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Structural Comparison of Asphaltenes of Different Origins by Using Multiple-stage Tandem Mass Spectrometry Weijuan Tang, Matthew R. Hurt, Huaming Sheng, James S. Riedeman, David J. Borton, Peter Slater, and Hilkka I. Kenttamaa Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501242k • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 8, 2015

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Structural Comparison of Asphaltenes of Different Origins by Using Multiple-stage Tandem Mass Spectrometry

WeijuanTang,a Matthew R. Hurt,b HuamingSheng,a James Riedeman,a David J. Borton,c Peter Slater,d Hilkka I. Kenttämaa* a a

Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907. bChevron Research Company, Richmond, CA 94802. cLeco Corporation, 3000 Lakeview Avenue, St. Joseph, MI49085. dConocoPhillips

* To whom correspondence should be addressed. E-mail: [email protected]

Submitted to Energy & Fuels

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Abstract In this work, six petroleum asphaltene samples of different geographical origins were studied using atmospheric pressure chemical ionization (APCI) in positive ion mode in a linear quadrupole ion trap mass spectrometer (LQIT). APCI doped with carbon disulfide reagent was selected as the ionization method as it has been previously demonstrated to generate stable molecular ions with no fragmentation for asphaltene molecules. The mass spectra measured using this approach revealed the apparent molecular weights (MWs) of the molecules in the asphaltene samples. The results show that petroleum asphaltenes from the American continent, Europe and China have similar apparent molecular weight distributions, ranging from 200 up to 1450 Da, with slightly different apparent average MWs ranging from 570 to 700 Da. Further, molecular ions with eight randomly selected mass-to-charge ratios (m/z) ranging from m/z 500 up to m/z 808 were isolated for each asphaltene sample and subjected to collisionally activated dissociation (CAD) at the same collision energy to examine their structures. The CAD mass spectra (MS2 experiment) provided information on the maximum total number of carbons in the alkyl chains and the smallest possible size of the aromatic cores in the ionized molecules. Additionally, MS3 experiments were performed to investigate the fragmentation patterns of the fragment ions generated in the MS2 experiments. The results obtained support the island structural model for these asphaltenes. Moreover, molecules of greater MWs are shown to have more carbons in alkyl chains (ranging from 17 to 41) but the minimum core size is fairly constant.


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Introduction Asphaltenes, the heaviest components in crude oil, are defined by their solubility: asphaltenes are insoluble in n-alkanes and soluble in aromatic solvents, such as toluene, benzene, and pyridine.1,2 They are extremely complex mixtures containing molecules with multiple fused aromatic rings, alkyl chains, heteroatoms and metals.1 Asphaltenes are problematic to the petroleum industry since they reduce oil recovery, precipitate in transport pipelines, adsorb on refinery equipment, and foul catalysts used in crude oil conversion.2-4 Moreover, the depletion of conventional lighter crude oils necessitates the utilization of heavier crude oils that have a higher concentration of asphaltenes.5 In order to address the above problems and facilitate the discovery of beneficial uses for asphaltenes, an in-depth understanding of the molecules that comprise asphaltenes is necessary.6 A wide range of analytical methods have been used to interrogate the bulk properties of asphaltenes. For example, nuclear magnetic resonance (NMR) spectroscopy sheds light on various molecular parameters, including the relative abundance of aliphatic and aromatic carbons;7and X-ray absorption near edge structure (XANES) technique can be used to identify different classes of sulfur and nitrogen compounds.8,9 The molecular weight distribution of asphaltenes remains controversial.10-12 Vapor pressure osmometry and size-exclusion chromatography have yielded molecular weights of several thousands of Daltons.13-15 However, fluorescence depolarization and mass spectrometry have indicated an average molecular weight of only 450-850 Da.1,16-27 The disparity of these measurements has been suggested to result from the tendency of asphaltenes to aggregate.27,28 In the past decades, two structural models have been debated for asphaltene molecules: the island model and the archipelago model.16,19-22,29 The island model (sometimes referred to as the continental model30) has only one aromatic core with peripheral alkyl chains, whereas the archipelago model has multiple aromatic cores that are bridged by alkyl chains and may also contain peripheral alkyl chains.20 Multiple experimental methods, such as time-resolved fluorescence depolarization, Taylor diffusion, and NMR spectroscopy provide strong support for the island model.16,18-23,31,32 However, the presence of archipelago structures has been demonstrated by NMR spectroscopy and average structural parameter calculations,33 mass spectrometry,34 and thermal cracking of asphaltenes.35,36 Mass spectrometry provides an important analytical tool for the characterization of asphaltenes at the molecular level, yet it faces many challenges. Asphaltenes have a tendency to degrade and aggregate upon introduction into the gas phase.28,37-39 Dissolution of asphaltenes for mass spectrometry experiments requires a careful choice of a solvent.28,40 Ionization bias is another concern because of the highly complex composition of asphaltenes.41 Electrospray ionization is suitable for ionizing polar constituents of asphaltenes with a high heteroatom content, while


