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Direct Separation and Quantitative Determination of (n-, Iso-)Alkanes in Neat Asphalt Using Urea Adduction and High-Temperature Gas Chromatography (HTGC)† Daniel A. Netzel* and Joseph F. Rovani Western Research Institute, 365 North 9th Street, Laramie, Wyoming 82070 ReceiVed June 5, 2006. ReVised Manuscript ReceiVed October 19, 2006
Urea adduction and high-temperature gas chromatography were used to separate and quantify the amount of normal plus iso-alkanes (n-, iso-)alkanes in 13 neat asphalts. Treating neat asphalt directly with urea eliminated the separation steps necessary for extracting the saturate fraction containing the alkanes from the asphalt. The amounts of n-alkanes including 2-methyl and 3-methyl isomers for the 13 asphalts ranged from 0 to 1.84 wt %. Saturate fractions from three asphalts were obtained by adsorption-desorption chromatography. The amounts of (n-, iso-)alkanes and the highly branched plus cyclic alkanes were determined also for these fractions using urea adduction and were found to be in good agreement with the direct determination of (n-, iso-)alkanes in the neat asphalts. The amount of (n-, iso-)alkanes was found to correlate with the crystalline wax content measured by differential scanning calorimetry, but the method does not account for all of the crystalline wax content. In addition, the amount of (n-, iso-)alkanes does not account for the large n-alkane methylene signals observed in the hydrogen-1 and carbon-13 nuclear magnetic resonance spectra for the asphalts. No correlation was found between the amount of (n-, iso-)alkanes in asphalt and the fracture temperature. However, an apparent correlation does exist between the highly branched plus cyclic alkanes in asphalt and the fracture temperature.
Introduction Low-temperature cracking is one of the primary modes of failure for asphalt pavement. This type of failure mode has been attributed, in part, to paraffin waxes. These paraffin waxes are commonly divided by structure and behavior into macrocrystalline, microcrystalline, and amorphous waxes. On the molecular level, macrocrystalline waxes, which form large crystals, are linear alkanes with less than 40 carbons. Microcrystalline waxes are linear alkanes with carbon numbers greater than 40 and give rise to small crystals. Highly branched alkanes with or without alicyclic and/or aromatic substituents are less likely to form crystals and, thus, give rise to amorphous phase (noncrystalline waxes) in asphalt. A number of methods1 have been developed to determine the total wax content in asphalt. Turner and Branthaver2 reported that differential scanning calorimetry (DSC) is most suited to crystalline wax determination. Carbon-13 (13C) nuclear magnetic † Disclaimer: The mention of specific brand names does not imply endorsement by the Western Research Institute or the Federal Highway Administration. The contents of this paper reflect only the views of the authors, who are responsible for the facts and accuracy of the data presented. This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for its contents or use thereof. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration. This paper does not constitute a standard, specification, or regulation. * To whom correspondence should be addressed: 2526 Sky View Lane, Laramie, Wyoming 82070. E-mail:
[email protected]. (1) Freund, M.; Csiko´s, R.; Keszthelyi, S.; Mo´zes, G. Y. Developments in Petroleum Science. In Paraffin Products. Properties, Technologies, Applications; Mo´zes, G. Y., Ed.; Elsevier: Amsterdam, The Netherlands, 1982; Chapter 14. (2) Turner, T. F.; Branthaver, J. F. DSC Studies of Asphalts and Asphalt Components. In Asphalt Science and Technology; Usmani, A. M. Ed.; Marcel Dekker: New York, 1997; pp 59-101.
