Molecular Structure Relationships of Asphaltenes in Polar

732

Energy & Fuels 2002, 16, 732-741

Solubility/Molecular Structure Relationships of Asphaltenes in Polar and Nonpolar Media E. Buenrostro-Gonzalez,† S. I. Andersen,‡ J. A. Garcia-Martinez,† and C. Lira-Galeana*,† Thermodynamics Research Laboratory, Branch of Molecular Engineering, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, C.P. 07730. Mexico, D.F., Me´ xico, and Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark Received September 17, 2001

Asphaltenes separated from a Maya type of crude oil were suspended in toluene and later fractionated by solubility in a polar (acetone) and a nonpolar (n-heptane) precipitating solvent. The two sets of derived fractions were characterized using size exclusion chromatography (SEC), elemental analysis, Fourier transform infrared spectroscopy (FTIR), and synchronous fluorescence spectroscopy and proton nuclear magnetic resonance (1H NMR) spectroscopy. The results show that the acetone-precipitated asphaltene fractions have larger structural differences compared with those of n-heptane. The average size of the aromatic and aliphatic-substitutions regions of each fraction was also found to correlate with asphaltene solubility in such a way that the smaller the aromatic region and the larger the aliphatic substitutions, the greater the solubility. These correlations may provide further explanations as to the role of structural properties on the solubility of asphaltenes in polar and nonpolar media.

Introduction Asphaltenes belong to a complex class of compounds with a significant impact on the economy of the oil industry. Asphaltene precipitation and further deposition may cause losses in oil production and raise maintenance costs,1-4 provoke catalyst deactivation,5 and the fouling of lines or processing equipment,6,7 just to mention a few problems. To understand precipitation, the idea of asphaltene molecular structure and its relation to the observed macroscopic behavior of asphaltene-containing fluids has been the subject of valuable research work aimed at providing a connection between (average) asphaltene structures and the bulk properties of these fluids.8-16 * To whom correspondence should be addressed. E-mail: clira@ www.imp.mx. † Thermodynamics Research Laboratory. ‡ Department of Chemical Engineering. (1) Leontaritis, K. J.; Mansoori, G. A. Int. J. Pet. Sci. Eng. 1989, 1, 229. (2) Shields, D. Offshore 2000, Sept, 84. (3) Pan, H.; Firoozabadi, A. AIChE J. 2000, 46, 416. (4) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden, J. Energy Fuels 2001, 15, 979. (5) Ware, J. C; Dolbear, G. E. Fuel Sci. Technol. Int. 1990, 8, 575. (6) Wiehe, I. A.; Kennedy, R. J.; Dickakian, G. Energy Fuels 2001, 15, 1057. (7) Wiehe, I. A.; Kennedy, R. J. Energy Fuels 2000, 14, 60. (8) Yen, T. F., Chilingarian, G. V., Eds. Asphaltenes and Asphalsts, 1 Developments in Petroluem Science, 40 A; Elsevier Science: New York, 1994. (9) Andersen, S. I. Fuel Sci. Technol Int. 1994, 12, 1551. (10) Andersen, S. I. Fuel Sci. Technol Int. 1995, 13, 579. (11) Sheu, E. Y., Mullins, O. C. Eds. Asphaltenes: Fundamentals and Applications; Plenum Pub. Co.: New York, 1995. (12) Yen, T. F.; Chilingarian, G. V., Eds. Asphaltenes and Asphalsts, 2 Developments in Petroluem Science, 40 B; Elsevier Science: New York, 2000.

Polydispersed after precipitation,8,17-19 asphaltenes are highly sensitive to the extent of aggregation at which they are obtained and studied. This fact provides limitations to the average-structure approach, since only localized structures are considered.20-23 An alternative way of studying asphaltenes may involve the fractionation of these materials into a number of “discrete fractions,” from which extended characterization experiments could provide further information as to the role of aggregation on solubility and phase separation. Although analytical procedures for fractionating asphaltenes into a number of subfractions have been reported in the literature (e.g., by chromatographic separations,24-26 dialysis fractionation,27 liquid-liquid extraction,28 or precipitation/dissolution experiments in mixed solvent systems9,29), the relation between sub(13) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6. (14) Groenzin, H.; Mullins, O. Energy Fuels 2000, 14 , 667. (15) Buenrostro-Gonzalez, E.; Espinosa-Penã, M.; Andersen, S. I.; Lira-Galeana, C. Pet. Sci. Technol. 2001, 19, 299. (16) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972. (17) Bunger, J. W., Li, M. C., Eds. Chemistry of Asphaltenes; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1984. (18) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1998; Chapter 10, p 419. (19) Domin, M.; Herod, A.; Kandiyoti, R.; Larsen, J. W.; Lazaro, M. J.; Li, S.; Rahimi, P. Energy Fuels 1999, 13, 552. (20) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. Scanning Microsc. 1994, 8, 463. (21) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225. (22) Peng, P.; Moralez-Izquierdo, A.; Hogg, A.; Strausz, O. P. Energy Fuels 1997, 11, 1171. (23) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M.; Kowalewski, I.; Behar, F. Energy Fuels 1999, 13, 228.

