Energy Fuels 2010, 24, 3990–3997 Published on Web 06/08/2010
: DOI:10.1021/ef1003057
Modeling Free-Radical Reactions, Produced by Hydrocarbon Cracking, with Asphaltenes Alexander Peraza, Morella Sanchez, and Fernando Ruette* Centro de Quı´mica, Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Apartado 21827, Caracas, Venezuela Received March 23, 2010. Revised Manuscript Received May 25, 2010
Radical adsorptions and reactions produced in oil hydrocarbon cracking with an asphaltene model molecule were carried out with CATIVIC and MOPAC programs. Density functional theory (DFT) calculations were also performed for comparison. Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) reaction mechanisms, of H•, CH3•, and CH3CH2• radicals with adsorbed H•, were studied on asphaltene unsaturated fragments. Qualitative results indicated that it is possible that radical adsorptions on the asphaltene surface lead to bond activations (surface deformation). The following adsorption energy trend was found for the three different programs: H• > CH3• > CH3CH2•. Approximate reaction energy barriers were evaluated. The ER mechanism has smaller reaction barriers and higher exothermic reactions than the LH mechanism, which means that ER is energetically more feasible than LH. These results indicate that, at low temperatures and high pressure, the asphaltene surface may play an important role in the recombination of free radicals.
alkenes and a H• free radical
1. Introduction
CH3 CH2 • f CH2 ¼ CH2 þ H•
Asphaltene molecules are polyaromatic compounds with side-chain groups of heterocyclic rings composed of carbon, hydrogen, nitrogen, sulfur, and oxygen atoms. These compounds exist in crude oils as monomers and micelles, whose sizes fall in colloidal ranges.1 Flocculation of these species may occur when oil solubility is reduced, having a negative effect on oil production, transportation, and refinery processes of the petroleum industry.2 Understanding asphaltene interaction with other oil components is of great importance, because it may permit the development of some technological alternatives that could help unravel the above-mentioned troubles. One particular case in refinery processes may occur when formed radicals by thermal cracking interact with asphaltenes. In refinery processes, heavy hydrocarbons are broken down into simpler molecules (e.g., light hydrocarbons) by bond scission in the precursors. The cracking rate and the end products depend upon the temperature and catalyst presence. Initiation reactions produce free radicals that normally involve breaking carbon-carbon bonds rather than carbonhydrogen bonds. Usually, the thermal cracking process follows a homolytic mechanism; i.e., bonds break symmetrically. Thus, pairs of free radicals are formed; for example CH3 CH3 f 2CH3 •
Other radical reactions, such as addition, recombination, and disproportionation may also occur. The radical-asphaltene interaction, as far as we know, has not been studied yet, from the theoretical and experimental points of view. However, some experimental evidence given by Marquaire et al.3 show the inhibition of hexadecane pyrolytic decomposition by the presence of tetraline and that hydronaphthalenic molecules affect the thermal stability of hydrocarbons, such as n-alkanes.4 Thus, they conclude that the stability of petroleum reservoirs is enhanced by hydronaphthalenic supramolecules, such as, asphaltenes and resins. On the other hand, Fukuyama and Terai5 proposed that mesoporous activated carbons, rich in polyaromatic hydrocarbons (PAHs), provide adsorption sites for free radicals generated during thermal cracking, preventing in this way coke formation from asphaltene. Theoretical results by Mujica et al.6 show that open-shell van der Waals molecules are more stable than the corresponding closed-shell molecules, and they suggested that reactive free radicals should be encapsulated by asphaltene molecules. Recent work7 with CATIVIC code8 on asphaltene fragments of Venezuelan crude showed that radical adsorption may occur on them. In addition, adsorptions lead to surface
ð1Þ
Then, hydrogen abstractions may take place, such as CH3 • þ CH3 CH3 f CH4 þ CH3 CH2 •
(3) Bounaceur, R.; Scacchi, G.; Marquaire, P.-M.; Domine, F.; Brevart, O.; Dessort, D.; Praider, B. Ind. Eng. Chem. Res. 2002, 41, 4689–4701. (4) Burkle-Vitzthum, V.; Michels, R.; Bounaceur, R.; Marquaire, P.-M.; Scacchi, G. Ind. Eng. Chem. Res. 2005, 44, 8972–8987. (5) Fukuyama, H.; Terai, S. A. Pet. Sci. Technol. 2007, 25, 231–240. (6) Mujica, V.; Nieto, P.; Puerta, L.; Acevedo, S. Energy Fuels 2000, 14, 632–639. (7) Peraza, A.; Sanchez, M.; Ruette, F. J. Comput. Methods Sci. Eng. 2010, in press. (8) Ruette, F.; Sanchez, M.; Martorell, G.; Gonzalez, C.; A~ nez, R.; Sierraalta, A.; Rinc on, L.; Mendoza, C. Int. J. Quantum Chem. 2004, 96, 321–332.