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atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization are more suitable for ionizing the nonpolar constituents.37,42-43,44 A variety of additional desorption/ionization methods have been utilized for asphaltene analysis, such as matrix-assisted laser desorption/ionization,14 field desorption/field ionization,45 and laser-induced acoustic desorption/electron ionization.39 In this work, positive ion mode APCI doped with carbon disulfide (CS2) was used to evaporate and ionize six petroleum asphaltene samples of different geographical origins in a linear quadrupole ion trap (LQIT) mass spectrometer. CS2 reagent has been demonstrated previously to generate stable molecular ions for asphaltenes.25,46-47 Furthermore, multi-stage tandem mass spectrometry was employed to examine the structures of asphaltene molecules by subjecting the selected molecular ions to collisionally activated dissociation (CAD). In addition to the apparent molecular weight distribution (MWD) and apparent average MW, structural information, including the approximate maximum number of carbons in alkyl chains and minimal sizes of the aromatic cores, was obtained.

Experimental Section Chemicals. The petroleum asphaltene samples from Bohai (China), Maya (Mexico), Claire (UK),Surmont (Canada), Montana (US), and McKittrick (US) were provided by ConocoPhillips. The asphaltenes had been isolated using exactly the same procedure. Carbon disulfide (>99.9 %) and heptane (>99.9 %) were purchased from SigmaAldrich (St. Louis, MO) and used without further purification. The asphaltene samples were stirred in heptane and sonicated for 1 hour, followed by filtration and drying under inert gas flow to remove the heptane-soluble maltene content. This procedure resulted in less than 1% weight loss, which suggests that the samples are mainly composed of real asphaltenes. Indeed, comparison of the mass spectra of these samples to untreated asphaltenes samples provided the same results. Instrumentation. A Thermo Scientific linear quadrupole ion trap (LQIT) was used for mass spectrometric analysis. The asphaltenes were dissolved in CS2 at a concentration of 0.5 mg/mL. The sample solutions were introduced into the APCI source via direct infusion from a Hamilton 500 μL syringe through the instrument’s syringe pump at a flow rate of 20 µL/min and ionized via positive ion mode APCI (at 300°C) by using CS2 as a dopant so that only molecular ions were generated.25,46-47 Molecular ions with eight randomly selected mass-to-charge (m/z) ratios ranging from m/z 500 up to m/z 808 were isolated using an isolation window of 2 Da (±1 Da), and subjected to CAD at an energy of 35 arbitrary units. The use of an isolation window of 2 Da (±1 Da) resulted in isolated ion populations that may contain isomeric and isobaric ions. This large window was used due to the relatively low ion signals measured for these complex mixtures; use of a narrower window reduced the signal greatly. This is justified as we have previously demonstrated that the fragmentation patterns and main fragment ions of ionized asphaltenes are independent of the size of the isolation window as long as it is equal or less than 2


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Da.25 The data were processed by using Thermo Xcalibur software. All measurements were repeated at least four times. The averaged results are reported in Table 1.

