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Characterization of Sulfide Compounds in Petroleum: Selective Oxidation Followed by Positive-Ion Electrospray Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Peng Liu,† Chunming Xu,*,† Quan Shi,*,† Na Pan,† Yahe Zhang,† Suoqi Zhao,† and Keng H. Chung‡ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China, and Well Resources Inc., Edmonton, Alberta, Canada A novel analytical method for identifying sulfides in petroleum and its fractions was developed. Sulfides in petroleum were selectively oxidized into sulfoxides using tetrabutylammonium periodate (TBAPI) and identified by positive-ion electrospray ionization (ESI) Fourier transformion cyclotron resonance mass spectrometry (FT-ICR MS). A variety of model sulfur compounds were examined to evaluate the selective oxidization and ionization efficiencies for sulfur compounds in petroleum. Two fractions, straight-run diesel and saturates of Athabasca oilsands bitumen were investigated using this approach. The oxidization process was highly selective for sulfides from thiophenes and aromatic hydrocarbons. Oxidation generated sulfoxides were ionized by positive-ion ESI and analyzed by FT-ICR MS. Mass spectra revealed the composition characteristics of sulfides in the diesel by contrasting the double bond equivalence (DBE) and carbon number distribution of sulfur compounds before and after oxidation. The abundant sulfides in the straight run diesel and saturates fraction of oilsands bitumen had DBE values of 1-3 and 1-4, respectively. A variety of sulfur compounds are present in petroleum, includingthiols,sulfides,disulfides,thiophenes,andbenzothiophenes.1,2 The sulfur compounds impact the oil refining process and the use of refined products. For example, sulfur-containing products can inhibit catalysis by competitive adsorption on the catalytic surface3 and release SOx gases when products are combusted.4-6 As a result, processes for the removal of sulfur in petroleum have been developed. * To whom correspondence should be addressed. Phone: 8610-8973-3738. Fax: 8610-6972-4721. E-mail:
[email protected] (C. X.);
[email protected] (Q. S.). † China University of Petroleum. ‡ Well Resources Inc. (1) Nishioka, M. Energy Fuels 1988, 2, 214–219. (2) Liao, Y.; Geng, A.; Huang, H. Org. Geochem. 2009, 40, 312–320. (3) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Fuel 1997, 76, 329–339. (4) Eastmond, D. A.; Booth, G. M.; Lee, M. L. Arch. Environ. Contam. Toxicol. 1984, 13, 105–111. (5) Liang, F.; Lu, M.; Birch, M. E.; Keener, T. C.; Liu, Z. J. Chromatogr., A 2006, 1114, 145–153. (6) Sinninghe Damste, J. S.; Kock-Van Dalen, A. C.; De Leeuw, J. W.; Schenck, P. A. J. Chromatogr., A 1988, 435, 435–452. 10.1021/ac1010553 2010 American Chemical Society Published on Web 06/29/2010
Hydrodesulfurization (HDS) is commonly used in refineries worldwide.7 However, it is not capable of removing all of the sulfur species.8 A better understanding of the nature and distribution of organic sulfur containing functional groups in petroleum will improve HDS processes resulting in better environmental acceptability of crude oil-derived products since they will contain less sulfur. Many analytical techniques have been explored to characterize sulfur species in petroleum. The preferred analytical technique is high resolution gas chromatography (GC) coupled with sulfurselective detectors such as a flame photometric detector (FPD),9 pulsed flame photometric detector (PFPD),10 atomic emission detector (AED)11,12 and sulfur chemiluminescence detector (SCD).13,14 However, since only volatile compounds can be analyzed by GC, this technique is not appropriate for identification of sulfur compounds in a high boiling fraction like vacuum gas oil (VGO) or vacuum residue. Furthermore, the results do not distinguish between the different classes of sulfur containing compounds. Gas chromatography coupled to mass spectrometry (GC-MS) has also been used to identify sulfur