Energy & Fuels 2005, 19, 2371-2377
2371
X-ray Diffraction of Subfractions of Petroleum Asphaltenes Simon Ivar Andersen* Department of Chemical Engineering, building 229, Technical University of Denmark, DK-2800 KGS, Lyngby, Denmark
J. Oluf Jensen Department of Chemistry, building 207, Technical University of Denmark, DK-2800 KGS, Lyngby, Denmark
James G. Speight CD & W Inc., P.O. Box 1722, Laramie, Wyoming 82073-1722 Received February 11, 2005. Revised Manuscript Received July 6, 2005
New data are reported from powder X-ray diffraction (XRD) investigations of fractions obtained from separation of the bulk heptane-asphaltenes into subfractions using toluene-heptane mixtures. The latter indicate that when removing large parts of the heptane-asphaltene fraction by extraction with solvent mixtures having an increased toluene content, the stacking distance (or intensity of the 002 peak) or other parameters derived are not particularly changed. Hence, this infers that no increase in molecular interaction takes place by removal of more soluble species. This is confirmed as the stack diameter is insensitive to subfractionation, as observed from the 100 band. The stacking of asphaltenes has previously been inferred from powder X-ray diffraction of solid asphaltenes. The XRD patterns usually reveal three different broad peaks where the stacking of the asphaltene constituents is determined from the 002 peak that sometimes occurs as a shoulder (rather than as a distinct peak) on a peak related to alkyl ordering. The possibility of stacking as a major interaction is discussed in terms of structural data, as is the quantitative information and sensitivity of this when the necessary curve resolution and fitting routines are used.
Introduction In the petroleum industry it is well-known that the asphaltene fraction or part of this can cause problems due to phase separation, leading to plugging of tubing, pipes, valves, catalyst beds, and reactor flow lines. The asphaltene constituents are generally described as forming colloidal aggregates that are dispersed in the oil by polar substances.1,2 Under specified conditions, phase separation occurs and is believed to be due to flocculation of the aggregates and, hence, precipitation from the oil. To predict the flocculation phenomenon, it is important to be able to account not only for the flocculating forces but also for the forces keeping the aggregates together. It is not known if the aggregates are an existing entity in the oil or if these are formed as the oil (being the solvent phase) becomes more hostile, thereby forcing the asphaltene constituents to associate. This can happen over geological time during maturation of crude oil (a phenomenon known as “gas deasphalting in the reservoir”), during recovery from the * Corresponding author. Phone: +45 45 27 2113. E-mail:
[email protected]. (1) Nellensteyn, F. J. In The Science of Petroleum; Oxford University Press: London, 1938; Vol. IV, p 2760. (2) Pfeiffer, J. P.; Saal, R. N. J. J. Phys. Chem. 1940, 44, 139-143.
reservoir, and also during refining. Hence, there could be a range of states in oil going from molecularly dissolved to associated state, depending on solvent properties and interactions with other compounds in the oil. The proposed structure of the asphaltene aggregate (sometimes misleadingly referred to as the asphaltene micelle) was based on the used of powder X-ray diffraction. Basically, two features were found in the diffraction patterns that, in normal diffraction terms, are almost featureless. These features have been referred to as the γ-peak and the 002 peak. The first is assigned to paraffinic material, in which ordering of alkyl chains or naphthenic portions of a vast number of molecules occurs, while the 002 peak is assigned to aromatic material.1,3 In another work,4 X-ray diffraction studies of asphaltenes and carbenes (a high molecular weight polar fraction of petroleum usually formed from the asphaltenes as a result of thermal processes) exhibited chain orientation but no graphite structure. On the other hand, there are reports1,5 of graphite-like structures within the asphaltene fraction. In addition, studies (3) Taxler, T. N. AsphaltsIts Composition, Properties and Uses; Reinhold Publ. Corp.: New York, 1961. (4) Alexanian, C.; Louis, M. Compt. Rend. 1950, 231, 1233, citation by Traxler (ref 3).