ð2Þ
with the formation of other radicals. It could be followed by radical decomposition, where free radicals break apart into *To whom correspondence should be addressed. E-mail: fruette@ ivic.gob.ve. (1) Priyanto, S.; Mansoori, G. A.; Suwono, A. Chem. Eng. Sci. 2001, 56, 6933–6939. (2) Mansoori, G. A. J. Petrol. Sci. Eng. 1997, 17, 101–111. r 2010 American Chemical Society
ð3Þ
3990
pubs.acs.org/EF
Energy Fuels 2010, 24, 3990–3997
: DOI:10.1021/ef1003057
Peraza et al.
using bond strength and distance changes with CATIVIC8 and MOPAC33 programs in the first part of section 3. In the second part, comparisons to density functional theory (DFT) calculations by evaluation of adsorption energies and bond distances were further analyzed. To finish the section, Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) reaction paths were also studied for H• interactions with the above-mentioned radicals. As a final point, conclusions and comments were enumerated in section 4.
distortions and electronic charge transfers from adsorbates to asphaltene surfaces. There are many evidence of radical interaction with aromatic compounds. Theoretical studies of H• radical adsorption on PAHs and graphite9-15 have been broadly studied besides molecular hydrogen formation.16-23 These processes have been experimentally corroborated.24,25 In addition, formations of small molecules on PAHs have been calculated recently.26,27 The free-radical interaction with carbon nanotubes by Galano28 show that these carbon compounds operate as free-radical scavengers and free-radical sponges. Leszczynski et al.29 found H• strong chemisorption on sidewall single-walled carbon nanotubes. Also, CH3• adsorbed on PAHs,30 O atoms on graphite,31 and the formation of CO2 on aromatic carbon-based materials32 have been reported. In this work, the interaction and reactions of free radicals with hydrogenated asphaltene fragments is performed by parametric quantum method (PQM) codes adapted for surface reaction processes. A model asphaltene molecule was employed, and its fragments were selected for adsorption of H•, CH3•, and CH3CH2• radicals (see reactions 1-3 shown above). This work is organized as follows: A brief description of the programs, model, and a technique for the calculation of approximated reaction paths and barriers are presented in section 2. Interactions of these radicals with model asphaltene fragments were considered by analyzing surface distortion
2. Theoretical Tools and Molecular Models PQMs were used here because they are considerably faster than DFT calculations; however, some DFT calculations34 were performed for comparison. CATIVIC8 and MOPAC33 programs were adapted to study surface-adsorbate interactions using a theoretical tool for evaluating bond strengths or diatomic bond energies (DBEs),35 coming from a natural total energy partition of PQMs in mono- and diatomic terms. A model molecule taken as an average asphaltene present in Tia Juana Venezuelan heavy crude oil was selected, as in previous works.7,36,37 According to McCaffrey et al.,38 this model molecule corresponds to an archipelago type that consists of smaller aromatic groups connected by aliphatic bridges. This model was divided into several fragments, as expected to happen after a mild pyrolysis.39 The fragments F1, F2, F3, and F4 of this model are displayed in Figure 1. Note that they contain heteroatoms such as N, O, and S, and the alkyl chains were replaced by H atoms, to saturate the corresponding carbon atoms. Calculations of approximated reaction barriers for the LH mechanism were carried out scanning the distance between two separated A and B adsorbed species on adjacent surface sites located on the x axis. A schematic picture of this procedure is presented in Figure 2. Each potential energy curve was obtained by performing calculations point by point of A adsorbate along perpendicular lines (see parallel lines to the y axis in Figure 2) to the line that joins both adsorbed species. The z coordinate of A, all coordinates of B, and surface were optimized, except for 6 degrees of freedom far from the reaction site, to avoid surface movement. In each case, an energy minimum is found and the pathway for the association reaction and an approximated transition-state barrier are obtained. For the ER mechanism, the A• radical is approached perpendicularly from the gas phase to a site on the surface where B is adsorbed. At each A-B distance, full optimization is performed,
