Characterization of Acid-Soluble Oxidized Asphaltenes by Fourier

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Characterization of acid soluble, oxidized asphaltenes by FTICR-MS: insights on oxycracking processes and asphaltene structural features Renzo C. Silva, Jagoš R. Radovi#, Farouq Ahmed, Ursula Ehrmann, Melisa Brown, Lante A Carbognani Ortega, Stephen R. Larter, Pedro Rafael Pereira-Almao, and Thomas B.P. Oldenburg Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02215 • Publication Date (Web): 03 Dec 2015 Downloaded from http://pubs.acs.org on December 8, 2015

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Characterization of acid soluble, oxidized asphaltenes by FTICR-MS: insights on oxycracking processes and asphaltene structural features Renzo C. Silva1*, Jagoš R. Radović1, Farouq Ahmed2, Ursula Ehrmann3, Melisa Brown1, Lante Carbognani Ortega2, Steve Larter1, Pedro Pereira-Almao2, Thomas B.P. Oldenburg1 1

PRG, Department of Geoscience, University of Calgary, 2500 University Drive NW, T2N 1N4,

Calgary, AB, Canada. 2

Catalysis for Bitumen Upgrading Group, Department of Chemical and Petroleum Engineering,

2500 University Drive NW, T2N 1N4, Calgary, AB, Canada. 3

Chemical Engineering Department, Simon Bolivar University, Caracas 1080, Venezuela.

ABSTRACT The dissolution of organic matter into water via oxidative processes, named oxycracking, has been practiced for a long time for the removal of organic pollutants, in which oxygen induces breakage and functionalization of organic molecules. Recently, oxycracking has been explored as an alternative approach to handling the increased amount of solid residues produced in oil sands upgrading activities that involve carbon rejection in solvent deasphalting units. This study uses an asphaltene rich feedstock, operationally known as petroleum pitch, isolated from an

ACS Paragon Plus Environment 1

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Athabasca bitumen vacuum residue, which was submitted to oxycracking reactions at 200ºC and 220ºC. The feed and the water soluble fractions isolated at pH 1, termed acid soluble oxidized asphaltenes (ASOA), were analyzed by ultrahigh resolution mass spectrometry (FTICR-MS, Fourier transform ion cyclotron resonance mass spectrometry) using electrospray and atmospheric pressure photoionization ion sources. FTICR-MS analysis revealed extensive oxidation of all compound classes originally present in the asphaltene rich feed. Double bond equivalent (DBE) distribution plots show that sequential carboxylation (formation of a carboxyl group) occurs progressively with increasing reaction temperature, leading to the incorporation of up to 15 oxygen atoms per molecule, whereas simultaneous decarboxylation reactions produce a CO2-rich gas phase. ASOA samples also show lower overall carbon number distributions than the asphaltene feed, which is direct evidence of C-C bond cleavage during the oxycracking process. In addition, molecular fragments detected in ASOA after carbon-carbon bond cleavages, showed not only lower carbon numbers but also lower DBEs per molecule, consistent with a more dominantly archipelago architecture for the parent asphaltene molecules.


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INTRODUCTION Traditionally, asphaltenes have been the villains of the oil industry for causing flow assurance problems by aggregation and precipitation upon changes of temperature, pressure and oil composition during oil production.1–4 Asphaltenes are also reported to strengthen oil and water emulsions, poison refining catalysts and increase coke formation during processing.5–8 As a consequence of asphaltene definition based on solubility regime (insoluble in light n-alkanes such as heptane and pentane but soluble in aromatic solvents like toluene, benzene or pyridine), the chemical variability among asphaltenes extracted from different oils by different methods is large,9–13 as is their different behavior in solution.14–16 Scientists have tackled the problem of asphaltene aggregation and precipitation by means of phase behavior modeling,17–21 where a detailed molecular characterization of asphaltene building blocks is fundamental to provide structural insights needed for accurate models.22,23 Asphaltenes are considered the most refractory fraction of a refinery feedstock, and as the oil industry shifts toward heavy oil inventories due to shortage of light oil reserves, production of vast quantities of asphaltenes is expected worldwide. For instance, asphaltenes are produced within the province of Alberta, Canada, as a consequence of oil sands production and upgrading activities involving carbon rejection in solvent deasphalting units; one Alberta’s facility was designed for processing up to 2,500 tons/day asphaltenes.24 Rough estimates indicate that heptane/pentane insoluble asphaltenes can represent respectively up to 40 %wt. of an Athabasca bitumen vacuum residue.12 While this material can represent a fuel, source of chemical feedstocks for hydrogen production or carbon product manufacture, in general it is viewed as a waste product. A highly functionalized material such as this however can potentially be a transport vector for heavy metals or in situ process aids. In this sense, the challenge of managing such waste comes with the


