Energy & Fuels 2003, 17, 135-139
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Molecular Dynamics Simulation of the Heat-Induced Relaxation of Asphaltene Aggregates Toshimasa Takanohashi,*,† Shinya Sato,† Ikuo Saito,† and Ryuzo Tanaka‡ Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 16-1, Onogawa, Tsukuba 3058569, Japan, and Central Research Laboratories, Idemitsu Kosan Co., Ltd., Kamiizumi 1280, Sodegaura 2990293, Japan Received June 5, 2002
The energy-minimum conformation calculated by molecular mechanics-molecular dynamics simulation for the asphaltene obtained from the vacuum residue of Khafji crude oils showed that structures aggregated through several noncovalent interactions are the most stable. Changes induced in aggregated structures by heating were investigated using molecular dynamics calculations. The simulation showed that the hydrogen bond between asphaltene molecules dissociated at 523 K, while aromatic-aromatic stacking interactions appeared to be stable. At 673 K, however, some stacking interactions could be disrupted, but some stable aggregates remained even at this high temperature where some decomposition reactions would be expected to occur. Simulations on two model compounds were carried out to investigate the effects of aliphatic chains and polar functional groups on the stability of asphaltene aggregates during heating. Aliphatic chains and polar functional groups contributed to the stability of aggregates; in simulations of “imaginary” structures in which the original structure was modified by removing the aliphatic side chains and then replacing heteroatoms with carbon, dissociation occurred at lower temperatures at to greater extents than for the original structure; van der Waals interactions between aliphatic chains acted cooperatively to stabilize the asphaltene aggregates.
Introduction Many structural analyses of petroleum vacuum residues (VR) have been carried out. Generally, VR is separated into maltene and asphaltene fractions, which have been characterized with techniques such as NMR and mass spectrometry. Asphaltenes are more aromatic and have a higher metal content than maltenes, but maltenes have more aliphatic moieties. Despite these studies, however, the true molecular weight of asphaltenes has not yet been determined. Vapor pressure osmometry (VPO) and size exclusion chromatography (SEC) yielded average molecular weights as high as 4000-10000 amu,1-4 and on the basis of these data it has been suggested that average model structures of asphaltenes are composed of aromatic clusters as large as 10-20 of aromatic rings.5,6 In contrast, laser desorption mass spectrometry gave a molecular weight of 400 amu7 and recent fluorescence depolarization measurements produced an average molecular weight of 750 amu for asphaltenes.8 * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST). ‡ Central Research Laboratories. (1) Storm, D. A.; DeCanio, S. J.; DeTar, M. M.; Nero, V. P. Fuel 1990, 69, 753. (2) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12, 51. (3) Reynolds, J. G.; Biggs, W. R. Acc. Chem. Res. 1988, 21, 319. (4) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31, 530. (5) Yen, T. F. Energy Sources 1974, 1, 447. (6) Speight, J. G. Fuel 1970, 49, 76. (7) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290.
Surface tension,9-12 calorimetry,13 or viscosity14 measurements suggested that, depending on the concentration and the type of solvent, asphaltenes form aggregated structures much like micelles in organic solvents. Because they have higher aromaticity, polar functional groups, and molecular weight than maltenes, asphaltene aggregated structures are thought to form through noncovalent aromatic-aromatic, electrostatic, and van der Waals interactions. The formation of aggregates may exaggerate the apparent molecular weight if aggregates are not dissociated in solvents used in VPO and SEC. Asphaltenes may be responsible for the detrimental formation of coke-precursor and may deactivate catalysts. The relation between aggregation of asphaltenes and formation of coke-precursors is not well understood, however. Many average model structures for heavy carbonaceous materials have been constructed on the basis of structural data. Computer modeling of asphaltenes15,16 and coal extracts17,18 has shown that aggregated struc(8) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (9) Rogel, E.; Leon, O.; Torres, G.; Espidel, J. Fuel 2000, 79, 1389. (10) Andersen, S. I.; Speight, J. G. Fuel 1993, 72, 1343. (11) Sheu, E. Y.; Tar, M. M. D.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299. (12) Mohamed, R. S.; Ramos, A. C. S.; Loh, W. Energy Fuels 1999, 13, 323. (13) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14, 38. (14) Storm, D. A.; Barresi, R. J.; DeCanio, S. J. Fuel 1991, 70, 779. (15) Murgich, J.; M., J. R.; Aray, Y. Energy Fuels 1996, 10, 68. (16) Rogel, E. Colloids Surf., A 1995, 104, 85. (17) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8, 395.