compounds in petroleum12,15 and coal.16 The limiting factor with this technique is that the mass detector is not sulfur-selective. Therefore, sulfurcontaining compounds must be enriched by liquid chromatographic (LC) separation to avoid interferences from other hydrocarbons present. In addition, traditional high resolution MS has inadequate resolution to distinguish individual mass peaks for the full mass range of crude oil. (7) Soleimani, M.; Bassi, A.; Margaritis, A. Biotechnol. Adv. 2007, 25, 570– 596. (8) Monticello, D. J. CHEMTECH 1998, 38–45, July. (9) Chakhmakhchev, A.; Suzuki, M.; Takayama, K. Org. Geochem. 1997, 26, 483–489. (10) Gao, L.; Liu, P.; Gu, T.; Xu, C.; Hai, L.; Shi, Q. J. Fuel Chem. Technol. (Chinese) 2009, 37, 183–188. (11) Andersson, J. T.; Schmid, B. J. Chromatogr., A 1995, 693, 325–338. (12) Depauw, G. A.; Froment, G. F. J. Chromatogr., A 1997, 761, 231–247. (13) Nyle´n, U.; Delgado, J. F.; Jaras, S.; Boutonnet, M. Fuel Process. Technol. 2004, 86, 223–234. (14) Zeng, X.; Lin, J.; Liu, J.; Yang, Y. Chin. J. Anal. Chem. 2006, 34, 1546– 1551. (15) Stratiev, D. S.; Shishkova, I.; Tzingov, T.; Zeuthen, P. Ind. Eng. Chem. Res. 2009, 48, 10253–10261. (16) Gryglewicz, G.; Rutkowski, P.; Yperman, J. Fuel Process. Technol. 2002, 77-78, 167–172.
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Recently, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)17 has been successfully used for petroleum analysis, leading to a new field of “petroleomics”.18-21 The ultrahigh resolution and mass accuracy of FT-ICR MS enables the assignment of an unique elemental composition to each peak in the mass spectrum of petroleum samples. Combined with various soft ionization techniques, including low voltage electron impact ionization (EI),22-24 electrospray ionization (ESI),25-31 atmospheric pressure chemical ionization (APCI),32 atmospheric pressure photoionization (APPI),33-36 atmospheric pressure laser ionization (APLI),32,37 field desorption/field ionization (FD/FI),38-40 matrix-assisted laser desorption ionization (MALDI),32 this technique is capable of determining the composition of a wide range of complex species. The use of FT-ICR MS for the analysis of sulfur compounds in petroleum has been reported elsewhere.32,34,37,41,42 To enhance the detectability by ESI mass analysis of the nonpolar sulfur compounds in vacuum residua, Mu¨ller et al.41 converted sulfur compounds into methylsulfonium salts by reacting them with iodomethane in the presence of silver tetrafluoroborate. The (17) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Mass Spectrom. Rev. 1998, 17, 1–35. (18) Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 53–59. (19) Rodgers, R. P.; Schaub, T. M; Marshall, A. G. Anal. Chem. 2005, 77, 20A– 27A. (20) Rodgers, R. P.; Marshall, A. G. Petroleomics: Advanced Characterization of Petroleum-Derived Materials by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry(FT-ICR MS); Springer: New York, 2005; pp 63-93. (21) Marshall, A. G.; Rodgers, R. P. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18090–18095. (22) Guan, S.; Marshall, A. G.; Scheppele, S. E. Anal. Chem. 1996, 68, 46–71. (23) Fu, J.; Purcell, J. M.; Quinn, J. P.; Schaub, T. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Rev. Sci. Instrum. 2006, 77. (24) Fu, J.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G.; Qian, K. N. Energy Fuels 2006, 20, 661–667. (25) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505–1511. (26) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492–498. (27) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 4145– 4149. (28) Barrow, M. P.; Headley, J. V.; Peru, K. M.; Derrick, P. J. J. Chromatogr., A 2004, 1058, 51–59. (29) Barrow, M. P.; McDonnell, L. A.; Feng, X.; Walker, J.; Derrick, P. J. Anal. Chem. 2003, 75, 860–866. (30) Smith, D. F.; Rodgers, R. P.; Rahimi, P.; Teclemariam, A.; Marshall, A. G. Energy Fuels 2009, 23, 314–319. (31) Shi, Q.; Xu, C.; Zhao, S.; Chung, K. H.; Zhang, Y.; Gao, W. Energy Fuels 2010, 24, 563–569. (32) Panda, S. K.; Andersson, J. T.; Schrader, W. Angew. Chem., Int. Ed. 2009, 48, 1788–1791. (33) Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2006, 78, 5906–5912. (34) Purcell, J. M.; Juyal, P.; Kim, D. G.