Heavy Petroleum Composition. 4. Asphaltene Compositional Space

Asphaltenes and maltenes are defined operationally by solubility (in, e.g., heptane). Asphaltenes self-associate in solution and form putative nanoaggregates composed of approximately 6–10 asphaltene monomers per subunit. Bulk measurements indicate that asphaltenes are more aromatic than maltenes and contain more heteroatoms and metals (nitrogen, sulfur, oxygen, nickel, and vanadium). Numerous direct imaging, molecular diffusion, and mass spectral results agree that asphaltenes and maltenes are defined by similar, overlapped carbon number ranges, drastically restricting the acceptable carbon number and aromaticity “compositional space” for asphaltene compounds. Thus, when viewed by a plot of aromaticity versus carbon number for a given heteroatom class, asphaltenes must occupy different compositional space than maltenes because they share the same carbon number range but differ in bulk aromaticity and solution phase behavior. Boduszynski’s work supported overlapping asphaltene/maltene molecular weights, and he proposed that “high boiling does not necessitate high molecular weight” [Boduszynski, M. M.Composition of heavy petroleums. 1. Molecular weight, hydrogen deficiency, and heteroatom concentration as a function of atmospheric equivalent boiling-point up to 1400 °F (760 °C). Energy Fuels 1987, 1 (1) 2−11]. However, his limited mass spectral resolution precluded direct molecular-level confirmation. Current mass spectral results combined with results published in parts 1 (10.1021/ef100149n), 2 (10.1021/ef1001502), and 3 (10.1021/ef3018578) of this series provide the basis for a continuum in petroleum structure and composition in support of the Boduszynski model and confirm that asphaltene molecules share the same carbon number range with their maltene counterparts but are simply more aromatic. Thus, the compositional space for maltenic and monomeric asphaltene species is now known. Part 3 (10.1021/ef3018578) provided evidence for asphaltene aggregate formation at concentrations below that required for most mass spectral analyses, suggesting that, at these concentrations, the majority of asphaltenes are locked in aggregate structures and, therefore, undetected as monomers. Here, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) 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 differentiate asphaltene and maltene monomeric molecular compositions.