X-ray Photoelectron Spectroscopy Analysis of Hydrotreated

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Cite This: Energy Fuels 2017, 31, 10706-10717

X‑ray Photoelectron Spectroscopy Analysis of Hydrotreated Athabasca Asphaltenes Héctor J. Guzmán,* Fernanda Isquierdo, Lante Carbognani, Gerardo Vitale, Carlos E. Scott, and Pedro Pereira-Almao Schulich School of Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada S Supporting Information *

ABSTRACT: C7 asphaltenes from Athabasca crude oil were hydrotreated using a commercial NiW/Al2O3 catalyst and analyzed using X-ray photoelectron spectroscopy. Results showed that the catalyst performed better in the removal of sulfur rather than nitrogen and that sulfur concentration is higher at the surface in comparison to the bulk structure. Moreover, the sample after reaction had a higher oxygen content in comparison to the untreated asphaltene, indicating poor hydrodeoxygenation performance of the catalyst in addition to a higher reactivity toward atmospheric oxygen at the surface of the asphaltene. The latter is proposed to arise once the layer of resin covering the asphaltene is partially converted during the hydrotreating process.

1. INTRODUCTION Asphaltenes are defined as the fraction of crude oil that is insoluble in n-paraffins, typically n-C5 and n-C7, but soluble in aromatic solvents such as toluene and benzene. It has been reported that extraction of crude oils and bitumen is extremely dependent on reservoir conditions and the natural occurring viscosity of the material, which has been correlated to the type of molecules present in the crude oil matrix, mainly resins and asphaltenes. Some researchers have noted that the distribution of such compounds in the bituminous layer on the sand grains present in the subrock also offers an influence in the quality of the extraction process.1 Nevertheless, understanding the dynamic equilibria played by these two fractions will offer the advantage of assessing the role that such molecules have in the stability and properties of crude oil and bitumen,2 providing the foundation to improve the quality of extraction and the upgrading parameters in order to achieve the desired properties in the oil and its different fractionation products. One key factor that plays a major role is the identification of the molecules present in the asphaltene and resin fractions and how they interact with each other. There are many studies in the open literature dealing with sampling and identification of many different compounds present in those fractions and the changes that they undergo when submitted to different treatments and solvent extraction procedures. Thus far, many families of compounds have been identified by diverse techniques involving solvent extraction procedures, fractionation, and reactions and their subsequent identification by many analytical techniques: gas chromatography−mass spectrometry (GC-MS),3−7 size exclusion chromatography,8 and NMR9 or by direct measurement using spectroscopy techniques, such as infrared (IR),10,11 X-ray absorption near-edge spectroscopy (XANES),12−14 nearUV/visible absorption spectroscopy,15 time-of-flight secondaryion mass spectrometry (TOF-SIMS),16,17 Auger electron spectroscopy, 18 and X-ray photoelectron spectroscopy (XPS),19−25 although with a limited scope and some of them with errors.26 Many of these techniques have been discussed in more detail by Merdrignac and Espinat.27 In each case, such © 2017 American Chemical Society

studies have answered many questions about the chemical nature of asphaltenes but still fall short from what it is required to provide breakthrough knowledge in terms of processing and converting such a fraction, if possible, into more desirable products. Hydrotreating of asphaltenes is one of the many paths currently researched to improve the quality of such fraction and convert it into a more economically attractive product. Within this scope, XPS offers the advantage of identifying the functional groups present at the surface of the solid and the possibility of quantifying the changes carried out by the catalyst in such macromolecules. Most of the XPS studies cited so far have dealt only with the characterization of asphaltenes in a superficial fashion, but an in-depth study of asphaltene and resin fractions at different take-off angles is lacking. Such an approach has proven to shed light into the dynamics of the catalysis conversion and in the overlooked fact that precipitation of asphaltenes is highly dependent on manipulation conditions, parameters which are not taken into account by traditional precipitation methods and obviously influence the properties and behavior of the material.