resonance (NMR) has been used to quantify the crystalline and amorphous phases in asphalt.3,4 Michon et al.3 reported a linear correlation between the weight percent of crystalline methylene carbons determined by 13C NMR and the weight percent of crystalline wax as determined by DSC. In the same paper, the weight percent of mobile, amorphous aliphatic carbons was shown to correlate with the fracture temperature (a lowtemperature cracking parameter) for different asphalts reported by Jung and Vinson.5 In all of the investigations, the total amounts and carbon-number distributions of n-alkanes and branched alkanes were not determined. Normal and branched alkanes have been separated, identified, and quantified in some petroleum products using the techniques of urea adduction and high-temperature gas chromatography (HTGC). HTGC was used to study the occurrence of high-molecular-weight hydrocarbons, waxes, and asphaltenes in crude oils.6,7 Urea-adduct formation was originally thought to be specific for normal alkanes, although many exceptions are now known.7,8 The specificity occurs through the formation of a hexagonal urea lattice around the linear alkane chains and branched alkanes with only a few methyl substituents. Highly branched and cyclic alkanes and aromatics do not possess the critical size and (3) Michon, L. C.; Netzel, D. A.; Turner, T. F.; Martin, D.; Planche, J.-P. Energy Fuels 1999, 13, 602-610. (4) Netzel, D. A.; Miknis, F. P.; Wallace, J. C.; Butcher, C. H.; Thomas, K. P. Molecular Motions and Rheological Properties of Asphalts: An NMR Study. In Asphalt Science and Technology; Usmani, A. M., Ed.; Marcel Dekker: New York, 1997; Chapter 2. (5) Jung, D.; Vinson, T. S. Transp. Res. Rec. 1993, 1417, 12-20. (6) Hsieh, M.; Philp, R. P. Org. Geochem. 2001, 32, 955-966. (7) Takemodo, K.; Sonoda, N. Inclusion Compounds of Urea, Thiourea and Selenourea. In Inclusion Compounds; Atwood, J. L., Davis, J. E. D., Nicol, D., Eds.; Academic Press: New York, 1984; Chapter 2, Vol. 2, pp 47-67. (8) Redlich, O.; Gable, C. M.; Dunlop, A. K.; Millar, R. W. J. Chem. Phys. 1950, 14, 150.
10.1021/ef060256b CCC: $37.00 © 2007 American Chemical Society Published on Web 12/10/2006
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symmetry for adduct formation. Numerous investigations have been conducted on the structure of urea adducts, also known as urea inclusion compounds.9-11 The urea-adduction methodology has been used to separate and quantify n-alkanes in a few petroleum-related products. Cookson and Smith12 used urea adduction to separate the n-alkanes from the branched plus cyclic alkanes in petroleumand coal-derived diesel fuels. Deparaffination of light crudes using the urea-adduction technique was reported by Nwadinigwe and Eze.13 Nwadinigwe and Nwobodo14 compared the molecular sieve and urea adduction as analytical techniques to determine the amounts of normal alkanes in light crudes. These authors found that the urea-adduction technique performed better than the molecular sieve technique for isolating n-alkanes. An investigation to determine the normal paraffins in petroleumheavy distillate by urea adduction and gas chromatography was conducted by Marquart et al.15 Khan and Medhwal16 studied the effectiveness of some compounds, such as alcohols, acids, ketones, and amides, as activators in the formation of inclusion compounds between urea and n-alkanes in a gas oil fraction derived from Ankleawar crude oil. Although the uses of the urea-adduction technique have been reported for some liquid petroleum products, the technique has not been used to separate and quantify the amount of normal and branched alkanes in asphalt. The amounts of n-alkanes and branched alkanes in asphalt would be useful in assessing the molecular-structural character of the low-temperature cracking parameter, the crystalline wax, and resolve the issue of the large methylene resonance signals observed in 1H and 13C NMR spectra of an asphalt.17 That is, whether the methylene resonance signals are due to the presence of large amounts of n-alkanes or whether they are due to a long methylene carbon chain length attached to alicyclic and/or aromatic structures. Experimental Section Asphalt Samples. Two sets of neat asphalts were treated directly with urea. The first set of asphalt samples consisted of asphalts from North Africa (Ashtart), Venezuela (Laguna), Africa (Mandji), Italy (Marghera), and Middle East (Safaniya). The five asphalt samples were provided by Elf-Antar France. The properties of these asphalts have been described in a paper by Michon et al.3 The second set of asphalts consisted of the eight Strategic Highway Research Program (SHRP) core asphalts obtained from the Material Research Library (MRL):18 Canada (Lloydminster, AAA-1; and Redwater, AAC-1), U.S.A. (Wyoming Sour, AAB-1; California Coastal, AAD-1; West Texas Sour, AAF-1; California Valley, AAG-1; and West Texas Intermediate, AAM-1), and Venezuela (Boscan, AAK-1). (9) Cookson, D. J.; Rolls, C. L.; Smith, B. E. Fuel 1989, 68, 788-792. (10) Harris, K. D. M. J. Solid State Chem. 1993, 106, 83-98. (11) Brustolon, M.; Maniero, A. L.; Marcomini, A.; Segre, U. J. Mater. Chem. 1996, 6, 1723-1729. (12) Cookson, D. J.; Smith, B. E. Anal. Chem. 1985, 57, 864-871. (13) Nwadinigwe, C. A.; Eze, S. O. Fuel 1990, 69, 126-128. (14) Nwadinigwe, C. A.; Nwobodo, I. O. Fuel 1994, 73, 779-782. (15) Marquart, J. R.; Dellow, G. B.; Freitas, E. R. Anal. Chem. 1968, 40, 1633-1637. (16) Khan, H. U.; Medhwal, D. C. Sci. Technol. 1993, November, 415417. (17) Jennings, P. W.; Pribanic, J. A.; Desando, M. A.; Raub, M.F.; Stewart, F.; Hoberg, J.; Moats, R.; Smith, J. A.; Mendes, T. M.; McGrane, M.; Fanconi, B.; VanderHart, D. L.; Manders W. F. Binder Characterization and EValuation by Nuclear Magnetic Resonance Spectroscopy, SHRP-A335; Strategic Highway Research Program, National Research Council: Washington, D.C., May 1993. (18) Jones, D. R., IV SHRP Materials Reference Library: Asphalt Cements: A Concise Data Compilation, SHRP-A-645; Strategic Highway Research Program, National Research Council: Washington, D.C., May 1993.
Netzel and RoVani Saturate Fractions. The saturate fractions from asphalts AAA1, AAB-1, and AAM-1 were obtained using the standard SAPA separation scheme19 with a slight modification. The aluminum oxide used in this chromatographic separation was activated at a temperature from 50 to 100 °C higher than in the standard procedure. The (n-, iso-)alkanes and the highly branched plus cyclic alkanes in these fractions were separated using urea adduction and quantified gravimetrically. Reagents and Reference Materials. ACS-reagent grade (99%) urea, n-octacosane (99%), and squalane (99%) were obtained from Aldrich Chemical Co. n-Hexacontane (>98%) and n-tetracontane (>99%) were obtained from Fluka Chemie AG. Polywax 500 used as a calibration standard was obtained from Accustandard (ASTMP-0051N). Urea-Adduction Methodology. The qualitative and quantitative assessment of (n-, iso-)alkanes contained in asphalts required the adaptation of the standard urea-adduction methodology reported in the literature for determining n-alkanes in liquid fossil fuel materials.20 The amount of alkanes in the saturate fraction was quantified gravimetrically, and for the asphalt, HTGC was used for quantifying the alkanes. The HTGC technique not only gives the total amount of (n-, iso-)alkanes but also the relative percents of normal alkanes and the isomers 2-methyl and 3-methyl branched alkanes. Saturate Fractions. A total of 1 g of the saturate fraction and 0.1 g of n-octacosane (n-C28) were accurately weighed ((0.0001 g) into a small beaker. A total of 5 mL of CH3OH was added to dissolve the mixture. n-Octacosane was used as a carrier for the small amount of (n-, iso-)alkanes in the saturate fraction. A total of 10 g of urea was weighed into a 125 mL Erlenmeyer flask. A total of 5 mL of CH3OH and 85 mL of CHCl3 were added to the flask to dissolve the urea. The two solutions were combined, stirred for 2 h at room temperature, and allowed to stand overnight. The solution containing the precipitated urea adduct of (n-, iso-)alkanes, excess urea, and the highly branched and cyclic alkanes is filtered using a glass-fritted funnel. The precipitated adduct was washed 6 times with 10 mL of CHCl3 and allowed to air dry. The dried adduct was transferred to a 250 mL separatory funnel. A total of 50 mL of CHCl3 and 100 mL of distilled water were added to the separatory funnel, moderately shaken, and allowed to stand until the two immiscible liquids separated. The CHCl3 layer containing the (n-, iso-)alkanes was transferred into a weighed beaker. The CHCl3 was removed by heating; the beaker was reweighed; and the amount of (n-, iso-)alkane in the saturate fraction was determined. The filtrate containing the highly branched and cyclic alkanes was combined with the 6 times CHCl3 washing and transferred to a 250 mL separatory funnel containing 50 mL of water. The separatory funnel was shaken moderately and allowed to stand until the immiscible liquids separated. The CHCl3 layer was transferred to an accurately weighed beaker and heated to remove the solvent, and the beaker was reweighed to determined the amount of the highly branched plus cyclic alkanes. The percents of (n-, iso-)alkanes and the highly branched plus cyclic alkanes were determined from the weights of the respective alkanes and the initial weight of the saturate fraction. Asphalt Samples. A total