High-Resolution Mass Spectrometric Analysis of a Vanadyl Porphyrin

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Energy & Fuels 1997, 11, 602-609

High-Resolution Mass Spectrometric Analysis of a Vanadyl Porphyrin Fraction Isolated from the >700 °C Resid of Cerro Negro Heavy Petroleum† R. D. Grigsby* and J. B. Green BDM-Oklahoma, Inc., National Institute for Petroleum and Energy Research, P.O. Box 2565, Bartlesville, Oklahoma 74005 Received November 12, 1996. Revised Manuscript Received February 4, 1997X

A vanadium-enriched fraction from the >700 °C resid of Cerro Negro heavy petroleum was analyzed by high-resolution, low-energy, electron-ionization mass spectrometry (low-eV HR/MS). This fraction, isolated in a prior investigation by liquid chromatographic methods, was known to contain 19 500 ppm vanadium, or 4.6 wt % of the total vanadium in the whole resid. On the basis of UV-visible absorption spectra, the concentration of porphyrinic vanadium in the fraction was previously estimated to be 11 000 ppm or 56.4 wt % of the total vanadium. Because the porphyrinic vanadium accounted for only about one-half of the total, an independent method was needed to characterize the nonporphyrinic vanadium in the fraction. Low-eV HR/MS with sample introduction by probe microdistillation was selected because of its capability for characterizing aromatic and polar compounds in low-volatility, high molecular weight mixtures. Etioporphyrins (CnH2n-28N4VO) with molecular weights ranging from 487 to 879 were found to be the most abundant vanadium-containing compounds in the fraction. Deoxophylloerythroetioporphyrins (CnH2n-30N4VO) with molecular weights ranging from 499 to 863 were the second most abundant. In addition, other compound types having the formula CnH2n+zN4VO, where z ranges from -32 to -50, excluding -46, were found. Additional saturated and aromatic rings present in compounds with more negative z numbers would appreciably alter their UV-visible absorption spectra relative to those of etioporphyrins. That is, the response of the other porphyrin types would be lower than that of etioporphyrins at 570 nm, the wavelength used for the determination of vanadyl porphyrins. Thus, the broad distribution of vanadyl porphyrin types identified in the fraction by low-eV HR/MS strongly suggests that at least a part of the apparent “nonporphyrinic vanadium” thought to be present in the fraction is, in fact, porphyrinic and is explained by the use of molar absorptivities based solely on those of etioporphyrins to calculate total porphyrin content.

Introduction Vanadium and nickel complexes in petroleum are significant not only as biomarkers but because they poison catalysts used in refining processes. For these reasons, as well as others, characterization of these types of compounds is receiving much attention from geochemists and others involved with petroleum exploration and refining. A considerable amount of uncertainty exists as to the nature of nonporphyrinic metal complexes in petroleum. Size exclusion liquid chromatographic (LC) separation has consistently indicated significantly higher average molecular weight ranges for nonporphyrinic as compared with porphyrinic complexes.1-4 However, it is not clear whether this apparent difference reflects true relative molecular size or whether it simply results from greater tendency for nonporphyrins to form aggregates † Presented in part at the 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portland, OR, May 12-16, 1996. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Biggs, W. R.; Fetzer, J. C.; Brown, R. J.; Reynolds, J. G. Liq. Fuels Technol. 1985, 3, 397-421. (2) Reynolds, J. G.; Biggs, W. R. Fuel Sci. Technol. Int. 1986, 4, 779798. (3) Biggs, W. R.; Brown, R. J.; Fetzer, J. C. Energy Fuels 1987, 1, 257-262. (4) Reynolds, J. G.; Jones, E. L.; Bennett, J. A.; Biggs, W. R. Fuel Sci. Technol. Int. 1989, 7, 625-642.

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which effectively behave as large molecules.5,6 There is also disagreement over the coordination sites surrounding the metal ions. For example, NOS2 coordination has been suggested for nonporphyrinic vanadium in Cerro Negro heavy oil.7 Perhaps the most definitive evidence points to tetradentate N4 coordination analogous to that of porphyrinic complexes.8,9 Part of the confusion may stem from the common practice of using visible absorption spectra for differentiating porphyrins from nonporphyrins. Potentially, porphyrinic complexes in aggregates or those with polar or cyclic substitution may exhibit sufficiently suppressed response or altered spectra so as to appear primarily nonporphyrinic.9 For example, even substitution with a cycloalkyl group such as in deoxophylloerythroetioporphyrin (DPEP) results in significantly lower visible absorption compared with simple alkylated (etio) porphyrin types.10 (5) Frakman, Z.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1988, 4, 171-179. (6) Nguyen, S. N.; Filby, R. H. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 384-401. (7) Reynolds, J. G.; Gallegos, E. J.; Fish, R. H.; Komlenic, J. J. Energy Fuels 1987, 1, 36-44. (8) Malhotra, V. M.; Buckmaster, H. A. Fuel 1985, 64, 335-341. (9) Goulon, J.; Retournard, A.; Friant, P.; Goulon-Ginet, C.; Berthe, C.; Muller, J.-F.; Poncet, J.-L.; Guilard, R.; Escalier, J.-C.; Neff, B. J. Chem. Soc., Dalton Trans. 1984, 1095-1103.

