972
Energy & Fuels 2001, 15, 972-978
The Overriding Chemical Principles that Define Asphaltenes Eduardo Buenrostro-Gonzalez,† Henning Groenzin,‡ Carlos Lira-Galeana,† and Oliver C. Mullins*,‡ Instituto Mexicano del Petroleo, and Schlumberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877 Received February 26, 2001. Revised Manuscript Received April 10, 2001
Asphaltenes are defined in terms of their solubility classification. This operational definition combined with the previous controversy over asphaltene molecular weight have obscured the governing chemical and structural parameters that define the asphaltene fraction. Here, asphaltenes are investigated by several techniques to elucidate relations between structure and properties. In particular, the asphaltene molecular size is compared to the ratio of aromatic to saturated carbon. The conclusion is obtained that asphaltene molecular structure is governed by the balance between the propensity of fused aromatic ring systems to stack via π-bonding, reducing solubility, vs the steric disruption of stacking due alkane groups, increasing solubility.
Introduction Asphaltenes represent an enigmatic yet very important class of compounds.1-4 For instance, asphaltenes strongly influence the rheology of paving materials.3,4 Asphaltenes impact the production,5 transportation, and refining of crude oils.1-4 The growing market of the production of crude oil in deep water environments is particularly sensitive to the complex phase behavior of crude oils.5-7 The substantial and growing importance of asphaltenes mandates detailed understanding of these materials. However, their very definition impedes characterization. Asphaltenes are defined by a solubility classification.1-4 A common but not unique definition is that they are the materials in carbonaceous sources that are n-heptane insoluble and toluene soluble. Different source materials such as widely varying petroleum crude oils and their thermally derived resid fractions, as well as different coals, can yield asphaltenes of somewhat different properties thereby hindering uniform characterization by solubility classifications. Nevertheless, these differences can lead to a tremendous understanding of asphaltenes and of their source crude oils or other materials as is shown herein. In any event, the operational definition of asphaltenes is very useful for petroleum chemistry because this definition incorporates the most aromatic fraction of crude oil. * Corresponding author. † Instituo Mexicano del Petroleo. ‡ Schlumberger-Doll Research. (1) Bitumens, Asphalts, and Tar Sands; Chilingarian, G. V., Yen, T. F., Eds.; Elsevier Scientific Publishing Co.: New York, 1978. (2) Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society, Washington, D.C, 1984. (3) Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Pub. Co: New York, 1995. (4) Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Publishing Co.: New York, 1998. (5) Shields, D. Offshore, Sept., 2000, pp 84-86, (6) Pan, H.; Firoozabadi, A. AIChE J. 2000, 46, 416. (7) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden, J.; Energy Fuels, accepted.
A second factor that has relegated asphaltenes to the enigmatic, is that there has been a 20-year controversy over the order of magnitude of asphaltene molecular weight.8,9 Clearly, if the molecular weight is unknown to within a factor of 10, it would be very difficult to establish general relationships between molecular structure and physical properties. Colligative measurements such as vapor pressure osmometry (VPO) of asphaltene molecular weight tend to give large estimates, but are known to suffer from asphaltene aggregation. Use of elevated temperatures and excellent solvents for asphaltenes yields smaller values of molecular weight indicating that aggregation is an important issue.10,11 Mass spectroscopy yields molecular weight values of ∼700 amu for petroleum asphaltenes12-14 but suffer from possible fragmentation and volatilization issues. Recently, fluorescence depolarization (FD) studies have shown that the molecular weights obtained by mass spectroscopy are correct.8,9 These fluorescence measurements are performed in extreme dilution obviating concern for aggregation. There are no fragmentation or volatilization issues that can arise. The fact that asphaltenes are only ∼700 amu and not giant molecules means that they are much more tractable than previously believed. Furthermore, the FD experiments indicated that each asphaltene molecule possess one or two chromophores. In addition, both optical15 and carbon Raman X-ray spectroscopy16 indicate that isolated benzene rings are not a significant (8) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237. (9) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (10) Anderson, S. I. Fuel Sci. Technol. Int. 1994, 12, 51. (11) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Fernandez, A.; Perez, P.; Caetano, M. Fuel 1999, 78, 997. (12) Boduszynski, M. M. Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society, Washington, D.C, 1984; Chapter 2. (13) Boduszynski, M. M. Energy Fuels 1988, 2, 597. (14) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290.
