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Evidence of naturally-occurring vanadyl porphyrins containing multiple S and O atoms
Fuel 239 (2019) 1258–1264
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Evidence of naturally-occurring vanadyl porphyrins containing multiple S and O atoms
T
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Kuangnan Qiana, , Thomas R. Fredriksena, Anthony S. Mennitoa, Yunlong Zhanga, Michael R. Harpera, Shamel Merchanta, J. Douglas Kushnericka, B. McKay Ryttingb, ⁎ Peter K. Kilpatrickb, a b
ExxonMobil Research and Engineering Company, Annandale, NJ 08801, United States Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, United States
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Petroporphyrins FTICR-MS Petroleum separations
A host of vanadyl petroporphyrins containing multiple sulfur and oxygen atoms and their combinations (CcH2c+ZN4VOoSs, s = 0–3, o = 1–4, Z = −28 to −70) in a vacuum residue were observed by a combination of extensive enrichment of petroporphyrins and detailed analysis using ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Many of the petroporphyrin species are being observed for the very first time. The patterns of the porphyrin type distribution revealed dramatic differences in the sulfur and oxygen atom incorporations and their impact on Z number. For each sulfur atom addition, the average Z-number was reduced by ∼4–7, implying that sulfur may be incorporated by the additions of thiophene or cyclic thiophene moieties. In contrast, oxygen addition had little to no impact on the average Z number, implying that oxygen may be incorporated as carbonyl or hydroxyl groups. The porphyrin-based structures were found to contain as many as 8 aromatic rings in addition to the porphyrin macrocycle. Petroporphyrins in longer wavelength absorption fractions were found to contain more heteroatoms and more condensed aromatic rings.
1. Introduction Petroporphyrins (or metallopetroporphyrins) are an important class of geological biomarkers used in petroleum exploration applications
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[1–4]. These molecules are also the primary host of metals, such as vanadium and nickel, in petroleum and have implications in petroleum refining processes [5–8]. Petroporphyrins have been the subject of research since the structures were first proposed by Treibs in 1936 [1–9].
Corresponding authors. E-mail address: [email protected] (K. Qian).
https://doi.org/10.1016/j.fuel.2018.09.115 Received 20 July 2018; Received in revised form 9 September 2018; Accepted 22 September 2018 Available online 29 November 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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2.2. Fourier transform ion cyclotron resonance mass spectrometry
Vanadyl and nickel porphyrins are two main porphyrin types which are derived from their biological precursors, such as chlorophyll, hemes, and chlorins found in nearly all living organisms. These cyclic tetrapyrrolic structures assume the general chemical formula, CcH2c+ZN4VO and CcH2c+ZN4Ni. The chemical formula of vanadyl porphyrins was first confirmed by ultra-high resolution mass spectrometry in 2001 using electrospray ionization [10]. The confirmation of nickel porphyrin chemical formula came much later in 2010 [11], when nickel porphyrins were first enriched by cyclograph separation and then analyzed by FTICR-MS via atmospheric pressure photoionization (APPI). Direct detection of nickel porphyrins in oil seeps without preseparation was later demonstrated by ultra-high resolution FTICR-MS analysis [12]. Sulfur-containing vanadyl (VOS) porphyrins were first reported in 2008 in a petroleum asphaltene sample [13]. The VOS porphyrins were later confirmed by multiple laboratories [14–16]. More recently, porphyrins containing additional oxygen and nitrogen atoms were reported [17–20]. Unlike sulfur, additional nitrogen and additional oxygen are believed to be incorporated on the side chains as NH2 and ether or carboxylic acid, respectively. The nature of organic V and Ni in petroleum continue to be studied intensely because both elements have not been quantitatively accounted for based on porphyrin structures. In addition, the molecular weight distributions of metal petroporphyrins appear to be different from that of hydrocarbons. In general, the molecular weight distributions (MWD) of petroporphyrins are much narrower than that of hydrocarbons and with a lower MWD peak position [13]. On the other hand, all data generated so far are consistent with the porphyrin backbones, which grows in a similar fashion as other petroleum compound families. For example, the core structure can grow in both aromatic and naphthenic rings. Recently, a collaborative work between ExxonMobil and the University of Notre Dame had successfully generated purified vanadyl petroporphyrin fractions [21]. These fractions were characterized by a number of analytical techniques, showing high purity enrichment. In this work, four purified vanadyl fractions with varying maximum UV Soret absorbance were subjected to more examinations to understand porphyrin compositions and their molecular weight distributions. Field desorption ionization time-of-flight mass spectrometry (FD TOF-MS) was applied to confirm the molecular weight distribution of the petroporphyrins observed by APPI FTICR-MS. Detailed examination of ultra-high resolution FTICR-MS data revealed a host of new vanadyl porphyrins containing multiple sulfurs and oxygens and their combinations. The pattern of the porphyrin type distribution showed dramatic differences in sulfur and oxygen incorporations.
