Measured and Simulated Electronic Absorption and Emission Spectra

Energy & Fuels 2009, 23, 1169–1177

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Measured and Simulated Electronic Absorption and Emission Spectra of Asphaltenes† Yosadara Ruiz-Morales*,‡ and Oliver C. Mullins§ Programa de Ingenierıá Molecular, Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas Norte 152, Mexico, D.F. 07730, Mexico, and Schlumberger-Doll Research, Cambridge, Massachusetts 02139 ReceiVed August 14, 2008. ReVised Manuscript ReceiVed September 27, 2008

The electronic absorption and emission spectra of petroleum asphaltenes are distinct and characteristic; indeed, asphaltene “color” is one of the canonical properties of asphaltenes. Moreover, each π electron has a fixed oscillator strength; thus, each aromatic carbon and nitrogen is represented in the absorption spectrum. Nevertheless, these spectral origins have not previously been thoroughly explored. Most importantly, there has been significant controversy regarding the types of polycyclic aromatic hydrocarbons (PAHs) contained in asphaltenes, with grossly different PAH distributions being reported. Here, we show that the main features of the absorption and emission spectra of asphaltenes are accounted for using a distribution of 523 PAH model compounds incorporating heteroatom substitution. This distribution is discussed in terms of PAH classes. In particular, we have used a distribution with 7 fused aromatic rings as most likely along with a slow drop off for contributions from smaller and larger PAHs. In addition, a “tail” of large PAHs is assumed. Another PAH distribution consisting of very small PAHs proposed in the literature is shown to be very inconsistent with measured asphaltene spectra. In particular, small PAHs absorb strongly in the UV spectrum but not in the visible spectrum; asphaltenes show a much smaller contrast between UV and visible absorption as shown herein. That is, if the π oscillator strength is not in the near-infrared and visible spectra, then it must be in the UV spectrum.

Introduction The asphaltenes1 are a very important class of petroleum, impacting virtually all aspects of resource use. Optimization of production and processing of crude oils behooves proper chemical understanding of the asphaltenes. The vision of petroleomics, including establishment of structure-function relationships of crude oils and asphaltenes, mandates determination of asphaltene molecular structure. Nevertheless, many basic features of the molecular architecture of asphaltenes have been the subject of long debate. Asphaltene molecular weight had been controversial; however, this debate is largely resolved. Two primary measurements have been employed to resolve this issue: molecular diffusion and mass spectroscopy. Time-resolved fluorescence depolarization (TRFD) measurements indicated that asphaltenes have rapid rotational relaxation and indicated most likely molecular weights of ∼750 g/mol.2-8 Moreover, the rotational diffusion of small, blue-emitting chromophores is 10 † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. E-mail: yruiz@ www.imp.mx. ‡ Instituto Mexicano del Petro ´ leo. § Schlumberger-Doll Research. (1) Asphaltene, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007. (2) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and structure. J. Phys. Chem. A 1999, 103, 11237–11245. (3) Groenzin, H.; Mullins, O. C. Molecular sizes of asphaltenes from different origin. Energy Fuel 2000, 14, 677. (4) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. The overriding chemical principles that define asphaltenes. Energy Fuels 2001, 15, 972. (5) Buch, L.; Groenzin, H.; Buenrostro-Gonzalez, E.; Andersen, S. I.; Lira-Galeana, C.; Mullins, O. C. Effect of hydrotreatment on asphaltene fractions. Fuel 2003, 82, 1075.

times faster the large red-emitting chromophores; they are not cross-linked in asphaltenes.2-8 That is, the TRFD results indicated that there is only one or perhaps two polycyclic aromatic hydrocarbon (PAH) chromophores per asphaltene molecule.2-8 This surprising result was very controversial and is a central feature of the work presented herein. Other diffusion measurements agree with the TRFD results regarding molecular size. Taylor dispersion performed on the same samples obtained the same results as the TRFD samples.9 Nuclear magnetic resonance (NMR) diffusion measurements also obtained consistent results.10 Fluorescence correlation spectroscopy (FCS) showed consistency with TRFD, particularly with regard to coalderived versus petroleum asphaltenes.11-14 (6) Groenzin, H.; Mullins, O. C.; Eser, S.; Mathews, J.; Yang, M.-G.; Jones, D. Asphaltene molecular size for solubility subfractions obtained by fluorescence depolarization. Energy Fuels 2003, 17, 498. (7) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Molecular size and weight of asphaltene and asphaltene solubility fractions from coals, crude oils and bitumen. Fuel 2006, 85, 1. (8) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and weight by time-resolved fluorescence depolarization. Asphaltene, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 2. (9) Wargadalam, V. J.; Norinaga, K.; Iino, M. Size and shape of a coal asphaltene studied by viscosity and diffusion coefficient measurements. Fuel 2002, 81, 1403. (10) Freed, D. E.; Lisitza, N. V.; Sen, P. N.; Song, Y.-Q. Asphaltene molecular composition and dynamics from NMR diffusion measurements. Asphaltene, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 11. (11) Andrews, A. B.; Guerra, R.; Sen, P. N.; Mullins, O. C. J. Phys. Chem. A 2006, 110, 8095. (12) Guerra, R.; Ladavac, K.; Andrews, A. B.; Sen, P. N.; Mullins, O. C. Diffusivity of coal and petroleum asphaltenes monomers by fluorescence correlation spectroscopy. Fuel 2007, 86, 2016–2020.

