Compositional and Structural Characterization of Waxes Isolated from

Energy & Fuels 2006, 20, 653-660

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Compositional and Structural Characterization of Waxes Isolated from Bitumens Xiaohu Lu* and Per Redelius Nynas Bitumen, SE-149 82 Nynashamn, Sweden ReceiVed October 18, 2005. ReVised Manuscript ReceiVed January 26, 2006

Waxes isolated from different bitumens were investigated with respect to their chemical compositions and structural characteristics. Isolation of waxes was performed using a distillation method (European standard EN 12606-1) and a method based on size exclusion chromatography. To characterize bitumen waxes, various techniques were used, including DSC, HTGC, GC-MS, FIMS, NMR, and WAXD. The study showed that bitumen waxes were complex mixtures of hydrocarbons structured as n-alkanes (C15-C57) and isoalkanes, cycloalkanes, and aromatics, which could be larger than C57. Proportions of different groups of these compounds were strongly dependent on bitumen origins and were also influenced by separation methods. In most cases, bitumen waxes were primarily composed of paraffinic hydrocarbons but differed greatly in the content and distribution of n-alkanes. There were also cases indicating a significant contribution of naphtenic hydrocarbons and aromatics to the wax fraction. Differences between bitumen waxes were further demonstrated by the thermal analysis. The waxes with a predominance of n-alkanes generally melted (and crystallized) over a narrower temperature range. The sharpness of wax crystallization/melting peaks decreased with increasing nonnormal alkanes, particularly cycloalkanes and aromatics. Experimental data also suggested that n-alkanes or compounds with long alkyl chains were essential fractions to the formation of regular structures (crystals) in bitumen waxes.

Introduction The term wax generally refers to all waxlike solids and liquids found in nature and to those individual organic substances that crystallize on cooling and melt on heating.1 It has been demonstrated that, in crude oils, waxes may be paraffin type or microcrystalline type, depending on source of the crude oil.2-5 Unlike paraffin waxes that consist primarily of n-alkanes, microcrystalline waxes are dominated by cycloalkanes and isoalkanes. Waxes from crude oils may also contain small amounts of aromatic carbon, as well as oxygen, present as acid or ketone.2 In bitumen, which is a material produced by distillation of crude oils or found in nature as natural asphalt, wax is the generic name given traditionally to straight-chain saturated hydrocarbons6 and is known as paraffin wax. The crystalline nature and nonsticky character of paraffin waxes was believed to cause negative effects on bitumen quality and on asphalt performance, such as being sensitive to permanent deformation (or rutting), prone to low-temperature cracking, and having poor * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Warth, A. B. The Chemistry and Technology of Waxes; Reinhold: New York, 1956. (2) Musser, B. J.; Kilpatrick, P. K. Molecular characterization of wax isolated from a variety of crude oils. Energy Fuels 1998, 12 (4), 715-715. (3) Carbognani, L.; DeLima, L.; Orea, M.; Ehrmann, U. Studies of large crude oil alkanes. II. Isolation and characterization of aromatic waxes and waxy asphaltenes. Pet. Sci. Technol. 2000, 18, 607-634. (4) Kumar, S.; Agrawal, K. M.; Fischer, P. Identification of acyclic isoprenoid hydrocarbons in wax derived from tank bottom sludge. Energy Fuels 2004, 18, 1588-1594. (5) Kane, M.; Djabourov, M.; Volle, J.-L.; Lechaire, J.-P.; Frebourg, G. Morphology of paraffin crystals in waxy crude oils cooled in quiescent conditions and under flow. Fuel 2003, 82, 127-135. (6) Whiteoak, D. The Shell Bitumen Handbook; 1990.

adhesion to aggregates.7 Accordingly, a limit on wax content has been taken as one of the specification criteria for bituminous materials in some countries.8,9 There is, however, no reason to believe that the structure of the waxes in bitumen, which are the fractions with the highest boiling point from the crude oil, should be less complicated than that in the crude oil. In recent work it was shown that bitumen wax, defined as the crystallizing material on the cooling of bitumen, not only consists of n-alkanes but also other types of molecules which were not further analyzed.10 Waxes differing in chemical composition can lead to different effects on bitumen properties and asphalt performance.11,12 The presence of crystallizing material in bitumen could easily be verified by differential scanning calorimetry (DSC). This also confirms the complex nature of the wax in bitumen, since the crystallization and melting take place during a large temperature range. However, DSC alone does not permit a precise determination of the amount of wax since the crystallization enthalpy is not known unless separation and characterization of the isolated wax have been done. (7) Richter, F. Effect of waxes on bitumen quality. Oil Gas Eur. Mag. 2002, 2, 35-38. (8) Specifications for paVing grade bitumens; European Standard EN 12591; 1999. (9) Technical specifications for construction of highway asphalt paVements; JTG F40-2004; Ministry of Communications: China, 2004. (10) Redelius, P.; Lu, X.; Isacsson, U. Nonclassical wax in bitumen. Road Mater. PaVement Des. 2002, 3 (1), 7-21. (11) McKay, J. F.; Branthaver, J. F.; Robertson, R. E. Isolation of waxes from asphalts and the influence of waxes on asphalt rheological properties. Preprints, DiVision of Petroleum Chemistry, 210th National Meeting of the American Chemical Society, Chicago, IL, Aug 20-25, 1995; American Chemical Society: Washington, DC; pp794-798. (12) Edwards, Y.; Redelius, P. Rheological effects of waxes in bitumen. Energy Fuels 2003, 17 (3), 511-520.