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Energy Fuels 2007, 21, 2869–2874. (35) Qian, K.; Edwards, K. E.; Mennito, A. S.; Walters, C. C.; Kushnerick, J. D. Anal. Chem. 2010, 82, 413–419. (36) Haapala, M.; Purcell, J. M.; Saarela, V.; Franssila, S.; Rodgers, R. P.; Hendrickson, C. L.; Kotiaho, T.; Marshall, A. G.; Kostiainen, R. Anal. Chem. 2009, 81, 2799–2803. (37) Schrader, W.; Panda, S. K.; Brockmann, K. J.; Benter, T. Analyst 2008, 133, 867–869. (38) Schaub, T. M.; Hendrickson, C. L.; Quinn, J. P.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2005, 77, 1317–1324. (39) Stanford, L. A.; Kim, S.; Klein, G. C.; Smith, D. F.; Rodgers, R. P.; Marshall, A. G. Environ. Sci. Technol. 2007, 41, 2696–2702. (40) Smith, D. F.; Schaub, T. M.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Anal. Chem. 2008, 80, 7379–7382. (41) Mu ¨ ller, H.; Andersson, J. T. Anal. Chem. 2005, 77, 2536–2543. (42) Saroj, K.; Panda, S. K.; Schrader, W.; Andersson, J. T. Anal Bioanal Chem 2008, 392, 839–848.
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methylsulfonium salts were characterized using positive-ion ESI FT-ICR MS. Initially, Purcell et al.33 coupled APPI to FT-ICR MS and applied the technique to the analysis of a Middle East crude oil. Most of the nonpolar classes were observed by APPI but not detected by ESI. The most notable of these compounds were those containing sulfur. Schrader et al.37 demonstrated the potential of APLI FT-ICR MS for the analysis of nonpolar aromatic heterocyclic compounds including sulfur-containing compounds in crude oils. A recent study32 investigated phenylation and methylation procedures for the polyaromatic sulfur hydrocarbons by ESI, MALDI, APCI, APPI, and APLI. It was concluded that methylation does not discriminate between polyaromatic sulfur compounds; the variation in the MS data is dependent on the ionization technique.32 Sulfur-containing compounds were identified together. The previous research studies noted above could not obtain the DBE distribution of the different types of sulfur compounds (sulfides and thiophenes) present. There are several methods for characterizing the sulfide and thiophenic compounds from petroleum available and these are reported elsewhere.43-46 In general, the sulfur compounds are derived by chemical processes and identified by various analytical techniques. Green et al.44 described a derivatization/NMR method which offers potential for identifying and quantifying both sulfide and thiophenic forms of sulfur in petroleum. The sulfides and thiophenes were methylated with 13C-enriched methyl iodide in the presence of silver tetrafluoroborate to form methyl sulfonium salts. The methylated petroleum sample was then analyzed by 13C NMR spectroscopy. Payzant et al.45 developed an improved method for the isolation of the sulfide and thiophenic classes of sulfur compounds from petroleum. The method is based on the ability to selectively oxidize the sulfur atom in a particular chemical environment. The oxidized products are easily separated from the resulting mixture by chromatography and reduced back to their corresponding sulfides and thiophenic compounds. The reagent tetrabutylammonium periodate (TBAPI) in a toluene/methanol solution oxidizes the sulfides to highly polar sulfoxides without affecting the thiophenic compounds. In this study, a new approach for sulfides characterization was investigated. Model sulfur compounds and a straight-run diesel were used to evaluate the selectivity and the conversion of sulfides. This technique was then used to characterize sulfides in a saturates fraction of Athabasca oilsands bitumen. EXPERIMENTAL SECTION Model Sulfur Compounds and Petroleum Products. Model sulfur compounds, including