Compositional Analysis of Oil Residues by Ultrahigh-Resolution

Article pubs.acs.org/EF

Compositional Analysis of Oil Residues by Ultrahigh-Resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Timo Kekal̈ aï nen,† Jaana M. H. Pakarinen,† Kim Wickström,‡ Vladislav V. Lobodin,§ Amy M. McKenna,§ and Janne Jan̈ is†,* †

Department of Chemistry, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland; Technology Centre, Neste Oil Oyj, P.O. Box 310, 06101 Porvoo, Finland; § National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States ‡

S Supporting Information *

ABSTRACT: Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry was used for compositional analysis of polar and asphaltene fractions of complex oil residues. The samples were collected before and after the processing of oil in a residue hydrocracking unit, in which the feed oil was the vacuum distillation residue of the crude oil, and the product sample was the residue collected after the processing. From the asphaltene fraction, as many as ∼26 000 peaks were detected by atmospheric pressure photoionization and more than ∼33 000 peaks by positive-ion electrospray ionization (ESI), with up to 18 distinct heteroatom classes identified. Negative-ion ESI provided complementary information through selective ionization of acidic compounds. The detected species were sorted based on heteroatom class, carbon number and aromaticity (double bond equivalence, i.e. number of rings + double bonds to carbon). The N1 class compounds were predominant in both fractions of the feed and product oils. The sulfur-containing compounds were mainly degraded or removed during the processing as expected. Vanadyl porphyrins (heteroatom class N4O1V1), detected in the asphaltene fraction of the feed oil, were not observed in the product oil fractions that is consistent with their efficient removal. Increase in the aromaticity for the most heteroatom classes was generally noticed in both polar and asphaltene fractions.

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resins/asphaltenes (SARA) separation.17 Characterization of asphaltenes is difficult owing to their chemical complexity and nonvolatile nature, and the fact that asphaltenes tend to selfassociate even at low concentrations.14 The compositional studies of asphaltenes have still been successfully performed by using ultrahigh-resolution mass spectrometry.18−23 Petroleum also includes traces of metals, among which the most abundant and undesirable are vanadium and nickel. These metals are present mainly as organometallic porphyrin complexes24,25 that are detrimental to petroleum processing. For example, vanadium can poison catalysts even at very low concentrations (≤0.01 wt %). Thus, there is a need for detailed characterization of vanadium and nickel compounds for development of oil demetalation.10,26−29 The concentration of metals increases with the content of asphaltenes, and also when the amount of sulfur increases.27 Generally, the higher the aromaticity and heteroatom content of petroleum the higher the level of metals. Compositional knowledge of refinery feedstock is essential for predicting how petroleum will behave in reservoirs and during processing, i.e., chemical composition should correlate with the properties and behavior of petroleum and its products. The term petroleomics refers to the characterization of all possible constituents of petroleum.30−33 The most compre-

INTRODUCTION The complexity of petroleum presents considerable challenges to its refining and processing as well as for its chemical analysis. The compositional complexity of petroleum increases gradually from light, low boiling point compounds to the most complex, nondistillable residues with increasing atmospheric equivalent boiling point (AEBP).1−6 Residues of distillations are the heaviest and the most complex part of petroleum, and hence the most problematic for the upgrading processes. Heavy petroleum fractions and distillation residues are typically enriched with metal- and heteroatom-containing (i.e., NxOySz) compounds. These compounds have significant implications for oil refining. One of the main concerns is a catalyst deactivation that leads to the loss of active sites. The deactivation could result from the catalyst poisoning by strongly adsorbed species, formation of coke or metal deposits or pore constriction and blockage.7−10 Furthermore, these compounds, among others, contribute to corrosion,11 and nitrogen and sulfur oxides, which are released during the combustion of oil, are the two major atmospheric pollutants.12,13 The vacuum residues of petroleum contain asphaltenes which are regarded as the most complex molecules in petroleum. Asphaltenes have a high degree of aromaticity, very high boiling points, and high content of heteroatomcontaining compounds and metals.14−16 They are defined as the nonvolatile fraction of petroleum that is soluble in toluene, but insoluble in alkanes (e.g., pentane).14 Asphaltenes are easy to isolate from the vacuum residue by saturates/aromatics/ © 2013 American Chemical Society