High resolution mass spectrometric analyses were conducted on a 15 Tesla Bruker solariX XR FTICR-MS (Bruker Daltonics Inc., Billerica, MA, USA). About 0.5 mg of the petroporphyrin sample was dissolved in 5–20 mL of toluene to form a 25–100 ppm solution. Concentration was adjusted to achieve adequate response. The solution was introduced into an APPI source using an integrated stock syringe pump with a 250 µL syringe (Hamilton Co., Reno, NV, USA). The flow rate was controlled at 120 µL/h. The APPI source was manufactured by Syagen Technology Inc. (Tustin, CA, USA), and is comprised of a heated capillary needle and a krypton UV lamp with ionization energy of 10.6 eV. Nitrogen was used as both the nebulizing and the drying gas. The nebulizing gas flow rate was set between 1 and 1.5 L/min while the drying gas flow rate was set between 3 and 5 L/min. The flow rates were adjusted to maximize APPI-FTICR-MS signals. The nebulizing gas temperature was set at 420 °C to maximize the vaporization of asphaltene molecules while maintaining a stable spray current. Toluene was used as both the solvent and the chemical ionization agent. Toluene was ionized by direct photoionization while asphaltene molecules (including metalloporphyrins) were ionized by charge transfer with toluene ions. During the FTICR-MS analysis, the acquisition mass range was set between m/z 300 and 3000. The ion accumulation time varied between 40 and 60 ms, adjusted for response. The time-of-flight was set to 1.2 ms. The funnel Rf was set to 115 Vpp. The excitation energy was ramped linearly from 18.0 to 90.2 percent, relative to effective maximum, up to a selected transition mass of m/z 1000. The full excitation frequency range was 76.7–769.3 kHz. The dataset size was set to 8 Megawords. Two hundred data sets were co-added to generate the final spectrum. Bruker Data Analysis (DA) software was used to find and calibrate the mass-to-charge peak list with signal-to-noise (S/N) ratio > 6. Data was internally calibrated using a homologous mass series. The calibrated peak list was exported to a table of mass and intensity. Peaks falling outside of a Kendrick mass defect series were removed, as well as any background or noise peaks. The “cleaned” and calibrated mass peak list was further analyzed for identification of VO porphyrin molecules. 2.3. Field desorption ionization time-of-flight mass spectrometry FD TOF-MS was conducted on a Waters GCT instrument. About 0.1 μL of the liquid was applied onto an FD emitter (from Linden ChroMasSpec) using a syringe. The FD emitter was then inserted into the ion source of a mass spectrometer near the extraction electrodes. The emitter (at ground voltage) was ∼1.5 mm away from a pair of extraction rods held at high potential (−12 kV), producing very high electric fields (∼10−7−10−8 V/cm) around the tips of the carbon dendrites. FD emitter current was manually ramped from 0 to 45 mA during data collection. Ions generated by FD were subjected to analysis by a reflectron ToF MS with an effective path length of 1.2 m. The voltage pulse was applied at a frequency of 30 kHz. A full spectrum was generated every 33 μs. The mass range was set at 200–3000 Da. A summed spectrum was collected every second in a centroid mode.
2. Experimental materials and methods 2.1. Samples Four petroporphyrin samples, each having a different Soret band maximum, were selected from a collation of petroporphyrins purified by Rytting et al in the 2018 work [21]. The details of the separation procedure can be found in the “M3 Petroporphyrin Enrichment and Purification” section. Fig. 1 shows a simplified separation flow scheme. In brief, a vacuum residue (M3) was subjected to a series of separation procedures comprising Soxhlet solvent extractions, extrographic and chromatographic separations using silica and alumina columns. The separations produced multiple fractions of purified petroporphyrins exhibiting a large range of Soret band maxima. Of these purified petroporphyrins, samples with Soret band maxima of 407, 411, 418, and 421 nm were analyzed in detail by FD-TOF MS and APPI-FTICR MS. The four samples are referred to in this work as fractions M3-407, -411, -418 and -421, respectively. Fig. 2 showed an overlay of UV spectra of the four samples.
3. Results and discussion 3.1. Molecular weight distributions Fig. 3 shows the molecular weight distributions of M3-407, -411, -418 and -421, respectively, by two different ionization methods and on two different instruments. On the left side, mass spectra were obtained by FD ToF-MS. The average mass resolution of ToF-MS is about 5000 [22]. In this work mass resolution (R) and mass resolving power (RP) are used interchangeably and defined as R = M/ΔMFWHM where M is the mass and ΔMFWHM is the mass peak width (full width at half 1259
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Fig. 1. Isolation scheme of four purified vanadyl porphyrin fractions examined in this work. The fraction numbers reflect the maximum wave length of the Soret absorption band of respected fractions.
composition. Fig. 4(a) displays the detailed APPI-FTICR mass spectra of sample fraction M3-421 at 4 nominal masses of 549, 689, 829, and 969 (mass increment 140 is equivalent of 10 CH2), respectively. Isolation of vanadyl porphyrins greatly simplified sample matrix and eliminated many potential mass overlaps. Assignments of chemical formula for mass peaks with a nominal mass of 969 and signal to noise ratio > 6 are shown in Table 1. The formula assignments of major mass peaks are also given in Fig. 4(a). Unique chemical formula can be assigned with mass error less than 0.2 mDa. Here, vanadyl porphyrins are described using general chemical formula, CcH2c+Z NnVOoSs where Z is the hydrogen deficiency index and c, n, o, s are the number of carbon, nitrogen, oxygen, and sulfur, respectively. Another commonly used terms to describe unsaturation of petroleum molecules is Double Bond Equivalence (DBE). There is a simple relation between Z and DBE as shown in Eq. (1)
Z= −2 × (DBE−1) + n Fig. 2. Overlaid UV–Visible spectra of M3-407, -411, -418, and -421, normalized for ease of comparison.
(1)
Here n is the number of nitrogen atoms in a molecule. In this work, we use Z VOoSs to shorthand the core compositions, for example C33H30N4VO is a member of −36 VO family. C60H66 VOS2 is shortened as −54 VOS2. The complexity of the mass spectra increases as the mass increases. Unsaturation and number of sulfur atoms per molecule also increase with mass. Mass 549 shows one major vanadyl porphyrin compound, C33H30N4VO (in −36 VO family). Mass 689 shows the most abundant peak to be a sulfur containing porphyrin C41H42N4VOS (−40 VOS). Mass 829 revealed two abundant peaks, C52H50N4VOS (−54 VOS) and C54H58N4VO (−50 VO). There are a total of 10 mono-isotope and two 13 C2 isotope vanadyl porphyrins found at nominal mass 969 (Table 1). Four major types are indicated in Fig. 4(a), C58H54N4VOS3 (-62 VOS3), C60H62N4VOS2 (-58 VOS2), C62H70N4VOS (-54 VOS) and C64H78N4VO (-50 VO), respectively. To the best of our knowledge, this is the first time that vanadyl petroporphyrins containing multiple sulfurs (number of sulfur atoms > 1) and multiple sulfur/oxygen combinations have ever been observed experimentally. The possible core structures of the porphyrin types are illustrated in Fig. 4(b). “−36 N4VO” can be assigned as benzo deoxophylloerythroetioporphyrin (benzo-DPEP). “−40 VOS” can be expressed as DPEP
maximum). On the right side, mass spectra were obtained by APPI on solariX FTICR-MS. Mass resolution at 600 is ∼1 million. APPI had exhibited limitations in ionizing high boiling vacuum residue molecules (up to ∼1300°F) [23]. Field desorption has been shown in the past to ionize high molecular weight hydrocarbon molecules, such as polywax and polystyrenes with molecular weights up to 5000 g/mol [24]. In Fig. 3, the patterns of the mass profile by the two techniques are very similar. The mass distributions peaked around m/z 550–600. Fraction M3-421 showed bimodal distributions by both techniques. In APPI FTICR-MS, no mass peaks were observed beyond m/z 1200. Although FD TOF-MS show signal beyond 1200, the signal to noise is low. It is safe to say that APPI FTICR-MS can detect the majority of petroporphyrins in the sample.
