Heavy Petroleum Composition. 4. Asphaltene Compositional Space

Jan 17, 2013 - Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306, United States. §...
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Heavy Petroleum Composition. 4. Asphaltene Compositional Space Amy M. McKenna,†,‡ Alan G. Marshall,*,†,‡ and Ryan P. Rodgers*,†,‡,§ †

Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, 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, United States § Future Fuels Institute, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States ABSTRACT: 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.

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INTRODUCTION

methods vary with precipitation conditions (e.g., precipitating solvent and geographic origin).13−19 For example, “pentaneasphaltenes” (C5-asphaltenes) exhibit different chemical and physical properties than “hexane-asphaltenes” or “heptaneasphaltenes”.2,20 Despite general agreement as to bulk elemental composition and other bulk properties of asphaltenic materials,5−7,21,22 controversy still exists over asphaltene molecular properties, such as structure and molecular weight,23−25 critical to predict asphaltene phase behavior and aggregation.25−27 Asphaltene molecular architecture remains controversial, with two dominant structural models in the literature. Dickie and Yen3,4 proposed that the primary structural motif in asphaltenes consists of fused aromatic and naphthenic ring core structures with alkyl side chains. On the basis of time-resolved fluorescence depolarization (TRFD) measurements, Mullins concluded “the predominant but not only asphaltene molecular architecture consists of a single, somewhat large PAH with cycloalkane, branched- and straight-chain substituents, often

Asphaltene deposition because of pressure or reservoir fluid compositional changes poses a significant challenge to the oil industry based on impedance of pipeline flow, oil transport, and refinery processing limitations attributed to asphaltene solution phase behavior.1−7 Deposition prevention and removal require significant capital investment to the global oil industry and detailed molecular knowledge of the asphaltene fraction.8 Asphaltenes precipitate from petroleum upon pressure, temperature, and fluid compositional changes (thermodynamic processes) and can cause deposition determined by flow shear rate, surface characteristics, and particle size and interactions.8−11 The operational definition that distinguishes asphaltenes and maltenes inherently leads to controversy, because fractions are based on solubility and, therefore, encompass a wide variety of molecular structures and functional groups. Asphaltenes and maltenes are operationally defined by their respective insolubility and solubility in straight-chain alkane solvents (e.g., n-pentane, n-hexane, and n-heptane).2,4,12 Standard asphaltene isolation methods (e.g., IP 143/90 and ASTM D4055) for asphaltene precipitation depend upon precipitation conditions, and asphaltene species isolated by different © 2013 American Chemical Society

Received: October 28, 2012 Revised: January 16, 2013 Published: January 17, 2013 1257

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the first direct measurement of aggregate molecular weight51 and reported stable aggregate formation at concentrations required for most mass spectral techniques, thereby limiting the observed asphaltenes to only those not locked in nanoaggregates, in further support of the Boduszynski model. The most compositionally diverse fraction of heavy oil, namely, the residue that remains after the highest distillation temperature, challenges conventional analytical techniques because of its low volatility, high polarity, and extreme compositional diversity. Detailed molecular characterization requires ultrahigh resolution (m/Δm50% > 800 000 at m/z 400, in which Δm50% is the full mass spectral peak width at halfmaximum peak height) achievable only by FT-ICR MS.47 A total of 50−80% of their molecular elemental compositions are thought to contain at least one heteroatom, and roughly half of the elemental compositions contain two or more heteroatoms, with a carbon number ranging from C35 to C100.7,21,52 Probe FI analysis has shown that 95+% by weight can be volatilized in vacuum during MS analysis.42 Molecular-level characterization by FT-ICR MS has been applied to characterize asphaltenes in situ,53 during deep hydroconversion,54 by pressure or solvent precipitation methods,55 after saturates, aromatics, resins, and asphaltenes (SARA) fractionation,56,57 and vanadyl porphryin enrichment reported for bitumen asphaltenes.58 The Boduszynski model imposes limits on molecular weight, composition, and structure of distillable molecules (maltenes) and predicts that the same rules apply to nondistillables (asphaltenes).59 If validated, that compositional “continuum” model could apply to all crude oils and extend the compositional patterns established for the lightest distillate fractions to the (much less readily characterized) high boiling and nondistillable residues. We present atmospheric pressure photoionization (APPI) mass spectral analysis of asphaltenes isolated from the vacuum bottom residue from Middle Eastern heavy crude oil. Molecular differences between maltenes and asphaltenes revealed by FTICR MS serve to link solution-phase behavior with observed compositional differences. Low-resolution (linear quadrupole trap) MS confirms that asphaltene molecular weight (400 < m/ z < 2500) is not abnormally high and agrees with previously published results. Here, ultrahigh-resolution FT-ICR MS enables individual component identification and demonstrates that monomeric asphaltenes occupy similar molecular weight distributions within the same heteroatom class (defined by carbon number) but are more aromatic than maltene counterparts. Thus, the compositional space of (monomeric) asphaltenes is now defined. The fundamental difference in maltene and asphaltene fractions is aromaticity and not carbon number, and the compositional space occupied by maltenes and asphaltenes must be anchored to bulk elemental analysis. Here, we provide the first direct molecular characterization of the subset of asphaltenes that is not locked in nanoaggregate structures and define the compositional space of monomeric asphaltenes and maltenes.