of 10 g of asphalt was accurately weighed ((0.0001 g) into a 400 mL beaker. A total of 20 ( 0.1 mg of n-C28 and 5 ( 0.1 mg of n-C60 accurately weighed on weighing paper are transferred to the beaker. n-C28 and n-C60 were used to quantitatively assess both the recovery of low- and highmolecular-weight alkanes from the asphalt and to monitor the limit of detectability of the HTGC. A total of 100 mL of HPLC-grade CHCl3 was added to the beaker, and the mixture was stirred and heated to the boiling point of CHCl3 (61 °C) and then cooled to (19) Corbett, L. W. Anal. Chem. 1969, 41, 576-579. (20) Weiss, F. T. Determination of Normal Paraffins in Heavy Fractions. In Chemical AnalysissDetermination of Organic Compounds: Methods and Procedures; Ewing, P. J., Kolthoff, I. M., Eds.; Wiley-Interscience: New York, 1970; Chapter 1, Vol. 32, p 14.
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Table 1. Relative Area Percent of Normal Alkanes in a Standard Toluene Solution by HTGC
alkane
initial relative weight percent
n-octacosane n-tetracontane n-hexacontane
43.2 29.7 27.0
HTGC relative area percent run 1 run 2 44.4 28.3 27.3
43.8 28.3 27.8
∼40 °C. A total of 100 mL of the stock solution (0.1 g/mL) of urea in CH3OH was slowly added to the warm mixture of asphalt and the two reference n-alkanes. This solution was then stirred and heated to 49 °C (the boiling point of the mixture) for 1 h. The beaker is removed from the hot plate and was allowed to stand for 16 h without a cover to allow for the evaporation of the solvents and the crystallization of the excess urea and the urea adduct of the (n-, iso-)alkanes. The evaporation was allowed to continue until near dryness. The asphalt in the urea-adduct mixture was then redissolved with 100 mL of CHCl3. The solution containing the excess urea crystals and urea-adduction crystals was vacuum-filtered using an accurately weighed 60 mL, 10-20 µm glass-fritted filter funnel. The crystals were washed with 100 mL of CHCl3 to completely remove any residual asphalt. The funnel containing the crystals was allowed to stand for 16 h to remove any residual CHCl3. The crystals in the funnel were washed first with 10 mL of distilled water at room temperature followed by 80 mL of hot distilled water (∼70-80 °C). The water dissolves the excess urea and releases the (n-, iso-)alkanes from the urea-adduct crystals. The funnel containing the solid wax-like (n-, iso-)alkanes was placed in a vacuum desiccator for 16 h. Drierite is used as a desiccant. The funnel was accurately weighed repeatedly until a constant weight was obtained. The precipitate (n-, iso-)alkanes was dissolved in the filter funnel using 30 mL of hot toluene (∼90 °C) and collected in a small accurately weighed beaker. The toluene was removed first by roomtemperature evaporation and, if necessary, by gentle heating. The beaker is again accurately weighed, and the amount of (n-, iso-)alkanes recovered was determined. The n- and iso-alkanes recovered were then redissolved in toluene and injected into the HTGC to the determine the amounts of n-C28, n-C60, and (n-, iso-)alkanes, the carbon-number distribution, and the relative amounts of n-alkanes and the 2-methyl and 3-methyl isomers. HTGC. An HP5890 series II gas chromatograph was retrofitted with a 15 m × 0.32 mm × 0.1 µm film thickness DB-1 hightemperature column, operated at an initial temperature of 201 °C and ramped to 385 °C. The injection port was a flow inversion cup design operated in the split mode and run at a temperature of 375 °C. The detector was a flame ionization detector at 395 °C. Helium was used as the carrier gas. Samples (1 µL) were injected using an autosampler. The qualitative performance of the HTGC technique was evaluated using Polywax 500 and the conditions sited above. The carbon-number distribution of the n-alkanes was found to be in good agreement with the distribution reported by Liang and Hsu.21 The quantitative performance of the HTGC technique was evaluated using a mixture of known amounts of n-C28, n-C40, and n-C60 in toluene. Table 1 gives the relative weight percent of the n-alkanes in the mixture and the relative area percent determined by HTGC. The agreement is quite good. To assess the recovery and quantitation of the n-alkanes using urea-adduction and HTGC methodologies together, urea adduction was performed on a mixture of weighed amounts of n-C28, n-C40, and n-C60 dissolved in CHCl3. A reasonably good agreement was obtained between the known relative weight percent of the n-alkanes in the mixture and the relative area percent determined by HTGC. The numerical results and a further discussion can be found in the Federal Highway final report.22 (21) Liang, Z.; Hsu, C. S. Energy Fuels 1998, 12, 637-643.