10.1021/ef050039v CCC: $30.25 © 2005 American Chemical Society Published on Web 11/01/2005
2372
Energy & Fuels, Vol. 19, No. 6, 2005
on a large number of asphaltenes6 reported X-rayderived aromaticity for asphaltenes in the range 0.210.51, well in agreement with other data reported for the aromaticity of the asphaltene fraction. Recently, the X-ray data on asphaltenes from four different Arab oils have been reported,7 and the derived structural data were very similar for these asphaltenes, although the diffraction patterns were different in terms of intensity of the 002 peak compared to the γ-peak. The latter was apparently not reflected in the data treatment. Furthermore, other workers8 have also presented X-ray data on one asphaltene showing similar trends. In the present work, the use of X-ray diffraction is revisited for a number of asphaltene fractions to focus on the ability of this method to derive knowledge of the molecular structuring of nanoaggregates in solid asphaltenes. To investigate deeper into the asphaltenes, we especially examined fractions of asphaltenes obtained by precipitation of the asphaltenes in solvent mixtures of toluene and heptane. The latter procedure would show if there is a difference when moving toward the less-soluble material and also if the less soluble material is more aromatic. Hence, this could reveal if aromaticity and therefore stacking of aromatic parts of molecules is the main driving mechanism of precipitation in the heptane deasphalting process. Often the X-ray diffraction patterns in the literature, as well as those derived in this work, suffer from the lack of a welldefined baseline. Therefore, we have also examined the effect and magnitude of data handling, using curveresolving software, on the ultimate data. Finally results and correlations are compared with those of other material (available from the literature) showing graphitic stacking. The knowledge that may be derived from this type of analysis is of importance in any attempt to understand the interaction of asphaltenes at a molecular level, for example, to design better inhibitors for remediation or the modeling of asphaltene stability in thermodynamic or colloidal frameworks. Experimental Section Asphaltenes were recovered from crude oils using heptane as well as mixtures of toluene and heptane by mixing 30 mL of precipitant per gram of oil and mixing with ultrasonication. The suspension was then left at room temperature overnight. The asphaltenes were recovered by filtration on membrane filters and washed with the appropriate solvent mixture. The solids were recovered after dissolution in toluene and evaporation of the solvent. Solids were washed with the same solvent using ultrasonication and centrifugation to remove coprecipitated oil components. The fractions were dried under nitrogen, in a vacuum, at 70 °C. X-ray diffraction patterns were recorded on Philips powder diffractometer (PW 1729-1820-1710) with Cu KR radiation (λ ) 1.5418 Å), variable slit, and steps of 2θ ) 0.02° and 50 s/step. All solvents were HPLC grade. DBLab A/S analyzed the sample for carbon and hydrogen. (5) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33, 1587-1594. (6) Dickie, J. P.; Yen, T. F. Anal. Chem. 1982, 39, 1487-1852. (7) Shirokoff, J. W.; Siddiqui, M. N.; Ali, M. F. Energy Fuels 1997, 11, 561-565. (8) Bouhadda, V.; Bendedouch, D.; Sheu, E.; Krallafa, A. Energy Fuels 2000, 14, 845-853.
Andersen et al.
Figure 1. X-ray diffraction pattern of asphaltene 1 and 0, 17, and 30% vol of toluene in heptane.
Figure 2. X-ray diffraction pattern of asphaltene 2 and 0, 17 and 30% vol of toluene in heptane.
Figure 3. X-ray diffraction pattern of asphaltene 3 and 0, 17, and 30% vol of toluene in heptane. The four oils were stabilized crude oils from different locations worldwide.