10.1021/ef0100449 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/19/2001
Chemical Principles that Define Asphaltenes
contributor to asphaltene structures. The picture that emerges is that asphaltene molecules primarily consist of a single fused aromatic ring system with peripheral alkyl substituents. Combining this information with the wealth of knowledge regarding molecular structural parameters of asphaltenes obtained from IR, NMR, EPR, XANES, SANS, SAXS, and other techniques has generated new optimism that asphaltene science is on the verge of significant advancement.1-5,17 In this paper, we explore the aspects of asphaltene chemical structure that define the asphaltene fraction. We explore a diverse set of asphaltenes obtained from different crude oils, a coal, and a resid. We use fluorescence depolarization, fluorescence emission spectroscopy, 13C NMR, and IR to perform asphaltene characterization. Several specific checks are performed to characterize the response of the FD system. The aim is not to characterize coal asphaltenes as a class. Instead, we attempt to use this diverse set of asphaltenes to identify key chemical parameters that leave their imprint on the corresponding identity of the asphaltene. In particular, we establish that there is a strong dependence of molecular size on the ratio of aromatic to saturate carbon. This is explained in terms of basic chemical principles that are illustrated with properties of a series of alkyl-aromatics. Other structural relations are also explored. Experimental Section The three virgin crude oil asphaltene samples were prepared from a Kuwaiti crude oil (UG8), API gravity 29, a Venezuelan crude oil (Ven20), API gravity 10, and a Mexican Gulf of Mexico sample (KU), API gravity 19. The Mexican asphaltene (KU) sample was separated from a Mexican Gulf of Mexico Maya crude oil following the procedure described in ref 17. These asphaltene samples are n-heptane asphaltene samples. We note that no effect was observed on the sample properties due to reprecipitation of the asphaltenes, an issue of concern for some asphaltene workers. The preparation of these asphaltenes, UG8,9 Ven20,9 and the KU17 have been described previously. The Arabian Medium Heavy vacuum resid n-heptane asphaltene18 was supplied to us from Texaco by Dr. Eric Y. Sheu. The coal asphaltene was supplied to us by Professor M. Iino at Tohoku University.19 This coal asphaltene sample was obtained from a bituminous coal sample, Tanito Harum from Indonesia. The asphaltene sample was prepared from the coal liquifaction residue. The pyridine-soluble fraction was isolated, and its toluene-soluble fraction was then isolated; the n-hexane asphaltenes of this fraction were then collected. The PTI C-72 system plus the PTI A-720 fluorescence spectrometer was used to measure fluorescence spectra of the samples. The PTI C-72 system with a PTI GL-3300 nitrogen laser and PTI GL-302 dye laser was used to measure fluorescence depolarization, as described previously.8,9 Very low concentrations of asphaltenes in toluene (∼6 mg/L) are used, preventing aggregation effects. (15) Ralston, C. Y.; Mitra-Kirtley, S.; Mullins, O. C. Energy Fuels 1996, 10, 623. (16) Bergmann, U.; Groenzin, H.; Mullins, O. C.; Fetzer, J.; Cramer, S. P.; manuscript in preparation. See also, Bermann, U.; Mullins, O. C.; Cramer, S. P. Anal. Chem. 2000, 72, 2609. (17) Buenrostro-Gonzalez, E.; Espinosa-Pen˜a, M.; Andersen, S. I.; Lira-Galeana, C. Pet. Sci Technol. J., in press. (18) Sheu, E. Y.; Storm, D. A. Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Pub. Co: New York, 1995. Chapter 1. (19) Iino, M.; Takanohashi, T. Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Pub. Co.: New York, 1995; Chapter 6.