10.1021/ef800663w CCC: $40.75  2009 American Chemical Society Published on Web 11/26/2008

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Mass spectroscopy has played a major role in determining asphaltene molecular weights. Boduszynski employed field ionization mass spectroscopy, showing that the asphaltene molecular weights are below 1000 Da for the most part.15 Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry has consistently shown that the most likely molecular weights of asphaltenes are on the order of 750 Da.16,17 Atmospheric pressure chemical ionization has shown similar molecular weights of asphaltene fractions along with expected systematics.18 Field desorption ionization of asphaltenes shows slightly higher molecular weights of asphaltenes; nevertheless, as noted by the authors, these results are in agreement with other mass spectral techniques.19 Laser desorption ionization (LDI) had previously yielded inconsistent results; this was traced to severe gas-phase aggregation if plasma densities are too high.20,21 If low laser brightness and low asphaltene surface concentrations are used, results consistent with all other mass spectral methods and all diffusion measurements are obtained for both petroleum20 and coal-derived asphaltenes.21 Recent experiments on a LDI variant, two-step laser desorption ionization (L2MS), employing an IR laser to desorb the sample and a weak UV laser to ionize the neutral asphaltene plume, show small molecular weights consistent with the other techniques.22,23 These experiments confirm the findings of gas-phase aggregation in uncontrolled LDI experiments.20,21 Indeed, the smaller asphaltene molecular weights are important in precluding previously proposed polymeric structures of asphaltenes that contained large numbers of large PAHs. Such structures are now known to be in error.24 The question arises (13) Schneider, M. H.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Asphaltene molecular size by fluorescence correlation spectroscopy. Energy Fuels 2007, 21, 2875–2882. (14) Andrews, A. B.; Shih, W.-C.; Edwards, J.; Norinaga, K.; Mullins, O. C. Coal asphaltene molecular size by fluorescence correlation spectroscopy. Energy Fuels, manuscript submitted. (15) Boduszynski, M. M. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society: Washington, D.C., 1981; Chapter 7. (16) Rodgers, R. P.; Marshall, A. G. Petroleomics: Advanced characterization of petroleum derived materials by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Asphaltene molecular size and weight by time-resolved fluorescence depolarization. Asphaltene, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 3. (17) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G.; Yen, A.; Asomaning, S. Mass spectral analysis of asphaltenes. I. Compositional differences between pressure-drop and solvent-drop asphaltenes determined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20, 1965–1972. (18) Merdrignac, I.; Desmazieres, B.; Terrier, P.; Delobel, A.; Laprevote, O. Analysis of raw and hydrotreated asphaltenes using off-line and on-line SEC/MS coupling. In Proceedings of Heavy Organic Deposition, Los Cabos, Baja California, Mexico, 2004. (19) Qian, K.; Edwards, K. E.; Siskin, M.; Olmstead, W. N.; Mennito, A. S.; Dechert, G. J.; Hoosain, N. E. Desorption and ionization of heavy petroleum molecules and measurement of molecular weight distributions. Energy Fuels 2007, 21 (2), 1042–1047. (20) Martıńez-Haya, B.; Hortal, A. R.; Hurtado, P. M.; Lobato, M. D.; Pedrosa, J. M. Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates. J. Mass Spectrom. 2007, 42, 701–713. (21) Hortal, A. R.; Hurtado, P. M.; Martıńez-Haya, B.; Mullins, O. C. Molecular weight distributions of coal and petroleum asphaltenes from laser desorption ionization experiments. Energy Fuels 2007, 21, 2863–2868. (22) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Two-step laser mass spectrometry of asphaltenes. J. Am. Chem. Soc. 2008, 130, 7216–7217. (23) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Asphaltene molecular weight distribution determined by twostep laser mass spectrometry. Energy Fuels, manuscript submitted. (24) Mullins, O. C.; Martinez-Haya, B.; Marshall, A. G. Contrasting perspective on asphaltene molecular weight. This comment vs the overview of A. A. Herod, K. D. Bartle, and R. Kandiyoti. Energy Fuels 2008, 22, 1765–1773.