with heteroatom content; asphaltene nitrogen is entirely contained within the PAH in pyrrolic structures and to a lesser extent pyridinic structures”.6,7,21 Here, asphaltenes are described as dispersed or suspended (or both) in crude oils and solvents as molecules, nanoaggregates, and clusters of nanoaggregates,28 with an average molar mass of ∼750 g/mol, diameter of ∼1.5 nm, and nanoaggregates containing ∼6 monomers that can further self-associate to form clusters that contain ∼8 nanoaggregates.7 However, controversy exists over the suitability of TRFD to detect the presence of polymethylene bridges between aromatic and non-aromatic pendant structures, which suggest the presence of bridged structures in the majority of asphaltenes.10,11,29,30 Asphaltenes have been referred to as the soluble fraction of kerogen in crude oil,31 with molecular structures comparable to their geochemical precursor, the kerogens.32−36 Pyrolysates produced from the aromatic fraction of asphaltenes yield a series of multiple polycyclic aromatic hydrocarbon (PAH) compounds that contain 1−5 (or more) rings, because pyrolysis conditions cannot account for carbon−carbon bond cleavage to account for small PAH compounds derived from larger condensed aromatic structures.30,37,38 Asphaltene characterization at the molecular level39−41 is critical to address the asphaltene structural debate. A host of analytical measurements converge on asphaltene monomer molecular weights less than 2 kDa.1,6,7,13,28,42−45 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is currently the only analytical technique capable of direct speciation (at the level of molecular formula assignment) of individual components in complex petroleum samples, especially polar and heavy oil fractions, such as asphaltenes. Although elemental composition does not by itself yield structural information, it does provide visualization of carbon number and aromaticity patterns within compositional heteroatom “classes” (i.e., CcHhNnOoSs molecules with the same n, o, and s) to reveal compositional differences between solubility-defined asphaltenes and maltenes from the same parent oil. Those compositional differences, in turn, determine the chemical and physical solution phase behavior of petroleum fractions (“petroleomics”).46,47 The first attempt to characterize heavy oil and its fractions at the molecular level culminated in a series of papers and later a book by Boduszynski et al., who proposed a compositional continuum for heavy petroleum fractions, grounded in the premise that “high boiling does not necessarily imply high molecular weight”.1,42−45,48 A plot of carbon number versus atmospheric equivalent boiling point of petroleum compounds reveals that, for a defined carbon number, petroleum compounds span a wide boiling point range.1 Conversely, for a defined boiling point, petroleum compounds exhibit a wide carbon number distribution. 1,42 In parts 1 (10.1021/ ef100149n) and 2 (10.1021/ef1001502) of this series, we provided molecular evidence in support of the Boduszynski continuum model for heavy vacuum gas oil (HVGO) boiling range (part 1) and to the limit of conventional vacuum distillation in heavy crude oil (part 2).49,50 We reported that petroleum composition is indeed continuous in carbon number, aromaticity, heteroatom progression, and boiling point based on ultrahigh-resolution FT-ICR MS characterization of heavy petroleum distillates and that compositional differences between boiling fractions (cumulatively more than 100 000 elemental compositions) obey the simple rules proposed by Boduszynski et al.1,42−45 Part 3 (10.1021/ef3018578) provided

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EXPERIMENTAL SECTION

Sample Preparation. All solvents were high-performance liquid chromatography (HPLC)-grade and purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO). Middle Eastern heavy crude oil49,50 vacuum residue (593+ °C) was supplied by General Electric Global Research (Niskayuna, NY) and fractionated according to IP 143/90. The crude has an American Petroleum Institute (API) gravity of 28° (at 60 °F), contains 3 wt % sulfur, a total acid number (TAN) of 0.2, 1258

