Energy Fuels 2010, 24, 1135–1138 Published on Web 12/15/2009
: DOI:10.1021/ef9009108
Study of the Asphaltene Aggregation Structure by Time-Resolved Fluorescence Spectroscopy Rodrigo da Silva Souza, David E. Nicodem, Simon J. Garden, and Rodrigo J. Corr^ea* Instituto de Quı´mica, Universidade Federal do Rio de Janeiro, Avenida Athos da Silveira Ramos, 149, CT, Bloco A, 21941-909 Cidade Universit aria, RJ, Brazil Received September 15, 2009. Revised Manuscript Received November 3, 2009
Asphaltene is defined as a fraction of petroleum that is insoluble in light linear alkane solvents, such as pentane or heptane, and soluble in toluene. This fraction of petroleum is responsible for serious problems in petroleum extraction, transportation, and refining. It aggregates and precipitates in well pipes and pipelines, reducing flow and increasing downtime for cleaning and removal. Fluorescence spectroscopy has been used to evaluate asphaltene size and aggregation. The time-resolved fluorescence decay profile of complex mixtures, such as asphaltenes, can be better evaluated by a distribution analysis method rather than the use of two exponential decays. Further, the time-resolved fluorescence decay profiles of asphaltene show that, even at a concentration of 0.08 g/L of asphaltene in toluene, the asphaltene compounds are aggregated, and using a simple kinetic model, we can conclude that the asphaltene compounds are dimerized via π-π stacking or other intermolecular forces.
can be described as aromatic groups joined by bridges, such as -(CH2)n-, -S-, -C(O)-O-, and -O.2-13 It is interesting to note that, for the methylenic bridges, one can find 6-23 carbon atoms. Although the aromatic, polar, and heteroatomic contribution to the asphaltene composition is well-established, the size and nature of aggregation in this fraction is still a polemical discussion. The measurement of the size distribution of the individual asphaltene molecules is complicated by its tendency to aggregate, and mean molecular weights as low as 500 Da and as high as 4800 Da have been reported.5,6,10,14-20 Although aggregation is generally considered to be due to aromatic π-π stacking, the model most used is based on analogy with a micelle and values of critical micelle concentration (cmc) in the
Introduction Petroleum is responsible for 36.4% of human energy use and is the largest single energy source.1 With the depletion of the light oils, the use of heavier petroleums is inevitable. These oils contain a higher concentration of asphaltene. Asphaltene is defined as that fraction of petroleum that is insoluble in light linear alkane solvents, such as pentane or heptane, and soluble in toluene. This fraction of petroleum is responsible for serious problems in petroleum extraction, transportation, and refining. It is the heaviest, least biodegradable fraction of petroleum and is implicated in the stabilization of water-oil emulsions. It aggregates and precipitates in well pipes and pipelines, reducing flow and increasing downtime for cleaning and removal. Additionally, it contains the highest concentration of metals, deactivates refinery catalysts, and forms coke. Because of its importance in operations involving petroleum, it has been intensively studied. Many different methodologies have been used, and the results are quite consistent. Probably the most accepted property is a C/H ratio of 1:1.2, where the carbon atoms are almost 40% aromatic, and this fraction shows a high sulfur and nitrogen content, reaching 6 and 3% (w/w), respectively. Further, a number of fused aromatic rings, varying from 4 to 10, in the asphaltene macrostructure have been wellestablished using 13C nuclear magnetic resonance (NMR), Raman spectroscopy, infrared (IR) spectroscopy, exclusion gas chromatography (GC), pyrolysis GC, scanning tunneling microscopy, and fluorescence depolarization. Structural units
(5) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103 (50), 11237–11245. (6) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14 (3), 677–684. (7) Mckay, J. F.; Amend, P. J.; Cogswell, T. E.; Harnsberger, P. M.; Erikson, R. B.; Lathan, D. R. Petroleum asphaltenes: Chemistry and composition. In Analytical Chemistry of Liquid Fuel Sources; American Chemical Society: Washington, D.C., 1978; Advanced Chemistry Series, Vol. 170, Chapter 9, pp 129-142. (8) Nomura, M.; Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S. Energy Fuels 1999, 13 (2), 287–296. (9) Pacheco-Sanchez, J. H.; Zaragoza, I. P.; Martı´ nez-Magadan, J. M. Energy Fuels 2003, 17 (5), 1346–1355. (10) Siuniayev, R. Physical Chemistry of Colloids and Interfaces in Oil Production; Editions Technip: Paris, France, 1992; pp 271-272. (11) Speight, J. G. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 1994, 39 (2), 200–203. (12) Tanaka, R.; Sato, E.; Hunt, J. H.; Winans, R. E.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18 (4), 1118–1125. (13) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39 (8), 2916–2924. (14) Andersen, S. I. J. Liq. Chromatogr. 1994, 17 (19), 4065–4079. (15) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12 (1), 51–74. (16) Andersen, S. I.; Stenby, E, H. Fuel Sci. Technol. Int. 1996, 14 (1-2), 261–287. (17) Boduszinski, M. M. Energy Fuels 1988, 2 (5), 597–613. (18) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12 (6), 1290–1298. (19) Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976, 55, 227– 232. (20) Storm, D. A.; Sheu, E. Y. Fuel 1995, 74 (8), 1140–1145.