10.1021/ef0102317 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/21/2002

Asphaltenes in Polar and Nonpolar Media

Energy & Fuels, Vol. 16, No. 3, 2002 733

Figure 1. Procedure for asphaltene fractionation. Table 1. Properties of the Maya Crude Oil API density (dead oil) SARA analysis (wt %) saturates aromatics resins asphaltenes (from n-C7)

Table 2. Identification of the Precipitated Fractions

19 18.5 31.9 37.9 11.7

fraction structural data and solubility in solvents of varying polarity has not yet been addressed or established. In this research, asphaltenes precipitated from a Maya-type crude oil are characterized as a collection of subfractions. A fractionation procedure based on reprecipitating asphaltenes from toluene solutions is employed. n-Heptane and acetone are used as reprecipitating solvents. The aim of this research is to study the correlation between the solubility and the molecular structure of the subfractions in solvents of varying polarity. As will be shown, the differences between the fractions of each solvent-precipitant pair have allowed us to gain an insight into the chemical polydispersity of asphaltenes and to suggest precipitation mechanisms based on the molecular characteristics of the fractions and the nature of the host media. Experimental Section Asphaltene Separation from Crude Oil. The total asphaltenes from a Maya-type crude oil (Table 1 shows the properties of this crude) were first precipitated with n-heptane using a modified IP-143 method which consists of a redissolution of the n-heptane-precipitated asphaltenes in a solution of methylene chloride (10 wt %). A subsequent reprecipitation step with n-heptane is then performed, following an exhaustive asphaltene washing process using excess n-heptane. At each of the washing steps, a 20-min ultrasonic shaking of the asphaltene/n-heptane suspension is achieved followed by a centrifugation process which separates the supernatant (e.g. n-heptane + n-heptane solubles) from the n-heptane insolubles. This shaking-centrifugation cycle was repeated until the supernatant became transparent with a pale yellow color that did not change after the various washing cycles. Asphaltene Fractionation. The procedure is shown schematically in Figure 1. The asphaltene separated from crude oil was first dissolved in toluene at a concentration of 2.3 wt (24) Selucky, M. L.; Kim, S. S.; Sinner, F.; Strausz, O. P. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981; Chapter 6, p 83. (25) Jacobs, F. S.; Filby, R. H. Fuel 1983, 62, 1186. (26) Cyr, N.; McIntyre, D. D.; Toth, G.; Strausz, O. P. Fuel 1987, 66, 1709. (27) Acevedo, S.; Escobar, G.; Ranaudo, M. A.; Pinãte, J.; Amorıń, A. Energy Fuels 1997, 11, 774. (28) Andersen, S. I. Pet. Sci Technol. 1997, 15, 185. (29) Kaminski, T. J.; Fogler, S. H.; Wolf, N.; Wattana, P.; Mairal, A. Energy Fuels 2000, 14, 25.

n-heptane (wt %) toluene (wt %)

TE4

TE5

TE6

TE7

TE8

TE9

TESOL in solution

40

50

60

70

80

90

90

60

50

40

30

20

10

10

TASOL TA2 TA3 TA4 TA5 TA6 TA7 TA8 TA9 in solution acetone (wt %) toluene (wt %)

20

30

40

50

60

70

80

90

90

80

70

60

50

40

30

20

10

10

%. Sufficient titrant (e.g., n-heptane or acetone) was added to precipitate the first asphaltene fraction. The system toluene/ precipitant/asphaltene was left in the dark for 24 h. The precipitate was then collected by pouring off the supernatant after centrifugation for 30 min at 3000 rpm. The supernatant was collected in a clean centrifuge tube and additional precipitant was then added to it. This process of adding precipitant to the supernatant and then collecting the asphaltenes by centrifugation was repeated until the precipitant concentration reached 90 wt % in the toluene-precipitant mixture. n-Heptane and acetone were chosen as precipitants since (a) asphaltenes are insoluble in both solvents,30,31 (b) these solvents are easy to eliminate from the precipitated fractions, and (c) they represent two kinds of interaction, namely, nonpolar and weakly polar. The first asphaltenes to precipitate from both the toluene/ acetone and the toluene/n-heptane systems did so at a toluene/ precipitant weight ratio of 80/20; however, in the case of the toluene/n-heptane fractionation, the amount of insoluble material precipitated with 20 and 30 wt % n-heptane was so small that it was not possible to obtain its characterization. The separated fractions were labeled to identify the solvent/ precipitant pair and the precipitant weight fraction (T stands for “toluene”, E stands for “n-heptane,” and A stands for “acetone”, see Table 2). In the case of the TASOL and TESOL fractions, they correspond to the asphaltene molecules that remained in solution with 90% of acetone and heptane, respectively. Size Exclusion Chromatography (SEC). The SEC experiments were conducted on a mixed polystyrene/polydivinylbenzene Waters styragel HR-4E column with a molecular weight distribution capacity of 50-100000 g/mol, using tetrahydrofurane (THF) as a mobile phase. All the injected samples had a concentration of 5 wt % in THF. The flow rate was fixed at 1 mL/min, at a temperature of 30 °C. Relative average molecular weights were calculated using polystyrene standards with molecular weights between 400 and 37000 g/mol. The eluent was analyzed in a Waters 996 diode array system with a UV-vis detection range between 200 and 700 nm. (30) Speight, J. G. Studies on Bitumen Fractionation-(A) Fractionation by a Cryoscopic Method; (B) Effect of Solvent Type on Asphaltene Solubility; Information Series 84; Alberta Research Council: Edmonton, 1979. (31) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149.

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Energy & Fuels, Vol. 16, No. 3, 2002

Buenrostro-Gonzalez et al.