Ruiz-Morales and Mullins Table 1. Distribution Used in the Absorption Profile and Number of Systems FAR family

number of calculated compounds

distribution

3FAR 4FAR 5FAR 6FAR 7FAR 8FAR 9FAR 10FAR 11FAR 12FAR 13FAR 14FAR 15FAR

2 5 28 27 41 39 71 46 41 34 61 64 64

0.20 0.40 0.60 0.80 1.00 0.80 0.60 0.40 0.20 0.18 0.16 0.14 0.12

as to whether asphaltene molecules predominantly have one large PAH per molecule or several smaller PAHs per molecule. Specifically, if one has a seven-membered fused ring system (∼28 aromatic carbons) along with a slightly larger fraction of alkane carbon (say 32 alkyl carbons), along with half a sulfur atom on average per molecule and H/C atomic ratio of 1:12, the resulting molecular weight is ∼800 Da. Consequently, a distribution of PAHs centered at 7 fused aromatic rings would yield observed molecular-weight distributions of asphaltenes. The first reports of a single PAH per asphaltene molecule as the dominant molecular architecture were from the TRFD diffusion studies.2-8 These results remain controversial. Nevertheless, when the TRFD studies first reported rather small molecular weights of asphaltenes, these results were viewed in many quarters as erroneous; now they are the rule. From that vantage, the TRFD results regarding molecular architecture need to be considered carefully. The LDI studies showing extensive gas-phase aggregation of asphaltenes and coronene are suggestive of large PAHs in asphaltenes.20,21 Interestingly, direct molecular imaging of asphaltene molecules by scanning tunneling microscopy (STM) clearly showed 6-7 fused rings as most probable,25 but for reasons unknown, these results have been largely ignored. These STM results have been supported by high-resolution transmission electron microscopy (HRTEM) imaging.26 Indeed, the HRTEM study showed 1 nm length scale for the petroleum asphaltene PAHs as did the STM study. In addition, the HRTEM study showed that coal-derived asphaltenes have smaller PAHs (∼0.7 nm). This result is consistent with the 1/2 size of coal-derived asphaltenes compared to petroleum asphaltenes found by TRFD,3,4,7,8 Taylor dispersion,9 FCS,12,14 properly controlled LDI,21 and L2MS.23 Recently, very strong evidence has emerged supporting a single PAH molecular architecture as the dominant species for asphaltenes.27 Asphaltene molecules have been isolated in the gas phase and fragmented by collision with helium gas. The resulting molecular fragments have been shown to decrease substantially in carbon number as expected. Most importantly, the aromaticity of the large fragment remains virtually unchanged.27 To date, a variety of molecular-weight ranges and heteroatom-containing classes of asphaltenes have all shown (25) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. Scan. Microsc. 1994, 8, 463. (26) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. C. Probing order in asphaltenes and aromatic ring systems by HRTEM. Energy Fuel 2002, 16, 490. (27) McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Marshall, A. G. Atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry for detailed compositional analysis of petroleum. In the 9th International Conference on Petroleum Phase Behavior and Fouling, Victoria, British Columbia, Canada, June 15-19, 2008; Abstract 17.

Absorption and Emission Spectra of Asphaltenes

Energy & Fuels, Vol. 23, 2009 1171

Table 2. Structure, Carbon Ratio, and HOMO-LUMO Gap Wavelength for Some of the 523 Calculated PAH Compounds