Weighed samples of the (n-, iso-)alkanes separated from the asphalt were prepared for HTGC using a stock solution of the internal standard squalane in toluene (0.25 mg/mL). The sample vials were placed in an aluminum block on a hot plate at 90 °C until needed. Heating was necessary to prevent any high-molecularweight alkanes from solidifying. Figure 1 gives the high-temperature chromatogram of the (n-, iso-)alkanes from the asphalt AAC-1. The first large peak at a retention time of 2.34 min is due to the internal standard, squalane. The peaks for n-C28 and n-C60 are at retention times of 3.02 and 20.38 min. The large peaks with retention times from 1 to 27 min are due to the normal alkanes. The smaller peaks between the large peaks are the iso-alkanes. The squalane integrated area was used to calculate the amounts of the low-molecular-weight n-C28 and the high-molecular-weight n-C60 recovered from the urea adduction of the asphalt. The baseline used for area integration was from the start to the end of the peak for the standards, squalane, n-C28, and n-C60. For the (n-, iso-)alkanes, the baseline used followed the curved contour for all of the peaks. The weight percent of (n-, iso-)alkanes in the neat asphalt was calculated from the sum of the peak areas and the total amount of asphalt used in the experiment. A complete description for calculating the percent of (n-, iso-)alkanes in asphalt from the chromatographic areas and weights of materials is given in the Federal Highway report.22
Results and Discussion Saturate Fractions. Table 2 lists the saturate yields obtained at the Western Research Institute (WRI) and the yields reported in the MRL report18 for asphalts AAA-1, AAB-1, and AAM-1. In all cases, the WRI yields are higher than the MRL values. The higher yields are the result of the higher temperature treatment of the aluminum oxide, which was the parameter changed in the separation method. A large difference between the saturate yields is noted for asphalt AAM-1. The hightemperature gas chromatograms of the saturate fraction from asphalt AAM-1 show that the higher temperature treatment of the aluminum oxide results in more extraction of normal and branched alkanes. The chromatograms for the three asphalts are given in the Federal Highway quarterly report.23 The urea-adduction procedure was used to separate the (n-, iso-)alkanes from the more highly methyl-substituted branched plus cyclic alkanes for the saturate fractions of asphalts AAA-1, AAB-1, and AAM-1. The weight percent of (n-, iso-)alkanes and the highly branched plus cyclic alkanes are also given in Table 2. Neat Asphalts. Figure 1 shows the high-temperature gas chromatogram of (n-, iso-)alkanes separated from asphalt AAC1. The high-temperature gas chromatograms of the (n-, iso-)alkanes separated from the other asphalts in sets 1 and 2 are given in the appendix of the Federal Highway report.24 The chromatograms show that asphalt from different sources can be grouped according to the carbon chain-length distribution and the amount of (n-, iso-)alkanes. Asphalts Laguna, AAA-1, AAD-1, AAF-1, AAG-1, and AAK-1 have, in general, (22) Robertson, R. E.; Branthaver, J. F.; Harnsberger, P. M.; Petersen, J. C.; Dorrence, S. M.; McKay, J. F.; Turner, T. F.; Pauli, A. T.; Huang, S.-C.; Huh, J.-D.; Tauer, J. E.; Thomas, K. P.; Netzel, D. A.; Miknis, F. P.; Williams, T.; Duvall, J. J.; Barbour, F. A.; Wright, C.; Salmans, S. L.; Hansert, A. F. Fundamental Properties of Asphalts and Modified Asphalts, Volume II: Final Report, New Methods; FHWA-RD-99-213. U.S. Department of Transportation, Federal Highway Administration: McLean, VA, 2001. (23) Western Research Institute. Fundamental Properties of Asphalts and Modified Asphalts. Quarterly Technical Report, August 16-November 15, 2001; Federal Highway Administration, contract number DTFH61-99C00022, November 2001; pp 183-193.
