Selective Methylation of Sulfides in Petroleum for Electrospray

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Selective Methylation of Sulfides in Petroleum for Electrospray Ionization Mass Spectrometry Analysis Haidong Li, Xiu Chen, Jianxun Wu, Yahe Zhang, Xuxia Liu, Quan Shi, Suoqi Zhao, Chunming Xu, and Chang Samuel Hsu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02756 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Selective Methylation of Sulfides in Petroleum for Electrospray Ionization Mass Spectrometry Analysis Haidong Li,1 Xiu Chen, 1 Jianxun Wu, 1 Yahe Zhang, 1 Xuxia Liu, 1 Quan Shi,1* Suoqi Zhao,1 Chunming Xu1 and Chang Samuel Hsu1, 2*

1. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China 2. (a) Petro Bio Oil Consulting, Tallahassee, FL 32312; (b) Department of Chemical and Biomedical Engineering, Florida A&M University/Florida State University, Tallahassee, FL 32310

ABSTRACT A method for selective methylation of sulfides (particularly aliphatic sulfides) but not thiophenes in petroleum has been developed for positive-ion electrospray ionization (ESI) mass spectrometry. The new method uses only CH3I without AgBF4 to produce methyl sulfonium ions of sulfides for molecular composition analysis. It provides simplicity and saves time compared to a previously developed methylation method using CH3I/AgBF4 to methylate all sulfur compounds into ionic species analyzable by positive-ion ESI. The composition of sulfides by this method agrees with that of the sulfide fraction obtained from CH3I/AgBF4 methylation/demethylation. Hence, the separation of sulfides from thiophenes through elaborate procedures is eliminated. The new method was validated by model compounds and has been successfully applied to the analysis of sulfides in diesel, vacuum gas oil, crude oil, and atmospheric pressure distillation residue. When combined with the previously developed CH3I/AgBF4 method, the new method may have the potential to differentiate reactive sulfur, such as sulfides, from non-reactive sulfur, such as thiophenes. This could provide a means to develop correlations between sulfur types and corrosion, and possibly for other studies.

ACS Paragon Plus Environment

Energy & Fuels

C22

CnH2n+1S

12

Petroleum+CH3I

S1

8

+ESI FT-ICR MS

C20

Sufides in Petroleum

10

DBE

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1. INTRODUCTION To meet product specifications, sulfur compounds must be removed from petroleum fractions. The burning of sulfur-containing petroleum-derived fuels generates sulfur oxides; these in turn form unhealthy particulates and are responsible for acid rain. During petroleum refining, sulfur compounds poison certain catalysts. Certain sulfur compounds induce corrosion in pipeline, tanks, and process equipment. The characterization of sulfur components in petroleum remains an important topic. Significant challenges still remain due to the chemical complexity of petroleum.[1-3] Han et al.[3] reviewed the numerous methods for characterizing sulfur components in petroleum. A key method is x-ray absorption near-edge structure spectroscopy (XANES) with synchrotron radiation.[4, 5] However, this method cannot distinguish between sulfur components at molecular level. Generally, speciation of the sulfur compounds in crude oils or refining streams is determined via gas chromatography-mass spectrometry (GC-MS), gas chromatography-gas

chromatography

(GC-GC),

and

liquid

chromatography-mass

spectrometry (LC-MS).[3, 6] In recent years, ultrahigh resolution mass spectrometry (MS), such as Fourier-transform ion cyclotron resonance (FT-ICR) MS[7-15] and Orbitrap MS[16, 17], has become widely used for molecular characterization of complex mixtures, especially for nonvolatile heavy petroleum fractions. Electrospray ionization (ESI) is an associated ionization technique.[14, 18, 19] However, sulfides and thiophenes cannot be readily ionized by ESI due to their low polarity. Methylation of these sulfur compounds is a use method for converting these nonpolar sulfur compounds into polar compounds. Green et al.[20] developed a method for the qualitative and quantitative analyses of sulfur compounds in petroleum. The sulfur compounds were converted to their corresponding methyl sulfonium salts by 13C-enriched methyl iodide (13CH3I) facilitated by silver tetrafluoroborate (AgBF4) for 13C NMR analysis. Muller et al.[21] developed a similar method for the characterization of sulfur-containing aromatics in vacuum residues. The sulfur compounds were converted to strong polar methyl sulfonium salts by silver tetrafluoroborate (AgBF4) and methyl iodide (CH3I) for analysis by positive-ion ESI FT-ICR MS. However, this method cannot differentiate sulfides and thiophenes because both of them are methylated. Wang et al.[22] developed a method to selectively separate sulfides and thiophenes by a methylation/demethylation separation scheme. It became one of the most sophisticate methods to analyze sulfides and thiophenes in petroleum. However, the method is laborious and time-consuming because it requires a variety of reactions and separation procedures prior to ESI FT-ICR MS analysis.[23]


