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Energy & Fuels 2005, 19, 998-1002
Electronic Structure Properties of Dibenzofurane and Dibenzothiophene Derivatives: Implications on Asphaltene Formation I. Garcı´a-Cruz,*,† J. M. Martı´nez-Magada´n,† R. Salcedo,‡ and F. Illas†,§ Programa de Ingenierı´a Molecular, Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Colonia San Bartolo Atepehuaca´ n, Me´ xico D. F., 07730, Me´ xico, Instituto de Investigaciones en Materiales, Ciudad Universitaria, Universidad Nacional Auto´ noma de Me´ xico, Me´ xico, D. F., 04510, Me´ xico, and Departament de Quı´mica Fı´sica i Centre especial de Recerca en Quı´mica Teo` rica, Universitat de Barcelona i Parc Cientı´fic de Barcelona, Martı´ i Franque` s 1, 08028 Barcelona, Spain Received July 16, 2004. Revised Manuscript Received January 26, 2005
The electronic structure of a series of dibenzofurane and dibenzothiophene derivatives has been studied by means of hybrid density functional theory. The molecular structures of these compounds were obtained through full geometry optimization and then characterized as potential energy surface minima through the pertinent vibrational analysis. The analysis of the frontier orbitals together with the study of aromaticity allows us to make predictions about the reactivity of these molecules and to compare them to the recent findings on carbazole derivatives. Since dibenzofurane, dibenzothiophene, and carbazole are likely to be molecular moieties present in crude oil, their possible role in the formation of asphaltene-like molecules is discussed.
Introduction The asphaltene fraction of crude oil is the cause of many technological problems arising mainly from the deposition of solids in different parts of equipment during oil extraction, transportation, storage, and processing.1-3 The molecular structure of asphaltenes is largely unknown, but it has been proposed that it contains polyaromatic condensed rings with short aliphatic chains and polar heteroatoms such as nitrogen, oxygen, and sulfur.4 It has also been suggested that the stability of asphaltenes in crude oil is due to the presence of some neutral polar substances, resins among them, already present in the crude oil.5-9 Zajac et al.10 and Groenzin and Mullins11 have proposed molecular models for asphaltene units. Indeed, asphaltene molec* To whom correspondence should be addressed. E-mail: igarcia@ imp.mx. † Instituto Mexicano del Petro ´ leo. ‡ Ciudad Universitaria. § Universitat de Barcelona i Parc Cientı´fic de Barcelona. (1) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker Inc.: New York, 1998. (2) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237. (3) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (4) Nalwaya, V.; Tangtayakom, V.; Piumsomboon, P.; Fogler, H. S. Ind. Eng. Chem. Res. 1999, 38, 964. (5) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1749. (6) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1758. (7) Pfeiffer, J. P. H.; Saal, R. N. J. Phys. Chem. 1940, 44, 139. (8) Murgich, J.; Rogel, E.; Leon, O.; Isea, R. Petroleum Sci. Technol. 2001, 19, 437. (9) Scotti, R.; Montanari, L. In Asphaltenes, Fundamentals and Applications; Sheu, E. Y., Mullins, O., Eds.; Plenum Press: New York, 2001. (10) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. V. Scanning Microsc. 1994, 8, 463. (11) Groenzin, H.; Mullins, O. C. Petroleum Sci. Technol. 2001, 19, 219.