2. EXPERIMENTAL METHODOLOGY 2.1. Materials. Toluene of HPLC grade from Sigma-Aldrich was used as received. Air, Ar, H2, He, and N2 of UHP grade were purchased from Praxair Canada. A commercial NiW/Al2O3 catalyst was used for the tests, and the asphaltenes samples were precipitated from a commercial pitch provided by Nexen-CNOOC Ltd. Details about sample and catalyst preparation and sample separation after reaction are reported by Isquierdo.28,29 2.2. Precipitation of Asphaltenes. The precipitation of the n-C7 asphaltenes samples used in this study were obtained using the procedure reported elsewhere.28,30 The samples were analyzed as received without any pretreatment. The samples were named as follow: asphaltenes before reaction (C7 ATH BR); asphaltenes after reaction Received: June 29, 2017 Revised: August 29, 2017 Published: August 29, 2017 10706

DOI: 10.1021/acs.energyfuels.7b01863 Energy Fuels 2017, 31, 10706−10717

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Figure 1. Setup for the angle-resolved analysis in a PHI VersaProbe 5000 (courtesy of Physical Electronics, Inc., modified).

Figure 2. Depth profiled at different takeoff angles (courtesy of Physical Electronics, Inc., modified). analysis, such depth can increase up to 30 nm. Normally, with a destructive depth profiling (sputtering), this number can increase up to 1000 nm. The setup of the PHI Versa-probe XPS instrument used is shown in Figure 1. When acquiring signal at different angles, the angle of the sample holder (surface normal) with respect to the analyzer is increased while the X-ray source is kept in a fixed position33 as illustrated in Figure 2. This angle is called the take-off angle of the photoelectrons. The photoelectrons thus are originated from an increased localized zone as take-off angle increments. Spectra were fitted using a Shirley background. Gaussian and Lorentzian contributions were allowed to vary independently to compensate for broadening of the Full Width to Half-Maximum (FWHM) due to changes in the chemical environment of the asphaltenes. Energy resolution was measured using the 3d5/2 Ag peak. A fwhm value of 0.75 was obtained with an asymmetric background (pass energy of 23.5 eV). In the case of the S region, the peak-fitting conditions used were the following:

(C7 ATH AR); asphaltenes after reaction + resins (C7 ATH AR + resins); and C7 resins (resin). 2.3. Hydrotreatment. The precipitated asphaltenes were treated with a commercial NiW/Al2O3 catalyst in a batch reactor (Parr). The treatment was performed at 380 °C with 800 psi of H2 for a total reaction time of 6 h.28,29 2.4. X-ray Photoelectron Spectroscopy Analysis. The XPS experiments were performed in a PHI VersaProbe 5000 (Physical Electronics, Inc.) under a vacuum of 10−9 Pa (10−10 Torr). The spectra were taken using a monochromatic Al source (1486.6 eV) at 50 W, with a beam diameter of 200.0 μm and 15 kV. For the high-resolution spectra, a pass energy of 23.5 eV and an energy step of 0.1 eV were employed, whereas for the survey spectra, a pass energy of 187.85 eV with an energy step of 1 eV were used instead. The samples were loaded in a custom-made sample holder and pressed on In foil, and spectra taken with a double neutralization, i.e., a low-energy electron beam and low-energy Ar+ beam. The binding energies are reported relative to C 1s at (284.8 ± 0.2) eV, and peak fitting was performed using the Multipak 9.2.0.5 software (Physical Electronics, Inc.). For the destructive depth profiling experiments, the sputtering protocol involved 10 min of Argon bombardment at 45°, 2 kV, 1.5 μA 2 × 2 mm (∼10.5 nm/min). Calibration was performed with a wafer SiO2/Si having a SiO2 layer of 100 nm. 2.5. Angle-Resolved Analysis. Because of the reactive nature of asphaltene molecules, angle-resolved (AR) XPS was chosen as a method of analysis in order to study the surface layer of such compounds in a nondestructive manner.31 In this case, when acquiring data from different angles the procedure is called an angled-resolved profiling, and it is used to determine the composition of the different layers comprising the solid as a function of depth.32 It is estimated that the sampling depth of the XPS technique is about 10 nm; but when using angle-resolved

• Difference in binding energy between the S 2p3/2 and S 2p1/2 signals ≈ 1.18 eV • Area ratio for the S 2p3/2/S 2p1/2 signals = 0.5 • Ratio of the full width to half-maximum S 2p1/2/S 2p3/2 signals ≈ 0.9−1.2 (depending on the species)

3. ANALYSIS OF ASPHALTENES USING X-RAY PHOTOELECTRON SPECTROSCOPY The changes that occurred at the surface of the asphaltene can be appreciated in Figure 3 along with other two samples for comparison purposes. At first look, the enormous change in the 10707