Ruiz-Morales and Mullins Table 2. Continued

similar behavior. Moreover, model compounds with intermediate-length alkane chains have been shown to fragment readily, giving loss of carbon number and aromaticity.27 These results are recent, and complications could easily arise; nevertheless, there is now overwhelming evidence that, at the least, asphaltene molecules with a single PAH are present in ample quantity. PAHs can be investigated by their characteristic electronic absorption and fluorescence emission spectra. The relation of these PAHs to crude oils and asphaltenes can be established. In particular, systematic rules governing the optical absorption spectra of PAHs in terms of the number of fused rings, their geometry, and their heteroatom content have been developed.28-31 To understand the optical properties of asphaltene PAHs, it is essential to determine their molecular geometry by measurement. This has been accomplished by carbon X-ray Raman spectroscopy.32-34 It has been established that aromatic sextet carbon dominates asphaltene PAHs; this is most likely due to the significantly enhanced stability of sextet carbon over that of isolated double-bond carbon (within the Clar representation).28-31 With asphaltene PAH geometry understood, it is now possible to decipher the known optical properties35 of crude oils and asphaltenes. In particular, it has been shown that asphaltene electronic absorption is linear in asphaltene concentration over (28) Ruiz-Morales, Y. Molecular orbital calculations and optical transitions of PAHs and asphaltenes. Asphaltene, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 4. (29) Ruiz-Morales, Y. HOMO-LUMO gap as an index of molecular size and structure for polycyclic aromatic hydrocarbons (PAHs) and asphaltenes: A theoretical study. J. Phys. Chem. A 2002, 106, 11283. (30) Ruiz-Morales, Y. The agreement between clar structures and nucleus-independent chemical shift values in pericondensed benzenoid polycyclic aromatic hydrocarbons: An application of the Y-rule. J. Phys. Chem. A 2004, 108, 10873. (31) Gutman, I.; Ruiz-Morales, Y. Note on the Y-rule. Polycyclic Aromat. Compd. 2007, 27 (1), 41. (32) Bergmann, U.; Mullins, O. C. Carbon X-ray Raman spectroscopy of PAH’s and asphaltenes. Asphaltene, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 5. (33) Bergmann, U.; Mullins, O. C.; Cramer, S. P. Carbon raman X-ray spectroscopy of asphaltenes. Anal. Chem. 2000, 72, 2609. (34) Bergmann, U.; Groenzin, H.; Mullins, O. C.; Glatzel, P.; Fetzer, J.; Cramer, S. P. Carbon K-edge X-ray Raman spectroscopy supports simple yet powerful description of aromatic hydrocarbons and asphaltenes. Chem. Phys. Lett. 2003, 369, 184.

a very wide range (from 5 mg/L to 5 g/L); thus, significant absorption from charge-transfer complexes is ruled out.35,36 In addition, it has been shown that the major characteristics of the optical absorption and fluorescence emission spectra of asphaltenes are understandable in terms of PAHs with most probable 7 fused rings and a distribution with 4-10 rings.36,37 In this paper, we use the results of molecular orbital (MO) calculations on 523 selected PAHs to create an ensemble of possible asphaltene molecular structures. These PAHs are selected in terms of having acceptable ratios of isolated doublebond carbon/sextet carbon. We use a simple distribution of these PAHs as a model distribution for asphaltene PAHs. The distribution has 7 fused rings as the most probable, with a symmetric distribution falling off to 3 and 11 fused rings, respectively (with no smaller ring systems). We also include a large fused ring “tail” of up to 15 fused aromatic rings. This distribution is used to model a molecular-weight distribution. This PAH distribution coupled with the individual PAH absorption spectra are used to create a weighted optical absorption profile for the model asphaltene. In addition, this PAH distribution is coupled with the individual highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transition energies and with the energy gap law to derive a predicted fluorescence emission spectrum. Good agreement with experimental data is obtained in all of these curves. In addition, a similar analysis is performed on a very different, recently proposed asphaltene molecular geometry. Large discrepancies for that are obtained. Overall implications of these comparisons are described. Computational Methods The theoretical methods used herein have been described previously.28,36,37 The geometry optimization of the PAH systems was accomplished by performing force-field-based minimization (35) Mullins, O. C. Optical interrogation of aromatic moieties in crude oils and asphaltenes. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Publishing Co.: New York, 1998; Chapter 2. (36) Ruiz-Morales, Y.; Wu, X.; Mullins, O. C. Electronic absorption edge of crude oils and asphaltenes analyzed by molecular orbital calculations with optical spectroscopy. Energy Fuels 2007, 21, 944. (37) Ruiz-Morales, Y.; Mullins, O. C. Polycyclic aromatic hyodrocarbons of asphaltenes analyzed by molecular orbital calcualtions with optical spectroscopy. Energy Fuels 2007, 21, 256.



Table 3. Theoretical and Experimental HOMO-LUMO Gap

Figure 1. Quantum yield profile of the PAHs used in the emission spectra was made to obey the energy gap law.