Fourier Transform Infrared Spectroscopy (FTIR). The instrument used was a Nicolet 710 spectrophotometer operated at a setting of 32 scans at a resolution of 4 cm-1. The samples were prepared with the film-spreading technique. Proton Nuclear Magnetic Resonance (1H NMR). These experiments were performed on a JEOL Eclipse-300 spectrometer operating at 300 MHz. Tetramethylsilane (TMS) was used as a reference for zero displacement and CDCl3 as a solvent. The sample concentrations were between 20 and 10 mg/mL, depending on the amount of each fraction. Synchronous Fluorescence Spectroscopy (SF). The samples were analyzed with the aid of a Perkin-Elmer LS40-B spectrometer using a spectral range from 300 to 575 nm, with a constant wavelength difference of 20 nm. To avoid aggregation, the fractions were diluted in anhydrous spectroscopicgrade toluene at concentrations of 5 mg/L. Figure 2. SEC-UV-vis chromatograms of TE fractions.

Results and Discussion As shown below, there are some remarkable differences between the two series of solvent systems, which may be explained by the chemical nature of the species and their interactions with the medium. SEC of the Separated Asphaltene Fractions. The polarity of the asphaltenes, their tendency to association and the fact that they are a mixture of different species, are factors that strongly affect the results of this technique.9,19,32-34 The polar molecules of the asphaltene fraction tend to adsorb onto the chromatographic column, thus causing that part of the sample to elute later than it would normally and the underestimation of the molecular weight averages. The association of asphaltene molecules to form aggregates causes that part of the sample to elute earlier than otherwise and the overestimation of the molecular weights. The presence of a broad variety of molecular structures in the asphaltene fractions makes it difficult to find a calibration standard which can reflect their heterogeneity. Although conventional polystyrene standards were used in this research to generate the calibration curve, it is clear that due to their flexible and linear structure they have limitations to represent the asphaltene fractions. As a consequence of all the above-mentioned, the number and weight averages calculated by SEC do not necessarily correspond to physical molecular or aggregate weights. Hence, the SEC results are primarily applicable when a trend is needed, but their absolute values have to be carefully considered in. In this research SEC is used as a tool to compare the trends among the separate fractions in order to get an indication of how the nature of the medium affects the extent of the association of the asphaltene “components” of the separated fractions. The averages shown in Table 3 give only an approximate idea of the molecular weight of the asphaltenes. The SEC-UV-vis chromatograms at 410 nm of the asphaltene fractions, series TA and TE, are shown in Figures 2 and 3, respectively. The fractions precipitated with acetone show differences in their retention times whereas for the fractions precipitated with n-heptane, all curves are grouped under the highest absorption chromatogram showing a minimum difference among (32) Andersen, S. I. J. Liq Chromatogr. 1994, 15, 4065. (33) Altgelt, K.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker: New York, 1994. (34) Nali, M.; Manclossi, A. Fuel Sci.Technol. Int. 1995, 13, 1251.

Figure 3. SEC-UV-vis chromatograms of TA fractions. Table 3. Average Molecular Weights from SEC-UV-Vis fractions

Mn

Mw

polydispersity

TE4 TE5 TE6 TE7 TE8 TE9 TESOL TA2 TA3 TA4 TA5 TA6 TA7 TA8 TA9 TASOL

1016 786 824 834 1004 959 861 781 793 913 1159 1427 1157 899 883 335

4359 3180 3166 3171 3400 3057 2677 3185 3539 4174 4361 5003 2739 1955 1802 706

4.3 4.0 3.8 3.8 3.4 3.2 3.1 4.1 4.5 4.6 3.8 3.5 2.4 2.2 2.0 2.1

their retention times. Since retention times are related to the molecular size (e.g. molecular weight) distribution of the species in the sample, this behavior shows a differentiation in molecular size distribution among the fractions precipitated with acetone. Such differentiation was not observed by the fractions precipitated with n-heptane. By definition of the different averages obtained from molecular weight distributions35 (see the corresponding formulas in the appendix), the number average molecular weight (Mn) is affected mainly by the low molecular weights of the sample, whereas the weight average (35) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997; Chapter 1.