Received: October 30, 2012 Revised: February 27, 2013 Published: March 4, 2013 2002

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The mass percentage for each fraction in the feed/product oils are presented in Table 1 and the content of some elements in the unfractionated feed and product oils are shown in Table 2.

hensive method for petroleomics is high-resolution mass spectrometry (MS), especially Fourier transform ion cyclotron resonance (FT−ICR) technique. High-field FT−ICR MS provides the highest mass resolving power (m/Δm50% typically greater than 1 000 000, where Δm50% is the mass spectral peak full width at half-maximum) and mass accuracy (ppblevel).34−36 Thus, it is possible to identify, sort, and simultaneously monitor thousands of elemental compositions without any prior physical separation. The use of several ionization techniques with FT−ICR MS provides complementary information about different species present in petroleum samples. Electrospray ionization (ESI) is one of the most used ionization techniques. ESI selectively ionizes polar, heteroatom-containing species; acidic species by negative-ion and basic species by positive-ion ESI. However, atmospheric pressure photoionization (APPI) enables characterization of nonpolar compounds, such as polycyclic aromatic hydrocarbons (PAHs), and polycyclic sulfur-containing aromatic compounds (PASHs) and sulfides. Positive-ion APPI can form both protonated molecules and radical cations. APPI coupled to FT−ICR MS has been successfully used to characterize nonpolar compounds in petroleum samples.37−40 However, due to the formation of two types of ions, the complexity of a given mass spectrum increases further. Only the FT−ICR can achieve resolving power high enough to separate the overlapping peaks in a very complex petroleum sample; the mass difference of only 720 μDa between the two species has recently been demonstrated.41 The lighter fractions are much easier to upgrade than the residue part of petroleum. However, rapidly depleted world’s oil reserves keep the oil companies motivated to refine also the most problematic part of petroleum, such as oil distillation residues, into useful end products. The knowledge of the chemical constituents in the feed and the product oils is therefore crucial. The aim of this study was to investigate the composition of the heavy distillation residue of a crude oil (feed oil) that is used for production of various end products in a hydrocracking unit, mainly sulfur-free diesel fuel. The main tasks of the unit are to produce lighter petroleum distillates from the residue oil by cracking and hydrogenating heavy hydrocarbons, and to remove sulfur, nitrogen, and metal compounds with the assistance of catalyst and hydrogen gas. In addition, the vacuum distillation residue from this hydrocracking unit (further used as a heavy fuel oil) was also studied to see whether it contains any remaining heteroatom compounds and/or trace metals as compared to the feed oil.

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Table 1. Percentages of Saturates, Aromatics, Polars and Asphaltenes in the Feed/Product Oils after Separationa

feed product a

saturates (wt %)

aromatics (wt %)

polars (wt %)

asphaltenes (wt %)

13 20

35 37

38 28

14 15

Separation by the modified ASTM D2007 method.

Table 2. Elemental Analysis of the Unfractionated Feed/ Product Oils

feed product

carbon (wt %)

hydrogen (wt %)

nitrogen (wt %)

sulfur (wt %)a

88.1 91.6

7.4 6.1

1.4 1.5

2.7 1.0

a

Sulfur content was determined with a separate method than the CHN content; therefore, the total amounts are not equal to 100 wt %.

