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Quantitative Molecular Characterization of Petroleum Asphaltenes Derived Ruthenium Ion Catalyzed Oxidation (RICO) Product by ESI FT-ICR MS Xibin Zhou, Suoqi Zhao, and Quan Shi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02533 • Publication Date (Web): 20 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016
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Energy & Fuels
Quantitative Molecular Characterization of Petroleum Asphaltenes Derived Ruthenium Ion Catalyzed Oxidation (RICO) Product by ESI FT-ICR MS
Xibin Zhou†‡, Suoqi Zhao†, Quan Shi†* † State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249 China ‡ College of Basic Science, Liaoning Medical University, Jinzhou, Liaoning 121001, China
Abstract Molecular structure of heavy petroleum could be investigated by the composition of its ruthenium ion catalyzed oxidation (RICO) products. However, the interpretation of the results was not comprehensive due to the limited compositional information obtained solely by gas chromatography (GC) analysis. In this study, a semi-quantitative method based on electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was established and applied for the molecular characterization of RICO products. Thousands of polar compounds were detected by negative ion ESI FT-ICR MS in the RICO products of the Canadian oil sands bitumen derived asphaltenes. Besides alkyl carboxylic acids, naphthenic acids with 1-5 naphtha rings, nitrogen and sulfur-containing carboxylic acids, and acidic compounds with multi-oxygen atoms were observed. The upper carbon number limit of alkyl moieties connected to the aromatic cores of the asphaltenes was found up to 60, which is much higher than the results derived from GC analysis. Normal and isomer alkyl carboxylic acids, as well as naphthenic acids were quantitatively analyzed separately. The quantitative results of alkyl carboxylic acids from ESI FT-ICR MS were well agree with the GC results. The FT-ICR MS results indicate that additional compositional information could be obtained from RICO analysis. In addition, the method is instructive for the development of quantitative analysis technology for petroleum molecular characterization based on ESI FT-ICR MS.
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1. Introduction Heavy fossil fuels, such as heavy petroleum, oil sand bitumen, coal, etc., are important energy resource of the world. The molecular structures of fossil fuels is an essential study issue for chemists, since the physicochemical properties and reaction behavior of these complex mixtures are deeply correlated to their composition and chemical structure.1 Most modern instrumental analysis techniques have been used for the composition and/or structure analysis of heavy fossil fuels, such as nuclear magnetic resonance (NMR)2, 3, high resolution transmission electron microscopy (HRTEM)4, X-ray absorption near-edge structure (XANES)5, 6, X-ray diffraction (XRD)2, fluorescence correlation spectroscopy (FCS)7, size exclusion chromatography (SEC)8, low-temperature atomic force microscopy,9 and mass spectrometers with various ionization techniques.10 One of the well known chemical methods used for molecular characterization of heavy fossil fuels is ruthenium ion catalyzed oxidation (RICO).11 RICO reaction can selectively oxidize aromatic carbon to CO2 while leave saturated carbon essentially unaffected.11 Typical reactions of RICO are illustrated in Scheme 1.