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opportunity to develop innovative approaches aimed at turning asphaltenes into a more useful or disposable material, while limiting greenhouse gases emissions. Organic matter dissolution in water, via oxidative processing, has been practiced for a long time for the removal of organic pollutants, while using the thermal energy arising from the exothermic oxidative reactions involved.25 Production of water soluble fractions from oxidation of petroleum fractions has been proposed via air oxidation,26 as well as of light hydrocarbons (oxygenated gasoline).27 A common feature of high temperature oxidative processes is oxygeninduced breakage of organic molecules, a process known under the broad term ‘oxycracking’. Currently, the state-of-art analytical technique for asphaltene and associated heterocompound compositional analysis is the ultrahigh resolution mass spectrometer, with a variety of ionization techniques available. The principal tool that has been applied is Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), which allows the detection of several thousand peaks representing species with unequivocal molecular formulae present in asphaltenes.28–35 Still, many asphaltene structural features have not been fully elucidated. Recent discussions using several analytical techniques, try to differentiate asphaltene composition between two major structural architecture end members:21,23,36 archipelago configurations, where aromatic and heteroaromatic ring cores are dispersed along the molecule linked by aliphatic bridges;15,37–39 island structures, where a central single aromatic or heteroaromatic core is home to a variety of saturated alkyl chains and other bonded elements;40–42 or a continuum between archipelago and island models.43,44 The compositional continuum of heavy petroleum has been studied in a series of FTICR-MS experiments, suggesting that this controversy could be somewhat resolved if archipelago structures are bridged by cycloalkane rather than linear alkane linkages.30,45 On the other hand, recent experiments using laser desorption laser ionization mass


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spectrometry (L2MS) and surface-assisted laser desorption/ionization mass spectrometry (SALDI), which are considered to provide unbiased measurements of all asphaltene species free from aggregation effects, suggested the island geometry as the dominant architecture.29 The present work uses ultrahigh resolution mass spectrometry analysis of acid solubilized oxidized asphaltene fractions produced from oxycracking reactions to examine structural features of the parent asphaltene moiety. Moreover, FTICR-MS data analysis was also targeted to provide the most comprehensive compositional characterization of this water-soluble material, setting up a compositional baseline for future research focusing on creating more useful water soluble asphaltene derived fractions.

1. MATERIAL AND METHODS Asphaltene feed. Industrial precipitation of asphaltene enriched fractions with light petroleum fractions, typically condensates, is usually carried out under temperatures spanning the 140-180ºC range and using solvent to sample ratios varying in the range 4-8:1 by wt.; however, detailed operational conditions are often confidential. Based on the elemental composition and resin content, the material studied here (operationally known as petroleum pitch) was found to grossly resemble n-C5 asphaltenes precipitated in the laboratory in a weight ratio of 13:1, used without further treatment, i.e., without Soxhlet extraction.12 Elemental analysis of the asphaltene rich feed revealed 81.7 % wt. of carbon, 8.2 %wt. of hydrogen, 1.6 %wt. of nitrogen and 7.1% of sulfur, while the n-C5 asphaltenes precipitated in the laboratory showed 80.9%, 8.3%, 1.2% and 8.5%, CHNS, respectively.12 The asphaltene feed is fully soluble in benzene, and its resin content, as determined by Soxhlet extraction using n-heptane, is 10%


