Characterization of Acidic Compounds in Vacuum Gas Oils and Their

Aug 13, 2012 - College of Chemical Engineering, China University of Petroleum, ... Mass Spectrometry: Keystone between Petroleomics, Metallopetroleomi...
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Characterization of Acidic Compounds in Vacuum Gas Oils and Their Dewaxed Oils by Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry Xiaohui Li,† Jianhua Zhu,*,† Bencheng Wu,† and Xinhua Mao‡ †

College of Chemical Engineering, China University of Petroleum, Beijing 102249, People’s Republic of China China Petroleum Engineering and Construction Corporation, Beijing 100120, People’s Republic of China

‡

ABSTRACT: The acidic components in vacuum gas oil (VGO) of cut-2, cut-3, and cut-4 and their corresponding dewaxed oils from a refinery were characterized by negative-ion electrospray ionization (ESI) Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). The acidic heteroatom compounds, N1, N2, N1O1, N1O2, O1, and O2 class species, were identified. Among them, the abundance of O2, O1, and N1 class species were much higher than other acidic class species and characterized by double-bond equivalent (DBE) values and carbon numbers. The N1, N2, N1O1, and N1O2 classes were only identified in VGO cuts instead of dewaxed VGOs. The composition of acidic compounds in VGOs and dewaxed VGOs were correlated with increased boiling point and varied in DBE and carbon numbers. For all identified naphthenic acids, polycyclic naphthenic acids were dominant in VGO cuts, while in their dewaxed oils, naphthenic acids with single-ring were dominant. The most abundant O1 class species had DBE values of 4 corresponding to alkyl phenols in three VGO cuts as well as in three dewaxed VGOs. The abundant N1 class species in three VGO fractions were centered at DBE values of 9 and 12, and these species were likely carbazoles and benzocarbazoles, respectively, while the N1 class species with a DBE of 9 were dominant in three dewaxed VGOs.

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INTRODUCTION Today, the exploration and utilization for heavy and inferior crude oil are more and more emphasized by many countries in the world since the conventional oil reserves are declining year by year due to the unavoidable fossil fuel scarcity, and the heavier and more inferior crude oils may be a nice choice for some refineries due to the relatively lower price of so-called “opportunity crude oils”.1−3 One of the most important characteristics of heavy and inferior crude oils is their higher total acid number (TAN) besides higher density, viscosity, and sulfur content, etc. compared with conventional crude oil.2,4 As one important marker, the TAN value is used to gauge the acidity of oils, defined as the mass of potassium hydroxide (in milligrams) required to neutralize acidic components in 1 g of crude oil.2 Also, higher TAN value means that there exist more acidic substances which are responsible for liquid phase corrosion in crude oil transport through pipelines and in refinery processing1−3,5,6 and a higher TAN value of oil products. As the supply of light crude oils is exhausted, reliance will shift to highly acidic heavy oils that are high in heteroatoms from the United States, China, Eastern Europe, and Venezuela.1 In recent years, with a larger amount of heavy and inferior crude oils chosen to be the feedstock of many refineries in China,1,7 high TAN value of distillates, such as atmospheric gas oil (AGO), vacuum gas oil (VGO), which directly originated from the high TAN value of crude oil itself, may be a challenge to the quality of oil products. In lube oil processing, the solvent dewaxing process is considered as an important technique for the removal of the wax from the feedstock in the temperature range of the desired pour point and is usually followed by the solvent extraction process, which is used to remove undesirable components such as aromatics and other low viscosity index materials.8 After the two sequences of refining techniques to © 2012 American Chemical Society