ular models usually contain a resin-like moiety into the polycyclic aromatic structure. In addition, it has been proposed that asphaltene species can be formed by an oligomerization-like process involving a crude oil fraction containing a particular family of resins,9,12 although the molecular mechanism is also unknown. However, it is worth pointing out that it has been also proposed that the asphaltene is a macromolecular solute that is found solvated and monodispersed; in this case, the presence of resins would be irrelevant.13 Still, Goual and Firoozabadi14 state that the asphaltene and resins are polar and may associate and form micelles. In fact, in a petroleum fluid, asphaltenes and resins coexist and may be found in the form of monomers and also as micelles. In a micelle, the micellar core is formed by the self-association of asphaltene molecules, and resins absorb onto the core surface to form a shell that also contains an oil fraction.15 The properties and the relative concentration of asphaltenes and resins govern the formation of micelles. When the resins are desorbed from the micellar core surface, they give rise to the formation of asphaltene phases. Hence, resins are the asphaltenes natural solvent. It has been proposed that resins are essential in the asphaltene aggregation because they attach to asphaltene micelles through their polar heads and hence stretch their aliphatic groups outward to form a steric-stabilization layer surrounding asphaltenes.5 The role of resins derived from compounds (12) Mujica, V.; Nieto, P.; Puerta, L.; Acevedo, S. Energy Fuels 2000, 14, 632. (13) Hirshberg, A.; de Jong, L. N. J.; Schipper, B. A.; Meijer, J. G. Soc. Pet. Eng. J. 1984, 24, 283. (14) Goual, L.; Firoozabadi, A. AIChE J. 2002, 48, 2646. (15) Pacheco-Sa´nchez, J. H.; A Ä lvarez-Ramı´rez, F.; Martı´nezMagada´n, J. M. Energy Fuels 2004, 18, 1676.
10.1021/ef049829f CCC: $30.25 © 2005 American Chemical Society Published on Web 03/03/2005
Dibenzofurane and Dibenzothiophene Derivatives
such as the carbazole, dibenzofurane (DBF), and dibenzothiophene (DBT) in asphaltene aggregations is unknown. The present work aims to extend previous work on carbazole derivatives16,17 to DBF and DBT, thus providing electronic structure information about these compounds that may be used to shed light on this complex asphaltene aggregation problem. The main idea here is to isolate the chemical effects derived from a given substitution in the DBF and DBT moieties. However, one has to be aware that the π system of this moiety is effectively involved in the conjugation with the rest of the aromatic part of the asphaltene molecule. This prevents using the DBF and DBT moieties to extract information concerning long-range interactions such as electronic excitations. Yet, valuable information about the possible asphaltene formation oligomerization mechanism described in refs 9 and 12 can be acquired by careful analysis of the molecular structure of DBF and DBT and its derivatives. The analysis of a series of local properties of substituted carbazole derivatives16 permitted us to make predictions about the reactivity of these compounds in electrophilic and nucleophilic reactions. It was found that among the family of compounds studied, the hydroxyl derivative was the most reactive for an electrophilic aromatic substitution, whereas the nitrile-substituted compound was the one predicted to be less active. Also, in the context of a nucleophilic reaction, the carbazole derivatives with deactivant substituents having accessible LUMOs were predicted to be the most reactive. Following these preliminary studies on carbazole derivatives,16,17 density functional theory calculations have been carried out for a series of DBF and DBT derivatives as representative of the resin moiety contained in the asphaltene molecular models previously mentioned. This study will permit us to check the influence of these heteroatoms on the reactivity of these compounds and their possible role in the formation of asphaltenes. The discussion involves reactivity indexes, which are essentially local in nature, and as pointed out previously, the main hypothesis is that electronic modifications induced by the presence of a given substituent in the DBF and DBT derivatives can be used to infer the reactivity of these resin moieties in asphaltene formation. It is important to point out that the main goals of these studies are to isolate the chemical effects derived from a given substitution in the resin moiety and consider this molecular fragment as embedded in the rest of the molecular framework in the same way as embedded cluster models are usually employed in surface science and heterogeneous catalysis models.18-22 This strategy permits one to concentrate (16) Garcia-Cruz, I.; Martı´nez-Magada´n, J. M.; Guadarrama, P.; Salcedo, R.; Illas, F. J. Phys. Chem. A 2003, 107, 1597. (17) Poater, J.; Garcı´a-Cruz, I.; Illas, F.; Sola`, M. Phys. Chem. Chem. Phys. 2004, 6, 314. (18) Bagus, P. S.; Illas, F. The Surface Chemical Bond in Encyclopedia of Computational Chemistry; Schleyer, P. V., Allinger, N. L., Clark, T., Gasteiger, J., Kollman, P. A., Schaefer, H. F., III, Schreiner, P. R., Eds.; John Wiley & Sons: Chichester, UK, 1998; Vol. 4, p 2870. (19) Illas, F.; Sousa, C.; Gomes, J. R. B.; Clotet, A.; Ricart, J. M. Theoretical Aspects of Heterogeneous Catalysis. In Progress in Theoretical Chemistry and Physics: Vol. 8; Chaer-Nascimento, M. A., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001; pp 149-181. (20) Sauer, J. Chem. Rev. 1989, 89, 1999. (21) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. Rev. 1994, 94, 2095.
