Distribution of Sulfides and Thiophenic Compounds in VGO

Jun 7, 2011 - Detailed elemental composition and distribution of sulfides and thiophenic compounds in four subfractions of Kazakhstan vacuum gas oil (...
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Distribution of Sulfides and Thiophenic Compounds in VGO Subfractions: Characterized by Positive-Ion Electrospray Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Peng Liu,† Quan Shi,*,† Na Pan,† Yahe Zhang,† Keng H. Chung,‡ Suoqi Zhao,† and Chunming Xu*,† † ‡

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Well Resources Inc., 3919-149A Street, Edmonton, Alberta, Canada T6R 1J8

bS Supporting Information ABSTRACT: Detailed elemental composition and distribution of sulfides and thiophenic compounds in four subfractions of Kazakhstan vacuum gas oil (VGO) were determined by positive ion electrospray (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The sulfides in VGO subfractions were selectively oxidized into sulfoxides using tetrabutylammonium periodate (TBAPI). The sulfur compounds in the oxidized VGO subfractions were reacted by methylation to form methylsulfonium salts and were then characterized. Elemental composition and distribution of sulfides and thiophenic compounds in the VGO subfractions were characterized by their double bond equivalents (DBE) values and carbon numbers before and after the oxidation reactions. The results showed that the S1 class species with DBE values of 6 and greater are likely thiophenic compounds, while those with DBE values less than 6 are sulfides. As boiling point of VGO increased, the abundance of thiophenic compounds increased. DBE value and carbon number of the compounds also increased.

’ INTRODUCTION The petroleum industry faces increasing challenges in converting heavy oil fractions into valuable products, such as gasoline, jet fuel, and diesel, as the demand for petroleum distillates increases. Vacuum gas oil (VGO) is a heavy feedstock produced by vacuum distillation for hydrocracking and fluid catalytic cracking processes to manufacture transportation fuels.1 3 VGO contains various organo-sulfur compounds, including sulfides, polysulfides, thiols, thiophenes, and alkyl-substituted isomers of thiophenes with aromatic rings.4 6 These sulfur compounds are detrimental to oil refining processes. They also react to form sulfur oxides (SOx) in combustion flue gas which are environmentally regulated compounds. Hydrodesulfurization (HDS) is a common refining process used to remove organo-sulfur compounds in petroleum feedstocks.7 9 Even with highly active catalysts and severe process operating conditions, HDS is not capable of removing all sulfur compounds.10 To optimize the HDS process, it is necessary to have a better understanding of the types and distribution of organic sulfur compounds in the feedstock. Many analytic techniques have been used to characterize sulfur compounds in petroleum derived streams. Sulfur compounds in light petroleum distillates can be readily analyzed using gas chromatography (GC) with sulfur specific detectors.11 Liquid chromatographic (LC) separation and GC coupled with mass spectrometry (MS) has also been used to identify sulfur compounds in petroleum12 and coal.13 Although these techniques have been successfully used to obtain detailed information on sulfur compounds in light distillates, they are inadequate for identifying sulfur compounds in heavy (high boiling point) petroleum fractions. This is because while the volatile compounds can be analyzed by GC, a single high resolution GC r 2011 American Chemical Society

column is not capable of separating the more than 10 000 sulfur compounds present in heavy petroleum fractions.4 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)14 has been successfully used for petroleum analysis, leading to the new field of “petroleomics”15 18 The ultrahigh resolution and mass accuracy of FT-ICR MS enables the assignment of a unique elemental composition to each peak in the mass spectrum of petroleum samples. FT-ICR MS has been coupled with various ionization techniques to characterize complex petroleum mixtures without fragmentation,31,39 42 including low voltage electron impact ionization (EI),19 21 electrospray ionization (ESI),22 29 atmospheric pressure chemical ionization (APCI),30 atmospheric pressure photoionization (APPI),31 34 atmospheric pressure laser ionization (APLI),30,35 field desorption/field ionization (FD/FI), 36 38 and matrixassisted laser desorption ionization (MALDI).30 The use of FT-ICR MS to characterize sulfur compounds in petroleum and its fractions has been reported elsewhere.30,32,35,41,43 46 Because the sulfur compounds in petroleum are not sufficiently acidic or basic, they cannot be effectively ionized by ESI.32 M€uller et al.43 proposed an analytical method for nonpolar sulfur species, in which sulfur compounds in vacuum residue were derivatized followed by ESI FT-ICR MS analysis. In previous work,46 the conversion and selectivity of methylation reaction for various sulfur compounds were investigated by gas chromatograph coupled with pulse flame photometric detector (GC-PFPD). The results showed that the sulfur compounds in the diesel react easily with iodomethane to form Received: December 4, 2010 Revised: June 4, 2011 Published: June 07, 2011 3014