VGO, the dewaxed VGO as the candidate of base oil is obtained. The acidic components are responsible for the TAN value of oil; however, usually only the TAN value of oil is concerning superficially rather than the molecular level information about the acidic components in oil; therefore, some advanced analytical approaches are needed to reveal some detailed information about the lube oil processing. In recent years, mass spectrometry has been increasingly used for crude oil studies. The elemental composition of a petroleum sample can be quickly determined by a C, H, N, S analysis, but the further reliable assignment of elemental compositions to mass spectrometric signals, however, requires high mass resolution and mass accuracy. Electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) has been successfully employed to analyze polar compounds in petroleum and its fractions because of its high resolving power, which means that it could resolve thousands of species in a single mass spectra allowing for unambiguous determination of elemental composition.6,9 Positive-ion ESI could selectively ionize heteroatom species, such as basic nitrogen compounds,1,10−15 while negative-ion ESI could be used for the ionization of carboxylic acids (e.g., naphthenic acids, fatty acids), phenols and “neutral nitrogen” species (e.g., pyrrole homologues)1,10,12−14,16−19 without derivatization or preconcentration of the sample and with minimal sample consumption.6 At present, a lot of work has been completed by different research teams and individuals from some countries for characterizing acidic compounds in petroleum by employing Received: March 7, 2012 Revised: August 9, 2012 Published: August 13, 2012 5646

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emitter voltage, 4.5 kV capillary column front end voltage, and −320 V capillary column end voltage. Ions accumulated for 0.01 s in a hexapole. The delay was set to 1.2 ms to transfer ions to an ICR cell by the electrostatic focusing of transfer optics. The mass range was set at m/z 150−800. The data size was set to 4 M words, and time-domain data sets were coadded from 128 data acquisitions. Mass Calibration and Data Analysis. The mass spectra obtained were calibrated internally according to the most abundant homologous series compounds. All masses were then converted to the Kendrick mass scale.6 The Kendrick-sorted masses with relative abundance greater than 6 times the standard deviation of the baseline noise were imported into an Excel spreadsheet for identification. Data analysis was implemented by using custom software which had been described by some references.1,6,9,19 Generally, a two-mass scale-expanded segment in the middle of the mass spectra was selected for calibration during data analysis, and then detailed identification of each peak would be carried out.6,20 Compounds of a homologous series would be identified according to the Kendrick mass defect (KMD), and the essence was that compounds with the same heteroatom composition and number of rings plus double bonds (DBE), but different numbers of CH2 groups, have identical Kendrick mass defects.6,9,19,24

all kinds of analytical instruments especially ESI FT-ICR MS. Mapolelo et al.6 showed that polar species in petroleum and related products ranging from weakly acidic molecules, such as pyrrolic nitrogen compounds, as well as more strongly acidic molecules, such as carboxylic acids, could all be selectively ionized by negative-ion electrospray and be identified efficiently. Smith et al.4 utilized ESI FT-ICR MS to characterize heavy vacuum gas oil distillation cuts of Athabasca bitumen for investigating the evolution of acidic species under standard distillation conditions and determining whether these acidic species could undergo structural changes as a function of temperature. However, to our knowledge, there is still no literature about studying the acidic components in lube oil processing by ESI FT-ICR MS; therefore, this paper mainly concentrated on the compositional changes of acidic components during the lube oil processing. The acidic compounds in VGO cuts and their corresponding dewaxed oils were analyzed by negative ESI FT-ICR MS. On the basis of the analysis results provided by ESI FT-ICR MS, some useful detailed information about the compositional changes of acidic components from VGOs to dewaxed oils could be learned about, which may help to know the occurrence, distribution, as well as abundance of acidic species in VGOs and dewaxed VGOs, and may be promising to provide some guidance to refine the base oil of lubricants from VGO distillates.

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RESULTS AND DISCUSSION Mass Distribution. A broadband negative-ion electrospray 9.4 T FT-ICR MS identified thousands of acidic species in VGO cuts and their dewaxed oil samples, as shown in Figure 1.

EXPERIMENTAL SECTION

Samples. Three VGOs and their corresponding dewaxed oil samples were collected from the no. 3 refinery of the Yanshan Petrochemical Company of SINOPEC, and their parent crude oil was the mixture of Daqing and Jidong crude oil to a certain proportion from the oilfields in China. Three VGO samples were cut-2, cut-3, and cut-4 from the vacuum distillation unit, respectively. The boiling ranges of VGO cut-2, cut-3, and cut-4 were 360−400 °C, 400−450 °C, and 450−490 °C, respectively. In practice, the overlap in the boiling range between fractions was inevitable.4 The TAN values of the six oil samples were measured according to ASTM D-664, and the results were listed in Table 1.