Results and Discussion X-ray diffraction patterns of the samples (Figures 1-4) show that the samples represent broad superimposed contributions around 2θ ) 20°, 25°, and 44°. The two former peaks have been assigned to the γ-peak and the 002 peak, while the latter peak appears to be more speculative, although it is often assigned to be an indication of the diameter of the graphitic sheets in graphite. Also a peak at 2θ ) 53° could be seen in two samples, and a weak indication of a broad peak centered at approximately 2θ ) 80° is also observed. These broad features indicate that the ordering is very dispersed in these systems, but average lattice plane distances (d) can be calculated according to the Bragg equation:
λ ) 2d sin 2θ X-ray diffraction patterns of well-defined (thermal) carbon and graphitic systems usually exhibit nicely
X-ray Diffraction of Petroleum Asphaltenes
Energy & Fuels, Vol. 19, No. 6, 2005 2373
Figure 4. X-ray diffraction pattern of asphaltene 4 and 0 and 10, 17, and 30% vol of toluene in heptane.
spaced sharp peaks, indicating the planes of crystallization in purely crystallinic material. In the present case, this type of sharp response is also observed in the asphaltene 4 (20% toluene) and the asphaltene 2 (20% toluene) samples superimposed on the “asphaltene diffraction pattern” due to the apparent presence of long chain crystallized n-paraffins (wax), which have been observed to coprecipitate with the asphaltenes in some cases. This is also observed in infrared spectra at 720 cm-1, where a splitting of the methylene “wag” peak is (in some cases) observed in asphaltene samples, indicating formation of ordered crystals of long alkyl chains. The intensity of this infrared peak indicates that this type of contribution is small, assuming that the intensity and contribution of the physical effect are proportional. Using the Scherrer function, the lattice Lx parameters [x ) c (thickness) or a (diameter)] were calculated
Lx ) Kx λ/β cos θ where Kx is the shape factor (approximately 0.9 for the c-axis and approximately 1.84 in the a-axis), β is the bandwidth at half-height, and θ is the Bragg angle of the diffraction peak. The shape constant for the a-axis is for a two-dimensional lattice.9 The X-ray diffraction patterns of the asphaltene fractions examined in the present work have the same broad features as those reported in the literature. The more amorphous the material under investigation, the more peak broadening is observed in the diffraction patterns. It has been reported10 that thermal coke has approximately 24% of ordered structures and the coke showed more regular 002 peaks than normally observed in the asphaltenes. However, thermal processes produce ordered carbon and graphite, and coke is at the lower end of the thermal treatment, but ordering is expected and observed. The evaluation of the classical XRD-derived aromaticity (fa) based on the γ-peak and 002 peak area Ax ratios are found in most cases to be around 0.2:
fa ) A002/(A002 + Aγ) This is not in agreement with the aromaticity as determined by 13C nuclear magnetic resonance, which produces aromaticity values in the range of 0.4-0.6.
Table 1. Reported Magnitudes of XRD-Derived Parameters of Different Carbon and Hydrocarbon Materials material
d002 (Å)
graphitized Assam coking coal single-crystal graphite high-purity graphite ThermalGraph turbostratic film semicoke (H/C 0.75) Hassi-Messaoud asphaltenes Baxterville asphaltene Arab Berri (H/C 1.02) RT-asphaltene
3.338 3.354 3.357 3.364 3.437 3.52 3.56 3.57 3.60 3.7
Lc (Å)
La (Å)
ref
220 458 710 16.4 35 16.5 19 22.7 14.3
110 324 990 40.4 9.5 10 13.0 9.9
25 26 10 26 9 16 8 5 7 15
Furthermore, the X-ray data also conflict with data from elemental analysis and comparison of hydrogen/carbon atomic ratios. However, the XRD data could indicate that only a fraction of aromatic carbon actually is found in structures capable of stacking, as previously also discussed by Ebert.11 The 002 peak for most asphaltenes is positioned around 2Θ ) 25°, indicating an interlayer spacing of approximately 3.55 Å. Different interlayer distances have been compiled from the literature (Table 1) to put this number in context. As observed, the fully ordered graphite structure range around 3.354 Å, and amorphous carbon had an interlayer distance (d002 ) 3.55 Å) that may undergo some ordering during heat treatment as the interlayer distance decreases (d002 ) 3.44 Å). Hence, the interlayer distance for asphaltenes falls in the range of very amorphous materials of little order. Of course, it may be expected that the smaller the twodimensional extent of the molecular moieties responsible for the stacking, the less ordered the structure will appear. This intuitive result has been confirmed by others12-14 using the following relationship between interlayer distance and diameter La:
d002 ) 3.354 + 7.4/La Therefore, as La decreases, the spacing increases. This (9) Hishiyama, Y.; Nakamura, M. Carbon 1995, 22, 1399-1403. (10) Suresh Babu, V.; Seehra, M. S. Carbon 1996, 34, 1259-1265. (11) Ebert, L. B. Fuel Sci. Technol. Int. 1990, 8(5), 563-569. (12) Takahashi, H.; Kuroda, H.; Akamatu, H. Carbon 1965, 2, 432. (13) Maahs, H. G. Carbon 1969, 7, 509. (14) Kinoshita, K. Carbon Electrochemical and Physicochemical Properties; Wiley-Interscience Publication: New York, 1987.