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Table 1. Yield and Sulfur Concentration of VGO Subfractions

Figure 1. Simulated distillation curves of VGO subfractions.

methylsulfonium salts at room condition. Schrader et al.35 used APLI FT-ICR MS for analyzing nonpolar aromatic heterocyclic compounds including sulfur compounds in crude oils. Al-Hajji et al.41 coupled APPI to FT-ICR MS and applied the technique to analyze two hydrocracked heavy feedstocks, heavy vacuum gas oil (HVGO) and demetallized oil (DMO). The mono-, di-, and trisulfur species were identified in HVGO and DMO, with disulfur species being the most abundant sulfur compounds in the DMO. However in previous studies, only the molecular formulas of sulfur compounds in petroleum were obtained. The identification of the different types of sulfur compounds (sulfides and thiophenic compounds) in petroleum and its fractions is still a challenge. A novel analytical method has been developed for identifying sulfides in petroleum and its fractions.47 Sulfides in petroleum are selectively oxidized into sulfoxides using tetrabutylammonium periodate (TBAPI) and characterized using positive-ion ESI FTICR MS. The purpose of this study was to determine the different types of sulfur compounds in VGO subfractions with various boiling point ranges using this technique.

’ EXPERIMENTAL SECTION Materials. Four subfractions of vacuum gas oil (VGO) derived from Kazakhstan crude oil were provided by the PetroChina Dushanzi refinery. The VGO subfractions were obtained from the side-draws of a commercial vacuum distillation unit at increasing boiling point ranges: VGO-1 was the lowest and VGO-4 was the highest. The simulated distillation (ASTM D7213) data for VGO subfractions are shown in Figure 1. The yield and sulfur concentrations of the VGO subfractions are shown in Table 1. As expected, the sulfur concentration increased as the VGO boiling point increased. Selective Oxidization of Sulfides in VGO. The procedure for selective oxidation of sulfides in VGO has been described elsewhere.47 In summary, 80 mg of VGO was mixed with 25 mL of toluene, 5 mL of methanol, and 0.2 g of TBAPI (Sigma Aldrich Chemistry) 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 for 48 h. The reacted mixture was transferred to a separator funnel. The organic phase of the mixture was extracted three times with 40 mL of water to remove the remaining TBAPI. The solvent was removed from the organic phase by vacuum rotary evaporation.

subfraction

yield, wt %

sulfur, wt %

VGO-1

4.34

0.36

VGO-2 VGO-3

8.42 9.55

0.48 0.59

VGO-4

2.93

0.79

Methylation of Sulfur Compounds. The procedure for methylation of sulfur compounds in VGO has been reported elsewhere.48 Samples (100 mg) of VGO before and after oxidation were diluted with 2 mL of dichloromethane (CH2Cl2). Fifty microliters of methyl iodide was added to the sample solution. The mixture was immersed in an ultrasonic bath at room temperature for 5 min. While mixing, 2 mL of silver tetrafluoroborate (20 mg/mL) in 1,2-dichloroethane was added. A yellow 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. 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 dichloromethane from the solution. Five milliliters of toluene was used to remove the unreacted oil and obtain the methyl sulfonium salts. FT-ICR MS Analysis. Methylsulfonium salts (10 mg) were diluted with 1 mL of dichloromethane. Five microliters of the methylsulfonium salt solution was further diluted with 1 mL of toluene/methanol/ dichloromethane (3:3:4 v/v/v) solution. Glassware was used to handle and transfer the solvent, except for the stainless steel pistons in the 100 μL Hamilton syringes. 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 3.0 kV emitter voltage, 3.5 kV capillary entrance 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 0.3 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 m/z 200 900. Four MW data sets were acquired. Sixty-four scans were coadded to enhance signal-to-noise ratio. A square sine bell multiplication apodization was carried out, followed by a single zero-fill before fast Fourier transform and magnitude calculation.47 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.49,50 Measured masses were converted from the IUPAC mass scale to the Kendrick mass scale. The Kendrick mass defect (KMD) was calculated.51 Molecular formulas of two neighboring even and odd normal masses were assigned on the basis of mass measurement to (1 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 3015

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Figure 2. GC-PFPD chromatograms of VGO-1 to VGO-4 (1: C1 benzothiophene (BT); 2: C2 BT; 3: C3 BT; 4: >C3 BT; 5: dibenzothiophene (DBT); 6: C1 DBT; 7: C2 DBT; 8: gC3 DBT). Figure 4. Narrow segment of the ultrahigh-resolution mass spectrum in Figure 3 at a representative odd mass m/z 395. Note: the peak marked with an asterisk was not identified.