© 1997 American Chemical Society

Vanadyl Porphyrin Fraction from Heavy Petroleum

Mass spectrometric (MS) methods have been applied to porphyrin characterization since before 1961.11-25 The bulk of the early work was conducted on demetalated fractions, and reported carbon number ranges were often limited to C40 and below. More recently, some workers have extended MS methods to metal-containing porphyrinic homologues having more than 60 carbons.16,17 On the other hand, attempts at MS identification of nonporphyrinic species have not yet yielded unequivocal results.26 Coupled chromatographic/MS techniques have been used extensively for porphyrin analysis. GC/MS has been applied largely to the analysis of demetalated porphyrin extracts, frequently in conjunction with chemical derivatization procedures.27-29 This approach yields extremely detailed compositional information. In addition, LC/MS has made significant progress toward becoming a routine tool for porphyrin characterization and screening.30-34 Although many of the novel ionization techniques, tandem MS/MS approaches, and coupled GC/MS or LC/ MS systems employed for porphyrin characterization may ultimately prove useful toward characterization of nonporphyrinic metal complexes, at least a general idea of nonporphyrin composition must be available before most of these techniques can be applied successfully. Prior work in this laboratory has resulted in the isolation of two concentrates with significantly enriched vanadium contents from Cerro Negro (Orinoco Belt, (10) Popp, B. M.; Hayes, J. M. Energy Fuels 1993, 7, 185-190. (11) Hood, A.; Carlson, E. G.; O′Neal, M. J. In The Encyclopedia of Spectroscopy; Clark, G. L., Ed.; Reinhold: New York, 1960; p 613. (12) Hood, A. In Mass Spectrometry of Organic Ions; McLafferty, F. W., Ed.; Academic Press: New York, 1963; pp 603-605. (13) Baker, E. W. J. Am. Chem. Soc. 1966, 88, 2311-2315. (14) Baker, E. W.; Yen, T. F.; Dickie, J. P.; Phodes, R. E.; Clark, L. F. J. Am. Chem. Soc. 1967, 89, 3631-3639. (15) Yen, T. F.; Boucher, L. J.; Dickie, J. P.; Tynan, E. C.; Vaughan, G. B. J. Inst. Pet. 1969, 55, 87-99. (16) Blumer, M.; Rudrum, M. J. Inst. Pet. 1970, 56, 99-106. (17) Barwise, A. J. G.; Whitehead, E. V. In Advances in Organic Geochemistry 1979; Douglas, A. G., Maxwell, J. R., Eds.; Pergamon: Oxford, U.K., 1980; pp 181-192. (18) Sundararaman, P.; Gallegos, E. J.; Baker, E. W.; Slayback, J. R. B.; Johnston, M. R. Anal. Chem. 1984, 56, 2552-2556. (19) Johnson, J. V.; Britton, E. D.; Yost, R. A.; Quirke, J. M. E.; Cuesta, L. L. Anal. Chem. 1986, 58, 1325-1329. (20) Strong, D.; Filby, R. H. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 154-172. (21) Van Berkel, G. J.; Glish, G. L.; McLuckey, S. A. Energy Fuels 1990, 4, 720-729. (22) Schurz, H. H.; Busch, K. L. Energy Fuels 1990, 4, 730-736. (23) Ali, M. F.; Perzanowski, H.; Bukhari, A.; Al-Haji, A. A. Energy Fuels 1993, 7, 179-184. (24) Pena, M. E.; Manjarrez, A.; Campero, A. Fuel Process. Technol. 1996, 46, 171-182. (25) Brodbelt, J. S.; Cooks, R. G.; Wood, K. V.; Jackson, T. J. Fuel Sci. Technol. Int. 1986, 4, 683-698. (26) Fish, R. H.; Reynolds, J. G.; Gallegos, E. J. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 332-349. (27) Marriott, P. J.; Gill, J. P.; Evershed, R. P.; Hein, C. S.; Eglinton, G. J. Chromatogr. 1984, 301, 107-128. (28) Gill, J. P.; Evershed, R. P.; Eglinton, G. J. Chromatogr. 1986, 369, 281-312. (29) Blum, W.; Richter, W. J.; Eglinton, G. J. High Res. Chromatogr. 1988, 11, 148-156. (30) McFadden, W. H.; Bradford, D. C.; Eglinton, G.; Hajbrahim, S. K.; Nicolaides, N. J. Chromatogr. Sci. 1979, 17, 518-522. (31) Sundararaman, P.; Vestal, C. Org. Geochem. 1993, 20, 10991104. (32) Eckardt, C. B.; Carter, J. F.; Maxwell, J. R. Energy Fuels 1990, 4, 741-747. (33) Rosell-Mele, A.; Carter, J. F.; Maxwell, J. R. J. Am. Soc. Mass Spectrom. 1996, 7, 965-971. (34) Van Berkel, G. J.; Quinones, M. A.; Quirke, J. M. E. Energy Fuels 1993, 7, 411-419.

