Energy Fuels 2010, 24, 1809–1812 Published on Web 03/04/2010
: DOI:10.1021/ef9012714
Determination of the Number Average Molecular Mass of Asphaltenes (Mn) Using Their Soluble A2 Fraction and the Vapor Pressure Osmometry (VPO) Technique S ocrates Acevedo,*,† Karina Guzman,† and Omar Ocanto‡ † Universidad Central de Venezuela, Facultad de Ciencias, Escuela de Quı´mica 40756, Caracas 1053, Venezuela, and Universidad Nacional Experimental Polit ecnica, Vicerectorado Barquisimeto, Departamento de Ingenierı´a Quı´mica, Venezuela
‡
Received October 31, 2009. Revised Manuscript Received December 25, 2009
Fractions A1 and A2 have been isolated from asphaltenes using the para-nitrophenol (PNP) method,20,21 with fraction A2 being characterized as one with a tendency for aggregation smaller than asphaltenes and the opposite being observed for fraction A1. Hence, measuring molecular mass (MM) properties of fraction A2 would suggest MM properties of asphaltenes where aggregation is either smaller or insignificant. In this work, using several samples of asphaltenes and different solvents and temperatures, we investigated the aggregation tendency of fraction A2 using the vapor pressure osmometry (VPO) technique. When measurement of the number average molecular mass (Mn) was performed in toluene and o-dichlorobenzene (ODB), Mn ≈ 1000 g mol-1 was found. Evidence for fraction A2 aggregation was detected in chloroform and nitrobenzene. We found that ODB was the best solvent for VPO, where a constant value close to 1000 g mol-1 was measured for fraction A2 when the temperature was changed from 80 to 120 °C. In all cases studied, the Mn ratio of fractions A1/A2 was greater than the one suggesting that aggregation properties of asphaltenes are mainly due to fraction A1.
vapor pressure osmometry (VPO),13-15 and size-exclusion chromatography (SEC),21,22,31 among others. A good review of these topics up to 2005 can be found in the literature.22 Earlier references can be found in the book by Bunger and Li.19 Although the gap between reported MM values has been reduced considerably since the early days,19 today, the gap between average values maintains a heated debate between those sustaining mass values around 600 g mol -11,2,4,8-12 and others sustaining values around 2000 g mol-1 or higher.7,13-18 In recent years, we succeeded in separating asphaltenes in fractions A1 and A2, whose main difference was solubility in toluene at room temperature;20,21 fraction A1 has a solubility close to 0.09 g L-1, whereas fraction A2 has a solubility close to the one found for asphaltenes, being between 50 and 100 g L-1 depending upon the sample. Among other things, the very low solubility of fraction A1 explains the very low threshold or onset of aggregation of asphaltenes reported to be close to 50 mg L-1,23 with latter confirmed by other
Introduction With asphaltenes being a complex mixture of high molecular mass (MM) and a strong tendency to form aggregates in both polar and nonpolar solvents, determination of the average MM and size of asphaltenes has become a challenge for researchers in the field. Along the years, many techniques have been employed, such as fluorescence correlations,1,2 diffusion techniques,3-6,25 several versions of mass spectra,7-13 *To whom correspondence should be addressed. E-mail: socrates.