2374
Energy & Fuels, Vol. 19, No. 6, 2005
Andersen et al.
Table 2. Crystallite Parameters of Heptane Asphaltenes Derived from Best Fit to XRD Data Using Gaussian Distributions sample
d002 (Å)
dγ (Å)
Lc (Å)
La (Å)
fa
H/C
asph 1 F asph 2 OA asph 3 MOT asph 4 Alaska
3.51 3.480 3.537 3.496
4.28 4.87 4.33 4.31
31 16 33 36
14 12 11 11
0.16 0.29 0.08 0.23-0.29a
0.94 1.16 1.05 1.14
a
Table 4. XRD Parameters for Heptane Asphaltenes and Subfractions Derived Using Lorentzian Distributions (determined with Origin) sample asph 1 F asph 2 OA
Deviation from individual curve-fitting sessions. asph 3 MOT asph 4 Alaska
a
Figure 5. Curve resolution of diffractogram of asphaltene 2 (pure heptane). Table 3. XRD Parameters (Å) for Heptane Asphaltenes and Subfractions Derived Using Gaussian Distributions (determined with Origin) sample asph 1 F asph 2 OA asph 3 MOT asph 4 Alaska
a
% tol in precipitant
d002 (Å)
dγ (Å)
Lc (Å)
La (Å)
fa
0a 17 30 0a 10 17 0a 10 17 0a 10 17 20
3.497 3.507 3.507 3.507 3.490 3.466 3.507 3.507 3.508 3.49 3.52 3.45 3.44
4.695 4.262 4.246 4.901 4.847 4.819 4.32 4.31 4.30 4.64 4.85 4.77 4.96
32.7 31.8 33.3 16.3 16.3 18.2 32.1 32.5 32.7 18.5 16.3 17.0 19.4
16.7 16.6 15.8 12.5 12.5 12.5 11.1 11.4 11.4 12.8 13.4 10.4 12.5
0.14 0.14 0.14 0.30 0.32 0.27 0.08 0.10 0.09 0.29 0.34 0.29 0.25
Heptane-asphaltene generated by modified IP143.
correlation, however, fails to work with very small disklike structures such as those obtained herein, as it predicts much larger diameters given the spacing measured. For the four heptane-asphaltenes we do, however, observe that the smaller the La the larger the interlayer spacing. Results for one fitting session on the four heptane asphaltenes using a Gaussian approach is reported in Table 2 for comparison. Figure 5 shows a common result for the curve resolution applied. Note the slight differences between data reported in Table 2 and below in Tables3 and 4 are a result of operator bias. As observed in the X-ray diffraction patterns reported herein, fitting of theoretical distributions is needed in order to get any quantitative information from the traces. In the present case, Gaussian and Lorentzian fitting procedures were used by applying the MicroCal Origin software. The three major bands γ, 002, and 100
% tol in precipitant
d002 (Å)
dγ (Å)
Lc (Å)
La (Å)
fa
00a 17 30 00a 10 17 00a 10 17 00a 10 17 20
3.51 3.52 3.53 3.49 3.52 3.49 3.545 3.537 3.541 3.60 3.58 3.56 3.55
4.566 4.587 4.654 4.83 4.7.4 4.84 4.462 4.462 4.479 4.60 4.79 4.75 4.65
23.6 22.6 21.5 16.3 14.9 16.3 22.3 23.6 22.3 13.6 13.5 15.4 16.1
14.0 13.8 11.7 15.1 14.8 14.3 15.9 16.5 16.8 15.1 13.7 13.1 13.7
0.29 0.30 0.34 0.19 0.24 0.22 0.20 0.22 0.23 0.34 0.32 0.30 0.24
Heptane-asphaltene generated by modified IP143.