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Netzel and RoVani
Figure 1. High-temperature gas chromatogram of the (n-, Iso-)alkanes separated from asphalt AAC-1 using urea adduction. Table 2. Weight Percent of Saturates in Neat Asphalts and the Weight Percent of (n-, Iso-)alkanes and Highly Branched Plus Cyclic Alkanes in Saturate Fractions for Asphalts AAA-1, AAB-1, and AAM-1
asphalt AAA-1 AAB-1 AAM-1
weight percent of saturates in neat asphalt 13.7 (WRI) 10.6 (MRL) 13.2 (WRI) 8.6 (MRL) 9.94 (WRI) 1.9 (MRL)
weight percent of alkanes in saturate fraction highly branched (n-, iso-)alkanes plus cyclic alkanes 1.04 ( 0.2
92.7 ( 9.3
7.07 ( 1.0
91.9 ( 9.2
2.58 ( 8.0
99.0 ( 9.0
(n-, iso-)alkanes distributions ranging from n-C26 to n-C50, with a maximum at approximately n-C29. Asphalts Marghera, AAB1, Mandji, and Safaniya have much broader distributions of (n-, iso-)alkanes, with maxima around n-C36. Asphalts Ashtart, AAC-1, and AAM-1 have the maximum n-alkane distribution at n-C42, n-C45, and n-C48, respectively. These three asphalts also have the highest amount of iso-alkanes relative to the n-alkanes. The relative percents of n-alkanes and iso-alkanes (combined 2-methyl and 3-methyl isomers) for the two sets of asphalts are given in Table 3. The relative percents were determined by summing the peak heights measured manually from the high-temperature gas chromatogram for the asphalt. There is little variation in the relative percents for the normal and isomeric alkanes for the asphalts in set 1 but a considerable variation for the asphalts in set 2. Table 4 lists the weight percent (via HTGC) of (n-, iso-)alkanes separated from the asphalts. The weight percent of the (n-, iso-)alkanes in the asphalts ranges from 0 to 1.84. Duplicate runs were made for three of the asphalts: Ashtart, AAC-1, and AAK-1. The errors associated with the (n-, iso-)alkane determinations for these asphalts range from (0.05 to (0.11% of the average measured value. These values are assumed to represent the errors associated with the other asphalts. Table 5 gives the percent recovered for n-C28 and n-C60 by urea adduction for each of the asphalts. The percent recovered (24) Western Research Institute. Fundamental Properties of Asphalts and Modified Asphalts. Draft Final Report, Interpretive Report; Federal Highway Administration, contract number DTFH61-99C-00022, submitted for review, October 2003; Vol. 1, pp 629-641.
Table 3. Relative Percent of n-alkanes and iso-alkanes for Asphalts in Sets 1 and 2 asphalt set 1 asphalts Ashtart (2) Laguna Mandji Marghera Safaniya set 2 asphalts AAA-1 AAB-1 AAC-1 (1) AAD-1 AAF-1 AAG-1 AAK-1 (2) AAM-1
relative percent of n-alkanes
relative percent of iso-alkanes
66 74 70 75 74
34 26 30 25 26
74 79 71 72 81 00 86 62
26 21 29 28 19 00 14 38
Table 4. Weight Percent of (n-, Iso-)alkanes and Crystalline Wax for Set 1 and 2 Asphalts
a
asphalt
weight percent of (n-, iso-)alkanes
weight percent of crystalline waxa
Ashtart (1) Ashtart (2)b Laguna Mandji Marghera Safaniya AAA-1 AAB-1 AAC-1 (1) AAC-1 (2)b AAD-1 AAF-1 AAG-1 AAK-1 (1) AAK-1 (2)b AAM-1
1.62 1.84 0.22 1.38 1.01 0.65 0.10 0.83 0.60 0.70 0.70 0.74 0.00 0.55 0.66 0.10
5.6 0.2 3.8 2.6 1.9 0.4 2.5 3.0 0.7 1.9 0.0 0.4 3.2
b
From ref 3. Repeat.
for the low-molecular-weight alkane, n-C28, varies from 68 to 120%, and the percent recovered for the high-molecular-weight alkane, n-C60, varies from 63 to 117%. In general, the percent recovered for n-C60 is less than the percent recovered for n-C28. A loss of 1-3 mg of these materials (starting amounts of 20 and 5 mg for n-C28 and n-C60, respectively) because of the multistep separation procedure can easily account for the magnitude and spread in the percent recovered.