Energy & Fuels, Vol. 11, No. 3, 1997 603

Venezuela) heavy petroleum.35 The >700 °C resid used as the starting material for the separations contained 994 and 228 ppm total V and Ni, respectively, and comprised 45.4 wt % of the whole crude oil. It was obtained using a short-path, thin-film (“molecular”) distillation apparatus which subjected the material to relatively mild conditions (0.002 Torr and 350 °C) for approximately 30 s.36 The thermal stress experienced by materials in this type of distillation is less than that occurring, for example, in a typical gas chromatographic analysis; therefore, it is unlikely that any appreciable thermal decomposition occurred.37,38 The resid contained the vast majority of metals present in the whole crude; only 3 wt % of either Ni or V distilled over at atmospheric equivalent boiling points below 700 °C (1292 °F).39 The whole resid was subjected to a sequence of three types of LC separations: initial separation by nonaqueous ion exchange, subfractionation using a bonded phase alkylpyridylsilica column, followed by further separation of selected subfractions on silica.35 On the basis of the assumption of one V per molecule and an average molecular weight of 500 g/mol, the two final concentrates with greatest relative enrichment of vanadium contained an estimated 19 and 13 wt % vanadium-containing compounds, which comprised, respectively, 4.6 and 0.5 wt % of the total vanadium present in the whole resid. The former concentrate, designated as fraction N-P2BS2, was selected for further investigation because it comprised the larger proportion of vanadium present in the crude and also because it exhibited the larger relative vanadium concentration of the two concentrates. Using an average molar absorptivity (570 nm) determined from Beers law plots of vanadyl etioporphyrins and vanadyl octaethylporphyrin in 1,2-dichloroethane (2.5 × 104 L/(m cm)), the porphyrinic vanadium in fraction N-P2BS2 was determined to be 11 000 ppm.35 Since the total V was 19 500 ppm, the UV-visible measurement indicated that only about one-half (56.4 wt %) of the vanadium was of porphyrinic type. Thus, a key objective of the current work was to account for any vanadium species that would contribute to the substantial apparent “nonporphyrinic character” of the fraction. For this reason, the primary analytical technique selected was high-resolution, low-energy, electronionization mass spectrometry (low-eV HR/MS) in conjunction with sample introduction by probe microdistillation. We felt that this technique would provide the most comprehensive and unequivocal analysis possible of fraction N-P2BS2. An example of the application of this methodology to the analysis of high-boiling petroleum distillates is described in the literature.40 (35) Pearson, C. D.; Green, J. B. Energy Fuels 1993, 7, 338-346. (36) Green, J. A.; Green, J. B.; Grigsby, R. D.; Pearson, C. D.; Reynolds, J. W.; Shay, J. Y.; Sturm, G. P., Jr.; Thomson, J. S.; Vogh, J. W.; Vrana, R. P.; Yu, S. K.-T.; Diehl, B. H.; Grizzle, P. L.; Hirsch, D. E.; Hornung, K. W.; Tang, S.-Y.; Carbognani, L.; Hazos, M.; Sanchez, V. Analysis of Heavy Oils: Method Development and Application to Cerro Negro Heavy Petroleum; Topical Report NIPER-452, NTIS No. DE900000200; NTIS: Springfield, VA, 1989; Chapter 1. (37) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; M. Dekker: New York, 1994; Chapter 3. (38) Rosscup, R. J.; Bowman, D. H. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1967, 12, A77-A81. (39) Pearson, C. D.; Green, J. A.; Green, J. B.; Anderson, R. P. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1989, 34, 282-291. (40) Schronk, L. R.; Grigsby, R. D.; Scheppele, S. E. Anal. Chem. 1982, 54, 748-755.

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Table 1. Ions in the Series CnH2n-28N4VO Identified in the 70-eV Mass Spectrum of Fraction N-P2BS2a exptl massb

rel intc

theor massd

766.6286 752.6225 738.6202 724.6401 710.6287 696.6270 682.6182 668.6248 654.6342 640.6286 626.6279 612.6252 598.6299 584.6260 570.6260 556.6247 542.6251 528.6245 514.6255 500.6253 486.6259 472.6248 458.6203

0.63 1.65 1.64 1.23 1.93 2.41 4.10 6.35 5.25 8.80 11.04 13.97 16.90 29.14 51.40 70.80 100.00 82.23 32.22 8.22 4.88 2.00 1.64

766.6263 752.6263 738.6263 724.6263 710.6263 696.6263 682.6263 668.6263 654.6263 640.6263 626.6263 612.6263 598.6263 584.6263 570.6263 556.6263 542.6263 528.6263 514.6263 500.6263 486.6263 472.6263 458.6263

average std dev

diff, mmue

error, ppm

2.300 -3.800 -6.100 13.800 2.400 0.700 -8.100 -1.500 7.900 2.300 1.600 -1.100 3.600 -0.300 -0.300 -1.600 -1.200 -1.800 -0.800 -1.000 -0.400 -1.500 -6.000

3.00 -5.05 -8.26 19.04 3.38 1.00 -11.87 -2.24 12.07 3.59 2.55 -1.80 6.01 -0.51 -0.53 -2.87 -2.21 -3.41 -1.55 -2.00 -0.82 -3.17 -13.08

-0.039 4.539

Figure 1. 70-eV mass spectrum of fraction N-P2BS2. Probe temperature: 317 °C. Peaks at m/z 529, 543, 557,... belong to the etio (CnH2n-28N4VO) series.