[email protected]. (1) Andrews, A. B.; Guerra, R. E.; Mullins, O. C.; Sen, P. N. J. Phys. Chem. A 2006, 110 (26), 8093–8097. (2) Schneider, M. H.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Energy Fuels 2007, 21 (5), 2875–2882. (3) Norinaga, K.; Wargardalam, V. J.; Takasugi, S.; Iino, M.; Matsukawa, S. Energy Fuels 2001, 15 (5), 1317–1318. (4) Badre, S.; Carla Goncalves, C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11. (5) Durand, E; Clemancey, M.; Lancelin, J. M.; Verstraete, J.; Espinat, D.; Quoineaud, A. A. J. Phys. Chem. C 2009, 113 (36), 16266–16276. (6) Durand, E; Clemancey, M.; Lancelin, J. M.; Verstraete, J.; Espinat, D.; Quoineaud, A. A. Energy Fuels 2008, 22 (4), 2604–2610. (7) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Energy Fuels 2004, 18, 1405–1413. (8) Quian, K.; Edwards, K. E.; Siskin, M; Olmstead, W. N.; Mennito, A. S.; Dechert, G. J.; Hoosain, N. Energy Fuels 2007, 21, 1042–1047. (9) Becker, C.; Qian, K.; Russell, D. H. Anal. Chem. 2008, 80, 8592– 8597. (10) Smith, D. F.; Schaub, T. M.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2007, 1309–1316. (11) Hortal, A. R.; Martı´ nez-Haya, B.; Lobato, M. D.; Pedrosa, J. M.; Lago, S. J. Mass Spectrom. 2006, 41, 960–968. (12) Hortal, A. R.; Hurtado, P.; Martinez-Haya, B.; Mullins, O. C. Energy Fuels 2007, 21 (5), 2863–2868. (13) Acevedo, S.; Gutierrez, L. B.; Negrı´ n, J. G.; Pereira, J. C.; Mendez, B.; Delolme, F.; Dessalces, G.; Broseta, D. Energy Fuels 2005, 19, 1548–1560. (14) Wiehe, I. A. J. Dispersion Sci. Technol. 2007, 28, 431–435. (15) Le on, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6–10. r 2010 American Chemical Society
(16) Strausz, O. P.; Safarik, I.; Lown, E. M. Energy Fuels 2009, 23, 1555–1562. (17) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 2008, 22, 4312–4317. (18) Acevedo, S.; Escobar, G.; Ranaudo, M. A.; Rizzo, A. Fuel 1998, 77, 853–858. (19) Speight, J. G.; Moschopedis, S. E. Chemistry of Asphaltenes; Bunger, J., Li, N. C., Eds.; American Chemical Society: Washington, D.C., 1981; Advances in Chemistry Series 195, Chapter 1. (20) Gutierrez, L. B.; Ranaudo, M. A.; Mendez, B.; Acevedo, S. Energy Fuels 2001, 15, 624–628. (21) Acevedo, S.; Castro, A.; Negrin, J. G.; Fernandez, A.; Escobar, G.; Piscitelli, V.; Delolme, F.; Dessalces, G. Energy Fuels 2007, 21, 2165– 2175. (22) Sato, S.; Takanohashi, T.; Tanaka, R. Energy Fuels 2005, 19, 1991–1994. (23) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Fernandez, A.; Perez, P.; Caetano, M. Fuel 1999, 78, 997–1003. (24) Andreatta, G.; Goncalves, C. C.; Buffin, G.; Bostrom, N.; Quintella, C. M.; Arteaga-Larios, F.; Perez, E.; Mullins, O. C. Energy Fuels 2005, 19, 1282–1289.
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: DOI:10.1021/ef9012714
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researchers using different techniques. Thus, fraction A1 promotes aggregation of asphaltenes in toluene at room temperatures at concentrations close to their solubility. General characteristics of fractions A1 and A2, such as elemental analysis and nuclear magnetic resonance (NMR) spectra, are similar to asphaltenes and cannot justify the above difference in solubility.21,29 However, in comparison to fraction A2, fraction A1 has a smaller H/C ratio, suggesting more ring condensation for fraction A1. In the case reported (Furrial),29 fraction A1 has 5 more double bond equivalents (DBEs) than fraction A2. Here, DBEs are totally or partially equivalent to aliphatic rings. As reported earlier,21 this suggests that fraction A2 is likely to contain flexible molecules, whereas fraction A1 is likely to contain a rigid core with substituted aliphatic chains. The above similarity between asphaltenes and fraction A2 could be used as a tool in the way that measuring the number average molecular mass (Mn) for fraction A2 would be equivalent to measure Mn for asphaltene free from aggregation. Aggregation problems would then be linked to the presence of fraction A1. Molecular flexibility in fraction A2 opposed to rigidity in fraction A1 has been the argument employed to justify the above large solubility difference. Molecular flexibility hinders proper aggregate pairing and also leads to a large negative entropy of aggregation. Octylated asphaltenes (OAs) are a good example of this entropy effect;18 these are asphaltene derivatives where a number of n-octyl groups (n-C8H17) are incorporated to asphaltenes, resulting in a product soluble in n-heptane and n-pentane. The high solubility of OAs is the result of the lost in conformational entropy (flexibility) when two OAs try to form aggregates. In other words, the change in chemical potential μ0OA for the formation of aggregates is positive (unfavorable) because the entropy change ΔS0OA is negative and large. Thus, the following balance is positive even for relatively large negative ΔH0OA: 0 0 -TΔSOA >0 μ0OA ¼ ΔHOA
In this paper, we present VPO measurements of asphaltenes and their fractions A1 and A2 measured in four different solvents and at different temperatures. As shown below, because of the above weak tendency of aggregation of fraction A2, when measured in toluene and o-dichlorobenzene (ODB) at different temperatures, fraction A2 afforded relatively low Mn close to 1000 g mol-1. Evidence for fraction A2 aggregation was found in both nitrobenzene and chloroform. Experimental Section Asphaltenes were obtained from the crude oil after the addition of 60 n-heptane volumes using reported methods.28 In all cases, resins were thoroughly extracted in Soxhlet with boiling n-heptane until the emerging liquid became clear. Asphaltenes were fractionated into fractions A1 and A2 by dissolving 8 g L-1 in a saturated solution of para-nitrophenol (PNP) in cumene as described earlier.20 Yields for these fractions (A1 and A2) were Furrial, 70-30; CN, 57-35; and Hamaca, 57-40. FA and EA were asphaltenes from Hamaca residue (500þ) and treatment of FA, respectively. These were used as received from PDVSA-INTEVEP.