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wt., comparable to 8% wt. for the n-C5 asphaltenes obtained elsewhere.12 Immediately before FTICR-MS analysis, the asphaltene rich feed was diluted in toluene to a concentration of 10 mg/mL (stock solution). Oxycracking reactions and acid soluble oxidized asphaltenes (ASOA) sample preparation. The ground asphaltene feed (4 g) was placed in a batch 0.5 L Parr reactor containing 160 mL of water at pH 13 (4 mL KOH 5N), and submitted to an oxidative atmosphere (oxygen) at 5 MPa during two hours of constant temperature and agitation. Two temperatures were tested, 200oC and 220oC. In each experiment, aqueous and solid phases were separated by vacuum filtration using a 1.5 µm pore size Whatman glass microfiber filter. Reactor and stirrer were carefully rinsed with distilled water and the resulting aqueous samples were filtered. All filtrates were combined and the fractions labeled as water soluble oxidized asphaltenes (WSOA). Aliquots of 100 mL of the WSOA solutions were acidified to pH 1 with concentrated HCl, stirred for 1h at 80 oC and filtered after overnight settling. The filtrates, labeled as acid soluble oxidized asphaltenes (ASOA), were brought to dryness in a rotary evaporator. Known amounts of the resulting solids (about 35mg) were mixed with 4 mL MeOH-BF3 inside a capped vial, kept at 60ºC for 4 hours under stirring; after cooling, 4 mL of water were added and methylated products were extracted with methylene chloride (DCM, 3x4 mL). An aliquot of each methylated ASOA extract was reconstituted in exactly 10.0 mg/mL toluene 1:1 methanol (stock solution). The two methylated ASOA extracts produced were labeled as ASOA 200 and ASOA 220, accordingly to reaction temperature. It is important to highlight that the term ASOA is a label for oxidized asphaltenes soluble in water at pH 1, since insoluble compounds at this pH were filtered out. Water insoluble oxidized asphaltenes at pH 1 or higher are out of scope of this work.


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FTICR-MS Analysis. Samples were analyzed using a 12 T, Bruker SolariX mass spectrometer, using a couple of different ionization modes. For APPI-P (atmospheric pressure photoionization in positive ion mode) analysis, an aliquot of the sample stock solutions was further diluted to 0.25 mg/mL in toluene and then infused into the ionization source using a syringe pump set to deliver 200 µL/h. A krypton lamp at 10.6 eV was used as the ion source. Transfer capillary temperature and nebulizer pressure were set to 400oC and 1.5 bar, respectively. For ESI-N (electrospray ionization in negative ion mode) analysis, sample stock solutions were further diluted to 0.25 mg/mL in toluene 1:1 methanol and doped with 2% of ammonium hydroxide. Electrospray ionization parameters included a flow rate of 200 µL/h, 4 kV of capillary voltage and nebulizer pressure of 1.0 bar. In addition, every sample prepared for FTICR-MS analysis was spiked with 10 µL of a standard mixture, which consists of a set of compounds with known concentrations, targeted to assess internal calibration efficiency. The instrument was tuned using a reference Athabasca whole bitumen sample. Ions ranging from m/z 150 to 1300 were accumulated over 50 ms in the collision cell before being transferred to the ICR cell. For each sample, two hundred transients of 8 million points in time domain were collected and summed to improve the experimental signal/noise ratio. The averaged resolving power (m/∆m50%) for peaks detected between m/z 397 and 403 was higher than 870,000, and mass accuracy assessed using the internal calibration standards showed absolute errors lower than 130 ppb. FTICR–MS raw data were processed using the CaPA v.1 (Aphorist Inc.) software package. Compositional boundaries, in terms of element atom content abundances for the fitting algorithm, were set to C4-95H0-200N0-3S0-3O0-25K0-1Na0-1Cl0-1 and double bond equivalent (DBE) ranges were set between 0 and 60, which is a measure of hydrogen deficiency due to double


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bonds and/or cyclic structures. Peaks with signal-to-noise ratio higher than 4 were assigned, based on highly accurate m/z measurements (assignment errors lower than 400 ppb) and on stable isotopic pattern (up to the fourth most intense isotopic peak), whenever possible. In order to manipulate and visualize the several layers of compositional data generated in each FTICRMS analysis, the software Ragnarök v.1.4 (Aphorist Inc.) was employed.

