Partitioning of Crude Oil Acidic Compounds into Subfractions by

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Partitioning of Crude Oil Acidic Compounds into Subfractions by Extrography and Identification of Isoprenoidyl Phenols and Tocopherols Yahe Zhang,† Quan Shi,*,† Aiqun Li,† Keng H. Chung,‡ Suoqi Zhao,† and Chunming Xu*,† † ‡

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China Well Resources, Incorporated, 3919-149A Street, Edmonton, Alberta T6R 1J8, Canada

bS Supporting Information ABSTRACT: Crude oil was subjected to extrography to obtain various acid compounds in multiple subfractions. Negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) and gas chromatography
mass spectrometry (GC
MS) were used to determine the acid compounds in the crude oil subfractions. Isoprenoidyl phenols with molecular formulas of C27H48O and C28H50O, which were previously deduced as sterol-type compounds, were highly enriched in a subfraction and confirmed by GC
MS. The mass peak with a molecular formula of C27H46O2 was identified as δ-tocopherol. The eluting sequence of the various compounds in crude oil was N1 carbazoles, followed by O1 class acid compounds, then O2 class acid compounds, and finally, N1O2, O3, and O4 class acid compounds. The results show that extrography is an adequate separation technique for partitioning crude oil acid compounds into various subfractions.

’ INTRODUCTION The issues resulting from the presence of organic acids in crude oil have attracted a lot of attention with the increased production of high-acid crude oils. Although the organic acids account for a relatively small amount of total crude oil, the acidic components cause many operating problems.1 Naphthenic acids and phenols are the two most common oxygen-containing acid compounds found in crude oil. Tremendous efforts have been devoted to understanding and solving operational problems related to naphthenic acid systems, such as corrosion, toxicity, geochemistry, emulsion, and deposition. 2 Phenols, the major oxygen-containing compounds in coal tar,3 are known to be associated with the oxidative stability of crude oil4 and biodiesel.5 Additional investigation of the molecular composition and distribution of organic acids in the complex organic mixtures is required to better understand how to resolve the issues in production. However, the characterization of oxygen-containing acid compounds in crude oil is challenging using bulk hydrocarbon analysis techniques. Because of their low concentration in crude oil, isolation of organic acids is carried out prior to analysis. Various techniques for isolating acid species from the complex organic mixtures have been developed, including distillation,1,6 base extraction,7
12 solid-phase extraction (SPE),9,13
27 and high-performance liquid chromatography (HPLC).28,29 However, there are discrepancies in the findings of acid species composition obtained by the various separation techniques. The partitioning of acids in the various fractions during the separation processes is not well-understood. This is dependent upon the acid species composition, column efficiency, low ionization selectivity, and low mass resolving power of conventional mass spectrometry (MS). Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance (FT-ICR) MS has high ionization selectivity, r 2011 American Chemical Society

unparalleled resolving power, and high mass accuracy.30
34 It is a powerful analytical technique capable of detecting acid species and neutral nitrogen compounds present in trace quantities in crude oil without prior sample pretreatment.8,14,21 It has been used to determine the composition of acids in the subfractions from crude oil by distillation,1,6,11 base extraction,7,8,11,35,36 saturates, aromatics, resins, and asphaltenes (SARA) fractionation,14 ion-exchange resin,37 and a modified two-step liquid column chromatography method.14,21 In a previous paper,1 the two abundant oxygen-containing compounds found in the 460
500 °C fractions of a crude oil were C27H48O and C28H50O. These compounds have a doublebond equivalent (DBE) value of 4 and cause a high total acid number (TAN) of crude oil. Because the crude oil sample was associated with a Cenozoic source rock in the presence of abundant hopanoic acids, it was deduced that these compounds were likely cholesterols. To substantiate this finding, this paper extends the work on a high TAN crude oil by subjecting it to extrography,24 expanding from McLean and Kilpatrick’s interpretation,38 to prepare multiple subfractions containing oxygen compounds with various acidities and confirming the oxygencontaining compounds in crude oil using gas chromatography
mass spectrometry (GC
MS) and the salient features of the negative-ion ESI FT-ICR MS. The extrographic subfractions will be subjected to future studies to determine the target acid compound classes responsible for corrosion issues. The partitioning of acid species in various subfractions from the extrography is also discussed.