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on the local properties of the resin fragment and thus extract important conclusions about the role of substituents. Of course, the relevance to asphaltene aggregation will depend on the validity of the hypothesis that asphaltene formation could arise by interaction of a resin moiety and a polycyclic aromatic fragment, probably with aliphatic chains. With the previous ideas in mind, the aim of this work is to study the aromaticity and inductive effect on several DBF- and DBT-type derivatives. It is important to clarify that this work is mainly devoted to the study of DBF and DBT derivatives with substituents in position 1. This choice follows from the resin models proposed by Murgich et al.23,24 However, since it is known that the stable derivatives for this type of molecules are those with substituents in position 4, a representative list of the corresponding derivatives has been included in the study even if their electronic and geometric properties could be different. On the other hand, the study of the reactivity of the organosulfured compounds derived from fossil fuels is of great interest as pointed out by Gates et al.25 and Kabe et al.26 Hence, the list of compounds studied in this work includes dibenzofurane (DBF) and dibenzothiophene (DBT) and their various derivatives such as methyl dibenzofurane (DBF-CH3), hydroxyl dibenzofurane (DBF-OH), bromide dibenzofurane (DBF-Br), nitrile dibenzofurane (DBFCN), acetyl dibenzofurane (DBF-COCH3), 4-methyldibenzofurane (DBF-4-methyl), 4,6-dimethyl-dibenzofurane (DBF-4,6-dimethyl), carboxyl dibenzofurane (DBFCOOH), methyl dibenzothiophene (DBT-CH3), hydroxyl dibenzothiophene (DBT-OH), bromide dibenzothiophene (DBT-Br), nitrile dibenzothiophene (DBT-CN), acetyl dibenzothiophene (DBT-COCH3), 4-methyl-dibenzothiophene (DBF-4-methyl), 4,6-dimethyl-dibenzothiophene (DBF-4,6-dimethyl), and carboxyl dibenzothiophene (DBF-COOH). In both types of heterocyclic compounds, we have considered one additional derivative having two substituents with different chemical character. This is the one containing acetyl and methyl (acetyl-DBF-CH3 and acetyl-DBT-CH3) groups both on the same ring and in positions Y (CH3) and R1 (COCH3) as shown in Figure 1. Computational Details An accurate structural study of a series of DBF and DBT derivatives described next has been carried out using density functional theory (DFT) quantum chemical techniques. In particular, we make use of the well-known hybrid B3LYP method27 and the standard 6-31++G** basis set. All calculations were carried out using the Gaussian98 package.28 The series of molecules studied in the present work is DBF and DBT derivatives where substitutents in these heterocycle rings are the same as were used in previous studies.16,17 These are CH3, OH, Br, CN, COOH, COCH3, 4-methyl, and 4,6dimethyl. A schematic representation of these compounds is (22) Martı´nez-Magada´n, J. M.; Cua´n, A.; Castro, M. Int. J. Quantum Chem. 2002, 88, 750. (23) Murgich, J.; Rodrı´guez, J.; Aray, Y. Energy Fuels 1996, 10, 68. (24) Murgich, J.; Abanero, J. A. Energy Fuels 1999, 13, 278. (25) Gates, B. C.; Topsoe, H. Polyhedron 1997, 16, 3213. (26) Kabe, T.; Ishihara, A.; Tajima, H. Ind. Eng. Chem. Res. 1992, 31, 1577. (27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (28) Frisch, M. J. et al. Gaussian 98 Revision A.7; Gaussian Inc.: Pittsburgh, PA, 1998.