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abundance distributions and graphical DBE or H/C ratio versus carbon number images.

and vanadium and nickel contents of 43 and 13 ppm. The distillation procedure and distillation properties have been described elsewhere.49 Briefly, 500 mL of n-heptane was added to the heavy oil sample (10 g), refluxed for 1 h in a 1 L round-bottom flask, and stored in the dark (12 h). The solids (asphaltenes) were isolated by gravity filtration through Whatman (Kent, U.K.) 2V-grade, 8 μm filter paper. Hot n-heptane was added to the asphaltene residue to complete the transfer of solids. The filter paper with the asphaltenes was then refluxed with n-heptane at a rate of 3−5 solvent drops/min for ∼120 min until the solution was colorless to remove entrained maltenes that co-precipitated with the asphaltenes. Asphaltenes were then completely desorbed from the filter paper with toluene at the same reflux conditions. The asphaltene and maltene fractions were desolvated under dry nitrogen, weighed, and redissolved in toluene to produce stock solutions of 2 mg/mL. Both asphaltene and maltene fractions were further diluted to 250 μg/ mL in toluene prior to APPI FT-ICR MS analysis with no additional modification. Instrumentation. APPI. A custom-built adapter60 interfaced the APPI source (ThermoFisher Scientific, San Jose, CA) to the front stage of a custom-built 9.4 T FT-ICR mass spectrometer (see below). The sample flows through a fused silica capillary at a rate of 50 μL/min and is mixed with nebulization gas (N2 at 50 kPa) inside a heated vaporizer operated at 300 °C for parent crude and maltenes and 350 °C for asphaltenes.61 After nebulization, gas-phase neutral analytes exit the heated vaporizer region as a confined jet and a krypton vacuum ultraviolet gas discharge lamp (Syagen Technology, Inc., Tustin, CA) produces 10−10.2 eV photons (120 nm). Toluene serves as both solvent and dopant and increases analyte ionization.62,63 Chargeexchange and proton-transfer reactions occur between ionized toluene and neutral analytes through collisions with toluene at atmospheric pressure inside the APPI source.64,65 FT-ICR MS (9.4 T). Samples were analyzed with a custom-built FTICR mass spectrometer66 equipped with a 22 cm horizontal room temperature bore 9.4 T magnet (Oxford Instruments, Abingdon, U.K.) and a modular ICR data station (Predator).67 Positive ions generated at atmospheric pressure enter the skimmer region (∼2 Torr) through a heated metal capillary, pass through the first radio frequency (rf)only octopole, pass through a rf-only quadrupole, and are accumulated68 (250−1000 ms) in a second octopole equipped with tilted wire extraction electrodes for improved ion extraction and transmission.69 Helium gas introduced during accumulation collisionally cools ions prior to transfer through two rf-only octopoles (total length of 119.5 cm) into an open cylindrical Penning ion trap (9.4 cm inner diameter × 30 cm long). Octopole ion guides were operated at 2.0 MHz and 240 Vp−p rf amplitude. Broadband frequency chirp excitation (70−700 kHz at a sweep rate of 50 Hz/μs and amplitude of 350 Vp−p) accelerated the ions to a cyclotron orbital radius detected by differential current induced between two opposed electrodes inside the ICR cell.70 The experimental event sequence was controlled by a Predator data station.67 Multiple (100−300) time-domain acquisitions were averaged for each sample, Hanning-apodized, and zero-filled once prior to fast Fourier transform and magnitude calculation.71 Linear Ion-Trap MS. Positive-ion APPI broadband mass spectra were acquired with a linear ion-trap mass spectrometer (ThermoFisher Corp., San Jose, CA) equipped with an IonMaxx ion source (ThermoFisher Corp., San Jose, CA) under experimental conditions similar to those described above. Mass Calibration and Data Reduction. ICR frequencies were converted to ion masses based on the quadrupolar trapping potential approximation,72,73 with internal calibration based on a homologous series of alkyl-aromatic ions differing by integer multiples of 14.015 65 Da (mass of CH2). Elemental compositions were assigned on the basis of accurate mass and, in the case of sulfur-containing species, confirmed by identification of the 34S isotopologue peak at a 1.9958 Da (mass difference between 34S and 32S) higher in mass than the monoisotopic peak. For each elemental composition, CcHhNnOoSs, the heteroatom class, type [double bond equivalents (DBE) = number of rings plus double bonds involving carbon],74 and carbon number, c, were tabulated for subsequent generation of heteroatom class relative

