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Separation and Characterization of Sulfoxides in Crude Oils Limin Ren, Jianxun Wu, Qian Qian, Xuxia Liu, Xianghai Meng, Yahe Zhang, and Quan Shi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03494 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019
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State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
ABSTRACT: A good understanding of the chemical composition of organic sulfur compounds (OSCs) in petroleum is necessary to develop suitable hydrogenation catalysts and optimize desulfurization refining processes. In this study, sulfur species in crude oils derived from different geological sources were characterized by Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The O1S1 class species (with one oxygen and one sulfur atoms in the molecule) were strongly responded in positive-ion ESI MS and were considered as sulfoxides. These compounds were separated into polar fractions by chromatographic separation and reduced by LiAlH4 to obtain corresponding sulfur compounds. The O1S1 class species were confirmed as sulfoxides, which have similar distribution in carbon number and double bond equivalent (DBE) to sulfides in the oils. However, molecular composition of sulfoxides in different crude oil were largely different. Polar sulfur compounds amount to a considerable proportion (>27%) of total sulfur for all investigated crude oils, while sulfoxides just occupied a small proportion (0.08% to 5.9% of total sulfur for the investigated oils) of the polar sulfur compounds. Other polar sulfur compounds were found resistant to hydrodesulfurization and should be concerned in the following studies.
1. INTRODUCTION Organic sulfur compounds (OSCs) are the most common heteroatom species found in petroleum and exist in a variety of chemical structures, generally described as mercaptans, disulfides, sulfides, thiophenes, sulfoxides, sulfones. They must be reduced to a low level in refining operations to produce clean transportation fuels. Good understanding of the chemical composition and transformation mechanism of OSCs is necessary to develop suitable hydrogenation catalysts and optimize desulfurization refining processes.[1] Generally, sulfides and thiophenes are the major forms of OSCs in crude oils. Numerous methods have been developed for the separation[2-5] and characterization[6-15] of sulfides and thiophenes since 1960s, which lead to a good understanding of them both in the molecular structures and sulfur content.[2, 3, 5, 9, 16-26] Sulfoxides are another form of OSCs and ubiquitous in petroleum, but these compounds are often neglected during OSCs analysis. The main reason is that only a few sulfoxides in petroleum can be directly detected by gas chromatography coupled with sulfur selective detection or mass spectrometry (MS) due to complex matrix interferences (including hydrocarbons, sulfides and thiophenes, et al.),[27]resulting that knowledge of the molecular composition of sulfoxides is limited. Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is a powerful tool to access polarizable compound classes in complex mixtures and has been widely applied to characterize the molecular composition of petroleum fractions.[28-32] Acidic molecular species (e.g., carboxylic acid) and basic species (e.g., pyridinic nitrogen) can be selectively ionized by ESI in the negative-ion and positive-ion mode, respectively. sulfoxides are weak bases, therefore, can be ionized in positive-ion electrospray.[33] In the past 20 years, various petroleum samples including crude oils,[31, 34-37] syncrude oil[38], diesel,[33] VGO,[39] resins and asphaltenes,[18] have been characterized by positive-ion ESI FT-ICR MS. The results suggest that the O1S1 class species, protonated or sodiated, ubiquitously existed and generally present high relative abundance, especially in crude oils and distillates. Hughey et al.[33] found that the O1S1 class species showed high relative abundance in positive-ion ESI FT-
ICR mass spectra of diesel fuels, and the OS−OS dimers were easily formed at high sample concentration. However, almost all the published works focused on basic nitrogen compounds and the compositions of the O1S1 class species were rarely discussed. Moreover, the relative abundance of the O1S1 class species cannot reflect their actual sulfur content in the original samples due to the competitive ionization of different heteroatom class species, thus there is also no valuable quantitative data for the O1S1 class species. In addition, the O1S1 class species are generally considered to be sulfoxides according to their low double bond equivalence (DBE) values, but the chemical structure of the O1S1 class species has not been unambiguously identified. Chromatographic methods can give valuable information for precise chemical identification and quantification. Sulfoxides have been separated from petroleum fractions by cation-exchange resin chromatography.[40] However, the molecular composition is not well characterized and the experimental procedure is laborious and time-consuming. Considering the large difference in the polarity of sulfoxides, sulfides and thiophenes, sulfoxides in crude oils can be theoretically isolated by liquid chromatography when suitable adsorbent was used. Silica gel and alumina are commonly used adsorbents. However, in silica and alumina adsorption chromatography, the separation of sulfides, thiophenes and sulfoxides is poor and the total sulfur recovery is usually low due to the strong adsorption of silica and alumina.[18] The purpose of this work is to investigate the chemical composition of polarizable sulfur species, especially the sulfoxides in crude oils. FT-ICR MS was used to analysis the sulfur compound composition and trace their whereabouts in the separation process.