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Garcı´a-Cruz et al. Table 1. HOMO and LUMO Energies and HOMO-LUMO Gap (∆) of Dibenzofurane (DBF) and Dibenzothiophene (DBT) Derivatives at B3LYP/6-31++G** Theory Level
Figure 1. Schematic representation of dibenzofurane (DBF) and dibenzothiophene (DBT) derivatives studied in the present work. given in Figure 1 where X stands for either O or S heteroatoms, Y represents H or any of the substituents stated previously, R represents H and CH3, and R1 represents H, CH3, and COCH3. Notice that CH3- and OH- are electron-releasing species and hence promoters of electrophilic aromatic substitution, whereas CN-, CH3CO-, and COOH- are present as electron-withdrawing species and consequently electrophilic aromatic substitution deactivants; Br is considered a special case with low deactivant power. All geometries have been fully optimized, and the resulting structures have always been characterized as true minima of the potential energy surface through the pertinent vibrational analysis. From the study of carbazole derivatives, it has been concluded that the main reactivity trends can be obtained by the analysis of frontier orbital energies and aromaticity measures,16 although the latter have to be handled with special care.17 Hence, the analysis of the reactivity of DBF and DBT derivatives is based in the HOMO and LUMO frontier orbital energies and on the nuclear independent chemical shift (NICS) defined by von Rague´ Scheleyer.29 The NICS has been computed for all the molecules studied in the present work in an attempt to quantify their aromatic character. The NICS has been calculated using the continuous set of a gauge transformation method30,31 under the same conditions of the method and basis used in the geometry optimization. Particularly, NICS has been computed at the center of each benzenoid ring of the DBF and DBT derivatives. To complete the picture of the electronic structure of these compounds, the natural bond order (NBO) analysis32-34 has also been carried out for all molecules studied in this work.
Results and Discussion For carbazole, the optimum geometry predicted at the present level of theory was in good agreement with available experimental data.16 This strongly suggests that the level of theory chosen is also adequate to carry out the analysis of the reactivity of the DBT and DBF derivatives. Three possibilities for the chemical pathway have been considered; those are a free radical reaction, (29) Schleyer, P. v. R.; Maerker, C.; Dransfield, A.; Jia, H.; EikemaHommes, N. J. R. v. J. Am. Chem. Soc. 1996, 118, 6317. (30) Keith, T. A.; Bader, R. F. W. Chem. Phys. Lett. 1993, 210, 223. (31) Keith, T. A.; Bader, R. F. W. Chem. Phys. Lett. 1992, 194, 1. (32) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (33) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (34) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1.
molecule
HOMO (eV)
LUMO (eV)
∆ (eV)
DBF DBF-CH3 DBF-OH DBF-Br DBF-CN DBF-COOH DBF-COCH3 DBF-4-methyl DBF-4,6-dimethyl acetyl-DBF-CH3 DBT DBT-CH3 DBT-OH DBT-Br DBT-CN DBT-COOH DBT-COCH3 DBF-4-methyl DBF-4,6-dimethyl acetyl-DBF-CH3
-6.31 -6.21 -6.11 -6.44 -6.76 -6.81 -6.58 -6.23 -6.15 -6.47 -6.07 -5.96 -5.96 -6.17 -6.45 -6.44 -6.26 -6.00 -5.94 -6.27
-1.34 -1.23 -1.31 -1.52 -2.17 -2.14 -2.02 -1.23 -1.14 -1.94 -1.34 -1.24 -1.35 -1.51 -2.15 -1.98 -1.82 -1.26 -1.19 -1.94
4.96 4.97 4.79 4.92 4.59 4.68 4.56 4.99 5.02 4.53 4.73 4.72 4.62 4.66 4.30 4.45 4.44 4.74 4.75 4.33