Energy & Fuels, Vol. 15, No. 4, 2001 973
Figure 1. The fluorescence depolarization data for the KU asphaltene. The excitation photon is polarized vertically (V) or horizontally (H) and the fluorescence emission photon is selected for either vertical or horizontal polarization, giving four combinations. The fluorescence lifetime decay is evident in the decrease of fluorescence intensity after the laser firing at t ∼ 67 ns. The decay of the fluorescence anisotropy due to molecular rotation is observed as the merging of the VV and HV curves. This anisotropy and its fitted curve are also shown. Both the fluorescence depolarization system8,9 and the theory applied to asphaltenes8 have been described previously. Here, we give a brief account. The output of a nitrogen-laserpumped dye laser is used to excite fluorescence in a dilute (∼6 mg/L) asphaltene sample. The incident laser beam is linearly polarized (either vertically or horizontally in the lab frame) defining the axis of quantization. The (excited) asphaltene molecules undergo rotational diffusion and at some point in time emit a fluorescence photon each. The fluorescence collection channel detects only the desired polarization, again either vertical or horizontal in the lab frame. The detection system includes a box car integrator which is scanned in time to cover the range of fluorescence decay times. The initial polarization anisotropy is reduced by this rotational motion. By collection of the four polarization possibilities, verticalvertical, vertical-horizontal, horizontal-vertical, and horizontal-horizontal, the dependence of the optical system throughput can be eliminated. Figure 1 shows the data that were acquired for the KU asphaltene. The initial separation between the vv and vh essentially gives anisotropy and the merging of these two curves (reduction of separation of these two curves) at later times gives the decay of the anisotropy. In addition, to the decay of the anisotropy, both curves decrease showing the decay of the fluorescence. Figure 1 also shows the fitted decay curve to the experimental data. The ability to measure the decay rate is evident. The 13C NMR measurements were performed on a JEOL Eclipse model 300 NMR apparatus operating at a 13C frequency of 75 MHz. The asphaltene samples were prepared as 100 mg/cm3 CDCl3 solutions. TMS was used as a zero-shift reference. To use the 13C NMR spectra in a quantitative manner, the spectra were subjected to an inverse gated decoupling technique to suppress the NOE effect, and chromium acetyl-acetonate (0.01 M in the final solution) was added to ensure complete nuclear magnetic moment relaxation between pulses. Figure 2 shows the 13C NMR spectra for the KU, Ven20, UG8, and Coal asphaltene samples. The IR measurements were performed with the aid of a Nicolet model 710 IR-FT apparatus operating between wavelength intervals of 400 to 4000 cm-1. For these measurements,
974
Energy & Fuels, Vol. 15, No. 4, 2001
Buenrostro-Gonzalez et al.
Figure 2. The 13C NMR spectra of the asphaltenes. The petroleum asphaltenes show appreciable peak areas for both saturated and aromatic carbon while this coal asphaltene is dominated by aromatic carbon absorption. This great contrast is very useful in identifying key molecular parameters that define asphaltenes. a film spreading procedure was applied to all samples. The films were prepared by making dilute asphaltene/dichloromethane solutions (5 g/L) that were later dispersed in drops on the surface of a sample holder. Heating to 50 °C prior to film formation was performed. Figure 3 shows the IR-FT spectra for the above-mentioned asphaltene samples.
Results The fluorescence depolarization data were analyzed as described previously8,9 using eq 1:
τc )
Vη kT
(1)
where τc is the rotational correlation time, V is the molecular volume, η is the solvent viscosity, k is the Boltzman constant, and T is temperature. Figure 4a plots τc obtained for the different asphaltenes vs the fluorescence emission wavelength. Note that all petroleum samples are fairly similar showing a large range of τc across the visible emission wavelengths. Figure 4b shows the fluorescence emission spectra obtained for these asphaltenes; the fluorescence intensity profiles show that the visible spectral range is appropriate for asphaltene analysis. The large range of asphaltene τc’s shows the large variation of molecular size for the asphaltenes.9 Detailed
Figure 3. The IR spectra of the asphaltenes. The coal asphaltene exhibits a greater diversity of chemical structures including appreciable hydrogen attached to aromatic carbon.