3.2. Resolution and identification of vanadyl porphyrins Among the four samples, M3-421 showed the most complex 1260
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Fig. 3. (a) Mass spectra of four purified vanadyl porphyrin fractions by FD TOF-MS (left) and (b) by APPI FTICR-MS (right). Similar mass profiles observed by two different ionization methods on two different mass spectrometers.
Fig. 4. (a) Zoomed-in mass spectral segments of sample M3-421 fraction at nominal mass 549, 689, 829 and 969. Spectra complexity increases with the increase of nominal mass. (b) Illustrative vanadyl porphyrin structures. Increase in sulfur and decrease in Z-number can be explained by incorporations of thiophenes. 1261
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Table 1 Assignments of chemical formula for components at nominal mass 969. The masses are that of molecular ions (one electron mass less than that of neutral). Experimental Mass (Da)
Signal Magnitude
Signal/Noise
Resolving Power
Theoretical Mass (Da)
Mass Errors (mDa)
Chemical Formula
Z-Number
969.28945 969.30718 969.34352 969.37083 969.3766 969.37989 969.39773 969.43403 969.4616 969.46717 969.47045 969.56096
3,165,220 2,817,061 5,182,502 2,298,919 7,492,647 16,610,343 2,839,592 5,383,209 3,652,650 5,022,762 16,790,570 5,182,509
9.4 8.1 17 6.2 25.6 59.8 8.2 17.7 11.2 16.4 60.5 17
728,128 624,312 644,942 865,994 745,941 688,922 646,111 667,621 720,752 781,675 662,013 688,915
969.28938 969.30714 969.34353 969.37097 969.37654 969.37991 969.39765 969.43406 969.46150 969.46707 969.47044 969.56097
0.07 0.04 −0.01 −0.14 0.06 −0.02 0.08 −0.03 0.10 0.10 0.01 −0.01
C58H54N4VOS3 C58H54N4VO3S2 C59H58N4VO2S2 C5813C2H60N4VOS2 C63H58N4VOS C60H62N4VOS2 C60H62N4VO3S C61H66N4VO2S C6013C2H68N4VOS C65H66N4VO C62H70N4VOS C64H78N4VO
−62 −62 −60 −60 −68 −58 −58 −56 −56 −64 −54 −50
fused with a benzothiophene. “−50 VO” can be assigned as fluorantheno etioporphyrin (fluorantheno-etio). “−54 VOS”, “−58 VOS2” and “−62 VOS3” can be described by additions of multiple thiophene units to the fluorantheno-etio. It should be noted that all structures displayed in this paper are for illustrative purposes. They are one of the many possible structures that are consistent with the Z-number specified and are not unique. Although the core structures are expressed as single cores, we recognized that high molecular weight petroleum molecules can contain both single core and multi-core structures. The differentiation of the two type of structures may be explored by fragmentation techniques, such as Collision-Induced Dissociation (CID) experiments. The subject is beyond the scope of this work but will be further studied in the future.
deficiency). Since APPI generates singly charged ions for the porphyrin samples, the term of “molecular weight” and “mass-to-charge ratio (m/ z) is used interchangeably in this paper. The intensity of each species is expressed by the color scheme of the map (red most intense and white least intense). The top row shows those species of vanadyl petroporphyrins containing multiple oxygen atoms (VO, VO2, VO3 and VO4). The numbers in the boxes are the intensity weighted average Z numbers. As the number of oxygen atoms increases, the average Z number remains relatively constant, implying that oxygen incorporation does not introduce unsaturation or additional hydrogen deficiency, and that most of the oxygen-containing petroporphyrins have roughly the same Z number or homologous serious. It is generally believed that oxygen functionalities (abundant in original biological precursor) were removed first during diagenesis and later were reintroduced during bacterial degradation. At present we suspect that oxygen atoms were incorporated as carbonyl or hydroxyl groups. The bottom row of Fig. 5 shows the FTICR-MS data of vanadyl petroporphyrins containing multiple sulfur atoms (VO, VOS, VOS2 and VOS3). In contrast to oxygen incorporations, the average Z number declines appreciably as the number of sulfur atoms in the vanadyl
3.3. Multiple sulfur and oxygen-containing vanadyl porphyrins Fig. 5 shows vanadyl petroporphyrins containing multiple sulfur and oxygen atoms in a two dimensional image display. Each box represents a class of vanadyl petroporphyrins. In each box, the X-axis is molecular weight and the Y-axis is the Z number (or hydrogen
Fig. 5. Vanadyl porphyrin compositions of sample M3-421 with increasing O and S incorporations. The numbers in the boxes are the intensity weighted average Z numbers. 1262
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Fig. 6. An overview of 12 vanadyl porphyrin types in M3-421. The multiple sulfur and sulfur/oxygen porphyrins (VOo=2-4 Ss=2-3) in boxes 6–12 are, to our knowledge, the first observations reported in the literature.
petroporphyrins increases, implying that each sulfur atom addition introduces more unsaturation. The average Z reduction per 1 sulfur incorporation ranges from about 4– 7. Structurally, the reduction in Z can be explained by incorporating a thiophene structure to the porphyrin molecules (ΔZ = −4 if fused; ΔZ = −6 if connected via a biaryl linkage). Fig. 6 presents an overview of all 12 porphyrin classes in sample M3-421 with incorporated oxygen and sulfur atoms observed by FTICRMS. The upper left box (1) is the traditional vanadyl porphyrins (VO) that have been studied and reported extensively. We observed Z number as negative as −68 for VO porphyrins, which is equivalent of the porphyrin base structure attached by a six aromatic ring system. The single sulfur-containing porphyrins (VOS) in box (5) have also been reported and studied since 2008 [11–14,16,20]. Multiple oxygen vanadyl porphyrins (VO2–VO4) were reported recently by other laboratories [14,20]. The multiple sulfur and sulfur/oxygen vanadyl porphyrins (VOo=2-4Ss=2-3) in boxes (6)–(12) are, to our knowledge, the first observations reported in the literature. Up to 3 sulfur and 3 additional oxygen atoms per molecule were observed. The compositional shapes of these porphyrins types are very similar, indicating similarities in aromatic, naphthenic and alkyl growth. Again, we observed that additions of sulfur atoms introduce unsaturation while addition of oxygen do not introduce unsaturation.