Table 1. TAN Values of VGO Cuts and Their Dewaxed Oils sample VGO cut

dewaxed VGO cut

TAN, mg KOH/g oil 2 3 4 2 3 4

0.24 0.37 0.44 0.03 0.05 0.10

Sample Preparation for ESI FT-ICR MS Analysis. The sample preparation method for the analysis of acidic species in VGO cuts and their dewaxed oil samples by negative-ion electrospray FT-ICR MS has been previously reported.1,6,16,17,19−22 For each oil sample, 10 mg was completely dissolved in 1 mL of toluene. For each mixture solution, it was diluted to 0.2 mg/mL with a mixture solvent of toluene/methanol (1:3 v/v). For each diluted solution, 5 μL of 28% NH4OH was then added to facilitate the deprotonation of acidic species to yield [M − H]− ions.6,23 The toluene and methanol were both analytical-reagentgrade and were distilled several times and kept within glass bottles with ground glass stoppers. All chemicals used were from Beijing Chemical Works in China. ESI FT-ICR MS Analysis. Samples were analyzed with a Bruker apex-ultra FT-ICR MS equipped with an actively shielded 9.4 T superconducting magnet. The sample was infused by an Apollo II electrospray source at a flow rate of 180 μL/h using a syringe pump. The operating conditions for negative-ion formation were 4.0 kV

Figure 1. Broadband negative-ion ESI FT-ICR mass spectra of VGO cuts and dewaxed oils. Samples were analyzed at the equivalent concentration instrumental operating conditions.

The mass spectra of each VGO contained almost the same number of ion peaks compared to the mass spectra of their corresponding dewaxed oil. (The broadband of VGO cut-2, 3, and 4 included altogether 22 844, 22 814, and 24 699 peaks, respectively, and their corresponding dewaxed oil included 22 183, 22 646, and 25 305 peaks, respectively). The ions counted as peaks were 6-fold larger than the baseline noise. The ESI FT5647

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ICR mass spectra shows that the molecular weight of identified compounds increased with increasing boiling point of the VGO fraction. This is in agreement with previous FT-ICR MS analyses by other authors.1,4,25,26 At first sight, it was apparent that the spectra of oil samples included several intense anion peaks, and these peaks were orders of magnitude higher than the others, representing the phenomenon of suppression effects, where several other factors than the analyte concentration affect the ESI response, e.g., ionization efficiency and solvation energy of the analyte.27 This effect was greater for dewaxed VGOs than for VGOs. The abundance-axes of the spectra of dewaxed oils were scaled so that the smaller signals could be seen. With the choice of some mass points with a better signal-tonoise ratio (SNR), it can be seen that there are multiple mass peaks in the range of one unit mass spoint when the mass spectra was unfolded further, as an example, mass point 345 Da, VGO cut-4 and dewaxed VGO cut-4. The importance of ultrahigh mass resolving power and mass accuracy in the determination of compositional differences in vacuum distillates is evident from Figure 2. From Figure 2, it could be found that Figure 3. Class species distribution for VGO cuts and their dewaxed oils based on negative-ion ESI FT-ICR MS.

boiling range of VGO increased, while N1 compounds exhibited the opposite trend. Similar results were also found by Smith et al.4 when they characterized heavy vacuum gas oil distillation cuts of Athabasca bitumen by ESI FT-ICR MS, and they concluded that this was attributed to the increase in multiheteroatom containing classes with increasing boiling range. Also, similar trends for O2 and N1 compounds occurred in the three dewaxed VGOs. In terms of O1 compounds, a decrease for the relative abundance was observed in VGOs from cut-2 to cut-4; however, conversely, an increase was observed in dewaxed VGOs. The distribution of heteroatomic classes was determined by dividing the sum of the relative abundances of all species of a given class by the relative abundance of the most abundant compound class in the mass spectra. Although this approach did not take into account suppression effects and differences in ionization efficiency, we felt that it was feasible to roughly compare classes for different oil samples of VGO as well as dewaxed VGO, respectively. It was reasonable to assume that their effect on each VGO and dewaxed VGO was the same since relatively similar mixtures were compared as well as the same measurement conditions were used.27 The sequence of the relative abundance of O2 class species in VGO cuts was observed from Figure 3 as follows: VGO cut-2 > VGO cut-3 > VGO cut-4; however, the TAN value as Table 1 shows is the opposing trend, and this could be explained from the following two aspects. On the one hand, O2 species merely contain two oxygen atoms, and they are not necessarily acids. The TAN value not only depends on the O2 class species but also associates with other acidic compounds, which means that the TAN value is a comprehensive result of the functioning of all acidic compounds rather than just being proportional to the content of O2 class species. Some research has demonstrated that TAN is no longer considered to be a reliable indicator to quantify the presence of naphthenic acids, which are believed to be largely responsible for oil acidity, so it is not reasonable to predict the quantity of O2 class species just relying on the