dibutyl disulfide, 2-phenyl1,3-dithiane, dihexyl sulfide, phenyl sulfide, 3-n-octadecylthiophene, dibenzothiophene (DBT) and n-dodecyl sulfide and phenanthrene were dissolved in a toluene/methanol (5:1 vol/vol) mixture to obtain a 100 mg/L solution for each compound. (43) Wang, L.; Green, T. K. Acta Petrol. Sin. (Petrol. Process. Sec.) 1998, 14, 80–86. (44) Green, T. K.; Whitley, P.; Wu, K.; Lloyd, W. G.; Gan, L. Z. Energy Fuels 1994, 8, 244–248. (45) Payzant, J. D.; Mojelsky, T. W.; Strausz, O. P. Energy Fuels 1989, 3, 449– 454. (46) Xu, H.; Shi, Q; Chen, Y. Acta Petrol. Sin. (Petrol. Process. Sec.) 1999, 15, 28–38.
Phenanthrene was used as an internal standard to evaluate the conversion of sulfur species. The model sulfur compounds were obtained from Sigma Aldrich Chemistry. Analytical-grade toluene and methanol were obtained from Beijing Chemical Reagents Company and were distilled twice before use. A straight-run diesel (204-363 °C) derived from Kazakhstan crude oil was provided by the PetroChina refinery. The saturates fraction of Athabasca oilsands bitumen was obtained by saturates/ aromatics/resins/asphaltenes (SARA) fractionation using the standard SH/T 0509-92 method (Chinese Standards for Petroleum and Natural Gas Industry). The amount of sulfur in the straightrun diesel and bitumen derived saturates fractions was 0.12 and 0.7 wt %, respectively, determined using an ANTEK 7000 Pyrofluorescence Analyzer (ANTEK Instruments). Selective Oxidization of Model Sulfide Compounds and Sulfides in Petroleum Fractions. The procedure for selective oxidation of sulfur compounds was based on that of Payzant et al.45 with some modification in the amount of regents used and reaction time. The sulfur containing samples used in this study were 10 mL model sulfur compounds solutions, 80 mg diesel, and 80 mg saturates fraction of bitumen. Each sulfur sample was mixed with 25 mL toluene, 5 mL methanol and 0.2 g TBAPI in a 100 mL round-bottom flask equipped with a reflux condenser and a magnetic stirrer. The mixture was stirred and refluxed at room temperature. The reacted mixture was transferred to a separatory funnel. The organic phase of the mixture was extracted three times with 40 mL high-purity water to remove the remaining TBAPI. The solvent was removed from the organic phase by a rotary evaporator to obtain the sulfides and unreacted compounds. Methylation of Sulfur Compounds. A 100 mg sample of straight-run diesel before and after oxidation was diluted with 2 mL dichloromethane (CH2Cl2). Fifty microlitres methyl iodide was added to the sample solution. The mixture in a beaker was immersed in an ultrasonic bath at room temperature for 5 min. While mixing, a solution of 20 mg/mL silver tetrafluoroborate and 2 mL 1,2-dichloroethane was added. A yellowish brown precipitate formed immediately. The reaction mixture in the beaker remained in the ultrasonic bath for an additional 5 min to allow the reaction to continue. Then the reaction mixture solution was stored in the dark for 48 h. A mixture consisting of methyl sulfonium salts, silver iodide, silver tetrafluoroborate and unreacted oil was obtained after nitrogen was used to remove the 1,2-dichloroethane and the dichloromethane from the solution. Five milliliters of toluene was used to remove the unreacted oil and obtain the methyl sulonium salts. Sample Preparation. The 10 mg oxidized oil sample was diluted with 1 mL toluene. Twenty microlitres of the oxidized oil sample solution was further diluted with 1 mL toluene/methanol (5:5 vol/vol) solution. The final solution was spiked with 5 microlitres formic acid to promote electrospray ionization. Methylsulfonium salts (10 mg) were diluted with 1 mL dichloromethane. Five microliters of the methylsulfonium salt solution was further diluted with 1 mL toluene/methanol/ dichloromethane (3:3:4 vol/vol/vol) solution. Glassware was used to handle and transfer the solvent, except for the stainless steel pistons used in the 100 µL Hamilton syringes.