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Alternatively, selective oxidation followed by positive-ion ESI FT-ICR MS is an effective method for sulfide analysis in petroleum.[24] Sulfides are selectively oxidized into corresponding sulfoxides using tetrabutylammonium periodate (TBAPI) and then identified by ESI FT-ICR MS. However, it is also time-consuming and complicated. Recently, Ren et al.[25] developed a method entailing oxidizing into sulfoxides for FT-ICR MS analysis by “direct analysis in real time” (DART) with spray injection. It is a rapid analysis method, but the DART ionization source is expensive and not commonly available in mass spectrometry laboratories. This work focused on a selective methylation method for sulfides in which only CH3I is used instead of previously reported methylation methods using both CH3I and AgBF4. The selectivity of sulfide conversion is confirmed and evaluated by model compounds and a straight run vacuum distillation gas oil (VGO). For demonstration, the new method is also applied to a diesel, a distillation residue, and a crude oil. 2. EXPERIMENTAL SECTION 2.1 Reagents and Samples Analytical-grade dichloromethane (DCM), n-hexane (n-C6), toluene, methanol, acetonitrile (ACN) were obtained from Beijing Chemical Reagents Company. They were distilled for purification and stored in glass containers before use. Methyl iodide (CH3I), silver tetrafluoroborate (AgBF4), 4-dimethylaminopyridine (DMAP), 7-azaindole, diphenyl sulfide, di-n-butyl sulfide, di-n-hexyl sulfide, di-n-dodecyl sulfide, phenylethyl sulfide, dibenzyl sulfide, 3-dodecylthiophene, 5-methylbenzo[b]thiophene, dibenzothiophene, and hexadecane (n-C16) were purchased from J&K Chemical Ltd. Four oil samples were used in the study: (1) a vacuum distillation gas oil (VGO) fraction (S: 2.70 wt%) obtained from heavy crude in a Kuwait refinery, (2) a diesel fraction (180-350 C, S: 1.55% ), (3) an atmospheric pressure distillation residue (＞350 C; S: 5.80 wt%) derived from Oman crude oil, and (4) a low sulfur crude oil (0.25 wt%) from a Sudan oil field. 2.2 Selective Methylation by CH3I only without AgBF4 Nine equimolar model sulfur compounds (0.20 mmol each) were dissolved in 10 mL of toluene in a beaker, followed by addition of CH3I (1.50 mL, 12.0 mmol). A 20 µL of the mixture was diluted with 1 mL toluene/methanol (1:3 vol/vol). The final solution was analyzed by


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positive-ion ESI FT-ICR MS and gas chromatography-mass spectrometry (GC-MS). For VGO analysis, a 10 mg sample was dissolved in 1 mL toluene in a beaker flask. A 20 µL of the toluene solution was diluted with 1 mL toluene/methanol (1:3 vol/vol), followed by the addition of 50 L CH3I. The solution was introduced into the positive-ion ESI FT-ICR MS ion source at 180 L/h. The same method was applied to the diesel, distillation residue and crude oil samples. 2.3 Isolation of Three Sulfidic Methyl Sulfonium Salts To assess the methylation efficiency of the new method and its comparison with the previous methylation method, a separated sulfide fraction from the VGO according to the previously developed methylation and demethylation method[22] was used. Briefly, the sulfides and thiophenes in the VGO were methylated to methyl sulfonium salts by AgBF4 and CH3I, the polar methyl sulfonium salts were then separated from the mixture by filtration. The thiophenic sulfonium salts were first demethylated to thiophenes by 7-azaindole. The remaining sulfidic sulfonium salts, called “Methyl Sulfonium Salts A”, were separated by filtration on a sodium sulfate column to elute off thiophenes by rinsing with n-hexane. Thereafter, the Methyl Sulfonium Salts A was demethylated to the sulfide fraction by DMAP. The sulfide fraction obtained above was dissolved in 10 mL of dichloromethane (DCM) in a beaker, followed by addition of CH3I (1.50 mL, 12.0 mmol). The anhydrous Na2SO4 particles (25 g, 100 mesh) were then added. The DCM and CH3I in the were removed by rotary evaporation at room temperature, leaving methyl sulfonium salts and unreacted sulfides adsorbed on the surface of anhydrous Na2SO4 particles. The Na2SO4 was transferred to a 60 cm × 8.0 mm glass column where n-hexane was added to elute the soluble fraction into a clean and previously weighed flask. Thereafter, a 40 mL aliquot of DCM was added to elute the n-hexane insoluble sulfidic methyl sulfonium salts, called “Methyl Sulfonium Salts B”. The unreacted sulfides in the n-hexane soluble fraction were methylated to sulfonium salts by CH3I/AgBF4, called “Methyl Sulfonium Salts C”. The separation scheme of Methyl Sulfonium Salts B and Methyl Sulfonium Salts C is shown in Figure 1. All methyl sulfonium salts (A, B, and C) were analyzed by positive-ion ESI FT-ICR MS after dissolving each of the salts in DCM except Methyl Sulfonium Salts B, which was already in DCM, and then diluted the DCM solutions with methanol for positive-ion ESI FT-ICR MS analysis.