Figure 2. Mass scale expansion at a nominal mass of 345 Da for VGO cut-4 and its dewaxed oil.

both the number of mass peaks and the identified class species of dewaxed VGO cut-4 are less than that of VGO cut-4, and this implies that some acidic components have been removed from VGO in the lube oil processing. Heteroatom Class Composition. Figure 3 exhibits the relative abundance of the negative-ion heteroatom class species in the VGOs and dewaxed VGOs. N1, N2, N1O1, N1O2, O1, and O2 classes were identified by the negative-ion mass spectra. The N1, O1, and O2 class species were dominant in the negative ion ESI mass spectra, while N2, N1O1, and N1O2 had much lower abundance. The acidic N1O1 and N1O2 class was observed in all VGO samples, while those were not found in dewaxed VGO cuts. Also the N2 class existed exclusively in VGO cut-3 and cut4. This indicated that the heteroatom class composition was more complex in the heavy distillates, consistent with previous literature.1 Heteroatom class N2, N1O1, and N1O2 species were not found in dewaxed VGO, and this was because most of them were removed from VGO when processing base oil by refining and could not be detected. It could also be found that from Figure 3, O2 compounds decreased in relative abundance as the 5648

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Figure 4. Broadband negative-ion ESI FT-ICR mass spectra of identified the O2, O1, and N1 class species in VGO cuts and dewaxed oils.

measurement of the TAN value.2,5,28 Of course, the TAN value directly related to the relative abundance of the O2 class species was reported by some authors. Hughey et al.17 made a comparison of the identified compound classes between different crude oil, and they found that the relative abundance of the O2 class species mirrored the TAN values; the sequence of the relative abundance of the O2 class species was Chinese crude (1.84 mg KOH/g) > Middle Eastern crude (0.32 mg KOH/g) > North American crude (0.09 mg KOH/g). On the other hand, the ESI response factors may have varied enormously between different acidic species or even between different acids, whereas the TAN value is a measurement of the molar concentration of acid-functions in the sample; therefore, the O2 class species abundance may not be directly compared without prior identification and established response factors for O2 compounds. The fundamental basis of electrospray measurement of TAN is that electrospray signal is directly proportional to the level of acid in the sample, which in turn relates to the KOH needed to neutralize the acid. TAN measurement by ESI is based on the quantification of all acid species in the sample in reference to an internal standard compound. Qian et al.29 reported the development of ESI-MS for quantitative characterization of

petroleum acids and rapid determination of TAN and TAN BP distributions, and they assumed a uniform response factors for all acid molecules in the TAN calculation; stearic acid was used as an internal standard. They illustrated the correlation between the total ESI MS response and TAN values, and the results demonstrated that a linear correlation existed for high-TAN crudes (TAN > 0.9 mg KOH/g), and the ESI response leveled at roughly a constant value for low-TAN crudes. In our study, the TAN value of VGO samples are less than 0.45 mg KOH/g, and dewaxed VGO samples are even less than 0.1 mg KOH/g. Therefore, according to the study of Qian et al., it will be difficult to precisely establish ESI response factors to describe the relation between TAN and mass spectrometric observations in our study by using a similar method. This interesting work needs to be further implemented in our future work continuously. Mass Distribution and the Abundance of O2, O1, and N1 Class Species. Figure 4 shows the broadband negative-ion ESI FT-ICR mass spectra of the identified O2, O1, and N1 class species from VGOs and dewaxed VGOs. The most abundant peaks of N1 class compound for VGO cut-2 to cut-4 were at m/ z 320, 342, and 412, respectively, and dewaxed VGOs were at m/z 320, 376, and 530, respectively. For the O1 class 5649