Results and Discussion In this study, APCI doped with CS2 was used to ionize asphaltenes in the positive ion mode so that only stable molecular ions were generated. 25,46-47 Based on the measured mass spectra, the apparent molecular weight distributions (MWD) were determined for six petroleum asphaltene samples originating from Bohai, Maya, Claire, Surmont, Montana and McKittrick. All of the studied asphaltenes from the American continent, Europe and China have similar apparent MWD, ranging from 200 up to 1400 Da. The apparent average molecular weight (AVG MW), which was calculated using Equation 1,25 varies slightly among samples with different geographical origins (Table 1). A typical monomodal apparent MWD with an AVG MW of ~615 Da is shown Figure 1 for Maya asphaltenes. ∑ ( m/ z) x ar ea( peak) AVG MW=

Equation 1

∑ ar ea( peak)

Figure 1. APCI mass spectrum showing the apparent MWD and AVG MW of the Maya asphaltene sample. Structural features of the asphaltene molecules were examined by performing MS and MS3 experiments of the isolated molecular ions. For each sample, molecular ions with eight (randomly selected) mass-to-charge ratios (m/z 500, 515, 606, 626, 634, 704, 736, and 808) were isolated and subjected to collision-activated dissociation (CAD) at the same nominal collision energy. The fragmentation patterns of molecular ions of the same m/z derived from the different asphaltene samples were compared. All the ions consistently show a similar decay pattern (for example, see Figure 2 for the fragmentation pattern of the molecular ions of m/z 634 ± 1 of Surmont asphaltenes), with a dominant methyl radical loss, less favored ethyl radical loss, even less favored propyl radical loss, and so on, with the larger alkyl radical losses always being less favored than the smaller ones. The reasons for this 2


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fragmentation behavior that is characteristic for asphaltenes but has not been observed for any model compounds are under investigation. It should be noted here that asphaltenes are known to contain abundant cycloalkane structures. The fragmentation of ionized asphaltenes with such structures is expected to lead to losses of ethylene molecules. Indeed, a closer examination of the mass spectra shown below, including that in Figure 2, indicates that there is a peak at one unit higher m/z value than that of the fragment ion formed upon loss of an ethyl radical, which suggests that ethylene losses do occur for these asphaltene samples. Hence, the presence of cycloalkane structures is indicated.

Figure 2. Fragmentation pattern of molecular ions of m/z 634 ± 1 of Surmont asphaltene sample. For comparison purposes, MS2 CAD mass spectrum of the molecular ions of m/z 500 ± 1 derived from the Maya asphaltenes sample and MS3 CAD mass spectrum ＋ of the [M-CH3] fragment ions of m/z 485 ± 1 that were formed from the above molecular ions are shown in Figures 3 and 4. The MS2 CAD mass spectrum (Figure 3) looks very much like that shown in Figure 1, supporting our statement that all molecular ions yielded similar fragmentation patterns. Interestingly, the MS3 mass ＋ spectrum (Figure 4) of the [M-CH3] fragment ions (m/z 485) derived from the molecular ions show similar decay patterns as the molecular ions themselves (Figure 3) although not quite as smooth, as some smaller fragment ions have greater ＋ abundances than slightly larger fragment ions. This may be due to the [M-CH3] ion population being a some-what simpler mixture of isobaric and isomeric ions than the molecular ions. Similar to molecular ions, the two major fragment ions (those with an even m/z-value) formed from the [M-CH3]＋ ions correspond to losses of a methyl and ethyl radical (m/z 470 and 456, respectively). Hence, these fragmentations do not obey the Even Electron rule48 that is commonly obeyed by fragmentations observed in mass spectrometry (i.e., even-electron ions fragment to yield even-