(9) Jeloaica, L.; Sidis, V. Chem. Phys. Lett. 1999, 300, 157–162. (10) Zecho, T.; G€ uttler, A.; Sha, X.; Jackson, B.; K€ uppers, J. J. Chem. Phys. 2002, 117, 8486–8492. (11) Sha, X.; Jackson, B. Surf. Sci. 2002, 496, 318–330. (12) Volpe, M.; Cleri, F. Surf. Sci. 2003, 544, 24–34. (13) Ferro, Y.; Teillet-Billy, D.; Rougeau, N.; Sidis, V.; Morisset, S.; Allouche, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, No. 085417-1-8. (14) Boukhvalov, D. W.; Katsnelson, M. I.; Lichtenstein, A. I. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, No. 035427-1-7. (15) Casolo, S.; Løvvik, O. M.; Martinazzo, R.; Tantardini, J. F. J. Chem. Phys. 2009, 130, No. 0504704. (16) Ferro, Y.; Marinelli, F.; Allouche, A. J. Chem. Phys. 2002, 116, 8124–8131. (17) Ferro, Y.; Marinelli, F.; Allouche, A. Chem. Phys. Lett. 2003, 368, 609–615. (18) Morisset, S.; Aguillon, F.; Sizun, M.; Sidis, V. Chem. Phys. Lett. 2003, 378, 615–621. (19) Bachellerie, D.; Sizun, M.; Teillet-Billy, D.; Rougeau, N.; Sidis, V. Chem. Phys. Lett. 2007, 448, 223–227. (20) Rauls, E.; Hornekær, L. Astrophys. J. 2008, 679, 531–536. (21) Dumont, F.; Picaud, F.; Ramseyer, C.; Girardet, C.; Ferro, Y.; Allouche, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, No. 233401-1-4. (22) Hirama, M.; Ishida, T. A.; Aihara, J.-I. J. Comput. Chem. 2003, 24, 1378–1382. (23) Hirama, M.; Tokosumi, T.; Ishida, T.; Aihara, J.-I. Chem. Phys. 2004, 305, 307–316. (24) Roman., T.; Di~ no, W. A.; Nakanishi, H.; Kasai, H.; Nobuhara, K.; Sugimoto, T.; Tange, K. J. Phys. Soc. Jpn. 2007, 76, No. 114703-1-4. Otero, R.; (25) Hornekær, L.; Rauls, E.; Xu, W.; Sljivan canin, Z.; Stensgaard, I.; Lægsgard, E.; Hammer, B.; Besenbacher, F. Phys. Rev. Lett. 2006, 97, No. 186102-1-4. (26) Allouche, A.; Jelea, A.; Marinelli, F.; Ferro, Y. Phys. Scr. 2006, T124, 91–95. (27) Rodrı´ guez, L. S.; Ruette, F.; Sanchez, M.; Mendoza, C. J. Mol. Catal. A: Chem. 2010, 316, 16–22. (28) Galano, A. J. Phys. Chem. C 2008, 112, 8922–8927. (29) Kaczmarek, A.; Dinadayalane, T. C.; Łukaszewicz, J.; Leszczynski, J. Int. J. Quantum Chem. 2007, 107, 2211–2219. (30) Unterreiner, B. V.; Carissan, Y.; Klopper, W. Chem. Phys. Chem. 2006, 7, 1311–1321. (31) Incze, A.; Pasturel, A.; Chatillon, C. Surf. Sci. 2003, 537, 55–63. (32) Orrego, J. F.; Zapata, F.; Truong, T. N.; Mondrag on, F. J. Phys. Chem. A 2009, 113, 8415–8420. (33) (a) Stewart, J. J. P. MOPAC2009; Stewart Computational Chemistry: Colorado Springs, CO, 2008; http://openmopac.net/. (b) Stewart, J. J. P. J. Mol. Model. 2007, 13, 1173–1213.
(34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian, Inc., Wallingford, CT, 2004. (35) Ruette, F.; Poveda, F. M.; Sierraalta, A.; Rivero, J. Surf. Sci. 1996, 349, 241–247. (36) Machı´ n, I.; Ruette, F.; Sierraalta, A.; de Jes us, J. C.; Rivas, G.; Higuerey, I.; C ordova, J.; Pereira, P. J. Mol. Catal. A: Chem. 2005, 227, 223–229. (37) Rosales, S.; Machı´ n, I.; Sanchez, M.; Rivas, G.; Ruette, F. J. Mol. Catal. A: Chem. 2006, 246, 146–153. (38) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Energy Fuels 2004, 18, 1377–1384. (39) Gray, M. R. Energy Fuels 2003, 17, 1566–1569.
3991
Energy Fuels 2010, 24, 3990–3997
: DOI:10.1021/ef1003057
Peraza et al.
adsorption sites on the surface, screenings of X• adsorption located initially at about 1.5 A˚ from the surface were carried out. Calculations of about 250 points were performed on the aromatic region in the x-y plane for each fragment. Optimization calculations in each point were restricted to avoid translation and rotation of the surface fragment. Fixed coordinates (6 degrees of freedom) correspond to atoms far from the adsorption site; however, the rest of the surface atoms and X• were fully optimized. The most stable adsorption sites are presented in Figure 1 surrounded by circles: H•, as Hads; and CH3• and CH3CH2•, as Rads. 3.1.1. Adsorption of H•. Results of DBEs for C1-Hads and N-Hads using CATIVIC (from -80 to -94 kcal/mol) and MOPAC (from -70 to -86 kcal/mol) are presented in Table 1 and indicate that Hads is strongly bonded to asphaltene fragment surfaces. The most stable adsorptions occur at H-edge sites (those that contain a C-H bond), except in fragment F1, where adsorption is on the N atom. Adsorptions produce carbon rehybridization from a sp2 character to a sp3 character. This creates a surface tension that is lower in H-edge sites than in the rest of PAH sites, because the H atom may easily leave the surface plane. These results suggest that H• leads to hydroaromatic formation (hydrogenated PAHs) that may act as intermediates for hydrogen transfer to free radicals and may help avoid polymerization, as proposed by Uglev and Kam’yanov.40a The hydrogen-donation process also occurs in carbon liquefaction40b and depends upon the lability of the hydroaromatic H-C bond. Our results may explain the findings by Marquaire et al.4 that suggest that the thermal stability of petroleum will significantly increase with the amount of asphaltenes, because these compounds are precursors of hydronaphthalenic species. The deformation of surface fragments may be a measure of adsorbate-surface interactions. In this sense, changes in DBEs (ΔDBE) and BDs (ΔBD) for the asphaltene surface because of adsorption are also reported in Table 1. The selected bonds (C1-C2, C1-C3, C1-H1, N-C1, and N-C2 are those adjacent to the most stable adsorption sites (see atom labels in Figure 1). It is observed that Hads-C1 and Hads-N bond formations lead to a DBE weakening and a BD elongation of the selected bonds. It means that these bonds are activated. The planar structure is significantly modified, except for adsorption on F1, as indicated by Δz (variation of surface planarity) calculated values by CATIVIC. In the case of F1, H• is adsorbed parallel to the asphaltene plane on the N atom. The average value of 0.32 A˚ indicates a trend of C1 carbons to be tetrahedral. This value compares well to other theoretical calculations for the graphite (0001) surface11 (0.36 A˚) and coronene24 (0.35 A˚) using DFT programs. The total ΔDBE for N-Cn (n = 1 or 2) and C1-Cn (n = 2 or 3) of F1-F4 fragments show bond decreases of about 46, 49, 49, and 48 kcal/mol for CATIVIC and 40, 52, 50, and 50 kcal/mol for MOPAC, respectively. Note that calculated ΔDBEs in both programs are similar and reflected on the same ΔBD average value that corresponds to an enlargement of about 0.08 A˚. The C-C bond elongations are due to a transformation from double- to single-bond character. Although the C1-H1 bond changes follow the same trends (i.e., bond strength decrease and bond distance increase), their values are much smaller than in C-C cases. 3.1.2. Adsorption of CH3•. Results of bond strength and distance changes are shown in Table 2 for CH3• adsorption on the most stable sites of each fragment. Those are indicated
Figure 1. Structure of model asphaltene fragments (F1-F4). Adsorption sites are indicated by circles: Hads for H• radicals and Rads for CH3• and CH3CH2• radicals. Adsorption site atoms of Hads and their nearest neighbors are shown with the corresponding atomic symbols and numbers.
Figure 2. Schematic representation of the potential energy surface for the LH mechanism. The A þ B symbol corresponds to separated adsorbates that react on the asphaltene surface to give the A-B compound. The dark line that crosses the bottom of each potential energy curve matches the reaction path.
except for atoms of A and B that directly interact (atoms that form a surface-adsorbate bond) and for six atomic coordinates of the asphaltene fragment far from the reaction site, to elude a surface displacement.
3. Results and Discussion 3.1. Surface Deformation Because of Radical Adsorption. As mentioned above, the adsorption of X• radicals (X = H•, CH3•, and CH3CH2•) is analyzed in terms of the adsorption energy (Eads), bond strengths (DBEs), and adsorbatesurface bond distance (BD). To obtain the most stable 3992
Energy Fuels 2010, 24, 3990–3997
: DOI:10.1021/ef1003057
Peraza et al.
Table 1. Diatomic Bond Energy Changes (ΔDBE) for H• Adsorption on the Most Stable Adsorption Sites (kcal/mol)a ΔDBE (kcal/mol) and ΔBD (A˚)
ΔDBE (kcal/mol) and ΔBD (A˚)
bond
CATIVIC
MOPAC
bond
N-Hads N-C1 N-C2
F1 -80.3 [1.02] 30.6 (0.06) 15.6 (0.02)
-70.0 [1.01] 29.5 (0.07) 10.08 (0.02)
C1-Hads C1-C2 C1-C3 C1-H1 Δz
Δz
(0.01)
C1-Hads C1-C2 C1-C3 C1-H1 Δz
F3 -94.2 [1.13] 30.0 (0.10) 19.2 (0.07) 0.4 (0.01)
-83.1 [1.11] 33.3 (0.11) 16.6 (0.08) 0.7 (0.03)
C1-Hads C1-C2 C1-C3 C1-H1 Δz
(0.30)
CATIVIC
MOPAC
F2 -92.9 [1.13] 21.6 (0.09) 27.1 (0.09) 0.3 (0.01)
-85.8 [1.11] 22.9 (0.09) 29.3 (0.11) 4.4 (0.03) (0.20)
F4 -94.1 [1.13] 9.4 (0.05) 38.7 (0.12) 0.4 (0.01)
-84.3 [1.12] 7.24 (0.05) 43.2 (0.13) 1.18 (0.03) (0.45)
Bond distance changes (ΔBD) and surface planarity variation (Δz), in the aromatic region, are shown in parentheses (A˚). Values in brackets correspond to Hads-asphaltene BDs. Δz values were calculated with CATIVIC. a
Table 2. ΔDBE Values (kcal/mol) for CH3• and Asphaltene Fragments Because of Adsorptiona ΔDBE (kcal/mol) and ΔBD (A˚)