FT-ICR MS Analysis. The samples were analyzed using a Bruker apex-ultra FT-ICR MS equipped with a 9.4 T actively shielded superconducting magnet. The sample solution was infused via an Apollo II electrospray source at 180 µL/h using a syringe pump. The operating conditions for positive ion formation were -2.5 kV emitter voltage, -3.0 kV capillary column front end voltage, and 320 V capillary column end voltage. Ions accumulated for 0.01 s in a hexapole with 2.4 V DC voltage and 300 Vp-p RF amplitude. The quadrupole (Q1) was optimized to obtain a broad range for ion transfers. An argon-filled hexapole collision cell was operated at 5 MHz and 400 Vp-p RF amplitude, in which ions accumulated for 0.2 s. The extraction period for ions from the hexapole to the ICR cell was set to 1.3 ms. The rf excitation was attenuated at 11.75 dB, and used to excite ions over the range of 200-750 Da. Four M data sets were acquired. Sixty four scans were coadded to enhance signal-to-noise ratio and dynamic range. The square sine bell multiplication apodization was carried out, followed by a single zero-fill before fast Fourier transformation and magnitude calculation. Mass Calibration and Data Analysis. The mass spectrometer was calibrated using sodium formate. Mass peaks with a relative abundance greater than 6 times the standard deviation of the baseline noise level were exported to a spreadsheet. Data analysis was performed using custom software, which has been described elsewhere.31,47 Measured masses were converted from the IUPAC mass scale to the Kendrick mass scale. The Kendrick mass defect (KMD) was calculated.48 Molecular formulas of two neighboring even and odd normal masses were assigned on the basis of mass measurement to ±1.5 ppm. Formulas were also confirmed/eliminated unequivocally by the presence/absence of the corresponding nuclide containing one 13C. For an assigned class species, compound types with various DBE values were identified by the difference of integer multiples of H2. A DBE range of 0-50 was allowed. Members of a homologous series differ by integer multiples of CH2, each homologous series was identified by the assigned single members with an additional limit of KMD tolerance of 0.0015. For each series, elemental compositions were assigned by use of a mass calculator program limited to molecular formulas consisting of up to 100 12 C atoms, 2 13C, 200 1H, 2 14N, 5 16O, 3 32S, and 1 34S. If there is a peak series in the KMD plot of unassigned peaks, one of the peaks will be identified manually, followed by automatic search for other species of this class. GC-MS Analysis. A Thermo-Finnigan Trace DSQ GC-MS coupled with a HP-5MS column (60 m × 0.25 mm × 0.25 µm) was used to analyze the composition of model sulfur compounds before and after the oxidation reactions. The GC oven was maintained at 50 °C for 1 min, and increased to 190 at 20 °C/min. It was increased to 210 at 2 °C/min, and further increased to 310 at 20 °C/min. It was then kept constant at 310 °C for 10 min. The sample was injected and the EI ionization source was operated under 70 eV ionization energy. The Mass range was set to 35-500 Da at a 1 s scanning interval. The ion source temperature was 200 °C. The ion current was 250 µA. (47) Shi, Q; Dong, Z.; Zhang, Y.; Zhao, S.; Xu, C. Chin. J. Instrum. Anal. 2008, 27 (Supplement 1), 246–248. (48) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. Anal. Chem. 2001, 73, 4676–4681.