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According to the separation scheme described above, three sulfonium salts (A, B, and C) were obtained. B and C can be considered as subfractions of A. These samples were used to evaluate the selectivity of the methylation by CH3I without AgBF4.

Sulfide Fraction CH3I, DCM Methylation Products （1）by n-C6

Unreacted Sulfides

（2）by DCM

Methyl Sufonium Salts B

AgBF4, CH3I Methyl Sufonium Salts C

Figure 1. Separation scheme for the methyl sulfonium salts B and C.

2.4 FT-ICR MS Analysis All methyl sulfonium salts were analyzed by FT-ICR MS. The experiments were carried out in a Bruker Apex-Ultra FT-ICR MS equipped with a 9.4 T actively shielded superconducting magnet. The operating parameters were: spray shield voltage, 4.5 kV; capillary column front end voltage, 5 kV; time-of-flight, 1.0 ms for VGO and crude oil, 0.9 ms for diesel, and 1.1 ms for distillation residue; and collision cell accumulated time, 1.0 s. The ICR was operated at m/z 150-800 mass range, 18 dB attenuation, and 4M data size. The spectra were superimposed 64 times to improve the signal-to-noise ratio. The FT-ICR MS mass calibration, data acquisition, and processing were reported elsewhere.[26, 27] 2.5 GC-MS Analysis The model sulfur compounds before and after methylation were analyzed by a Thermal Scientific TSQ 8000 Evo GC-MS equipped with a HP-5 MS fused silica column (60 m × 0.25


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mm × 0.25 μm). The GC oven was held at 80 °C for 2 min, programmed to 250 °C at 8 °C/min then to 310 °C at 30 °C/min and finally held at 310 °C for 10 min. The injector and transfer line temperatures were held at 300 °C. Helium was used as carrier gas at a flowing rate of 1 mL/min. The MS ion source was held at 230 °C for 70 eV electron-impact ionization. The mass range was 35-500 with a 0.5 s scan period.

3. RESULTS AND DISCUSSION 3.1 Methylation of Model Compounds Figure 2a exhibits the positive-ion ESI FT-ICR mass spectrum of the model compounds prepared with CH3I. Sulfide molecules (M) were methylated to form methyl sulfonium ions as [M + CH3]+ in the positive-ion ESI MS. It can be seen from the mass spectrum that the m/z 153.0731, 161.1357, 201.0731, 217.1983, 229.1045, and 385.3860 ions correspond to phenylethyl sulfide, di-n-butyl sulfide, diphenyl sulfide, di-n-hexyl sulfide, dibenzyl sulfide, and di-n-dodecyl sulfide methylation mass peaks, respectively. The 3-dodecylthiophene, 5methylthianaphthene and dibenzothiophene did not yield methylated ions. According to Otsuki’s calculation, the electron density of the sulfur atom in sulfides is higher than that in thiophenes.[28] Methyl iodide can react with sulfides at room temperature without a promoter, but not with thiophenes. Figures 2b and 2c exhibit the GC-MS results of the model compounds before and after CH3I addition, respectively. The n-C16 was used as an inert internal reference. When conducting the methylation reactions with CH3I at room temperature, the intensities of sulfides ， di-n-butyl sulfide, di-n-hexyl sulfide, di-n-dodecyl sulfide, phenyl ethyl sulfide, diphenyl sulfide, and dibenzyl sulfide shown in Figure 2b are reduced in Figure 2c, whereas the

intensities

of

thiophenes,

3-dodecylthiophene,

5-methylbenzo[b]thiophene,

and

dibenzothiophene shown in Figure 2b are almost unchanged in Figure 2c. Therefore, the positive-ion ESI FT-ICR MS and GC-MS analysis demonstrated that the sulfides were selectively methylated with the addition of CH3I.