ΔDBE (kcal/mol) and ΔBD (A˚)
bond
CATIVIC
MOPAC
bond
CATIVIC
MOPAC
C10 -C4 C10 -C20 C10 -C30 C10 -H10 C4-H2 C4-H3 C4-H4
F1 -91.1 [1.55] 30.2 (0.11) 26.3 (0.09) 1.3 (0.01) 0.70 (0.02) 4.4 (0.02) 1.8 (0.02)
-105.1 [1.53] 36.1 (0.11) 25.0 (0.09) 8.4 (0.04) 6.9 (0.03) 8.0 (0.04) 7.3 (0.04)
C10 -C4 C10 -C20 C10 -C30 C10 -H10 C4-H2 C4-H3 C4-H4
F2 -97.7 [1.52] 22.6 (0.10) 31.8 (0.11) 2.4 (0.03) 0.50 (0.02) 1.0 (0.02) 0.9 (0.02)
-106.0 [1.54] 27.9 (0.09) 38.1 (0.12) 13.4 (0.04) 6.2 (0.03) 8.3 (0.04) 7.1 (0.04)
C1-C4 C1-C2 C1-C3 C1-H1 C4-H2 C4-H3 C4-H4
F3 -97.2 [1.52] 21.5 (0.10) 32.0 (0.12) 1.3 (0.02) 0.70 (0.02) 1.2 (0.02) 1.2 (0.02)
-104.0 [1.54] 23.8 (0.09) 38.8 (0.12) 5.3 (0.03) 6.9 (0.03) 8.5 (0.04) 8.0 (0.04)
C10 -C4 C10 -C20 C10 -C30 C10 -H10 C4-H2 C4-H3 C4-H4
F4 -98.7 [1.52] 20.9 (0.09) 31.6 (0.12) 4.5 (0.03) 1.2 (0.02) 0.8 (0.02) 1.0 (0.02)
-105.3 [1.54] 24.8 (0.09) 37.7 (0.12) 8.5 (0.04) 7.8 (0.04) 6.7 (0.03) 6.6 (0.03)
a
Values in parentheses correspond to ΔBD (A˚). Values in brackets correspond to adsorbate-surface BDs (A˚).
in Figure 1 by a circle and the symbol Rads and are specified in Table 2 as C10 for F1, F2, and F4 fragments and as C1 for the F3 fragment. The nearest neighbors to C10 and C1 are displayed as C20 , C30 , and H10 and C2, C3, and H1, respectively. Atoms labels as C4, H2, H3, and H4 correspond to the CH3• radical (see the CH3 picture in Figure 1). Values in Table 2 suggest stronger bond strength adsorption in CH3• (average values of -96 and -105 kcal/mol) than in H• (average values of -90 and -71 kcal/mol) for CATIVIC and MOPAC, respectively. CH3-surface bond lengths in both methods are similar, in the range of 1.52-1.55 A˚. Note that the C10 -C4 and C1-C4 bond distances correspond to a standard single C-C bond length (1.54 A˚) in hydrocarbons. Total changes on nearest-neighbor bond strengths to the adsorption site (C10 -C20 þ C10 -C30 and C1-C2 þ C1-C3) show that these bonds are weakened by average values of around 54 and 63 kcal/mol by CATIVIC and MOPAC, respectively. It means that a greater surface distortion than in the case of H• (average value of 48 kcal/mol) occurs. In fact, the average bond distance elongation of nearest neighbors to C10 and C1 is 0.11 A˚, which is longer than in H• adsorption (0.09 A˚). Small adsorbate bond activations are observed. For CH3, the C4-Hn (n = 2, 3, or 4) bond strengths decrease by average values of about 1 and
7 kcal/mol and bond lengths enlarge by about 0.02 and 0.04 A˚ for CATIVIC and MOPAC, respectively. 3.1.3. Adsorption of CH3CH2•. The most stable adsorption sites of CH3CH2• are the same as those of CH3• (Rads). Asphaltene atom labels are shown in Figure 1 together with those of CH3CH2•. Similar results to CH3• adsorption are found (see Table 3). The C1-C4 and C10 -C4 bond strength and bond length are slightly smaller and longer than the CH3• bond strength and bond length, respectively; for example, MOPAC gives -100 and -105 kcal/mol and 1.54 and 1.55 A˚ for CH3• and CH3CH2•, respectively. In addition, similar CH3CH2• distortions are observed in CATIVIC and MOPAC; i.e., the C4-C5 bonds are elongated by 0.06 A˚, and their bond strengths decrease by about 10 kcal/mol. In the case of C-H bonds (C4-H2 and C4-H3), ΔDBE and ΔBD are small but different for CATIVIC and MOPAC: 8 and 2 kcal/mol and 0.04 and 0.02 A˚, respectively. 3.2. Comparison for Adsorption Energy. Adsorption energy values (Eads) for these radicals on F1-F4 fragments were calculated by CATIVIC, MOPAC, and DFT. A comparison of Eads, given in Table 4, with bond strengths (DBEs) in Tables 1-3 calculated by CATIVIC and MOPAC indicates a large difference between these properties. This is due to surface and adsorbate distortions. It means that adsorption energy is not only a measurement of the adsorbate-surface 3993
Energy Fuels 2010, 24, 3990–3997
: DOI:10.1021/ef1003057
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Table 3. ΔDBE Values (kcal/mol) for CH3CH2• and Asphaltene Because of Adsorptiona ΔDBE (kcal/mol) and ΔBD (A˚)
ΔDBE (kcal/mol) and ΔBD (A˚)
bond
CATIVIC
MOPAC
bond
CATIVIC
MOPAC
C10 -C4 C10 -C20 C10 -C30 C10 -H10 C4-C5 C4-H2 C4-H3
F1 -95.3 [1.55] 27.2 (0.11) 23.6 (0.10) 1.3 (0.02) 11.1 (0.06) 2.0 (0.02) 1.5 (0.02)
-99.5 [1.55] 35.6 (0.11) 24.3 (0.09) 7.51 (0.04) 10.38 (0.06) 9.35 (0.04) 8.45 (0.04)
C10 -C4 C10 -C20 C10 -C30 C10 -H10 C4-C5 C4-H2 C4-H3
F2 -97.0 [1.54] 22.6 (0.10) 31.8 (0.11) 3.9 (0.04) 10.5 (0.06) 0.7 (0.02) 1.6 (0.02)