Figure 3. Broadband positive-ion ESI FT-ICR mass spectra of methylsulfonium salts derived from VGO-1 to VGO-4. Samples were analyzed at the equivalent concentration and instrumental operating conditions. program limited to molecular formulas consisting of up to 100 12C 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-PFPD Analysis. GC-PFPD analysis was performed using the BF3420 GC (Beijing Analytical Instruments Company) equipped with a PFPD detector (OI Analytical 5380). The sample was injected into a capillary column (DB-1MS, 30 m  0.25 mm  0.25 μm) and was held for 3 min at 50 °C. The oven temperature was increased to 300 at 10 °C/min and then held at 300 °C for 30 min. The injector temperature was 280 °C.

’ RESULTS AND DISCUSSION GC-PFPD Analysis of VGO Fractions. Figure 2 shows the GCPFPD chromatograms of the four VGO subfractions. The GCPFPD spectrum of VGO-1 exhibited a small hump with poorly resolved groups of peaks. This indicates that low molecular weight sulfur compounds were present, such as alkyl substituted benzothiophenes and dibenzothiophenes, similar to those found in straight run middle distillates.52 The GC-PFPD spectra of VGO-2 and VGO-3 had a larger and wider hump as the VGO fraction became heavier (higher boiling point). This indicates that more complex sulfur compounds were present in the heavy VGO fractions. The initial boiling point of VGO4 was above 350 °C (see Figure 1), which is higher than the operating temperature of the GC. Hence, the sulfur compounds in VGO-4 were

not volatile and could not be identified by GC-PFPD analysis. This result confirmed that GC-PFPD analysis is not suitable for analyzing sulfur compounds in heavy petroleum fractions. Positive-Ion ESI FT-ICR MS. Figure 3 shows the broadband positive-ion ESI FT-ICR mass spectra of methylsulfonium salts derived from VGO-1 to VGO-4. The most abundant peaks of VGO-1, VGO-2, VGO-3, and VGO-4 were at m/z 310, m/z 370, m/z 440, and m/z 520, respectively. The ESI FT-ICR mass spectra show that the molecular weight of sulfur compounds increased with increasing boiling point of the VGO fraction. This is in agreement with previous FT-ICR MS analyses.20,53 55 Figure 4 shows expanded mass scale at m/z 395 of FT-ICR mass spectra shown in Figure 3. The mass resolving power at m/z 395 was greater than 530 000 (m/Δm50%, in which Δm50% is the magnitude-mode mass spectral peak full-width at half-maximum peak height). The achieved mass resolving power at m/z 700 was 300 000 for VGO-4, which was sufficient to separate the 3.4 mDa split (the mass difference differentiating C3 from SH4, see Supporting Information Figure S-1). Elemental compositions of these compounds can be identified by accurate mass analysis. The various sulfur compounds identified include S1, S2, and O1S1 class species. The S1 class species were the most abundant (see Supporting Information Figure S-2). The data in Figure 4 also show that the DBE values of the sulfur compounds increased with increasing boiling point temperature of VGO fraction. Distribution of Sulfides and Thiophenes. Sulfides in petroleum fractions can be selectively oxidized into sulfoxides using tetrabutylammonium periodate (TBAPI) to form sulfoxides without affecting the thiophenes. The fractions can then be analyzed by positive-ion ESI FT-ICR MS.47 The mass spectra obtained can be used to map the DBE and carbon number distribution of sulfoxides. These in turn, reveal the composition characteristics of sulfides in petroleum fractions. However, the positive-ion ESI FT-ICR MS analysis showed that sulfoxides were present in the unreacted VGO subfractions (O1S1 class species, see Supporting Information Figure S-3). Therefore, to validate that O1S1 class species were derived from sulfides, the positive-ion ESI FT-ICR MS analyses were performed on the methylsulfonium salts43 samples before and after the oxidation reactions. Figure 5 shows the broadband FT-ICR mass spectrum of the methylsulfonium salts derived from VGO-2 before and after 48 h 3016

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Figure 5. Broadband positive-ion ESI FT-ICR mass spectra of methylsulfonium salts derived from VGO-2 before and after 48 h selective oxidation. The insert shows the expanded mass scale at m/z 381. The samples were analyzed at the equivalent concentration and instrument operating conditions. Note: the peak marked with an asterisk was not identified.