VPO Measurements Between 1 and 6 g L-1 of sample was dissolved in the corresponding analytical-grade solvent (benzene, chloroform, nitrobenzene, and ODB). Measurements at each concentration were averages of five or more measurements. Pyrene was used as the standard to find the equipment constant KE for each solvent at each temperature. Once KE was known, Mn was found by extrapolation at infinite dilution KE ð2Þ Mn ¼ ΔV C C ¼0 In eq 2, the term ΔV is the equipment reading that corresponds to the linear extrapolation of ΔV/C values evaluated at the experimental concentration C.
ð1Þ
Similar to the OAs, both effects, relative low enthalpy of aggregation (small in absolute value) and relative large and negative conformational energy, tend to keep fraction A2 as monomers in solution; on the contrary, rigidity of A1 promotes aggregation (more negative enthalpy; see ref 28), and this leads to a negative entropy much smaller in absolute values than the one found for A2. Molecules belonging to fraction A1 have been referred to as continental type because the core is formed by fused rings both aromatic and aliphatic, whereas molecules in fraction A2 have been referred to as archipelago type or rosary type because the polycyclic units are interconnected by aliphatic chains. Examples of both can be found in the literature.21,26,27,29-32
Results Figure 1 is an example of the many plots performed. This case corresponds to CN asphaltenes (asphaltenes from the Cerro Negro crude oil). As shown here distance between cut points in the ΔV/C axes, corresponding to fractions A1 and A2 are much larger than the experimental errors (equivalent to 20% of Mn in the worst case). As shown above, yields for fraction A2 are between 30 and 40% of the total mass of asphaltenes. In some cases, yields of fractions A1 and A2 do not add up to 100% because of the presence of trapped resins, which were not removed by the Soxhlet treatment. These yields show that fraction A2 is an important part of asphaltene composition. Tables 1-4 show the Mn results of the present work. Nitrobenzene at 100 °C was the condition used in Table 1. As shown in these tables, all A1/A2 afforded values higher than 1, showing that fraction A1 has a higher aggregation tendency than fraction A2. The same result was obtained when using chloroform (Table 2) and ODB (Table 4) and different temperatures. Note that, regardless of high polarity, a relatively high value (1700 g mol-1) was obtained for fraction A2 in the CN case (Table 1). Except for this case, all asp/A2 were significantly higher than 1. Note that the same trends observed for asphaltenes from crude oils were
(25) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. Fuel 2006, 85, 1465–1472. (26) Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy Fuels 1999, 13, 278–286. (27) Gawrys, K. L.; Blankenship, G. A.; Kilpatric, P. K. Langmuir 2006, 22, 4487–4497. (28) Acevedo, S.; Layrisse, I.; Mendez, B.; Rivas, H.; Rojas, A. Fuel 1985, 64, 1741–1747. (29) Acevedo, S.; Escobar, O.; Echevarria, L.; Gutierrez, L. B.; Mendez, B. Energy Fuels 2004, 18, 305–311. (30) Akbarzadeh, K.; Bressler, D. C.; Wang, J.; Gawris, K. L.; Gray, M. R.; Kilpatrick, P. K.; Yarranton, H. W. Energy Fuels 2005, 19, 1268– 1271. (31) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809– 822. (32) Murgich, J. Mol. Simul. 2003, 29, 451–461.
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Acevedo et al. Table 3. Mn Values Measured in Toluene at 50 °C for Sample CNa Mn (g mol-1) sample
asphaltene
CN
A1
4200
b
A2
As/A2c
1200
3.5
a
All values were determined at infinite dilution using pyrene as the standard. b Sample insoluble in toluene. c Mn relationships; errors were less than 20%.