11
Aromatic-attached aliphatic appendages are converted to their carboxylic acid derivatives, the aromatic carbon at the site of attachment is oxidized to carboxylic carbon. Polycyclic aromatic compounds afford benzene mono- to penta- carboxylic acids due to the electron-withdrawing property of carboxyl group prevent the oxidation of the last aromatic ring and gives some insight into the mode of aromatic condensation in the asphaltene molecules.12
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R
RICO
HOOC
R RICO
RICO HOOC R
HOOC
HOOC
COOH + COOH
R
R
COOH
R COOH
Scheme 1 The common techniques that used for RICO products analysis are GC and GC-MS.12 Quantitative results for the distribution of n-alkyl groups attached to aromatic carbons and the distribution of polymethylene bridges connecting two aromatic units of heavy fossil fuels can be obtained by GC and/or GC-MS. Strausz et al.12, 13 pointed out the type and the amount of benzene carboxylic acids can reflect the structure of aromatic core in original asphaltenes. However, the GC and GC-MS were limited by the operation temperature and the separation ability of GC, as a result, the RICO products with high boiling point and those have chromatographic peaks submerged in the unsolved humps (e.g., naphthenic acids, nitrogen and sulfur-containing carboxylic acids) cannot be detected of characterized by GC analysis. Electrospray ionization (ESI) is a common ionization technique used in modern MS, which ionizes polar compounds and have no limited by boiling point.14-17 The combination of ESI and FT-ICR MS was considered as an efficient approach for detailed molecular characterizing of RICO products. Wang et al. 18 have characterized the coal derived RICO products by positive ion ESI FT-ICR MS. In this study, the asphaltenes of Canada oil sand bitumen was subjected to RICO decomposition. The RICO products were analyzed by GC, GC-MS, and ESI FT-ICR MS, respectively. The purpose of this study is to investigate the composition of the RICO products which were ignored by general GC analysis, and try to develop quantitative method for the molecular composition of the RICO products. 2. Experimental
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2.1 Chemicals and materials. Analytical grade n-pentane, toluene, methanol, carbon tetrachloride (CCl4), acetonitrile (CH3CN), dichloromethane (CH2Cl2), tetrahydrofuran (THF), sodium periodate (NaIO4) were obtained from Beijing Chemical Reagents Company and were purified by distillation. Ruthenium trichloride (RuCl3) was obtained from J&K Chemical Ltd. Nineteen fatty acids and the stearic-d35 acid (Table 1) were used as model compounds to examine ionization efficiency and response factors in ESI FT-ICR MS analysis. Table 1. Model compounds used for quantitative analysis. Name
Molecular formula
Purity
Name
Molecular formula
Purity
Hexanoic acida
C6H12O2
≥99.5%
Palmitic acidd
C16H32O2
98%
Heptanoic acidb
C7H14O2
>98%
Stearic acidb
C18H36O2
>98%
Caprylic acidc
C8H16O2
99%
Nonadecanoic acidd
C19H36O2
99%
Nonoic acida
C9H18O2
>99%
Eicosanoic acidd
C20H40O2
99%
Decanoic acidd
C10H20O2
99%
Heneicosanoic acide
C21H42O2
99.5%
Undecanoic acidd
C11H22O2
99%
Docosanoic acidd
C22H44O2
95%
Lauric acidd
C12H24O2
99%
5β-Cholanic acidf
C24H40O2
≥99%
Tridecanoic acidd
C13H26O2
97%
Cerotic acidb
C26H52O2
>95%
Myristic acidd
C14H28O2
99%
Hentriacontanoic
C31H62O2
≥99%
Pentadecanoic
C15H30O2
99%
Stearic-d35 acidg
C18D35HO2
D% > 98%
a
Aladding Industrial Inc.
b
Tokyo Chemical Industry Co.
c
J&K Chemical Ltd.
d
Acros Organics. e Dr. Ehrenstorfer GmbH. f Sigma-Aldrich. g ISOTEC.
The asphaltenes (nC7) was separated from the vacuum residue (> 560 °C) of a Canadian oil sands bitumen by saturates/aromatics/resins/asphaltenes (SARA) fractionation using the standard method (ASTM D2007-93). The bulk properties of the asphaltenes are listed in Table 2.19
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Table 2. Bulk properties of the asphaltenes Yields, wt%
C, wt%
H, wt%
S, wt%
N, wt%
O, wt%
H/C
fa*
18.49
76.72
7.61
8.65
1.23
2.93
1.19
0.50
*
fa: The ratio of aromatic hydrogen atoms to all four types of hydrogen atoms