Sample Preparation. Methanol and toluene (both HPLC grade) as well as formic acid (in highest available purity) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO). Tetramethylammonium hydroxide (28% in methanol) was procured from Fisher-Scientific (Hampton, NH). The FeedPOL and the ProductPOL samples were dissolved in toluene:methanol mixture (1:1, v/v) to give a final concentration of 250 μg/mL, whereas the FeedASP and the ProductASP samples were dissolved in toluene:methanol mixture (3:1, v/v) to yield the same final concentration of 250 μg/mL. Formic acid was added to the dissolved samples to a concentration of 1.0 v % for positive-ion electrospray ionization (ESI) FT−ICR MS analysis and tetramethylammonium hydroxide was dosed to a concentration of 0.25 v % for negative-ion ESI FT−ICR MS. All samples were dissolved in toluene to yield a final concentration of 250 μg/mL without further modification prior to APPI FT−ICR MS analysis. Mass Spectrometry and Data Analysis. Experiments were conducted with a custom-built FT−ICR mass spectrometer equipped with a passively shielded 9.4 T superconducting magnet of 225 mm bore diameter (Oxford Instruments, Abingdon, Oxfordshire, U.K.)42 and a compensated seven segment open cylindrical (94 mm i.d.) ICR cell.43 Instrument control and data analysis were performed with a modular ICR data station.44 The conditions of ESI source were optimized to generate ions for petroleum samples. A fused-silica micro ESI needle with a 50 μm inner diameter (ID) was used to deliver each sample to the ionization source via a syringe pump at a rate of 400 nL/min under typical ESI conditions (2.4 kV; tube lens, 350 V and heated metal capillary current, 4 A). The APPI source (Thermo-Fisher Scientific, San Jose, CA) was coupled to the first pumping stage of the mass spectrometer (see below) through a custom-built interface.37 The tube lens was set to 50 V to minimize fragmentation of thermal ions, and heated metal capillary current of 4.5 A. A Hamilton gastight syringe (2.5 mL) and syringe pump were used to deliver the sample (50 μL/min) to the heated vaporizer region (300 °C) of the APPI source, where N2 sheath gas (50 psi) facilitated nebulization, while the auxiliary port remained plugged. Gas-phase molecules flow out of the heated vaporizer in a confined jet and photoionization is initiated by a krypton vacuum ultraviolet gas discharge lamp (10−10.2 eV photons, 120 nm). FT−ICR mass spectra were externally calibrated with Agilent (Palo Alto, CA) HP mix. Broadband frequency-sweep excitation (720−100 kHz at a sweep rate of 50 Hz/μs and 360 Vp‑p amplitude) was followed by broadband detection (6.6 s data acquisition period to yield 8 Mword time-domain data with a low mass Nyquist cutoff of at m/z 228). 100 time-domain transients were summed, Hanning-apodized, and zero-

EXPERIMENTAL SECTION

Oil Samples. The oil samples were provided by Neste Oil Oyj (Porvoo, Finland). The residue of the vacuum distillation of Russian crude oil (referred to as the feed oil) and the residue from the hydrocracking unit (referred to as the product oil) were analyzed. Both the feed and the product oils were separated as follows. First, the asphaltene fraction was removed from the oil by precipitation with npentane. Then the n-pentane soluble fraction (i.e., maltene fraction) was chromatographically separated, using a modified ASTM D2007 method, yielding saturate, aromatic, and polar fractions. In this study, only the polar and the asphaltene fractions were analyzed. Most heteroatom compounds and metals are especially enriched in these two fractions and are therefore a major interest for the current refinery process development. In summary, the four samples were analyzed: the polar fraction of the feed oil (FeedPOL), the polar fraction of the product oil (ProductPOL), the asphaltene fraction of the feed oil (FeedASP), and the asphaltene fraction of the product oil (ProductASP). 2003

