Heavy Petroleum Composition 4. Asphaltene Compositional Space

Energy Fuels, Just Accepted Manuscript, Publication Date (Web): January 17, 2013, DOI: 10.1021/ef301747d
Heavy Petroleum Composition 4. Asphaltene Compositional Space
Abstract
Asphaltenes and maltenes are defined operationally by solubility. Asphaltenes self-associate in solution and form putative nanoaggregates, composed of approximately six to ten asphaltene monomers per subunit. Bulk measurements indicate that asphaltenes are more aromatic than maltenes, and contain more heteroatoms and metals.
Numerous direct imaging, molecular diffusion, and mass spectral results agree that asphaltenes and maltenes are defined by similar, overlapped carbon number ranges, restricting the acceptable carbon number and aromaticity "compositional space" for asphaltene compounds. Consequently, when viewed by a plot of aromaticity versus carbon number for a given heteroatom class, asphaltenes must occupy different compositional space than maltenes.  This is due to the fact that they share carbon number range, but differ in bulk aromaticity and solution phase behavior.
Boduszynski’s work supported overlapping asphaltene/maltene molecular weights. He proposed that "high boiling does not necessitate high molecular weight." However, his limited mass spectral resolution precluded direct molecular level confirmation. Current mass spectral results combined with results published in parts I, II, and III of this series provide the basis for a continuum in petroleum structure and composition in support of the Boduszynski model.  They confirm that asphaltene molecules share carbon number range with their maltene counterparts, but are simply more aromatic. The compositional space for maltenic and asphaltenic species, therefore, is now known. Part III provided evidence for asphaltene aggregate formation at concentrations below that required for most mass spectral analyses.  Thisindicates that at these concentrations, the majority of asphaltenes are locked in aggregate structures and therefore undetected as monomers. Here, Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry results confirm that asphaltenes and maltenes of the same heteroatom class exhibit higher aromaticity than maltenes of the same carbon number, limited by the highest possible aromaticity for a stable planar aromatic structure, and clearly differentiates asphaltene and maltene molecular compositions.
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