*To whom correspondence should be addressed. E-mail: rcorrea@ iq.ufrj.br. (1) World Energy Council. 2007 Survey of Energy Resources, 2007. (2) Barteau, M. A.; Watson, B. A. Ind. Eng. Chem. Res. 1994, 33 (10), 2358–2363. (3) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. Fuel 2003, 82, 817–823. (4) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. J. Pet. Sci. Eng. 2003, 37, 135–143. r 2009 American Chemical Society
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Energy Fuels 2010, 24, 1135–1138
: DOI:10.1021/ef9009108
Souza et al.
Figure 1. Asphaltene fluorescence lifetime distribution. Excitation, 467 nm; emission, 511 nm.
range of 0.05-0.075 g/L have been found in toluene. To the best of our knowledge, the direct detection of asphaltene aggregation has not been observed nor is there an appropriate model of its macrostructure, despite the huge amount of work published on asphaltenes. Fluorescence spectroscopy has been used to evaluate asphaltene size and aggregation. Recent work using timeresolved fluorescence and depolarization had indicated that there was a single aromatic core with 4-10 fused rings (which correlates with results using other methods) and a molecular weight in the range of 500 Da. These results have recently been criticized and the use of fluorescence spectroscopy questioned in an elegant study by Strausz and co-workers.21 Fluorescence depolarization is used to measure polymer mobility,22,23 and therefore, its use as a probe of molecular weight is questionable. Although asphaltenes are not composed solely of fluorescent aromatic compounds, this technique permits evaluation of the components that do fluoresce and, hence, the results are representative of the behavior of this complex mixture. As will be seen from the data presented here, the analysis of asphaltene fluorescence lifetimes using only two lifetimes is inadequate. However, as shown here, fluorescence lifetime studies can generate valuable insight into important properties of asphaltene. In this paper, we show that fluorescence lifetime decay measurements can be used to directly follow asphaltene aggregation and, consequently, we have proposed a new simple model for aggregation and the nature of this interaction.
boundary lifetimes and a smooth distribution of pre-exponential factors. This procedure was performed using the FAST version 1.8.1 program, supplied by Edinburgh Analytical Instruments. The fitting function uses up to 100 lifetime values to fit the fluorescence decays.