2. RESULTS AND ANALYSIS FTICR-MS spectra and data analysis. ESI-N coupled to high resolution mass spectrometers has been used frequently for complex organic mixture analysis, targeting the polar acidic compounds that can be deprotonated in the ion source.46–48 APPI-P has also been used for the analysis of complex organic mixtures, allowing the detection of less polar compounds, such as aromatic hydrocarbons and sulfur compounds.45,49–51 In this study, unless otherwise stated, both techniques show complementary trends and thus only selected plots are illustrated for further discussion. Mass spectra in both modes are shown in Figure 1. All detected peaks were produced from singly charged ions, as confirmed by their isotopic pattern. Considering results in both modes, less than 10% of summed peak intensities was left unassigned. The asphaltene feed does not produce ions below m/z 300 in either mode, but peaks extend up to m/z 800. Klein et al.52 reported ESI-N spectra, ranging from m/z 320 to 750, when comparing n-C7-precipitated asphaltenes to those fractions induced by pressure drop from a crude oil. Significantly different from the asphaltene feed, m/z 300 to 800, the ASOA samples show a mass spectral range of m/z 200 to 700 in both ionization modes, suggesting that the molecular weight range from feed and products are also different. Although the sensitivity was not optimized for the range below m/z


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200, signals detected indicate that ASOA samples could have a molecular weight range extending to even smaller components. A detailed inspection of the peaks between m/z 499.0 and 499.4, exemplified in Figure 2 (ESI-N), attests to the significant compositional difference between the ASOA samples and the raw asphaltene rich feed. Asphaltene feed. Figure 3a shows the heteroatom class distribution for the asphaltene feed as analyzed using the APPI-P source. In a class distribution plot, intensities of peaks from a given class, i.e. same heteroatom content on an atomic basis, are summed and presented as a percentage of the summed intensity of all assigned peaks. Although both protonated (PRO) and radical (RAD) ions are expected in APPI-P spectra,49,51,53 the asphaltene feed exhibited mostly radical ions. The few PRO ions that were assigned, failed to create classes with averaged S/N higher than 5 and/or with more than 15 peaks, thus they were rejected from the results and are ignored here in further discussions. Such observations may indicate a lack of acidic groups that are able to transfer protons to other species in the ionization chamber. Furthermore, the sample was dissolved in toluene, which contributes to the lack of protonation reactions that might be seen in methanol or other protic solvents. The absence of more highly oxygenated classes O>1 in Figure 3a may also indicate the absence of carboxylic groups in the asphaltene rich feed material. Compound class distributions from ESI-N analysis (Figure S-1) show low ion abundances for heteroatom classes O2, SO2 and NO2, potentially containing deprotonation sites such as carboxylic acid groups. All these observations indicate that the asphaltene feed under study has low initial acidity. Heteroatom classes S1-2, largely dominate the species abundances plots in APPI-P mode (Figure 3a), followed by the hydrocarbons (class HC). This ionization mode is known to favor sulfur-containing species,49,50 thus classes with species containing one up to four sulfur atoms