2. MATERIALS AND METHODS 2.1 Samples and Reagents. Three high-sulfur crude oils including a Kuwait crude oil, a Venezuela heavy oil and a China crude oil were used in this study. The former two crude oils are of marine origin and the last one is of lacustrine origin. The detailed information refers to Table 1. Analytical grade n-hexane (nC6), toluene (Tol), methanol (MeOH) and dichloromethane (DCM) were obtained from Beijing
Chemical Reagents Company, which were distilled for purification before use. Silver tetrafluoroborate (AgBF4), methyl iodide (MeI), lithium aluminium hydride (LiAlH4) and dioxane were obtained from J&K Chemical Ltd. Dioxane was distilled to remove water before use. TLC grade diatomite (CAS-No.: 91053-39-3) was obtained from Aladdin Inc. and cleaned using Soxhlet extraction with methanol for 24 h before use. 2.2 Separation and Identification of the O1S1 Class Species. The procedure for separation of OSCs from crude oils by diatomite extrography is shown in Figure 1. Thirty milligram of crude oil was dissolved in 8 mL of DCM in a 50 mL round bottom flask, followed by addition of 4 g purified diatomite. The mixture was dried by a rotary evaporator and the crude oil was evenly dispersed on diatomite after DCM completely removed; then the dried mixture was packed into a 1-cm-o.d. glass column. Fifty milliliters each of n-hexane, toluene/ methanol mixture (4:1, v/v), and methanol were successively passed through the column with eluent flow rates of 1−2 mL min−1 and the OSCs in crude oil were separated into three fractions. The criterion for the switching of each fraction is that the eluate becomes colorless. Here, we define Fraction 1 as nonpolar sulfur fraction eluted by n-hexane; Fraction 2 as polar sulfur fraction eluted by toluene/methanol mixture (4:1, v/v); and Fraction 3 as residue eluted by methanol. The solvent type and amount have been optimized to separate sulfoxides, as well as other polar sulfur species from non-polar sulfur species such as sulfides and thiophenes. Being high polarity, the O1S1 class species were eluted in Fraction 2. To confirm whether the O1S1 class species correspond to sulfoxides or not, the Fraction 2 was subjected to LiAlH4 reduction. The Fraction 2 was concentrated by a rotary evaporator and placed in a 50-mL round-bottom flask, and then 60 mg of LiAlH4 and 15 mL of dried dioxane were added to the flask. After being equipped with a reflux condenser, the mixture was stirred with a magnetic bar and refluxed for 1 h. At the end of the reaction, the reaction flask was cooled in ice-water and the excess LiAlH4 was destroyed by the cautious addition of water (10 mL) followed by n-hexane (15 mL), toluene (10 mL), and a few mL of hydrogen chloride aqueous solution (2 N). The resulting mixture was stirred at room temperature for ca. 15 min, after which the solution became a suspended emulsion which consists
of an organic phase and an aqueous phase. The layers were separated by centrifugation and the organic phase was collected. The remaining aqueous phase was extracted with another 15 mL of n-hexane and 10 mL of toluene, and the organic phase was collected and combined with the above organic phase. This procedure was repeated once again.[3] The combined organic phase was labeled as Reduction products as shown in Figure 1. The reduction products were further separated by diatomite extrography. Same with the procedure for separation of OSCs from crude oils, elution with n-hexane to obtain non-polar sulfur compounds which was named as Sub-Fraction 1; elution with toluene/methanol mixture (4:1, v/v) to obtain polar sulfur compounds, which was named as Sub-Fraction 2. All fractions mentioned above were concentrated by a rotary evaporator and followed by readjusting to a final volume of 5 mL in toluene for ESI FT-ICR MS and elemental analysis. The sulfur content of crude oils and all fractions was determined using an ANTEK 7000 pyrofluorescence analyzer (ANTEK Instruments) according to the ASTM D5453 method. The calculation of fraction yield in total sulfur for each fraction was described in Table S1 (see Supporting Information). 2.3 FT-ICR MS Analysis. All samples were analyzed using a Bruker Apex ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet. Nonpolar sulfur fractions were methylated before FT-ICR MS analysis. The procedure for the methylation of sulfur compounds has been described elsewhere.[5] The crude oils and all fractions were introduced as a solution in toluene/methanol (v/v: 1:1 for positive-ion; 1:3 for negative-ion) via an Apollo II electrospray at 250 μL/h using a syringe pump. Ions were accumulated for 0.01 s in a hexapole with 2.4 V DC voltage and 300 Vp-p RF amplitude. The optimized mass for Q1 was 250 Da. The extraction period for ions from the hexapole to the ICR cell was set to 1.1 ms. The rf excitation was attenuated at 13.75 dB and used to excite ions over the range of m/z 200-1000. The FT-ICR data were acquired in 4M word with a transient time about 2.3 s, resulting in a resolving power of roughly 500,000 at m/z 400. The signal-