-0.379 6.870

a Probe temperature: 317 °C. b Experimental Kendrick mass. Relative intensity. d Theoretical Kendrick mass. e Difference in millimass units.

c

We should emphasize that the LC fraction analyzed in this investigation comprises only 4.6 wt % of the total V present in the >700 °C resid, and owing to the LC separation sequence used, it is definitely not representative of the vanadyl compounds present as a whole. On the other hand, procedures which could conceivably bias or alter the distribution of vanadyl species presents notably, demetalation, asphaltene precipitation, crystallization, “heart-cutting”, or the likeswere strictly avoided. Simply stated, the objective of the investigation was to determine the comprehensive distribution of vanadium compounds in fraction N-P2BS2.

Figure 2. Low-eV mass spectrum of fraction N-P2BS2. Probe temperature: 286 °C. Peaks at m/z 501, 515, 529,... belong to the etio (CnH2n-28N4VO) series.

Experimental Section Fraction N-P2BS2 was analyzed by high-resolution mass spectrometry using a Kratos MS-50 mass spectrometer (Kratos Analytical Instruments) operated at an accelerating potential of 8 kV. All spectra were acquired at 10 K resolving power and recorded with a Kratos DS-55 data-acquisition system. The fraction was introduced by a quartz direct probe with linear control of temperature by a programmer (Masspec, Inc.). Ions were generated by 70-eV and low-energy (∼10 eV) electron ionization. The low energy was set by reducing the filament-to-block potential until a ratio of at least 10:1 was achieved for the intensities of the molecular-ion and M-15 peaks of mixed xylenes (I106/I91 > 10). This mixture was introduced through an all-glass expansion volume inlet (R. J. Brunfeldt Co.) operated at 307 °C. The electron current was 30 µA for both energy settings. A preliminary run at 70 eV was made to optimize experimental conditions and to identify major ions in the spectra. The ion-source temperature was 300 °C. Perfluorokerosene-H (PCR Inc.), introduced through the expansion-volume inlet, was used as a mass-marking reference. An unweighed amount of the fraction was loaded into a capillary and then into the probe. Temperature of the probe was increased linearly at 10 °C/min from 64 to 364 °C, where it was held constant for seven scans. Spectra were scanned from m/z 800 to 135 at 100 s/decade. A total of 30 scans were recorded. Perfluorokerosene

Figure 3. Elimination curve for m/z 529 (C31H34N4VO) from the low-eV mass spectra of fraction N-P2BS2. R2 ) 0.985. TIMAX ) 576.5 ( 1.1 K. Area ) (1091 ( 130) × 104 arbitrary units. (PFK) ions were identified up to m/z 717 and used to assign masses to time centroids in the spectra of the fraction by performing linear regression on a polynomial equation, as described previously.40,41 Table 1 shows the agreement between experimental and theoretical masses for ions identified in the CnH2n-28N4VO series (etioporphyrins) in the 70-eV spectrum of the fraction recorded at a probe temperature of 317 °C. After ions in the etioporphyrin series were identified from m/z 459 to 711 (based on PFK reference ions), the masses of four higher homologues from the same series, m/z 725, 739, 753, and 767, were added to the reference-mass file, whereupon the calculations were repeated. Thus, it was possible to make exact-mass assignments to all ions in the spectrum from m/z 405 to 767. All mass assignments in the 70-eV and low-eV spectra were based on the Kendrick mass scale, in which the mass of CH2

Vanadyl Porphyrin Fraction from Heavy Petroleum

Figure 4. Elimination curve for m/z 879 (C56H84N4VO) from the low-eV mass spectra of fraction N-P2BS2. Simpson’s rule integration. Area ) 25 × 104 arbitrary units. is defined as integer 14.41-44 This scale is convenient for expressing the masses of ions in the mass spectra of complex mixtures, such as petroleum, that consist largely of compounds in homologous series whose members differ by CH2. Thus, theoretical masses in Table 1 all have the same fractional mass, i.e., 0.6263, because the masses of all members of the homologous series differ by exactly 14. In addition to masses and relative intensities, Table 1 lists the millimass unit (mmu) differences between experimental and theoretical masses and the errors in parts per million. The average difference in mass is -0.039 ( 4.539 mmu, and the average error is -0.379 ( 6.870 ppm. The relatively large error of 19.04 ppm (13.800 mmu) observed for m/z 725 occurred because the peak was split by the data-acquisition system. For fraction N-P2BS2, the chromatographic behavior, elemental analysis, and UV-absorption spectra were sufficient to limit the elements to C, H, N, O, and V. Nickel was considered, as well, but only traces of compounds containing this element were found in the high-resolution mass spectra, in keeping with the relatively small amount of Ni (206 ppm) found previously in the fraction.35 Given the additional information available about the fraction together with the close fit between experimental and theoretical masses, the only reasonable elemental formulas for the ions listed in Table 1 are those belonging to the CnH2n-28N4VO series. For the low-eV experiment, the ion-source temperature was increased to 400 °C and the mass range was changed to allow scanning from m/z 1003 to 325. The fraction was loaded directly into the probe from a solution in dichloromethane (9 mg/mL). Two 10 µL aliquots were loaded with evaporation of the solvent by a stream of N2 after each load. Then, the fraction was rinsed down twice with 10 µL of dichloromethane with evaporation after each rinse. This procedure ensured that the fraction was deposited in the bottom of the probe and far enough away from the ion source (∼4 cm) to avoid being heated by the ion source. The weight of the fraction in the probe was calculated to be 180 µg. Probe temperature was programmed at 10 °C/min from 183 to 455 °C. A total of 27 scans were recorded over a period of 33 min, and of these, scans 5-21 (probe temperatures 219-405 °C) were processed with computer programs described previously.40-42 A mixture of halogenated compounds was introduced through the expansion-volume inlet while the low-eV spectra were being recorded. This mixture provided reference masses up to m/z 458 and ion intensities for monitoring changes in ion (41) Scheppele, S. E.; Grindstaff, Q. G.; Grigsby, R. D.; McDonald, S. R.; Hwang, C. S. Int. J. Mass Spectrom. Ion Phys. 1983, 49, 179209. (42) Grigsby, R. D. In Novel Techniques in Fossil Fuel Mass Spectrometry, ASTM STP 1019; Ashe, T. R., Wood, K. V., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1989; pp 172-193. (43) Scheppele, S. E.; Chung, K. C.; Hwang, C. S. Int. J. Mass Spectrom. Ion Phys. 1983, 49, 143-178. (44) Kendrick, E. Anal. Chem. 1963, 35, 2146-2154.