Results and Discussion The fluorescence decay of a Brazilian asphaltene in toluene (0.08 g/L) analyzed using the method of lifetime distributions is shown in Figure 1. As can be seen, a wide time distribution varying in the range of 0.100-10 ns is found, which is expected for a complex mixture, such as petroleum fractions. The most interesting feature of the distribution is that it is centered on two maxima: 0.196 and 1.792 ns. At still lower concentrations (below 0.01 g/L), only the longer lifetime component is observed. The distribution of the single broad peak, from 1 to 10 ns, is quite well-represented by a fit using only two exponential factors, giving lifetimes of 2 and 7 ns as previously reported.23-26 We have found that complex mixtures, such as the asphaltene solutions that give broad lifetime distributions, can give good correlations of the decay using two monoexponential decays, and in the present study, the two lifetimes straddle the distribution maximum. When the asphaltene concentration is varied from 0.016 to 10 g/L, the concentration range where aggregation occurs,5,10,27-30 two distributions are observed: the first is a short lifetime component (SLC) peak, with a distribution of lifetimes from 0.1 to 1 ns, and the second is a long lifetime component (LLC) that varies from 1 to 10 ns. The lifetime of the maximum of each peak and the relative areas are given in Table 1. As the concentration of asphaltene increases, the relative concentration of the SLC increases and its mean lifetime decreases, as would be expected for an increased degree of association between the various asphaltene chromophores, while on the other hand, the LLC component concentration decreases, while its lifetime distribution remains constant. It is worthwhile noting that, at an asphaltene concentration of 10 g/L, the
Materials and Methods All solvents were purchased from Sigma-Aldrich and used without further preparation. Asphaltenes were extracted from a Brazilian oil from Bacia de Campos, Rio de Janeiro, Brazil, by diluting the crude oil 1:40 in n-heptane and keeping the mixture in the dark for 2 days. After this period, the asphaltenes were removed from the solution by vacuum filtration and dried under vacuum. Fluorescence decays were monitored at room temperature in a front face configuration, using an Edinburgh FL 900 singlephoton counting apparatus. The decay profiles were reconvoluted by nonlinear least-squares routines minimizing X2. The lifetime distribuition analysis assumes that a fluorescence decay can be fitted by a series of exponential functions with predetermined
(24) Ralston, C. Y.; Mitra-Kirtley, S.; Mullins, O. C. Energy Fuels 1996, 10, 623–630. (25) Buenzostro-Gonzales, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972–979. (26) Groenzin, H.; Mullins, O. C. Energy Fuels 2003, 17, 498–503. (27) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142 (2), 497–502. (28) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14 (1), 38– 42. (29) Deo, M. D.; Oh, K.; Ring, T. A. J. Colloid Interface Sci. 2004, 271, 212–219. (30) Shew, E. Y.; de Tar, M. M.; Storm, D. A.; Decannio, S. J. Fuel 1992, 71, 299–302.
(21) Strausz, O. P.; Lown, E. M.; Morales-Izquierdo, A. Energy Fuels 2008, 22, 1156–1166. (22) Yamamoto, M.; Horinaka, J.; Maruta, M.; Ito, S. Macromolecules 1999, 32, 1134–1139. (23) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11.
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Table 1. Effect of the Asphaltene Concentration (in Toluene) on the Total Fluorescence Decay Distributiona short lifetime decay
Table 3. Temperature Effect on the SLC Lifetime and Concentration for an Asphaltene Concentration of 0.08 g/La
long lifetime decay
short lifetime decay
long lifetime decay
asphaltene (g/L)
%
τ (ns)
%
τ (ns)
temperature (°C)
%
τ (ns)
%
τ (ns)
10.0 3.0 0.4 0.08 0.016
40 28 19 11 0
0.196 0.235 0.282 0.381
60 72 81 89 100
1.796 2.170 2.340 2.337 2.255
5 10 15 20 25 35
37 26 24 19 17 0
0.173 0.191 0.199 0.330 0.407
63 74 76 81 83 100
2.398 2.380 2.129 2.302 2.450 2.246
a
Excitation, 467 nm; emission, 511 nm. a
Excitation, 467 nm; emission, 511 nm.
Table 2. Effect of the Solvent Composition on the Total Fluorescence Decay Distribution for an Asphaltene Concentration of 0.08 g/La short lifetime decay
long lifetime decay
toluene/cyclohexane ratio
%
τ (ns)
%
τ (ns)
100:0 80:20 60:40 40:60 20:80
10 15 19 22 30
0.454 0.373 0.277 0.183 0.177
90 85 81 78 70
2.258 2.481 2.339 2.311 2.402
a
Excitation, 467 nm; emission, 511 nm.