Table 2 Petroleum compound classes (% Abundances) observed by APPI FTICR-MS.
HC 1S 2S 1O 1S 1O 2S 1O 4N 1O 1V 4N 2O 1V 4N 3O 1V 4N 4O 1V 1S 4 N 1O 1V 2S 4N 1O 1V 3S 4N 1O 1V 1S 4N 2O 1V 2S 4N 2O 1V 1S 4N 3O 1V 2S 4N 3O 1V 1S 4N 4O 1V Un-assigned Peaks
M3-407
M3-411
M3-418
M3-421
0.2 0.0 0.0 0.0 0.1 0.0 57.1 3.9 1.2 0.0 24.6 5.0 0.0 1.7 0.0 0.3 0.0 0.0 5.8
0.3 0.1 0.1 0.1 0.1 0.1 48.0 8.0 3.4 0.7 24.0 4.3 0.1 4.1 0.4 1.5 0.0 0.1 4.9
0.1 0.1 0.1 0.0 0.2 0.1 40.6 9.5 4.9 1.2 23.4 4.4 0.1 5.2 0.6 3.4 0.4 0.4 5.3
0.2 0.0 0.0 0.0 0.1 0.0 32.0 7.1 3.1 0.5 32.6 6.6 0.4 6.9 0.9 3.1 0.1 0.3 6.2
wavelength of peak Soret band increases, more multiple sulfur and oxygen containing species have been observed. Fraction M3-421 showed the most abundant sulfur and oxygen containing vanadyl porphyrins. Fig. 7 shows the Z-number distributions of VO and VOS porphyrins for the four vanadyl petroporphyrin fractions with Soret band peaked at 407, 411, 418 and 421 nm, respectively. VO porphyrins (top) start at Z = −28 (etio) and end around Z = −68 (7-aromatic ring VO porphyrins). Benzo DPEP VO porphyrins are the most abundant structures for all fractions but M3-421. The latter contains much higher levels of
3.4. Comparison of Compositions of Vanadyl Porphyrin Fractions with Different UV Absorptions Table 2 summarized all petroleum compound classes detected in the four fractions by APPI FTICR-MS. ∼95% mass peaks in Fig. 3(b) can be assigned. The un-assigned mass peaks were found to be mostly noise. The non-porphyrins compounds are very low abundance (< 1%), again demonstrating the high purity of the porphyrins fractions. As UV 1263
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Fig. 7. Z-number distributions of VO and VOS porphyrins in M3-407, 411, 418 and 421 fractions. The average unsaturation (Z-number or hydrogen deficiency) and relative sulfur content increase with the increase of UV absorbance.
VOS porphyrins. VOS porphyrins (bottom) start at Z = −38 (benzothiophene attached to etio) and end around Z = −72 (8-aromatic ring VOS porphyrins). The distribution peaked around Z = −44, corresponding to a dibenzothiopheno etio (DBT-etio). Again we remind reader that all structures displayed are for illustrative purposes. They are one of the many possible structures that are consistent with the Znumber and are not unique. As wavelength increases, relative concentrations of DPEP and benzo-DPEP structure decreases while DBTetio increases. The hydrogen deficiencies of the fractions increase with the UV absorption wavelength.
ExxonMobil Research and Engineering Company, Mobae Afeworki, Steven P. Rucker and William E. Riedinger. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusions New vanadyl porphyrin compound classes, containing multiple sulfur and oxygen atoms (up to 3S and 3 additional O) and their combinations, were observed in purified porphyrin fractions by APPI FTICR-MS. Molecular weight distributions of the porphyrins were confirmed by FD ToF-MS. Sulfur incorporation introduced more unsaturation to the molecules. The reduction of average Z per sulfur addition implies that sulfur may be incorporated by addition of thiophene functional groups to the porphyrins. On the other hand, oxygen incorporation did not significantly change the average Z value, implying that oxygen is being incorporated as carbonyl or hydroxyl groups. Four fractions with different UV absorption wavelengths were examined by FTICR-MS. The lower UV wavelength fractions contain more DPEP and Benzo DPEP structures. The higher UV wavelength fractions contain more heteroatom vanadyl porphyrins with more condensed porphyrin structures. Up to 8 aromatic rings can be present in these porphyrins.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgement Authors appreciate reviews and valuable feedbacks by colleagues of
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Treibs A. Angew Chem 1936;49:682. Rankin JG, Czernuszewicz RS, Lash TD. Org Geochem 1995;23:419. Vanberkel GJ, Quinones MA, Quirke JME. Energy Fuels 1993;7:411. Gallango O, Cassani F. Org Geochem 1992;18:215. Hamidi Zirasefi M, Khorasheh F, Ivakpour J, Mohammadzadeh A. Energy Fuels 2016;30:10322. Bridjanian H, Khadem Samimi A. Pet Coal 2011;53:13. Sakanishi K, Yamashita N, Whitehurst DD, Mochida I. Catal Today 1998;43:241. Rankel LA. Prepr - Am Chem Soc Div Pet Chem 1993;38:343. Quirke JME. The Porphyrin Handbook. Academic Press; 2000. p. 371. Rodgers RP, Hendrickson CL, Emmett MR, Marshall AG, Greaney M, Qian K. Can J Chem 2001;79:546. Qian K, Edwards KE, Mennito AS, Walters CC, Kushnerick JD. Anal Chem (Washington, DC, U. S.) 2010;82:413. McKenna AM, Williams JT, Putman JC, Aeppli C, Reddy CM, Valentine DL, et al. Energy Fuels 2014;28:2454. Qian K, Mennito AS, Edwards KE, Ferrughelli DT. Rapid Commun Mass Spectrom 2008;22:2153. Zhao X, Liu Y, Xu C, Yan Y, Zhang Y, Zhang Q, et al. Energy Fuels 2013;27:2874. Gao YY, Shen BX, Liu JC. Energy Sources Part A 2012;34:2260. McKenna AM, Purcell JM, Rodgers RP, Marshall AG. Energy Fuels 2009;23:2122. Putman JC, Rowland SM, Corilo YE, McKenna AM. Anal Chem (Washington, DC, U. S.) 2014;86. 10708. Liu T, Lu J, Zhao X, Zhou Y, Wei Q, Xu C, et al. Energy Fuels 2015;29:2089. Liu H, Mu J, Wang Z, Ji S, Shi Q, Guo A, et al. Energy Fuels 2015;29:4803. Zhao X, Shi Q, Gray MR, Xu C. Sci Rep 2014;4:5373. Rytting BM, Singh ID, Kilpatrick PK, Harper MR, Mennito AS, Zhang Y. Energy Fuels 2018;32:5711. Qian K, Dechert G. J Anal Chem 2002;74:3977. Qian K, Edwards KE, Mennito AS, Saeger RB. U.S. Patent 9,490,109 filed February 12, 2014, and issued November 8, 2016. Qian K, Edwards KE, Siskin M, Olmstead WN, Mennito AS, Dechert GJ, et al. Energy Fuels 2007;21:1042.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Evidence of naturally-occurring vanadyl porphyrins containing multiple S and O atoms