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Figure 5. Plots of DBE as a function of the carbon number for O2 class species from negative-ion ESI FT-ICR mass spectra of VGO cuts and their dewaxed oils.

compound, from VGO cut-2, cut-3, and cut-4, the most abundant mass peaks were at m/z 387, 429, and 485, respectively, and the peaks at m/z 387, 443, and 541 were for dewaxed VGO cut-2, cut-3, and cut-4, respectively, were most abundant. This demonstrated that the molecular weight of the O1 and N1 compounds increased with increasing boiling point of the VGO fraction, which was in agreement with the whole negative ESI FT-ICR mass spectra in Figure 1. In addition, it was also revealed that O1 compounds tended to be higher molecular weight compared to N1 compounds at the same boiling point of the VGO fraction. Furthermore, comparing the N1 and O1 class species in VGO and the corresponding dewaxed VGO, basically and approximately, dewaxed VGO shifted the distribution toward higher masses, indicating that the relatively lower molecular weight com-

pounds were easily removed during the base oil refining processes. It was apparently observed that the spectra of VGOs and their dewaxed oils included intense carboxylic acid anion peaks at m/z 255 and 283, representing deprotonated palmitic and stearic acids.27 Some references proved that most of these ions were due to impurities with the polypropylene tube of the electrospray source being the main suspect for their release.17,27 Although these intense impurity signals caused some interference to the analysis and discussion, it could be still concluded that the O2 class of species moved to a high molecular range with an increased boiling point of VGOs and their dewaxed oils. Also, at the same boiling range, the O2 compounds were distributed in the highest mass range, followed by O1 and N1 sequentially. 5650

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Figure 6. Plots of DBE as a function of the carbon number for the O1 class species from negative-ion ESI FT-ICR mass spectra of VGO cuts and their dewaxed oils.

DBE versus Carbon Number Distributions for O2 Class Species. Figure 5 shows DBE versus carbon number distributions for the O2 class of compounds from positive-ion ESI FT-ICR mass spectra for each VGO distillation cut and corresponding dewaxed oil. At first sight, the C18 fatty acid has a considerable relative abundance presented in the plots and suppresses other O2 class compounds, which appear to be invisible. Some references also showed that C16 and C18 fatty acids are often present in the negative-ion ESI mass spectra as well-known contaminants.27 The suppressing effect and the complexity of ionization efficiency are usually very difficult to be eliminated. Although these effects produce a negative influence on the analysis of O2 class compounds, some useful information still could be obtained approximately and qualitatively. In order to scale up the iso-abundance maps so that the other O2 class species could be visible, C18 fatty acid was deleted and the remaining O2 class species showed up in the inset maps; the C18 fatty acid was not involved in the following discussion. Also naphthenic acids have good selectivity in the ESI ionization source, and other species would be suppressed and some of them might be undetectable due to limited dynamic range of the FT-ICR MS. From Figure 5, the O2 class species were distributed over a relatively wide range of DBE and carbon number, suggesting that the molecular structures of individual O2 class species were significantly different. The abundant O2 class species moved to a high carbon number and DBE values with increasing of the

boiling point of the distillate fraction. For all identified naphthenic acids, the high relative abundances of the O2 class species were at DBE values of 2 and 3 and a carbon number of 24−29 in VGO cut-2, and at DBE value of 5 and a carbon number of 28−34 in VGO cut-3, while in VGO cut-4, the O2 class species were concentrated at DBE values of 5 and 6, carbon number 30−37. O2 class species with DBE values of 2− 7 are naphthenic acids with 1−6 naphthenic rings, and O2 class species with higher DBE values are likely multiring naphthenic acids and/or aromatic acids. Crude oils containing 4 and 5 ring naphthenes with special biologic skeleton structures, such as hopane and sterane, are commonly used as biomarkers in the field of geochemical science.28 For all identified naphthenic acids in dewaxed VGOs, basically and approximately, the most abundant is mainly monoring naphthenic acids; therefore, it could be concluded that the amount of multiring naphthenic acids were significantly removed during the base oil refining process. DBE versus Carbon Number Distributions for O1 Class Species. The plots of DBE as a function of the carbon number for the O1 class species in three VGO samples and their dewaxed oils are shown in Figure 6. In three VGO cuts, the dominant homologous series of O1 class species had a DBE value of 4, and these species were considered as alkyl phenols,20 with the possible structures shown in the figure. On the other hand, because negative ESI is less effective for ionization of hydroxyl compounds compared to carboxyl acids, the high 5651