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Figure 3. XPS signals for the different asphaltene samples analyzed.

results suggest that any treatment performed to precipitated asphaltenes will invariably need to convert the resin layer before reaching the asphaltene “core”. The binding energies for the most common functionalities reported in asphaltenes are summarized in Figure 4. From the atomic percentages (Figure 5), it can be concluded that during hydrotreatment the sulfur species were the ones that were converted the most, which is in agreement with the proposed mechanism of asphaltene conversion proposed by Asaoka and Nakata,38 indicating that the sulfur linkages are the weakest ones and more reactive toward cracking. This behavior might also explain the increase in the oxidation state for the sulfur species. Despite the fact the hydrotreatment was carried out under reducing atmosphere it seems that those sulfur functionalities are extremely sensible to oxygen once exposed to air.3 It would be interesting to address how the remaining species in the molecules rearranged themselves to stabilize the converted asphaltene. On the other hand, the atomic concentration of carbon remained stable, indicating that, probably, only cracking of the alkyl chains occurred,39 with lesser contributions from reactions, such as gasification,40 although around 1% of coke formation was detected.29 Additionally, nitrogen compounds were barely affected, most likely because they are part of the polyaromatic structures rather than of the alkyl chains; for instance, to be able to convert them, high severity conditions should be employed.41 Despite the fact that NiW catalysts have powerful hydrogenating capabilities, it is well-known that to be able to perform HDN the aromaticity of the molecule has to be broken first.42

C 1s and S 2p signals indicates that the converted asphaltenes showed more oxidized species than its untreated counterpart. It has been speculated in the literature that such changes can occur once the treated asphaltene is exposed to air.30 According to the results obtained this seems to have occurred. On the other hand, several authors have reported the increase of oxygen and nitrogen content when asphaltenes are hydrotreated with NiMo catalysts. Song et al.34 explained that such behavior might be the consequence of the type of molecules (heavier or lighter) containing the oxygen groups. Heavier molecules will tend to be adsorbed better as the pore size of the catalyst is increased in detriment of lighter molecules, hence showing a competitive conversion. Similar results were obtained with NiW catalysts.35 The increase in oxygen content in these cases would be a matter of a concentration effect. The authors never addressed the changes at the surface of the asphaltenes (oxidation). It is well-established that resins play a fundamental role in the suspension of asphaltenes in crude oils forming a steric layer that provides stabilization.36,37 There are indications that during the extraction process, this resin layer coprecipitates along with the asphaltene molecules. Assuming the former scenario, a portion of that resin layer will be located at the surface of the asphaltene and it will be converted first when submitted to hydrotreatment. In order to corroborate such claims, a sample was prepared mixing isolated C7 resin with the asphaltene obtained after reaction. This sample has a concentration of 25% wt of resins (discussed in a previous work30). From Figure 3, the resemblance of the spectra for the asphaltenes before reaction, asphaltene + resins, and the pure isolated resins samples are significantly high. These 10708


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Figure 4. Representative functionalities found in asphaltenes (not an actual representation of an asphaltene molecule).

Figure 5. Atomic percentages for the different analyzed samples.

The usual metals found in asphaltenes, Fe, V, etc., were not detected with this set of samples; only the resin fraction showed the presence of Si (0.3%; Figure 5). When the concentration of elements at the surface with respect to the bulk are compared (Table 1), it can be noticed that

the nitrogen concentrations are extremely similar; although it is not the case for the sulfur content, especially for the asphaltenes, where the differences amount to about 1.5%. More sulfur is concentrated at the surface of the molecules and most probably is due to the presence of the sulfur linkages that stabilize the 10709


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Figure 6 shows the C 1s signal for the analyzed samples; the peak at 284.8 eV is normally assigned to adventitious carbon and it is used as a reference for the energy scale. In this case, due to the nature of the asphaltenes, it is assigned to the C−C bonds present in such samples, although it is indistinguishable if this carbon is in aromatic or aliphatic form. In addition, the binding energy values detected at about 285.4 eV have been reported for C−S bonds,44 which, according to Asaoka and Nakata,38 are the main constituents of aliphatic chains. This is supported by the fact that a change from 65 to 26% occurred in the signal after catalytic treatment. This also indicates that the binding energy for the C−S bonds in these species are lower than what is normally reported, falling under the area of the 284.8 peak. According to Isquierdo,28,29 one the main characteristics of the catalytic treatment with NiW-based catalysts is the removal of aliphatic chains from the asphaltenes and the subsequent

