The Compositional and Structural Continuum of Petroleum from Light

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Chapter 6

The Compositional and Structural Continuum of Petroleum from Light Distillates to Asphaltenes: The Boduszynski Continuum Theory As Revealedby FT-ICR Mass Spectrometry Martha L. Chacón-Patiño,1 Steven M. Rowland,1,2 and Ryan P. Rodgers*,1,2,3 1National

High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States 2Florida State University Future Fuels Institute, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States 3Department of Chemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306, United States *E-mail: [email protected].

Forty years ago, Boduszynski published the first part in a series of manuscripts that addressed the composition, and compositional progression of heavy oil. It concluded that crude oil composition increases gradually and continuously in aromaticity, molecular weight, and heteroatom content as a function of boiling point. Remarkably, the Continuum Model was inferred from field ionization mass spectral data that lacked the requisite resolution to uniquely identify elemental compositions across the observed mass range. However, combined with boiling point trends of light distillate classes, molecular weight distributions, boiling point cuts, and chromatography, Boduszynski assembled a series of manuscripts that described the Continuum Model of Petroleum. Herein, we revisit the same topic and employ the same analytical methods, only now armed with state-of-the-art Fourier Transform Ion Cyclotron Resonance Mass Spectrometers (FT-ICR MS) that can readily resolve and uniquely identify

© 2018 American Chemical Society Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

molecular formulas to tens-of-thousands of individual petroleum species in a single analysis. We tested the Continuum Model for hundreds-of-thousands of species identified by mass spectrometry, from light distillates to asphaltenes, and found no deviations. In the process, we collected data that supports the low molecular weight of petroleum (< 2000 Da), defines the maltene continuum, highlights the effect of aggregation on mass spectral analysis, identifies and overcomes selective ionization, and confirms that asphaltenes are composed of abundant island and archipelago structures. In this chapter, we review FT-ICR MS petroleum characterization efforts that led to the conclusion that the Boduszynski Continuum Model is correct and thus applicable to both the compositional and structural continuum of petroleum.

1. Introduction The effective conversion of petroleum distillates into valuable products through refinery processes requires comprehensive knowledge of structure and composition (1). The task is not simple; it requires combined efforts from several disciplines: chemical separations, analytical instrumentation, petroleum/organic chemistry, and “big-data” analysis (2–4). The difficulties largely arise from the increased molecular complexity of the high-boiling fractions. As the boiling point increases, the molecular weight distribution broadens and the heteroatom content increases (5). Thus, the compositional complexity of high-boiling distillates, “non-distillables”, and asphaltenes is immense, and thus requires the use of ultrahigh-resolution Fourier Transform Ion Cyclotron Resonance mass spectrometry (FT-ICR MS) to obtain accurate molecular level information not provided by “bulk” techniques such as elemental analysis or nuclear magnetic resonance spectroscopy (6). FT-ICR MS, discovered 45 years ago by Marshall and Comisarow (7, 8), has enabled a detailed molecular-level understanding of the composition of petroleum (9). In the early 2010s, FT-ICR MS demonstrated that carbon number, aromaticity, heteroatom content and complexity, gradually and continuously increase as a function of boiling point, known as the Boduszynski continuum (10). Remarkably, Mieczyslaw M. Boduszynski proposed this compositional continuum principle nearly 30 years prior. He was able to overcome the limitations of analytical instrumentation at that time and proposed a unified model to describe the molecular composition of petroleum distillate fractions (5). Boduszynski based his model on atmospheric equivalent boiling point (AEBP), bulk elemental analysis, and molecular weight measurements by field desorption / ionization low-resolution mass spectrometry (5, 11). He proposed rules to describe the dependence of boiling point on molar mass and elemental composition of petroleum species. Summarized, the Boduszynski model dictates that diverse compounds with similar molecular weight cover a broad range of boiling point; conversely, a narrow distillation cut contains a 114 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