10.1021/ef0201275 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/05/2002
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Table 1. Ultimate Analysis and Properties of Khafji Asphaltene properties
Khafji asphaltene
wt % in VR
14.2
elemental analysis, wt % carbon hydrogen sulfur nitrogen oxygen H/C (-)
83.0 7.8 6.6 1.0 1.6 1.1
metal, ppm Ni V
200 550
density, g/cm3
1.168
tures are the most stable conformations. Evaluations of asphaltene aggregation, such as interactions with resins or solvents, and a thermodynamic approach based on the aggregated model structures have been investigated.19-21 Dissociation of the aggregated structure by heating and/or soaking in solvent is not well understood. If aggregation is so strong that that the structure cannot be dissociated during heating, it could be responsible for the formation of coke precursor. In the present study, the asphaltene obtained from Khafji VR was modeled, and the stable conformation was determined by molecular mechanics (MM) and molecular dynamics (MD) calculations. The most stable aggregated structure obtained was “heated” computationally to temperatures of 373-673 K in order to see structural changes in the aggregates and to see whether the aggregates dissociated. In addition, the effect of polar or substituted alkyl groups on the stability of the model structures during heating was also investigated.
Takanohashi et al. Table 2. Structural Parameters Observed for Khafji Asphaltene, and Values Estimated for Its Model Structure structural parameters
observed
model
0.08 0.21 0.51 0.20
0.09 0.22 0.49 0.20
0.52 0.11 510
0.51 0.12 507
1H
NMR, (-) Haa HRb Hβc Hγd 13C NMR, (-) fae fai3f Mwusg, amu
a Hydrogen attached on aromatic rings. b Hydrogen attached on R-position carbons. c Hydrogen attached on β-position carbons. d Hydrogen attached on peripheral methyl carbons. e Aromatic carbons per total carbons. f Quaternary aromatic carbons shared with three aromatic rings per total carbons. g Number-average molecular weight of aromatic unit structure.
Experimental Section Sample. Asphaltene (n-heptane-insolubles) obtained from fractionation of Khafji VR with n-heptane was used as the sample. The ultimate analysis and several properties are listed in Table 1. Structural Analyses. Solution 1H NMR and 13C NMR spectra were measured in DCCl3 (99.5%) on a JEOL Lambda 500 NMR spectrometer with TMS as the internal standard. To analyze the distribution of molecular weights, laser desorption mass spectrometry (LD/MS) was performed on a Kratos Kompact MALDI III linear/refrectron time-of-flight mass spectrometer. 2000 scans were accumulated for each sample.
Computer Simulation Average Model Structure. A model structure was constructed using Sato’s method22 based on the structural parameters obtained from analytical data (Table 2). One average model structure is shown in Figure 1. The model is composed of three different molecules constructed to express a range of reasonable chemical structures, the average values of which are consistent with the structural parameters obtained experimentally. (18) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1998, 12, 1168. (19) Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy Fuels 1999, 13, 278. (20) Rogel, E. Energy Fuels 2000, 14, 566. (21) Rogel, E. Langmuir 2002, 18, 1928. (22) Sato, S. Sekiyu Gakkaishi 1997, 40, 46 (in Japanese).
Figure 1. Average model structures of Khafji asphaltene.
It was assumed that the bonding types of sulfur, nitrogen, and oxygen are thiophenic and sulfide, pyridinic, and hydroxyl groups, respectively. Energy-Minimum Conformation. An energy-minimized conformation for the average model structure in Figure 1 was determined using MM and MD methods. The Cerius2 software package (version 4.2, Molecular Simulation, Inc.) was run on an OCTANE graphic work station (Silicon Graphics Inc), and the DREIDING 2.02 package23 was used for force field calculations. Periodic boundary conditions (PBC) were used to simulate the bulk properties of the model structure.24 The potential energy for an arbitrary geometry of a molecule is expressed as a combination of bonded torsions, which depend on the covalent bonds of the structure, and nonbonded interactions, which depend (23) Mayo, S. L.; Olafson, B. D.; Goddard ,W. A., III. J. Phys. Chem. 1990, 94, 8897. (24) Takanohashi, T.; Nakamura, K.; Iino, M. Energy Fuels 1999, 13, 922.
