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Energy & Fuels 1999, 13, 552-557
A Comparative Study of Bitumen Molecular-Weight Distributions M. Domin,† Alan Herod,† Rafael Kandiyoti,‡ John W. Larsen,*,§ M-J. Lazaro,‡ Shang Li,§ and Parviz Rahimi| Department of Pharmaceutical and Biological Chemistry, School of Pharmacy, Brunswick Square, London, U.K. WC1N 1AX, Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology, and Medicine, Prince Consort Road, London, U.K. SW7 2BY, Department of Chemistry, 6 East Packer, Lehigh University, Bethlehem, Pennsylvania 18015, and National Centre for Upgrading Technology, 1 Oil Patch Drive, Suite A202, Devon, Alberta, Canada T6G 1A8 Received April 23, 1998
A sample of Athabasca bitumen was fractionated by preparative size-exclusion chromatography (SEC), and the molecular-weight distribution of the five fractions and the original sample were determined by SEC in NMP solvent using a calibration based on polystyrene standards, by 252Cf plasma desorption mass spectrometry (PDMS), and by laser desorption and matrix-assisted laserdesorption mass spectrometry (LDMS and MALDI) and the average molecular weights were measured by vapor pressure osmometry (VPO). A comparison of the results shows that VPO, MALDI, and SEC gave similar results while PDMS showed the same trend. The molecular-weight distribution of this 525+ °C fraction is continuous up to and perhaps beyond a molecular weight of 15 000.
Introduction It has proven difficult to obtain molecular-weight distributions or even average molecular weights for asphaltenes and bitumens. This fundamental property of these mixtures is important in almost all physical and chemical measurements made on them. Here we report the size-exclusion chromatographic fractionation of a bitumen and the measurement of its number average molecular-weight distribution by size exclusion chromatography (SEC) using N-methylpyrrolidinone (NMP) solvent (polystyrene calibration standards) by 252Cf plasma desorption mass spectrometry (PDMS), by laserdesorption mass spectrometry (LDMS), and by matrixassisted laser desorption ionization mass spectrometry (MALDI). All of the techniques except PDMS give similar results. PDMS only qualitatively reproduces the trend established by the other methods. VPO and other colligative property measurements require a solvent in which the bitumen is not associated under the measurement conditions. This produces a race between the apparatus sensitivity and the association of the bitumen. If the instrument sensitivity allows measurements at concentrations so low that complete dissociation occurs, reliable average molecular weights can be obtained. Most common are several measurements made on associating systems at decreasing concentrations and thus decreasing degrees of associa* Author to whom correspondence should be addressed. † School of Pharmacy. ‡ Imperial College of Science, Technology, and Medicine. § Lehigh University. | National Centre for Upgrading Technology.
tion.1 Extrapolation to infinite dilution then yields the desired average molecular weight. This extrapolation assumes a constant slope, a risky assumption for systems in which even changes in the sign of the slope have been observed.2 Also, many changes can be hidden in an average molecular weight. The full molecularweight distribution is more desirable. The technique most thoroughly explored for obtaining bitumen and asphaltene molecular-weight distributions is SEC, and its problems are well documented.3-5 These problems are both fundamental and operational. On a fundamental level, the separation is based on molecular size, not mass. For mixtures of molecules of differing shapes, the size-mass relationship may be complex. The technique requires a standard to calibrate the column; a standard that must mimic closely the shape of the unknown to be studied. The consensus is that no available commercial polymer standards are suitable for bitumens or asphaltenes. Sorption by the organic polymer column is perhaps the most serious operational problem.3 Polar compounds have lower solubilities than nonpolar molecules in the cross-linked polystyrene column packing. Because of this solubility difference, nonpolar molecules have an additional retention mech(1) Slade, P. E., Ed.; Polymer Molecular Weights; Marcel Dekker: New York, 1975. (2) Given, P. Personal communication. (3) Mulligan, M. J.; Thomas, K. M.; Tytko, A. P. Fuel 1987, 66, 1472-1480. (4) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Fuel 1984, 63, 1556-1560. (5) Champagne, P. J.; Emmanuel, M.; Ternan, M. Fuel 1985, 64, 432-425.
10.1021/ef980065b CCC: $18.00 © 1999 American Chemical Society Published on Web 04/08/1999
Study of Bitumen Molecular-Weight Distributions
Energy & Fuels, Vol. 13, No. 3, 1999 553
Table 1. Composition and Molecular Weights of Fractions elemental (wt %) GPC fraction feed 1 2 3 4 5 loss additivity
wt % 15.87 22.32 20.95 15.65 23.62 1.59
M h n VPO (g/mol)
M h n PDMS (g/mol)
M h w PDMS (g/mol)
GPC MW
MALDI max peak intensity
630 790 590 540 540 530
1020 1360 990 840 830 800
430 3200 1005 480 390 370