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RESULTS AND DISCUSSION Asphaltene Molecular Weight Distribution. Molecular weight distributions derived from experimental FT-ICR mass spectra can be truncated and/or distorted during injection of ions from an external accumulation ion trap to the ICR cell by time-of-flight (TOF) mass discrimination, Coulomb repulsions, and resonant excitation. The truncation is especially problematic for heavy crude oil fractions whose components span several hundred to a thousand daltons in molecular weight and often contain more than 100 species per nominal mass.49,75 Furthermore, high signal-to-noise (desirable for accurate molecular weight distribution measurement) must be obtained through signal averaging, because ion−ion interactions resulting from confinement of more than ∼1 000 000 ions in a Penning trap can result in peak splitting, peak coalescence, and reduced accuracy in determining the cyclotron frequency from which the ion mass-to-charge ratio is calculated.70,76,77 TOF dispersion during ion injection into the ICR cell limits the maximum detected m/z range. Therefore, a lower resolution mass analyzer (e.g., TOF and linear quadrupole trap), without such dispersion, provides a reference molecular weight distribution, to guide optimization of FT-ICR MS instrumental parameters to best represent the molecular weight distribution for a complex petroleum sample. Simply, the low-resolution mass spectrometer provides a more accurate measure of the molecular weight distribution, whereas the higher resolution FT-ICR MS provides molecular identities. Figure 1 shows broadband positive-ion APPI linear quadrupole ion-trap mass spectra of the maltene (top) and asphaltene

Figure 1. Broadband positive-ion APPI linear quadrupole ion-trap mass spectra of the n-heptane maltene fraction (top) and insoluble asphaltene fraction (bottom) isolated from a Middle Eastern heavy crude oil (593+ °C) residue. The observed molecular weight distribution is maximal at m/z 650 for the maltene fraction and m/z 1200 for the asphaltene fraction. Both fractions span approximately the same total range (350−3000 Da), with most of the components below 2 kDa for each fraction.

(bottom) fractions isolated from the vacuum bottom residue (593+ °C) of a Middle Eastern heavy crude oil. Both fractions span similar molecular weight distributions (200 < m/z < 2800), centered at m/z 1200 for asphaltenes and m/z 750 for maltenes. However, despite efforts to reduce or eliminate contributions from non-covalent complexes (dimers) to the mass spectrum, their presence cannot be completely elimi1259

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mass-scale expansion spanning a 45 mDa window (top of Figure 3) illustrates resolution of three isobaric splits (each 1.1 mDa, corresponding to C4 versus SH313C) that differ in mass by roughly twice the mass of an electron. Thus, at routine achieved mass resolving power = 800 000 at m/z 400, broadband molecular speciation of asphaltenic materials is resolution-limited to species below ∼1000 Da. Relationship between Mass Defect and Composition. Each atomic isotope has a unique mass defect [namely, the difference between its nominal (nearest integer) mass and its exact mass];82,83 therefore, every different combination of atoms (elemental composition, CcHhNnOoSs...) can be uniquely identified by its mass. Moreover, each additional ring or double bond to carbon reduces the number of hydrogen atoms by 2 and, thus, the mass defect by 0.015 65 Da. Crude oil is composed of a homologous molecular series, each with a distinct heteroatom composition but differing in the degree of alkyl substitution.42,52 Thus, the molecular mass defect can be used to differentiate between compounds of the same heteroatom “class” (NnOoSs) but different hydrogen deficiency (defined as z in an elemental composition, CcH2c+zNnOoSs). A more aromatic molecule contains more double bonds and, thus, lower mass defect than a less aromatic (or more aliphatic) molecule. Moreover, oxygen and sulfur atoms (16O, 18O, 32S, and 34S) found in high relative abundance in asphaltenes and heavy crude oil have large negative mass defects and are thus readily differentiated from other molecules. Asphaltene Mass Defect. Bulk hydrogen deficiency and heteroatom content are higher for asphaltenes than for their parent oil.5,7,21 Therefore, a comparison of mass defect distributions of species across the same m/z range reveals global compositional and structural differences between asphaltenes and maltenes of the same molecular weight. Asphaltenes, with a reported H/C ratio of ∼1,21 should exhibit less positive mass defects than the parent crude oil. Figure 4 shows the identical, ∼50 mDa mass-scale-expanded segment for the parent residue (593+ °C) (top) and asphaltenes (bottom) at m/z 553. Molecular formulas for parent residue compounds (that include both maltenes and asphaltenes) translate to mass defects between ∼0.30 and 0.60 Da, but asphaltene molecules of the same nominal molecular weight have mass defects of ∼0.20−0.55 because of their higher heteroatom enrichment and higher aromaticity (lower H/C ratio), as expected from their bulk properties. Molecular Composition of Asphaltenes: DBE versus Carbon Number Images. The tens of thousands of elemental compositions obtained from FT-ICR MS analysis of complex mixtures may be compactly visualized in two-dimensional images as previously reported.84,85 In addition, calculation of the H/C ratio from neutral-assigned elemental compositions provides a second indicator of aromaticity. Asphaltene fractions typically contain between 1 and 6% sulfur by weight, depending upon the parent crude oil.13,20 Because ionization in APPI produces radical cations and protonated ions from aromatic moieties, it is therefore an excellent ionization method for asphaltene fractions, shown to be enriched in condensed aromatic and multi-heteroatomic molecules.54 Figure 5 shows isoabundance-contoured plots of DBE versus carbon number for the S2 (top) and S3 (bottom) classes, for the parent crude distillate residue, and its isolated asphaltene and maltene monomeric fractions. Surprisingly, although distillation residues typically contain 10−50% by weight heptane-insoluble (asphaltene) material, the image for the 593+ °C residue (that