molecular weight (Mw) is affected mainly by the high molecular weights, because it not only depends on the number of species (molecules or aggregates) of each weight, as does Mn, but also of the weight of each one of the species. Thus, the Mn of each fraction is affected mainly by the molecular weights of the nonassociate molecules (monomers), and the Mw is affected mainly by the aggregate weights and the molecular weights of the biggest monomers. Then the Mw and the polydispersity (e.g., Mw/Mn) could be used as indices to get an idea of the extent of association so that the aggregation tendency of the different fractions can be compared. It is known that the calculated molecular weight averages based on the chromatograms obtained from UV-vis responses (like those shown in the Figures 2 and 3) depend on the detector wavelength setting.9 To overcome this difficulty in some way, the values of the Mw, Mn, and polydispersity of the TA and TE fractions series shown in Table 3 had to be obtained by averaging the values calculated from the chromatograms at 310, 410, 500, and 600 nm. From Table 3, it can be seen that Mn does not change significantly among the different fractions (20% variation between the lowest and highest values); Mw varies in a slightly broader range (39% between the lowest and highest values). Mw shows a clear tendency to diminish with increasing solubility. This is an indication that the aggregation (the size and number of aggregates) tends to diminish as the solubility of the fractions increases. In the case of the TA-fraction series, Mn and Mw vary notably with the solubility (a 79% variation of Mn between its highest and lowest values is observed, while Mw varies 86% within these limits). These results indicate a pronounced effect of the polarity of the medium on the tendency of association of each fraction. Mw shows a clearer tendency in relation to solubility than does Mn. According to the values of Table 3, in the TA2-TA6 fractions, the tendency to association is important, but the presence of big molecules is very possible as well. The “tailing effect” due to the adsorption of the asphaltene molecules onto the chromatographic column is notable in the chromatograms of the more insoluble fractions, TA2, TA3, and TA4 (Figure 3), causing the low values of Mn in these fractions. “Tailing” diminishes as the solubility of the fractions increases: this could be the cause of the gradual increment in Mn and Mw observed from TA2 to TA6. In the TA7 to TASOL fractions the presence of high weight species (molecules or aggregates) tends to decrease drastically; in these fractions the molecules become smaller with low or no tendency to associate as the solubility increases. The chromatogram of the TASOL fraction shows a bimodal distribution which is due to a very high content of low molecular weight species, among which are the metalloporphyrins. These results allow us to see that the molecular weight of the asphaltene fractions cannot always be directly related to the solubility, making it necessary to consider the particular details of the molecular weight determination technique employed and the different aspects of the molecular structure of asphaltenes (polarity, aromaticity, and aliphaticity) and their interactions,


Figure 4. Peak at 410 nm in the UV-vis spectrum of TASOL fraction.

so that the relationship between the solubility, the molecular size and the extent of the association can be explained. Although polydispersity varies very little in the TE series with a tendency to decrease as the solubility increases, values are maintained over 3; in fact, all the fractions have an important degree of association. In the TA series, polydispersity varies in a wider range with values between 4.6 and 2. This has two different tendencies: in the more insoluble fractions (between TA2 and TA6), it increases with solubility; this is probably due to an increment in the number and size of aggregates that coexist with the nonassociate molecules, but in the fractions between TA6 and TASOL polydispersity drops, thus reflecting a decrease in the association tendency of the molecules of these fractions. In general, it is clear that n-heptane is not able to separate asphaltenes in fractions that have important differences in their tendency to associate. On the other hand, the precipitation with acetone, which implies an increment in the polarity of the medium, has a strong effect on the distribution of the molecules among the different fractions, causing remarkable differences in molecular weight and polydispersity. Determining Relative Metalloporphyrin Content by UV-Vis. The metalloporphyrins which are concentrated in the asphaltene fraction36 absorb energy in a region of the UV spectrum known as the Soret band around 410 nm.9,37,38 In the chromatograms of the fractions that remain in solution (e.g., TASOL and TESOL, Figures 2 and 3) around an elution time of 10.55 min there is a “bump” which could possibly correspond to the Va- and Ni-metalloporphyrin molecules that are concentrated in the more soluble fractions. In the UV spectrum corresponding to an elution time of 10.5 ( 0.5 min there is a peak centered at 410 nm (Figure 4) appearing for some of the fractions whose area might be considered proportional to the metalloporphyrins concentration in the fraction.38 The results of the integration of the Soret band at that elution time for all fractions (Figure 5) indicate a clear concentration of this kind of molecules in the more soluble fractions TASOL and TESOL, but especially in the former. The (36) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1998; Chapter 6, p 325. (37) Yokota, T.; Scriven, F.; Montgomery, D. S.; Strausz, O. P. Fuel 1986, 65, 1142. (38) Sugihara J. M.; Bean R. M. J. Chem. Eng. Data 1962, 7, 269.

736



Figure 5. Area of the Peak at 410 nm in the UV-vis Spectra for all fractions.

Figure 7. Heteroatom/(C+H) ratios for TA fractions. Table 5. Assignments of the IR Spectrum Bands functional groups OH (stretch, free) Iintermolecular H bond(-OH, NH-) C-H (stretch, CdC and aromatics) CH3 (asym and symm. stretch) CH2 (asym and symm stretch) ester (R-COO-R, Ar-COO-Ar) ketona (CdO stretch) aldehydes (CdO stretch) acid (CdO stretch) R-COOH or Ar-COOH amide (CdO stretch)

Figure 6. Heteroatom/(C+H) ratios for TE fractions. Table 4. Elemental Composition (wt %) of the TE and TA Fractions TE4

TE5

TE6

TE7

TE8

TE9

TESOL

C H N O S

81.5 7.3 1.2 2.1 7.3

81.5 7.0 1.4 3.1 7.0

81.6 7.5 1.3 1.9 7.3

81.7 7.5 1.2 1.8 7.4

81.7 7.7 1.1 1.6 7.4

81.7 7.7 1.1 1.8 7.3

81.5 7.6 1.9 1.8 7.2

TA3

TA4

TA5

TA6

TA7

TA9

TASOL

C H N O S

82.5 8.1 1.3 1.6 6.5

80.6 7.5 1.4 3.6 6.9

79.2 7.9 1.3 4.8 6.8

78.7 7.9 1.2 5.3 6.9

82.0 8.2 1.3 1.0 7.5

81.2 7.7 1.0 n.d.a n.d.