to-noise ratio and dynamic range were improved by summing 64 times of domain transient for each mass spectrum. Internal and external mass calibration were performed using an identified mass series in the samples covering a mass range from 200 to 800 Da, such as Na1O1S1 and/or N1, which are present in relatively high abundance in the mass spectra. After calibration, the mass accuracy of all samples is less than 0.5 ppm and the plot of error versus m/z of Kuwait crude oil obtained from positive-ion ESI FT-ICR mass spectrum was shown in Figure S1 in the supporting information. Data analysis was performed using a custom software. The detailed data processing has been described elsewhere.[41]
3. RESULTS AND DISSCUTION 3.1 Composition and Distribution of O1S1 Class Species in Crude Oil and Its Fractions. Three crude oils derived from different geological sources were studied. The geological information, total sulfur contents, sulfur recovery yields in all fractions are listed in Table 1. The data for the Kuwait oil was averaged by three repeated experiments and the relative standard deviations were also listed in Table 1. The yields were calculated based on the measured sulfur content.
Figure 2 shows the broadband and expanded positive-ion ESI FT-ICR mass spectra of the Kuwait crude oil and its three fractions. Segments of m/z 407 and 408 were randomly selected to show the compositional difference among the samples partially because they are located in the central area of the spectra. As illustrated in Figure 1, fractions were eluted from a diatomite column by different solvents. Since diatomite has weak adsorption, the separation mainly based on the solubility of the solvents. Scattered mass peaks in the spectrum of Fraction 3 indicates that the components in this fraction cannot be ionized by positive-ion ESI. The profiles of the broadband mass spectra of the crude oil and Fraction 1, Fraction 2 showed similar distribution in a range of m/z 200-700. However, the expanded mass spectrum segments (Figure 2(b) and 2(c)) exhibited the
different composition of these samples and the successful separation of O1S1 class species from nitrogen-containing species. Figure 3 shows the summarized relative abundance of the major class species assigned from the positive-ion ESI mass spectra. The Na1O1S1 class species showed a very high relative abundance, these compounds should be sodium adducts of O1S1 compounds. It was known that polar compounds are prone to produce sodium products in positive-ion ESI MS analysis. For example, sodium adducts have been observed for petroporphyrins[42] and sulfoxides[33]. Figure S2 shows the DBE versus carbon number distributions for the Na1O1S1 and O1S1 class species in Kuwait crude oil. The distributions of carbon number and DBE value of Na1O1S1 class species are quite similar to that of O1S1 class species. Therefore, the Na1O1S1 class species should be the sodium adducts of the O1S1 class species. The purpose of the separation is to separate sulfoxides, as well as other polar sulfur species from non-polar sulfur species such as sulfides and thiophenes, which are dominant in petroleum fractions. Non-polar sulfur compounds cannot be ionized in conventional ESI analysis, to confirm whether sulfides and thiophenes were co-eluted into Fraction 2, methyl derivatization were performed on the three isolated fractions prior to positive-ion ESI FT-ICR MS analysis. Figure 4 (a) and 4 (b) shows the broadband and expanded positive-ion ESI FT-ICR mass spectra of the methyl derivatization products of the three fractions. The results from expanded mass spectra at m/z 407 clearly showed that the S1 and S2 class species were isolated into Fraction 1, and the O1S1 class species were enriched into Fraction 2. The S1 and S2 class species should correspond to sulfides and thiophenes; the O1S1 class species should correspond to Na1O1S1 class species found in the spectra of the samples before methylation. The results were further confirmed by the relative abundance of the S1, S2 and O1S1 class species assigned from the mass spectrum of Fraction 1 and Fraction 2 as shown in Figure 4 (c) and 4 (d). In summary, the sulfur compounds in the Kuwait crude oil can be clearly separated into three
fractions by diatomite extrography. Nonpolar sulfur compounds including the S1, S2 and N1S1 class species (corresponding to conventional and basic nitrogen-containing sulfides and thiophenes) were eluted with n-hexane into Fraction 1; the O1S1 class species were eluted with a mixture Tol/MeOH (4:1, v/v) into Fraction 2 almost with sulfides and thiophenes free; Fraction 3 were eluted with MeOH into Fraction 3, defined as residue with few mass peaks.
