Energy & Fuels 2005, 19, 1991-1994
1991
Molecular Weight Calibration of Asphaltenes Using Gel Permeation Chromatography/Mass Spectrometry Shinya Sato* and Toshimasa Takanohashi National Institute of Advanced Industrial Science and Technology, Energy Technology Research Institute, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
Ryuzo Tanaka Idemitu Kosan Co., Ltd., 1280 Kamiizumi, Sodegaura, Chiba 299-0293, Japan
Received December 16, 2004. Revised Manuscript Received June 19, 2005
Molecular weight (MW) is one of the most important properties of asphaltene. To determine the precise average MW using gel permeation chromatography (GPC), the MWs obtained for three different asphaltenes using GPC (MWps) and GPC-MSD (MWms) were compared. MWms proved independent of asphaltene type and had values similar to MWps for a MW of ∼900 but was significantly different in the lower-MW range, with a linear correlation between MWms and MWps. More-precise average MW values can be obtained using a revised calibration curve based on three new chemicals to convert MWps to MWms.
Introduction Molecular weight (MW) is one of the most important properties used to study asphaltenes. It has been measured via several methods, including vapor-pressure osmometry (VPO),1,2 mass spectroscopy (MS),3 gel permeation chromatography (GPC), and combinations of these techniques.4,5 Although VPO provides a value for a number-averaged MW, it supplies no information on how the range of MW values is distributed. Furthermore, the observed MW is dependent on the solvent used in the determination.6 GPC gives the distribution of the MW range from which both weight-averaged and number-averaged MW values can be calculated. However, because the calibration curve between retention time and MW is derived from a polystyrene mixture, it is widely believed that the MW determined using this method is not the actual MW, but rather a polystyreneequivalent MW.7,8 Generally, pericondensed polyaromatic hydrocarbons have a tendency to elute later than expected, so that the number-averaged MW determined for asphaltene is too low. * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Schabron, J. F.; Pauli, A. T.; Rovani, J. F. Fuel 2001, 80, 529537. (2) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916-2924. (3) Douda, J.; Llanos, M. E.; Alvarez, R.; Bolanos, J. N. Energy Fuels 2004, 18, 736-742. (4) Peramanu, S.; Pruden, B. B.; Rahimi, P. Ind. Eng. Chem. Res. 1999, 38, 3121-3130. (5) Nari, M.; Manclossi, A. Fuel Sci. Technol. Int. 1995, 13, 12511264. (6) Cunico, R. L.; Sheu, E. Y.; Mullins, O. C. Pet. Sci. Technol. 2004, 22, 787-798. (7) Evans, N.; Haley, T. M.; Mulligan, M. J.; Thomas, K. M. Fuel 1986, 65, 694-703. (8) Merdrignac, I.; Truchy, C.; Robert, E.; Guibard, I.; Kressmann, S. P. Pet. Sci. Technol. 2004, 22, 1003-1022.
Many determinative techniques using MS have been developed recently, including field-desorption mass spectroscopy (FD-MS),3 laser-desorption mass spectroscopy (LD-MS),9 atmospheric-ionization mass spectroscopy (API-MS),6 etc. Because MS normally does not use solvents, there is no solvent effect and the distribution of MW is obtained directly. However, problems arise in ensuring that the ionization efficiency is flat throughout the MW range and that fragmentation is negligible.10 Electron-ionization mass spectroscopy is the most popular API-MS method used as a detector in high-performance liquid chromatography (HPLC); however, its sensitivity to nonpolar compounds is poor, so the solvents generally used for GPC cannot be used. In addition, the equipment is expensive and requires careful handling. In contrast, GPC is inexpensive and easy to maintain. Where suitable standards are available, other than polystyrene, GPC has the merit of providing data over the entire MW range without the limitations that are inherent in MS. We have studied the aggregation behavior of asphaltene using molecular modeling and molecular dynamic techniques, where the determination of the MW distribution is a primary consideration.11-17 This paper investigates the correlation between the MW derived using polystyrene standards (MWps) and that determined using MS (MWms), over a small molecular range, and it compares the data obtained with the GPC/MS results. Our results indicate the need to use new MW standards for calibrating MWms. (9) Fujii, M.; Yoneda, T.; Satou, M.; Sanada, Y. Sekiyu Gakkaishi 2000, 43, 149-156. (10) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809-822. (11) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127-134.