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Figure 2. Two local minimum conformations for the model structures of Khafji asphaltene.
only on the distance between atoms. The bonded terms consist of bond length torsion (Eb), bond angle torsion (Eθ), dihedral angle torsion (Eφ), and inversion (Ei), while the nonbonded terms consist of van der Waals (Evdw), electrostatic (Eel), and hydrogen bond (Ehb) energies. The total energy E is the simple sum of these energies:
E ) Eb + Eθ + Eφ + Ei + Evdw + Eel + Ehb
(1)
The MM and MD calculations were repeated to minimize the total potential energy of the model structure. Several local minimum conformations were obtained (Figure 2). For both conformations, the aggregated structure obtained as the stable conformation consisted of aromatic clusters that formed gross layered structures and were surrounded with alkyl side chains, in agreement with results reported elsewhere.15,16,19,20 The total energies for the two conformations were 260.9 and 276.6 kcal/mol for conformations (a) and (b), respectively. Finally, conformation (a), which is the most energy-minimum structure, was selected as the representative model structure for Khafji asphaltene and was used for the subsequent simulations described below. A density simulation was conducted on the final model to check the validity of the aggregated structure. The details of the simulation procedure have been described elsewhere.25 For styrene25 and coal models,26-29 the densities estimated using this approach were in good agreement with experimental values. The estimated density for the energy-minimum conformation in Figure 2a was 1.10 g/cm3, in agreement with the observed value of 1.16 g/cm3. (25) Nakamura, K.; Murata, S.; Nomura, M. Energy Fuels 1993, 7, 347. (26) Murata, S.; Nomura, M.; Nakamura, K. Energy Fuels 1993, 7, 469. (27) Dong, T.-L.; Murata, S.; Miura, M.; Nomura, M.; Nakamura, K. Energy Fuels 1993, 7, 1123. (28) Nakamura, K.; Takanohashi, T.; Iino, M.; Kumagai, H.; Satou, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003. (29) Takanohashi, T.; Kawashima, H. Energy Fuels 2002, 16, 379.
Figure 3. Flowchart of calculation procedure.
MD Simulation for Stability of the Aggregated Structure. The position of each atom in the model structure depends on the temperature, and the dynamics of conformational structural changes that occur during the MD calculation can be observed as “snapshots.“ To investigate the stability of the aggregated structure when heated, the MD calculation was conducted under periodic boundary condition (PBC) in the range 373-673 K. The flowchart for the calculation procedure is shown in Figure 3. The MD was performed under constant-pressure and -temperature condition (constant NPT). The charges were updated every 0.01 ps during MD using the method of charge equilibration proposed by Rappe and Goddard.30 The size of cell and the potential energy are changeable. If the interaction energy between molecules is relatively low, structures may dissociate. Thus, we can estimate the degree of the stability of the aggregated structure at a given temperature. The distances between atoms on several interaction sites in the model structure in Figure 2a were used as the parameter to evaluate the structural change; for example the stacking interaction between aromatic clusters a and b in Figure 2a was evaluated from the minimum of distances between internal carbon atoms in aromatic clusters. (30) Rappe, A. K.; GoddardIII, W. A. J. Phys. Chem. 1991, 95, 3358.
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In addition, two “imaginary” simulations were carried out. First, all aliphatic side-chain carbons were removed from the original model structure shown in Figure 2a, and MD was carried out in the range 300 K- 673 K. If aliphatic side-chains contribute to the stability of asphaltene aggregates, the aggregated structure should be more easily dissociated after the removal of the aliphatic chains. Next, all sulfur, oxygen and nitrogen atoms were replaced with carbons, and the MD was carried out in the same temperature range; this allowed the effects of polar functional groups on the stability of the aggregated structure to be evaluated.