T
⁎
Kuangnan Qiana, , Thomas R. Fredriksena, Anthony S. Mennitoa, Yunlong Zhanga, Michael R. Harpera, Shamel Merchanta, J. Douglas Kushnericka, B. McKay Ryttingb, ⁎ Peter K. Kilpatrickb, a b
ExxonMobil Research and Engineering Company, Annandale, NJ 08801, United States Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, United States
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Petroporphyrins FTICR-MS Petroleum separations
A host of vanadyl petroporphyrins containing multiple sulfur and oxygen atoms and their combinations (CcH2c+ZN4VOoSs, s = 0–3, o = 1–4, Z = −28 to −70) in a vacuum residue were observed by a combination of extensive enrichment of petroporphyrins and detailed analysis using ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Many of the petroporphyrin species are being observed for the very first time. The patterns of the porphyrin type distribution revealed dramatic differences in the sulfur and oxygen atom incorporations and their impact on Z number. For each sulfur atom addition, the average Z-number was reduced by ∼4–7, implying that sulfur may be incorporated by the additions of thiophene or cyclic thiophene moieties. In contrast, oxygen addition had little to no impact on the average Z number, implying that oxygen may be incorporated as carbonyl or hydroxyl groups. The porphyrin-based structures were found to contain as many as 8 aromatic rings in addition to the porphyrin macrocycle. Petroporphyrins in longer wavelength absorption fractions were found to contain more heteroatoms and more condensed aromatic rings.
1. Introduction Petroporphyrins (or metallopetroporphyrins) are an important class of geological biomarkers used in petroleum exploration applications
⁎
[1–4]. These molecules are also the primary host of metals, such as vanadium and nickel, in petroleum and have implications in petroleum refining processes [5–8]. Petroporphyrins have been the subject of research since the structures were first proposed by Treibs in 1936 [1–9].
Corresponding authors. E-mail address: [email protected] (K. Qian).
https://doi.org/10.1016/j.fuel.2018.09.115 Received 20 July 2018; Received in revised form 9 September 2018; Accepted 22 September 2018 Available online 29 November 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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2.2. Fourier transform ion cyclotron resonance mass spectrometry
Vanadyl and nickel porphyrins are two main porphyrin types which are derived from their biological precursors, such as chlorophyll, hemes, and chlorins found in nearly all living organisms. These cyclic tetrapyrrolic structures assume the general chemical formula, CcH2c+ZN4VO and CcH2c+ZN4Ni. The chemical formula of vanadyl porphyrins was first confirmed by ultra-high resolution mass spectrometry in 2001 using electrospray ionization [10]. The confirmation of nickel porphyrin chemical formula came much later in 2010 [11], when nickel porphyrins were first enriched by cyclograph separation and then analyzed by FTICR-MS via atmospheric pressure photoionization (APPI). Direct detection of nickel porphyrins in oil seeps without preseparation was later demonstrated by ultra-high resolution FTICR-MS analysis [12]. Sulfur-containing vanadyl (VOS) porphyrins were first reported in 2008 in a petroleum asphaltene sample [13]. The VOS porphyrins were later confirmed by multiple laboratories [14–16]. More recently, porphyrins containing additional oxygen and nitrogen atoms were reported [17–20]. Unlike sulfur, additional nitrogen and additional oxygen are believed to be incorporated on the side chains as NH2 and ether or carboxylic acid, respectively. The nature of organic V and Ni in petroleum continue to be studied intensely because both elements have not been quantitatively accounted for based on porphyrin structures. In addition, the molecular weight distributions of metal petroporphyrins appear to be different from that of hydrocarbons. In general, the molecular weight distributions (MWD) of petroporphyrins are much narrower than that of hydrocarbons and with a lower MWD peak position [13]. On the other hand, all data generated so far are consistent with the porphyrin backbones, which grows in a similar fashion as other petroleum compound families. For example, the core structure can grow in both aromatic and naphthenic rings. Recently, a collaborative work between ExxonMobil and the University of Notre Dame had successfully generated purified vanadyl petroporphyrin fractions [21]. These fractions were characterized by a number of analytical techniques, showing high purity enrichment. In this work, four purified vanadyl fractions with varying maximum UV Soret absorbance were subjected to more examinations to understand porphyrin compositions and their molecular weight distributions. Field desorption ionization time-of-flight mass spectrometry (FD TOF-MS) was applied to confirm the molecular weight distribution of the petroporphyrins observed by APPI FTICR-MS. Detailed examination of ultra-high resolution FTICR-MS data revealed a host of new vanadyl porphyrins containing multiple sulfurs and oxygens and their combinations. The pattern of the porphyrin type distribution showed dramatic differences in sulfur and oxygen incorporations.