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filled once, prior to fast Fourier transform and magnitude calculation. ICR frequencies were converted to ion masses based on the quadrupolar trapping potential approximation.45 Each spectrum was further internally calibrated with respect to an abundant homologous alkylation series differing in mass by integer multiples of 14.01565 Da (mass of a CH2 unit) confirmed by isotopic fine structure based on the “walking” calibration equation.36 Experimentally measured masses (for the peaks with a relative abundance greater than six standard deviations of baseline rms noise, 6σ) were converted from the International Union of Pure and Applied Chemistry (IUPAC) mass scale to the Kendrick mass scale to identify homologous series for each heteroatom class (i.e., species with the same heteroatom content, differing only by their degree of alkylation).46,47 Peak assignments were performed by Kendrick mass defect analysis as previously described.48 For each elemental composition (CcHhNnOoSs) the heteroatom class, degree of aromaticity (double bond equivalence, DBE = c − h/2 + n/2 + 1) and carbon number, c, were tabulated for subsequent generation of heteroatom class relative abundance distributions and graphical isoabundance-contoured DBE vs carbon number images with SigmaPlot 9.0 (Systat Software Inc., San Jose, CA). Protonated molecules exhibit half-integer DBE values and may thus be distinguished from radical cations with integer DBE values.

700 (ProductASP) peaks, indicating greater compositional complexity relative to polar fractions. Molecular weight distributions, centered between 400 < m/z < 600 for the polar and 350 < m/z < 550 for the asphaltene fractions, remain similar before and after the processing. However, the increased number of detected peaks in the both fractions indicates that the processing is likely to transform some of the heteroatomcontaining compounds to the other heteroatom compound classes rather than to remove these compounds from the oil. The most notable difference in the mass spectrum of FeedASP was the appearance of highly abundant peaks observed within 500 < m/z < 600 that differ in mass by 14.10565 Da (mass of a CH2 unit). These signals correspond to vanadyl porphyrins (heteroatom class N4O1V1), with DBE values ranging from 17 to 24, as previously reported.49 In contrast, the ProductASP sample contained only a trace amount of these compounds, which indicates effective removal of vanadyl porphyrins through the processing (Figure 2A). Vanadium in petroleum can be

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RESULTS AND DISCUSSION Positive-Ion Atmospheric Pressure Photoionization. Broadband positive-ion APPI FT−ICR mass spectra of the polar and asphaltene fractions of both feed and product oils are shown in Figure 1. The mass spectra of FeedPOL and ProductPOL contained ∼18 800 and ∼21 100 peaks with signal magnitude greater than six times the baseline rms noise, while the asphaltene fractions contain ∼24 100 (FeedASP) and ∼25

Figure 2. (A) DBE distribution for N4O1V1 class compounds in asphaltene fractions, and (B) possible core structures of vanadyl porphyrins found in petroleum.49.

found in the porphyrin structures,50−52 with the two main structures being etioporphyrin (Etio) and deoxophylloerythroetioporphyrin (DPEP) with DBE values of 17 and 18, respectively. Reported core structures of some N4O1V1 species are presented in Figure 2B.26,49,53 The concentration ratio of DPEP to Etio is an oil maturity indicator; more mature crude oils contain a higher amount of DPEP vanadyl porphyrin.25,50 Vanadyl porphyrins have also been observed previously in petroleum samples.49,53−57 In addition, the FeedPOL fraction

Figure 1. Broadband positive-ion APPI FT−ICR mass spectra of the feed and the product oils. 2004

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The increased abundance of HC, N1, and also N2 species after the processing could indicate efficient removal of the other classes, or conversion of multiheteroatomic (e.g., NS, NOx, SOx, etc.) species to HC, N1, or N2 species. Nitrogen species have a considerable impact to the refinery processes due to the poisoning of catalysts by the coke formation.7,9 Hydrodenitrogenation occurs only at saturated rings, which means that the nitrogen-containing unsaturated heterocyclic rings must first be saturated before the carbon−nitrogen bond cleavage can take place.59−61 Figure 4 shows isoabundance-contoured DBE vs carbon number plots for the HC and N1 class species detected with APPI. These data clearly show that the DBE values and the carbon numbers of the N1 species in the both fractions remained nearly unaltered, which is consistent with high stability of these compounds. Of course, some multiple heteroatom compounds (e.g., NSx, N2, NOS, NOx) have likely

also contained trace amounts of vanadyl compounds (Figure 3A). According to the literature, vanadyl porphyrins are slightly