positioned at approximately 2θ ) 20, 25, and 44° were used as the initial estimates. On the high 2θ side of the 100 peak, the 004 band may be observed in some cases at 2θ approximately around 53°. Also, estimates of the peak width and intensity were needed to initiate the regression. Baselines are a major issue in the X-ray diffraction patterns of asphaltene as observed in this work and elsewhere (for example, see ref 15). On the low 2θ side, the baseline is not sufficiently well defined, so one must use the high value end of the X-ray diffraction pattern and fix this as a constant baseline. This introduces the potential for a reasonable assumption, inspired guesswork, or statistical inaccuracies that may influence the outcome in favor of the operator. The change in baseline, however, had in most cases very little influence on the relative results such as the aromaticity. In all cases manual refinements were added to enhance regression coefficients. But, this compromises the change of peak position as well as width, leading to, as yet, a real but indeterminate degree of operator bias involved that does, indeed, affect the results. Tables 3 and 4 reports the different values obtained using the two approaches. In general, the Gaussian approach gives a slightly narrower d002. In the final procedure, the following steps are employed: (a) select a baseline at the lowest value in the range 2θ ≈ 70°; (b) fit the 100 band while giving initial values to other peaks; (c) fit the γ-peak and the 002 peak with initial estimates that aim at reproducing the shape of the summed peak; (d) manually optimize the peak position and width while regressing the peak; and (e) finally perform a regression analysis of each individual peak position. This procedure does not include consideration of the valley between the 002 peak and the 100 peak that, in some Gaussian cases, required an additional peak at 2θ ≈ 30° that has not been assigned to any crystallite component. Individually initiated fits following this procedure would indeed affect the final result. Lorentzian fits did estimate the valley correctly at the low 2θ side of the γ-peak, and the Gaussian approach showed much better fits than the Lorentzian approach. Lorentzian fits, however, required less manual refinement, as the midvalley point was fit so well. Hence, an almost automatic analysis could be made. However, the final results for the various asphaltenes (15) Siddiqui, M. N.; Ali, M. K.; Shirokoff, J. Fuel 2002, 81, 51-58.
X-ray Diffraction of Petroleum Asphaltenes
Figure 6. Relationship between crystallite dimension and bandwidth. Lc is the thickness of the crystallite and La the diameter in Å.
and their fractions are very different and very dependent on the shape selected. In all cases, lower aromaticity values (fa) were obtained compared to our NMR spectroscopic knowledge, pointing toward 40-50% of aromatic carbon in most hetpane-asphaltenes. To get an aromaticity of approximately 0.5, which corresponds to data produced by other methods, the data fits were found to be very poor. Asphaltene 3 does indeed look, from the appearance of the XRD profile, as apparently having a large 002 contribution. However, the γ-peak is very broad and hence the 002 contribution is very small, making the aromaticity in the range of only 0.08 when using the Gaussian shape, while the Lorentzian shape gave an aromaticity of approximately 0.2. The same is seen for asphaltene 1, having an fa of about 0.16 when fitted in one way and 0.27 using an almost similar procedure using the Gaussian distribution. On the other hand, asphaltene 2 has an apparent fa of about 0.29 (Gaussian distribution) but displays a very distorted diffraction pattern at low angles that has a strong impact on the quality of the fit. For asphaltene 4, attempts at fitting an individual peak gave an aromaticity varying from 0.23 to 0.29 for the sample with the highest hydrogen/ carbon atomic ratio. The two crucial parameters to understand crystallite size Lc and La were very sensitive to the bandwidth at half-height, as observed in Figure 6, where a theoretical relation between Lx and βx has been calculated. It is evident that Lc is sensitive to small changes in β, i.e., a change from 2θ ) 3° to 5° causes the stack height to decrease from 30 to 15Å. The sheet diameter data are less sensitive and remain almost constant around 11-15 Å, and only for the narrow 100 band will an effect be crucial. Again, we observe that the basic results are very different also in terms of trends when changing from Gaussian to Lorentzian shape. Basically, there is no rule for selecting one over the other; hence, the selection biases the results. Gaussian peak profiles have been used by others.16 The fitting procedure may indeed be oversimplified in that graphitization normally involve a formation of asymmetric 002 peaks formed by several contributions. Asphaltene Subfractions. The asphaltene fraction is composed of a range of molecules that are grouped by heptane insolubility. Therefore, it was of interest to investigate subfractions of these to understand different aspects of association and phase separation. It has been (16) Alvarez, A. G.; Martinez-Escandell, M.; Molina-Sabio, M.; Rodrı´guez-Reinoso, F. Carbon 1999, 37, 1627-1632.