Figure 6. Plots of double bond equivalence (DBE) as a function of carbon number for the S1 class species derived from positive-ion ESI FT-ICR mass spectrum of the methylsulfonium salts derived from VGO subfractions before and after oxidation. The largest dots denote the most abundant S1 class species in the sample; “a” and “b” are methylsulfonium salts before and after oxidized VGO fractions.

of oxidation reactions. The mass spectra had a similar molecular weight range of 250 500 Da for the samples before and after oxidation. The insert in Figure 5 shows the expanded mass scale at m/z 381. The sulfide compound C25H49S1 with 2 DBE in the unreacted sample almost disappeared after oxidation, indicating that sulfides were oxidized to sulfoxides. A peak assigned as C24H45O1S1 with 3 DBE was in the mass spectrum of the oxidized sample. The peak assigned as C26H37S1 with 9 DBE was thiophenic compound. This compound, which appeared in the methylsulfonium salts of unreacted and oxidized samples, is refractory to oxidation reactions. Unique composition formulas were assigned to the spectral peaks on the basis of accurate mass values and the corresponding homologue series. Figure 6 shows the relative abundance maps of DBE as a function of carbon number for the S1 class species in the methylsulfonium salts derived from four VGO subfractions before and after oxidation reactions. The carbon numbers on the x-axis of Figure 6 include all of the carbon atoms for each sulfur compound molecule in the derivatization groups of the methylated oil samples. These had one additional carbon atom compared to the molecules in the original oil samples. For the

unreacted VGO subfractions, as the boiling point of VGO subfraction increased, the carbon numbers of the most abundant sulfur compounds increased from 15 25 carbon numbers for VGO-1 to 27 45 for VGO-4. Similarly, the DBE of the most abundant S1 class species shifted to higher values as the boiling point of VGO subfraction increased. The plot of DBE vs carbon number of the S1 class species in the methylsulfonium salts derived from the unreacted VGO-1 shows that the S1 class species had a bimodal distribution with maxima at DBE values of 2 5 and 6 7 over the carbon number range of 15 25. This suggests that the S1 class species had two main types of molecular core structures; likely sulfide and thiophenic. For the methylsulfonium salts derived from the oxidized VGO-1, the most abundant S1 class species with DBE values of 6 and 9 are likely benzothiophenes (BTs) and dibenzothiophenes (DBTs), respectively.56 However, the low relative abundance S1 class species with carbon number greater than 20 and DBE value less than 6 are likely sulfides. The sulfur compounds with 0 DBE were aliphatic sulfides. Those with DBE values of 1 and 2 were one and two cyclic-rings sulfides, respectively. The S1 class species with DBE values of 1 and 2 were also identified in 3017

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Energy & Fuels the methylsulfonium salts derived from the oxidized sample. The presence of sulfides in the oxidized sample is likely due to incomplete oxidation of sulfides to sulfoxides despite the long reaction time.47 The S1 class species with 3 DBE are likely tricyclic-rings sulfides. Because tricyclic-ring sulfides and alkyl thiophenes have the same DBE values, the distribution of relative abundance of these species overlapped each other in the DBE vs carbon number plots.45,47,54 However, the S1 class species with 3 DBE, which were abundant in the methylsulfonium salts derived from the unreacted VGO-1, were relatively scarce in the salts derived from the oxidized VGO-1. This suggests that alkyl thiophenes were not present in VGO-1, which is in agreement with a previous finding.57 The S1 class species with DBE values of 4 or greater are likely cyclic-rings or aromatic sulfides. For the remainder of the high boiling point VGO subfractions, the relative abundances of S1 class species with DBE values of less than 6 in the methylsulfonium salts derived from the oxidized samples were significantly low. The relative abundance of S1 class species with DBE values of 6 and greater in the methylsulfonium salts derived from the unreacted and oxidized samples had similar distribution patterns. 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 sulfides. However the DBE and carbon number distribution of thiophenes in these three VGO subfractions were significantly different. The most abundant thiophenes in VGO-2, VGO-3, and VGO-4 had 6 7 DBE and 20 30 carbon number, 6 10 DBE and 22 35 carbon number, and 6 13 DBE and 25 45 carbon number, respectively. Benzothiophene has 6 DBE. The addition of one and two fused aromatic rings to benzothiophene yields dibenzothiophene with 9 DBE and benzonaphthothiophene with 12 DBE, respectively. In VGO-3 and VGO-4, another abundant S1 class species had 7 8 DBE. These compounds are likely benzothiophenes with one and two fused cycloalkane rings. The S1 class species with DBE values of 10 and 11 are likely the benzo homologues with 7 and 8 DBE, respectively.

’ CONCLUSIONS Sulfide and thiophenic compounds in four subfractions of Kazakhstan VGO were subjected to chemical derivatization and characterized by positive-ion ESI FT-ICR MS. The S1 class species were the most abundant sulfur compounds in the four VGO subfractions. The S1 class species with DBE values of 6 and greater are likely thiophenes, while those with DBE values less than 6 are sulfides. Sulfide compounds were abundant in low boiling VGO subfractions. As boiling point of VGO increased, the abundance of thiophenic compounds increased, along with their DBE value and carbon number. The minimum core structure of thiophenic compounds present in the four VGO subfractions were benzothiophenes with 6 DBE. Thiophenes with 3 DBE were not present in these samples. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S-1 to S-3. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: 8610-8973-3738. Fax: 8610-6972-4721. E-mail: [email protected]. cn (Q. S.); [email protected] (C. X.).

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