82.2 7.6 2.6 n.d. n.d.

a

n.d. ) not determined.

metalloporphyrins of the total asphaltene studied do not precipitate with acetone. These results confirm that most of the metalloporphyrins are not chemically bound to the asphaltene molecule but coprecipitate due to polarity, as reported elsewhere. 9 Monitoring Polarity by Elemental Analysis. It is possible to know how heteroatoms are distributed among the fractions by using elemental analysis results. The elemental composition of each fraction is shown in Table 4. The heteroatom/hydrocarbon ratios N/CH, S/CH, and O/CH (Figures 6 and 7) indicate how the heteroatom proportion varies among the fractions; this may be roughly correlated with the content of polar functional groups, leading to the polarity of the host hydrocarbon

auinones (OC-cicle-CO) CdC conjugated CdC aromatic CH3 o CH2 (bending) CH3 (bending) ester (C-O stretch) ether (C-O-C) aliphatic aromatic sulfoxide aromatic C-H (deformation) 1 adjacent H 2 adjacent H 3 adjacent H 4 adjacent H 5 adjacent H alkyl chains (CH2)n; n >) 4 a

absorption bands (cm-1)a 3650-3500 3500-3200 3050-3000 2950 and 2872 2920 and 2850 1750-1730 1735-1705 1740-1730 1760 or 1720 Free: 1690-1650. Associative: 1650-1640. 1650 1650 and 1600 1600 1460 1379 ∼1300 and ∼1050 1150-1070 1275-1200 1034 (1060-970) 900-860 860-800 810-750 770-735 770-730 or 710-690 731

From refs 39-41.

molecules. In the TA series, the O/CH ratio changes widely, reaching its maximum in the TA6 fraction, whereas the S/CH and N/OH ratios change much less between fractions; they rather tend to increase as the solubility in the acetone rich medium increases. The N/CH ratio shows a notable increment in the TASOL fraction, which could be due to the concentration of metalloporphyrin molecules in this fraction. According to these results, the TA6 fraction is the highest polar fraction of this series, followed by the fraction that remains in solution with 90 wt % acetone, TASOL. On the other hand, the heteroatom ratios for the TE fractions change slightly with the solubility of the fractions, except for the O/CH ratio, which has a notable increment in the low soluble fraction TE5. For the remaining fractions the ratios have uniform values. Polar Structures in the Fractions by FTIR Spectroscopy. In Table 5 a summary of the assignments of the IR spectrum used in this paper is presented.



Figure 8. The FTIR spectra of TE fractions.

(TESOL and TASOL) show a smooth and broad peak at around 3250 cm-1, indicating the presence of molecules with groups and N-H and O-H groups in these fractions. In the region between 1800 and 1640 cm-1, which corresponds to carbonyl groups, such as carboxylic acids, ketones, aldehydes, esters, and amides, the fractions TE5 and TESOL show a well-defined peak around 1732 cm-1, which could correspond to ester groups, R-COO-R or Ar-CO-O-Ar. In the more soluble fractions, TE8 to TESOL, there appears a small peak around 1650 cm-1 probably due to some class of amide in its associate form. On the other hand, in the TA series, the fractions TA2, TA5 and TA6 have a peak at 1724 cm-1, small in TA2 and TA5 but very big in TA6, which could correspond to an aldehyde group or to a nonassociated form of carboxylic acid (Ar-COOH or R-COOH) or, considering a light shift of the spectra, to a ketone group. The more soluble fraction, TASOL, presents an important peak at 1648 cm-1 next to the aromatic CdC peak at 1600 cm-1. It is difficult to give a specific assignation to the 1648 cm-1 peak because of the extension and overlap of several bands in the region between 1650 and 1600 cm-1of the IR spectrum. The absorption at 1648 cm-1 may be assigned to CdO stretch of quinones bridged to acidic hydroxyl, CdO stretch of amides or CdC from conjugate double bonds systems. The identification of this 1648 cm-1 peak requires additional research. In the region between 1300 and 900 cm-1, the fractions TE5 and TESOL show peaks at 1287 and 1259 cm-1, respectively, which probably correspond to the vibration stretching of the C-O bond of the ester group whose carbonyl part was identified in the corresponding region; however, it is also possible that this peak is related to an ether group (C-O-C) or aromatic amines one (Ar-N). The TE5 fraction has a peak at 1125 cm-1 that would correspond to a sulfone function (R-SO2R′), but these kinds of structures are present in asphaltenes in very low concentrations.43 In the TA fractions the fraction TA2 shows a peak at 1137 cm-1 which is difficult to identify because it could correspond to the C-O bond stretching in the ester group, but the corresponding carbonyl peak is not present. Another possibility is that the peak could correspond to a phenol group (Ar-OH) or even to an ether group; however, the high intensity of this peak makes it difficult for it to be related to any of these functional groups; it may correspond to an inorganic impurity in the asphaltene fraction. A peak at 1301 cm-1 appearing in the TA6 fraction could be caused by the intermolecular association of carboxylic acid groups forming dimers, which is in accord with the peak corresponding to “free” carboxylic acid at 1724 cm-1. All the fractions in the two series show a peak around 1032 ( 5 cm-1 that corresponds to a sulfoxide group. This peak is better defined in the fractions TESOL, TA6, and TASOL.

Figure 9. The FTIR spectra of TA fractions.