10.1021/ef049672r CCC: $30.25 © 2005 American Chemical Society Published on Web 07/16/2005
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Energy & Fuels, Vol. 19, No. 5, 2005
Communications
Table 1. Properties of Asphaltenes Value Maya, Khafji, Iranian oil sand, AS-MY AS-KS Light, AS-IL AS-OB
property elemental analysis (wt %) carbon hydrogen sulfur nitrogen oxygen (by difference) H/C atomic ratio metals content (wt ppm) nickel vanadium manganesea density (g/cm3)
82.0 7.5 7.1 1.3 1.2 1.10
82.2 7.6 7.6 0.9 1.1 1.11
83.2 6.8 5.9 1.4 1.5 0.98
80.3 7.7 8.3 1.1 2.6 1.15
390 1800 4000 1.177
200 550 4000 1.168
390 1200 2400 1.167
935 386 5900 1.241
Figure 1. Effect of different fragmentor settings (([) 150, (0) 200, (b) 300, and (O) 400) on Maya asphaltene.
a Molecular weight determined by vapor-pressure osmometry (VPO).
Experimental Section Samples and Standards. Residua after the vacuum distillation of three crude oils at >500 °C were obtained. The asphaltenes were isolated by adding a 20:1 excess of n-heptane to each residue at 25 °C. The suspension was stirred for 1 h at 100 °C in an autoclave, and then cooled, filtered, and allowed to stand at 25 °C overnight. The precipitates was washed twice with n-heptane and dried. The yields of asphaltenes (precipitates) of Maya (AS-MY), Khafji (AS-KF), Iranian Light (AS-IL), and Athabasca oil sand bitumen (AS-OB) were 24.9, 14.2, 6.3, and 19.6 wt %, respectively.11,17 Their properties are shown in Table 1. Polystyrene standards for MW calibration were prepared by mixing four different polystyrenes (supplied by Tosoh Co., Ltd.) that had MWs of 19 700, 9500, 2800, and 580 amu. Other chemicals from Aldrich Co., Ltd., were tested as new calibration standards. Analysis. The GPC system that was used consisted of an Agilent Series 1100 LC/MSD SL system. It consisted of a binary pump, an autosampler, a column compartment, a diodearray detector (DAD), and an atmospheric photoionization mass selective detector (APPI-MSD). The GPC column used was made by Shodex (model K402.5HQ); it had an exclusion limit of 20 000 amu, an internal diameter of 4.6 mm, and a column length of 250 mm. In most cases, the samples analyzed consisted of 20 µL of 0.2 wt % asphaltene in chloroform (CHCl3). Elution was performed with CHCl3 at a flow rate of 0.3 mL/min and a temperature of 40 °C.
Figure 2. Comparison of chromatograms obtained using a diode-array detector (DAD) (denoted by the thick line) and a mass selective detector (MSD) of Maya asphaltene at a wavelength of 300 nm on the DAD and a fragmentor setting of 150 (denoted by the thin line).
Figure 3. Recorded values of molecular weight versus retention time for Maya asphaltene at a fragmentor setting of 150.
Results and Discussion Sensitivity of APPI-MSD. The APPI-MSD system is accessible to nonpolar compounds, and it readily accepts CHCl3 as an eluentsunlike the ESI-MSD systemsenabling GPC-MSD to be applied to MW analysis. In the GPC-MSD system, the ionization efficiency is controlled by a parameter called the “fragmentor”. At higher fragmentor settings, heavier molecules are ionized, but more fragmentations occur. To find the setting best suited to our study, AS-MY was analyzed over a range of fragmentor settings of 150-400. The area distributions for fragmentor settings of 150, 200, 300, and 400 are shown in Figure 1, with the area sum(12) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 17, 135-139. (13) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Pet. Sci. Technol. 2003, 21, 491-505. (14) Zhang, Y.; Takanohashi, T.; Sato, S.; Kondo, T.; Saito, I.; Tanaka, R. Energy Fuels 2003, 17, 101-106. (15) Zhang, Y.; Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2004, 18, 283-284.