-100.8 [1.55] 27.0 (0.09) 37.6 (0.12) 12.8 (0.04) 10.4 (0.06) 7.4 (0.04) 8.4 (0.04)
C1-C4 C1-C2 C1-C3 C1-H1 C4-C5 C4-H2 C4-H3
F3 -96.8 [1.54] 21.8 (0.10) 32.1 (0.12) 3.2 (0.02) 10.4 (0.06) 2.8 (0.02) 1.7 (0.02)
-98.8 [1.55] 22.7 (0.09) 38.8 (0.12) 4.6 (0.04) 10.7 (0.06) 9.9 (0.04) 8.4 (0.04)
C10 -C4 C10 -C20 C10 -C30 C10 -H10 C4-C5 C4-H2 C4-H3
F4 -97.4 [1.54] 21.4 (0.09) 32.0 (0.12) 4.8 (0.03) 10.8 (0.06) 1.1 (0.02) 1.1 (0.02)
-100.3 [1.55] 24.3 (0.09) 37.3 (0.12) 8.0 (0.04) 10.7 (0.06) 8.0 (0.04) 8.0 (0.04)
a
Values in parentheses correspond to ΔBDs (A˚). Brackets correspond to adsorbate-asphaltene BDs (A˚).
Table 4. Eads Values (kcal/mol) for Adsorption of H•, CH3•, and CH3CH2• on F1-F4 Fragments Calculated by CATIVIC, MOPAC, and DFTa adsorbate
CATIVIC
MOPAC
DFT
CATIVIC
MOPAC
DFT
H• CH3• CH3CH2•
-64.6 [1.02] -41.9 [1.52] -23.8 [1.54]
F1 -60.4 [1.01] -28.1 [1.53] -20.6 [1.55]
-45.0 [1.01] -12.4 [1.56] -8.4 [1.57]
-49.6 [1.13] -34.7 [1.52] -16.9 [1.54]
F2 -47.0 [1.11] -27.4 [1.54] -19.4 [1.55]
-34.4 [1.11] -12.6 [1.55] -8.7 [1.56]
H• CH3• CH3CH2•
-56.0 [1.13] -41.5 [1.52] -23.7 [1.54]
F3 -52.2 [1.11] -31.8 [1.54] -23.7 [1.55]
-35.4 [1.10] -16.4 [1.56] -12.1 [1.57]
-55.1 [1.13] -40.4 [1.52] -21.9 [1.54]
F4 -54.9 [1.12] -37.6 [1.54] -29.6 [1.55]
-35.7 [1.10] -13.4 [1.55] -9.4 [1.56]
a
Brackets correspond to BDs (A˚).
functional45 gave adsorption energy values very similar to this work (≈ -60 kcal/mol). Similar results were derived by Roman et al.46 with H• adsorption energy (about -55 kcal/ mol). Results of average adsorbate-substrate interatomic distances indicate that CATIVIC and MOPAC gives values of 0.03 and 0.02 A˚, respectively, shorter than DFT. Qualitative results from both CATIVIC and MOPAC indicate that Eads absolute values for these radicals follow the same trend: H• > CH3 • > CH3CH 2•, as reported previously.7 The same tendency is followed by DFT results. 3.3. Surface Reactions between Adsorbed H• and Radical Fragments. Reactions between X• radicals (X• = H•, CH3•, and CH3CH2•) in the gas phase and adsorbed species (Hads) on a selected fragment (F4) were performed considering different mechanisms (ER and LH). To study these reactions, the fragment F4 was selected because adsorption energy differences between fragments are, in general, small (see Table 4). 3.3.1. ER Mechanism. A screening was performed starting at about 5 A˚ above adsorbed Hads with steps of 0.06 A˚ for the ER mechanism. The fragment is in the xy plane, and the radical X• is coming in the z axis. All of the atoms of the radical X• are allowed to optimize, except the z coordinate of its interacting atom (H and C4 in CH3• and CH3CH2•). The fragment surface is completely optimized, except two atoms far from the adsorption site, to avoid rotation and the approaching of the fragment to the X• radical. Potential
bond but also effects produced by adsorption on the surface and in the adsorbate. Previous work41 of radical adsorptions on the coronene cation calculated with DFT and CATIVIC indicated that Eads absolute values are rather smaller than bond adsorption energy (BAE). It was found that a comparison of BAE between both methods was quite reasonable. CATIVIC and MOPAC programs overestimate adsorption energies with respect to DFT. Two possible causes of this difference may be explained as follows: (a) PQMs for C-H and C-C bonds have to be parametrized for these types of adsorbate-substrate compounds. (b) DFT fails in the description of radical states, as proposed in several works.42-44 In a previous work,41 comparisons of CATIVIC and DFT programs for adsorption of H•, CH2•, and CH3• on positively charged coronene showed very small differences in adsorption energy values. These may be explained because the adsorption product (adsorbate plus substrate) is not a radical. On the other hand, results by Allouche et al.26 for H• on the (0001) basal plane using periodic DFT with pseudopotential and the Perdew-Burke-Ernzerhof correlation (40) (a) Uglev, V. V.; Kam’yanov, V. F. Petrol. Chem. 2000, 40, 121– 125. (b) Kayima, Y.; Futamura, S.; Mizuki, T.; Kajioka, M.; Koshi, K. Fuel Process. Technol. 1986, 14, 79–90. (41) Ruette, F.; S anchez, M.; Castellanos, O.; Sosc un, H. Int. J. Quantum Chem. 2010, 110, 743–754. (42) Kaplan, I. G. Int. J. Quantum Chem. 2007, 107, 2595–2603. (43) Gervasio, F. L. Comput. Phys. Commun. 2007, 177, 27–29. (44) Chermette, H.; Ciofini, I.; Mariotti, F.; Daul, C. J. Chem. Phys. 2001, 114, 1447–1453. (45) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868.