High resolution mass spectrometric analyses were conducted on a 15 Tesla Bruker solariX XR FTICR-MS (Bruker Daltonics Inc., Billerica, MA, USA). About 0.5 mg of the petroporphyrin sample was dissolved in 5–20 mL of toluene to form a 25–100 ppm solution. Concentration was adjusted to achieve adequate response. The solution was introduced into an APPI source using an integrated stock syringe pump with a 250 µL syringe (Hamilton Co., Reno, NV, USA). The flow rate was controlled at 120 µL/h. The APPI source was manufactured by Syagen Technology Inc. (Tustin, CA, USA), and is comprised of a heated capillary needle and a krypton UV lamp with ionization energy of 10.6 eV. Nitrogen was used as both the nebulizing and the drying gas. The nebulizing gas flow rate was set between 1 and 1.5 L/min while the drying gas flow rate was set between 3 and 5 L/min. The flow rates were adjusted to maximize APPI-FTICR-MS signals. The nebulizing gas temperature was set at 420 °C to maximize the vaporization of asphaltene molecules while maintaining a stable spray current. Toluene was used as both the solvent and the chemical ionization agent. Toluene was ionized by direct photoionization while asphaltene molecules (including metalloporphyrins) were ionized by charge transfer with toluene ions. During the FTICR-MS analysis, the acquisition mass range was set between m/z 300 and 3000. The ion accumulation time varied between 40 and 60 ms, adjusted for response. The time-of-flight was set to 1.2 ms. The funnel Rf was set to 115 Vpp. The excitation energy was ramped linearly from 18.0 to 90.2 percent, relative to effective maximum, up to a selected transition mass of m/z 1000. The full excitation frequency range was 76.7–769.3 kHz. The dataset size was set to 8 Megawords. Two hundred data sets were co-added to generate the final spectrum. Bruker Data Analysis (DA) software was used to find and calibrate the mass-to-charge peak list with signal-to-noise (S/N) ratio > 6. Data was internally calibrated using a homologous mass series. The calibrated peak list was exported to a table of mass and intensity. Peaks falling outside of a Kendrick mass defect series were removed, as well as any background or noise peaks. The “cleaned” and calibrated mass peak list was further analyzed for identification of VO porphyrin molecules. 2.3. Field desorption ionization time-of-flight mass spectrometry FD TOF-MS was conducted on a Waters GCT instrument. About 0.1 μL of the liquid was applied onto an FD emitter (from Linden ChroMasSpec) using a syringe. The FD emitter was then inserted into the ion source of a mass spectrometer near the extraction electrodes. The emitter (at ground voltage) was ∼1.5 mm away from a pair of extraction rods held at high potential (−12 kV), producing very high electric fields (∼10−7−10−8 V/cm) around the tips of the carbon dendrites. FD emitter current was manually ramped from 0 to 45 mA during data collection. Ions generated by FD were subjected to analysis by a reflectron ToF MS with an effective path length of 1.2 m. The voltage pulse was applied at a frequency of 30 kHz. A full spectrum was generated every 33 μs. The mass range was set at 200–3000 Da. A summed spectrum was collected every second in a centroid mode.
2. Experimental materials and methods 2.1. Samples Four petroporphyrin samples, each having a different Soret band maximum, were selected from a collation of petroporphyrins purified by Rytting et al in the 2018 work [21]. The details of the separation procedure can be found in the “M3 Petroporphyrin Enrichment and Purification” section. Fig. 1 shows a simplified separation flow scheme. In brief, a vacuum residue (M3) was subjected to a series of separation procedures comprising Soxhlet solvent extractions, extrographic and chromatographic separations using silica and alumina columns. The separations produced multiple fractions of purified petroporphyrins exhibiting a large range of Soret band maxima. Of these purified petroporphyrins, samples with Soret band maxima of 407, 411, 418, and 421 nm were analyzed in detail by FD-TOF MS and APPI-FTICR MS. The four samples are referred to in this work as fractions M3-407, -411, -418 and -421, respectively. Fig. 2 showed an overlay of UV spectra of the four samples.
3. Results and discussion 3.1. Molecular weight distributions Fig. 3 shows the molecular weight distributions of M3-407, -411, -418 and -421, respectively, by two different ionization methods and on two different instruments. On the left side, mass spectra were obtained by FD ToF-MS. The average mass resolution of ToF-MS is about 5000 [22]. In this work mass resolution (R) and mass resolving power (RP) are used interchangeably and defined as R = M/ΔMFWHM where M is the mass and ΔMFWHM is the mass peak width (full width at half 1259
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Fig. 1. Isolation scheme of four purified vanadyl porphyrin fractions examined in this work. The fraction numbers reflect the maximum wave length of the Soret absorption band of respected fractions.
composition. Fig. 4(a) displays the detailed APPI-FTICR mass spectra of sample fraction M3-421 at 4 nominal masses of 549, 689, 829, and 969 (mass increment 140 is equivalent of 10 CH2), respectively. Isolation of vanadyl porphyrins greatly simplified sample matrix and eliminated many potential mass overlaps. Assignments of chemical formula for mass peaks with a nominal mass of 969 and signal to noise ratio > 6 are shown in Table 1. The formula assignments of major mass peaks are also given in Fig. 4(a). Unique chemical formula can be assigned with mass error less than 0.2 mDa. Here, vanadyl porphyrins are described using general chemical formula, CcH2c+Z NnVOoSs where Z is the hydrogen deficiency index and c, n, o, s are the number of carbon, nitrogen, oxygen, and sulfur, respectively. Another commonly used terms to describe unsaturation of petroleum molecules is Double Bond Equivalence (DBE). There is a simple relation between Z and DBE as shown in Eq. (1)
Z= −2 × (DBE−1) + n Fig. 2. Overlaid UV–Visible spectra of M3-407, -411, -418, and -421, normalized for ease of comparison.