broad molar mass range (5, 11). Figure 1 illustrates the principle, as proposed by Boduszynski (5). For a given homologous series, for example, paraffins (CnH2n+2) shown on the far left, the boiling point increases continuously as a function of molecular weight. The same is true for all homologous series, however, compounds that contain naphthenic rings, fused aromatic moieties, and/or heteroatom-containing functionalities, are offset to higher boiling point. Thus, for a defined boiling point range (x-axis), the compounds with the highest molecular weight are paraffins, followed by naphthenic species, aromatic hydrocarbons, polar heteroatom-containing compounds, and finally polar/polyaromatic species. Armed with homologous series plotted in such a manner, combined with molecular weight distributions obtained by FD-MS, Boduszynski realized that for a defined boiling point, the vertical progression from the saturates downward, one compound family to the next (Figure 1), resulted in a decrease in ~2-3 carbon atoms per molecule. Hence, S- and N-containing aromatics should exhibit ~2-3 fewer carbon atoms than their hydrocarbon counterparts. Boduszynski predicted that for a given boiling point, a polycyclic aromatic compound, substituted with multiple heteroatomic functionalities, would have ~6-7 fewer carbon atoms than the analogous hydrocarbon. This premise captured two remarkable compositional features of petroleum. First, as the boiling increases, the mass range will broaden. Second, on the basis of bulk H:C ratio, MS data, and heteroatom concentration, the molecular weight of asphaltenes (operationally defined as the petroleum fraction insoluble in n-pentane/n-heptane but soluble in toluene/benzene) should not exceed ~2000 g/mol (5, 12–14). The Boduszynski model reveals compositional trends that demonstrate that the terms “heavy” and “high boiling” are incorrectly used as synonyms for “high molecular weight.” Boduszynski demonstrated that the high boiling point of distillation residues is a consequence of increased intermolecular interactions, derived from a higher concentration of polyaromatic moieties and heteroatom-containing functionalities capable of acid/base interactions and hydrogen bonding. Thus, Boduszynski challenged the idea that high boiling equaled high molecular weight. He went on to address the “non-distillable” nature of distillation residues/asphaltenes and used chemical separations to extend the characterization of petroleum. In 1987, Boduszynski found that once a distillation residue is separated, specific solubility fractions are, in fact, distillable. Boduszynski and coworkers hypothesized that all compounds in crude oil (those not in an aggregated state) are soluble in heptane and only aggregated species exhibit insolubility (5, 15–17).

2. FT-ICR Mass Spectrometry Enables a Definitive Test of the Boduszynski Continuum Model Petroleomics emerged in the early 2000s as a Proteomics counterpart, with the goal to predict the behavior and economic value of petroleum from detailed molecular-level information (18–21). The term was jokingly suggested by Dr. Carol Nilsson, in a hallway conversation at the National High Magnetic Field Laboratory, but the term stuck and has been used regularly ever since. 115 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Petroleomics was not a new idea at the time the term was coined; it is rooted in the work of Quann and Jaffe (17), and was renewed with the recent developments in high field FT-ICR MS and soft ionization techniques. The ultra-high resolving power and mass accuracy, only accessible by FT-ICR MS, allow forif she the baseline resolution and the molecular formula assignment of tens of thousands of compounds within a single petroleum sample. Such performance is crucial, as complex fractions derived from heavy oils, such as asphaltenes, can exhibit more than 500 peaks in less than one nominal mass. Ultrahigh resolving power (m/Δm50% > 750,000 at m/z 500) is critical for the resolution and the accurate identification of isobaric peaks, which differ in composition by, for example, 12C4 versus SH313C and 12C3 versus SH4, which differ in mass by 1.1 and 3.4 mDa (19). In routine FT-ICR MS analysis of heavy petroleum derived fractions, it is possible to resolve and confidently assign elemental compositions to over 25,000 mass spectral peaks in a single sample, by one ionization method (22). By taking advantage of the mass accuracy and the spacing patterns between spectral peaks, CH2 (14.01565 Da) and H2 (2.01565 Da) units, it is possible to deconstruct the compositional complexity into hundreds of overlapping homologous series (23).