Received: June 10, 2011 Revised: September 29, 2011 Published: September 29, 2011 5083

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Figure 1. Extrographic fractions with recycling of the solvents and different recycling times.

Figure 3. Total ion chromatograms from GC
MS of crude oil subfractions: A, molecular ion m/z 388 of C27H48O; B, molecular ion m/z 402 of C27H46O2; C, molecular ion m/z 280; D, molecular ion m/z 294; PA, phthalic anhydride; C16H32O2, palmitic acid; and C18H36O2, stearic acid.

Figure 2. Yields of crude oil subfractions 1
9.

’ EXPERIMENTAL SECTION Sample Description. The crude oil sample was obtained from the Liaohe oilfield in Bohai Basin, China. The TAN of the crude oil was 4.67 mg of potassium hydroxide (KOH) g
1. The nitrogen, oxygen, and sulfur contents were 0.65, 0.85, and 0.30 wt %, respectively. Preparation of Subfractions by Extrography. The preparation of the modified silica for column chromatography has been described elsewhere.13,24 A total of 3 g of KOH was dissolved in 60 mL of isopropanol. The KOH
isopropanol solution was mixed with 30 g of silica (100
200 mesh, Qingdao Haiyang Chemical Co., Ltd., Shandong, China) and 100 mL of chloroform (CHCl3). After standing for 5 min, the suspension was slurried into the column of the Soxhlet extractor, as shown in Figure 1. All KOH was retained on the silica and reacted to form potassium silicate.24 The potassium silicate packing was rinsed with 50 mL of CHCl3 and recirculated for 10 min. A total of 4 g of the crude oil sample was transferred onto the top of the potassium silicate packing and then topped with 2 g of silica. The crude oil sample was separated into nine subfractions according to the fractionation scheme shown in Figure 1. The first five diluted subfractions (fractions 1
5) were obtained by recirculating 50 mL of CHCl3 in the Soxhlet extractor and withdrawing subfractions sequentially at various time intervals. Each of the subfractions was 50 mL. After that,

a solvent mixture, CHCl3/HCOOH (4:1, v/v) was used to rinse acidic fractions. Diluted subfractions (fractions 6
9) were obtained by recirculating the CHCl3/HCOOH solvent mixture in the Soxhlet extractor and withdrawing subfractions sequentially at various time intervals. It was expected that the neutral, basic, and weak acid species would be eluted into fractions 1
5, whereas strong acid species and polar organic compounds were expected to elute into fractions 6
9. The solvent in the diluted subfraction was removed by a vacuum rotatory evaporator. The solvent-free subfractions were collected and weighed. The total weight of subfractions accounted for 82 wt % of the crude oil sample. The yields of subfractions are shown in Figure 2. The 18 wt % yield loss is likely due to evaporation of the light components of crude oil during the vacuum rotator evaporation of solvents and/or irreversible adsorption of crude oil components onto the alkali-treated silica column. GC
MS Analysis. A Thermo-Finnigan Trace GC 2000 coupled with a DSQ mass detector was used to analyze the composition of the subfractions. The mass spectrometer was equipped with an electron impact source at 70 eV ionization energy and set to scan from 35 to 500 Da in 1 s. A HP-5MS (30 m  0.25 mm  0.25 μm) fused-silica capillary column was used for GC
MS analysis. The programmed oven temperature for GC
MS analysis was held at 80 °C for 1 min, increased to 220 °C at 5 °C/min, and then increased to 310 °C at 8 °C/min, and held constant at 310 °C for 12 min. The injector was maintained at 300 °C in splitless mode. Derivatization of Phenols. A total of 10 mg of each subfraction was dissolved in a solution that consisted of 0.5 mL of acetonitrile and 15 μL of silylation agent [N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA)]. The mixture was stirred continuously and kept at 40 °C for 30 min. ESI FT-ICR MS Analysis. The sample preparation procedure for ESI FT-ICR MS analysis has been described elsewhere.14,21 The sample was dissolved in toluene and diluted to 0.2 mg mL
1 with toluene/ methanol (1:3, v/v), and then 15 μL of NH4OH was added to facilitate the deprotonation of acid species and neutral nitrogen compounds to yield [M
H]
ions.8 MS analyses were performed using a Bruker 5084