a The experimental data were taken from UV-vis spectra or fluorescence emission spectra. In some specified cases, also the data from the fluorescence excitation spectra are reported for comparison purposes. b Roos, B. O.; Andersson, K.; Fu¨lscher, M. P. Chem. Phys. Lett. 1992, 5, 192. c Garrat, P. J. Aromaticity; John Wiley and Sons: New York, 1996; p 39. d Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237. e George, G. A.; Morris, G. C. J. Mol. Spectrosc. 1968, 26, 647. f Salama, F.; Allamandola, L. J. J. Chem. Phys. 1991, 94, 6964. g Berlman, L. B. Handbook of Fluorescence Specta of Aromatic Compounds; Academic Press: New York, 1971. h McKay, J. F.; Latham, D. R. Anal. Chem. 1972, 44, 2132. i Fluorescence excitation spectra. j The Sadtler Standard Spectra, Ultraviolet-Visible. Sadtler Research Laboratories, Philadelphia, PA, 1966. k Spectral Atlas of Polycyclic Aromatic Compounds; Karcher, W., Fordham, R. J., Dubois, J., Glaude, P. G. M., Ligthart, J. A. M., Eds.; Reidel: Dordrecht, The Netherlands, 1990. l McKay, J. F.; Latham, D. R. Anal. Chem. 1972, 44, 2132. m Acree, W. E., Jr.; Tucker, S. A. Polycyclic Aromat. Compd. 1991, 2, 75. n Nakashima, K.; Yashuda, S.; Ozaki, Y.; Noda, I. J. Phys. Chem. A 2000, 104, 9113. o Sadtler, Fluorescence Spectra Sadtler. Research Laboratories, Inc., Philadelphia, PA, 1973.

using the energy minimization panel in Cerius2, version 4.6, and the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS)38,39 consistent force field, as provided in the Cerius2 package.40 The excited electronic states of the PAH and PAC systems were calculated using the ZINDO/S41

Figure 2. (Top) Fluorescence emission intensities (quantum yields) of crude oils obey the energy gap law. (Bottom) All crude oils from the very light (small cutoff λ) to very heavy (large cutoff λ) are seen to obey this law. The cutoff wavelength is arbitrarily defined to be the wavelength where there is a 2 mm optical path length and an (electronic) absorbance of 1.

method, as provided in the Gaussian 98 package42 and using the COMPASS force field (FF) geometry-optimized structures.28,36,37 In previous studies,28 we carried out the validation of the best combination of theoretical methods (optimization method/excited states calculation method and optimization method/single-point calculation method) that agree better with the experimental fluorescence emission data of PAHs. Thus, to find the best method for the geometry optimization of the structures, we tested three different methods: (1) using COMPASS FF-based minimization, (2) using the semi-empirical PM3 method, and (3) using density (38) Sun, H.; Ren, P.; Fried, J. R. Comput. Theor. Polym. Sci. 1998, 8, 229. (39) Sun, H. J. Phys. Chem. B 1998, 102, 7338. (40) Molecular Simulation Incorporated. Cerius2 Modeling Environment, release 4.0; Molecular Simulations Incorporated, San Diego, CA, 1999.


Figure 3. (Top) Modeled optical absorption spectrum composed of the spectra of 523 PAH spectra weighted as shown in Table 1. Seven fused aromatic rings are most probable. (Bottom) Measured optical absorption spectra of four petroleum asphaltenes.

Figure 4. Optical absorption and fluorescence emission spectra of anthracene show line widths that are significantly broader than simply the HOMO-LUMO (0,0) transition. The vibrational state quantum numbers are given in parentheses for the excited and ground states, respectively. This broadening is due to the Franck-Condon factor, the projection of the initial vibrational state into many final vibrational states.

functional theory and the B3LYP functional. The excited electronic states, including the frontier orbital π-π* transition, were calculated also using different methods: (1) the semi-empirical electronic structure method ZINDO/S, (2) the semi-empirical PM3 method, (3) a single-point calculation at the Hartree-Fock self-consistent field level, (4) a single-point calculation with the density functional