Energy & Fuels, Vol. 11, No. 3, 1997 605

Figure 5. Examples of structures for vanadyl porphyrins: (a) C32H34N4VO; (b) C32H36N4VO. source extraction efficiency caused by fluctuations in total ion current during the run.40 This effect was found to be relatively small, as indicated by an average correction factor of 1.001 ( 0.026. Assignment of masses to time centroids in the low-eV spectra above m/z 458 was based on the identification of CnH2n-28N4VO molecular ions in the 70-eV spectra. Ions in this homologous series were then used along with molecular ions from the halogenated mixture to provide reference masses in the low-eV spectra up to m/z 949. Time centroids were converted to masses using the same linear-regression method as used for the 70-eV spectra. The material remaining in the probe at the end of the loweV experiment was found to be completely soluble in dichloromethane, indicating that no appreciable thermal decomposition had occurred. The total vanadium present, as measured by atomic absorption spectrophotometry, was 1.17 µg. This comprised one-third of that present initially (180 µg × 19 500 ppm of V ) 3.51 µg of V). Thus, approximately two-thirds of the V-containing compounds in the fraction distilled from the probe into the ion source of the mass spectrometer.

Results Figure 1 shows the 70-eV mass spectrum of fraction N-P2BS2 recorded at a probe temperature of 317 °C. Major ions noted at m/z 529 and higher are molecular ions in the CnH2n-28N4VO series. In addition to highermass molecular ions, the spectrum shows an appreciable number of fragment ions below m/z 520. For example, m/z 500 and 514 represent fragments having elemental formulas of C29H29N4VO and C30H31N4VO, respectively. Fragmentation has to be taken into consideration if quantities of components in a mixture are to be determined. Thus, the overall pattern of molecular-ion intensities observed in Figure 1 does not necessarily provide a quantitative representation of the composition of the fraction, even when changes in the pattern with probe temperature are taken into account. On the other hand, by using low-energy ionization to suppress fragmentation, the relationship between molecular-ion intensity and quantity (response factor) is simplified and becomes approximately constant for components in a homologous series.40,45 In contrast, the low-eV spectrum shown in Figure 2 is much simpler than the 70-eV spectrum shown in Figure 1. Almost no peaks are seen below m/z 500, and those having masses two and four units less than peaks in the CnH2n-28N4VO series correspond to molecular ions for CnH2n-30N4VO and CnH2n-32N4VO homologues, respectively. Intensities of peaks having masses one and two units higher than peaks in the CnH2n-28N4VO series (45) Scheppele, S. E.; Grizzle, P. L.; Greenwood, G. J.; Marriott, T. D.; Perreira, N. B. Anal. Chem. 1976, 48, 2105-2113.

606 Energy & Fuels, Vol. 11, No. 3, 1997

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Table 2. Results from the Nonlinear Regression Analysis of Low-eV Mass Spectral Data for Fraction N-P2BS2 z no. -28

m/z

C no.

487 501 515 529 543 557 571 585 599 613 627 641 655 669 683 697 711 725 739 753 767 781 795 809 823 837 851 865 879

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

TIMAX, K

576.5 ( 1.1 581.0 ( 2.2 582.9 ( 0.7 587.1 ( 1.1 591.6 ( 0.8 597.8 ( 0.8 602.0 ( 0.8 606.2 ( 0.7 607.8 ( 0.9 610.4 ( 0.6 614.1 ( 2.6 617.9 ( 4.6 616.9 ( 3.9 623.7 ( 9.8 625.0 ( 10.5 627.6 ( 12.4 632.8 ( 7.8 634.1 ( 6.0 640.2 ( 7.8 644.1 ( 6.7 643.9 ( 5.0 644.7 ( 1.7

sum

area/104

pts

45 80 261 1091 ( 130 1514 ( 199 1261 ( 78 1075 ( 155 980 ( 100 927 ( 94 906 ( 128 821 ( 107 705 ( 139 569 ( 48 452 ( 38 428 ( 71 382 ( 45 351 ( 74 322 ( 80 305 ( 110 296 ( 72 247 ( 59 211 ( 93 181 ( 81 145 ( 49 89 ( 13 72 50 40 25 13833

9 11 16 17 17 17 17 16 15 16 14 14 14 13 14 13 12 13 13 12 12 11 10 9 6 6 6 5 6

z no. -30

m/z

C no.