Figure 4. Correlation between molecular weights determined using mass spectroscopy (MWms) and polystyrene standards (MWps). Molecular weights were obtained at retention times of 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, and 14 min. Points A, B, and C mark the chosen calibration standards described in Figure 5.
marized every 100 amu. No significant difference is observed in the curves obtained for fragmentor settings of 150 and 200. Above 200, the curves shift toward smaller MW values, although no additional ionization occurred in the higher MW range. These results led to a fragmentor setting of 150 being used in subsequent experiments.
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Energy & Fuels, Vol. 19, No. 5, 2005 1993
Figure 5. Selected calibration standards for converting molecular weight from MWps to MWms: A, decacyclene (MW ) 451); B, 2,9,16,23-tetra-t-butyl-29H,31H-phthalocyanine (MW ) 739); and (C) 2,4,6-tris(3′,5′-di-t-butyl-4′-hydroxybenzyl)mesitylene (MW ) 775).
Figure 2 shows the chromatograms obtained using both DAD at 300 nm and APPI-MSD. The chromatogram from DAD starts after ∼8 min or at 20 000 amu, whereas that from the APPI-MSD does not commence until ∼10.5 min, at ∼2000 amu. Clearly, APPI-MSD has poor sensitivity in the high-MW range. Molecular Weight Distribution. Figure 3 shows the record obtained from AS-MY with the x- and y-axes displaying the retention time and MW, respectively. The ridge line crosses the calibration curve obtained from the polystyrene standards at ∼900 amu; however, toward lower MW values, the MWms values are higher than the MWps values expected from the calibration curve. MW becomes independent of the retention time after 13 min, and, consequently, MWps has a tendency to be underestimated, compared to MWms. Because MWps is calculated from the retention time through a calibration curve obtained by polystyrene, MWps is correct only when a sample has the same adsorbing power to a GPC column as polystyrene. However, the authors reported that the smaller asphaltene molecule has the higher carbon aromaticity,18 which suggests that the smaller asphaltene molecule has stronger adsorption power than does polystyrene. This results in a delay in retention time and a MW value that is much lower than expected. On the other hand, an APPI mass detector detects a protonated molecular ion (MH+). MWms is determined from m/z, regardless of the retention time. For these reasons, MWms is more reliable than MWps, and such a gap is observed (see Figure 3). Figure 4 shows the correlation between MWps and MWms of the three asphaltenes at every 0.5 min in the retention time in the range of 11-14 min; the broken line shows the calibration curve obtained from polystyrene. No significant difference exists in the relationship found among the different asphaltenes, and there is a linear correlation between MWps and MWms, which differs from the polystyrene calibration curve.16 New Calibration Standard. The results in Figure 4 indicate that a more-precise picture of MW distribution, as determined by GPC, is possible if the calibration curve were revised to bridge existing gaps. Three (16) Zhang, Y.; Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. J. Jpn. Pet. Inst. 2004, 47, 32-36. (17) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Energy Fuels 2004, 18, 1405-1413. (18) Sato, S.; Takanohashi, T.; Tanaka, R. In Heavy Hydrocarbon Resources Characterization, Upgrading, and Utilization; Nomura, M., Rahimi, P. M., Koseoglu, O. R., Eds.; ACS Symposium Series 895; American Chemical Society: Washington, DC, 2005; pp 65-74.
Figure 6. Molecular weight distributions of asphaltene from oil sand bitumen (AS-OB) before and after MW correction.
appropriate standards for such a curve were selected after testing several pure compounds, including polyaromatics and porphyrins (Figure 5). The MWps and MWms of the three are shown in Figure 4 (denoted as A, B, and C). In the revised calibration, the MWms of an asphaltene is given to be ∼450 amu, even where MWps is 0 amu. Application of the revised calibration curve to the asphaltenes that we tested allows the observed MWps to be converted to a more-precise MW value. Nevertheless, additional investigations are necessary before the technique can be applied to other fractions, such as maltene, although the reliability of the original calibration curve obtained from polystyrene at a MW range of >900 amu has yet to be investigated. Figure 6 shows the GPC chromatograms of AS-OB. Line A is the chromatogram before the correction. It indicates a MW range of
