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Associative π-π Interactions of Condensed Aromatic Compounds with Vanadyl or Nickel Porphyrin Complexes Are Not Observed in the Organic Phase Cindy-Xing Yin,† Xiaoli Tan,† Klaus Mu¨llen,‡ Jeffrey M. Stryker,§ and Murray R. Gray*,† Departments of Chemical and Materials Engineering, and Chemistry, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada, and Max Planck Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed February 7, 2008. ReVised Manuscript ReceiVed April 6, 2008
Association of vanadium and nickel petroporphyrin compounds with aromatic groups is commonly invoked as an important mechanism for aggregation of asphaltenes. In this study, the interaction of vanadium and nickel octaethylporphyrin with condensed aromatic compounds and bridged aromatic compounds was investigated. Neither UV-vis nor fluorescence spectroscopy detect any interactions between porphyrins and aromatic ring groups in either the ground or excited states. The lack of detectable binding of these model compounds suggests that the association of the majority of the vanadium and nickel petroporphyrins in crude oils with the asphaltene fraction may be due to other functionality appended to the porphyrin ring, rather than favorable π-π stacking interactions of aromatic rings with the porphyrin core itself.
Introduction Vanadium and nickel exist in crude oil or bitumen at concentrations up to 1320 ppm, depending upon the source.1,2 These chelated metals are difficult to remove from the petroleum and cause a range of problems, such as catalyst poisoning and solid sludge formation. The heaviest fraction of heavy oil or bitumen, also the most metal-rich fraction, is represented by asphaltenes. All of the vanadium and nickel species in crude oil bear a coordination geometry similar to the metalloporphyrins, as determined by X-ray absorption fine structure (XAFS) spectroscopy.3 Therefore, both vanadium and nickel petroporphyrins are in tetrapyrrole-like structures, some more structurally elaborated than others. Approximately half of the vanadium and nickel petroporphyrins can be identified and quantified by their characteristic ultraviolet-visible (UV-vis) spectra, an intense Soret band around 400 nm and two visible bands at lower energy.4 The remaining vanadium and nickel compounds, which we term the non-Soret petroporphyrins, are defined by an absence of distinct UV-vis spectroscopic bands. All of the petroporphyrins are generally believed to chelate or nonco* To whom correspondence should be addressed. Telephone: 780-4927965. Fax: 780-492-2881. E-mail:
[email protected]. † Department of Chemical and Materials Engineering, University of Alberta. ‡ Max Planck Institut fu ¨ r Polymerforschung. § Department of Chemistry, University of Alberta. (1) Yen, T. F. Chemical aspects of metals in native petroleum. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science: Ann Arbor, MI, 1975; pp 1-30. (2) Filby, R. H. The nature of metals in petroleum. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science: Ann Arbor, MI, 1975; pp 31-58. (3) Miller, J. T.; Fisher, R. B.; van der Eerden, A. M. J.; Koningsberger, D. C. Structural determination by XAFS spectroscopy of non-porphyrin nickel and vanadium in Maya residuum, hydrocracked residuum, and toluene-insoluble solid. Energy Fuels 1999, 13 (3), 719–727. (4) Smith, K. M. General features of the structure and chemistry of porphyrin compounds. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier Scientific Publishing Company: Amsterdam, The Netherlands, 1975; pp 1-28.