Figure 1. Effect of molecular structure on boiling point. Reproduced with permission from reference (5). Copyright 1987 American Chemical Society. (see color insert) 116 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 2. Top panel: Positive APPI 9.4 T FT-ICR mass spectrum of the interlaboratory sample Petrophase 2017 Asphaltenes, with characteristic spacing patterns of 14.01565 Da and 1.01565 Da. Bottom panel, left: High resolving power, mass accuracy, and spacing trends allow for the elemental assignment of thousands of peaks. Bottom panel, right: Representation of molecular level information by a color-coded isoabundance contoured plot of DBE versus carbon number; the relative abundance is normalized within the compound class N1, which refers to all the species that contain C, H, and only one N atom. (see color insert)

Figure 2 presents a positive-ion Atmospheric Pressure Photo-ionization (APPI) FT-ICR mass spectrum of an interlaboratory asphaltene sample (Petrophase 2017 Asphaltenes), (Top panel) (24). A mass spectral zoom inset exposes two predominant spacing patterns, given by additions of CH2 and H2 (23, 25). Specialized software algorithms take advantage of the mass accuracy provided by FT-ICR MS and the spacing patterns to generate peak lists with the assigned elemental compositions (Bottom panel, left) (26). The vast amount of molecular level information, derived from a single mass spectrum, is represented in color-coded isoabundance contoured plots of double-bond equivalents (DBE, number of rings + double bonds in a molecule) versus carbon number (Bottom 117 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

panel, right) for a specific class of molecules. DBE is plotted on the y-axis and is a measure of the aromaticity of the compounds (DBE = C – H/2 + N/2 + 1), where C, H, and N are the numbers of carbon, hydrogen, and nitrogen in the molecular formula. Thus, a higher DBE denotes a higher aromaticity. Carbon number is plotted on the x-axis; and at constant DBE, an increasing carbon number denotes a greater CH2 content, hence increasing the extent of alkyl substitution. The color scale represents the relative abundance, which is usually normalized within each compound class (27, 28). Several publications highlight the advantage of DBE versus carbon number plots to compare and sort samples, track the effect of refinery process on petroleum-derived materials, and evaluate the performance of chromatographic separations applied to petroleum (29–33).

3. Compositional and Structural Continuum of Petroleum Revealed by FT-ICR MS 3.1. A Molecular-Level Test of the Boduszynski Model Through the FT-ICR MS Analysis of Athabasca Bitumen HVGO Distillates A conclusive demonstration of the Boduszynski continuum model requires ultrahigh resolution mass spectrometry to determine the elemental composition of each ionizable compound from petroleum distillation fractions (10). Impressively, based on low-resolution mass spectrometry data, Boduszynski proposed a general model to describe the composition of petroleum, with the general premise that different compounds with similar molar masses occupy a wide boiling point range. Conversely, a narrow boiling cut can exhibit a wide molar mass range (5). Figure 3 illustrates the progression of the compositional space of the hydrocarbon class from Athabasca Bitumen heavy vacuum gas oil (HVGO) distillates. From the initial boiling point (IBP)-343 °C fraction to the boiling cut 500-538 °C, there is a sequential increase in the abundance weighted average carbon number from 20 to 40 carbon atoms. A close examination of the carbon number distribution reveals an important compositional feature, more evident for the distillation cuts 375400 °C, 400-425 °C, 425-450 °C, and 450-475 °C: homologous series with high DBE are slightly shifted toward lower carbon number. This behavior illustrates a fundamental principle of the Boduszynski model: within a distillation cut, high DBE species must exhibit lower carbon number and vice versa (5, 10).

3.1.1. High Boiling Point Due To Increased Heteroatom Content Figure 4 shows the contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for the Athabasca Bitumen distillation cuts 343-375 ºC, 375-400 ºC, 400-425 ºC, and 425-450 ºC. It illustrates the effect of increased heteroatom content on the progression of carbon number within a distillation cut. Each sulfur addition, HC class to S1, and S1 to S2, leads to a decrease of ~2-3 carbon atoms within a given boiling cut. It is important to consider that compounds that exhibit a higher aromaticity or higher content of heteroatoms such as S, N, and O exhibit stronger attractive intermolecular forces and thereby concentrate in 118 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

high boiling cuts, but the increased heteroatom content lowers their carbon number relative to the HC class (5, 34). For example, the class HC of the distillation cut 425-450 °C, exhibits an abundance-weighted average carbon number of 30. The addition of one sulfur atom decreases the average carbon number to 28; finally, the addition of a second sulfur atom decreases the average carbon number to 26 (10).