(1)
Here n is the number of nitrogen atoms in a molecule. In this work, we use Z VOoSs to shorthand the core compositions, for example C33H30N4VO is a member of −36 VO family. C60H66 VOS2 is shortened as −54 VOS2. The complexity of the mass spectra increases as the mass increases. Unsaturation and number of sulfur atoms per molecule also increase with mass. Mass 549 shows one major vanadyl porphyrin compound, C33H30N4VO (in −36 VO family). Mass 689 shows the most abundant peak to be a sulfur containing porphyrin C41H42N4VOS (−40 VOS). Mass 829 revealed two abundant peaks, C52H50N4VOS (−54 VOS) and C54H58N4VO (−50 VO). There are a total of 10 mono-isotope and two 13 C2 isotope vanadyl porphyrins found at nominal mass 969 (Table 1). Four major types are indicated in Fig. 4(a), C58H54N4VOS3 (-62 VOS3), C60H62N4VOS2 (-58 VOS2), C62H70N4VOS (-54 VOS) and C64H78N4VO (-50 VO), respectively. To the best of our knowledge, this is the first time that vanadyl petroporphyrins containing multiple sulfurs (number of sulfur atoms > 1) and multiple sulfur/oxygen combinations have ever been observed experimentally. The possible core structures of the porphyrin types are illustrated in Fig. 4(b). “−36 N4VO” can be assigned as benzo deoxophylloerythroetioporphyrin (benzo-DPEP). “−40 VOS” can be expressed as DPEP
maximum). On the right side, mass spectra were obtained by APPI on solariX FTICR-MS. Mass resolution at 600 is ∼1 million. APPI had exhibited limitations in ionizing high boiling vacuum residue molecules (up to ∼1300°F) [23]. Field desorption has been shown in the past to ionize high molecular weight hydrocarbon molecules, such as polywax and polystyrenes with molecular weights up to 5000 g/mol [24]. In Fig. 3, the patterns of the mass profile by the two techniques are very similar. The mass distributions peaked around m/z 550–600. Fraction M3-421 showed bimodal distributions by both techniques. In APPI FTICR-MS, no mass peaks were observed beyond m/z 1200. Although FD TOF-MS show signal beyond 1200, the signal to noise is low. It is safe to say that APPI FTICR-MS can detect the majority of petroporphyrins in the sample.
3.2. Resolution and identification of vanadyl porphyrins Among the four samples, M3-421 showed the most complex 1260
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Fig. 3. (a) Mass spectra of four purified vanadyl porphyrin fractions by FD TOF-MS (left) and (b) by APPI FTICR-MS (right). Similar mass profiles observed by two different ionization methods on two different mass spectrometers.
Fig. 4. (a) Zoomed-in mass spectral segments of sample M3-421 fraction at nominal mass 549, 689, 829 and 969. Spectra complexity increases with the increase of nominal mass. (b) Illustrative vanadyl porphyrin structures. Increase in sulfur and decrease in Z-number can be explained by incorporations of thiophenes. 1261
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Table 1 Assignments of chemical formula for components at nominal mass 969. The masses are that of molecular ions (one electron mass less than that of neutral). Experimental Mass (Da)
Signal Magnitude
Signal/Noise
Resolving Power
Theoretical Mass (Da)
Mass Errors (mDa)
Chemical Formula
Z-Number
969.28945 969.30718 969.34352 969.37083 969.3766 969.37989 969.39773 969.43403 969.4616 969.46717 969.47045 969.56096
3,165,220 2,817,061 5,182,502 2,298,919 7,492,647 16,610,343 2,839,592 5,383,209 3,652,650 5,022,762 16,790,570 5,182,509
9.4 8.1 17 6.2 25.6 59.8 8.2 17.7 11.2 16.4 60.5 17
728,128 624,312 644,942 865,994 745,941 688,922 646,111 667,621 720,752 781,675 662,013 688,915
969.28938 969.30714 969.34353 969.37097 969.37654 969.37991 969.39765 969.43406 969.46150 969.46707 969.47044 969.56097
0.07 0.04 −0.01 −0.14 0.06 −0.02 0.08 −0.03 0.10 0.10 0.01 −0.01
C58H54N4VOS3 C58H54N4VO3S2 C59H58N4VO2S2 C5813C2H60N4VOS2 C63H58N4VOS C60H62N4VOS2 C60H62N4VO3S C61H66N4VO2S C6013C2H68N4VOS C65H66N4VO C62H70N4VOS C64H78N4VO
−62 −62 −60 −60 −68 −58 −58 −56 −56 −64 −54 −50
fused with a benzothiophene. “−50 VO” can be assigned as fluorantheno etioporphyrin (fluorantheno-etio). “−54 VOS”, “−58 VOS2” and “−62 VOS3” can be described by additions of multiple thiophene units to the fluorantheno-etio. It should be noted that all structures displayed in this paper are for illustrative purposes. They are one of the many possible structures that are consistent with the Z-number specified and are not unique. Although the core structures are expressed as single cores, we recognized that high molecular weight petroleum molecules can contain both single core and multi-core structures. The differentiation of the two type of structures may be explored by fragmentation techniques, such as Collision-Induced Dissociation (CID) experiments. The subject is beyond the scope of this work but will be further studied in the future.
deficiency). Since APPI generates singly charged ions for the porphyrin samples, the term of “molecular weight” and “mass-to-charge ratio (m/ z) is used interchangeably in this paper. The intensity of each species is expressed by the color scheme of the map (red most intense and white least intense). The top row shows those species of vanadyl petroporphyrins containing multiple oxygen atoms (VO, VO2, VO3 and VO4). The numbers in the boxes are the intensity weighted average Z numbers. As the number of oxygen atoms increases, the average Z number remains relatively constant, implying that oxygen incorporation does not introduce unsaturation or additional hydrogen deficiency, and that most of the oxygen-containing petroporphyrins have roughly the same Z number or homologous serious. It is generally believed that oxygen functionalities (abundant in original biological precursor) were removed first during diagenesis and later were reintroduced during bacterial degradation. At present we suspect that oxygen atoms were incorporated as carbonyl or hydroxyl groups. The bottom row of Fig. 5 shows the FTICR-MS data of vanadyl petroporphyrins containing multiple sulfur atoms (VO, VOS, VOS2 and VOS3). In contrast to oxygen incorporations, the average Z number declines appreciably as the number of sulfur atoms in the vanadyl
3.3. Multiple sulfur and oxygen-containing vanadyl porphyrins Fig. 5 shows vanadyl petroporphyrins containing multiple sulfur and oxygen atoms in a two dimensional image display. Each box represents a class of vanadyl petroporphyrins. In each box, the X-axis is molecular weight and the Y-axis is the Z number (or hydrogen
Fig. 5. Vanadyl porphyrin compositions of sample M3-421 with increasing O and S incorporations. The numbers in the boxes are the intensity weighted average Z numbers. 1262
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Fig. 6. An overview of 12 vanadyl porphyrin types in M3-421. The multiple sulfur and sulfur/oxygen porphyrins (VOo=2-4 Ss=2-3) in boxes 6–12 are, to our knowledge, the first observations reported in the literature.