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could be detected each with more than 2% of total ion abundance. Figures 3b and 3c show modified Kendrick plots for classes HC (RAD) and S (RAD). Modified Kendrick plots show all components within a given heteroatom compound class, as a plot of pseudohomolog C# versus DBE (adapted from nominal Kendrick mass versus Kendrick mass defect plots54,55), with intensity color-coding normalized to the most intense peak within the compound class.51 Both classes HC (RAD) and S (RAD) exhibit similar maximum intensities in the range of DBE 19-26 and C#32-42. Since the asphaltene feed was obtained from a vacuum distillation residue, its molecular composition shows notable depletion of low DBE and low C# species, which would be located at the bottom left of the plots. Notably, maximum intensities in Figures 3b and 3c do not lie along the DBE compositional boundary limit for polycyclic aromatic hydrocarbons,56 plotted as DBE= C# * 0.9. The distance of maximum intensities in the modified Kendrick plots to the PAH boundary limit line is on average, fourteen methylene units (Fig 3b), representing the general alkylation level on the aromatic core(s), saturated bridges between aromatic moieties, less condensed aromatic core configurations or a combination of these three factors. Both Figures 3b and 3c also show a wide range of molecular formulae detected in classes HC (RAD) and S (RAD) with reasonable intensities (dark blue, approximately 40% of total ion abundance), from DBE 8-32 and C#22-60. ASOA samples. Figure 4 illustrates the class distribution detected in methylated sample ASOA 200, using APPI-P as the ion source. The compositional complexity is remarkable. In total, 64 compound classes can be identified (119, considering related species identified as both RAD and PRO ions), most of them counting for less than 3% of total ion abundance. In contrast to the asphaltene feed, radical species are no longer the only species identified but the protonated ions now do represent the dominant ionization products within a class. Figure 4 also shows that


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all original compound classes present in the asphaltene rich feed are affected by sequential oxidation, with an upper limit of 15 oxygen atoms included per molecule. The same oxidation pattern is observed in ASOA samples in both ionization modes APPI-P and ESI-N (Figure S-2). Such complexity demands another compositional sorting procedure, on top of those traditionally used in FTICR-MS data processing (classes, DBE and C#). Thus, we have classified heteroatom classes into groups of classes, based on a fixed non-oxygen heteroatom content plus a variable oxygen heteroatom number. For instance, the sum of RAD ion (PRO ignored for clarity) intensities of classes O1 to O15 would count as the group of classes Ox. This scheme provides a simpler comparison of asphaltene feed and ASOA samples (Figure 5). The most abundant groups of classes present in the asphaltene feed - Sx, NSx, Nx and HC - are no longer present in ASOA samples, with the dominant compound classes are now the oxidized analogs SOx, S2Ox, NSOx, NOx, N2Ox, Ox, in a different order of relative abundance. For instance, the most abundant group in the ASOA samples is Ox, likely generated, in part, from oxidation of parent hydrocarbons. Even though Sx compounds showed combined intensities more than three times higher than HC in the asphaltene feed, SOx and S2Ox combined intensities in ASOA, are still lower than Ox . Discrimination at the ion source, favoring the ionization of sulfur-compounds over hydrocarbons, might explain the distorted Sx abundance in the asphaltene feed. On the other hand, it is possible to speculate that cracking of a C-C bond of a hydrocarbon during oxycracking reactions, could generate two oxidized species contributing to the Ox classes. In addition, despite the relative abundance of classes S3-4 in the asphaltene feed, no S3-4Ox group of heteroatom classes were detected in the ASOA suggesting heteroaromatic cores in the feed material, contain 2 or less, sulfur atoms.


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Oxycracking effects. In order to further investigate the effects of oxycracking on the asphaltene feed, the compositional data generated by FTICR-MS analysis has been explored in greater depth. It was already demonstrated above, that many compound classes were extensively oxidized and the oxidized and methylated ASOA samples show more peaks in the low m/z range when compared to the asphaltene feed. One example of a compound class shift expected during an oxycracking reaction is the production of a carboxyl group on a hydrocarbon.25 That would result in a change of heteroatom class (+O2) and DBE (+1) for a given molecule. The DBE distribution plots of ASOA 200 in ESI-N (Figure 6), show classes O4, O6, O8, O10 and O12 exhibiting their maximum relative intensity at consecutive DBEs, apart one unit from another, 6,7,8,9 and 10, respectively. This observation indicates a sequential carboxylation process (in this paper we use the term carboxylation to refer to formation of a carboxyl functional group, but not necessarily addition of a CO2 species), where oxidation takes place at multiple molecular sites allowing a molecule to incorporate up to 15 atoms of oxygen during an oxycracking reaction, depending on the reaction severity. The same general pattern could also be detected in other groups of compound classes such as the SOx and NOx (data not shown), heterocompound classes. In addition, within a given heterocompound group, the two DBEs showing the highest relative abundances are commonly separated from each other by 3 DBE units. As an example, class O4 shows the two highest relative species abundances for compounds with DBE 6 and 9. Assuming these molecules are affected by sequential carboxylation, DBE 6 species would then represent dicarboxylic, monoaromatic compounds, while the DBE 9 species are likely, dicarboxylic, diaromatic acids. In this sense, class O6 DBE 7 would then possibly represent a series of tricarboxylic monoaromatic acids and class O6 DBE 10 a series of tricarboxylic diaromatic acids. It is worth noting that the untreated asphaltene feed APPI-P FTICR-MS