Ruiz-Morales and Mullins theory and the B3LYP functional, and (5) the time-dependent density functional theory to calculate the transition energy. We concluded that the best agreement between the theory and experiment is observed for the case of the FF/ZINDO calculations.28 We have calculated the UV-vis spectra of PAH and polycyclic aromatic hydrocarbons with a heteroatom with N and S atoms in their structure (PAC) with 3-15 fused aromatic rings (3FAR-15FAR). We have only calculated the UV-vis spectra of neutral PAHs with fused six-membered rings, that is, benzenoid-type PAHs and their PACs. Some of the PACs contain in their structure one nitrogen atom in its pyridinic43 form or one sulfur atom. In the case of the nitrogen heteroatom, one carbon atom and its hydrogen, in the PAH compounds, are substituted by a nitrogen atom. The substitution is made in several places of the structure, and for each PAC, the UV-vis spectrum is calculated. For the case of PAC systems with a sulfur atom, the sulfur atom was located in a bay region to form a five-membered ring with thiophenic sulfur, which is the dominant form of aromatic sulfur in asphaltenes.44,45 The excited states of a total of 523 PAH and PAC systems have been calculated. The total number of systems (PAHs and PACs) calculated for each FAR family is given in Table 1. The number of calculated spectra is the sum of the number of systems for each FAR family. The observed experimental aphaltene fluorescence emission spectra, which exhibit significant intensity in the range of 400-550 nm, reflect the nature and type of fused aromatic ring (FAR) structures present in asphaltenes. In previous studies,28,29,36,37 the HOMO-LUMO gap for a wide number of PAHs with a different number of FARs (from 1FAR to 15FAR) and different spatial ring configurations were correlated with the experimental optical fluorescence and absorption spectra of asphaltenes. From this correlation, it was concluded that two fundamental parameters largely control PAH and asphaltene optical properties: (1) the number of fused rings and (2) the ratio of isolated double-bond carbon/sextet carbon. Asphaltene are largely but not entirely sextet carbon.29 XRRS studies have shown that there is roughly one isolated double bond per sextet ring in asphaltene PAHs; therefore, the bulk of asphaltenes show a carbon ratio of ∼0.333.32-34 In particular, we find29 that (1) acenes are not allowed as asphaltene PAH region based on stability and (2) fully resonant PAHs are not allowed either based on their high-energy transitions (unless unrealistically large-ring systems are assumed). That is, the fully resonant systems are colorless or pale yellow, unlike asphaltenes; (3) almost fully resonant pericondensed structures are stable and compatible with the large volume of optical absorption and emission data; and (4) mostly FARs with 5-10 fused rings and with 2-4 π sextets satisfy the requisite of stability, the requisite optical absorption and emission transitions, and the FAR size constraints imposed by direct molecular imaging and measurement of the rotational diffusion constants. Only certain PAHs with certain π-electronic distribution, in sextets and double bonds, with certain geometry and certain type of condensation, are most likely to be candidates for the aromatic (41) Zerner, M. C.; Correa de Mello, P.; Hehenberger, M. Int. J. Quantum Chem. 1982, 21, 251. (42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7, Gaussian, Inc., Pittsburgh, PA, 1998. (43) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Geroge, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252–258. (44) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182. (45) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53–57.


Figure 5. (Top) Simulated fluorescence emission spectrum of asphaltene using the PAH distribution in Table 1. (Bottom) Measured emission spectra of several asphaltenes and one condensate are shown.

Figure 6. Molecular-weight distribution obtained for the PAH distribution in Table 1. Most importantly, one PAH per molecular is assumed. Excellent agreement is obtained with extensive literature.

core of asphaltenes. In general, for the case of the 5FAR-10FAR families, we selected PAH systems that fulfill the asphaltene experimental constraints (optical transitions range, STM, HRTM, and molecular weight) and present a π-electron density distribution that agrees with their optical transitions and stability. In general, we mainly selected PAHs with a carbon ratio around 0.16-0.7, which exhibit intense optical properties in the region of 400-600 nm. The carbon ratio around 0.3 is closed to the asphaltene experimental carbon ratio. The distribution of the electronic π density and, thus, the carbon ratio was obtained using the Y rule.28-31


Figure 7. (Top) Simulated absorption spectrum and (bottom) fluorescence emission spectrum for model asphaltene compounds reported in ref 46. The corresponding proposed asphaltene molecules contain primarily small PAHs, thereby giving rise to little or no vis-NIR absorption and only UV fluorescence. These results are in stark contrast to what is actually observed for asphaltene absorption spectra (Figure 3) and asphaltene fluorescence emission spectra (Figure 5).

In general, we tried to select PAHs close to the asphaltene carbon ratio and/or that present optical properties around 400-600 nm. For the case of 3FAR, we selected the only known isomers phenanthrene and anthracene, and for 4FAR, we selected pyrene and its PAC compounds. From a previous study,35 we concluded that the population of aromatic cores with 11FAR-15FAR in asphaltenes is low. Thus, for the 11FAR-15FAR families, we selected PAHs with a carbon ratio between 0 and 0.5333, which show some optical properties in the range of 400-600 nm. Lineal and zigzag structures were also considered for the 11FAR-15FAR systems. In Table 2, the structure, FAR, carbon ratio, and HOMO-LUMO gap wavelength for some of the 523 calculated structures are presented. In addition, the PAHs were calculated for a recently proposed asphaltene molecule of strongly contrasting properties,46 where 10 aromatic groups are considered to be present in the asphaltene representation. The excited states of these 10 PAHs (see Table 3) were calculated with the same methodology explained above. This proposed molecule is representative of the “archipelago” structure, with many small isolated PAHs. The molecular weights for the archipelago molecules are too high by roughly a factor of 5 or more and, therefore, are not representative of asphaltenes. Nevertheless, we compare these structures with our larger proposed PAHs. (46) Sheremata, J. M.; Gray, M. R.; Dettman, H.; McCaffrey, W. C. Energy Fuels 2004, 18, 1377–1384.