analysis of the τc shows that the mean petroleum asphaltene molecular size is ∼17.5 Å. Comparison with known molecular sizes as well as direct comparisons of τc’s for various model compounds shows the mean molecular weights are ∼750 amu but vary somewhat depending on the corresponding crude oil.8,9 Figure 4 shows that this coal asphaltene is quite different than those derived from crude oils. We want to make use of this fact to understand the correspondence between asphaltene structure and properties. It is difficult to use only petroleum asphaltenes for this purpose because they tend to span a fairly narrow range in comparison. Figure 4 also shows that the single chromphores represent a large fraction of the total molecular size. The small blue-emitting chromophores rotate much faster than, thus are not attached to, the red-emitting larger chromophores. Thus, each chromophore represents a significant fraction of the total molecular size. The picture emerges that an asphaltene molecule possesses a fused aromatic core with peripheral alkyl substituents. Figure 5 shows that there is a good correlation between the τc’s and the fluorescence emission maxima for the asphaltenes. This result is expected; larger chromophoric groups lead to long τc’s and to longer fluorescence emission maxima.20 Coal asphaltene molecules (at least from this sample) are smaller than our petroleum asphaltene molecules. These independent results are self-consistent. (20) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings Pub. Co.: Menlo Park, CA, 1978.
Chemical Principles that Define Asphaltenes
Energy & Fuels, Vol. 15, No. 4, 2001 975
Figure 6. The CH2/CH3 ratio scales with Caro/Csat for asphaltene molecules. Asphaltene molecules with longer alkane chains have a larger CH2/CH3 ratio and a larger fraction of saturated carbon. Table 1. Spectral Values for the Asphaltene Samples
Figure 4. The fluorescence analysis of the asphaltenes. (a) The rotational depolarization times (τc) for the asphaltenes. The small τc’s indicate asphaltene molecular structures are small (∼750 amu). The order of magnitude variation of τc’s vs excitation wavelength indicates that there is ∼1 chromophore/ asphaltene molecule, red chromophores which are known to be larger are in larger molecules. (b) The fluorescence emission profile of asphaltenes provides the spectral range of interest for depolarization studies.
Figure 5. The rotational correlation times τc’s scale with the emission maxima. Red chromophores which are known to be larger are in larger molecules. The larger asphaltene molecules have larger chromophores. This result indicates that the asphaltene chomophore is an appreciable fraction of the entire molecule, meaning there is a single chromophore per asphaltene molecule.
Figure 6 shows the correlation of the Caro/Csat ratio from 13C NMR vs the CH3/CH2 IR peak area ratio for
asphaltene
〈τc〉 ns
〈λem〉 nm
Caro/Csata,b
CH2/CH3c,d
Ku Ven20 UG8 resid coal
0.6741 0.6418 0.5282 0.5259 0.1796
493 490 493 470 451
1.14 0.94 1.09 1.31 7.21
2.484 2.277 2.557 2.48 1.896
a C 13C NMR aromatic carbon area integration (160-110 aro ) ppm).21 b Csat ) 13C NMR saturated carbon area integration (6010 ppm).21 c CH2 ) area of the 2920 cm-1 band from IR-FT spectra. d CH3 ) area of the 2950 cm-1 band from IR-FT spectra; area ) ∫Av dν; in the absorbance (Aν) vs absorption-frequency (ν) curve.
the asphaltenes. Clearly the coal asphaltene stands out in both axes. The coal has a much higher fraction of aromatic carbon. This is directly evident in the 13C NMR spectra in Figure 2. In addition, this is born out by the very small fraction of methylene groups. Since the alkane fraction is small in the coals, the chain lengths must be short, and alicyclic groups must be few in number for the coal asphaltene. The NMR and IR data are consistent, the coal asphaltene has a much smaller fraction of alkyl carbon than our petroleum asphaltenes. Table 1 gives the data presented in the previous figures. Figure 7 shows the correlation between the asphaltene τc’s and the Caro/Csat ratios derived from 13C NMR. There is some variability among the petroleum asphaltenes along both axes. However, the salient feature is that the coal asphaltene is highly deficient in alkanes and has much smaller asphaltene molecules than the petroleum asphaltenes. This is a very important finding. Whole coals clearly have very large molecules, much larger than those of petroleum asphaltenes; the small size of the coal asphaltene molecules (in this coal) is not due to the lack of large molecules in the source material, nor to the inability to solvate these molecules in solvents stronger than toluene.19 The reason for the small size of the coal asphaltene molecules is that the absence of alkanes greatly reduces steric interference to aromaticring stacking and bonding in general. That is, to be classified as an asphaltene, the molecule must be toluene soluble. Without alkane steric disruption, the solubility of large aromatic ring systems drops very low.