Table 4. Mn Values Measured in ODB as a Function of Temperaturesa,b Figure 1. Plot of ΔV/C against C for asphaltenes (Cerro Negro or CN sample) and fractions A1 and A2. Solvent ODB at 80 °C. Note that differences between (ΔV/C)C=0 of values extrapolated at C = 0 are much greater than the experimental error. Similar results were found in all examined cases, even for those where error was the largest (20%).
Mn (g mol-1)
A1
A2
A1/A2b
asp/A2b,c
CN Furrial Hamaca FAd EAe
1800 2300 2050 2200 1100
2700 3300 2100 1700 990
1700 1200 1400 1100 590
1.6 2.8 1.5 1.5 1.7
1.1 1.9 1.5 2.0 1.9
All values were determined at infinite dilution using pyrene as the standard. b Mn relationship. Errors were less than 20%. c asp = asphaltenes. d Asphaltenes from a residue (feed). e Asphaltenes from plant processing of FA.
A1
A2
A1/A2b
asp/A2b
CN Furrial
3000 2600
2000 2400
1400 1500
1.4 1.6
2.1 1.7
A1/A2c
asp/A2c
80 100 120
1580 1450 1540
3190 2970 2600
1020 1200 1060
3.1 2.5 2.5
1.5 1.2 1.5
solvent
T (°C)b
asphaltene
A2
asp/A2c
toluene nitrobenzene chloroform
50 100 30 80 100 120
4200 1800 3000 1580 1450 1540
1200 1700 1400 1020 1200 1060
3.5 1.1 2.1 1.5 1.2 1.5
ODB
Table 2. Mn (g mol-1) Values Measured in Chloroform at 30 °C for Two Samplesa asphaltene
A2
Mn (g mol-1)
a
sample
A1
Table 5. Mn (g mol-1) Values for Asphaltenes and Fraction A2 Corresponding to CN, Measured at Different Temperatures and Different Solventsa
Mn (g mol-1) asphaltene
asphaltenes
a See footnotes in Table 1. b All measurements for the CN sample. c Mn ratio.
Table 1. Mn Values Measured in Nitrobenzene at 100 °C for Several Samplesa
sample
temperature (°C)
a
Values gathered from Tables 1-4. b Temperature. c Mn relationship between asphaltenes and fraction A2.
A1/A2 ratios are greater than 1, suggesting that aggregation for fraction A1 is much more important than for fraction A2 (except in toluene, where fraction A1 is not soluble and cannot be measured; see Table 3). For the solvents tried, toluene and ODB afforded the lowest values for fraction A2; these results gave an average Mn close to 1100 ( 90 g mol-1. However, significantly higher values were measured in nitrobenzene (Table 1; CN and Hamaca cases) and chloroform (Table 2), suggesting different degrees of fraction A2 aggregation in these solvents. Even so, the Mn ratio of asp/A2 was generally higher than 1, sustaining that fraction A2 has a smaller aggregation tendency than asphaltenes. The low or smaller aggregation tendency of fraction A2 compared to asphaltenes could be appreciated in Table 5, where the Mn range for CN asphaltenes measured in different solvents at different temperatures is about 4 times wider than the one found for A2. This was also the case for all other asphaltenes studied (see Tables 1-4). This is coherent with aggregation of asphaltenes promoted by fraction A1. ODB was the best solvent for VPO measurement of Mn for fraction A2. This is underlined for CN in Table 5. It is interesting that no significant change of Mn with the temperature was observed for fraction A2, whereas a Mn decrease was shown for the aggregate A1 fraction (see Table 4). Although Mn values for asphaltenes were apparently independent of the temperature (Table 4), this could be the result of a casual aggregation and dissociation of molecules during heating. The association behavior of pyrene compounds as flexible models for asphaltenes, using VPO and ODB as a solvent, has been reported.30 Authors found that, whereas the nonpolar compounds pyrene and 1,10-dipyrenil decane yielded the
a All values were determined at infinite dilution using pyrene as the standard; errors were less than 20%. b Mn relationship.