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Energy & Fuels, Vol. 21, No. 1, 2007 337
Table 5. Percent Recovery for n-Octacosane and n-Hexacontane asphalt
recovered n-octacosane (%)
recovered n-hexacontane (%)
Ashtart (1) Ashtart (2)a Laguna Mandji Marghera Safaniya AAA-1 AAB-1 AAC-1 (1) AAC-1 (2)a AAD-1 AAF-1 AAG-1 AAK-1 (1) AAK-1 (2)a AAM-1
81 89 117 88 89 108 118 107 68 70 112 102 121 116 115 96
70 79 110 71 86 97 117 82 63 95 94 89 95 89 96 66
a
Figure 3. Weight percent of (n-, iso-)alkanes versus the weight percent of crystalline wax for asphalts in sets 1 and 2.
Repeat.
Table 6. Calculated Weight Percent of (n-, Iso-)alkanes and Highly Branched Plus Cyclic Alkanes in Neat Asphalts and Measured Weight Percent of (n-, Iso-)alkanes in Neat Asphalts
asphalt AAA-1 AAB-1 AAM-1
calculated weight percent of alkanes in neat asphalts highly branched plus cyclic (n-, iso-)alkanes alkanes 0.14 0.11 average 0.13 0.93 0.61 average 0.77 0.26 0.05 average 0.15
12.7 9.8 average 11.3 12.1 7.9 average 10.0 9.8 1.9 average 5.9
measured weight percent of (n-, iso-)alkanes in neat asphalts 0.10 0.83 0.10
Figure 2. Weight percent of (n-, iso-)alkanes versus the weight percent of crystalline wax for asphalts in set 1.
The percent of (n-, iso-)alkanes determined directly for the neat asphalts AAA-1, AAB-1, and AAM-1 and the calculated percent of (n-, iso-)alkanes in the neat asphalts are given in Table 6. The weight percent of (n-, iso-)alkanes in neat asphalt was calculated from the data in Table 2. As shown in the Table 6, the average values for the calculated percents of (n-, iso-)alkanes in the neat asphalts agree remarkably well with the percent of (n-, iso-)alkane values determined directly for the three asphalts. Figure 2 shows a plot of the weight percent of the (n-, iso-)alkanes versus the weight percent of crystalline wax by DSC for the five asphalts in set 1. A second-order polynomial
forced thru the zero intercept was found to best fit the data (R2 ) 0.98). An average value was used for the Ashtart asphalt, with an error bar indicating the high and low values. A 1:1 correlation between the weight percent of (n-, iso-)alkanes and the weight percent of crystalline wax was not observed. Several reasons can account for the nonlinear correlation of less than 1:1: (1) the urea adduction may not be very effective for separating very high-molecular-weight (n-, iso-)alkane, and if appreciable amounts of these alkanes are present, these could easily form crystals detectable by DSC; (2) the moderately branched alkanes present do not form adducts with urea; however, they can form distorted crystals in the asphalt matrix and can also be measured by DSC; and (3) the temperature obtainable in HTGC limited the detection of (n-, iso-)alkanes to n-C80. Figure 3 shows a plot of the weight percent of (n-, iso-)alkanes versus the weight percent of crystalline wax for the asphalts in set 2, superimposed on Figure 2. Duplicate determinations of the (n-, iso-)alkanes for asphalts AAC-1 and AAK-1 are shown as the average values, with error bars representing the high and low values. Four of the asphalts in set 2 (AAA-1, AAB-1, AAF-1, and AAG-1) are within the experimental error of the data plotted for the asphalts in set 1. The asphalt AAM-1 position lies far removed from the other asphalts. The poor result obtained for this asphalt is attributed to experimental difficulties encountered during the filtration of the urea adduct, thus, resulting in a low value for the amount of (n-, iso-)alkanes. Aside from experimental difficulties encountered during the urea adduction of the (n-, iso-)alkanes in neat asphalt AAM-1, it is apparent that the amount of (n-, iso-)alkanes in asphalt AAM-1 is small based on the calculated amount from the saturate experiments (see Table 6). However, the 1H and 13C NMR spectra of asphalt AAM-1 indicate large amounts of long carbon chains. Obviously, other chemical constituents such as (n-, iso-)alkanes greater than n-C70 and/or