petroporphyrins increases, implying that each sulfur atom addition introduces more unsaturation. The average Z reduction per 1 sulfur incorporation ranges from about 4– 7. Structurally, the reduction in Z can be explained by incorporating a thiophene structure to the porphyrin molecules (ΔZ = −4 if fused; ΔZ = −6 if connected via a biaryl linkage). Fig. 6 presents an overview of all 12 porphyrin classes in sample M3-421 with incorporated oxygen and sulfur atoms observed by FTICRMS. The upper left box (1) is the traditional vanadyl porphyrins (VO) that have been studied and reported extensively. We observed Z number as negative as −68 for VO porphyrins, which is equivalent of the porphyrin base structure attached by a six aromatic ring system. The single sulfur-containing porphyrins (VOS) in box (5) have also been reported and studied since 2008 [11–14,16,20]. Multiple oxygen vanadyl porphyrins (VO2–VO4) were reported recently by other laboratories [14,20]. The multiple sulfur and sulfur/oxygen vanadyl porphyrins (VOo=2-4Ss=2-3) in boxes (6)–(12) are, to our knowledge, the first observations reported in the literature. Up to 3 sulfur and 3 additional oxygen atoms per molecule were observed. The compositional shapes of these porphyrins types are very similar, indicating similarities in aromatic, naphthenic and alkyl growth. Again, we observed that additions of sulfur atoms introduce unsaturation while addition of oxygen do not introduce unsaturation.
Table 2 Petroleum compound classes (% Abundances) observed by APPI FTICR-MS.
HC 1S 2S 1O 1S 1O 2S 1O 4N 1O 1V 4N 2O 1V 4N 3O 1V 4N 4O 1V 1S 4 N 1O 1V 2S 4N 1O 1V 3S 4N 1O 1V 1S 4N 2O 1V 2S 4N 2O 1V 1S 4N 3O 1V 2S 4N 3O 1V 1S 4N 4O 1V Un-assigned Peaks
M3-407
M3-411
M3-418
M3-421
0.2 0.0 0.0 0.0 0.1 0.0 57.1 3.9 1.2 0.0 24.6 5.0 0.0 1.7 0.0 0.3 0.0 0.0 5.8
0.3 0.1 0.1 0.1 0.1 0.1 48.0 8.0 3.4 0.7 24.0 4.3 0.1 4.1 0.4 1.5 0.0 0.1 4.9
0.1 0.1 0.1 0.0 0.2 0.1 40.6 9.5 4.9 1.2 23.4 4.4 0.1 5.2 0.6 3.4 0.4 0.4 5.3
0.2 0.0 0.0 0.0 0.1 0.0 32.0 7.1 3.1 0.5 32.6 6.6 0.4 6.9 0.9 3.1 0.1 0.3 6.2
wavelength of peak Soret band increases, more multiple sulfur and oxygen containing species have been observed. Fraction M3-421 showed the most abundant sulfur and oxygen containing vanadyl porphyrins. Fig. 7 shows the Z-number distributions of VO and VOS porphyrins for the four vanadyl petroporphyrin fractions with Soret band peaked at 407, 411, 418 and 421 nm, respectively. VO porphyrins (top) start at Z = −28 (etio) and end around Z = −68 (7-aromatic ring VO porphyrins). Benzo DPEP VO porphyrins are the most abundant structures for all fractions but M3-421. The latter contains much higher levels of
3.4. Comparison of Compositions of Vanadyl Porphyrin Fractions with Different UV Absorptions Table 2 summarized all petroleum compound classes detected in the four fractions by APPI FTICR-MS. ∼95% mass peaks in Fig. 3(b) can be assigned. The un-assigned mass peaks were found to be mostly noise. The non-porphyrins compounds are very low abundance (< 1%), again demonstrating the high purity of the porphyrins fractions. As UV 1263
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Fig. 7. Z-number distributions of VO and VOS porphyrins in M3-407, 411, 418 and 421 fractions. The average unsaturation (Z-number or hydrogen deficiency) and relative sulfur content increase with the increase of UV absorbance.
VOS porphyrins. VOS porphyrins (bottom) start at Z = −38 (benzothiophene attached to etio) and end around Z = −72 (8-aromatic ring VOS porphyrins). The distribution peaked around Z = −44, corresponding to a dibenzothiopheno etio (DBT-etio). Again we remind reader that all structures displayed are for illustrative purposes. They are one of the many possible structures that are consistent with the Znumber and are not unique. As wavelength increases, relative concentrations of DPEP and benzo-DPEP structure decreases while DBTetio increases. The hydrogen deficiencies of the fractions increase with the UV absorption wavelength.
ExxonMobil Research and Engineering Company, Mobae Afeworki, Steven P. Rucker and William E. Riedinger. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusions New vanadyl porphyrin compound classes, containing multiple sulfur and oxygen atoms (up to 3S and 3 additional O) and their combinations, were observed in purified porphyrin fractions by APPI FTICR-MS. Molecular weight distributions of the porphyrins were confirmed by FD ToF-MS. Sulfur incorporation introduced more unsaturation to the molecules. The reduction of average Z per sulfur addition implies that sulfur may be incorporated by addition of thiophene functional groups to the porphyrins. On the other hand, oxygen incorporation did not significantly change the average Z value, implying that oxygen is being incorporated as carbonyl or hydroxyl groups. Four fractions with different UV absorption wavelengths were examined by FTICR-MS. The lower UV wavelength fractions contain more DPEP and Benzo DPEP structures. The higher UV wavelength fractions contain more heteroatom vanadyl porphyrins with more condensed porphyrin structures. Up to 8 aromatic rings can be present in these porphyrins.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgement Authors appreciate reviews and valuable feedbacks by colleagues of
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