499 513 527 541 555 569 583 597 611 625 639 653 667 681 695 709 723 737 751 765 779 793 807 821 835 849 863

29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

TIMAX, K

600.0 ( 6.6 601.2 ( 5.4 605.0 ( 5.2 616.5 ( 4.3 613.5 ( 0.4 618.0 ( 2.2 619.3 ( 4.5 627.0 ( 4.0 626.8 ( 3.6 638.6 ( 3.1 636.2 ( 3.6 639.2 ( 4.3 645.4 ( 16.3 645.7 ( 16.0 646.9 ( 11.4 648.3 ( 12.7 650.8 ( 13.5

sum -32

539 553 567 581 595 609 623 637 651 665 679 693 707 721 735 749 763 777 791 805 819 833 847 861

32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

589 603 617 631 645 659 673 687 701 715 729 743 757 771 785 799 813 827 841

36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

608.0 ( 2.7 605.5 ( 2.5 611.8 ( 6.2 614.9 ( 5.1 621.9 ( 6.7 625.9 ( 6.5 624.7 ( 7.7 631.9 ( 4.7 637.8 ( 5.9 641.9 ( 13.8 644.0 ( 6.6 644.7 ( 14.3 648.0 ( 8.7 648.2 ( 6.2

656.3 ( 4.1 sum

-34

523 537 551 565 579 593 607 621 635 649 663 677 691 705 719 733 747 761 775 789 803 817 831

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

8

617.4 ( 1.0 628.3 ( 1.6 626.7 ( 2.8 632.3 ( 2.7 632.2 ( 3.8 647.8 ( 4.0 650.5 ( 2.9 650.5 ( 7.0 647.2 ( 8.7

22 36 62 112 ( 9 151 ( 39 196 ( 65 227 ( 41 264 ( 57 265 ( 51 252 ( 42 233 ( 67 197 191 183 ( 58 160 137 122 97 82 76

5 6 8 9 10 12 12 12 11 11 11 10 11 10 10 10 9 9 9 8 9

-38

648.4 ( 26.6 650.3 ( 11.8 651.4 ( 0.7 669.2 ( 0.3 638.0 ( 1.0

sum -40

657 671

41 42

area/104

pts

27 72 123 143 144 163 216 ( 95 258 ( 62 398 ( 58 519 ( 73 612 ( 39 595 ( 38 560 ( 80 477 ( 61 453 ( 55 403 ( 50 360 ( 51 316 ( 52 277 ( 107 261 ( 117 206 ( 79 170 ( 76 142 ( 67 121 77 59 40 7191

6 10 13 14 14 14 13 13 14 14 14 14 13 12 12 12 13 12 11 10 10 9 8 7 6 5 5

19 33 68 127 ( 54 166 ( 40 223 ( 78 259 ( 60 286 ( 69 320 ( 71 339 ( 69 316 ( 65 285 ( 65 276 ( 96 252 ( 77 236 ( 94 209 ( 83 192 ( 63 164 129 125 99 74 ( 49 50 50 4296

6 9 11 14 13 13 13 12 13 12 11 12 11 12 11 10 11 9 10 9 5 5 5 6

11 14 22 37 63 ( 32 65 ( 44 82 ( 8 76 ( 9 80 85 ( 12 77 78 60 58 56 51 39 30 17 999

5 5 7 9 9 7 10 9 9 8 7 9 7 7 6 6 6 6 5

25 36

6 6

Vanadyl Porphyrin Fraction from Heavy Petroleum

Energy & Fuels, Vol. 11, No. 3, 1997 607

Table 2 (Continued) z no.

m/z

C no.

845

54

TIMAX, K sum

-36

577 591 605 619 633 647 661 675 689 703 717 731 745 759 773 787 801 815 829 843

35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

625.3 ( 27.6 650.9 ( 1.2 635.4 ( 1.5 626.6 ( 1.6 648.6 ( 1.1 654.2 ( 1.8 645.7 ( 2.4 648.0 ( 4.1 650.8 ( 3.6 652.5 ( 2.2

sum

-44

681 695 709 723 737 751 765 779 793 807 821

43 44 45 46 47 48 49 50 51 52 53

area/104 48 3120 16 25 55 73 ( 20 119 ( 15 132 ( 16 136 ( 16 145 ( 9 164 ( 15 139 ( 45 134 ( 56 127 ( 64 107 ( 14 91 82 81 68 56 35 1783

pts

z no.