976
Energy & Fuels, Vol. 15, No. 4, 2001
Figure 7. The rotational correlation time scales with Caro/ Csat over a very larger range of values. This important result shows that asphaltene molecules lacking in alkane chains can have only small ring systems (to remain soluble in toluene). Molecules with long chains can have large ring systems (stacking is disrupted by alkane steric repulsion maintaining solubility).
Figure 8. Melting points of alkyl aromatics. Alkane substitution disrupts ring stacking lowering the melting point. Larger ring systems require increasingly longer alkane chains to effectively disrupt stacking. These simple principles govern asphaltene molecular identity.
Due to the small alkane fraction of this (and many other) coal asphaltenes, only small ring systems from the coal can be solvated in toluene and thus be included in the asphaltene fraction. The disruption of alkanes on aromatic ring stacking is evident in the melting point of different series of alkyl aromatics. Figure 8 shows the melting point of benzene and various n-alkyl benzenes.22 The addition of a single methyl group dramatically disrupts ring stacking necessitating a much lower temperature to form the solid. With the longer-chain phenyl n-alkanes the stacking (21) Dickinson, E. M. Fuel 1980, 59. (22) Melting point data collected from Handbook of Chemistry and Physcis, 63rd ed.; CRC Press: Boca Raton, FL, 1983; and from the Aldrich Chemicals Co. catalogue.
Buenrostro-Gonzalez et al.
disruption persists and eventually the melting points climb with sufficiently long chains as expected. Figure 8 shows that β-alkyl naphthalenes and β-alkyl anthracenes show the same trend. However, β-methyl group is insufficient to cause much stacking disruption for these larger ring systems. Addition of a β-ethyl group to naphthalene is sufficient to cause disruption as evidenced by the drop in melting point for β-ethyl naphthalene. For β-alkyl anthracene, larger chains are required for causing stacking disruption. β-methyl anthracene has almost the same melting point as anthracene, but β-ethyl anthracene has a lower melting point. The overriding chemical structural issues that define asphaltene chemical structures are (1) the propensity of aromatic rings to stack and π-bond via van der Waals interaction, and (2) the steric disruption which occurs due to peripheral alkyl substitution. These simple principles are operative in determining trends in melting point data of alkyl aromatics and are readily comprehensible. By knowing the molecular weight of asphaltenes, and knowing that most asphaltene molecules possess one or two chromophores,8,9 we are able to clarify the enigmatic asphaltenes to a considerable extent. We have more evidence that the above explanation is correct. Asphaltenes are known to aggregate; there is a question as to what concentrations this aggregation occurs. Micelle formation is thought to occur at the relatively high concentration of 0.5 g/L or more.18 However, incipient aggregation would correspond to dimer formation and would occur at much lower concentrations for the aphaltenes. Different optical measurements9,11 have indicated that asphaltene dimer formation occurs at concentrations in the range of 60 mg/L. Our experiments are typically performed at 6 mg/ L, one order of magnitude lower than where dimer formation starts with asphaltenes. However, for aromatics that stack more effectively, we may not be able to operate at a concentration low enough to obtain single monomers. Figure 9 shows the anisotropy curves obtained for two compounds. Both contain large fused aromatic ring systems. One of the ring systems has two peripheral substituents consisting of 13 carbon n-alkyl chains. This molecule readily exhibits a large anisotropy and a long decay time. The other compound (dibenzocoronene) does not possess any alkanes to disrupt ring stacking. In addition, despite the large size of this molecule, it does not exhibit any anisotropy. Dimer formation is often accompanied by electronic degeneracies20 that reduce the anisotropy sometimes to zero. We believe that this large molecule forms dimers at concentrations more than 10 times lower for the asphaltenes due to effective and unimpeded ring stacking. We have looked at several other large alkylated aromatics that do exhibit large anisotropy and other large unsubstituted aromatics that do not show any anisotropy. Specifically, N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide, N,N′-diphenyl-3,4,9,10-perylenetetracarboxylic diimide, N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide, and octaethyl prophyrin are all relatively large alkylated fused aromatic ring systems and all gave large fluorescence anisotropies. On the other hand, molecules with comparable fused ring
Chemical Principles that Define Asphaltenes
Figure 9. Fluorescence anisotropy of different aromatics. Larger fused ring systems with alkane chains are resistant to dimerization/aggregation at low concentrations (∼10 ppm) and exhibit large anisotropies. Comparable ring systems without alkane substitution lack stacking disruption and tend to aggregate, killing the anisotropy via electronic energy exhange mechanisms. At these concentrations all asphaltene solutions exhibit large anisotropies.