2.2 Ruthenium ion catalyzed oxidation The asphaltenes was dissolved in 20 mL CCl4 in a 100 mL flask, then 20 mL CH3CN, 20 mL water, 5 g NaIO4, and 20 mg RuCl3 were added to the flask. After equipped with condenser tube, the mixture was continuously stirred with a magnetic bar at 40 ˚C. The color of the mixture changed from black to pale slowly during the reaction. The reactions were terminated after 24 hours to ensure the reaction completely and reduce the further oxidation on aliphatic groups as far as possible. 20-22 At the end of the reaction, the solution became a suspended emulsion which consists of organic phase, acquous phase, and tiny particles of NaIO3. These three phase were homogeneously mixed under dramatically stir. A partial of this mixture (0.1 mL) was transfered into a 10 mL sample vial and the solvent was removed by nitrogen blowing at 40 °C. Then 5 mL THF was added to the vial to extract the produced acids in an ultrasonic bath. The solid salt was removed by centrifugation and the clean supernatant was use as the parent solution to analyze the whole product which used to compared with the product in the organic phase and that in the aqueous phase. The rest RICO product mixture was filtered and the precipitate was washed with 10 mL CH2Cl2. The aqueous and organic phases were separated by separating funnel. The aqueous phase being further extracted with CH2Cl2 (10 mL × 2). The organic phase and the CH2Cl2 solution were combined, after be desiccated with anhydrous Na2SO4, the liquid was transferred into a 100 mL volumetric flask and cooled to room temperature. One hundred microliter CCl4 solution of n-docosane (2.00 mg/mL) was added to the mixture as internal standard for quantitative analysis of large molecular alkyl carboxylic acids. Total volume of organic phase was determined to 100 mL with
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CH2Cl2. The aqueous phase was transferred into a 25 mL volumetric flask and the volume was determined at 25 mL with water. 2.3 GC and GC-MS Analysis The low molecular alkyl carboxylic acids (carbon number < 13) in the organic and that in the aqueous phase were directly analyzed using a HP-FFAP capillary GC column (30m × 0.25mm × 0.25µm) (Agilent Ltd). The carboxylic acids in the organic and the aqueous phases were derived into their methyl esters by diazomethane (CH2N2). The detailed operation and condition for GC and GC-MS analysis was described in Supporting Information (Page s2, 7, 8). 2.4 ESI FT-ICR MS Analysis A Bruker apex-ultra FT-ICR MS equipped with a 9.4 T superconducting magnet was used for the molecular characterization of the RICO products. General operation parameters were described in the Supporting Information. For normal analysis, the instrument parameters were optimized with a mass range of 200-600, which covered most abundant mass peaks for the RICO products. In the FT-ICR MS, ions are firstly accumulated in a hexapole trap and then be shot into the ion transfer optics towards the analyzer. The time of high and low m/z ions reached to the analyzer is inconsistent.23, 24 Although the instrument parameters were optimized, the discrimination for low or high m/z ions still exists. For this reason, two different instrument conditions (condition-I and condition-II) were used for quantitative analyzing of the low (100 Da to300 Da) and high (> 280 Da) molecular weight acids, respectively. Figure 1 shows the FT-ICR broadband mass spectrum of the mixed acids and stearoc-d35 acid under condition-I and condition-II. The standard compound of stearoc-d35 acid was consist of C18D35HO2, C18D34H2O2, and C18D33H3O2 with relative contents of 75.66%, 20.65%, and 3.69%, respectively. The peak of [C18D35O2]- was used as the internal standard. Segmentation acquisition and internal standard calibration enable the wide mass range quantitative analysis. The ESI FT-ICR MS conditions, mass calibration, and data analysis are described in Supporting Information (page s2-4).
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C C16 C12 13C14 C15 C11
Condition-I
C10
C18
C9 C8
IS
C7 * C6 * *
C26
C31
Condition-II
C21 C18 C20 C24
C19
IS C22
100
150
200
250
300
350
400
450
500
m/z
Figure 1. ESI(-) FT-ICR mass spectra of the 20 fatty acids (concentration of each acid is 2×10-7 M) under condition-I and condition-II. Note: the peaks marked with an asterisk are likely contamination. 2.5 Quantitative analysis of the RICO product by ESI FT-ICR MS Twenty fatty acids (one deuterium homolog used as internal standard) with carbon number range of 6 to 31 (m/z from 116 to 466) were used to investigate the linear range of ESI FT-ICR MS (Figure S3 and S4 in Supporting Information). Good linear relationship were observed when the total acids concentration≤ 4×10-5 mol/L. The concentration of each acid can be calculated by the intensity of acids, internal standard and corresponding response factors, as shown in Formula 1.
