Characterisation of subfractions of asphaltenes extracted from an oil sand using NMR, DEPT and MALDI-TOF

This paper reports the findings of an investigation into molecular structures and properties of an oil sand asphaltene sample and its subfractions. The asphaltene sample extracted from Buton Oil Sand (Indonesia) was fractionated stepwise into four subfractions by precipitation in a binary solvent system made from mixtures of dichloromethane/n-heptane with volumetric ratios of 30/70, 20/80 and 10/90, respectively. The average molecular structural parameters, including the average polycyclic aromatic hydrocarbon size, average side chain length and average molecular weight of the oil sand asphaltene sample and its subfractions, were measured and compared, using characterisation data obtained from nuclear magnetic resonance in combination with distortionless enhancement by polarisation transfer. The molecular weight distributions of the asphaltene samples were measured using a matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. The results indicated that the island molecular architecture was featured in all the asphaltene samples. The average polycyclic aromatic hydrocarbon size was found to be 7 rings for the least soluble subfraction, 6 rings for the other subfractions and the oil sand asphaltenes. The fractionation mechanism was dictated by polarity difference amongst subfractions as a result of relative luxuriance of the aliphatic parts. The use of C-13 NMR, DEPT and MALDI-TOF was shown to provide a useful means for characterisation and estimation of molecular structures of the asphaltenes.