Molecular representations of Permian-aged vitrinite-rich and inertinite-rich South African coals

Abstract Molecular representations for two Permian-aged South African coals, inertinite-rich Highveld (dominated by semifusinite) and vitrinite-rich Waterberg were constructed based on analytical data. High-resolution transmission electron microscopy (HRTEM) was used to determine the size and distribution of aromatic fringes, thereby affording the base aromatic skeleton for each coal model. Sulfur, nitrogen, oxygen and aliphatic side chains and crosslinks were added to the aromatic skeletons according to 13 C NMR and literature data. The individual molecules were assembled into three-dimensional structures and were in agreement with experimental data (NMR, mass spectrometry and elemental analyses data). These models were structurally diverse with a molecular weight ranging from 78 to 1900 amu. The vitrinite-rich coal model consists of 18,572 atoms and 191 individual molecules and the inertinite-rich coal model consists of 14,242 atoms and 158 individual molecules. These were the first molecular representations for South African vitrinite-rich and inertinite-rich coals. The inertinite-rich Highveld coal model was more aromatic with a larger portion of the aromatic carbons polycondensed. The vitrinite-rich Waterberg coal model was more aliphatic and contained more aliphatic side chains and longer aliphatic crosslinks. Although these coals have very similar average molecular structures according to the various analytical data, subtle differences in the experimental data lead to significant structural differences in the models.