The FTIR spectra of both fraction series (Figures 8 and 9) are very similar to the spectra of other asphaltenes in the literature.21,40,42 TA and TE fractions show a very low-band intensity in the range between 3600 and 3100 cm-1; however, the more soluble fractions

(39) Nakanishi, K. Infrared Absorption Spectroscopy. Practical; Nankodo Company Ltd.: Tokyo, 1972. (40) Christy, A. A.; Dahl B.; Kvalheim, O. M. Fuel 1989, 68, 430. (41) Rouxhet, P. G.; Robin, P. L.; Nicaise, G. In Kerogen; Durand, B. ed.; Technip: Paris, 1980; Chapter 6. (42) Yen, T. F.; Erdeman, J. G. Am. Chem. Soc., Div. Pet. Chem., Prepr. 1962, 7, 5. (43) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53.

738



Figure 10. SF spectra of TE fractions.

Figure 11. SF spectra of TA fractions. Table 6. Highest Peaks of the SF Spectra of the Asphaltene Fractions fraction TE4 TE5 TE6 TE7 TE8 TE9 TESOL TA2 TA3 TA4 TA5 TA6 TA7 TA8 TA9 TASOL a

highest peaks (nm) 397 392 412 401 398 389

419 410

398

416 418 418

423a 421 420 425 422 425 421

435 435 423 420 425 423 422

417 388

397

450 450

437 438 431 435

441 440

436 438 438 437

449 445 445

457 447

442

410

Boldface indicates maximun peak.

SF of the Fractions. Results from SF are used to gain insight concerning the distribution of polyaromatic structures among the fractions separated by the two different solvent mixtures. Figures 10 and 11 show the SF spectra of the TE and TA fractions series, and a summary of the maximum peak values of the spectrums for both fractions series is present in Table 6. Considering the fractions precipitated with n-heptane, the highest peaks (in bold type) of all fractions vary within a very narrow range, i.e., between 419 and 425

nm, pointing to a very uniform distribution of the aromatic structures among the TE fractions. The spectra shift slightly to shorter wavelengths following the concentration of the precipitant (Table 6), suggesting a decrease in the content of big aromatic systems and an increment in the content of small aromatic molecules as their solubility increases. In the case of the fractions precipitated with acetone, the wavelengths of the highest peaks of the SF spectra vary in a wider range, i.e., between 416 and 446 nm, which suggests a more varied distribution of aromatic structures among these fractions than in the case of the fractions precipitated with n-heptane. The spectra show an initial shift with regard to solubility (from short to long wavelengths between the fractions TA2 and TA6), which could be caused mainly by an increment in the polyaromatic structure size and additionally by the increment of the heteroatomic substitutions in these fractions (as was shown by the elemental analysis). These kinds of substitutions in the chromophore part of a molecule, especially those due to oxygen atoms, cause a decrease of the emission frequency44 and therefore a shift to longer wavelengths. On the other hand, the spectra of the more soluble fractions, TA6 to TASOL, show a shift from long to short wavelengths, which suggests an important decrease of the content of big aromatic systems. If Table 3 is compared with Table 6 and Figures 10 and 11, it can be observed that the intensity, at a constant weight concentration, and the shift of the highest peaks of the whole fluorescence spectra have a correlation with Mn, i.e., the intensity decreases and the peaks shift toward longer wavelengths with increasing Mn. A similar behavior of the fluorescence spectra of asphaltene fractions has been reported previously by Yokota et al. 37 Average Structural Parameters from 1H NMR. This technique has been used here to study the effect of the relationship between the aromatic and the aliphatic aspects of the asphaltene structure on the solubility of the different fractions by calculating different average structural parameters. The choice of the different integration domains for 1H NMR spectrum has been discussed at length in the literature.21,45-48 In this paper, the 1H NMR spectrum was divided into the following regions: Hγ (γ+ CH3), 0.5-1.0 ppm; Hβ (β+ CH2): HR, 1.0-1.6 ppm, HN 1.6-2.0 ppm; HR (R CH2), 2.0-4.0 ppm; and Har (CH aromatic), 6.0-9.0 ppm. Here, HR and HN refers to β-protons in aliphatic chains and naphthenic rings, respectively. The molecular parameters (Table 7) were calculated using the formulas proposed by Speight46 (Table 6) and the following information: elemental composition (Table 4), molecular weight average number (Mn, Table 3), and the integration of the corresponding regions of the 1H NMR spectra. The shape parameter,46 Φ, gives an estimate of the average shape of the aromatic sheets; low values indicate a tendency to small and less condensed struc(44) Guilbault, G. G. Practical Fluorescence; Marcel Dekker: New York, 1973. (45) Yen, T. F.; Erdeman, J. G. Am. Chem. Soc., Div. Pet. Chem., Prepr. 1962, 7, 99. (46) Speight, J. G. Fuel 1970, 49, 76. (47) Dickinson, E. M. Fuel 1980, 59, 290. (48) Gillet, S.; Rubini, P.; Delpuech, J. J.; Escalier, J. C.; Valentin, P. Fuel 1981, 60, 221.