Table 1. Weight Percentages for the Surface and the Bulk of the Studied Samples wt % (elemental analysis)

wt % (surface) sample

C

O

S

N

S

N

C7 ATH BR C7 ATH AR resins

86.04 85.79 87.73

4.68 5.45 4.27

7.76 5.41 5.81

1.52 2.19 0.75

6.1 4.03 5.13

1.13 1.43 0.77

different aromatic sheets as discussed by several authors.38,43 A similar trend is observed with nitrogen, although to a lesser extent and thus might be a consequence of an enhanced exposure of such functionalities at the periphery of the asphaltene core once the sulfur linkages were removed.

Figure 6. C 1s signals for the different analyzed samples. 10710


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Figure 7. Auger C KLL transition and first derivative signals for the different analyzed samples (appended numbers = calculated D values).

Rozada et al.56 reported that the intensity of the signal can increase after removal of oxygen atoms by thermal treatment. Other authors have reported the same.57 It seems that such effect is more relevant to the ordering of the material than to the chemical environment surrounding the carbon atoms, because Kelemen et al.55 analyzed a wide variety of asphaltenes without any significant difference; while in the work of Rozada, the samples studied were graphene oxides which tend to resemble highly oriented pyrolytic graphite (HOPG) after annealing, inclusive decreasing the FWHM values of the C 1s signals56 and the symmetry of the peak.55 The same effect has been observed for the Auger peak; it is highly sensitive to oxygen content.53 As a consequence, the D value will be usually grossly underestimated when this heteroatom is present. (2) In the Auger peak, changes observed in the A part (Figure 7) are usually associated with the appearance of new C−H bonds whereas, for the B part, the changes are associated with the formation of new π bonds. In principle, this is the general trend; nonetheless, the appearance of π−π* features in the B part and the broadening of the A part when, for example, HOPG is submitted to Ar+ sputtering, has been recorded.53 It seems that the cause of this behavior arises from the degree of disorder created in the sample and not from changes in the sp2 character; thus, in the case of amorphous materials the Auger method becomes highly unreliable. (3) Both methods are accurate within the limits of the calibration curve performed. For graphitic/graphenic materials the calibration curves match the expected evolution/chemical environment of the samples, inclusive, to the point of being highly accurate in comparison with other methodologies of assessing sp2/sp3 ratios, such as NMR or FTIR. It is yet to be determined if any of these methods will work with asphaltenes or with another type of molecules, probably polymers, with a high degree of heteroatoms contents. Figure 7 shows the Auger transition recorded for the samples in this work; unfortunately, high-resolution spectra of the region

increase in aromaticity, as reported by NMR measurements. Other authors have reported the same behavior.45 This observation is corroborated by the increase of the signal assigned to π e− in aromatic rings (∼290 eV)44,46,47 (Figure S1). The other interesting feature is the higher concentration of the C−C signal for the resin in comparison to the rest of the samples, which may correspond mainly to aliphatic carbon rather than aromatic as reported by Speight.36 Hydrotreating of pure resin samples will be interesting to address. Carbonates also have a contribution at about 290 eV, and for petroleum derivatives, such a species occurs mainly as calcite (CaCO3). Nonetheless, the survey spectra (Figure S14) for these samples did not show the presence of calcium or other metals (only the resin sample showed the presence of Si); therefore, its contribution can be ruled out. The rest of the signals were assigned as follows: ∼285 eV for ether or C−N bonds,48 ∼287 eV for C−O bonds (alcohols),49,50 ∼288 eV for ketones51 or amide52 groups, and ∼289 eV for carboxylic acids or esters.48 Assessing aromaticity using XPS spectra can be challenging at times. The literature is split regarding the best approach for analysis. Principally, there are two ways of determining the aromatic content: the first approach consists of analyzing the C 1s shake up satellite feature that normally arises between 290 and 293 eV.44 The second one consists of analyzing the Auger transition C KLL (C KVV) usually in its derivative form53,54 and determining a parameter called the D value, which is the distance between the maximum and the minimum in the derivative spectrum. Both methods offer advantages but also suffer shortcomings. Strictly speaking, the value that can be determined using either method is most specifically the sp2 character of the sample, not the aromaticity. CC bonds, either in an alkyl chain or in a cyclic molecule, will give rise to the same features. Some points to consider while using these methods are the following: (1) The shake up satellite feature is not very sensitive to the effect of heteroatoms contained in the molecules,55 although 10711