Concentration(acid) =
Peak Intensity (acid) × Concentration(Internal standard) Peak Intensity (Internal standard) × Response factor Formula 1
Generally, the response factors can be easily calculated by the peak intensity of each model acid and internal standard. However, We found that the response factor of
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acid changes obviously with the concentration of total acid. The response factors for 19 model acids to the internal standard (stearoc-d35 acid) at different total acid concentration are different. (the results are shown in Figure S5a and S5b, see Supporting Information). The results indicate that using consistant response factors for the analysis is not reliable. However, the total acid concentration and composition of acids in the real analyte are unknown, so how to obtain response factors of each acid for uncertain analyte concentration is critical. To test the exact response factors, we added model compounds into the real sample to eliminate the matrix effects for the response factor of fatty acids to the internal standard (stearic-d35 acid). The scheme is
shown in Figure 2. The 19 model acids mixture in the following
statements is labeled as M; stearic-d35 acid (d-C18 acid) used as internal standard is labeled as IS; The RICO product of Canada oil sand asphaltenes was used as actual analyte sample (S). Firstly, the linear relationship between intensity ratio of IS/analyte and concentration of IS in RICO product solution is investigated. As shown in Figure 2a, the peak intensity ratios of IS (stearic-d35) to C11 acid (I185) and IS to C19 acid (I297) are proportional to the concentration of IS, indicates that the responds between fatty acids and IS are linear in the acidic compounds matrix. The concentration of acids derived from RICO in the analyte solution should be determined by two factors: (1) ensuring that the mass spectrum have adequate signal to noise ratio and not interference by impurities from the solvent; (2) the concentration not too high to out of the linear range. The final solution of RICO products for ESI analysis was prepared by 10 µL RICO solution in vial 1 (Supporting Information, page 7) added to 1.0 mL toluene/methanol (1:3 in volume) solvent. The results were shown in Figure 2a, which shows good linearity.
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0.5
b
a S+IS
Id-c18 / I185
0.4
0.3
S+IS+M Condition I
I d-C18 / I 185 C17
0.2
0.0 0.0
IS
Condition I -7
5.0x10
-6
1.0x10
1.5x10
-6
2.0x10
-6
-6
2.5x10
Concentration (mol/L)
1.4 1.2
C17 IS
0.1
I d-C18 / I 297
Condition II
S+IS
S+IS+M
Id-c18 / I297
1.0 0.8 0.6
Condition II
0.4 0.2 0.0 0.0
-6
1.0x10
-6
2.0x10
3.0x10
-6
4.0x10
-6
-6
5.0x10
100
200
300
400
500
600
700
m/z
100
200
300
400
500
600
700
m/z
Concentration (mol/L)
Response Factors
c
d
6
Normalized
5 Increment 4
Condition I
Condition I
C17
3
IS
2 1 0 100
150
200
250
300
350
400
450
500
m/z
6 Response Factors
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5
Increment
Condition II
Condition II
4 3 2 1 0 100
150
200
250
300 m/z
350
400
450
500
100
200
300
400
500
600
700
m/z
Figure 2. a. linear relationship between the ratio of stearoc-d35 acid/analyte peak intensity and the molar concentration of stearoc-d35 acid in RICO product solution; b. ESI(-) FT-ICR mass spectra of RICO products of the asphaltenes in organic phase with internal standard (IS+S) and internal standard + mixed acids (IS+S+M) under condition-I and condition-II; c. The ESI(-) FT-ICR mass spectra of (IS+S) and (IS+S+M) under condition-I and condition-II. The internal standard was normalized; d. The curve fitting of response factors for the various acids relative to the internal standard in RICO product under condition-I and condition-II.