Table 7. Relationships for the Calculation of the Average Molecular Parametersa CS ) HT(H1/2 + H2/2 + H3/3) CA ) CT - CS CP ) HT(H5 + H1/2) CI ) CA - CAP CSAr ) HTH1/2 CN ) CS - (CSAr + CR) CR ) HT(H4/2 + H3/3)

Table 9. Shape Factors, Φ, of Condensed Aromatic Compounds

fa ) (CT - CS)/CT no. of aromatic rings ) (CI + 2)/2 n ) (CS - CN)/CSAr σ ) CSAr/CAP Φ ) CP/CA

a H ) Total integration area of the 1H NMR spectrum. H ) 1 HR/H, H2 ) Hβ/H, H3 ) Hγ/H, H4 ) HR/H, H5 ) Har/H. CT and HT ) total carbon and hydrogen atoms (from Mn and elemental analysis). CS ) total saturated carbons. CA ) total aromatic carbons. CP ) Peripheral carbon in a condensed aromatic sheet. CI ) Internal carbon atoms in a condensed aromatic sheet. CSAr ) Total saturated carbon atoms R to an aromatic ring. CN ) total naphthenic carbon atoms per molecule. CR ) Total paraffinic carbon atoms in locations other than R to an aromatic ring. fa ) aromaticity factor. n ) average number of carbon atoms per alkyl side chain. σ ) degree of substitution of the aromatic sheet. φ ) shape factor of the aromatic sheet. Relationships taken from ref 46.

Table 8. Average Molecular Parameters of the Asphaltene Fractions fa φ σ n n/no. of aromatic rings CR/no. of aromatic rings

TE4

TE5

TE6

TE7

TE8

TE9

TESOL

0.56 0.38 0.38 5 0.36

0.57 0.35 0.33 6 0.54

0.55 0.36 0.29 7 0.63

0.55 0.37 0.30 7 0.63

0.53 0.38 0.35 6 0.49

0.54 0.37 0.27 7 0.59

0.48 0.40 0.38 6 0.67

2.01

1.91

2.04

2.17

2.32

2.07

2.85

TA3 TA4 TA5 TA6 TA7 TA8 TA9 TASOL fa Φ σ n n/no. of aromatic rings CR/no. of aromatic rings

0.57 0.31 0.26 9 0.75

0.54 0.36 0.29 7 0.62

0.51 0.40 0.34 6 0.50

0.50 0.42 0.43 5 0.34

0.51 0.44 0.33 6 0.48

0.52 0.46 0.33 5 0.56

0.54 0.43 0.27 6 0.62

0.55 0.49 0.39 4 0.91

1.83 2.15 2.71 2.77 2.80 2.67 2.31

2.18

tures and high values indicate a tendency to big and highly condensed aromatic structures. To illustrate the relationship between the value of this parameter and the shape of possible ring structures of the aromatics more clearly, the shape parameters for a series of standard condensed aromatics are presented in Table 9. For the fractions precipitated with n-heptane, this parameter varies very little from one fraction to another with no clear tendency with regard to solubility, thus indicating an almost uniform distribution in size and shape of the polyaromatic structures among these fractions. This is in agreement with the results of the SF; however, in the case of fractions precipitated with acetone, the variation of the shape parameter is clear, increasing as it does following the solubility “order” of the fractions, indicating smaller and less condensed aromatic structures in those which are more soluble. These changes on the level of condensation of the aromatic region and the shift of the fluorescence spectra of the TA fractions, as was commented before, demonstrate a strong effect of the change of polarity of the

Figure 12. Total number of carbon atoms in aliphatic chains versus aromaticity for TE fractions.

solvent medium on the distribution of the polyaromatic structures among the fractions with different solubility. Figure 12 shows the correlation of the total number of carbon atoms in aliphatic chains vs the aromaticity factor for the fractions precipitated with n-heptane. There is a fairly defined tendency where more aliphatic carbon atoms together with less aromaticity means more solubility. Figure 13 shows the case of the fractions precipitated with acetone; here there are two tendencies: from TA3 to TA6 more aliphatic carbon atoms together with less aromaticity means more solubility, but from TA6 to TASOL the solubility follows an opposite tendency. Precipitation Mechanism in the Fractionation with Heptane. The relationship between aromaticity and the total number of aliphatic carbon atoms indicates

740


Figure 13. Total number of carbon atoms in aliphatic chains versus aromaticity for TA fractions.

that a larger number of aliphatic carbons with smaller aromaticity correspond to the more soluble fractions. Similarly, the size proportion between the aliphatic chains and the aromatic system (Table 8), represented by the ratio n/no. of aromatic rings, shows that the more soluble fractions have longer aliphatic chains in relation to the size of the aromatic system, thus suggesting a mechanism in which the solubility of the asphaltene molecules depends mainly on the relationship of the length of the aliphatic chains to the size of the aromatic sheets. The aliphatic chains introduce disruptions on aromatic-sheet stacking. These irregularities diminish the energy which is necessary to destroy the stacks so that more and longer chains in an asphaltene aggregate result in less aggregate stability, hence being easier to dissolve.16 Precipitation Mechanism in the Fractionation with Acetone. Among the fractions TA3 to TA6 the degree of substitution of the aromatic sheet, σ, increases (Table 8) and the aromaticity is smaller with more aliphatic carbons as the solubility of the fractions increases, which is in agreement with the mechanism described for the precipitation with n-heptane. The solubility of the asphaltene molecules in the fractions TA3 to TA6 depends not only on the aromaticity and aliphaticity but also on the polarity. With less than 50% of the acetone in the medium, the most polar asphaltenes with big polyaromatic structures do not precipitate because two forces act on them: (a) on one hand, their polar part is attracted by the medium that gradually becomes more polar, and (b) on the other, the aromaticity of the medium helps to disperse the aromatic sheets. But when the aromaticity of the medium decreases under a certain critical level (e.g., 50% toluene/50% acetone) the most polar molecules begin to precipitate because they have big aromatic systems whose natural tendency is to stack, and this tendency cannot be counteracted by the low aromaticity of the medium. The high polarity of the host media cannot maintain these molecules in solution by itself since their nature is more aromatic than polar. After the molecules with the biggest aromatic systems as well as the most polar-aromatic ones have precipitated, the following fractions that precipitated, TA7 to TASOL, have a smaller tendency to association due to a smaller content of big polyaromatic structures (fluo-