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Figure 7. Plots of DBE as a function of the carbon number for N1 class species from negative-ion ESI FT-ICR mass spectra of VGO cuts and their dewaxed oils.

observed, and this was the result of the refining process to VGO, and O1 class species also followed a similar trend. In all VGO samples, N1 class species with a DBE value of 9 and 12 had higher relative abundance. Species with a DBE value of 9 and 12 are usually regarded as carbazoles and benzocarbazoles, respectively.17,20,30 While, in each dewaxed oil sample, the N1 class species with a DBE value of 9 were dominant, this difference can be attributed to the selective removal of N1 class species during the refining process, more N1 class species with a DBE value of 12 were removed from VGO cuts.

abundance of the O1 class species are also likely due to high concentrations of sterol-like compounds in the oil sample.29 The O1 class species is also a major contributor to the TAN value of oil. It was clearly presented that an obvious increase in the carbon number of the abundant O1 class species for both VGOs and dewaxed VGOs from cut-2 to cut-4 was attributed to the increasing of boiling point of VGO fractions. The relative abundance of the O1 class species with DBE values of 4 and 5 increased and then decreased as the carbon number increased. Also it seems that the O1 class species in dewaxed VGOs tended to shift toward higher carbon numbers slightly compared to VGOs. The O1 class species with a DBE value of 4 were also observed as the most abundant in each dewaxed oil sample, which maybe implied that all kinds of O1 class species were removed at almost same removal efficiency from VGO cuts during refining processes for base oil. DBE versus Carbon Number Distributions for N1 Class Species. The N1 class species are another key species shown in negative ESI mass spectra. Figure 7 shows the iso-abundance maps of DBE as a function of the carbon number for N1 class species in each VGO fraction and its dewaxed oil. The abundant N1 class species shifted to higher DBE values and carbon numbers as the boiling point of the distillate fraction increased. A slight shift to higher carbon numbers for the N1 class species in dewaxed VGOs compared to VGOs was

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CONCLUSIONS Detailed molecular compositions of acidic compounds in three different boiling point VGO samples and their corresponding dewaxed VGOs were characterized by negative ESI FT-ICR MS. The acidic heteroatom compounds, N1, N2, N1O1, N1O2, O1, and O2 class species were identified. The distributions of the most abundant O2, O1, and N1 class species in each distillate fractions were determined. N1, N2, N1O1, and N1O2 class were identified in VGO cuts while not found in dewaxed VGOs. The distribution of acidic compounds shifted to higher carbon numbers and higher molecular weight gradually with the increase of the boiling point of the fraction. The composition of acidic species between VGOs and dewaxed VGOs was compared to find out the difference between them so that a 5652