3.2 Chemical Reduction of the O1S1 Class Species. Sulfoxides are weak bases and can be directly ionized in positive-ion ESI FT-ICR MS analysis,[33, 40, 43] therefore, the most probable structure of O1S1 class species presenting in positiveion ESI mass spectra should be sulfoxides, but the chemical structure of the O1S1 class species has not been unambiguously identified. To confirm whether the O1S1 class species in Fraction 2 exists in the form of sulfoxides or not, Fraction 2 was subjected to a reduction by LiAlH4 treatment, which can selectively convert the polar sulfoxides to the corresponding non-polar sulfides and thiophenes. Figure 5 shows the broadband and expanded (odd mass scale; the even mass scale was showed in Figure S3) positive-ion ESI FT-ICR mass spectra of Fraction 2 before (top) and after reduction with LiAlH4 (middle) and the methylation products of the reduction products (bottom). Both the broadband and expanded mass spectra obviously showed that the mass peaks of Na1O1S1 class species in Fraction 2 almost disappeared after LiAlH4 treatment, indicating that the Na1O1S1 class species were almost quantitatively converted to other forms, which cannot be ionized by positiveion ESI. After methylation of the reduction products, the S1 class species emerged, which should correspond to sulfides and/or thiophenes. Since sulfides and thiophenes are almost absent in Fraction 2 as shown in Figure 4, thus the sulfides and/or thiophenes are the reduction products of the Na1O1S1 class species. Therefore, the results confirmed that the Na1O1S1 (O1S1) class species in Fraction 2 existed in the form of sulfoxides.
Figure 6 shows the relative abundance plots of DBE versus carbon number for Na1O1S1 class species of Kuwait crude oil and Fraction 2, and the plot of DBE versus carbon number for S1 class species of methyl derivatization products of the reduction products of Fraction 2. The Na1O1S1 class species in the Kuwait crude oil had DBE values of 1-13 and carbon numbers of 10-50. The most abundant Na1O1S1 class species had DBE values varying from 1 to 7. DBE values of 1-3 should be 1 to 3 rings aliphatic sulfoxides, while DBE values of 4-7 probably correspond to polycyclic aliphatic sulfoxides or aromatic sulfoxides. The distribution of the S1 class species in the methylated reduction products is quite similar to that of the Na1O1S1 class species in both the crude oil and Fraction 2. The results indicate that not only the O1S1 (mainly sodium adduct) are sulfoxides, but also the discriminations on various sulfoxide compounds in the separation and LiAlH4 reduction processes are negligible.
3.3 Molecular Composition of Sulfoxides in Crude Oils. Two additional crude oils (from Venezuela and China, respectively) with high sulfur content but different geological sources were also characterized by positive-ion ESI FT-ICR MS. Figure 7 showed the plots of DBE versus the carbon number for the Na1O1S1 class species of the Kuwait, Venezuela and Chinese crude oil. As discussed above, the Na1O1S1 (O1S1) class species in the positive-ion ESI mass spectra mainly correspond to sulfoxides. Large differences in DBE distribution of sulfoxides from the three crude oils were observed. Compared with the Kuwait crude oil, sulfoxides in the Venezuela crude oil mainly existed in the form of aliphatic sulfoxides with 1 to 3 rings (DBE=1, 2, 3), but the distribution of sulfoxides in the Chinese crude oil is special: except for the occurrence of naphthenic sulfoxides with 1 to 2 rings (DBE=1, 2), the sulfoxides with DBE value of 5 and carbon number of 28-30 present very high relative abundance, these compounds should be sterane sulfoxides, because corresponding sterane sulfides have been identified in the oil (will be presented in another paper). The compositional difference can be explained by their different depositional environment. The Kuwait and Venezuela crude oil were of marine
depositional environment while the Chinese crude oil was of lacustrine. The major difference between the Kuwait and the Venezuela crude oil was the maturity, for which the Kuwait oil was more mature and exhibited a higher condensation degree. The Chinese crude oil was in low maturity, in which the sulfur compounds were generated from bacteria sulfate reduction (BSR)[44, 45]
instead of thermochemical sulfate reduction (TSR). The molecular composition of sulfides and
thiophenes in the three crude oils as shown in Figure S4 (see Supporting Information) also supports the interpretation.