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Figure 8. S 2p signals for the different analyzed samples.

behavior, if the thiophenic ring is opened, a similar functionality will be created, although because the aromatic rings were not converted, most likely the thiophenic sulfur reacted to form thiophene sulfones, once exposed to air, as the results suggest. The presence of these type of molecules have been reported elsewhere.58 Overall, the hydrogenation of the molecules during the course of the reaction should have occurred, but once the asphaltene was exposed to air, these functionalities reacted further to stabilize the structure. The evolution of asphaltenes when exposed to air has been reported, resulting in oxygen uptake, as well as sulfur, when exposed to elemental sulfur or sulfur containing minerals.36 Oxidation occurs naturally, although it seems that in the converted asphaltene the rate of oxidation is faster than in the naturally occurring structure, probably because of the partial removal of the resin layer and/or by further reaction of metastable aromatic free radicals generated during thermal cracking. It has been reported, for example, that in Athabasca asphaltenes between 40 and 70% of the oxygen-containing molecules are in the form of phenolic hydroxyl groups.3 In this case, Figure 9 shows that the most important contribution is located at about 533 eV, which is the value normally attributed to these functionalities,49,50 while ether groups appear at about 532 eV.44,62 Organic sulfites and sulfonic groups have been also reported at binding energies of about 533 eV.60 On the other hand, the contribution at about 531 eV corresponds to carbonyl groups, although sulfoxides and sulfone groups have been reported at the same values.44,60,62 Carboxylic acids, ester groups, and sulfates have similar binding energies at about 535 eV.63 Interestingly enough, a small signal appeared at 536.9 eV for the sample after treatment. Hao et al.64 have discussed that such a signal might correspond to absorbed molecular oxygen or water, which according to the results discussed thus far seems to be a plausible scenario.

could not be acquired and the region obtained from the survey spectra was used instead. Despite the fact of the poor resolution, different features in the spectra can be observed. Comparing the A part of the spectra, it can be noticed that the resin sample shows a higher intensity in such regions, which is in agreement with the reported chemical structure of resins, where they exhibit a higher degree of aliphatic chains in comparison to that of asphaltenes; such a region is similar to all the samples excepting the hydrotreated asphaltenes where such contribution seems to have decreased. In this case, the D values calculated for the analyzed samples are 14−15 eV; which, according to the reported values, would give a sp character close to that of diamond,54 which is erroneous. As mentioned earlier, the reason for these discrepancies, chemically speaking, has to do mainly with the presence of oxygen in the samples and other heteroatoms, although the influence of sulfur or nitrogen in the C KLL spectrum has not yet been addressed. Figure 8 shows the distribution of sulfur species for the analyzed samples. As reported in the literature,6,27,58 the dominant species in asphaltenes and resin fractions are thiophenic sulfur (∼164 eV) followed by sulfides (∼163 eV). In this study, hydrotreating the asphaltene caused a significant reduction in the superficial aliphatic chains (0.5% to 0%) along with thiophenic sulfur. Interestingly enough, as mentioned earlier, more oxidized species appeared for the converted asphaltene. The signal at around 165.4 eV can be assigned to sulfones, while the one between 166.5 and 167.7 eV can be assigned to a variety of functionalities: thiosulfates (RS-SO2OR′), ester sulfates (RO-SO2-OR′), or organic sulfites (RO-SOOR′);59−61 some of them form part of the pure resin as well. In addition, sulfoxides (∼165 eV)23 and sulfonic acids (∼168 eV)23 were already present in the asphaltene to begin with. From these results, it can be inferred that once the aliphatic sulfur links (thioethers38) are cleaved, the sulfur remains as the final group in the newly formed structure seeking to react with the H2 in the media and form thiol groups (∼163 eV). In a similar 10712


