Heavy Petroleum Composition. 5. Compositional and Structural Continuum of Petroleum Revealed

摘要

Twenty-five years ago, Boduszynski et al. conducted a comprehensive study of heavy oil composition and concluded that crude oil composition increases gradually and continuously with regard to aromaticity, molecular weight, and heteroatom content from the light distillates to non-distillables (the Boduszynski continuum model). Previous exhaustive characterization of heavy vacuum gas oil by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) provided compositional data that strongly supports the continuum model. However, when the molecular formulas obtained by FT-ICR MS for the distillates and asphaltenes from the same parent crude oil are plotted as double bond equivalents (DBE) versus carbon number, a gap appears between the compositional space of "asphaltenes" and "maltenes", in contradiction to the Boduszynski-Altgelt model. Here, a heavy distillate cut (atmospheric equivalent boiling point of 523-593 °C) is fractionated according to the number of aromatic rings by HPLC-2. The C7-deasphalted whole oil (C7-DAO), its pentane soluble/insoluble fractions, and each of their ring number fractions are comprehensively characterized by atmospheric pressure photoionization (APPI) FT-ICR MS and tandem mass spectrometry (MS/MS). The HPLC-2 fractions from both the C5-soluble and C5-insoluble C7-DAO represent a gradual and continuous progression that fills the compositional "gap" in carbon number and aromaticity between asphaltenes and maltenes as a function of the increasing aromatic ring number, as predicted by Boduszynski. MS/MS results indicate that each ring number fraction comprises both island and archipelago structural motifs. FT-ICR MS reveals a continuum in carbon number and aromaticity. The C5-insoluble C7-DAO components have a similar structure but with higher-order fused ring core structures and are composed of a higher proportion of archipelago structures than the C5-soluble C7-DAO components. Thus, fractionation by the aromatic ring number of "maltenic" and "asphaltenic" species from the C7-solubles from a high boiling distillate validates the compositional continuum of petroleum components, and MS/MS exposes the aromatic building blocks of "maltenic" and "asphaltenic" species (structural continuum) that comprise island and archipelago structural motifs.