Figure 3. Relative abundance of the most abundant heteroatom compounds in the polar (A) and asphaltene (B) fractions of both the feed and product oils identified by positive-ion APPI FT−ICR MS.

soluble in small alkanes (e.g., hexane and cyclohexane).58 In this work, deasphaltenation was performed with n-pentane as the solvent; therefore it is likely that the maltene fraction also contained small amounts of porphyrins which were then observed from the FeedPOL fraction. Figure 3 shows the positive-ion APPI FT−ICR MS heteroatom class distributions for all classes detected with >1% relative abundance (calculated from the sum of relative abundance of the peaks of each compound class divided by the total relative abundances of all the peaks in the spectra). The most abundant class detected in the FeedPOL fraction was N1, followed by N1O1 and N1S1, with most classes containing more than one heteroatom per molecule. In the ProductPOL fraction, the most abundant class was also N1 followed by hydrocarbon (HC) and N1O1 classes (Figure 3A). Figure 3B shows the heteroatom class distributions for the FeedASP and ProductASP fractions. The three most abundant classes in the FeedASP fraction were HC, N1 and S1. In the ProductASP, higher abundance for HC, N1, and N2 species was observed with a concomitant decrease of the other higher order heteroatom classes (except O1 and O2), similar to the polar fraction.

Figure 4. Isoabundance-contoured DBE vs carbon number plots for the N1 and HC class species detected with APPI. PAH planar limit is the highest DBE that a molecule can have at a given carbon number and still remain in a planar structure (DBE = #C × 0.893).62 2005

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produced more N1 species by removal of heteroatoms (S, N, and O). Figure 4 shows that the carbon number of the N1 compounds in the polar fraction increases without increased in the DBE. In contrast, in the asphaltene fraction the carbon number increases along with the increase of aromaticity. The same behavior was noticed with several heteroatom-containing classes in this study. Figure 4 also shows that the HC species had a high level of aromaticity with the DBE of the compounds approaching the PAH planar limit;62 the most abundant DBE series were between 20 and 30. The HC species with DBE values of less than 15 disappeared almost totally, but more unsaturated compounds seemed to be quite stable. The most abundant sulfur-containing classes in the feed oil were S1, N1O1S1, and N1S1 (Figure 3). After the processing, a decreased abundance of sulfur-containing species was observed in the both ProductPOL and ProductASP fractions, suggesting their targeted removal or conversion to other compounds (hydrodesulfurization). The only exceptions were the S2O1 and S2O2 compounds in the asphaltene fraction. Hydrodesulfurization happens more easily than the cleaving of nitrogen. A variety of sulfur compounds can be found in petroleum including thiols, sulfides, disulfides, thiophenes, benzothiophenes, and dibenzothiophenes.63−68 Most sulfur-containing compounds (∼60−70%) are nonpolar and therefore not efficiently ionized by ESI without derivatization.69−71 Lower ionization potentials inherent to highly conjugated ring systems increases photoionization efficiency for aromatic hydrocarbons and sulfur-containing species, which are abundant in heavy oil fractions. Therefore, APPI has readily been applied to characterize sulfur compounds present in petroleum samples.23,55,72 Isoabundance-contoured DBE versus carbon number plots for the S1 and S1O1 class are shown in Figure 5; these species were highly fused aromatic ring containing compounds. Interestingly, the S1 class of the ProductASP fraction shows much wider distribution of compounds in terms of their DBE and carbon number as compared to FeedASP; increased aromaticity and degree of alkylation suggest formation of new S1 class compounds from the other sulfurcontaining compounds. Somewhat biased results were generally observed considering the formation of protonated molecules and the corresponding radical cations for some of the compound classes. For example, the separate plots of [M + H]+ and [M]+• ions of the S1O1 compounds (Figure 6) shows that the species with lower aromaticity (lower DBE) did not form radical cations. The same was true for the other classes as well. This phenomenon could be seen with the HC and S1 species, but it was even more evident for compounds constituting polar fractions. Positive-ion Electrospray Ionization. Broadband positive-ion ESI FT−ICR mass spectra of the FeedASP and ProductASP fractions consisted of ∼33 100 and ∼21 600 peaks, respectively, indicating a considerable decrease in the number of different heteroatom-containing species (Figure S1 of the Supporting Information, SI). In contrast, the mass spectra of the FeedPOL and ProductPOL fractions consisted of ∼19 000 peaks and ∼18 200 peaks, respectively. Thus, the number of the peaks was nearly unaltered. Figure 7 shows the heteroatom class distribution for all detected species in the positive-ion ESI FT−ICR mass spectra. The asphaltene fraction is characterized by more diverse content of heteroatom classes, and on average, higher number of heteroatoms in each class than in the case of the polar fraction. In the both fractions, the most abundant species were nitrogen-containing (Nx) com-