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spectrum shows negligible signals from monoaromatic and diaromatic species (DBE 4 and 7, Figure 3b), suggesting that the carboxylic acids produced during oxycracking are the result of carboxylation/oxidation of fragments from the original feed asphaltene material. Other oxidation intermediates (such as ketones and alcohols) do not exhibit a pattern as clear as that observed with inferred carboxylation. It is expected that a higher reaction temperature would favor both accelerated oxidation and cracking, even though the individual mechanisms are still unclear. On a large scale where it is important to favor both the oxidation and cracking of the asphaltene molecules with a higher reaction temperature and ultimately produce more water soluble asphaltenes, it is equally important to minimize the amount of energy required to be put into the system as well as minimizing the amount of emitted CO2 produced. Figure 7 shows the relative abundances of oxygenated species for the group of classes SOx and Ox (PRO ions only). Although samples ASOA 200 and ASOA 220 exhibit the occurrence of the same compound classes, with the exception of classes SO15 and O16 – exclusively found only in the ASOA 220, the composition of ASOA 220 is generally shifted towards classes with higher oxygen contents. This suggests therefore, that increasing reaction temperature results in increasing net oxidation level of the produced fractions. A comprehensive screening of other factors influencing oxycracking reactions as well as the reactor setup is not part of this study, but will be addressed in a following paper in this series. Oxycracking is at present being studied as a further alternative for petrochemicals production, however, it is too early to being able to identify advantages and/or disadvantages compared to other well-known processes such as oxidative distillation and catalytic steam cracking processes; the later has also been investigated in our laboratories.57


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Evidence of C-C bond cleavages in the reactor are of most interest since it would provide insights on the structural features of asphaltenes. As discussed previously, usual data processing schemes sort molecular formulae generated by FTICR-MS analyses, by heteroatom class, then DBE and finally by carbon number. However, this approach is especially limited when sample complexity is very high, as is evident in this situation. The concept of groups of heteroatom classes (e.g. Ox), was already introduced earlier in this paper to help understand oxidation processes happening in the reactor. Figure 8a modifies the common compositional sorting approach even further, showing the distribution of species carbon number, independent of heteroatom class or DBE. Every point in this plot represents the relative abundance of the summed intensities of peaks whose assignments exhibit the same carbon number. This different perspective is particularly useful in this study since it partially filters out the effects of oxidation, which, in the absence of C-C bond cleavage, does not affect the carbon number of feed molecules. For instance, a C#30 assignment for a compound in the starting material would still appear as a C#30 assignment in the oxidized products, if only oxidation is expected to occur. However, as described in the experimental part, ASOA samples were also submitted to methylation reactions catalyzed by BF3. Since the methylation efficiency was not controlled, the effects of methylation on the C# distribution of ASOA species is a shift of species abundances to higher C# by an unknown amount (one C# for each successful methylation). For example, in the case where one compound from class O8 possesses four carboxylic acids in its structure, methylation reactions would increase the C# up to 4 units, depending on the reaction efficiency. That being considered, Figure 8a shows both ASOA samples exhibiting assignments with C# distributions in the products, being significantly lower than in the asphaltene feed. Similar trend can be observed with averaged molecular mass values, reported in Figure 1. This is direct