Ruiz-Morales and Mullins

Two absorption profiles were obtained: one that considers a mixture of the 523 PAH systems and another one considering a mixture of the 10 chromophores (see Table 3). To obtain the absorption profile for a mixture of the calculated 523 PAH, all of the calculated spectra for each FAR family were added up and the resulting intensity was divided by the number of compounds in the FAR family, see Table 1. Then, the resulting intensity was normalized according to the distribution presented in Table 1. The distribution that we selected is consistent with a variety of data, suggesting PAHs of 7 FARs dominate asphaltenes. This includes our own previous analyses of asphaltene optical absorption and fluorescence emission spectra.36,37 A plot of the intensity versus wavelength was obtained for each FAR family for a total of 13 plots (from 3FAR to 15FAR). Then, all 13 plots were added up and normalized according to Table 1. For the case of the chromophores presented in Table 3, the 10 calculated spectra were added up and the resulting intensity was divided by 10, which is the number of compounds in the mixture; the final data were plotted. Toobtainthefluorescenceemissionpredictions,theHOMO-LUMO transitions of the 523 PAHs were tallied and normalized by the PAH distribution in Table 1. These transitions were weighted by the quantum yield as obtained from the familiar energy gap law (eq 1). This law has been shown to apply to all crude oils, including very heavy oils.47 We conclude that we can use the corresponding parameters for asphaltenes.47 Both the measured frequency factor (1012) and the energy term (3500 cm-1) are reasonable for organics.47 The plot of the quantum yield versus excitation photon energy (HOMO-LUMO gap) was obtained with eq 1

Φ)

Φ0 A ∆E 1 + exp kF0 R

(

)

(1)

where Φ0 ) 0.85 is the quantum yield in the absence of internal conversion, kF0 ) 109 is the radiative rate for crude oils, A ) 1012, and R ) 0.434 eV (3500 cm-1). The HOMO-LUMO gap (∆E) for each of the 523 systems was used to calculate the quantum yield; the corresponding plot in a quantum yield versus HOMO-LUMO gap diagram is shown in Figure 1. Figure 2 shows the crude oil data that is consistent with this constraint.47 Equation 1 was also used for the emission profile of the smaller PAHs shown in Table 3. To obtain the fluorescence spectra, the corresponding HOMOLUMO gap, for each compound, was multiplied for the corresponding quantum yield and the corresponding distribution factor shown in Table 1. The resulting fluorescence intensities for the 523 PAHs were added up and presented in a fluorescence intensity versus corresponding fluorescence emission wavelength (nm). A Gaussian smoothing was used with a width of 40 nm for the HOMO-LUMO transition to obtain the smooth emission curve. The same procedure was performed for the 10 compounds presented in Table 3, but for this case, no distribution factor is used.

Results and Discussion Figure 3 shows the plot of the resulting optical absorption profile for the 523 PAHs (and PACs) for ground- and excitedstate transitions. The distribution of the PAHs is given in Table 1. Also plotted in Figure 3 are the spectra of four petroleum asphaltenes. The simulted and measured spectra show many common features. There is substantial near-infrared (NIR) electronic absorption. Indeed, it is difficult to account for such low-energy absorption in PAHs and PACs without PAH distributions that we have used. Again, the lack of any nonlinear optical absorption in asphaltenes strongly indicates that chargetransfer complexes do not contribute any significant absorption.36 (47) Ralston, C. Y.; Wu, X.; Mullins, O. C. Appl. Spectrosc. 1996, 50, 1563.