rescence spectra shifted to shorter wavelengths and smaller average number of aromatic rings). In these fractions, the main mechanism is again controlled by the relationship of the size of the aliphatic chains to the size of the aromatic system. The ratio n/no. of aromatic rings (Table 8) increases in an important way with the solubility. The decrease of the size of the polyaromatic structures in these fractions implies a smaller aromatic surface available for the stacking of the asphaltene molecules. The average length of the aliphatic substitutions increases because of the decrease in the degree of substitution. In this way, there is an important reduction of the extension of association which is reflected in a bigger solubility of the fractions, despite their greater aromaticity, and in a clear reduction of the Mw and the polydispersity (Table 3). In the TA7 to TASOL fractions, the polarity of the medium plays a secondary role in these more soluble fractions. Summary and Conclusions A gradual increase in the aliphatic character of the host fluid where asphaltenes are dispersed causes the redistribution of the asphaltene molecules in fractions whose average properties are very similar. The nheptane is not able to separate the asphaltenes in fractions that have important differences in their tendency to association. On the other hand, the precipitation with acetone, which implies an increment of polar character of the medium, has a greater effect on the distribution of the asphaltene molecules according to their solubility, generating remarkable differences in the average molecular weight and in polydispersity. This reflects important differences in the tendency of association of the separated fractions. When a nonpolar medium passes from being aromatic to mainly aliphatic, e.g., in the toluene/n-heptane system, the heteroatom distribution among the insoluble fractions varies very little with no clear tendency with regard to solubility; in this case, the polarity of the asphaltenes plays a minor role in the determination of the solubility of the fractions. On the other hand, when a nonpolar medium becomes mainly polar by the effect of a polar precipitant, e.g., in the toluene/acetone system, the increment of the polarity of the medium has an important effect on the heteroatom distribution. In this case, the polarity of the solute molecules has an important effect on the solubility and composition of the fractions. The solubility of the separated asphaltene fractions is not related directly and simply to average molecular weight of the fractions. According with the results showed in this paper the solubility appears to be more closely related to the aromaticity, aliphaticity, and polarity of the asphaltenes than to their dimension. The fluorescence of the separated asphaltene fractions, on the other hand, show a decrease of intensity and a gradual shift toward longer wavelengths with increasing number average molecular weight. The main molecular characteristic that favors association is the availability in the asphaltene molecules of sufficiently extended obstacle free aromatic sheets that facilitate the molecular recognition for π-bond stacking. The main molecular characteristic against association is the steric impediment or the steric


disruption that the aliphatic substitutions around the aromatic systems can offer against efficient stacking. Asphaltene precipitation from toluene solutions is attractive as a fractionation method given its simplicity, the quantity of material that can be processed and the low consumption of reagents; however the efficiency of this method is limited by the nature of the precipitant. In this case, acetone turns out to be quite superior to the n-heptane as precipitant agent since the fractions obtained with it are better differentiated than those obtained with n-heptane. Appendix Method of Calculation for Average Molecular Weights in Table 3. General Formulas for the Average Molecular Weights.33 The formula for number average molecular weight is

Mn )

Ni ∑NiMi ∑ N Mi ) N ) ∑xiMi ∑ i ∑ i

The formula for weight average molecular weight is

Mw )

Wi ∑WiMi ∑ W Mi ) W ) ∑xwiMi ∑ i ∑ i

The formula for polydispersity is

D)

Mw Mn

where Ni is the number of molecules, Wi is the weight


(mass), Mi is the molecular weight, xi is the molar fraction, and xwi is the weight fraction of the species i. The integration of the chromatograms shown in Figures 2 and 3 was made automatically by the software of the data acquisition unit of the SEC equipment. The software convert the measure absorbance into the area of the chromatogram corresponding to the fractions eluted. The absorbance measure by the diode array depends on the detector wavelength setting and the distribution of chromophores throughout the different molecules in the sample, but for a fixed detector wavelength the absorbance is assumed to be proportional only to the total number of chromophores following the Beer’s law. The elution time of each fraction is related to the molecular weight on the calibration curve: log(molecular weight) ) 9.364826-0.685428(elution time). The chromatogram area of the fraction i, Ai (product of the integration of the absorbance over the elution time), is transformed into the weight of the fraction, Wi (product of Ni by Mi), which transforms the formulas above into

Mn )

∑Ai Ai ∑Mi

and

Mw )

∑AiMi ∑Ai

Acknowledgment. This research was supported by the Mexican Institute of Petroleum under the grand FIES D.00077. We thank PEMEX E & P, Activo KuMaloob-Zaap, for providing sample of fresh maya crude oil. We thank Mrs. V. Aguilar-Iris and Miss M. RuizFigueroa from the IMP Spectroscopy Laboratory for performing the FTIR and synchronous fluorescence spectroscopies. EF0102317

Molecular Structure Relationships of Asphaltenes in Polar

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