Results and Discussion Thermal Stability of Aggregated Structures. Changes in three interaction sites were monitored during MD simulation on the aggregated structure in Figure 2a: the distances between carbon atoms on aromatic clusters a-b and b-c, and the distance of the hydrogen bond between two hydroxyl groups. If the structure dissociated with decreasing total energy, the distances become greater. Conversely, if further aggregation is more energetically favorable at a given temperature, changes in these distances should be small. Figure 4 shows changes in the three interaction sites during the MD simulation. At 300 K (Figure 4a), the distances of the hydrogen bond and the a-b cluster changed slightly and the b-c distance was essentially unchanged. Consequently, the aggregated structure was not dissociated, and the structure is stable at 300 K. At 523 K (Figure 4b), the two aromatic-aromatic interactions between a-b and b-c clusters do not appear to have changed significantly, suggesting that they are stable at 523 K. The distance of the hydrogen bond at 50 ps had changed significantly (∼10 Å), however, but after 60 ps the value became small again. Thus, while the hydrogen bond seems to dissociate at this temperature, the stability of the aromatic-aromatic interactions allows the hydrogen bond to reform. At 673 K (Figure 4 c), the change in distance of the b-c aromatic cluster was still small. Figure 5 shows a snapshot of the conformation at around 85 ps of the original structure (Figure 4c, arrow). The structure is still aggregated at a temperature at which some chemical reactions such as decomposition would occur. This result suggests that because the inside of the aggregate may not be accessible to hydrogen during decomposition reactions, undissociated aggregates may partially become a heavier fraction like a coke-precursor. Effect of Functional Groups on the Stability of the Aggregated Structure. Unlike bituminous coal, which has a smaller number (2-4) of aromatic rings and fewer aliphatic carbons,17,28 asphaltenes are characterized by a large number of long alkyl chains attached to aromatic ring systems. The difference in solubility in ordinary solvents of coals and asphaltenes is influenced by the number of alkyl chains. In addition, the number of polar nitrogen, sulfur, and oxygen functional groups also affects the solubilities of asphaltene and coal. It seems reasonable, then, that aliphatic side chains and polar groups can influence the stability of aggregates.
Figure 4. Changes in distances between aromatic-aromatic clusters (a-b and b-c in Figure 2a) and distance of hydrogen bond.
Figure 6 shows the result of an MD simulation at 623 K for the model asphaltene structure from which all aliphatic side chains were removed. A greater change in a-b distance was observed than that of original structure at 673 K shown in Figure 4c. Because dissociation occurs more readily and at a lower temperature in the absence of aliphatic side chains, they must contribute to the stability of aggregates. The calculated energy before and after removal showed that a decrease in interaction energy, mainly because of van der Waals interaction between aliphatic side-chain carbons, reduced the stability of aggregated structure. Entanglement of long aliphatic chains may also contribute to hold such aggregated structure.
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Figure 5. A snapshot of aggregated structure during MD calculation at 673 K.
Figure 7. Effect of polar groups on the stability of aggregates when the MD was conducted at 623 K.
Conclusions
Figure 6. Effect of alkyl chains on the stability of aggregates when the MD was conducted at 623 K.
Figure 7 shows the result of an MD simulation at 623 K for the model compound in which all heteroatoms were replaced with carbon. For coal models, polar functional groups attached to aromatic clusters have been reported to contribute to the strength of whole aggregate through cooperative aromatic-aromatic interactions.18,24,31 Figure 7 shows that a significant change occurs in the distance between the a-b cluster and the b-c clusters. In the original structure (Figure 2a), the two pyridinic nitrogens and two sulfurs could help stabilize b-c aggregates through electrostatic interactions; indeed, the value of the term for electrostatic energy (Eel in eq 1) greatly decreased after replacement of the heteroatoms with carbons. Comparing Figures 6 and 7, the effect of removing polar functional groups seems to be greater than that for removal of aliphatic chains. It is clear, however, that both functionalities act cooperatively to stabilize asphaltene aggregates, in addition to main aromaticaromatic stacking interactions. (31) Takanohashi, T.; Nakamura, K.; Iino, M. Energy Fuels 2000, 14, 393.
MM/MD calculations on a model for an asphaltene obtained from Khafji vacuum residue showed that aggregated structures of asphaltene molecules held together through several noncovalent interactions are the most stable conformation. Changes in the aggregated structures during heating were monitored during an MD simulation. At 673 K, a temperature at which some decomposition reactions may begin, part of the aromatic-aromatic stacking interactions still formed stable aggregates, indicating that such stable aggregates might become a heavier fraction like a cokeprecursor. Aliphatic chains and polar functional groups on the aromatic rings were found to contribute to the stability of aggregates. The effect of van der Waals intermolecular forces between aliphatic chains was estimated to be important for the stability of asphaltene aggregates during heating. It was found that several interaction forces can act cooperatively to stabilize asphaltene aggregates. Acknowledgment. Financial support for the work was supplied by the Proposal-Based International Joint Research Program, New Energy and Industrial Technology Development Organization. The authors thank Dr. R. E. Winans and Dr. J. E. Hunt of Argonne National Laboratory for obtaining the LDMS data and for their useful discussions. Helpful discussions with Professor M. R. Gray of University of Alberta and Dr. X. Ma of The Pennsylvania State University are also appreciated. EF0201275