Energy & Fuels, Vol. 19, No. 6, 2005 2375
established that phase transition (phase separation) or flocculation involving only a small fraction of the asphaltenes in an oil may cause immense technical problems during recovery and refining operations. Thus, subfractionation moves one a step closer to understanding various aspects of the asphaltene fraction, including the interactions of the multitude of different molecular species involved, of which only a part is aromatic, but all in reality are unkown in terms of exact hydrocarbon structure. In the present case, the subfractionation was performed using variations in heptane-toluene precipitant composition. Although amounts do vary, there is evidence that the average composition varies in a nonregular manner.17,18 As the asphaltene subfractions investigated are obtained from a distribution of molecules, it is assumed that the least soluble (sometimes referred to as “hard core”) asphaltenes are present in all samples, as they will be part of all fractions. In other words, as the toluene content of the precipitant decreases, more of the heptane-asphaltenes are added to the samples. Most of the samples exhibited an increase in the 002 peak with increasing toluene content in the heptane precipitant. This is in accordance with speculation that the mechanism of asphaltene insolubility in heptane is being caused by aromaticity and the affinity toward stacking of aromatic sheets.5 However, even though a large fraction of the lower molecular weight asphaltene constituents (therefore relatively more soluble) is removed from the less soluble asphaltenes, the diffraction patterns do not change dramatically. Changes in the fa are within the limits of experimental difference. For asphaltene 3, it was observed that only the intensity of the γ-band decreased slightly, whereas the rest of the diffraction pattern was essentially constant, even though 82% of the material had been removed by the fractionation procedure. This is in agreement with the constant values for the elemental analysis. The opposite was observed in asphaltene 4, which exhibited diffraction patterns that were dominated by the γ-band. The diffraction pattern of asphaltene 2 is also dominated by the γ-band, but the diffraction pattern differences from the heptane (bulk)-asphaltene fraction to the 20% toluene-C7 asphaltenes is irregular. For all asphaltenes examined, the 100 band was almost constant in both position and width, showing that any stack diameter remains the same through out the material. In asphaltene 4, the appearance of narrow peaks at 2θ ) 22° and 24° is, as mentioned above, related to the crystallization from the oil of paraffinic wax molecules that, in turn, is related to the room temperature separation, which may not remove entirely the heaviest wax components.19 It might be surmised that ordering occurs during solidification of asphaltenes, but there is no evidence to support the issue that the separated (solidified) structure is the structure of the asphaltene constituents in the dissolved state at the concentrations of asphalt(17) Buenrostro-Gonzalez, E.; Andersen, S. I.; Garcia-Martinez, J. A.; Lira-Galeana, C. Energy Fuels 2003, 16, 732-741. (18) Merino-Garcia, D. 2004, Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark. (19) Andersen, S. I., 1990, Ph.D. Thesis, Department of Physical Chemistry, Technical University of Denmark.
