Energy & Fuels 2001, 15, 1317-1318
1317
Measurement of Self-Diffusion Coefficient of Asphaltene in Pyridine by Pulsed Field Gradient Spin-Echo 1H NMR Koyo Norinaga,*,† Verina J. Wargardalam,† Susumu Takasugi,† Masashi Iino,† and Shingo Matsukawa‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan, and Laboratory of Marine Biochemistry, Tokyo University of Fisheries, Konan, Minato-ku, Tokyo 108-8477, Japan Received March 15, 2001. Revised Manuscript Received June 26, 2001 Asphaltenes are complex mixtures of naturally occurring organic compounds found in fossil fuels such as petroleum crude oils.1 A conventional definition of asphaltene is that being insoluble in n-alkanes, such as n-pentane and n-heptane, and soluble in toluene under certain conditions. Asphaltenes are known to readily associate and form aggregate, or micelles in organic solvents.1 This makes it very difficult to determine the molecular weights of asphaltene using conventional techniques such as vapor pressure osmometry or gel permeation chromatography. Many attempts have been so far made to reveal the self-association characteristics of asphaltenes.1 The diffusion coefficient is an important molecular characteristic that can yield information concerning molecular size, as well as aggregation states. There are many techniques which can be employed to measure diffusion coefficients, one of which is pulsed field gradient 1H NMR (PFG-1H NMR).2 The PFG 1H NMR measurements determine the displacement of proton in the time interval between the two gradient pulses applied in the pulse sequence. The displacement by the Brownian motion is usually related to the self-diffusion coefficient of the molecule of which the protons are part, no matter how many molecular species are simultaneously present or diffusing. Thus, this technique is useful for the analysis of complex mixtures such as natural organic compounds. To our knowledge, PFG NMR has not been applied in the past to study asphaltene solutions. The present communication reports on the preliminary results obtained from the measurement of self-diffusion coefficient of asphaltene in pyridine by PFG 1H NMR. The asphaltenes used in this study were derived from a vacuum residue of a crude oil from Kafji (Kuwait). A 3.0 g sample of the Kafji vacuum residue was extracted with 60 mL of n-heptane under ultrasonic (38 kHz) irradiation for 30 min at room temperature. The mixtures were subsequently centrifuged under 29000g for 30 min, and the supernatant was immediately filtered through a membrane paper with a pore size of 0.8 µm. The residue was repeatedly extracted with the fresh n-heptane in the same way, until the filtrate become almost colorless. The n-heptane insolubles were dissolved in toluene and filtered through a membrane paper with a pore size of 0.8 µm after the centrifugation. The asphaltenes were finally isolated from the filtrate by rotary evaporation and vacuum-drying at 353 K. The molecular mass distribution * Author to whom correspondence should be addressed. Fax: +8122-217-5655. E-mail:
[email protected]. † Tohoku University. ‡ Tokyo University of Fisheries. (1) Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998.
of the asphaltene was measured by laser desorptionionization mass spectrometry on a spectrometer (Thermoquest Co. Ltd. Vision 2000). The number averaged molecular mass and the weight averaged molecular mass of the asphaltene were 1800 and 2400 g/mol, respectively. The sample solutions were prepared by dissolving the asphaltene in pyridine at the concentrations ranged from 0.1 to 2.0 wt %. The self-diffusion coefficient (D) measurements were performed on a Brucker DRX 300 proton NMR spectrometer operating at 300 MHz for 1H equipped with a magnetic field gradient probe at 303 K. The sequence to measure diffusion is very similar to the spinecho pulse sequence,3,4 except that two gradient pulses are applied: one before the refocusing 180° pulse, and one after the 180° pulse. These two gradient pulses are identical in amplitude, G, and width, δ. They are separated by a time ∆ and are placed symmetrically about the 180° pulse. The function of the first gradient pulse is to dephase magnetization according to its position in the NMR tube. During the subsequent ∆ period, the spins are left to evolve and diffuse. Their chemical shift is refocused by applying the usual 180° pulse. At the end of the ∆ period, the spins that have diffused to a new location in the NMR tube will not get refocused by the second gradient and will therefore attenuate the signal. The relationship between the signal amplitude (A) obtained in the presence of a gradient amplitude G in the z direction and the diffusion coefficient D in the same direction is given by
A/A0 ) exp [-(Gγδ)2D(∆ - δ/3)]
(1)
where A0 is the signal amplitude at zero gradient and γ the gyromagnetic ratio (2.675 × 108 T-1 s-1 for 1H). In our experiments, δ and ∆ were fixed at 1.0 and 5.0 ms, respectively, and G was varied from 0.38 to 11.44 T m-1. Figure 1 shows the spin-echo 1H NMR spectra of pyridine solution of Kafji asphaltene (1 wt %) as a function of G. The observed spectra consist of contributions from both solute (asphaltene) and solvent (pyridine). Because the diffusivities of the two species differ greatly, the amplitude of each signal changes nearly individually with varying G. At G less than 3.02 T m-1, three peaks arisen from pyridine are predominant contributors to the observed spectra. The peaks diminish in size with increasing G and disappear almost completely at G ) 4.69. The two broad peaks assigned to the asphaltene are clearly observed beyond G ) 4.69. The G-dependent (2) von Meerwall, E. D. Adv. Polym. Sci. 1983, 54, 1. (3) Hahn, E. L. Phys. Rev. 1950, 80, 580. (4) Carr, H. Y.; Purcell, E. M. Phys. Rev. 1954, 94, 630.