an ionic pathway, or finally, aromatic fusion. In the first two cases, it is very important to consider the inductive effect supplied by the substituents. Therefore, the inductive effect can be indirectly estimated by the corresponding measures of aromaticity. Molecular orbital energies and the HOMO-LUMO gap of DBF and DBT derivatives are summarized in Table 1. Rigorously speaking, the Kohn-Sham orbital energies have only a precise meaning in the case of the HOMO.35 Nevertheless, they are commonly used to extract qualitative information about electronic structure and in particular about changes induced by substituents. In fact, the frontier orbital energies already permit us to see that in the case of molecules with promoter substituents (i.e., CH3- and OH-), the HOMO is practically the same in all cases, especially for DBT derivatives, and is destabilized with respect to the molecule without substituents. This behavior is strongly marked in DBT derivatives, but the trend is also present in the case of DBF although it is less apparent. On the other hand, there is also a remarkable similarity in the corresponding frontier orbital energies of the molecules with deactivant substituents, but in this case, the HOMO is stabilized with respect to unsubstituted DBF or DBT. The LUMO values follow again a trend, but in this case of molecules with a promoter substituent, the LUMO energy is very similar to that of the unsubstituted compounds. The molecules with deactivant substituent exhibit similar LUMO energies, and those are lower than the correspondent to the parent compounds. In the case of molecules with promoter substituents, this behavior results in larger HOMO-LUMO energy gaps. The observed trends in the HOMO and LUMO energies could perhaps be anticipated from the electron donor or acceptor character of the substituent. Nevertheless, calculations permit a better classification of the different substituted heterocycle rings. Concerning orbital energies, the trends in DBF and DBT are the same, and those are indeed the same previously found for the carbazole derivatives. (35) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989.
Dibenzofurane and Dibenzothiophene Derivatives
Energy & Fuels, Vol. 19, No. 3, 2005 1001 Table 3. Natural Bond Orbital Derived Charges of Dibenzofurane and Dibenzotiophene NBO//B3LYP/6-31G**
Figure 2. Relative variation of NICS on the substituted, heterocycle, and unsubstituted ring of DBF and DBT derivatives. Table 2. Calculated Nuclear Independent Chemical Shifts (NICS) in ppm of DBF and DBT Derivatives as Obtained at the B3LYP/6-31++G** Theory Level molecule DBF DBF-CH3 DBF-OH DBF-Br DBF-CN DBF-COOH DBF-COCH3 DBF-4-methyl DBF-4,6-dimethyl acetyl-DBF-CH3 DBT DBT-CH3 DBT-OH DBT-Br DBT-CN DBT-COOH DBT-COCH3 DBF-4-methyl DBF-4,6-dimethyl acetyl-DBF-CH3
substituted ring -12.92 -14.20 -13.85 -13.33 -13.18 -13.03 -12.79 -12.73 -12.51 -12.27 -13.41 -13.14 -12.43 -12.31 -12.20 -11.98 -11.89 -11.74
heterocyclic ring
nonsubstituted ring
-8.53 -8.54 -9.10 -9.14 -8.95 -8.91 -8.65 -8.39 -8.21 -8.16 -9.76 -9.68 -10.07 -10.30 -10.18 -10.06 -9.74 -9.41 -9.19 -9.08
-13.09 -13.21 -13.46 -13.45 -13.12 -12.99 -13.00 -13.04 -12.73 -13.20 -12.15 -12.43 -12.60 -12.71 -12.35 -12.07 -12.16 -12.08 -11.89 -12.46
NICS values obtained for the DBF and DBT derivatives are reported in Table 2. To better analyze the effect of the substituents, we report in Figure 2 the variation of the NICS in the substituted, heterocycle, and unsubstituted ring relative to the DBF and DBT parent compounds. For the electron-withdrawing substituents, the aromaticity on the heterocyclic ring exhibits the smallest variation when it is compared with that of the unsubstituted molecule, whereas the contrary holds for the electron donor derivatives. Relatively larger changes are found in the lateral phenyl rings of the DBF and DBT derivatives. The OH and Br derivatives exhibit the largest variation on the substituted ring as expected from electron donor substituents. The increase in aromaticity is a consequence of the increase in electronic density arising from this donor groups, and hence, these compounds will be reactive toward electrophilic substitution, the OH-substituted compound being predicted to be the one with highest reactivity toward this kind of reaction. The effect of the methyl group is smaller than the one expected from chemical intuition. This behavior was also found in the corresponding carbazole derivative and attributed to hyperconjugation.16 Both DBF-OH and DBT-OH derivatives have the largest NICS value on the substituted ring; this is indeed the same trend already found for the carbazole derivatives