6 6 9 9 11 11 9 11 11 10 10 9 10 9 9 8 8 8 7

5

21

5

651.8 ( 12.0

17 ( 35

6

645.3 ( 18.1 sum

10 ( 10 68

5

-42

-48

Mass spectra recorded as a function of probe temperature provide data for plotting curves of ion intensity vs temperature (elimination curves), and the areas under these curves are related to the quantities of the corresponding components in a mixture.40 When ion intensity is strong, as noted in Figure 2 for m/z 529 and higher-mass homologues, the curve is relatively smooth and capable of being analyzed by nonlinear-regression methods. Then, the parameters obtained from the regression analysis can be used to calculate the areas under the curves, the maximum heights of the curves, and the temperatures at which the maxima occur. Oftentimes, the curves have a skewed-Gaussian shape, depending on the nature and complexity of the mixture, and the data can be expressed by the skewed-Gaussian

685 699 713 727 741 755 769 783 797 811

43 44 45 46 47 48 49 50 51 52

TIMAX, K 650.2 ( 2.4 647.5 ( 4.9 627.2 ( 17.7 648.6 ( 6.9

641 655 669 683 697 711 725 739 753 767 781 795 809 823

40 41 42 43 44 45 46 47 48 49 50 51 52 53

663 677 691 705 719 733 747 761 775 789 803

42 43 44 45 46 47 48 49 50 51 52

650.4 ( 12.1

654.1 ( 12.0 sum

sum -50

correspond to those expected for the first and second 13C-isotopic peaks of this series.

C no.

sum

6

20

m/z

633 647 661 675 689 703 717 731 745 759 773

40 41 42 43 44 45 46 47 48 49 50

646.2 ( 20.6 645.9 ( 41.1

648.5 ( 11.2

650.8 ( 9.9 sum

area/104

pts

30 38 ( 3 44 47 ( 35 37 ( 14 39 42 ( 14 33 43 28 441

5 6 8 5 7 6 8 6 5 5

26 23 28 32 29 39 32 43 34 ( 22 33 28 20

5 7 7 5 6 8 6 6 6 7 6 5

19 ( 15 387

5

13

7

7

5

10 30

5

14 27 ( 31 50 ( 23 59 59 42 38 ( 17 37 26 16 ( 6 369

5 5 7 7 10 6 7 6 6 6

equation:

{[

I ) A exp -

]}

T-B C - D(T - B)

2

(1)

where I is ion intensity, T is absolute temperature, and A, B, C, and D are parameters evaluated by nonlinear regression. This equation is somewhat different from one described previously, and it is more easily handled by the regression-analysis program.40 Figure 3 shows a plot of experimental points and calculated curve for m/z 529 (C31H34N4VO) from the loweV mass spectra of fraction N-P2BS2. The curve appears almost Gaussian, and a smooth fit of the data is obtained, as noted by a goodness of fit (R2) equal to 0.985. The area under the curve is (1091 ( 130) × 104 arbitrary units, and the temperature of the maximum (TIMAX) is 576.5 ( 1.1 K. Estimates of 90% confidence intervals are included with the values.

608 Energy & Fuels, Vol. 11, No. 3, 1997

Even when the ion intensities are too small to allow data analysis by nonlinear regression (as determined by a lower R2 cutoff limit set at 0.8), a plot of intensity vs temperature can still be made, and the area under the curve can be obtained by Simpson’s rule integration. An example of such a curve is shown in Figure 4, in which data are plotted for m/z 879 (C56H84N4VO). The area under the curve is 25 × 104 arbitrary units. This curve represents the highest mass detected in the CnH2n-28N4VO series although it should be noted that the minimum number of data points was set at five in the data-analysis program, as required by the nonlinear regression. Thus, it is possible that molecular ions from higher-mass members of the series are present in the spectra and that the corresponding data were simply ignored by the program. A summary of the results from the nonlinear regression analysis of the low-eV data is given in Table 2. The mass numbers of ions in homologous series (columns 2 and 8) are identified with a particular series by the z numbers listed in columns 1 and 7. The corresponding carbon numbers are given in columns 3 and 9. Thus, a z number of -28 identifies the ions in column 2 as belonging to the CnH2n-28N4VO series (etioporphyrins). Absolute temperatures corresponding to the maximum heights of the curves (TIMAX) obtained from the nonlinear regression are listed in columns 4 and 10 together with estimates of 90% confidence intervals. Blanks in the columns indicate that data scatter was excessive, i.e., the nonlinear regression was not successful (R2 < 0.8) owing to low peak intensities. Areas under the curves in arbitrary units are listed in columns 5 and 11. Estimates of 90% confidence intervals are included when the nonlinear regression was successful; otherwise, a blank for the confidence interval indicates that the area was obtained from a Simpson’s rule integration of peak intensity vs absolute temperature. The number of points on the curves is listed in columns 6 and 12. Blanks in these columns indicate that the number was less than five, i.e., the ion was detected in less than five of the spectra. The sum of the areas under the curves for ions in each homologous series is listed in columns 5 and 11 at the end of each series. The areas in Table 2 are not isotopically corrected. At C40 the second 13C-isotope peak contributes 9.4% to the intensity of the 12C-peak two mass units higher. Thus, m/z 653 in the -30 z series contributes 9.4% of its intensity to the intensity of m/z 655 in the -28 z series. These corrections are of the same magnitude as the uncertainties in the 12C-peak intensities, themselves. Discussion Homologues in the -28 z and -30 z series (etio and DPEP, respectively) are the most abundant in the fraction, as noted by area sums of 13 833 and 7191, respectively, in Table 2. Typical DPEP and etio structures having 32 carbons are shown in Figure 5. Quantitation of these two types is important in geochemical investigations because the DPEP:etio ratio is known to correlate with the maturity of the crude source.5,25,46-48 Previous studies, based on low-eV mass spectrometry, (46) Philp, R. P. Mass Spectrom. Rev. 1985, 4, 1-54. (47) Gallegos, E. J. Mass Spectrom. Rev. 1985, 4, 55-85