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electron and not odd-electron ions). This is not entirely surprising as aromatic ions are the most common species that do not obey the rule.48 Moreover, the above observations also demonstrate that the fragment ions obtained from the molecular ions in the MS2 experiment via loss of a methyl, ethyl and propyl radicals are formed directly from the molecular ions and not via further fragmentation of larger fragment ions. For example, ions of m/z 457 are formed directly from molecular ions of m/z 500 instead of the fragment ions of m/z 485. A previous study demonstrated that this is also true for losses of alkyl radicals from protonated asphaltene molecules.24 However, the smaller fragment ions (m/z of 443 and below) have the same m/z-values for the molecular ions (Figure 3) and their [MCH3]＋ fragment ions (Figure 4) and hence may be formed via stepwise fragmentation.

Figure 3. MS2 CAD mass spectrum of ions of m/z 500 ±1 derived from the Maya asphaltene sample, with the minimal total number of carbons in alkyl chains and the estimated aromatic core size indicated.


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Figure 4.MS3 CAD mass spectrum of [M-CH3]＋ fragment ions of m/z 485 ±1 that were formed from molecular ions of m/z 500 ±1 derived from the same Maya asphaltene sample (Figure 3). The MS2 and MS3 results discussed above indicate that the fragmenting ion populations in all cases contain many isomeric and possibly isobaric ions with alkyl chains of differing lengths since no single structure can undergo losses of so many different alkyl radicals. Further, the results support the island model more than the archipelago structural model due to the absence of facile losses of large aromatic moieties that would be expected for archipelago structures. These findings are in agreement with earlier results obtained by tandem mass spectrometry for both protonated and molecular ions of asphaltenes.24,25 Based on MS2 CAD mass spectra of the selected asphaltene molecular ions, the approximate maximum total number of carbons in alkyl chains was determined by counting the number of carbons lost upon formation of the smallest detectable fragment ion (considered to be the ion with approximately 1% relative abundance from the most abundant ion in the mass spectrum). This value does not take into account any methyl or methylene groups that may still be attached to the aromatic core of the smallest fragment ion. The approximate minimum aromatic core size was estimated by counting the number of fused aromatic rings possible for the smallest detectable fragment ion. For molecular ions of m/z 500 ±1 derived from the Maya asphaltenes sample (Figure 3), the smallest detectable fragment ion has am/z value of 233. This ion can contain 4 fused aromatic rings after cleavage of alkyl chains as alkyl radicals or alkenes (containing a maximum of about 19 carbons). It should be noted that the above determination is based on the assumption that all non-aliphatic carbons are aromatic carbons, and all aromatic rings are fused. Further, the isomer mixture is likely to also contain molecules with fewer aliphatic carbons and larger aromatic cores but we cannot identify them by using this approach. Hence, this method is limited to providing insights into differences between the smallest aromatic cores and largest numbers of aliphatic carbons in asphaltene molecules in the samples of different geographic origins. The molecular weight and structural information obtained for the eight selected molecular ions derived from the six asphaltenes samples are summarized in Table 1. The approximate maximum total number of carbons in alkyl chains and the estimated minimum aromatic core size are given as a range of values obtained in replicate measurements. For visualization of the general trends, the maximum number of aliphatic carbons and the estimated minimum aromatic core sizes are shown as a function of the m/z-values of the molecular ions studied for the six asphaltenes samples respectively, in Figures 5 and 6. Average values and standard errors are provided. For asphaltene molecules with MWs ranging from 500 to 808 Da, the maximum total number of carbons in the alkyl chains ranges from 17 to 41, while the


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approximate minimum number of aromatic rings in the cores ranges from 3 to 7. These results are consistent with other reports suggesting that asphaltene aromatic moieties contain 4-10 fused rings, and the length of aliphatic chains covers a wide range up to 30-40 carbon atoms.49-50 Generally, molecules of greater MWs were found to have more carbons in alkyl chains, yet their smallest core sizes do not vary much, regardless of their geographic origins.