Figure 5. Isoabundance-contoured DBE vs carbon number plots for the S1 and S1O1 classes detected with APPI.

pounds. Basic nitrogen species in petroleum are mainly pyridines, quinolines, and acridines and their derivatives.73−78 Especially in the product samples the proportion of the Nx species was pronounced; the relative abundance of the Nx species was more than 80% in the ProductPOL sample, and almost 70% in the ProductASP sample. Thus, the results indicate that the processing could not completely remove N x compounds from the feed oil. The behavior of the Nx compounds was quite foreseeable, because similar results have been previously reported.79−82 An unexpected result was the observation of the HC species in the asphaltene fractions (Figure 7B). To the best of our knowledge, the HC species have not been earlier reported in the petroleum samples analyzed with positive-ion ESI. Owing to the intrinsically low proton affinities, the HC species are not usually detected with positive-ion ESI, but can efficiently be ionized with APPI. These compounds were highly polycyclic structures with DBE values approaching the PAH planar limit (Figure S2 of the SI).62 We anticipate that the protonation must have occurred via some proton transfer reaction(s), 2006

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polar heteroatom-containing compounds can be analyzed with negative-ion ESI. On average, compounds in the polar fraction seemed to have higher molecular weights than compounds in the asphaltene fraction, which is consistent with the positive-ion ESI measurements. The amount of the peaks was identical in the FeedPOL and ProductPOL samples (∼12 400 peaks). The results of the asphaltene fractions were totally different, since the number of detected species decreased substantially; there were ∼29 500 peaks in the FeedASP oil spectrum, but only ∼10 400 peaks in the ProductASP oil spectrum. The most abundant species were again the Nx species in all samples (Figure 8) with predominant N1 class. The compounds

Figure 6. Isoabundance-contoured DBE versus carbon number plots for [M + H]+ and [M]+• ions of the S1O1 class compounds in the polar fractions of the feed and product oils detected with APPI.

Figure 8. Relative abundance of the most abundant heteroatom compounds in the polar (A) and asphaltene (B) fractions of both the feed and product oils identified by negative-ion ESI FT−ICR MS.

containing one N atom, detected by deprotonation with negative-ion ESI, correspond to pyrroles, indoles, and carbazoles and their derivates.73−78 The relative abundance of the N2 class increased during the processing in the asphaltene fraction whereas the opposite was observed for the polar fraction. N1O1, which was one of the most abundant compound classes, changed to the opposite direction than the N2 species. Overall, there were a lot of NxOy species detected in both the polar and the asphaltene fractions and they seemed to be greatly resistant to removal by the processing, data consistent with the APPI as well as with the positive-ion ESI data. Interestingly, HC species were also detected from all samples (for DBE vs carbon number plots, see Figure S4 of the SI). Negative-ion ESI process relies on deprotonation of acidic species in petroleum. Therefore, when conventional solvent modification with ammonium hydroxide is applied to heavy oil fractions, the most acidic compounds (i.e., carboxylic acids and naphthenic acids) are most efficiently ionized. However, modification of the solvent system with tetramethylammonium hydroxide (TMAH) enhances the compositional diversity of