Figure 3. Color-coded isoabundance contoured plots of DBE versus carbon number for the hydrocarbon (HC) class for Athabasca bitumen HVGO distillate cuts (IBP-343 °C to 500-538 °C). Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

The Boduszynski continuum model predicts that for a given carbon number range, the addition of heteroatomic functionalities should increase the boiling point (5, 11, 13). Figure 5 demonstrates this premise. For a fixed range of carbon number (~20-35), each additional sulfur atom increases the boiling point by approximately 25 °C. Thus, the classes S1 and S2 boil between 400-425 ºC and 425-450 ºC. Importantly, both S2 and S1O1 classes are di-heteroatomic; however, the increased dipole moment due to oxygen (increased polarity) leads to a higher boiling point for the O1S1 species, which also exhibit much lower DBE values than the HC and S1-2 counterparts. Boduszynski predicted that a higher concentration of heteroatoms, such as N and O, promotes stronger intermolecular associations, such as hydrogen bonding and acid/base interactions, which result in an increased boiling point (5, 10). 119 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 4. Color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for Athabasca bitumen HVGO distillate cuts (343-375 °C, 375-400 °C, 400-425 °C, and 425-450 °C). Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

3.1.2. High Boiling Point Due To Heteroatom-Containing Functionalities Electrospray ionization (ESI) performed in negative-ion mode enables access to polar/acidic species, capable of hydrogen bonding (35–37). Figure 6 presents a comparison of the compositional space of the HC and S1 classes accessed by positive APPI, and the class O2 by negative electrospray ionization, for the distillation cuts 425-450 °C and 475-500 °C. Boduszynski and Altgelt proposed that for a given distillation cut, the presence of acidic functionalities must decrease aromaticity and carbon number (13, 38, 39). Thus, within a given boiling range, O2-containing species contain fewer carbon atoms and display much lower DBE values (aromaticity) than non-oxygenated species. For instance, the distillation cut 475-500 °C contains O2 species that exhibit a marked reduction in aromaticity, from DBE = 9 to DBE = 5, compared with the hydrocarbon class. A decrease in 3-4 units of DBE suggests that O2 species could have ~1 aromatic ring and/or ~1-4 naphthenic rings less than their hydrocarbon counterparts, which decreases the strength of London dispersion forces and potential π-π interactions (5, 10). However, the loss in DBE is offset by the ability to hydrogen bond. 120 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 5. Color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC, S1, S2, O1S1 with a fixed range of carbon atoms (~20-35) for Athabasca bitumen HVGO distillate cuts (343-375 °C, 375-400 °C, 400-425 °C, and 425-450 °C). The increased heteroatom content (S1 and S2) and finally the dipole moment induced by oxygen (O1S1) progressively increase the boiling point range at a constant carbon number range. Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

Figure 6. Color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC (positive APPI), S1 (positive APPI) and O2 (negative ESI) for the distillation cuts 425-450 °C and 475-500 °C. Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert) 121 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