SLC component corresponds to 40% of the total fluorescence decay, while at 0.08 g/L, the SLC component corresponds to only 11% of the total decay distribution. The LLC, which is present at all concentrations, clearly represents those components of asphaltene that absorb at 467 nm, the wavelength used for excitation in these experiments, and fluoresce at 511 nm. We attribute the SLC to the formation of an aggregate where the corresponding emission is that of the exciplex. This component is concentration-dependent in toluene and only seen at concentrations in which asphaltene is known to aggregate. When using asphaltene solutions of 0.08 g/L, the last concentration at which the SLC component is still observed in toluene, varying the solvent composition with different ratios of toluene/cyclohexane (a solvent in which asphaltene is known to aggregate) from 100:0 to 20:80 resulted in an increase of the SLC concentration from 10 to 30% of the total fluorescence lifetime decay and, further, caused a shortening in the lifetime decay of the SLC from 0.454 to 0.177 ns. These results are shown in Table 2. These results are analogous to the results obtained by varying the concentration of asphaltene in toluene, showing that the aggregation process can be induced by other methods known to result in aggregation. Dynamic excimers and exciplexes are usually formed in the excited state, but this cannot be the case here because, first, at the lowest asphaltene concentration used, in which the SLC is observed, and considering the lifetime of the LLC, a diffusioncontrolled process could not generate the quantity of SLC observed; second, the LLC lifetime does not change with the concentration; third, there is no indication of grow-in of any component; and fourth, the concentration of the short decay component is found to be dependent upon the temperature. At 5 °C, using the asphaltene solution of 0.08 g/L, the total fluorescence decay is 37% for the SLC and 63% for the LLC. The SLC concentration diminishes to 0% at 35 °C (Table 3). In a similar manner to Birks,31 an Arrhenius plot constructed using the relative SLC concentration at each temperature gives a calculated activation energy for the loss of the SLC of 4.6 kcal/mol (Figure 2), which is close to the value found for
Figure 2. Arrhenius plot for the SLC of the Bacia de Campos asphaltene. Scheme 1. Dimerization of Asphaltenea
a
L, an asphaltene molecule; S*, an asphaltene exciplex.
the pyrene excimer by both experimental and theoretical32-34 approaches. If the SLC was being formed in the excited state, then its concentration would not be expected to vary in this way. All of these observations are, however, completely compatible with the formation of a complex in the ground state by aggregation, which then absorbs and emits. If the aggregation observed was micellar in nature, a plot of the intensity of the SLC against the total asphaltene concentration would be expected to be linear,35-37 but this was not observed. On the other hand, a model in which the asphaltene molecules (L) form a dimer in the ground state and this dimer absorbs and emits as an exciplex (SLC) would be expected to follow Scheme 1 shown below. (32) Cinacchi, G.; Prampolini, G. J. Phys. Chem. C 2008, 112 (25), 9501–9509. (33) Podeszwa, R.; Szalewicz, K. Phys. Chem. Chem. Phys. 2008, 10, 2735–2746. (34) Sato, T.; Tsuneda, T.; Hirao, K. J. Chem. Phys. 2005, 123, 10436. (35) Rosenthal, K. S.; Koussaie, F. Anal. Chem. 1983, 55, 1115–1117. (36) Bradford, M. Anal. Biochem. 1978, 72, 248–254. (37) Chen, L. J.; Lin, S. Y.; Chern, C. S.; Wu, S. C. Colloids Surf., A 1997, 122, 161–168.
(31) Birks, J. B.; Dyson, D. J.; Munro, I. H. Proc. R. Soc. A 1963, 275, 575–588.
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: DOI:10.1021/ef9009108
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huge mixture of aromatic molecules, which are both interacting and non-interacting with their respective neighborhoods. From the calculated energy for π-stacking, it is clear that the polyaromatic rings involved are pyrene-like or larger structures.31-34 Further, for the interacting asphaltene components, the aggregation observed is initially due to ground-state dimer formation by π-π stacking or other intermolecular interactions (acid-base, hydrogen-bonding, or dipolar interactions) of the polyaromatic components of the asphaltene.38 Conclusions The time-resolved fluorescence decay profile of complex mixtures, such as asphaltenes, can be better evaluated by a distribution analysis method instead of two exponential decays. Further, the time-resolved fluorescence decay profiles of asphaltene show that, even at a concentration of 0.08 g/L of asphaltene in toluene, there is significant aggregation of the asphaltene compounds. Using a simple kinetic model, we can conclude that the asphaltene compounds are dimerized via π-π stacking or other intermolecular forces in the ground state.
Figure 3. Correlation between SLC and LLC concentrations.
Under these conditions, a plot of the concentration of the LLC2 against that of the SLC should be linear. This plot is shown in Figure 3, and indeed, a linear correlation was found. In light of the results presented here, we conclude that the fluorescence decay profile indicates that the asphaltene aromatic structure can be better understood in terms of a
Supporting Information Available: Asphaltene concentration effect in toluene and cyclohexane and temperature effect on asphaltene time-resolved fluorescence distribution. This material is available free of charge via the Internet at http:// pubs.acs.org.
(38) Murgich, J. Pet. Sci. Techol. 2002, 20, 983–997.
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