Heavy Petroleum Composition. 5. Compositional and Structural Continuum of Petroleum Revealed David C. Podgorski, †,‡ Yuri E. Corilo, † Leonard Nyadong, † Vladislav V. Lobodin, †,‡ Benjamin J. Bythell, † Winston K. Robbins, ‡,∥ Amy M. McKenna, †,‡ Alan G. Marshall, †,§ and Ryan P. Rodgers* ,†,‡,§ † National High Magnetic Field Laboratory, and ‡ Future Fuels Institute, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, United States § Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306-4390, United States * S Supporting Information ABSTRACT: 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. ■ INTRODUCTION In the final installment of this five-part series, we address the compositional “gap” exposed by the comparison of the molecular level information obtained from the exhaustive characterization of distillate fractions up to and including heavy vacuum gas oil to their corresponding heptane-insoluble asphaltenes. In doing so, we address the structural moieties (island and archipelago) of asphaltenes and maltenes and rationalize the absence of species that lie in compositional space [defined as a plot of double bond equivalents (DBE = number of rings plus double bonds involving carbon) versus carbon number] above previously observed (more aromatic) maltenes but below (most aromatic) asphaltenic species that lie just below the polycyclic aromatic hydrocarbon (PAH) planar limit line. 1,2 Asphaltenes are operationally defined by their insolubility in excess paraffin, designated by the n-alkane used for their precipitation (i.e., pentane or heptane). There is general agreement as to asphaltene molecular weight, 3−14 number of fused rings per PAHs in asphaltene monomers, asphaltene aggregation number, critical nanoaggregation concentration (CNAC) 15−17 (also addressed in part 3 of this series), and asphaltene cluster size. However, despite many reports to the contrary, 14,18−28 the specific structural elements (i.e., number and type of PAH building blocks) of asphaltenes and high boiling species (vacuum bottoms) remain unknown. 29 Literature reports that strongly support a predominately island structural model for asphaltenes (and vacuum bottoms) rely heavily on a fundamental property of asphaltenes, self- association. In parts 3 and 4 of this five-part series, we postulate that aggregation restricts the remaining continuum of asphlatenes to those that contain island-type structures, because those island-type species ionize as nanoaggregates whose masses exceed the upper mass limit of most mass analyzers. However, time-of-flight mass spectrometers can readily detect asphaltene aggregates (4−25 kDa) and confirm that, at concentrations above the CNAC, a portion of the asphaltenes in a whole crude oil is directly observable as nanoaggregates. The aggregation explains their absence in mass spectra below Received: October 26, 2012 Revised: December 22, 2012 Published: December 28, 2012 Article pubs.acs.org/EF © 2012 American Chemical Society 1268 dx.doi.org/10.1021/ef301737f | Energy Fuels 2013, 27, 1268−1276