3.1.3. The Continuous Progression of DBE Values as a Function of Increasing Boiling Point Demand Naphthenic Structures The characterization of “non-distillable” heteroatom-containing petroleum compounds, necessitates a comprehensive understanding of the molecular structure of species from low-boiling cuts. Boduszynski and Altgelt hypothesized that the continuum model, rooted in the “distillable” petroleum fractions, could be linearly extended to explain the molecular composition of distillation residues and asphaltenes (13, 38, 39). Figure 7 illustrates an application of the Boduszynski continuum model to understand petroleum structure. Figure 7 (top row) highlights the progression of DBE and carbon number for the class S1 as the boiling point increases for Athabasca Bitumen distillation cuts (10). For particular distillation cuts, homologous series with DBE values of 3, 6, 9, and 12 exhibit a prominent relative abundance when compared with intermediate values such as 2, 4, and 5. It is well known that S1 species with DBE of 3 are most likely alkylated thiophene derivatives. The addition of one and two benzene rings yields benzothiophene (DBE = 6) and dibenzothiophene (DBE = 9) homologous series, which are well recognized as abundant (stable) compounds in distillation cuts from sulfur-enriched crude oils (40–42). Further addition of benzene rings, following a catacondensed growing pattern (structures a and b, Figure 7 bottom panel), produce core structures with DBE 12 and 15 (43–45). The bottom DBE versus carbon number plot in Figure 7 is the combination of the top panel plots; it contains the molecular composition of the S1 species for all the HVGO cuts derived from Athabasca bitumen. Interestingly, the combined plot does not exhibit a prominent abundance of “magic” DBE values (DBE = 3, 6, 9, 12, 15, 17, …). Given that alkenes are not native structures in virgin crude oils, the intermediate DBE values of 4-5, 7-8, and 10-11, must be derived from cycloalkane ring addition to the aromatic core structures (46–48). Figure 7 is evidence of the tremendous structural diversity of petroleum and emphasizes that the degree and abundance of cycloalkane substitutions cannot be ignored in the structural continuum. The fact that high-boiling distillation cuts and vacuum residues yield considerable amounts of distillable 1-4 ring alkyl- and cycloalkylaromatics after thermal cracking processes, suggests that island-type structures are not always the dominant or sole structural motif in petroleum. A more indepth discussion about the debate of island- versus archipelago-type structures is found later in this chapter. The structural diversity of petroleum, highlighted in Figure 7, suggests the potential existence of species with multiple aromatic cores bridged by cycloalkane or heteroatom-containing 5-membered rings. If this hypothesis is correct, thermal cracking and/or mild pyrolysis would break these alkyl / cycloalkyl bridges to produce distillable products extensively reported for high-boiling distillation cuts, petroleum asphaltenes, and vacuum residues (10, 49–52). Thus, if the Boduszynski Model is correct, the structural diversity of these distillates provides insight into abundant structural motifs present in higher boiling distillates and residua.

122 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 7. Upper panel: Progression of the compositional space of the class S1 with an increase of boiling point for Athabasca Bitumen HVGO distillation cuts (343-375 °C, 375-400 °C, 400-425 °C, and 425-450 °C). Lower panel: Combined DBE versus carbon number plots for the class S1 for all Athabasca Bitumen distillation cuts. The lack of the distinctive DBE “magic” numbers, combined with the absence of alkenes in virgin crudes, requires the presence of naphthenic rings. Reproduced with permission from reference (10). Copyright 2010 American Chemical Society. (see color insert)

3.2. Extending the Boduszynski Continuum Model to the Limit of Distillation: FT-ICR MS of Heavy Vacuum Gas Oils from a Middle Eastern Heavy Crude Oil 3.2.1. Progression of Molecular Weight, Heteroatom Content, Carbon Number, and Aromaticity from 371 °C to the Limit of Distillation Up to this point, there is a clear correlation between the boiling range of light distillates and the continuous progression of carbon number, aromaticity, and the N, S, and O content of heteroatom-containing functionalities. The analysis highlighted the importance of cycloalkane substitution, through the disappearance 123 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