(46) Roman, T.; Di~ no, W. A.; Nakanishi, H.; Kasai, H.; Sugimoto, T.; Tange, K. Carbon 2007, 45, 203–228.
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The reaction takes place when the H 3 3 3 Hads distance is about 2.2 A˚. The reaction potential energy curve for recombination of CH3• with H• is shown in Figure 3b. A small barrier of about 3 kcal/mol is obtained, and an energy gain of about 40 kcal/mol is calculated. The reaction begins to occur at 2.6 A˚ above the Hads atom. In the case of CH3CH2•, the formation of CH3CH3 (see Figure 3c) goes on without a reaction barrier. There is the possibility of precursor species formation (physisorption) with a small barrier of about 4 kcal/mol, and the reaction is feasible because it is exothermic, greater than 45 kcal/mol. 3.3.2. LH Mechanism. In this case, both radicals are adsorbed on the surface and the following reactions were studied: ðaÞ Hads þ Hads f H2
energy curves calculated with CATIVIC are presented for radical recombination reactions ðaÞ
Hads þ CH3 • f CH4
ðbÞ
Hads þ CH3 •CH2 • f CH3 CH3
ðcÞ
ðbÞ
Hads þ CH3 CH2, ads f CH3 CH3
ðcÞ
as shown in Figure 4. These reactions are not thermodynamically feasible if the adsorbed H is maintained in the most stable site (Hads). Two Hads (one on C1 and other on C3) produce a total Eads of about -123 kcal/mol reported elsewhere.7 Note that this value is higher in magnitude than the H-H bond formation (≈ -104 kcal/mol). For this reason, the reaction is evaluated in sites C2 and C* indicated in the F4 fragment of Figure 1, whose calculated total Eads is about -90 kcal/mol. Otherwise, the reaction would be endothermic, and a high activation barrier is anticipated. The way that the reaction path is calculated was presented above in section 2, and details are shown in Figure 2. The molecular hydrogen formation reaction path is presented in Figure 4a. Results indicate a reaction barrier of about 14 kcal/mol and an exothermic process with a release of about 12 kcal/mol. The total calculated adsorption energy for CH3,ads in site C2 and Hads in site C* is about -65 kcal/ mol. The reaction path for methane formation is depicted in Figure 4b. Results show a small reaction barrier of about 6 kcal/mol and an exothermic process with an stabilization of about 33 kcal/mol. Note the formation of a precursor to the association process at about 1.9 A˚ is possible. In the case of Hads þ CH3CH2,ads, the calculated total Eads is about -43 kcal/mol. Results for CH3CH3 formation are shown in Figure 4c with a reaction barrier of about 7 kcal/mol and a stabilization of products (CH3CH3 þ fragment) of about 45 kcal/mol. Qualitative results of this section suggest that the recombination of radicals may preferably occur by the ER mechanism than the LH mechanism, because of a lower reaction barrier and more exothermic process for the former. In addition, the LH mechanism requires a high concentration of radicals to reach adsorption on C* and C2 sites, because the most stable adsorption sites are located on edge carbon atoms that contain hydrogen. There are experimental evidence of asphaltene participation in radical reactions because they inhibit radical polymerization.40,47 In general, aromatic compounds, such as maltenes, are able to reduce or delay coke formation from
Figure 3. Reaction energy curves for the ER mechanism of (a) H2, (b) CH4, and (c) C2H6 formations because of the reaction of H•, CH3•, and CH3CH2• with Hads, respectively.