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to the π system; therefore, delocalization of electrons does not occur, hence the effect in the binding energies. The binding energy at 401 eV can be assigned to graphitic nitrogen,69−71 a quaternary nitrogen,72−75 or protonated analog, while the value at 402.372,76 can be assigned to a N-oxide compound.69,77−79 Hydrotreating of the asphaltene seems to have targeted the basic nitrogen molecules (pyridine) instead of the nonbasic type (pyrrole) as usually found for other systems,41,80 which is a direct consequence of the preferred mode of adsorption of the basic nitrogen molecule (through the lone pair of electrons) versus the nonbasic nitrogen molecules (through the aromatic ring) over the catalyst surface. Interestingly enough, many of the nitrogen-containing groups reported in this work for asphaltenes have been found in compounds such as graphene oxides and humic acids, with which they share many similarities.81−85 Isquierdo28,29 reported that the treated asphaltenes exhibited a higher degree of stacking in comparison to the untreated sample, according to XRD results. Such behavior was triggered once the alkyl chains were removed during the hydrotreating process and also when the resin layer present at the surface of the molecules was converted. Once this occurred, the stabilization provided by the resins was no longer present making the asphaltenes more reactive toward oxygen, thus justifying the highly oxidized nature of the sample after reaction. 3.1. Athabasca C7 after Reaction + Resins (AngleResolved Analysis). Figure 11 shows the spectra acquired at two different take-off angles, 45 and 90°, with the former “closer” to the surface. It can be noticed that most of the oxidized sulfur species are present at the surface as seen in Figure 3, although when probing deeper into the sample, this behavior reverted; less of the oxidized sulfur species was detected, while, for example, a signal of an N-oxide compound appeared. Nevertheless, the amount of oxygen was higher at 90°, while the rest of the heteroatoms showed a decreased atomic percentage, although this can be explained by a concentration effect rather than a diffusion of oxygen into the bulk of the structure. Even after probing the sample at 90° the signals obtained did not resemble entirely the spectra for the sample after reaction, meaning that the layer of resins is thicker than a few nanometers or that the resins got intermixed with the asphaltene to such an extent that a clear boundary between the two fractions does not exist. 3.3. Athabasca C7 after Reaction + Resins (Depth Profile Analysis). Figure 12 shows the depth profiles for each one of the signals analyzed. As expected, the O 1s changed the most (from 5.78 to 1.2%), indicating preferential sputtering of that atom as usually found for metal oxides,86,87 while the concentration of nitrogen decreased slightly (from 0.96 to 0.6%). Lastly, the concentration of sulfur remained practically the same, whereas the carbon changed very little (Figure S8). This indicates that reduction by Ar sputtering mostly occurs to C−O bonds rather than to the S−O or N−O species. The variation determined for the atomic percentage of the carbon signal 4.91% would be in this case the sum of the concentrations of oxygen (4.58%) and nitrogen (0.33%) sputtered. Judging by the spectra acquired before sputtering (Figure 13), the changing nature of the sample becomes apparent. Either the resin and the asphaltene kept reacting after the initial analysis or the sample is highly inhomogeneous. The effect of Ar sputtering has been widely reported in the open literature, principally for metal oxides, where the main effect is the preferential removal of oxygen atoms with the subsequent reduction of the metal oxide. Preferential sputtering

Figure 9. O 1s signals for the different analyzed samples.

According to the open literature, alkyl carbazoles are the most important nitrogen-containing species present in asphaltenes.65 The pyrrole ring of the structure is the most intense signal detected by XPS,66 and it normally appears at a binding energy of 400 eV (Figure 10).67 The second most intense contribution is

Figure 10. N 1s signals for the different analyzed samples.

attributed to species containing a pyridinic ring at binding energies of 398 eV.68 It is worth mentioning the effect of hydrogen in the chemical shift; although hydrogen cannot be directly detected by XPS, its influence can be observed because it increases the ionic character of any given bond.50 For instance, it would be expected that the pyrrole nitrogen would have a higher binding energy than the pyridine nitrogen, although this is not the case. For the pyrrole system, the lone pair of electrons of the nitrogen atom is located in an unhybridized p orbital, which overlaps with the π system of the carbon ring participating, for instance, in the resonance. In contrast, the lone pair of electrons for the pyridine nitrogen are localized in a p orbital perpendicular 10713


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Figure 11. Angle-resolved regions for the Athabasca C7 asphaltene after reaction + resins.

Figure 12. Depth profile regions for the Athabasca C7 asphaltene after reaction + resins.

of S has been observed in transition-metal sulfides as well.88−91 According to Malherbe et al.87 and Mitchell et al.,86 such effect is influenced by two parameters: surface binding energies and mass difference effects. There are several reports of the damaging influence of argon sputtering in organic molecules, either by monatomic Ar sputtering or by Ar gas cluster ion beam (GCIB)

sputtering; the latter appears to a lesser extent.92,93 Nevertheless, studies with asphaltenes molecules are still lacking. Expanding the behavior observed for metal oxides and some organic molecules,94,95 the degradation of some of the oxygen functionalities is expected (C−O, S−O, CO, etc.), although as suggested by the results, removal of oxygen seems to have 10714


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Figure 13. Depth profile regions for the Athabasca C7 asphaltene after reaction + resins (before and after sputtering).