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Figure 4. Mass spectra of peaks A and B (before derivatization) and A0 and B0 (after derivatization). apex-ultra 9.4 T FT-ICR MS mass spectrometer. A sample solution was injected into the ESI source at 180 μL/h using a syringe pump. The operating conditions for negative-ion formation were a 3.5 kV emitter voltage, a 4.0 kV capillary column introduction voltage, and a
320 V capillary column end voltage. Ions accumulated for 0.01 s in a hexapole with 2.4 V direct current and 500 Vp
p radio frequency (RF) amplitude. The optimized mass for Q1 was 200 Da. An argon-filled hexapole collision pool was operated at 5 MHz and 700 Vp
p RF amplitude, in which ions accumulated for 0.02 s. The delay was set to 1.2 ms to transfer the ions to an ICR cell by the electrostatic focusing of transfer optics. The mass range was set at m/z 110
800. The data set size was set to 4 M words, and time-domain data sets of 64 acquisitions were co-added.

’ RESULTS AND DISCUSSION Characterization of Crude Oil Subfractions by GC
MS. Figure 3 shows the total ion chromatogram from the GC
MS scan of fractions 1
9. Most of fractions 1
3 consisted of saturated alkanes with 13
30 carbon atoms. In fraction 4, the two peaks labeled “A” and “B” were molecular ions with m/z 388 and 402, respectively. These molecular ions had high relative abundances, indicating that they were distinctly separated from the bulk of the crude oil sample. The FT-ICR MS analysis showed that the molecular formula for peak “A” was C27H48O and the molecular formula for peak “B” was C27H46O2. These molecules are found occasionally in the FT-ICR MS spectra of crude oils with a very high abundance. These are likely sterol-like compounds.1 The mass spectra of peaks “A” and “B” before and after derivatization with BSTFA are shown in Figure 4. The data indicate that “A”

and “B” peaks were phenolic compounds. Peak “A” had been previously identified as isoprenoidyl phenols, which are likely hydrothermally altered products of natural product precursors.39 Peak “B” was identified as δ-tocopherol with a molecular formula of C27H46O2 according to the National Institute of Standards and Technology (NIST) MS database. Because these compounds exhibit the characteristics of biomolecules, their presence in the crude oil would provide insight into the geochemistry of this crude oil sample, such as the origin of the product precursors, depositional environment, and the thermal maturity of the sediment. In Figure 3, fraction 5 had two peaks “C” and “D”, which were molecular ions with m/z 280 and 294, respectively. These substances also had high relative abundances. Derivatization of fraction 5 showed that they each contained a hydroxyl group. Fractions 6 and 7 had high relative abundances of phthalic anhydride, palmitic acid, stearic acid, and other fatty acids. Fractions 8 and 9 were predominantly naphthenic acids. GC
MS analyses showed the partitioning of various acid compounds in crude oil subfractions from the extrography method. Distribution of Heteroatom Class Species in Crude Oil and Its Subfractions by Negative-Ion ESI FT-ICR MS. Figure 5 shows the negative-ion ESI FT-ICR MS broadband (200
700 Da) mass spectra of the crude oil and its subfractions and relative abundances of heteroatom class species. The mass spectra of the crude oil and its subfractions were centered at m/z 400 with a mass resolving power of 300 000. Peaks in the range of m/z 200
700 Da with a relative abundance greater than 6 times the standard deviation of the baseline noise were exported to a spreadsheet and assigned 5085

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Figure 5. Negative-ion ESI FT-ICR MS broadband (200
700 Da) mass spectra of the crude oil and its subfractions and relative abundances of heteroatom class species.

Figure 6. Plots of the DBE as a function of the carbon number for O1 class species from the negative-ion ESI FT-ICR mass spectra of the crude oil and its subfractions. Chemical structures of C28H50O and C27H48O were identified.

with the molecular formulas by exact masses using custom software, which has been described elsewhere.40 Negative-ion ESI generates deprotonated molecular ions [M
H]
of acid species and neutral nitrogen compounds. The even-mass peaks in fractions 1, 2, and 7 were mainly N1 carbazoles. The odd-mass heteroatom species with a high relative abundance in fractions 3
6 are likely O1 phenols. The peaks in the crude oil and fractions 8 and 9 are likely O2 acidic compounds. The highest peaks in fractions 3 and 4 were determined to be C28H50O and C27H48O with the same DBE of 4 based on the high mass accuracy of FT-ICR MS. They are identified as isoprenoidyl phenols using GC
MS by derivatization with BSTFA.