Hads þ H• f H2
Hads þ CH3, ads f CH4
shown in Figure 3. CATIVIC results depicted in Figure 3a indicate that, for H recombination, there is a small activation barrier (0.7 kcal/mol) that is in excellent agreement with DFT results (0.03 eV).20 In addition, it is important to note that this reaction is thermodynamically feasible because there is an energy gain of about 52 kcal/mol. Note that a fragmentHads bond (-64.6 kcal/mol; see Table 4) is broken, and the formation of a H-H bond occurs (about -104 kcal/mol).
(47) Bukowski, A.; Milczarska, T. J. Appl. Polym. Sci. 1983, 28, 101– 1009.
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reported yet, either theoretical or experimental. Therefore, a theoretical analysis of interactions between low-molecularweight radicals and asphaltenes may be relevant to shed some light on possible reaction mechanisms that may occur in the hydrocarbon oil cracking. A review of qualitative results and comments obtained in this work are shown as follows: (a) Theoretical results by parametric (CATIVIC and MOPAC) and DFT methods strongly suggest that X• radicals can be bonded by aromatic fragments of the studied asphaltene. The carbon edge atoms of asphaltene that contain bonded hydrogen correspond to those of a higher bond interaction with X• radicals. These results and experimental results suggest that asphaltenes may act as temporary trappers of H•, CH3•, and CH3CH2• radicals at low temperatures. Thus, it hints that hydronaphthalenic species may formed and are able to transfer Hads radicals to improve the thermal stability of petroleum, as proposed in experimental results. (b) Adsorption of X• radicals on asphaltene aromatic regions leads to an important distortion of the asphaltene surface and the formation of strong radical-asphaltene bonds. Bond strengths around the adsorption site decrease about 50-70 kcal/mol, and bond lengths enlarge 0.1 A˚. The distortion in the direction perpendicular to the surface is also important (0.32 A˚). (c) Results show that parametric methods (CATIVIC and MOPAC) overestimate Eads values, as compared to the DFT method; however, adsorption energies follow a similar trend: H• > CH3• > CH3CH2•. In addition, surface-radical bond lengths are similar using different methods. (d) Calculations suggest that the ER reaction mechanism is more feasible than the LH reaction mechanism on the asphaltene surface. Reaction barriers are smaller in the ER mechanism than in the LH mechanism, and the former processes are more exothermic than the LH processes. (e) Note that effective collisions (bond formations) between asphaltenes and radicals are more likely than between radicals, because asphaltene fragment cross-sections are much bigger than radical cross-sections. Therefore, the chance that two radicals interact on the surface is higher than in the free space. For example, Hads desorption from the asphaltene surface is less likely than diffusion, particularly for Hads atoms to form Hads 3 3 3 Hads, Hads 3 3 3 CH3,ads, and Hads 3 3 3 C2H5,ads bonds. (f) It is also significant to consider other types of asphaltenes, for example, continental asphaltenes, with many aromatic rings gathered in a round shape. Those have many weak adsorption sites in the central part of their fragments, and diffusion may easily occur; therefore, the LH mechanism could be more viable. (g) It is important to note that, in this work, entropic effects were not considered. ΔG becomes negative only at very low temperatures, because ΔHads is negative, as shown by asphaltene-radical interaction energies. As the temperature rises, ΔG becomes positive because of entropic effects and, therefore, the radical adsorption probability decreases. On these conditions, other processes, such as coke formation and dehydrogenations, may preferentially occur. On the other hand, if the pressure is increased, the radical adsorption probability is augmented, because it may occur in well oils, where temperatures are not too high and pressures are very elevated. In this situation (very high pressure and relatively low temperature), free radicals from the thermal cracking of oil light fractions may adsorb on asphaltenes to favor radical recombination. It is relevant to emphasize that these proposed mechanisms (radical recombinations on asphaltene surfaces) may occur only on specific reaction conditions. (h) Other facts that may
Figure 4. Reaction energy curves for the LH mechanism of (a) H2, (b) CH4, and (c) C2H6 formations because of the reaction of Hads with Hads, CH3,ads, and CH3CH2,ads, respectively.
asphaltene.48 In addition, Kidena et al.49 reported that PAH solvents with high molecular weight are more effective for the degradation of asphaltene molecules, because of the radical acceptability. They also reported that asphaltenes seem to act as an inhibitor for the cracking of the lighter fraction in heavy oil. 4. Conclusions and Comments A qualitative theoretical work of asphaltene interaction with oil thermal cracking compounds (free radicals) is presented here. As far as we know, this study has not been (48) Rahimi, P. M.; Gentzis Fuel Process. Technol. 2003, 80, 69–79. (49) Kidena, K.; Usui, K.; Murata, S.; Nomura, M.; Trisnaryanti, W. J. Jpn. Petrol. Inst. 2002, 45, 214–215.
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influence the anticipated mechanism are activation barrier values for radical adsorption and desorption. In previous work,7 H• adsorption barriers at different asphaltene fragments were evaluated (about 2 kcal/mol), while desorption barriers have values as high as about 40 kcal/mol, using DFT calculations. It means that, from the kinetics point of view,
radical adsorption on asphaltene surfaces is favored against desorption. Acknowledgment. The authors gratefully acknowledge the support of the Ley Org anica de Ciencia, Tecnologı´ a e Innovaci on (LOCTI) program of research.
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