The rather modest increase in absorption from the NIR-vis spectra to the UV spectrum is also another hallmark of the optical absorption spectra of asphaltenes, and this is reproduced rather well in the simulated spectrum. Another important similarity between the simulated and measured spectra is the lack of spectral structure. The simulated spectrum shows slightly more structure than the measured spectra. Asphaltenes have been shown to have tens of thousands of components; therefore, it is not surprising that increasing the number of compounds by 2 orders of magnitude beyond the 523 calculated PAH structures removes the last vestiges of structure observed in the simulated spectrum. There is no Soret band evident in the absorption data; therefore, metalloporphyrins do not contribute significantly to these asphaltenes. For metalloporphyrins, the Soret band is near 25 000 cm-1. The one distinct difference between the simulated and measured spectra is that calculated absorption at the longest wavelengths is still too small. In part, this is explained by our use of narrow spectral lines. In fact, spectral lines of PAHs in solution are broadened substantially by several effects. One effect is the Franck-Condon factor, which corresponds to projection of a single vibrational state of the initial state into several vibrational states of the final state. Figure 4 shows an example of this substantial line broadening for anthracene. The quantized nature of the transitions makes identification of vibronic bands easy in this case. This line-broadening mechanism in electronic transitions was not taken into account in the simulated PAH spectra. Note the mirror symmetry of the absorption and emission spectra in Figure 4 showing the near equivalence of the complementary Franck-Condon factors for excitation and emission of the HOMO-LUMO transitions. However, the bulk of this line broadening is to the highfrequency side for electronic absorption. It is possible that a greater fraction of large-ring systems is needed to increase this absorption.36 Alternatively, it might be that some heteroatomcontaining components are not being properly taken into account. The five-membered nitrogen rings are somewhat difficult to calculate and might contribute to the longest wavelength absorption. There might be multiple heteroatomic species that absorb light at the long wavelengths. These are known to be components in asphaltenes;16,17 the corresponding optical implications are under investigation. Figure 5 shows the fluorescence emission profile corresponding to the same PAH population distribution that gives rise to the absorption spectrum in Figure 3. The fluorescence emission spectra are dominated by the HOMO-LUMO gap,47 and this is mandated in the simulated emission spectrum. The simulated and measured fluorescence spectra are rather similar. The structure of the simulated spectrum is largely a function of the narrow transitions that we employed and is easily smoothed as shown to reflect broadening from various mechanisms as shown. The Franck-Condon broadening would give a red shift to the simulated emission spectrum, making the match even better (cf. Figure 4). The distribution in Table 1 was used to obtain an estimated molecular-weight distribution. For each class of PAH characterized by a particular number of fused aromatic rings, we obtained a small range of aromatic carbon numbers. We then used known parameters for petroleum asphaltenes to obtain the molecular weights. We assumed the aromatic carbon/saturate carbon ratio is 45-55%. We assumed the H/C ratio is 1:1.2, and we assumed half a sulfur atom (on average) per asphaltene molecule. This gives the molecular-weight distribution shown in Figure 6. This


simulated molecular-weight distribution is very close to that observed by a variety of techniques. The distribution of PAHs in asphaltenes given in Table 1 is seen to match rather closely the optical absorption spectrum and the fluorescence emission spectrum. In addition, when assuming one PAH per molecule, this distribution gives the known molecular-weight distribution of asphaltenes. This analysis strongly indicates that the asphaltene molecular architecture with one PAH per molecule dominates. We now consider a contrasting molecular architecture for asphaltenes. Recently, a published report proposed many asphaltene molecular structures distinctly different46 from that proposed herein in Tables 1 and 2. These structures have molecular weights of many thousand of Daltons and, thus, are inconsistent with the literature cited herein. Nevertheless, it is instructive to consider the predicted optical absorption and fluorescence emission spectra from the proposed structural molecules that are considered to be present in the asphaltene representation (see Table 3). In particular, these proposed PAH ring systems are quite small. With small-ring systems, there are very few choices for ring geometry and, essentially, those alternatives are included herein. Consequently, the following analysis is representative and general for asphaltene molecules consisting of small-ring systems. Figure 7 shows these simulated spectra, which are in stark contrast to observed asphaltene absorption spectra (Figure 3) and asphaltene fluorescence emission spectra (Figure 5). It is very important to remember that there is a fixed oscillator strength of unity for each π electron. If the π electron does not absorb NIR and visible light, it must absorb UV light. As a consequence, the colorless molecules proposed in ref 46 have


very large absorption in the UV spctrum, giving an enormous contrast between visible and UV absorption as shown in Figure 7. This is not observed for asphaltenes (cf. Figure 3). Likewise, the fluorescence from these proposed small-ring systems is in the UV spectrum, as shown in Figure 7. This is similar to where condensates fluoresce as shown in Figure 5 but not similar to asphaltene fluorescence. Conclusions There is a growing body of literature pointing out that most probable asphaltene molecular weights are 750 Da, with a distribution of roughly 500-1000 Da. The molecular architecture of asphaltenes is the subject of current debate. Here, we show that a centroid of 7 fused rings in the asphaltene PAH distribution not only matches known molecular-weight distributions of asphaltenes but also gives optical absorption and fluorescence emission spectra that are quite close to those of asphaltenes. Proposing molecules that reproduce the NIR, visible, and UV absorption profile of asphaltenes is very difficult. Indeed, we show herein that one popular archipelago molecular model for asphaltenes gives sharply different spectral properties than observed. The analysis herein encompassing 523 PAHs strongly supports the findings that asphaltenes are dominated by a single PAH per molecule and that a centroid of 7 fused rings is the most probable asphaltene PAH. Acknowledgment. Y.R.-M. acknowledges the support under project D.00406 of the Instituto Mexicano del Petro´leo. EF800663W

Measured and Simulated Electronic Absorption and Emission Spectra

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