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Secondly, the solution of IS+S (2×10-10 mol of stearoc-d35 acid and 10 µL RICO solution were add to 1.5 mL toluene/methanol (1:3 in volume)) and IS+S+M (2×10-10 mol of stearoc-d35 acid, 10 µL RICO solution and 1×10-10 mol for each acid of 19 mixed-acids were add to 1.5 mL toluene/methanol (1:3 in volume)) were subjected to negative ion ESI FT-ICR MS analysis, respectively. The broadband FT-ICR mass spectra are shown in Figure 2b. The difference between spectra of IS+S and IS+S+M should be caused by the 19 acids in M. Thirdly, the intensity of each mass peaks in IS+S were normalized (adjusted) by the intensity of the n-C17 acid in the mass spectra of IS+S+M to eliminate the minor intensity difference of the n-C17 acid from the different analysis. Figure 2c shows the overlapped mass spectra of IS+S and IS+S+M after normalization. Except the 19 acids shown in Table 1, the peak intensity of other corresponding acids between the
IS+S and IS+S+M are coincided exactly. The intensities of peaks for the 19 acids in IS+S+M are higher than that in IS+S. These intensity increments were caused by the adding of mixed-acids (M). Fourthly, the values of mass peak intensity increment for the 19 acids, the intensity of IS, the concentration of each acid and IS were used to calculate the response factors of each acids in the RICO solution. Results were shown in Figure 2d and Figure S5 (see Supporting Information). The results indicate that the total acids concentration of the RICO product is within the linear range and the acids in the RICO product can be quantified by internal standard method. The curve of response factor under condition I and II in this RICO product solution was fitted by using a sum of two Gaussian functions (Supporting Information Formula S1-1, page s18). The response factor (y) is a function of m/z value (x) of the acids. The response factors for acids with different m/z value can be calculated by using this fitting formula. Since the calibration sensitivities of several mono-acids varied by a factor of 40, while ESI MS generally is unlimited by this issue. In this study, we adjusted the instrument operation parameters to enlarge the MS signal abundance of high mass end, which enable the detection of upper limit of the alkyl moieties in the asphaltenes. Figure 9a shows the RICO product mass spectrum in high mass mode, Figure 9b shows the iso-abundance plots of DBE as a function of carbon number for the O2 class species from the mass spectrum (Figure 9a). It shows the upper mass limit is about 1000, corresponding to a carbon number about 60. The maximum carbon number for On, SOx, and NOy class species are approximate. The upper limit of side chain carbon number of the asphaltenes is generally consistent with our previous work,30 in which reported the carbon number range of Canada VTB derived saturates is 22-85.
a
10 9 8
200
400
600
800
1000
m/z
7 6
DBE
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O2
b
5 4 3 2 1 0 10
15
20
25
30
35
40
45
50
55
60
65
Carbon Number
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Figure 9. a. FT-ICR mass spectrum (optimized for high mass end) of the RICO product; b. iso-abundance plots of DBE as a function of carbon number for the O2 class species derived from Figure 9a.
3.3 Quantitative analysis of the O2 class species in RICO products by ESI FT-ICR MS
GC was used to quantitative determination of n- alkyl acids with carbon number of 2-34 in the RICO product (Supporting Information Figure S6, page s12). The Quantitative method by GC is described in Supporting Information (Page s9-10). Meanwhile, considerable peaks of iso- and cyclic- alkanoic acids can be observed between the peaks of n- alkyl acids (as shown in Figure S7 in Supporting Information, page s13). The iso- and cyclic- alkanoic acids peaks with carbon number ≤ 10 can be roughly resolved in GC, which make it possible that calculated the total acid content (n- + iso- + cyclic-) for the acid with carbon number of 6, 7, 8, 9, and 10, respectively. At the same time, the total content was also obtained by ESI FT-ICR MS for the acids with carbon numbers from 6 to 10. Table 1 lists the comparison of the quantitative results from GC and ESI FT-ICR MS. The results from ESI FT-ICR MS are generally agree with the results from GC in the comparable range (C6-C10), which validates that the quantitative results by FT-ICR MS are acceptable.
Table 3. Comparison of the quantitative results obtained by GC and ESI FT-ICR MS for total C6-C10 acids (the values are peak intensity) Carbon Number
6
7
8
9
10
n-
8.80E-03
6.29E-03
5.20E-03
3.75E-03
2.77E-03
iso- and cyclic-
7.34E-03
4.91E-03
5.33E-03
6.08E-03
4.95E-03
Total
1.61E-02
1.20E-02
1.53E-02
9.83E-03
7.72E-03
ESI
n- and iso-
1.29E-02
1.03E-02
7.18E-03
5.90E-03
5.27E-03
(mmol/0.3g asphaltenes)
cyclic-
7.24E-04
1.81E-03
2.18E-03
1.26E-03
1.55E-03
GC (mmol/0.3g asphaltenes)
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Total
1.36E-02
1.21E-02
9.37E-03
7.15E-03
6.82E-03
The distribution of iso-alkanoic acids was also calculated from the GC and FT-ICR MS results. The branched acid with carbon number 4-7 can be exactly resolved and calculated from the GC chromatogram; the quantitative results for iso-alkanoic acids with carbon number 8-35 can be determined from the difference between FT-ICR MS result for acyclic- acids and GC result for n- alkanoic acids. The results were shown in Figure 10. The percentage of branched acids in acyclic acids is about 40.0 % (acetic acid and propionic acid were not took into account), indicating that branched alkyl groups occupied a large portion of side chains. 12 11
total alkyl carboxylic acids n-alkyl carboxylic acids
10 9
mmol/100g Asphaltenes
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8 7 6 5 4 3 2 1 0 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Carbon Number
Figure 10. Quantitative result of total and n-alkyl carboxylic acids in the RICO product.