2376
Energy & Fuels, Vol. 19, No. 6, 2005
enes in the crude oils. Indeed, other techniques have shown that asphaltenes dissolved in organic solvents exhibit particulate properties i.e., as finite shapes from scattering experiments.20,21 Recently, SANS was used to study aggregation in instable oil blends without solvent addition. However, the results did not indicate whether nanostructures of aggregate asphaltenes did exist in the nonblended crude oils. Other techniques have indicated that asphaltenes may be well-dispersed in virgin crude oils with no solid precipitation related production problems recorded. To relate the discussion of stacking of asphaltenes of complex nature to well-described molecules both in the pure solid state and in solution, we have examined the literature on large planar aromatic substances. It has been well-established that packing and stacking of aromatic molecules, and other carbon-like structures, takes place in solution (seen in fluorescence studies of pyrene or perylene19) and in the solid crystal structure. For pure polynuclear aromatic structures, certain areas of the molecule will promote either stacking or gliding of layers, the latter leading to the so-called herringbone crystalline structure.22 As part of the molecules can be assigned to different degrees of promotion of the stack or the glide (not promoting graphite like stacking), one can analyze the possibility of a specific crystalline structures based on knowledge of the molecular structural architecture. In the more complex molecules, the conformational bending of molecules and bond flexibility will affect the final result. Only carbon atoms shared by three aromatic rings have a 100% stacking influence, whereas all others only are assigned 50% stacking promotion. Finally, intramolecular sterically hindered atoms have less influence and substitutions, such as hydrogens on edges, basically lead to gliding. Only a few molecular types were found to obtain graphite-like smallest axis intermolecular distances. Examples often mentioned in connection with asphaltene stacking are coronene and similar large symmetric polyaromatic compounds used to support the evidence of aromatic stacking. However, the shortest molecular distance in these crystalline structures is in the range of 4.6-5.4 Å. This is a distance much larger than we observe in our experiments, where the substrates under examination are much closer to amorphous solid carbon structures. Other types go up to as much as 8 Å in intermolecular shortest distance. The only structural type forming distances below 4 Å is the so-called flattened herringbone structures or beta-structure, where the centers of molecules are displaced relative to each other in the crystalline structure formed. Graphite-like very closely stacked particles would be difficult to form as stacking should predominate over gliding actions in order to reach this packing density.22 Also the available molecular contact area should be large. In consideration of the hypothetical molecular structures of asphaltene constituents often reported, the large contact area may be present, but at the same time (assuming average molecular structures), only 50% or less of the carbon is situated in aromatic structures, (20) Roux, J. N.; Borseta, D.; Deme´, B. Langmuir 2002, 17, 50855092. (21) Mason, T. G.; Lin, M. Y. J. Phys. Chem. 2003, 119, 565-571. (22) Desiraju, G. R.; Gavezzotti, A. Acta Crystallogr. 1989, B45, 473-482.
Andersen et al.