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Table 1. Apparent MWD and AVG MW of Molecules in the Six Asphaltenes Samples as well as the Maximum Total Number of Carbons in Alkyl Chains and the Minimum Number of Aromatic Rings in the Cores for the Eight Selected Ions MWD AVG MW Ion of m/z 500 Maximum number of carbons inchains Minimum core size Ion of m/z 515 Maximum number of carbons inchains Minimumcore size Ion of m/z 606 Maximum number of carbons inchains Minimumcore size Ion of m/z 626 Maximum number of carbons inchains Minimum core size Ion of m/z 634 Maximum number of carbons inchains Minimum core size Ion of m/z 704 Maximum number of carbons inchains Minimum core size Ion of m/z 736 Maximum number of carbons inchains Minimum core size Ion of m/z 808 Maximum number of carbons inchains Minimum core size

Bohai 250-1450 702

Maya 200-1400 615

Claire 300-1500 681

Surmont 200-1350 575

22

17-21

21-22

17-21

17

21-22

3

4

3-4

4-5

5

3-4

23-24

18-19

23-24

16-18

13-14

22-23

3

5

3

5

7

3-4

29-30

28-30

28-30

27-28

25-28

28

3

3-4

3-4

4

4-5

4

30-31

31

31

30

28-31

30-31

3-4

3

3

4

3-5

3-4

30-32

30-32

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31-32

30

30

3-4

3-4

3-4

3-4

4

4

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34

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4

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4

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35-36

37

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4

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4

4

4

4

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40

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40

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5

5

5

5

5

5


Montana McKittrick 300-1400 200-1360 664 571

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45

Number of Aliphatic Carbons

40 35 Bohai

30

Montana 25

Maya

20

Claire Surmont

15

McKittrick

10 5 0 500

515

606

626

634

704

736

808

MW

Figure 5. General trend for the approximate maximum total number of carbons in alkyl chains as a function of MW of the molecules derived from the six asphaltene samples.

8 7

Aromatic Core Size

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6

Bohai Montana

5

Maya

4

Claire

3

Surmont

2

McKittrick

1 0 500

515

606

626

634

704

736

808

MW

Figure 6.General trend for the approximate minimum aromatic core size as a function of MW of the molecules derived from the sixasphaltene samples.


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Conclusions Examination of six petroleum asphaltene samples of different geographical origins by using positive ion mode APCI doped with CS2 and multi-stage tandem mass spectrometry allowed the determination of the apparent molecular weight distribution (MWD), apparent average molecular weight (AVG MW) as well as structural information for molecules in these samples. The asphaltene samples studied have similar apparent MWDs, ranging from 200up to 1450 Da. The apparent average MWs range from 570 up to 700 Da, and are dependent on the origin of the samples. Eight randomly selected molecular ions with m/z values ranging from m/z 500 up to m/z 808, derived from the different asphaltene samples, all show a similar fragmentation pattern, providing support to the island structural model of asphaltenes. Based on their fragmentation patterns, the fragmenting ion populations in all cases contain many isomeric and possibly isobaric ions with alkyl chains of differing lengths and aromatic cores with different numbers of aromatic rings since no single structure can explain the losses of so many different alkyl radicals as was observed. The approximate maximum total number of carbons (ranging from 17 to 41) in all alkyl chains generally increases with the increase of MWs of the asphaltene molecules (ranging from 500 to 808 Da). However, the minimum number of aromatic rings (ranging from 3 to 7) in the cores does not have an obvious correlation with the MWs of the asphaltene molecules. The findings of this study are obviously limited to those compounds in asphaltenes that can be ionized using the APCI (CS2) method.

Acknowledgements The authors thank ConocoPhillips for financial support of this work and for providing the asphaltene samples.

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