Thermal evolution of aliphatic and aromatic moieties of asphaltenes from coals of different rank: possible implication to the molecular architecture of asphaltenes

The compositional and structural changes that coal asphaltenes undergo with increasing thermal maturation were investigated using solid-state 13C NMR and Fourier transform infrared spectroscopy. The results show a gradual increase in the relative proportion of carbon content with a concomitant decrease in the hydrogen, sulphur and oxygen content. The amount of aromatic carbon also increases while the aliphatic carbon decreases with increasing maturity. Ester and carboxylic groups are particularly sensitive to maturation and decrease in relative abundance with increasing maturity while the amount of aromatic carbonyl groups increases. In general, the asphaltenes were observed to evolve towards a more thermally stable structure with increasing amounts of aromatic moieties and relatively lower amounts of aliphatic moieties. The results are in agreement with the ultimate dominance of the island molecular architecture (the Yen–Mullins model) in asphaltenes.