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evidence of C-C bond cleavage during the oxycracking reaction. While the asphaltene feed material shows abundances from C#20-68 (max at C#39), ASOA samples exhibit compounds ranging from C#6-45, peaking at C#24. Since there is no input of other organic matter in the reactor, such a comparison of products to starting material indicates that the smaller molecules seen in the ASOA samples must be coming from the reacted feed. Decarboxylation processes may lead to CO2 loss and are likely to happen at temperatures higher than 150oC, when under the influence of catalysis.58–60 A molecule subject to decarboxylation has its molecular assignment affected in several parameters, changing the heteroatom class (two oxygen atoms less) and decreasing one unit in both DBE and C# (shift to the left in Figure 8a). The competition between oxidation and decarboxylation processes is of practical interest to optimize reactor conditions but, as mentioned, it is out of this paper scope. Given the shift in the C# distribution of the asphaltene feed and ASOA samples (roughly 20 carbons on average), it is unlikely that, on average, molecules from the asphaltene feed went through 20 steps of decarboxylation (excluded on oxygen balance as well), resulting in the differences shown in Figure 8a. Thus, we have attributed the shift in C# distribution seen for the ASOA to be caused mainly by cracking rather than decarboxylation. Further experiments with this focus will be conducted to fully address the three competing process mechanisms involved in oxycracking reactions (oxidation, cracking and decarboxylation). Finally, Figure 8a shows differences between ASOA 220 and ASOA 200 in C# distribution, the former slightly shifted to higher C# values. This pattern is interpreted as an effect of extended oxidation happening at higher temperatures, generating more carboxylated oxidation sites, which are then leading to more methylation reactions occurring (increase in C#).


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The breakage of structural bridges between cyclic cores, in archipelago-type asphaltenes, will result in products with different DBE values when compared to their parent compounds, whereas island-type asphaltenes, with larger central cyclic cores, undergoing dealkylation without ring cleavage for example, will show loss of carbon but no change in DBE values. The DBE distributions presented in Figure 8b are independent of heteroatom class or carbon number, so all assignments with a given DBE value would contribute to the percentage of total ions. Oxidation of hydrocarbons to ketones or carboxylic acids generates molecules with higher DBE values as compared to the parent molecule, due to the creation of double bonds between carbon and oxygen. Even though the DBE distribution of ASOA samples is shifted towards high DBE numbers due to the obvious carboxylation processes occurring to the constituents of the feed sample, the asphaltene feed still shows higher abundances of higher DBE species when compared to ASOA samples. A shift to lower values in both DBE and C# distribution in the methylated oxycracked products fits perfectly with the presence of mainly archipelago-type asphaltenes in the feedstock, whose structures are broken through oxycracking yielding to highly oxidized smaller pieces of the parent asphaltene structures. For hypothetical island-type asphaltenes, the breakage of side chains would not affect the DBE distribution, and their oxidation (to carboxyl species) would shift the net DBE distribution to higher values for products. The net effect would be a DBE distribution shifted to the right in Figure 8, which is clearly not the case for the ASOA samples. It is important to remember however, only the acid soluble products (at pH=1) were considered for the analysis in this study. We cannot exclude the presence of island-type asphaltene species in the acid insoluble products of the reaction although such specificity would be surprising.


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3. CONCLUSIONS The compositional complexity of methylated, acid soluble, oxidized asphaltenes was unraveled by FTICR-MS analysis using APPI-P and ESI-N ionization modes. Changes in the traditional FTICR-MS data analysis were necessary to compare asphaltene feed and acid soluble oxidized asphaltenes fractions. From the analysis of the highly oxidized products, all compound classes present in the asphaltenes feed were subject to extensive oxidation (with inclusion of up to 15 oxygen atoms in one parent molecule) and to cracking, yielding lower molecular weight species. An increase in the reaction temperature of 20 oC yielded more extensive oxidation and higher abundances for molecules with higher oxygen content. In this sense, controlling reaction severity is key to tune product chemistries from oxycracking reactions. Furthermore, in order to mitigate by-product CO2 production resulting from decarboxylation reactions, targeted experiments are needed to assess the controlling factor behind all the mechanisms involved in oxycracking reactions: oxidation, cracking and competing decarboxylation processes. Current results were obtained from a batch reactor, but experiments on a continuous reactor are planned to assess all experimental variables affecting oxycracking reactions. Molecular fragments produced after carbon-carbon bond cleavages showed not only lower carbon numbers but also lower DBEs in the products, which is only possible if archipelago structural architectures are dominant in the feedstock asphaltene molecules. Further experiments are needed with archipelago-like model compounds to assess if structures bridged by cycloalkanes would yield similar results. Finally, FTICR-MS results were able to reveal the composition of one of the most complex water soluble organic mixtures so far reported in the literature, not only setting up a baseline for oxycracking product composition and properties (with applications to future use or


