Subfractionation and characterization of Mayan asphaltene

Mayan crude, residuum, and hydrocracked asphaltenes have been separated into two fractions by extended Soxhlet extraction in It-heptane. Although the solubility, composition, and molecular structures differ slightly, the greatest difference between the two asphaltene fractions is the degree to which they associate in solution. The vapor-phase osmometry molecular weight, molecular size by size-exclusion chromatography, and small-angle neutron scattering indicate that approximately 25% of Mayan asphaltene is not highly associated in aromatic solvents and, thus, is noncolloidal. By contrast, the remaining asphaltene forms large, rodlike colloidal particles in solution and has a higher apparent molecular weight. Although laser desorption mass spectrometry indicates that the molecular weight of the individual molecules in maltenes and asphaltenes is not very different, high-resolution mass spectrometry indicates that the size of the aromatic core of asphaltenes is significantly larger than those in maltenes. Furthermore, the tendency of the residuum fractions to form coke during thermal cracking is likely related to the size of the largest polyaromatic rings.