Grigsby and Green

Figure 6. Example of structure for benzo-DPEP (CnH2n-36N4VO) and suggested structure for CnH2n-50N4VO.

have shown that the ratio varies from 0.27 to 5.8 and that it decreases with increasing maturation.13,14 From the results in Table 2, a ratio of 0.52 is obtained for fraction N-P2BS2. However, because the fraction contains only 4.6 wt % of the total vanadium in the >700 °C resid, a ratio of 0.52 is not necessarily representative of the ratio expected for the whole resid or the whole Cerro Negro crude. Other vanadyl porphyrins are noted from the results in Table 2. In order of decreasing abundance, these range from types having z numbers from -32 to -50, excluding -46. The -36 z series corresponding to benzo-DPEP homologues is relatively abundant, as noted by an area sum of 1783. An example of a benzoDPEP structure is shown in Figure 6a. The existence of benzoporphyrin and benzo-DPEP porphyrin structures (-34 z and -36 z) is well documented in the literature.5,14,17,27,28 An overall view of the relative abundances of vanadyl porphyrins vs z number and carbon number is shown in Figure 7. The smooth carbon number distributions observed for most z series provide evidence that thermal degradation (i.e., dealkylation by thermal cracking) during isolation (e.g., distillation) and analysis did not occur. No types having a z number of -46 were found. However, types having z numbers of -48 and -50 were detected. These may be explained by structures with three benzo groups attached to the vanadyl DPEP form. A hypothetical structure for CnH2n-50N4VO is suggested in Figure 6b. Saturation of the double bond in the five-membered ring would produce structures having a z number of -48. We should emphasize that the structure shown in Figure 6b is for illustration purposes only, i.e., identification of compounds having an elemental formula of CnH2n-50N4VO in the high-resolution mass spectra is not sufficient to prove that structures of the type shown in Figure 6b actually exist in the fraction. The distribution of vanadyl porphyrins noted in Figure 7 may be sufficient to explain the apparent “nonporphyrinic vanadium” found in fraction N-P2BS2. That is, given the wide variety of types found, it is unlikely that their average molar absorptivity is adequately represented by the assumed value based on etioporphyrins used earlier to estimate vanadyl porphyrin content.35 Significant effects from cycloalkyl and benzo substitution on spectral characteristics of porphyrins have been documented.5,10,15,17 Specifically, cycloalkyl groups lower visible absorption coefficients (48) Filby, R. H.; Van Berkel, G. J. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 3-39.

Vanadyl Porphyrin Fraction from Heavy Petroleum

Energy & Fuels, Vol. 11, No. 3, 1997 609

Figure 7. Relative areas under elimination curves vs z number and carbon number for CnH2n+zN4VO compounds in fraction N-P2BS2. Results from low-eV mass spectra.

while benzo groups shift absorption maxima to longer wavelengths. Both of these effects would cause the apparent porphyrin visible absorption at 570 nm, the wavelength corresponding to λmax of etioporphyrins, to be lower. In this case, the effective average molar absorptivity for the Cerro Negro fraction appears to be approximately one-half that measured for pure etioporphyrins, since the apparent porphyrin V was only 56% of the total V, as noted in the Introduction. However, other organic vanadium compounds may be present in the fraction as minor components. As noted in the Experimental Section, one-third of the total vanadium remained in the material recovered from the probe with dichloromethane at the end of the low-eV experiment. One of the more novel findings from the investigation was the detection of compounds with z numbers more negative than -36, which is the most negative z value reported previously in the literature. Presumably, compounds belonging to the more negative z series are porphyrins substituted with progressively greater numbers of saturated and aromatic rings, as illustrated by the hypothetical structure in Figure 6b. Potentially, there exists a wider variety of porphyrinic types and carbon number ranges within each type than is presently recognized. These more highly substituted/more aromatic porphyrins may account for the higher molecular weight ranges, greater tendencies toward associa-

tion, higher resistance toward removal in refining processes, and other properties currently attributed to “nonporphyrinic” metal complexes in petroleum. Conclusions Etioporphyrins (CnH2n-28N4VO) having carbon numbers of e56 comprise the bulk of fraction N-P2BS2. Deoxophylloerythroetioporphyrins (CnH2n-30N4VO) with carbon numbers of e55 were the second most abundant. In addition, other cycloalkyl and aromatic forms having the formula CnH2n+zN4VO, where z ranges from -32 to -50, excluding -46, were found. These latter types would exhibit lower apparent UV-visible response at 570 nm because of altered absorption characteristics compared with those of vanadyl etioporphyrins. Their presence in the fraction indicates that at least a part of the apparent “nonporphyrinic” vanadium originally thought to be present is really contained in porphyrinic structures. Acknowledgment. We thank G. P. Sturm, Jr., for reviewing the manuscript and J. A. Green for determining vanadium in the probe residue. Funding for this project was provided by the U.S. Department of Energy under Contract No. DE-AC22-94PC91008. EF960205M