of “magic” DBE numbers as boiling point increased. In this section, there is a focus on distillation fractions that boil above 371 °C in an effort to demonstrate extension of the continuum model to the limit of distillation. Figure 8 (left) presents the molecular weight distributions (MWD) of the distillation cuts 371-510 °C, 510-538 °C, 538-593 °C and 593+ °C from a Middle Eastern Heavy crude oil. Figure 8 demonstrates that the center of the molecular weight distribution shifts towards higher m/z values as the boiling point increases. Although very similar to the results Boduszynski initially used to develop his model, the analytical capabilities afforded by FT-ICR MS allow assignment of the elemental compositions to nearly all (> 95%) mass spectral peaks detected. Combined with advances in soft ionization sources such as ESI, APCI and APPI, and continued progress of ultrahigh-resolution mass spectrometry, modern analytical methods allow for a definitive test of the Boduszynski Model to the limit of distillation and into the “non-distillables” (2, 5, 53, 54). Figure 8 also shows the heteroatom class distribution for all ionized/detected species with a relative abundance above 1%. In general, there is a decrease in the abundance of low-heteroatom containing compounds with an increased boiling point. Concurrently, the relative abundance of poly-heteroatomic classes such as N1S1, S2, and S3 increases as a function of boiling point. As postulated in the Boduszynski model, heteroatoms such as sulfur, nitrogen, and oxygen increase the strength of intermolecular interactions; thus, high boiling distillation cuts and residues are enriched with polyheteroatom-containing species (5, 13, 39, 55). Figure 9 presents the contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for the distillation series and the residue from the Middle Eastern heavy crude oil. For the hydrocarbon class, the progressive increase in boiling point yields a gradual increase in the abundance weighted average of carbon number and DBE. The compositional progression starts with the distillation cut 371-510 °C, with an average carbon number of 36 and DBE = 10. The following two fractions (510-538 °C and 538-593 °C) exhibit an increase of 6 and 16 carbon atoms, with only a slight increase in 2 DBE values. Finally, the “non-distillable” residue (593+ °C) is notably “heavier” with an average carbon number of 63, and more aromatic, with an average DBE of 16, as it is not a true boiling “cut”. The average increase of 6 DBE units suggests the addition of 2-fused aromatic rings to the core structures. Greater numbers of fused-aromatic rings lead to stronger intermolecular interactions, such as π-π stacking, which result in a higher boiling point (56, 57). The Boduszynski continuum model also predicts that distillation residues (and asphaltenes) have high boiling points due to a greater concentration of heteroatom-containing species that promote stronger interactions and nanoaggregation, and not necessarily because of higher molecular weight (5, 38, 39). Figure 9 also illustrates the impact of heteroatom content in the carbon number of high boiling point distillates. The effect of the addition of one and two sulfur atoms is more pronounced for the residue 593+ °C. The addition of two S-containing functionalities decreases the abundance weighted average carbon number from 63 to 56, as hypothesized by Boduszynski (5, 55). Thus, the Boduszynski Continuum Model accurately describes the molecular progression of high boiling and non-distillable species. 124 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 8. Molecular weight distributions (MWD) derived from positive APPI FT-ICR mass spectrometry (Left), and heteroatom class distribution (Right) of the distillation cuts 371-510 °C, 510-538 °C, 538-593 °C and 593+ °C from a Middle Eastern Heavy crude oil. Reproduced with permission from reference (55). Copyright 2010 American Chemical Society. (see color insert)

3.2.2. Implications of the Boduszynski Continuum Model for the Molecular Composition of Asphaltenes The Boduszynski continuum model predicts that the molecular weight of most petroleum compounds is less than 2000 g/mol. The reason for the “non-distillable” nature of asphaltenes is rooted in the increased concentration of aromatics and oxygen/nitrogen-containing functionalities (and perhaps sulfur as sulfides), and not in ever-increasing molecular weight. However, to be consistent with the continuum model, the increase of carbon number and aromaticity for “non-distillable” species should follow a linear extrapolation of the molecular compositions (determined by FT-ICR MS), established by the distillable fractions. The conventional wisdom at the time was to simply extrapolate to greater carbon number, but that is inconsistent with the continuum model proposed by Boduszynski. The discussion below aims to highlight the inconsistency in the extrapolation of DBE and carbon number from distillation data and the observed, molecular-level data from atmospheric / vacuum residues and asphaltenes. 125 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 10 illustrates the extrapolation of distillable compositional space (orange line) to an improbably high molecular weight, 1 MDa. The hydrogen to carbon ratio obtained by such extrapolation is ~1.4. Thus, a linear extrapolation of the distillable (maltene) continuum to greater carbon number neither accounts for the widely accepted H:C ratio of asphaltene samples (H:C ~0.90-1.15) nor their recently recognized low molecular weight (55, 58–63). If asphaltenes share the same carbon number range as their maltenic counterparts, their molecular compositional space must be displaced to greater DBE (aromaticity). So, do asphaltenes share the same carbon number range as maltenes?

Figure 9. APPI derived color-coded isoabundance contoured plots of DBE versus carbon number for the classes HC, S1, and S2 for “heavy” distillation cuts 371-510 °C, 510-538 °C, 538-593 °C, and 593+ °C, form a Middle Eastern heavy crude oil. Reproduced with permission from reference (55). Copyright 2010 American Chemical Society. (see color insert) 126 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 10. Combined DBE versus carbon number plot for the class S1 for the distillation cuts 371-510 °C, 510-538 °C, 538-593 °C, and 593+ °C, from a Middle Eastern heavy crude oil. The linear extrapolation of the continuum trend (orange dotted line) to higher carbon numbers cannot account for bulk asphaltene composition and confirmed molecular weight (55). However, extrapolation to higher DBE (aromaticity), within the same carbon number range, yields acceptable H:C ratios (red dotted line). Reproduced with permission from reference (55). Copyright 2010 American Chemical Society. (see color insert)