3.4 Quantitative Analysis of Sulfoxides in Crude Oils. Although sulfoxides are recognized as a critical type of sulfur-containing compounds in crude oils, the content, as well as the molecular composition, are not clear due to the lack of adequate testing methods and the immense petroleum complexity. The positive-ion ESI MS demonstrates that sulfoxides comprise a wide range in carbon number and DBE values; this distribution largely varies as a function of geological origin. However, the content of sulfoxides is unknown because it is difficult to obtain quantitative results based on the ESI-MS analysis. On the other hand, bulk elemental analysis is well suited for quantification. The sulfur content of the crude oils and their fractions were determined and listed in Table 1. The summed sulfur recoveries in the three fractions are more than 98% for all samples, indicating that almost all sulfur compounds were eluted out from the diatomite column. The sulfur content of sulfoxides listed in Table 1 is equivalent to that of Sub-Fraction 1 (as shown in Figure 1), implies sulfoxides were converted to sulfides and/or thiophenes by reduction with LiAlH4 in high yields, which is supported by the results shown in Figure 5 and Figure S5 (see Supporting Information). Sulfides and thiophenes reduced from sulfoxides were separated into Sub-Fraction 1 (as shown in Figure 1) by diatomite extrography. As shown in Table 1, the polar sulfur compounds in Fraction 2 account for a large proportion of the total sulfur. We also evaluated other six crude oils with various sulfur content and geological origins (see Table S1 in Supporting Information). Sulfur in polar fractions account for about 27-46
% of the total sulfur. However, although the O1S1 class species present high relative abundance in positive-ion ESI FT-ICR mass spectra, the sulfur content existed in the form of sulfoxide only accounts for 0.8% to 5.7% of the total sulfur, which is consistent with the results measured by spectroscopic methods, such as infrared spectrum (IR),[40] X-ray photoelectron spectroscopy (XPS)[46, 47] and X-ray absorption near-edge structure (XANES)[48]. In addition, the sulfur content of sulfoxides in crude oils is not proportional to the total sulfur. The Kuwait and Venezuela crude oils have a higher content of sulfoxides than that of the Chinese crude oil, implying that these crude oils may have undergone more serious oxidation in the sedimentary environment. In addition, the crude oils have been stored for 2 years before analysis, thus the sulfur content of sulfoxides could be higher than the original value due to the sulfoxides formation by the photo-oxidation and air oxidation of sulfur compounds during storage.[40, 49] Except for sulfoxides, most of polar sulfur compounds in Fraction 2 are hardly to be ionized by positive-ion ESI. Figure 8 shows the broadband and expanded negative-ion ESI FT-ICR MS mass spectra of the Kuwait crude oil and its fractions before and after LiAlH4 reduction. Sulfur containing class species of O1S1, O2S1, O3S1 and O4S1 were detected in the crude oil. The O1S1 class species should be thiophenic or sulfidic sulfur compounds with a hydroxyl function group according to the fact that they ionized by negative ESI and eluted into the Fraction 1 with the O1 class species. The O2S1, O3S1 and O4S1 class species were enriched in Fraction 2 in the separation. After LiAlH4 reduction, the relative abundance of O2 and O2S1 were reduced (see Figure S6 in Supporting Information), while the O3S1 and O4S1 class species were resistant to the reduction. The results indicate that the sulfur species have different chemical structures, but it cannot be identified just based on the ESI MS results. DBE=0 series of O3S1 and O4S1 were detected in the reduction product, which implies sulfonic acids and/or organic sulfates are existing in the oil. In addition, the reduction of these oxygen containing compounds could make the quantitative result of sulfoxides higher than the real value. We have carried out the hydrotreating experiments of the crude oil and its fractions. Results (shown in Table S2 in Supporting Information) showed that the non-sulfoxide polar sulfur species
are most resistant to the hydrodesulfurization. Following studies should focus on the molecular composition and structure of these compounds, especially a quantitative evaluation.