nated/discounted. In agreement with results from a host of prior analytical techniques,1,5,7,78−80 APPI linear ion-trap mass spectra show that most (∼90%) asphaltene molecular weights are below 2 kDa,1,42−45 suggesting that differences in molecular structure and chemical functionality and not molecular weight account for solubility differences between maltenes and asphaltenes. Asphaltene Compositional Complexity and Dynamic Range. Asphaltene molecular complexity challenges routine analytical techniques because of enormous compositional diversity that requires a resolving power sufficient to separate species that differ in mass by roughly twice the mass of an electron. The resolving power, m1/(m2 − m1) required to distinguish ions of a given compositional difference (e.g., 3.4 mDa for elemental compositions differing by C3 versus SH4) increases linearly with mass. However, the number of isobars (namely, same nominal mass but different exact mass) increases exponentially with molecular weight. Therefore, ultrahigh mass resolving power (800 000) is needed for unique assignment of elemental compositions at 1000 Da. For example, Figure 2, a

Figure 2. Zoom mass inset (∼30 mDa wide at m/z 499) of a positiveion APPI 9.4 T FT-ICR mass spectrum of the asphaltene fraction isolated from the 593+ °C vacuum bottom residue from a Middle Eastern heavy crude oil reveals closely spaced (1.1 mDa) doublets, abundant in asphaltene samples.

mass-scale-expanded zoom inset at m/z 499, highlights the resolution of two isobaric overlaps (1.1 mDa) critical for accurate elemental composition assignment. Such closely spaced mass splits are common in asphaltenes and, at this nominal mass, are revealed only at mass resolving power in excess of 500 000. Although the resolution achieved in broadband mode is sufficient for many elemental composition assignments, the richness in the number and types of closely spaced mass doublets is ultimately highlighted through massselected external ion accumulation, because resolving power and ICR dynamic range improve significantly with selected accumulation of ions within a defined m/z range (∼5−30 Da) by quadrupole mass filtering prior to external ion accumulation.68,81 The bottom of Figure 3 shows a ∼30 Da massisolated FT-ICR mass spectral segment for asphaltenes. The dynamic range improves by more than 100% and doubles the number of detectable species [∼214 identified mass spectral peaks (>6σ)] relative to the same segment of the corresponding broadband FT-ICR mass spectrum. Further 1260

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Figure 3. Mass-isolated 500 mDa segment at nominal m/z 563 of a positive-ion APPI 9.4 T FT-ICR mass spectrum of the asphaltene fraction isolated from the 593+ °C vacuum bottom residue from a Middle Eastern heavy crude oil. The dynamic range improvement afforded by mass selective quadrupole isolation results in 214 mass spectral peaks of magnitude greater than 6σ of baseline noise.

