oxidation between 50°C and 100°C resulted in only minor structural changes. During oxidation between 100°C and 250°C there were very significant changes, involving the diminution in the number of aliphatic CH structures, the complete loss of the exocyclic alkene function, the appearance of aromatic structures and the conversion of carboxylic acids to acid anhydrides. Above 250°C, aliphatic groups continued to decline in number, aromatic character continued to increase and spectroscopic features, attributable to acid anhydrides, achieved maximum intensity. The optical reflectance of resinite increased with increasing oxidation temperature and the trends in reflectance and fluorescence properties (colour, wavelength of maximum intensity and red/green quotient) could be ascribed, as reported in earlier studies, to: (1) chemisorption, which occurs between 25° and 100°C, and decomposition of chemisorbed components at 100–150°C; (2) oxycoal formation (150–300°C), in which oxygen complexes and oxidation rims develop; and (3) combustion (>350° C). The variations n mass, infrared peak ratio ( C H/ C= O), fraction of aromatic carbon ( f a) and fraction of hydrogen attached to aromatic carbon (H ar), with the variation in the temperature of oxidation mirrored the changes observed by optical microscopy. The results show that the physical changes observed by optical microscopy have a direct correlation with the molecular structural changes determined by FT-IR and NMR spectroscopy. Additionally, it has been observed that resinites underwent thermal aromatization, a phenomenon not well studied for these materials. The changes observed in oxidation structures can be related to reactions involving thermal dehydrogenation of the diterpenoid ring structures and those involving oxygen addition. Oxygen inclusion occurs at all temperatures above 100°C, whereas thermal aromatization occurs mainly above 250°C.