It is important to keep in mind that the history of asphaltene characterization is highlighted by controversy and ambiguous debates on molecular weight and structure (14, 64–66). For more than 50 years of asphaltene research, the Petroleum Community hypothesized that a high molecular weight should account for the “non-distillable” nature, strong aggregation and deposition, and increased content of N, O, S, and V/Ni in asphaltenes (14, 67–71). Boduszynski disagreed, and suggested that they share the same carbon number range as maltenes. Since 2000, results from time-resolved fluorescence depolarization (60, 72), atmospheric pressure mass spectrometry (73–76), and two-step laser mass spectrometry (77, 78), converged on molecular weights between ~250-1200 127 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

g/mol with an average around at 750 g/mol. Thus, the “low” molecular weight of petroleum asphaltenes, hypothesized by Boduszynski 30 years ago, is correct. Boduszynski provided experimental support of the initial premise of Dean and Whitehead, who speculated that 2000 g/mol was the upper molecular weight limit for most petroleum compounds (79, 80). As previously discussed, Figure 10 shows the rationale for predicting the compositional space of asphaltenes based on a linear projection of the molecular composition of the distillable species. The orange line highlights the abundance weighted average of H:C ratio of the S1 species in all distillation cuts and the 593+ ºC residue from Middle Eastern Heavy crude oil. The extrapolation of the orange line to molecular weights greater than 1 MDa results in a decrease in H:C as a function of increasing carbon number. However, at 1 MDa, the projected H:C ratio is ~1.39, which is considerably greater than the measured bulk H:C ratio for asphaltenes (0.90 800,000 at m/z = 500) and mass accuracy less than 1 ppm. Moreover, it must be performed quickly, on a chromatographic time scale. Such capabilities are only currently possible at the highest magnetic fields available for FT-ICR MS, those equipped with 21 T superconducting magnets (162). Successful demonstration of online HPLC/MS of petroleum samples was recently presented and exposes a wealth of structurally defined, molecular-level information in continued support of the Boduszynski Model (163–165). The mass spectral total ion chromatogram (TIC) was shown to match the evaporative light scattering detector response, but is the sum of ~1400 high resolution mass spectra acquired over the 80 minute LC separation. Abundant species are detected for 158 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

all the structures targeted in the analysis: saturates, 1-ring, 2-ring, 3-ring, 4-ring, 5+ring/polars, and finally, sulfides. Summation of the mass spectra across each structurally defined elution windows recapitulates the results obtained by off-line fraction collection followed by MS analysis and exposes the Boduszynski defined trends highlighted earlier in this chapter. Specifically, the ~3 carbon shift to lower carbon number between the HC and S1 class (within the same ring fraction), and within the HC and S1 classes, there is a ~30 carbon shift (to lower carbon number) as one progresses from the saturates (HC), sulfides (S1 class) through the increasing ring number fractions, to the 5+ring fraction. Such analysis captures the Boduszynski defined relationships between tens-of-thousands of petroleum species, across multiple heteroatom classes, in a structurally defined manner, in a single analysis. Most exciting, the compositional information provided by the method exposes the Boduszynski trends within a ring class. The 1-ring aromatic elution period begins with low DBE species and ends with species 5 DBE higher (the addition of 5 naphthenic rings), but as Boduszynski predicted, the higher DBE species contain 10 fewer carbons than those at lower DBE. Thus, all of the previously presented molecular-level results are now attainable within a ring class, as well as between ring classes, and the entire experiment takes less than 2 hours.

Acknowledgments The work presented herein is attributed to the petroleum scientific community, and specifically to all of the authors and coauthors of the referenced manuscripts. It was made possible by the gracious and continued support of Dr.’s Parviz Rahimi and Andrew T. Yen who provided invaluable samples and discussions over the years. Work supported by NSF Division of Materials Research (DMR- 1157490), the Florida State University, the Florida State University Future Fuels Institute, and the State of Florida. We would like to give a special thanks to Marianny Y. Combariza for providing the South American crude oil and Pierre Giusti and the organizers of Petrophase 2017 for providing the Petrophase 2017 asphaltene.

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