their insolubility in paraffinic solvents (e.g., n-heptane) and solubility in aromatic solvents (i.e., toluene), more highly aromatic compounds are expected to reside in the asphaltene fraction. Conversely, more aliphatic maltenes remain soluble in paraffinic solvents. PAH Planar Stability Limit. For a given carbon number, aromaticity for PAHs cannot exceed a maximum value that increases linearly with an increasing carbon number.86−88 Asphaltenes shown on right-hand-side of Figure 5 consist of condensed PAH systems that approach the theoretical limit (dashed line) for stable planar PAHs.54 Aromaticity beyond the planar stability limit requires “buckybowl”-type structures that have been synthesized89 but not yet found in fossil fuels. Asphaltene elemental compositions shown in Figure 5 consist of highly pericondensed aromatic structures.7,90 Similar structures have been reported for asphaltenes produced by deep hydroconversion, with condensed coke-like structures that approach the PAH stability limit.54 Asphaltene Compositional Space. The debate over asphaltene structure and molecular weight has been contentious.25,27,30,91,92 In contrast, there is widespread agreement as to a bulk H/C ratio (∼1) for asphaltenes and significantly higher for maltenes.7,21,22 Because FT-ICR MS produces tens

contains both maltenes and asphaltenes) is nearly identical to that for the maltene fraction. Abundance-weighted average carbon number and DBE values are displayed in the upper left corner in each image. Members of the maltenes S2 class average 55 carbons and DBE of 16, nearly identical to the parent residue (average C56 and DBE of 17). S3 class components also exhibit near-identical average carbon number and DBE values for the residue and maltene fractions. In contrast, the average carbon numbers for the S2 and S3 classes are lower (C43 and C41) for monomeric asphaltenes than for their corresponding maltenes and parent residue. Carbon number distributions for parent residue and maltenes range from C30 to C80 for both S2 and S3 classes, similar to those for their asphaltene counterparts (C25−C60). Although slightly lower in carbon number, asphaltene monomers span similar carbon number distributions as maltenes. However, asphaltenes have higher DBE values than maltenes of the same carbon number. Higher hydrogen deficiency corresponds to higher DBE (and thus higher boiling point) for asphaltenes relative to maltenes of the same carbon number. For example, S2 maltenes average C55 and DBE of 16, whereas S2 asphaltene monomers average C43 and DBE of 25. As expected, asphaltenes are more aromatic than maltenes. Because asphaltenes are defined by 1261

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Figure 4. Mass-scale-expanded FT-ICR mass spectral segment spanning ∼500 mDa for 593+ °C vacuum bottom residue from a Middle Eastern heavy crude oil (top) and its asphaltene fraction (bottom) facilitates visualization of the fundamental difference between maltene and asphaltene fractions. The asphaltene fraction exhibits less positive mass defects [the difference between mass calculated from elemental composition and the nominal (nearest integer) mass] relative to the maltene fraction because of higher DBE (higher aromaticity and hydrogen deficiency).

Figure 5. Isoabundance-contoured plots of DBE versus carbon number for members of the S2 and S3 heteroatom classes for 593+ °C Middle Eastern heavy crude oil residue (left) and its isolated maltene (middle) and asphaltene (right) fractions.

Figure 6. Isoabundance-contoured plots of H/C ratio versus carbon number for members of the S1 class for the maltene (left) and asphaltene (right) fractions of Middle Eastern heavy crude oil (593+ °C) residue reveal the difference in the H/C ratios of the two fractions and the absence of abundant species at or near H/C = 1.1 (the bulk ratio for asphaltenes).

of thousands of accurate masses (thus, distinct elemental compositions) for asphaltenes, a plot of DBE versus carbon number can be replotted as H/C ratio (another measure of hydrogen deficiency) versus carbon number. Figure 6 shows H/C ratio versus carbon number for monomers for the S1 class maltenes (left) and asphaltenes (right). H/C ratios and DBE values are calculated for the neutral molecule, M, corresponding to each gas-phase M+ • or (M + H)+ ion. Asphaltenes exhibit an average H/C ratio of 0.89, indicating more aromatic, alkyldeficient structures than maltenes (average H/C of 1.56). To define the compositional differences between the asphaltene and maltene fractions isolated from a 593+ °C residue, we combine data for the S1 class for the asphaltene and maltene fractions into a single DBE versus carbon number

image. Figure 7 shows composite DBE versus carbon number (left) and H/C ratio versus carbon number (right) images for S1 species detected separately for maltenes (taken from the middle of Figure 5) and asphaltenes (far right of Figure 5). Relative abundances are scaled with respect to the highest magnitude peak in each mass spectrum and summed. Superimposition of maltenes and asphaltenes into a single compositional image reveals global compositional trends for different solubility fractions from the same parent residue. Maltenes of a given carbon number are more highly alkylated (lower DBE and higher H/C ratio) than their asphaltene counterparts. Interestingly, the compositional space that should 1262