valently associate with aromatic asphaltene components by π-π interactions.5 Metals in smaller, simple porphyrinic structures similar to available model compounds are easier to remove by solvent extraction, while metals in larger non-Soret petroporphyrin molecules are more tightly associated with the asphaltene structures.6,7 Although debates exist about the typical molecular structure of asphaltenes, it is commonly believed that asphaltene molecules consist of structures incorporating 4-10 fused aromatic rings.8,9 Hence, it is important to study the interactions of petroporphyrins with fused-ring aromatic systems of different sizes. These interactions will help to elucidate the preferred binding sites of petroporphyrins in asphaltene mixtures and define the contributions of the petroporphyrins to the mechanism of asphaltene aggregation.10 Yen and co-workers first proposed an aggregation scheme for the macrostructure of petroleum asphaltenes, within which petroporphyrins could stack to the aromatic sheets of the asphaltene.5 Such stacking can be investigated directly using spectroscopic measurements on mixtures of pertinent model compounds in solution. Existing studies on the interaction of (5) Dickie, J. P.; Yen, T. F. Macrostructures of the asphaltic fractions by various instrumental methods. Anal. Chem. 1967, 39 (14), 1847–1852. (6) Reynolds, J. G.; Biggs, W. R. Application of size exclusion chromatography coupled with element-specific detection to the study of heavy crude oil and residua processing. Acc. Chem. Res. 1988, 21, 319– 326. (7) Bestougeff, M. A.; Byramjee, R. J. Chemical constitution of asphaltenes. In DeVelopments in Petroleum Science 40A (Asphaltenes and Asphalts 1); Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1994; pp 67-94. (8) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Subfractionation and characterization of Mayan asphaltene. Energy Fuels 1998, 12 (6), 1290–1298. (9) Ruiz-Morales, Y.; Mullins, O. C. Polycyclic aromatic hydrocarbons of asphaltenes analyzed by molecular orbital calculations with optical spectroscopy. Energy Fuels 2007, 21 (1), 256–265. (10) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. On the distribution of chemical properties and aggregation of solubility fractions in asphaltenes. Energy Fuels 2006, 20 (2), 705–714.
10.1021/ef800088c CCC: $40.75 2008 American Chemical Society Published on Web 06/21/2008
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Figure 1. Two asphaltene model compounds, PBP for the bridged aromatic structure and HBC-C12 for large fused aromatic core structure, and the metalloporphyrin complexes used in this study.
porphyrin complexes with fluorescent aromatic rings mostly focus on energy-transfer efficiency related to photosynthetic processes.11 Many of these investigations are limited to interactions within synthetic constructs assembled via covalent bonds.12 Only a few noncovalent self-assembled metal porphyrin aggregates (M ) Zn and Co) comprised of either free-base porphyrin or N-substituted aromatic arrays have been documented in the context of light-harvesting processes.13,14 To our knowledge, there is no experimental evidence for intermolecular interactions of metal porphyrin complexes with aromatic ring systems, aside from indirect speculations based on free-radical studies of asphaltenes.15
spectroscopy was carried out on a 500 MHz Varian spectrometer in CDCl3; all the chemical shifts were referenced to the solvent peak of CDCl3 at 7.260 ppm.
Results and Discussion
Experimental Section
Two divergent structural models for asphaltenes have been proposed: one is constructed from short alkyl chains bridging smaller fused aromatic ring clusters,16 and the other is constructed from large polycondensed aromatic ring systems with compositionally varied side chains.18,19 The interactions of two asphaltene model compounds with a range of metalloporphyrin complexes were investigated, specifically, 4,4′-bis-(2-pyren-1ylethyl)-[2,2′]bipyridinyl (PBP hereafter) as a model for bridged aromatic compounds and hexa-peri-hexabenzocoronene (side