10.1021/ef0100597 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/01/2001
1318
Energy & Fuels, Vol. 15, No. 5, 2001
Communications
Figure 1. Spin-echo 1H NMR spectra of pyridine solution of Kafji asphaltene (1 wt %) as a function of field gradient strength.
Figure 2. Plots based on eq 1.
changes in the amplitude of the signal arisen from aliphatic hydrogens of the asphaltene was analyzed to evaluate D of asphaltene (Figure 2). Each plot can be approximated by a single straight line, though asphaltenes are mixtures of organic compounds with different chemical structures and molecular size. In addition, the analysis using the data of aromatic peak area suggested that the aromatic and aliphatic components of the asphaltene diffuse at nearly same rate. D decreases with asphaltene concentration from 1.4 × 10-10 to 0.9 × 10-10 m2/s in the range 0.1-2.0 wt % as shown in Figure 3. The D of pure pyridine was also measured and found to be 1.6 × 10-9 m2/s at 303 K. The D of pyridine solvent was almost same as that of pure pyridine at asphaltene concentrations up to 2 wt %. Applying the StokesEinstein equation suggests that the hydrodynamic radius (sphere equivalent) of the asphaltene increases from 1.8 nm at 0.1 wt % to 2.8 nm at 2.0 wt %. The diffusion coefficients were also measured using the Taylor dispersion method.5 The procedure of the measurement is given elsewhere.6 The method is based on the dispersion of the probe component in a laminar flow of a solvent and provides the diffusion coefficient (D∞) of the asphaltene sample in pyridine at 303 K to be 2.4 × 10-10 m2/s, corresponding to the hydrodynamic radius of 1.1 nm. D∞ (5) Taylor, G. Proc. R. Soc. London, Ser. A 1953, A219, 186. (6) Wargadalam, V. J.; Norinaga, K.; Iino, M. Energy Fuels, in press.
Figure 3. D as a function of asphaltene concentration. A plot at concentration ≈ 0 represents D∞ determined by the Taylor dispersion technique.
is approximately two times larger than D at 0.1 wt %. D∞ would reflect an average character of nonassociated asphaltene molecules and is plotted at concentration ≈ 0 in Figure 3, since the Taylor dispersion was measured for the very diluted solution of asphaltene. Although there are presently some uncertainties in a direct comparison of the D given by PFG NMR with D∞ by Taylor dispersion, the molecular aggregates formation is one of the most reasonable explanations for the large difference between D∞ and D at 0.1 wt %. The onset of molecular aggregation of the asphaltene sample in pyridine would occur at concentrations less than 0.1 wt %. This is supported by the evidence that asphaltenes can form micelles at fairly low concentrations, for example, at 0.05 wt % for vacuum resid in pyridine.7 Acknowledgment. The authors are grateful to Dr. Hiroyuki Seki of Petroleum Energy Center for providing the crude oil samples and molecular mass distribution data of the asphaltene. This work was supported in part by a “Research for the Future Project” grant from the Japan Society for the Promotion of Science (JSPS), through the 148 Committee on Coal Utilization Technology. EF0100597 (7) Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299.