atom
DBF
DBT
C1 C2 C3 C4 C5 C6 C7 C8 O/S C10 C11 C12 C13 H14 H18
-0.28 -0.23 -0.25 -0.21 -0.21 -0.25 -0.23 -0.28 -0.47 0.33 -0.10 -0.10 0.33 0.25 0.24
-0.24 -0.23 -0.24 -0.21 -0.21 -0.24 -0.23 -0.24 0.41 -0.19 -0.07 -0.07 -0.19 0.25 0.24
studies earlier.16 The nonsubstituted ring presents smaller increases in aromaticity with a maximum in the case of the Br analogue and a minimum in the case of the carboxylic substituted molecule; both molecules with electronic promoter radicals have a discrete effect on the aromaticity of this ring. From the preceding analysis, one can conclude that DBF and DBT derivatives exhibit the same trends, although in the case of DBF, the NICS values are larger. In acetyl-DBF-CH3 and acetyl-DBT-CH3, the double substitution establishes, in practice, a competition between the two substituents so that the one with deactivator effect (i.e., the acetyl group) overcomes the methyl activator effect. The resulting values are -12.51 ppm for the substituted ring and -13.20 ppm for the free one for the DBF, while the DBT the values are -12.51 ppm for the substituted ring and -13.20 ppm for the free one. In both cases, the value for heterocyclic ring is -8.16 and -9.08, respectively; both values are smaller than the value for the furanic ring of dibenzofurane and the thiophenic ring of dibenzothiophen. Notice that this result is in agreement with the HOMO energy, which suggests that this is the compound with higher reactivity toward electrophilic aromatic substitution. To complete the analysis of the electronic structure of the DBF and DBT derivatives, we turn out attention to the NBO charges; these are reported in Table 3. For the DBF derivative, a large negative charge (-0.47e) predicted is found in the O atom. In the case of DFT, the charge in the heteroatom is positive (0.41e) as expected; here, the S atom on the tiophene ring appears to be the electrophilic center. The variation of the charge on the heteroatom with respect to the substituents (not shown) is consistent with both frontier orbital and aromaticity indexes. Conclusion Resin model compounds derived from dibenzothiophene and dibenzofurane could participate in condensation reactions with a polycyclic aromatic hydrocarbon thus leading to the asphaltene formation. This reaction is likely to occur spontaneously, especially considering the thermodynamic conditions of the oil well. The analysis of the reactivity of some different model compounds by means of theoretical calculations shows that the most available molecular orbitals to carry out this kind of
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reaction are the HOMOs of molecules with electronreleasing groups, particularly the cases of OH-, Br-, and, to a lesser extent, CH3- substitutions. The aromaticity analysis via NICS confirms this assumption, and furthermore, it is possible to establish that the hydroxylsubstituted molecule is the best target in dibenzothiophene as well as dibenzofurane in some cases. Finally, it is important to point out that the previous analysis is fully consistent with that previously reported for carbazole derivatives.16 Hence, it can be concluded that, in the case that asphaltene formation could proceed through a reaction of resins with polycyclic
Garcı´a-Cruz et al.
aromatic hydrocarbons, the chemical behavior of resins containing O-, S-, and N- will be similar. Acknowledgment. F.I. is grateful to the DURSI of the Generalitat de Catalunya and to the Spanish Ministerio de Ciencia y Tecnologı´a for financial support through projects Distincio´ de la Generalitat per a la Promocio´ de la Recerca Universita`ria and CICyT PB981216-CO2-01, respectively. EF049829F