4,4′-Bis-(2-pyren-1-ylethyl)-[2,2′]bipyridinyl was prepared, purified, and characterized as detailed elsewhere.16 Hexa-peri-hexabenzocoronene (side chain ) n-C12H25) was prepared by Dr. Daniel Wasserfallen according to the procedure developed by the Mu¨llen group.17 Dibenzofuran (Eastman Organic Chemicals) dissolved in diethyl ether was purified by NaOH (2 M), then water-washed, crystallized from an aqueous 80% ethanol mixture, and then dried under vacuum. All other chemicals were purchased from Aldrich and used as received. UV-vis spectroscopy was performed on a Photonics CCD Array UV-vis spectrometer or a Varian Cary 50 Scan UV-vis spectrometer using a quartz cell (diameter of 1 cm). Stock solutions (1 × 10-5-10-2 M) were prepared by dissolving the compounds into ACS-grade dichloromethane, unless noted otherwise. For UV-vis spectroscopic measurements, the diluted metalloporphyrin solution in n-hexane was mixed with 1-200 mol equiv of the aromatic compound added via a 10-100 µL pipet (the metalloporphyrin solution was diluted to the extent that the absorbance was less than 1); for steady-state fluorescence measurements, 1.5 mL of n-hexane (ACS-grade), combined with aliquots of 50 µL of the stock solution via a 10-100 µL pipet in a 1 cm path-length quartz cell, was monitored by fluorescence spectroscopy at a 4 nm bandwidth with a Photon Technology International MP1 fluorescence spectrometer or a Varian Eclipse fluorescence spectrometer at a 5 nm bandwidth. Nuclear magnetic resonance (NMR)
(11) Hori, T.; Nakamura, Y.; Aratani, N.; Osuka, A. Exploration of electronically interactive cyclic porphyrin arrays. J. Organomet. Chem. 2007, 692 (1-3), 148–155. (12) Giribabu, L.; Kumar, A. A.; Neeraja, V.; Maiya, B. G. Orientation dependence of energy transfer in an anthracene-porphyrin donor-acceptor system. Angew. Chem., Int. Ed. 2001, 40 (19), 3621–3624, and references therein. (13) Ambroise, A.; Li, J.; Yu, L.; Lindsey, J. S. A self-assembled lightharvesting array of seven porphyrins in a wheel and spoke architecture. Org. Lett. 2000, 2 (17), 2563–2566. (14) Sessler, J. L.; Wang, B.; Harriman, A. Photoinduced energy transfer in associated but noncovalently linked photosynthetic model systems. J. Am. Chem. Soc. 1995, 117, 704–714. (15) Yen, T. F.; Saraceno, A. J.; Erdman, J. G. Investigation of nature of free radicals in petroleum asphaltenes and related substances by electron spin resonance. Anal. Chem. 1962, 34 (6), 694–700. (16) Tan, X.; Fenniri, H.; Gray, M. R. Pyrene derivatives of 2,2′bipyridine as models for asphaltenes: synthesis, characterization, and supramolecular organization. Energy Fuels 2008, 22, 715–720. (17) Wu, J.; Fechtenkötter, A.; Gauss, J.; Watson, M. D.; Kastler, M.; Fechtenkötter, C.; Wagner, M.; Müllen, K. Controlled Self-Assembly of Hexa-peri-hexabenzocoronenes in Solution. J. Am. Chem. Soc. 2004, 126, 11311–11321. (18) Rakotondradany, F.; Fenniri, H.; Rahimi, P.; Gawrys, K. L.; Kilpatrick, P. K.; Gray, M. R. Hexabenzocoronene model compounds for asphaltene fractions: Synthesis and characterization. Energy Fuels 2006, 20 (6), 2439–2447.
Vanadyl or Nickel Porphyrin Complexes
Figure 2. Fluorescence emission spectra of 0.3 µM PBP upon addition of 0-5 mol equiv of VO(OEP) or Ni(OEP) in n-hexane (excitation wavelength of 342 nm), each shown with the corresponding normalized UV-vis spectra of metalloporphyrins to illustrate the inner-filter effects caused by the strong absorbance of the Soret bands.
chain ) n-C12H25, abbreviated as HBC-C12) as a model for the polycondensed aromatic compounds (Figure 1). UV-vis spectroscopy was used to study potential interactions between the metalloporphyrins and the asphaltene model compounds. For VO(OEP) or Ni(OEP), no interactions whatsoever were observed for binary mixtures consisting of a metalloporphyrin and an asphaltene model compound (PBP or HBC, see Figure S1-S4 in the Supporting Information for details). Consequently, strong association into heterodimers or larger aggregates does not occur in such systems. In comparison to UV-vis spectroscopy, fluorescence spectroscopy is sensitive to molecular dynamics and can provide information on weaker interactions between a fluorophore and other molecules in solution. By simple addition of 1-5 mol equiv of vanadyl octaethylporphine [VO(OEP) hereafter, in stock solutions of ca. 10-5 M in CH2Cl2] to a 0.3 µM solution of PBP in n-hexane, the fluorescence of the emission spectrum of PBP changes (Figure 2). A resonance valley appears precisely at the wavelength of the UV-vis Soret band of VO(OEP) at 405 nm (Figure 2). There is a similar but less significant effect when PBP is mixed with nickel octaethylporphine [Ni(OEP)]
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Figure 3. Fluorescence emission spectrum of 0.3 µM PBP and corrected fluorescence emission spectra of PBP upon addition of 1-5 mol equiv of VO(OEP) or Ni(OEP) in n-hexane (excitation wavelength of 342 nm) with eq 1.