4. CONCLUSIONS Researches relevant to the molecular composition of polar sulfur compounds in petroleum were carried out based on separation and high-resolution mass spectrometry analysis. The results showed that polar sulfur species amount to a large proportion (generally more than 1/3) of sulfur compounds in the crude oils from various regions of the word. Sulfoxides were found enriched in the polar fraction and can be analyzed by positive ion ESI FT-ICR MS. By LiAlH4 reduction, sulfoxides were quantitatively characterized and the result showed that these sulfur compounds just occupied a small proportion of the polar sulfur fraction (0.08% to 5.9% of total sulfur for the investigated oils). Sulfoxides have similar distribution in carbon number and double bond equivalent (DBE) to sulfides in the oils. However, molecular composition of sulfoxides in different crude oil were largely different. Polar sulfur compounds other than sulfoxide were resistant to hydrodesulfurization, of which molecular composition should be concerned in the following studies.
ACKNOWLEDGMENTS The research is supported by the National Natural Science Foundation of China (NSFC 41773038) and the Guangxi Key R&D Plan (No. 2017AB54014) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: +86 10 89739157 ORCIDs Yahe Zhang: 0000-0003-2573-568X Quan Shi: 0000-0002-1363-1237
Notes: The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information Figures of: plot of error versus m/z of Kuwait crude oil obtained from positive-ion ESI FTICR mass spectrum; relative abundance plots of DBE versus carbon number for Na1O1S1 and O1S1 class species of the Kuwait crude oil; Expanded positive-ion ESI FT-ICR mass spectra of Fraction 2 before and after reduction with LiAlH4 and the methyl derivatization products of the reduction products; relative abundance plots of DBE versus carbon number for the S1 class species of methyl derivatization products of the crude oils; expanded positive-ion ESI FT-ICR mass spectra of Fraction 2 and its subfractions; relative abundance plots of DBE versus carbon number of the Na1O1S1 class species of Fraction 2; relative abundance of heteroatom classes assigned from the negative-ion ESI FT-ICR MS mass spectra. Tables of: total sulfur content of crude oils and the sulfur yields of separated fractions; the removal rate (%) of sulfur of Venezuelan crude oil and its fractions after hydrotreating experiments. REFERENCES 1.
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Figure 1. Flow diagram for the separation of the sulfur compounds in crude oils (top). Note: the recovery yields of sulfur in all separation fractions for the Kuwait crude oil were listed in the flow diagram. The reaction scheme of LiAlH4 treatment (bottom).
Figure 2. Broadband (a) and expanded (b and c, the intensity of all peaks was normalized at the same scale, respectively) positive-ion ESI FT-ICR mass spectra of the Kuwait crude oil and its isolated fractions. Asterisks (*) denote species containing one Discontinued spiked peaks in (a) should be contaminants.
Figure 4. Broadband (a) and expanded (b) positive-ion ESI FT-ICR mass spectra of methyl derivatization products of the three isolated fractions from the Kuwait crude oil; relative abundance of various classes derived from the spectrum of Fraction 1 (c) and Fraction 2 (d).
Figure 5. Broadband (left) and expanded (right) positive-ion ESI FT-ICR mass spectra of Fraction 2 before (top) and after (middle) reduction with LiAlH4 and the methyl derivatization products of the reduction products (bottom).
Figure 6. Relative abundance plots of DBE versus carbon number for the Na1O1S1 class species of the Kuwait crude oil and Fraction 2, and the plot of DBE versus carbon number for the S1 class species of methyl derivatization products of the reduction products of Fraction 2.
Figure 7. Relative abundance plots of DBE versus carbon number for Na1O1S1 class species of the Kuwait crude oil, Venezuela crude oil, and Chinese crude oil.
Figure 8. Broadband (left) and expanded (right) negative-ion ESI FT-ICR mass spectra of the Kuwait crude oil, Fraction 1, and Fraction 2 before and after reduction with LiAlH4. Note: the intensity of all peaks was normalized at the same scale.
Table 1. Bulk properties and total sulfur content of crude oils and the sulfur recovery yields in all isolated fractions. Crude Oils Kuwait Boscan Bohai a Calculated
Depositional environment Kuwait Marine Venezuela Marine China Lacustrine Origin
Total S wt % 3.74 5.43 4.72
recovery yield Recovery yield =
b Each c
Fraction 1a Fraction 2 a %S %S b c 69.25 ±0.56 27.50b±1.13c 66.85 31.86 59.79 40.98
Sulfur content of each fraction * 100% Sulfur content of crude oil
element recovery yield is the average of three replicates determination.
Relative standard deviation of three replicates determination.