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(9) Zhu, X.; Shi, Q.; Zhang, Y.; Pan, N.; Xu, C.; Chung, K. H.; Zhao, S. Characterization of Nitrogen Compounds in Coker Heavy Gas Oil and Its Subfractions by Liquid Chromatographic Separation Followed by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2011, 25, 281−287. (10) Klein, G. C.; Rodgers, R. P.; Marshall, A. G. Identification of hydrotreatment-resistant heteroatomic species in a crude oil distillation cut by electrospray ionization FT-ICR mass spectrometry. Fuel 2006, 85 (14−15), 2071−2080. (11) Shi, Q.; Xu, C.; Zhao, S.; Chung, K. H.; Zhang, Y.; Gao, W. Characterization of Basic Nitrogen Species in Coker Gas Oils by Positive-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2010, 24, 563−569. (12) Fu, J. M.; Klein, G. C.; Smith, D. F.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Comprehensive compositional analysis of hydrotreated and untreated nitrogen-concentrated fractions from syncrude oil by electron ionization, field desorption ionization, and electrospray ionization ultrahigh-resolution FT-ICR mass spectrometry. Energy Fuels 2006, 20 (3), 1235−1241. (13) Bae, E.; Na, J.-G.; Chung, S. H.; Kim, H. S.; Kim, S. Identification of about 30 000 Chemical Components in Shale Oils by Electrospray Ionization (ESI) and Atmospheric Pressure Photoionization (APPI) Coupled with 15 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) and a Comparison to Conventional Oil. Energy Fuels 2010, 24 (4), 2563− 2569. (14) Smith, D. F.; Klein, G. C.; Yen, A. T.; Squicciarini, M. P.; Rodgers, R. P.; Marshall, A. G. Crude oil polar chemical composition derived from FT-ICR mass spectrometry accounts for asphaltene inhibitor specificity. Energy Fuels 2008, 22 (5), 3112−3117. (15) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Reading chemical fine print: Resolution and identification of 3000 nitrogen-containing aromatic compounds from a single electrospray ionization Fourier transform ion cyclotron resonance mass spectra of heavy petroleum crude oil. Energy Fuels 2001, 15 (2), 492−498. (16) Shi, Q.; Hou, D.; Chung, K. H.; Xu, C.; Zhao, S.; Zhang, Y. Characterization of Heteroatom Compounds in a Crude Oil and Its Saturates, Aromatics, Resins, and Asphaltenes (SARA) and Non-basic Nitrogen Fractions Analyzed by Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2010, 24, 2545−2553. (17) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K. N.; Robbins, W. K. Identification of acidic NSO compounds in crude oils of different geochemical origins by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2002, 33 (7), 743−759. (18) Smith, D. F.; Rodgers, R. P.; Rahimi, P.; Teclemariam, A.; Marshall, A. G. Effect of Thermal Treatment on Acidic Organic Species from Athabasca Bitumen Heavy Vacuum Gas Oil, Analyzed by Negative-Ion Electrospray Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry. Energy Fuels 2008, 23 (1), 314− 319. (19) Hughey, C. A.; Galasso, S. A.; Zumberge, J. E. Detailed compositional comparison of acidic NSO compounds in biodegraded reservoir and surface crude oils by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Fuel 2007, 86 (5−6), 758−768. (20) Zhang, Y.; Xu, C.; Shi, Q.; Zhao, S.; Chung, K. H.; Hou, D. Tracking Neutral Nitrogen Compounds in Subfractions of Crude Oil Obtained by Liquid Chromatography Separation Using Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2010, 24, 6321−6326. (21) Liu, P.; Xu, C.; Shi, Q.; Pan, N.; Zhang, Y.; Zhao, S.; Chung, K. H. Characterization of Sulfide Compounds in Petroleum: Selective Oxidation Followed by Positive-Ion Electrospray Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Anal. Chem. 2010, 82 (15), 6601−6606.

deeper understanding about lube oil processing was known. The refining process to VGO cuts shifted the molecular weight and carbon number distribution of the abundant O1 and N1 class species to higher ranges, changing the appearance of the VGO spectra. For all identified naphthenic acids, polycyclic naphthenic acids were dominant in VGO cuts, while in their dewaxed oils, naphthenic acids with single-ring were mainly dominant. The most abundant O1 class species had DBE values of 4 corresponding to alkyl phenols in three VGO cuts as well as in three dewaxed VGOs. The abundant N1 class species in three VGO fractions were centered at DBE values of 9 and 12, and these were likely carbazoles and benzocarbazoles, respectively, while N1 class species with a DBE of 9 were dominant in three dewaxed VGOs. These differences between VGOs and their corresponding dewaxed VGOs were due to the refining process when VGO was used to produce base oil.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 8610-8973-9029. Fax: 8610-8970-2776. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank all the help from members of the State Key Laboratory of Heavy Oil Processing, China University of Petroleum in Beijing, and thank the no. 3 refinery of the Yanshan Petrochemical Company of SINOPEC for providing the oil samples.