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Figure 1. Total ion chromatograms of model compounds with different reaction time. 1: dibutyl disulfide, 2: dihexyl sulfide, 3: phenyl sulfide, 4: 2-phenyl-1,3-dithiane, 5: DBT, 6: phenanthrene, 7: 3-noctadecylthiophene, 8: n-dodecyl sulfide.
Figure 3. Broadband positive-ion ESI FT-ICR mass spectrum of the diesel before and after selective oxidation by TBAPI. The insert shows the expanded mass scale spectrum at m/z 282-283. The samples were analyzed at the same concentration and experimental conditions. The abudant peak at m/z 227 is likely arised from the butyl diglycol acetate (namely, 2-(2-butoxyethoxy) ethyl acetate), a highboiling solvent, which commonly presents in the positive-ion ESI mass spectra as contamination.
Figure 2. Broadband positive-ion ESI FT-ICR mass spectrum of the oxidized model sulfur compounds after 60 h. The inserts show the expanded mass scale spectra at m/z 195, m/z 203, and m/z 213.
RESULTS AND DISCUSSION Selective Oxidization of Model Sulfur Compounds. The oxidation of the model sulfur compound mixture was carried out over 60 h. Figure 1 shows the total ion chromatograms for the various model sulfur compounds in the mixture before and after oxidation reactions with TBAPI. Figure 1 also shows the total ion chromatograms of phenanthrene which is less readily oxidized. This compound was used as an internal standard to track the conversion of the various model sulfur compounds. Figure 1 shows that the relative abundance of sulfide compounds, including dihexyl sulfide, 2-phenyl-1,3-dithiane, and n-dodecyl sulfide, decreased with increasing reaction time. These sulfide compounds did not completely react even after 60 h with excess TBAPI. This indicates that the complete oxidation of sulfide compounds requires a much longer reaction time than that reported by previous studies.45 Dibutyl disulfide and phenyl sulfide had a low oxidative reactivity among the model sulfide compounds tested. Thiophenic compounds such as dibenzothiophene and 3-n-octadecylthiophene were not oxidized. The oxidation reaction generated sulfoxides which, being highly polar molecules, cannot be analyzed by gas chromatography.49 (49) Payzant, J. D.; Montgomery, D. S.; Strausz, O. P. Tetrahedron Lett. 1983, 24, 651–654.
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Figure 4. Heteroatom class distribution (heteroatom content) for raw diesel and its different oxidation treatment products derived from positive-ion ESI FT-ICR mass spectrum.
Figure 5. Broadband positive-ion ESI FT-ICR mass spectrum of the methylsulfonium salts derived from the raw (top) and selective oxidation (for 72 h) diesel (bottom). The insert shows the expanded mass scale spectrum at m/z 271. The samples were analyzed at the same concentration and experimental conditions.
Figure 6. Plots of double bond equivalents (DBE) as a function of carbon number for the S1 class derived from the positive-ion ESI FT-ICR mass spectrum of the methylsulfonium salts of the raw and oxidized diesel samples (Figure 5). Note: The largest dot correspond to the most abundant S1 species in the sample. Even if the relative abundance of a given species is the same for two samples, its not always means the two samples have the same absolute abundance on the species.