Notes

occurred only with the C−O functionalities. The reason for this may lie in the fact that such functionalities are not part of a fused or aromatic ring as in the case of the S−O or N−O bonds. Functionalities that are not part of cyclic molecules will be the easiest to reduce; the rest will require extra energy to break the cyclic molecule apart first and form more stable compounds afterward.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC), Nexen-CNOOC Ltd, and Alberta Innovates-Energy and Environment Solutions (AIEES) for the for the financial support provided through the NSERC/NEXEN/AIEES Industrial Research Chair in Catalysis for Bitumen Upgrading. Also, the contribution of facilities from the Canada Foundation for Innovation; the Institute for Sustainable Energy, Environment and Economy; the Schulich School of Engineering; and the Faculty of Science at the University of Calgary are greatly appreciated.

4. CONCLUSIONS XPS analysis has shown the distribution of species in asphaltenes samples and how they change when submitted to hydrotreating conditions. The surface of asphaltenes seems to be inhomogeneous and greatly populated by a variety of functionalities with an increased concentration of sulfur according to surface analysis, although not vastly different from the concentration of heteroatoms normally found in the bulk. Scission of these sulfur species seems to be the easiest reaction that occurs during hydrotreating. On the other hand, results suggest that a layer of resin is still adsorbed at the surface of the asphaltenes even when using a careful precipitation protocol.36 Such a layer greatly impacts the stability of the asphaltene, making it more reactive toward air oxidation once removed. Understanding such resin− asphaltene interaction at the surface of the molecules and the type of functionalities oxidized at various levels of the asphaltenes (from surface to bulk) is key to designing better catalysts to convert these families of compounds at lower severities.

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(1) He, L.; Li, X.; Wu, G.; Lin, F.; Sui, H. Distribution of Saturates, Aromatics, Resins, and Asphaltenes Fractions in the Bituminous Layer of Athabasca Oil Sands. Energy Fuels 2013, 27, 4677−4683. (2) He, L.; Lin, F.; Li, X.; Sui, H.; Xu, Z. Interfacial sciences in unconventional petroleum production: from fundamentals to applications. Chem. Soc. Rev. 2015, 44, 5446−5494. (3) Frakman, Z.; Ignasiak, T. M.; Lown, E. M.; Strausz, O. P. Oxygen Compounds in Athabasca Asphaltene. Energy Fuels 1990, 4, 263−270. (4) Sagachenko, T. A.; Sergun, V. P.; Cheshkova, T. V.; Kovalenko, E. Y.; Min, R. S. Chemical nature of the oil and tarry-asphaltene components of natural bitumen from the Ashal’chinsk deposit in Tatarstan. Solid Fuel Chem. 2015, 49, 349−355. (5) Sergun, V. P.; Kovalenko, E. Y.; Sagachenko, T. A.; Min, R. S. Lowmolecular-mass asphaltene compounds from USA heavy oil. Pet. Chem. 2014, 54, 83−87. (6) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Strausz, O. P. Molecular structure of athabasca asphaltene: Sulfide, ether, and ester linkages. Energy Fuels 1997, 11, 1171−1187. (7) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M.; Kowalewski, I.; Behar, F. Structural features of Boscan and Duri asphaltenes. Energy Fuels 1999, 13, 228−247. (8) Carbognani, L.; Espidel, J. Preparative Subfractionation of Petroleum Resins and Asphaltenes. II. Characterization of Size Exclusion Chromatography Isolated Fractions. Pet. Sci. Technol. 2003, 21, 1705−1720.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01863. Analysis of all the studied asphaltenes, Athabasca C7 after reaction + resins (angle-resolved and depth profile analyses), survey scans, S2s region (PDF)

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REFERENCES

AUTHOR INFORMATION

ORCID

Héctor J. Guzmán: 0000-0003-1392-8361 Gerardo Vitale: 0000-0002-3142-0898 10715


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X-ray Photoelectron Spectroscopy Analysis of Hydrotreated

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