long carbon chain-length substituents on aromatic moieties are also contributing to the large methylene resonance signal observed in the NMR spectra.17 Asphalts AAC-1, AAD-1, and AAK-1 also are significantly removed from the polynomial curve representing the asphalts in set 1. The position of asphalts AAC-1 and AAK-1 were confirmed by repeating the urea-adduct experiment. The deviations of the data for these two asphalts from the polynomial curve may be due to measurement errors in the DSC experiments. A plot of the weight percent of the (n-, iso-)alkane in set 2 asphalts versus the fracture temperature of the asphalt is shown
338 Energy & Fuels, Vol. 21, No. 1, 2007
Netzel and RoVani
However, an apparent linear correlation is obtained (R2 ) 0.90) when the weight percent of the highly branched plus cyclic alkanes of the saturate fractions from asphalts AAA-1, AAB-1, and AAM-1 versus their fracture temperature is plotted (Figure 5). Similar data for the other asphalts in sets 1 and 2 were not available. A correlation is expected because it is the amorphous phase and not the crystalline phase that determines the lowtemperature cracking properties of asphalts.3 The reason is that the amorphous phase in the saturate fraction from asphalt consists of over 90% of highly branched plus cyclic alkanes with long carbon chain substituents. Highly branched and substituted cyclic alkanes contain more end groups per given molecular weight than do linear alkanes. Flexible end groups increase asphalt fluidity and, thus, lower both the fracture temperature and the asphalt glass transition temperature. Figure 4. Weight percent of (n-, iso-)alkanes versus the fracture temperature for asphalts in set 2.
Figure 5. Weight percent of highly branched plus cyclic alkanes in saturate fractions versus fracture temperatures for asphalts AAA-1, AAB-1, and AAM-1.
in Figure 4. Fracture temperature data for asphalts in set 1 are not available. The fracture temperature is a low-temperature cracking parameter of asphalts.5 The asphalts with little or no (n-, iso-)alkanes (asphalts AAA-1, AAG-1, and AAM-1) can be grouped together. Asphalts with greater amounts of (n-, iso)alkanes (asphalts AAB-1, AAC-1, AAD-1, AAF-1, and AAK1) can also be grouped. However, it can be seen in the figure that no apparent correlation exists between the measured amounts of (n-, iso-)alkanes (crystalline wax) in asphalts and their fracture temperatures.
Summary A method combining urea adduction with HTGC was developed that specifically quantifies (n-, iso-)alkanes in neat asphalts without first separating the saturate fraction from the asphalt. The amount of (n-, iso-)alkanes in asphalts ranges from 0 to 1.84 wt %. The method was also applied to the saturate fractions from three asphalts to quantify the (n-, iso-)alkanes and highly branched plus cyclic alkanes. The weight percent of highly branched plus cyclic alkanes in the saturate fractions was found to correlate with the fracture temperature of the asphalt. The weight percent of (n-, iso-)alkanes was found to correlate with the crystalline wax content measured by DSC, thus establishing the molecular type of the wax. However, no correlation was found for the weight percent of (n-, iso-)alkanes in asphalt with its fracture temperature. These results confirm previous studies showing that it is the amorphous phase and not the crystalline phase that correlates with the fracture temperature. The amount of (n-, iso-)alkanes in an asphalt was found to be less than 2 wt %. Thus, the large methylene signals observed in the 1H and 13C NMR spectra of an asphalt are mainly due to alicyclic and aromatic molecules with long carbon chain lengths. Acknowledgment. The authors express their appreciation to Jackie Greaser for typing the manuscript and to Fred Turner for the many helpful discussions and providing the graphics. The support of the Federal Highway Administration under FHWA contract number DTFH61-99C-00022 is also appreciated. EF060256B