Figure 2 shows FT-ICR mass spectrum of the oxidized sulfur compounds in the mixture. Ions and their sodium adducts ([MNa]+) corresponding to O1S1 class species can be distinctly identified in the mass spectrum. Although dibutyl disulfide and phenyl sulfide had a low oxidative reactivity, their corresponding sulfoxide products were detected in the mass spectrum (inserts in Figure 2). However, no sulfoxides corresponding to thiophenes and sulfones were detected. This indicates that the TBAPI was highly selective in oxidizing the sulfides to sulfoxides. Although the starting concentration of each sulfide compound was similar, the intensity of the peaks for each ion is different. This is due to different conversion and the ionization efficiencies, as well as the suppression effects of the ESI ionization technique. Oxidized Sulfides in Diesel. The straight-run diesel was oxidized with TBAPI for 72 h. Reaction samples were taken at 24, 48, and 72 h for ESI FT-ICR analysis. Figure 3 shows the FTICR mass spectrum of the straight run diesel before and after 72 h of oxidation reactions. The mass spectrum has a similar molecular weight range of 200-400 Da for the before and after oxidation samples. The inserts in Figure 3 show the expanded mass scale spectra at m/z 282-283. The most abundant peaks from the unreacted diesel were even mass peaks, corresponding to N1 class species. Those from the oxidized diesel were odd mass peaks, corresponding to O1S1 class species. Heteroatom class species identified in the mass spectra from the diesel samples at various reaction times are shown in Figure 4. The five abundant heteroatom class species identified were N1, N1O1, N1S1, NaO1S1 and O1S1. The N1 class species was the most abundant in the spectrum of the unreacted diesel sample. The O1S1 class species were dominant in the oxidized diesel samples and the amount increased with increasing reaction time. The NaO1S1 and O1S1 class species were considered the same class species as the sulfoxides (O1S1 class) were in the form of sodium adducts [M-Na]+. The unreacted diesel contained sulfoxides which may have been derived from oxidation of the sulfur species in the petroleum feedstock during storage.50,51 Although the sulfoxides in the oxidized diesel sample had a higher relative abundance in the mass peaks than that of the unreacted diesel sample, the distribution of O1S1 class species on the DBE versus carbon number plot (see Supporting Information Figure S-1) was similar for the two samples, one key
difference was the O1S1 class species with DBE ) 0 which had a very low relative abundance in the spectrum of oxidized sample. To validate that the O1S1 class species were derived from sulfides, the diesel before and after oxidation was reacted by methylation to form methylsulfonium salts41 and characterized by positive-ion ESI FT-ICR MS. Figure 5 shows the broadband FT-ICR mass spectrum of the methylsulfonium salts derived from the diesel before and after 72 h of oxidation reaction. The inserts in Figure 5 shows the expanded mass scale spectra at m/z 271. The sulfur compound C17H35S1 (DBE ) 1) in the unreacted sample almost disappeared in the oxidized sample. Conversely, a peak assigned as C17H35O1S1 (DBE)1) appeared in the mass spectrum of the oxidized sample (not shown). This indicates that sulfides were oxidized to sulfoxides. A peak assigned as C16H31O1S1 (DBE ) 2) was in the mass spectrum of the oxidized sample. The peak assigned as C18H23S1 (DBE)8) is for thiophenic sulfur compounds which are less readily oxidized and appeared in both the methylsulfonium salts of unreacted and oxidized samples. Figure 6 shows the iso-abundance map of DBE as a function of carbon number for the S1 class species from the positive-ion ESI FT-ICR mass spectrum shown in Figure 5. An additional carbon atom was attached to the sulfur atom, due to the methylation reaction. The most abundant S1 class species in the methylsulfonium salts of unreacted and oxidized samples had DBE values of 6 and 9, corresponding to benzothiophenes and dibenzothiophenes, respectively.52 The relative abundance of S1 class species with DBE value of 6 and greater in the two samples has a similar distribution. However, the relative abundance of S1 class species with DBE value less than 6 in the methylsulfonium salts of the oxidized sample were significantly lower. This suggests that the S1 class species with a DBE value of 6 and greater are likely thiophenic compounds, while those with a DBE value less than 6 are likely sulfide compounds. The S1 class species with DBE values of 1 and 2 are likely one and two cyclic-ring sulfides, which were identified (50) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53–57. (51) Green, J. B.; Yu, S. K. T.; Pearson, C. D.; Reynolds, J. W. Energy Fuels 1993, 7, 119–126. (52) Smith, D. F.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2008, 22, 3118–3125.
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Figure 7. Broadband positive-ion ESI FT-ICR mass spectrum of the selective oxidized saturates fraction of Athabasca oilsands bitumen. The insert shows the expanded mass scale spectrum at m/z 507. Note: the peaks marked with an asterisk likely arise from contamination.