and probed using fluorescence spectroscopy. In this case, a resonance valley appears at 390 nm. This change of fluorescence emission at the Soret band position is due to absorption by the metalloporphyrin even at concentrations as low as 10-6M, a spectroscopic artifact known as the inner-filter effect. A correction for the inner-filter effect is made according to the equation derived by Lakowicz.20,21 fcorrection ) 10(Aems)/2
(1)
The correction factor, fcorrection, is defined as the ratio of corrected fluorescence intensity over observed fluoresence intensity. Aem, representing the absorbance of the fluorescence emission (in cm-1), was measured with a UV-vis spectrometer, and s/2 ()0.5 cm) was the path length that the fluorescence emission light travels inside the cuvette assuming that the fluorescence is emitted in the middle of the cuvette. Overall, this correction is sufficient as long as the absorbance of the colored solution is less than 0.5. As illustrated in Figure 3 above, there was no detectable interaction between the bridged as(19) Groenzin, H.; Mullins, O. C. Molecular size and structure of asphaltenes. Pet. Sci. Technol. 2001, 19 (1 and 2), 219–230. (20) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006; pp 55-57.
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Figure 5. Normalized fluorescence emission spectra of 0.6 µM HBC upon addition of 1-5 mol equiv of VO(OEP) in n-hexane (excitation wavelength of 358 nm).
Figure 4. Fluorescence emission spectrum of 0.6 µM pyrene and corrected fluorescence emission spectra of pyrene upon addition of 1-2 equiv of VO(OEP) or VO(TPP) in n-hexane (excitation wavelength of 333 nm).
phaltene model compound, PBP, and the two model metalloporphyrin compounds, VOOEP or NiOEP, even though fluorescence spectroscopy clearly indicated self-association of PBP in a previous study.16 The apparent reduction of the fluorescence intensity as shown in Figure 2, therefore, is mostly due to secondary inner-filter effects. The pyrene ring present in PBP accounts for the fluorescent emission observed in Figure 3, as demonstrated by a control experiment conducted with pyrene itself (Figure 4). The addition of either VO(OEP) or another vanadyl porphyrin model compound, vanadyl tetraphenylporphyrin [VO(TPP)], had no effect on the fluorescent emission spectrum of pyrene (Figure 4). VO(TPP) is less capable of sustaining π-π interactions because of the steric influence of the perpendicular phenyl substituents on the porphyrin macrocycle. These results are consistent with the lack of detectable interactions between PBP and porphyrin compounds. The large fused aromatic hexabenzocoronene, with a core structure comprising 13 rings, HBC-C12 (Figure 1), was also evaluated for interactions with the porphyrin complexes. No interactions were detected with either VOOEP or NiOEP by UV-vis spectroscopy (Figure S2 and S4 in the Supporting Information). 1H NMR monitoring the chemical shifts of methine proton of Ni(OEP) (0.28-1.12 mM) also showed no shifts with or without 0.28 mM HBC-C12 (Table S1 in the
Supporting Information). As shown in Figure 5, no interactions of HBC with VOOEP or NiOEP (Figure S5 in the Supporting Information) were observed in the detection window of fluorescence emissions, even though alkyl HBC compounds experience self-association in solution.18 We conclude that associative interactions of HBC-C12 with VOOEP or NiOEP do not occur based on analysis by fluorescence spectrometry. A structurally diverse range of other fused aromatic ring compounds were also evaluated for π-π interactions with vanadyl and nickel porphyrin complexes using this method. Inhibition of the fluorescence signals was observed for mixtures containing naphthalene, anthracene, triphenylene, pentacene, and coronene at exactly the wavelengths illustrated in Figure 2. Similarly, oxygen- and sulfur-containing triaromatic compounds (dibenzofuran and dibenzothiophene) did not show any π-π interactions with vanadyl and nickel porphyrin complexes by fluorescence spectroscopy (Figure S6 in the Supporting Information). We conclude that these interactions arise from similar inner-filter