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radical cations (M+ •) and protonated analytes ([M + H]+) through charge-exchange and proton-transfer pathways.93 Another is that aggregation (known to occur even at the low concentration for MS) preferentially removes higher H/C ratio asphaltenes through nanoaggregate formation (see below). Future reports from this laboratory will systematically address that issue. Implications of Asphaltene Composition for Molecular Architecture. Boduszynski and Altgelt performed model calculations to account for the high hydrogen deficiency found in high-boiling petroleum fractions and concluded that asphaltenes contain three or more aromatic rings per cluster.42 Asphaltene molecules contain the same functionalities as their parent petroleum constituents but with more heteroatoms per molecule. We therefore constructed an H/C versus carbon number image for members of a polyheteroatomic class, S3 (Figure 9), exposing a periodic variation in carbon number at a Figure 7. Composite isoabundance-contoured plots of DBE versus carbon number (left) and H/C ratio versus carbon number (right) for members of the S1 class for the asphaltene and maltene fractions of Middle Eastern heavy crude oil (593+ °C) residue. The mass spectrum for each fraction is normalized separately to better highlight the compositional differences between the two fractions.

correspond to the highest relative abundance of asphaltenic species, 1.1 < H/C < 1.3, contains the lowest number and relative abundance of detected species (discussed in more detail later). Further evidence of the lack of highly abundant species in the “asphaltenic” H/C range is provided by Figure 8 that shows the

Figure 9. Isoabundance-contoured plot of the H/C ratio versus carbon number for members of the S3 class from the asphaltene fraction. The quantized carbon number intervals of 2−3 carbons at H/C ratios centered at 0.89 correspond to the addition of successive aromatic rings onto a condensed PAH core.

constant H/C ratio. Condensation of each additional fused ring to an existing PAH would account for such periodic variation and suggests pericondensed growth about aromatic cores. Asphaltene Aggregation. It is well known that asphaltene monomers self-associate into nanoaggregates at low concentrations (between 50 and 150 mg/L).6,21,94−97 Although the concentration required for MS is typically above the nanoaggregation concentration, some unaggregated asphaltene molecules are nevertheless observed between 200 and 2000 Da, as noted here. However, only those asphaltenes that are not locked into nanoaggregates are observed, because asphaltene aggregate formation has been reported by a host of analytical techniques at low concentrations [parts per billion (ppb)].13,30,37,53,78,98−104 Moreover, it is important to note that the asphaltenes not locked into nanoaggregates represent the most aromatic, hydrogen-deficient compounds, as supported by an average H/C ratio well below that for bulk asphaltenes and a periodic variation in carbon number at a constant H/C ratio. It has been suggested that asphaltenes selfassociate through π−π interactions between aromatic cores.80,103 However, if π−π interactions were the primary

Figure 8. Isoabundance-contoured plots of DBE versus carbon number (left) and H/C ratio versus carbon number (right) for members of the S1 class for the asphaltene fraction. The abundanceweighted average H/C ratio of 0.89 is calculated from the elemental compositions of the neutral species and is well below the bulk ratio for asphaltenes (H/C = 1.1, highlighted by the red dashed line).

DBE (left) and H/C ratio (right) versus carbon number images for S1 class compounds from asphaltenes. For S1 species with an average C44, the average H/C ratio is 0.89, below the accepted average bulk value for asphaltenes (H/C ≈ 1). Asphaltene molecular compositions observed by MS exhibit H/C ratios lower than the bulk value and indicate highly condensed aromatic structures with a low degree of alkylation. One explanation of the difference between bulk and MS-based H/C ratios is that APPI preferentially ionizes aromatics to produce 1263

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structure and molecular weight.42 The insoluble molecules contain numerous heteroatoms and metals and are responsible for the most adverse effects in crude oil refining; thus, a fundamental understanding of their chemical nature should increase crude oil processability. Boduszynski argued that “[asphaltene] precipitation with low-boiling hydrocarbons is a convenient shortcut for concentrating the most refractory petroleum constituents and determining their amount”, but in view of the many variations in such procedures (e.g., precipitant, sample/precipitant ratio, temperature, agitation, etc.), he recommended that the term insolubles replace asphaltenes, because “asphaltenes” defines a solubility fraction and not a structural compound class.1,42−45,48 Boduszynski published a controversial paper more than 30 years ago105 in which he proposed that all species in crude oil are soluble in heptane and that only the aggregated molecules are insoluble. That controversial premise is the subject of ongoing research and will be addressed in the next paper in this series.