Figure 8. Plots of double bond equivalents (DBE) as a function of carbon number for O1S1 class species derived from the positive-ion ESI FT-ICR mass spectrum of the oxidized saturates fraction of Athabasca oilsands bitumen (Figure 7).
in the methylsulfonium salts of the oxidized sample. The incomplete oxidation of sulfides to sulfoxides over long reaction time is also supported by the data in Figure 6. This was determined by comparing the distribution of relative abundance of the S1 class species in the methylsulfonium salts of unreacted and oxidized samples. Oxidized Sulfides in Saturates Fraction of Athabasca Oilsands Bitumen. The saturates fraction of Athabasca oilsands bitumen was oxidized with TBAPI for 2 h. The oxidized saturates fraction was analyzed by positive ESI FT-ICR MS. Figure 7 shows the FT-ICR MS broadband (m/z 200-750) spectrum for the oxidized saturates fraction. The unreacted saturates fraction did not yield a sufficient ion signal for positive ESI FT-ICR MS analysis. The abundant peaks with odd masses in the molecular weight range of 200-750 Da indicated that the O1S1 class species were dominant. Also shown in Figure 7 is the expanded mass scale spectrum obtained under the resolving power of 240 000 (m/∆m 50% at m/z 507). The peak assigned as C33H63O1S1 (DBE (53) Shi, Q. ; Pan, N. ; Liu, P. ; Chung, K. H. ; Zhao, S. ; Zhang, Y. ; Xu, C. Energy Fuels 2010, 24, 3014-3019.
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) 3) in the oxidized saturates fraction suggests that TBAPI is effective for oxidizing large molecular sulfides in heavy petroleum feedstock into sulfoxides. Figure 8 shows the iso-abundance plots of DBE as a function of carbon number for the O1S1 class species in the oxidized saturates fraction. The O1S1 class species in the oxidized saturates fraction were spread over a wide range of DBE (0-12) and carbon number (10-45). The most abundant species were centered at DBE values of 1-4 with carbon numbers of 17 to 30. The sulfur compounds with 0 DBE value are aliphatic sulfides. Those with DBE values of 1 and 2 are one and two cyclic-rings sulfides, respectively. Three cyclicring sulfides and alkyl thiophenes have the same DBE values. The distribution of relative abundance of these species overlapped in the DBE vs carbon number plots.52,53 Figure 8 shows that the sulfur compounds with DBE values of 6 and 9 did not have high relative abundance. This indicates that no benzothiophenes and dibenzothiophenes were present in the saturates fraction or were not oxidized to sulfoxides. This is expected since sulfur compounds with a DBE value of 3 are derived from sulfides with three cyclic-rings, but not from thiophenes. Sulfur compounds with DBE values of 4 and greater are likely be cyclic-rings or aromatic sulfides. CONCLUSIONS The results of this study illustrate a method for characterization of sulfides by selective oxidation and high-resolution FT-ICR MS. The advantage of this approach is the ability to characterize sulfides from thiophenic and/or other sulfur compounds. TBAPI can selectively oxidize the sulfides into sulfoxide in the presence of thiophenic compounds in a solvent or complex hydrocarbon matrix. Although the reaction time is longer than expected, this method is effective for obtaining a good selectivity and satisfactory sulfur conversion. This method has been demonstrated here with a middle distillate and saturates fraction of bitumen. Similar performance is anticipated for other complex mixtures such as crude oil and coal-derived liquids. ACKNOWLEDGMENT We thank Mr. Junhui He and Mrs. Xuxia Liu for assisting with the GC and total sulfur analysis. Dr. Zhiming Xu is thanked for providing the bitumen sample. This work was supported by the National Basic Research Program of China (2010CB226901). SUPPORTING INFORMATION AVAILABLE Figure S-1. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review April 22, 2010. Accepted June 17, 2010. AC1010553