Figure 7. Relative abundance of the most abundant heteroatom compounds in the polar (A) and asphaltene (B) fractions of both the feed and product oils identified by positive-ion ESI FT−ICR MS.

probably in the ion source region. The proportion of the HC class was higher in the product than in the feed oil sample. This result is in agreement with the APPI measurements (Figure 3). Although the processing had effectively removed/degraded the polar sulfur-containing compounds from the feed oil (see, for example classes NS, N2S, NOS, and NS2 in the asphaltene fractions), the NxOy class compounds seemed to moderately withstand the oil processing. Negative-Ion Electrospray Ionization. Broadband negative-ion ESI FT−ICR mass spectra of the polar and asphaltene fractions are presented in Figure S3 of the SI. Typically, acidic, 2007

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acidic species through selective ionization of five-membered ring hydrocarbons, as previously reported.83,84 On the basis of the negative-ion ESI data, the processing had effectively removed sulfur-containing compounds from the oil; the proportions of N1S1, N1O1S1, N2S1, and N1S2 species decreased remarkably. Considering the peak numbers between the FeedPOL and ProductPOL samples, it might be expected that the sulfur-containing compounds generated more Nx and NxOy species to the product oil. However, these compounds had been likely removed, and not degraded, from the asphaltene fraction given the dramatic decrease in the peak numbers between the FeedASP and ProductASP samples.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Finnish Funding Agency for Technology and Innovation (Tekes), Neste Oil Oyj, the University of Eastern Finland, and the Academy of Finland (grant 259901) is gratefully acknowledged. Neste Oil Oyj is thanked for supplying the oil samples for this study. National High Magnetic Field Laboratory (NHMFL) work was supported by NSF Division of Materials Research through DMR-0654118, Florida State University, and the State of Florida. The authors thank Dr. Ryan P. Rodgers (NHMFL) for fruitful discussions.

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CONCLUSIONS The aim for this study was to examine the efficiency of a residue hydrocracking unit to remove nitrogen-, oxygen-, and sulfurcontaining compounds as well as metals from the heavy feed oil to produce low-sulfur diesel and other end-products. Knowledge of the compositional changes between the feed and the product oils is important for the fine-tuning and development of the used processing units. Variation in the chemical composition of petroleum directly impacts all refinery processes. Two different fractions of the residue oils, namely polar and asphaltene fractions, were analyzed with APPI and ESI FT−ICR MS. Ultrahigh resolution FT−ICR mass spectrometry provides sufficiently high mass resolving power to resolve and identify different compounds in very complex petroleum samples. The utilization of two different ionization techniques (APPI & ESI) provides complementary information about different species in the samples. The most abundant compound classes were the Nx species in all oil samples, before and after the hydrocracking. With APPI, the most abundant heteroatom classes detected were N1 and hydrocarbon (HC) classes. These results indicate that N1 species are resistant to the applied oil processing. The amount of sulfur-containing compounds was clearly decreased during the processing but some of them were still detected from the both fractions of the product oil. Vanadyl porphyrins (heteroatom class N4O1V1) were detected from the asphaltene fraction of the feed oil but not from the either fraction of the product oil, which is consistent with their efficient removal. Vanadyl porphyrins are known to poison the upgrading catalysts and to assist coke formation, so that their detection and removal is of great importance for oil refinement.

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ASSOCIATED CONTENT

* Supporting Information S

Figure S1. Broadband positive-ion ESI FT−ICR mass spectra of the feed and the product oils. Figure S2. Isoabundancecontoured DBE/H:C-ratio vs carbon number plots for the HC class detected from asphaltene fraction with positive-ion (ESI FT−ICR MS). Figure S3. Broadband negative-ion ESI FT− ICR mass spectra of the feed and the product oils. Figure S4. Isoabundance-contoured DBE vs carbon number plots for the HC class detected with negative-ion ESI FT−ICR MS. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: janne.janis@uef.fi. 2008

dx.doi.org/10.1021/ef301762v | Energy Fuels 2013, 27, 2002−2009

Energy & Fuels

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