The abundant heteroatom compounds in fractions 1
9 were N1, N1O1, N1O2, O1, O2, O3, and O4 class species. Figure 5 showed that a sequence (yellow arrows) of various eluting compounds can be obtained by the extrography with different solvents. The N1 carbazoles eluted first, followed by O1 phenols, and finally, O2 and other acid species. In addition to O2 class species in fractions 8 and 9, N1O2, O3, and O4 acid species were also present. These could not be identified by GC
MS. Only trace quantities of N1 carbazoles remained in fractions 6 and 7, which might be the result from adsorption and/or inclusion on the alkali-treated silica column. It is possible to track the partitioning of crude oil acid compounds 5086

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Figure 7. Plots of the DBE as a function of the carbon number for O2 class species from the negative-ion ESI FT-ICR mass spectra of the crude oil and its subfractions. Chemical structures of C28H48O2 and C27H46O2 were identified.

in various extractable subfractions using negative-ion ESI FTICR MS. O1 Class Species. Figure 6 shows plots of the DBE versus the carbon number for O1 class species from the negative-ion ESI FT-ICR mass spectra of the crude oil and its subfractions. The O1 class species in crude oil had 4
15 DBEs and 17
45 carbon atoms. The two highest peaks with molecular formulas of C28H49O and C27H47O (DBE = 4) were distinct. They were isoprenoidyl phenols, as discussed previously. The partitioning of various O1 class species into subfractions is shown in Figure 6. Peaks with a high relative abundance in fractions 5
7 were either not ascertained or contaminants. O2 Class Species. Figure 7 shows plots of the DBE versus the carbon number for O2 class species from the negative-ion ESI FT-ICR mass spectra of the crude oil and its subfractions. The O2 class species in crude oil had 1
13 DBEs and 16
43 carbon atoms. The O2 class species with 1 DBE are likely fatty acids, while those with 2
7 DBEs are naphthenic acids with 1
6 naphthenic rings. The O2 class species that had 5
18 DBEs and 18
47 carbon atoms are likely multi-ring naphthenic acids and/or aromatic acids and/or phenols with one or two hydroxyl groups. The C16 and C18 fatty acids in fractions 1
5 are likely contaminants in the negative-ion ESI mass spectra, because they had a relatively high response compared to low-polarity compounds in crude oil.1,11 The high relatively abundant O2 class species had DBE value of 5 and 28 carbon atoms in fraction 3 and 27 carbon atoms in fractions 4 and 5, which are β- and/or γ-tocopherols and δ-tocopherol, respectively. The C16 and C18 fatty acids had high relative abundance in fractions 6 and 7. Most acid compounds concentrated in fractions 8 and 9 had similar DBEs and carbon number distribution to that in the crude oil.

’ CONCLUSION Negative-ion ESI FT-ICR MS and GC
MS were used to determine the composition of crude oil subfractions prepared by extrography. The results showed that acid compounds in crude oil were partitioned in various subfractions. The eluting sequence of various compounds in crude oil was N1 carbazoles, followed by O1 class acid compounds, then O2 class acid compounds, and

finally, N1O2, O3, and O4 class acid compounds. The peaks with high relative abundances and molecular formulas of C27H48O and C28H50O, which were previously deduced as sterol-type compounds, were successfully separated and identified as isoprenoidyl phenols. The mass peak with a molecular formula of C27H46O2 was identified as δ-tocopherol. The extrographic subfractions will be subjected to future studies to determine the effects of target acid compound classes responsible for TAN and corrosion issues.

’ ASSOCIATED CONTENT

bS

Supporting Information. NIST library search result of peak B in Figure 3 (Figure S1), mass error distribution in the full mass range (Figure S2), average mass spectra during the elution time of 14.64
42.58 min of fraction 9 (Figure S3), and pulsed flame photometric detector (PFPD) gas chromatogram for sulfur compounds of the crude oil (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

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

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