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Figure 2. Positive-ion ESI FT-ICR mass spectrum of the model compounds added with CH3I (a), and GC-MS chromatograms of the model compounds before (b) and after (c) CH3I addition. The peaks with asterisks are from contaminants. The model sulfur compounds have different conversion yields when reacted with CH3I, shown in Table 1. The conversions of di-n-butyl sulfide, di-n-hexyl sulfide, and di-n-dodecyl sulfide were 42%, 42%, and 31%, respectively. The conversions of phenylethyl sulfide, dibenzyl sulfide, and diphenyl sulfide were much lower at 5%, 7%, and 2%, respectively. The lower methylation yields of aromatic sulfides compared to aliphatic sulfides are expected for hyper-conjugation of π-electron in the aromatic rings with non-bonding electron on the sulfur atom, which reduces the electronic density of the sulfur atom for CH3+ ion addition. The electronic density of the sulfur atom in thiophenes is even lower due to full conjugation with adjacent aromatic rings; they require a promoter or moderator, such as Ag+, for methyl ion


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attachment. Therefore, it is further demonstrated that sulfides but not thiophenes are selectively methylated by the addition of CH3I only (without a promoter) for positive-ion ESI FT-ICR MS analysis.

Table 1. Conversions of model sulfur compounds for CH3I methylation without promoter Compound

Conversions (%)

Compound

Conversions (%)

n-Butyl sulfide

42

Dibenzothiophene

0

Phenylethyl sulfide

5

Benzyl sulfide

7

5-Methylthianaphthene

0

3-Dodecylthiophene

0

n-Hexyl sulfide

42

n-Dodecyl sulfide

31

Diphenyl sulfide

2

3.2 Selective ESI MS Analysis of Sulfides in VGO It is well-known that sulfur compounds cannot be readily ionized by ESI in either positiveion or negative-ion modes due to insufficient polarities. Hence, the sulfur compounds have to be methylated to form methyl sulfonium ions for the ESI MS analysis. Figure 3 shows the positive-ion ESI FT-ICR mass spectrum of the VGO with only CH3I addition. The relatively abundant peaks with dots correspond to methyl sulfonium ions, [M+CH3]+, of the DBE=1 S1 compounds shown in Figure 4(a).

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C29 200

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Figure 3. Positive-ion ESI FT-ICR mass spectrum of VGO added with CH3I, with a representative structure shown on the right.


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3.3 Comparison of CH3I Additions with and without AgBF4 Figure 4 displays the relative abundances of the S1 species in CH3I-methylated VGO and methyl sulfonium salts of A, B, and C. Amounts are expressed as the relative sizes of circles in a plot of DBE versus carbon number. The DBE values of the S1 species in VGO shown in Figure 4(a) range from 1 to 5, corresponding to 1- to 5-ring naphthenic sulfides or aromatic sulfides for DBE ≥ 4. The results are in good agreement with previous studies.[25] The abundances of 6 and 9 DBE compounds are very low, indicating that there is no methylated benzothiophenes or dibenzothiophenes, which are usually the most abundant sulfur species in petroleum. This demonstrates that this is an effect way of methylating only sulfides but not thiophenes in the VGO by adding only CH3I without AgBF4. Figures 4(b) shows the relative ion abundance plots of DBE versus carbon number for the S1 species in methyl sulfonium salts A. Methyl Sulfonium Salts A includes the sulfidic methyl sulfonium salts prepared by CH3I/AgBF4 methylation/demethylation that remain on the sodium sulfate column after removing thiophene compounds. The main DBE range and carbon number range of both figures are 1-4 and 19-30, respectively. The relative high abundance of the DBE=3 series implies that trace amounts of alkyl thiophenes remained in the methyl sulfonium salts after the separation.