mechanism for aggregate formation, the most aromatic species should have the greatest propensity to self-associate (and thus not observable as monomers), whereas those species are in fact excluded from the aggregate, remain monomeric, and are therefore accessible by mass spectral techniques.10,11 It is interesting and informative to summarize the compositional boundaries of a collection of asphaltene/maltene pairs from different crude oils. The summary exposes similarity in the location and slope of the border between maltene and asphaltene compositional space. Figure 10 provides a summary

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CONCLUSION

In parts 1 (10.1021/ef100149n) and 2 (10.1021/ef1001502) of this series, we provided definitive evidence that crude oil composition is continuous in molecular weight, structure, and boiling point up to the limit of distillation. In part 1 (10.1021/ ef100149n), we provided the first definitive test of the Boduszynski model. In part 2 (10.1021/ef1001502), we proposed abundance-weighted boundary lines that define maltene compositional space. Projection of the entire continuum compositional space of more than 110 000 elemental compositions to a higher carbon number could not attain the bulk H/C ratio for asphaltenes.49,50 Thus, asphaltenes are not an extension of maltene compositional space to higher and higher carbon number but an extension to higher degrees of aromaticity. Boduszynski showed that, although nondistillable residues have a broad molecular weight distribution, most petroleum heavy oil components do not exceed a molecular weight of approximately 2 kDa. Here, we have defined the molecular weight distribution by low-resolution MS and the compositional space of monomeric asphaltenes and maltenes in DBE and carbon number by ultrahigh-resolution MS. Asphaltenes exhibit higher aromaticity (higher DBE and lower H/C ratio) at an equivalent carbon number than maltenes. Solution-phase behavior differences between the two fractions support that conclusion, because asphaltenes precipitate from paraffinic solvents (i.e., heptanes/pentane) but are soluble in more aromatic solvents (i.e., toluene). The present results (in agreement with most other analytical techniques) indicate molecular weight less than ∼2 kDa for most crude oil components. Bulk composition and mass spectral techniques suggest that asphaltenes may be composed primarily of small molecules with highly condensed aromatic ring systems. Although all other evidence to date suggests that crude oil represents a continuum in composition, there is a pronounced gap between elemental compositions of asphaltenes and maltenes based on mass spectra below 2 kDa. Part 5 (10.1021/ef301737f) concludes this series of papers, provides further insight into the molecular characterization of asphaltenes, and attempts to address the asphaltene structural debate and implications toward predominant asphaltene selfassociation mechanisms.

Figure 10. Experimentally determined boundary (blue) between asphaltene (above) and maltene (below) compositional space yields a linear relationship between DBE and carbon number (DBE = 0.46 × carbon number) that equals a H/C ratio of 1.10.

of the compositional trend and raises serious questions about the analytical accessibility of asphaltenic species when measured at concentrations above the critical nanoaggregation concentration. Interestingly, the boundary [i.e., the line that separates the two abundant regions (asphaltene and maltene) in the lefthand-side of Figure 7] between the experimentally determined compositional space of asphaltenes (above) and maltenes (below) yields an equation that accurately describes a continuum of species with a H/C ratio of 1.1 (asphaltenes). The absence of highly abundant species in this compositional space (along and adjacent to the maltene/asphaltene boundary) is most likely due to the tendency of such species to form nanoaggregates. The compositionally specific aggregation depletes these species from the monomeric region of the mass spectrum. Thus, systematic “stripping” and isolation of species from the nanoaggregate or “insolubles”, as suggested by Boduszynski, appears to hold the greatest promise for addressing the compositional coverage and complexity of this “gap” material, an approach that will be highlighted in part 5 (10.1021/ef301737f) of this series. Asphaltene Definition. Tissot and Welte related the geochemical origin of crude oil to asphaltenes and referred to asphaltenes as the soluble fraction of kerogens in crude oil; therefore, asphaltene molecular structures should be similar to those for their geologic precursor, kerogen.31,32 However, Boduszynski et al. opposed the use of the term “asphaltenes” to refer to a specific type or class of compounds in crude oil as an operational definition and, instead, referred to the “insolubles” that comprise a wide variety of compounds of different 1264

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-850-644-0529 (A.G.M.); +1-850-644-2398 (R.P.R.). Fax: +1-850-644-1366. E-mail: [email protected]. edu (A.G.M.); [email protected] (R.P.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge General Electric Global Research for providing the Middle Eastern heavy crude oil residue applied in this work and Jade E. Velasquez for data processing. This work was supported by the National Science Foundation (NSF) Division of Materials Research through DMR-0654118 and the State of Florida.

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dx.doi.org/10.1021/ef301747d | Energy Fuels 2013, 27, 1257−1267