Understanding the characteristics of an Asphaltene extract of an oil sand by analyzing its subfractions using HRTEM, EELS, FTIR, and TGA

This article reports an experimental investigation into the aggregation characteristics of an oil sand asphaltene sample by analyzing its subfractions precipitated in a series of binary dichloromethane/n-heptane solvents. The asphaltene sample was extracted from Buton oil sand using the Soxhlet extraction and fractionated stepwise into four subfractions by precipitation in the binary solvent system. The subfractions were characterized using high resolution transmission electron microscopy (HRTEM) for average stacking number, interlayer spacing, and layer size; electron energy loss spectroscopy (EELS) for the degree of graphitization; Fourier transform infrared (FTIR) spectroscopy for functional groups; and thermogravimetric analysis (TGA) for thermal behaviour, respectively. The average stacking number decreased while the average interlayer spacing increased as the polarity of the subfractions was decreased. The average layer size across the four subfractions was slightly larger than 0.9 nm, implying an average polycyclic aromatic hydrocarbon (PAH) size of 6 ~ 7 fused rings. The graphitization and aromaticity decreased as the polarity of the subfractions was increased. The FTIR results showed the ratio of aromatic functional groups to aliphatic functional groups decreased in the subfractions, following the decreasing polarity. The amounts of residues from pyrolysis of subfractions in the TGA experiments indicated that the PAH content in the subfractions also decreased in accordance to the decreasing polarity. The results confirm that the aromaticity decreased from the least soluble fraction to the most soluble subfraction and the asphaltene subfractions aggregate through π stacking.