systems without alkyl chains exhibit no fluorescence anisotropy, benzo[c]naphtho[gra]tetracene, dibenzocoronene, dibenzo[a,j]pentacene, and naphtho[1,2,3,4-ghi]perylene. We believe all of these aromatic systems, are governed by the same general principles as asphaltenes. Of course, the asphaltenes possess a range of molecular sizes and will show specific solubility behavior which is different than that of the pure compounds. Nevertheless, the balance between the tendencies of fused aromatics to form π-bond molecular stacks, and the steric disruption to this stacking are, we believe, the most important molecular structural issues in defining that fraction of carbonaceous materials contained within the asphaltene solubility fraction. To validate further that we understand the fluorescence depolarization measurements, and to corroborate our conclusions reached above, we have performed a test involving the measurement of the τc of a single chromophore in two solutions of very different viscosities. Figure 10 shows the anisotropy decays for a toluene solution (η ) 0.56 cp) and an ethylene glycol solution (η )16.1 cp) of N,N′-ditridecyl-3,4,9,10-perylene-tetracarboxylic diimide. The much longer anisotropy decay time predicted by eq 1 is evident in the ethylene glycol solution. The ratio of the τc’s is 23.9 and of the ratio of the viscosities is 28.8. Equation 1 predicts that these two ratios should be the same. They are close but there is a discrepancy. We may have some error in the measurement of very long decay time for the ethylene glycol solution due to the fact that the anisotropy decay becomes comparable to the rate fluorescence decay rate. The determination of the molecular radius is obtained as the cube root of τc, so this discrepancy would have little effect on the radius estimation. In any event, the important point is that our measurements are producing expected trends. Finally, a feature that is noted in Figure 11, the absorption spectra of the asphaltene solutions, is that
Energy & Fuels, Vol. 15, No. 4, 2001 977
Figure 10. The effect of viscosity on fluorescence anisotropy of a dye in two solutions. The much higher viscosity of ethylene glycol slows molecular rotation allowing the anisotropy to remain large for long times. The fluorescence depolarization method behaves as expected corroborating all corresponding conclusions for asphaltenes.
Figure 11. The optical absorption spectra of the different asphaltenes used here. The Soret bands of the VO porphyrins and Ni porphyrins are evident especially for the Ven20 asphaltene. The metallo octaethyl porphyrins are comparable is size to the smallest asphaltene molecules.
the Venezuelan oil produces a very large Soret band indicating the presence of porphyrins. This is correlated with the relatively large metal content In fact, the size range of the smallest asphaltene molecules in this asphaltene is comparable to a porphyrin.8,9 Conclusions Several techniques are used to analyze a diverse set of asphaltenes. It is shown that π-bond stacking vs alkane steric disruption of stacking are the key molecular structural issues which determine which molecules will fall into the asphaltene solubility classification. These simple ideas substantially reduce the perceived complexity of asphaltenes and promise the comparable
978
Energy & Fuels, Vol. 15, No. 4, 2001
extensions for other relations between molecular structure and properties for asphaltenes. Development of these concepts relied on the recent resolution of key asphaltene molecular properties, in particular, the accurate determination asphaltene molecular weight. Acknowledgment. We are indebted to Professor Iino with Tohoku University for the coal asphaltene sample. We thank Dr. Eric Sheu, formerly with Texaco now with Zeneca Chemicals, for the resid asphaltene
Buenrostro-Gonzalez et al.
sample. Thanks also go to Pemex E&P in Ciudad del Carmen, Mexico, for providing a fresh KU oil sample. Mr. J. Garcia-Martinez from IMP provided excellent technical assistance in the 13C NMR studies. We thank Dr. John Fetzer of Chevron for providing us with several compounds consisting of large fused aromatic ring systems. EF0100449