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disposal of oxidized asphaltenes) but also shedding light on the debate of asphaltene architectures. We note the usefulness of different data visualization strategies to recover chemical process information from a diverse range of species detected by FTICR-MS analysis of complex organic mixtures from complex reaction systems.

AUTHOR INFORMATION

Corresponding author *Telephone: +1 403 210.6778. Fax: +1 403 220.8618. E-mail: [email protected] Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT

Figure S-1, ESI-N FTICR MS class distribution for the asphaltene feed. Figure S-2, ESI-N FTICR MS class distribution for the ASOA samples. The Supporting Information is available free of charge on the ACS Publication website at DOI: XX/efXXX.

ACKNOWLEDGEMENTS

This research was made possible in part by research support from the NSERC/Nexen/AIEES Industrial Research Chair in Catalysis for Bitumen Upgrading, the Canada Foundation for Innovation (CFI), the NSERC Canada Research Chairs, the Institute for Sustainable Energy, Environment and Economy (ISEE), the Schulich School of Engineering and the Department of Geoscience (University of Calgary). Aphorist Inc. is acknowledged for providing the software


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tools needed for FTICR-MS data analysis. Bruker Daltonics is thanked for continued support in the instrumentation.


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Figure 1. APPI-P and ESI-N FTICR mass spectra of asphaltene feed, ASOA 200 and ASOA 220. Averaged molecular mass (AMM) values are shown for each spectrum.


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Figure 2. Mass scale-expanded m/z 499.2 ± 0.2 segment obtained in ESI-N FTICR-MS. Major peaks are labeled accordingly to their assigned classes. ACS Paragon Plus Environment 25

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Figure 3. APPI-P FTICR MS results of the asphaltene feed. (a) Compound class distribution, and modified Kendrick plots of (b) class HC (RAD) and (c) class S1 (RAD) are shown. Dots following class labels refer to radical ions (RAD) while the absence of a dot refers to a protonated ion (PRO). PAH DBE limited calculated from Hsu et al.56 Color indicates intensities normalized to the most abundant pseudohomolog within the heteroatom class. ACS Paragon Plus Environment 26

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Figure 4. APPI-P FTICR-MS compound class distribution for sample ASOA 200. Dots following class labels refer to radical ions (RAD) while the absence of a dot refers to a protonated ion (PRO).


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Figure 5. Group of classes distribution for methylated ASOA samples and asphaltene feed, where `x` denotes a variable number of heteroatoms summed into each group of class, except for hydrocarbons (HC). Dots following class labels refer to radical ions (RAD), protonated (PRO) ions detected in ASOA samples were neglected for clarity.


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Figure 6. DBE distribution of classes O4, O6, O8, O10, O12 for the sample ASOA 200 analyzed in ESI-N mode. Abundances were normalized to each of the classes, i.e. sum of every series is 100%. Regular increments of one in the DBE number as the oxygen number increases by two strongly suggests oxidation is primarily producing carboxyl species.


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Figure 7. APPI-P Ox and SOX group of classes distribution, showing that sample ASOA 220 contains relatively more oxygen-rich molecules (distribution shifted to the right). Only protonated (PRO) ions are considered here for clarity reasons. ACS Paragon Plus Environment 30

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Figure 8. APPI-P overall (a) C# distribution and (b) DBE distribution plots. These plots are produced by sorting the desired attribute (C# and DBE respectively) prior to any other classification. Thus, while a point in (a) considers molecular assignments of any class or DBE at a given C#, a point in (b) is calculated considering assignments of any class and C# with a given DBE.


Characterization of Acid-Soluble Oxidized Asphaltenes by Fourier

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