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Figure 4. Relative ion abundance plots of DBE versus carbon number for the S1 species in the


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VGO added with CH3I (a) and methyl sulfonium salts A (b), B (c), and C (d). The yield of methyl sulfonium salts B is 13.6%, and the total yield of methyl sulfonium salts B and C is 97.5% (based on the weight of salt A). Figures 4(c) and 4(d) show the relative ion abundance plots of DBE versus carbon number of the S1 species in methyl sulfonium salts B and C, respectively. Methyl sulfonium salts B was formed from sulfidic fraction with only CH3I addition. Methyl sulfonium salts C was the AgBF4/CH3I methylation product of the unreacted sulfide compounds that may contain trace amounts of unreacted thiophenes in the CH3I/AgBF4 method. The S1 molecular composition in the VGO treated with CH3I (Figure 4(a)) is comparable with that of methyl sulfonium salts A (Figure 4(c)), while Figures 4(b) and 4(d) exhibit some trace amounts thiophenes of DBE ≥ 3; these remained after the reaction and separation steps in which methylation was aided by AgBF4. We can therefore conclude that the addition of CH3I without AgBF4 is an effective method for the selective characterization of sulfide compounds in petroleum, as well as other complex mixtures. The additional benefits of the only CH3I addition method include the following: it is convenient and time-saving because it does not require a promoter and does not depend on time-consuming reaction and separation procedures. Hence, it improves the accuracy of the compositional analysis by decreasing the loss of yields during separation. 3.4 Applications of Direct CH3I Addition in the Diesel, Crude Oil, and Residue The proposed direct CH3I addition method was demonstrated for distinguishing between sulfides in diesel, crude oil, and distillation residue. Figure 5 shows relative ion abundance plots of DBE versus carbon number of S1 species from positive-ion ESI FT-ICR mass spectra. The main range of DBE is 1-4 and carbon numbers are 15-21 for the diesel. There are more sulfide types in the crude oil and distillation residue. The main DBE range is 0-14 and carbon numbers are 15-50 for in the crude oil while the main DBE range of is 1-16 and carbon numbers are 23-60 for the distillation residue. Note the presence of aliphatic sulfides of DBE = 0 that are easily removed in refinery processes, evidenced by their absence in the diesel and distillation residue after catalytic and thermolytic reactions. The species of DBE > 4 are likely multinaphthenosulfides rather than thiophenes that are not ionized by CH3I as inferred by the model compound studies. Further confirmation of polynaphthenosulfides is difficult due to the lack of model compounds. However, the dominance of DBE = 3-7 with carbon number of 21-45 might


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indicate the presence of naphthenic sulfur biomarkers currently not discovered or unidentified. Saturated naphthene biomarkers with greater than 6 rings have been found and identified in heavy oils.[29] 16

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Oman Diesel 12

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Figure 5. Relative ion abundance plots of DBE versus carbon number for S1 species assigned from positive-ion ESI FTICR mass spectra of the diesel, crude oil, and residue added with CH3I. 4. CONCLUSIONS This paper presents a simple and selective methyl derivatization method for the analysis of sulfides in petroleum. Methyl iodide without a promoter, such as AgBF4, is added to the petroleum sample to selectively ionize sulfides for direct positive-ion electrospray (ESI) mass spectrometry analysis. The sulfide composition obtained by this method is consistent with that of isolated sulfide fractions from the CH3I/AgBF4 methylation/demethylation method. Compared to the CH3I/AgBF4 method, the CH3I method has the following benefits: it is convenient and quick, because it does not need a promoter to methylate sulfide components, thus avoiding time-consuming separation procedures to remove the promoter that can suppress other ions generated in ESI. Hence, it improves the accuracy of the compositional analysis of sulfide compounds by decreasing the loss of yields during separation. The method is validated by model compounds and has been successfully applied to the analysis of sulfides in diesel, VGO, crude oil, and atmospheric pressure distillation residue. Combining with previously developed CH3I/AgBF4, the two methods may have the potential to differentiate reactive sulfur, such as sulfides, from non-reactive sulfur, such as thiophenes,3 semi-quantitatively for the corrosion correlation and other studies. 

ACKNOWLEDGEMENTS The research is supported by the National Natural Science Foundation of China (NSFC


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41773038, U1463207, and 41503032) and the Guangxi Key R&D Plan (No. 2017AB54014).

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ORCIDs Yahe Zhang: 0000-0003-2573-568X Quan Shi: 0000-0002-1363-1237 Chang S Hsu: 0000-0003-4411-7860 Suoqi Zhao: 0000-0003-3707-2844 Notes The authors declare no competing financial interest.

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REFERENCES

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