effects in solution and that fluorescence spectroscopy indicates no significant interactions of the aromatics with the metalloporphyrins. In crude oils, vanadium and nickel are 5-6 times more concentrated in asphaltene fractions than in resin fractions.2,22 Isolation of the porphyrin components from asphaltenes can be performed by solvent extraction with yields of 95-98% of the total Soret-active porphyrin content, which accounts for approximately 30% of the total vanadium content of the sample.23 As determined by size-exclusion chromatography, these extractable petroporphyrins are smaller in molecular size compared to the non-Soret petroporphyrins.6 The lack of any observation of intermolecular interactions, as revealed by emission fluorescence spectroscopy, suggests that the vanadyl and nickel petroporphyrins by themselves do not exhibit detectable π-π interactions with a range of fused-ring aromatic compounds in (21) Zimmermann, U.; Skrivanek, T.; Lo¨hmannsro¨ben, H. G. Fluorescence quenching of polycyclic aromatic compounds by humic substances. Part 1. Methodology for the determination of sorption coefficients. J. EnViron. Monit. 1999, 1, 525–532. (22) Reynolds, J. G. Effects of asphaltene precipitation on the size of vanadium-, nickel-, and sulfur-containing compounds in heavy crude oils and residua. In DeVelopments in Petroleum Science 40A (Asphaltenes and Asphalts 1); Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1994; pp 233-248.
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solution. For the non-Soret petroporphyrins to bind competently, the tetrapyrrole ring system must be covalently attached to other structures to enhance binding to the asphaltene components by additional interactions, such as hydrogen-bonding and acid-base interactions.7,24 The strong aggregation that defines asphaltenes must arise from more specific electronic interactions among and within constituents and, possibly, from higher order aggregates promoted by the highly heterogeneous matrix. This type of association defines the challenge of accomplishing selective removal of the vanadium and nickel from crude oils and bitumens. For this elusive but attractive goal, a binding site with greater affinity for the vanadium tetrapyrrolic substructure must be identified to separate the metal-bearing components from the asphaltene matrix.25 Conclusions Using UV-vis and fluorescence spectroscopies, we have demonstrated that vanadyl and nickel porphyrin model com(23) Spencer, W. A.; Galobardes, J. F.; Curtis, M. A.; Rogers, L. B. Chromatographic studies of vanadium compounds from Boscan crude oil. Sep. Sci. Technol. 1982, 17, 797–819. (24) Nguyen, S. N. The nature and distribution of nickel(II) complexes in oil-sand asphaltenes. Ph.D. Thesis, Washington State University, Pullman, WA, 1986. (25) Andersen, S. I.; Keul, A.; Stenby, E. Variation in composition of subfractions of petroleum asphaltenes. Pet. Sci. Technol. 1997, 15 (7 and 8), 611–645.
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pounds do not have detectable associative interactions with fused aromatic rings in solution. Therefore, the non-extractable petroporphyrin fraction almost certainly bears affinity sites covalently substituted onto the porphyrin periphery. Acknowledgment. The authors sincerely thank Professor Cornelia Bohne at the University of Victoria for her help with corrections of the inner-filter effects. The authors gratefully acknowledge financial support from the Imperial Oil-Alberta Ingenuity Centre for Oil Sands Innovation and the Natural Sciences and Engineering Research Council. M.R.G. holds the NSERC/ Imperial Oil Chair in Oil Sands Upgrading and a Canada Research Chair. Supporting Information Available: UV-vis spectra of the non-interacting PBP and HBC with VO(OEP) or Ni(OEP), fluorescence spectra of HBC with the addition of Ni(OEP), fluorescence spectra of dibenzofuran and dibenzothiophene with the addition of VO(OEP)/Ni(OEP), and NMR data with no detectable interactions of hexabenzocoronene-C12 with Ni(OEP) in CDCl3. This material is available free of charge via the Internet at http://pubs.acs.org.
EF800088C