Finally, the yield of monoacids (in millimoles) of per 100 g asphaltenes were calculated from ESI FT-ICR MS and GC. The data are tabulated in Table S3 (page s19) and plotted as bar charts as shown in Figure 11. The acids with DBE = 1 are nand iso- alkanoic acid, in which the acetic acid is the most abundant compound; the acids with DBE = 2-6 should be naphthenic monoacids with cyclic ring number of 1-5. The percentage of naphthenic monoacids in total O2 class species is about 22% (C2-C5, 20 ACS Paragon Plus Environment
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acetic acid to pentanoic acid were not took into account). Compared to the n-alkyl groups, branched and alicyclid group have tertiary C−H bonds, so they have higher RICO reactivity and more easily suffer further oxidation. So, the quantitative results are more reliable reflect the feature of n-alkyl groups in asphaltenes, but exist some bias for iso- and cyclo-alkyl groups.
1 0.8 0.6 0.4 400
0.2 0
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3
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5 24
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1 42
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1 2 3 4 5 6
Figure 11. Quantitative result of total and cyclic monocarboxylic acids at whole mass range by combination of FT-ICR MS and GC results.
The equivalent carbon percent in the asphaltene for monoacid, di-n-alkanoic acid, benzene carboxylic acids, CO2 and the total carbon recovery were calculated and shown in Table S3. The quantified monoacids afford 20.1% carbon recovery. The carbon recovery of alkyl group calculated from the monoacids is 12.4% (the carbon on carboxyl group should be classified to aromatic carbon), while the percent aliphaticity of Canada oil sand asphaltene by Hazendonk is about 50% (calculated by solid 13C NMR).31 The average aliphatic chain length calculated from the quantitative results of monoacids is 1.73, which far lower than the value 4 in reference.31 The 42.7% total carbon recovery also indicated that present obtained quantitative result is one 21 ACS Paragon Plus Environment
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piece of a jigsaw. Unrecovered portion should include the di-iso-alkanoic acid, diacid naphthenic derivatives, the oxidation products of O2 and O4 species, SOx and NOy compounds.
4. Conclusions The asphaltenes derived from a Canada oil sand bitumen were subjected to RICO decomposition. The RICO products were analyzed by GC-MS, GC, and ESI FT-ICR MS, respectively. Three compound groups: On (n=2-12), SOx (x=3-12), and NOy (y=3-11) with various DBE values were identified by ESI FT-ICR MS. The O2 class species, which represents the single C-C bond-attached alkyl groups in the aromatic system are the most abundant compounds in the RICO products. Besides the alkyl carboxylic acids commonly detected by GC in RICO products, large molecular alkyl carboxylic acids with carbon number up to 60 and 1-5 ring naphthenic acids, as well as their sulfur and nitrogen substituted counterparts were detected by FT-ICR MS. A semi-quantitative method for the analysis of mono-carboxylic acids in RICO products by using ESI FT-ICR MS was established. Different operation modes for high and low mass ranges were carried out for the FT-ICR MS analysis to extend the detection mass range; mixed standard compounds with a wide molecular mass range were used to eliminate the mass discrimination of the FT-ICR MS and calculate the response factors by using standard addition method. The method is instructive for the development of quantitative analysis methodologies for petroleum molecular characterization based on ESI FT-ICR MS. The quantitative results from ESI FT-ICR MS were well agreement with that from the GC in a comparable mass range (C6-C10). Combination of the GC and FT-ICR MS results, the quantitative distribution of normal alkyl carboxylic acids, isoalkyl carboxylic acids, and 1-5 ring naphthenic acids in the whole mass range were obtained. Due to the iso- and cyclo- group could suffer secondary and tertiary oxidation during RICO reaction, the quantitative results are more reliable reflect the feature of n-alkyl groups in asphaltenes, but exist some bias for iso- and cyclo-alkyl groups. 22 ACS Paragon Plus Environment
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Acknowledgement The authors thank Mr. Peidong Wang for assisting with the GC analysis. This work was supported by the National Natural Science Foundation of China (NSFC, 21236009, 21376262, 21405069).
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