presented by the asphaltenes obtained from a petroleum residue (FA) and the treated FA sample (EA). Lower Mn values for both fractions A1 and A2 could be expected for treated samples (EA), in view of expected breaking of aliphatic chains under treatment conditions. Interactions between fractions A1 and A2 are unknown, and both dispersion of fraction A1 by fraction A2 components or aggregation between them could be expected.21 Hence, molecular mass of asphaltenes could not be anticipated from the Mn of fractions A1 and A2 shown in Tables 1-4. Relatively low values for fraction A2 close to 1000 g mol-1 were obtained in both ODB and toluene, and evidence for different degrees of aggregation was found in nitrobenzene (Table 1; CN and Hamaca) and chloroform (Table 2), where values significantly higher than 1000 g mol-1 were observed. In Table 5, we gathered measurements for CN collected from Tables 1-4; in general, the ratio of asp/A2 is significantly greater than 1, with the exception of the nitrobenzene case; note that the Mn range for asphaltenes amounts to about 2750 g mol-1 (from 4200 to 1450 g mol-1), whereas the one for fraction A2 amounts to about 680 g mol-1. Discussion Some common features can be seen from the above results; in the first place, regardless of the solvent and temperature, all 1811
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low or insignificant, afforded Mmax ≈ 400 atomic mass units; however, the reported spectra have a significant tail extending to 1000 atomic mass units and above.12 Thus, here too, the inequality Mn > Mmax is expected to hold. These data suggest that the above Mn ≈ 1000 g mol-1 found for fraction A2 in VPO would be equivalent to a lower Mmax in a mass spectrum. The results measured above in ODB and toluene and data from the literature suggest that Mn for fraction A2 should be around 1000 g mol-1; the presence of fraction A1 in the asphaltenes mixture promotes aggregation, leading to higher Mn values. A molecular mass close to 2000 g mol-1 was reported by our research group, using LDI-time of flight mass spectrometry (TOF MS). A good comparison to VPO values for the same asphaltenes measured in nitrobenzene was found.13 However, as shown above, these VPO values13 are too high because of aggregate formation promoted by fraction A1.
expected molar mass, the polar compounds (diketones, diols, and pyrenol) gave evidence of association. All but pyrene and pyrenol are compounds where two pyrenyl units are separated by a long chain of 10 or 12 carbon atoms, in other words, flexible compounds where aggregation is expected to be hindered (see the Introduction). Although all polar compounds gave different degrees of aggregation at high concentrations, the expected molar mass could be obtained as the experimental concentration approached 0 (in 0.6-20 g L-1). This is coherent with the results above where presumable flexible molecules (fraction A2), the same solvent, and extrapolation to infinite dilution were used. A study of the colloidal nature of asphaltenes and the molecular weight of covalent units reported a dissociation of asphaltenes from several thousands to roughly less than 1000 g mol-1. The reported dissociation was monitored in a SEC column for 14 days using dichloromethane as the solvent.31 A field desorption (FD) mass spectra study of heavy petroleum molecules was reported by Qian et al., where spectra were carefully measured to avoid aggregate formation in the gas phase during measurements.8 The authors measured both the spectra of combined maltenes and asphaltenes as well as the spectra of each component separately. Using the experimental composition of the mixture and the FD molecular mass distribution (MMD) of each component, the MMD of the mixture was calculated. The very good mach found between the experimental and calculated asphaltene-maltene mixture lead the authors to conclude that the MMD of asphaltenes measured by FD was not affected by dimer formation. These authors reported Mn close to 1200 g mol-1 for asphaltenes. As shown in Figure 7 of ref 8, the asphaltene band maximum (around 800 g mol-1) was well below this Mn. This appear to be a general property of the MMD of asphaltenes found when using mass spectra; that is, that Mn > Mmax, where Mmax is the MM found at the maximum of the MMD measured in the mass spectra. For instance, laser desorption ionization (LDI) spectra measured at low energy laser power, where aggregation is expected to be
Conclusions Mn close to 1000 g mol-1 in ODB and toluene was found for fraction A2 using the simple VPO technique. The A1/A2 Mn ratio was higher than 1 in all cases studied, suggesting that aggregation of asphaltenes is mainly promoted by the presence of very low soluble (toluene, room temperature) fraction A1, the presumably rigid-type asphaltene fraction.20,21,29 In comparison to asphaltenes, fraction A2 proved to have a much smaller tendency for aggregation. This is consistent with the presumed high flexibility of fraction A2 type molecules. According to the results above, it is concluded that Mn ≈ 1000 g mol-1 for fraction A2. This value compared well to other values reported for asphaltenes using conditions where aggregation is expected to be low or insignificant.8,12,31 Acknowledgment. The financial support provided by projects FONACIT (G2005000430) and CDCH (AI-03-12-5509-2004, PG-03-00-5732-2004, and PI-03-00-5648-2004) is gratefully acknowledged. We also thank Lic. Betilde Segovia for administrative assistance and PDVSA-INTEVEP for providing the samples employed in this study.
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