and only a fraction of these can be expected to be stacking promoting. In other words, this points in the direction of a large content of gliding promoters in these molecules. Adding to this the possibility of catacondensed structures dominating over peri-condensed structures in asphaltenes, as observed by infrared analysis in the 700-900 cm-1 region,23 strong stacking of asphaltene constituents should not be anticipated on this basis. Hence, the nanostructures observed and inferred by the 002 band are likely to represent only a small fraction of an asphaltene sample. Furthermore, application of subfractionation techniques using heptane-toluene mixtures did not reveal any difference in terms of structures of asphaltene of higher stacking propensity as the solvent strength was increased. Hence, the apparent amorphous state is not related to interference from lower molecular weight (higher solubility) asphaltene constituents, and the removal of these molecular species did not lead to an enhancement of the structuring based on aromaticity. This may, on the other hand, also question the fractionation power of heptane-toluene mixtures to distinguish between molecular types. Another interesting feature observed in stacked molecules of pigments is that the individual molecules in stacks are shifted such that only a fractional overlap between molecules exists. Ha¨dicke and Graser24 found that even small changes in side chain structure of perylene-based pigments could affect the stacking such that branching or alkyl chain length would affect the color observed, as well as the stacking pattern. By analogy to hypothetical asphaltene structures, it is obvious that if differences even in chain length of alkyl substitutes may affect the nanocrystals, then it could be very difficult to give a true interpretation of the average signal observed. One fact that is obvious when examining the literature on both pigments and polyaromatic systems is the mentioned partial overlap of molecules in these substances. In the original hypothetical asphaltene particle mode1, one observe the “coin” type of stacks with approximately 100% overlap. A series of regular polyaromatic compounds with low H/C was reported to give a so-called “beta” structure, mainly due to the distortion from planarity making stack formation difficult. These types of structures are mainly caused by asymmetric molecular structures similar to those seen in the asphaltene literature.22 Hence, from our knowledge of pure compounds, the coin-type stacking of asphaltenes of similar structure would not be favored. However, the special case of asphaltenes is probably to be found in the enhanced packing of very different molecular structures in terms of both size and functionality, in analogy with the closer packing of spheres of different size. This may explain the appearance of the 002 band, although as indicate above we believe this is only caused by a limited fraction of the sample. (23) Yen, T. F.; Wu, W. H.; Chilinar, G. V. Energy Sources 1984, 7, 203-235. (24) Ha¨dicke, E.; Graser, F. Acta Crystallogr. 1986, C42, 189-195. (25) Kumar, M.; Gupta, R. C. Fuel Proc. Technol. 1995, 43, 169176. (26) Adam, P. M.; Katzman, H. A.; Rellick, G. S.; Stupian, G. W. Carbon 1998, 36, 233-245.
X-ray Diffraction of Petroleum Asphaltenes
Conclusion Subfractions of asphaltenes have been obtained by extraction using heptane-toluene mixtures of increasing toluene content. Applying powder X-ray diffraction analysis to this indicates that there is little difference between different subfractions in the apparent solid aggregate structure. X-ray diffraction patterns obtained for petroleum asphaltenes do indicate that crystallinic planes are present. However, due to dispersions and a low degree of crystallinity, the peak broadening is significant. This could indicate that only a fraction of the asphaltenes exhibits the aromatic stacking behavior known from coke and graphite. The aromaticity obtained for all samples investigated is in the range of 0.2,which is not in accordance with our knowledge from nuclear magnetic resonance and elemental analysis nor with the notion of aromatic stacking as being the driving force in asphaltene deposition. When examining fractions, broadening is not diminished and no dramatic change is observed in the pattern supporting the idea that the pattern is indicative of very diffuse local ordering. Probably, this is also highly affected by the vast number of molecules present such that important interactions are difficult to detect due to the masking by other molecular types. In our opinion this indicates that in molecule modeling or interpretation of data based on a notion of a particular “asphaltene structure” one should be very careful in oversimplifying this very complex system. Hence, knowing the apparent high degree of hydrogen-
Energy & Fuels, Vol. 19, No. 6, 2005 2377
bonding capacity of asphaltenes reported in the literature, stacking is only one side of the issue, and more complex association schemes must be taken into account. Curve fitting in the analysis of these very dispersed diffractions patterns is a great improvement over manual fitting procedures, but the fitting procedures (manual or instrumental) especially for asphaltenes with the common distorted baseline features are subject to operator error or bias and to the type of peak distribution function employed. This, in the long run, affects the end results by a substantial margin, often by as much as 50%. Therefore, such data derived by X-ray diffraction procedures should be treated with caution and may not be accepted without question and without remembering that a substantial error may be linked to the derived parameters. On the basis of the analysis of the present data and the data reported in the literature, we find that significant evidence exists which indicates that only a fraction of the interactions among asphaltenic molecules may take place through stacking of aromatic sheets. This is indeed important in developing, for example, tailormade inhibitors for asphaltenes, as well as in the modeling of asphaltenes on a molecular scale. Acknowledgment. J.G.S. thanks DONGs Jubilæumslegat for financial travel support. S.I.A. thanks STVF for financial support under the talent project grant. EF050039V