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REFERENCES

(1) Stanford, L. A.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Characterization of Compositional Changes in Vacuum Gas Oil Distillation Cuts by Electrospray Ionization Fourier Transform−Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry. Energy Fuels 2006, 20 (4), 1664−1673. (2) Barrow, M. P.; McDonnell, L. A.; Feng, X. D.; Walker, J.; Derrick, P. J. Determination of the nature of naphthenic acids present in crude oils using nanospray Fourier transform ion cyclotron resonance mass spectrometry: The continued battle against corrosion. Anal. Chem. 2003, 75 (4), 860−866. (3) Barrow, M. P.; Headley, J. V.; Peru, K. M.; Derrick, P. J. Fourier transform ion cyclotron resonance mass spectrometry of principal components in oilsands naphthenic acids. J. Chromatogr., A 2004, 1058 (1−2), 51−59. (4) Smith, D. F.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Characterization of Athabasca bitumen heavy vacuum gas oil distillation cuts by negative/positive electrospray ionization and automated liquid injection field desorption ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2008, 22 (5), 3118−3125. (5) Slavcheva, E.; Shone, B; Turnbull, A. Review of naphthenic acid corrosion in oil refining. Br. Corros. J. 1999, 34 (2), 125−131. (6) Mapolelo, M. M.; Rodgers, R. P.; Blakney, G. T.; Yen, A. T.; Asomaning, S.; Marshall, A. G. Characterization of naphthenic acids in crude oils and naphthenates by electraspray ionization FT-ICR mass spectrometry. Int. J. Mass Spectrom. 2011, 300 (2−3), 149−157. (7) Yuan, Q. Progress and outlook on technologies for processing inferior crude oil in China. China Pet. Process. Petrochem. Technol. 2008, No. 1, 1−8. (8) Zaky, M. T.; Tawfik, S. M. Production of lubricating base oil from slop wax by different subsequent refining techniques. Fuel Process. Technol. 2011, 92 (3), 447−451. 5653

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Energy & Fuels

Article

(22) Mapolelo, M. M.; Stanford, L. A.; Rodgers, R. P.; Yen, A. T.; Debord, J. D.; Asomaning, S.; Marshall, A. G. Chemical Speciation of Calcium and Sodium Naphthenate Deposits by Electrospray Ionization FT-ICR Mass Spectrometry. Energy Fuels 2009, 23 (1), 349−355. (23) Qian, K. N.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Resolution and identification of elemental compositions for more than 3000 crude acids in heavy petroleum by negative-ion microelectrospray high-field Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15 (6), 1505−1511. (24) Wu, Z. G.; Rodgers, R. P.; Marshall, A. G. ESI FT-ICR mass spectral analysis of coal liquefaction products. Fuel 2005, 84 (14−15), 1790−1797. (25) Liu, P.; Shi, Q.; Pan, N.; Zhang, Y.; Chung, K. H.; Zhao, S.; Xu, C. Distribution of Sulfides and Thiophenic Compounds in VGO Subfractions: Characterized by Positive-Ion Electrospray Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2011, 25 (7), 3014−3020. (26) Fu, J.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G.; Qian, K. Nonpolar Compositional Analysis of Vacuum Gas Oil Distillation Fractions by Electron Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2006, 20 (2), 661−667. (27) Teravainen, M. J.; Pakarinen, J. M. H.; Wickstrom, K.; Vainiotalo, P. Comparison of the composition of Russian and North Sea crude oils and their eight distillation fractions studied by negativeion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry: The effect of suppression. Energy Fuels 2007, 21 (1), 266−273. (28) Jaffé, R.; Gallardo, M. T. Application of carboxylic acid biomarkers as indicators of biodegradation and migration of crude oils from the Maracaibo Basin, Western Venezuela. Org. Geochem. 1993, 20 (7), 973−984. (29) Shi, Q.; Zhao, S.; Xu, Z.; Chung, K. H.; Zhang, Y.; Xu, C. Distribution of Acids and Neutral Nitrogen Compounds in a Chinese Crude Oil and Its Fractions: Characterized by Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2010, 24 (7), 4005−4011. (30) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Mankiewicz, P. Acidic and neutral polar NSO compounds in Smackover oils of different thermal maturity revealed by electrospray high field Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2004, 35 (7), 863−880.

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dx.doi.org/10.1021/ef300318t | Energy Fuels 2012, 26, 5646−5654