10.1021/ef0503414 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/24/2006

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Table 1. Physical Parameters and Chemical Composition of Bitumen Samples physical propertiesa

bitumen samples

b

chemical compositionb

code

source

pen 25 °C (0.1 mm)

softening point (°C)

viscosity 135 °C (mm2/s)

saturates (%)

aromatics (%)

resins (%)

asphaltenes (%)

A B C D E F

Venezuela Mid-East Russia Mexico unknown China

214 205 180 101 86 73

39.1 39.2 40.1 45.1 46.4 46.4

283 225 202 325 181 274

7 9 7 9 11 8

49 60 58 56 55 43

23 17 21 17 19 33

21 14 14 18 15 16

a Measured by the European standard methods; penetration by EN 1426 (penetration), softening point by EN 1247, and kinematic viscosity by EN 12595. Determined using thin-layer chromatography with flame ionization detection (TLC-FID, Iatroscan).

By literature search, we can find a great number of publications on waxes in crude oils. On the contrary, very few investigations have been published to understand waxes of bitumen particularly with respect to their chemical composition and structural characteristics.3,10 In the present study, a variety of techniques are used to characterize bitumen waxes; those include DSC mentioned above, high-temperature gas chromatography (HTGC), gas chromatography-mass spectroscopy (GC-MS), field ionization mass spectroscopy (FIMS), nuclear magnetic resonance spectroscopy (NMR), and wide-angle X-ray diffraction spectrometry (WAXD). The isolation of waxes from bitumen is conducted by a standard method (EN 12606-1 or DIN 52015)13 and a method based on size exclusion chromatography (SEC). To demonstrate variations in wax composition, bitumens of different sources are selected. Experimental Section Bitumen Samples. Six bitumen samples of different sources, designated A-F, were selected. Typical physical properties and chemical compositions of the samples are described in Table 1. According to the European standard EN 12591, these bitumens may be classified as 160/220 (A, B, and C), 100/150 (D), and 70/100 (E and F), respectively. Separation of Wax from Bitumen. European Standard Method EN 12606-1. This method is based on DIN 52015, a German method used for determining the paraffin wax content of bitumen and bituminous binders.13 The test is carried out on two portions, each of 25 g of bitumen. A specified distillation process is used to obtain the distillate from the bitumen. The wax is obtained by dissolving the distillate in a diethyl ether/ethanol (50/50, v/v) solvent and crystallizing at a temperature of -20 °C. Method Based on Size Exclusion Chromatography (SEC). The method consists of two steps, which follows procedures proposed in refs 11 and 14. In the first step, larger or associating molecules (fraction SEC I) were removed from the bitumen using preparative SEC performed in toluene. The fraction containing small size and nonassociating molecules (fraction SEC II) was collected, and the solvent was evaporated, followed by dilution in toluene by a ratio of 5 g to 14 cm3, and further by 2-butanone with 11.25 cm3 for each g of dry small size fraction. The solution was cooled to -20 °C and held at that temperature for 1 h, then poured into a chilled funnel (vacuum) having a frit of 10-16 µm porosity. The filter cake was rinsed with 25 cm3 of cold 2-butanone, and the wax fraction was collected. Test Methods. Differential Scanning Calorimetry (DSC). Thermal characterization was carried out using a TA Instrument, model 2920 DSC. For each test, approximately 15 mg of bitumen waxes was hermetically sealed into a sample pan, then heated to 110 °C and kept at this temperature for 15 min. The data were recorded (13) Bitumen and bituminous binderssdetermination of the paraffin wax contentspart 1: method by distillation; European Standard EN 12606-1; 1999. (14) Petersen, J. C.; et al. Binder Characterization and EValuation, Vol. 4: Test Methods; SHRP-A-370; National Research Council: Washington, DC, 1994.

during cooling to -110 °C and heating to 110 °C, both at a rate of 10 °C/min. The measured thermal characteristics include temperatures and enthalpies for wax crystallization and wax melting. A similar DSC procedure was also used to determine wax content in bitumen.15 High-Temperature Gas Chromatography (HTGC). In HTGC, a Hewlett-Packard 5890 equipped with a flame ionization detector was employed. The column used was 5 m long, 0.53 mm in diameter, and with 0.1 µm film thickness. Two percent (w/w) solutions of bitumen waxes were prepared in CS2, and 1 mm3 of sample solution was injected to the column using an autoinjector. Tests were performed under a helium flow rate of 15 cm3/min and a temperature program of 40-430 °C at 10 °C/min. Gas Chromatography-Mass Spectroscopy (GC-MS). The equipment for GC-MS was a Hewlett-Packard 6890 gas chromatography coupled with a Hewlett-Packard 5973 mass selective detector. The GC column was a DB-5 silica capillary column (J&W Scientific, 30 m × 0.32 mm i.d. and phase thickness of 0.25 µm), and the oven temperature was programmed from 120 °C (hold 3 min) to 270 °C (hold 40 min) at 10 °C/min. Splitless injection was used at 300 °C. The system was operated in a constant flow mode at 2 cm3/min. The MS transfer line was operated at 300 °C. The ion source was kept at 230 °C. The mass spectrometer was scanned in electron impact mode (70 eV) from m/z 50 to 500 at a rate of three scans per s. Field Ionization Mass Spectroscopy (FIMS). FIMS analysis was performed in positive ion mode with a VG Prospec magnetic sector instrument equipped with field ionization ion source (extraction voltage 6 kV, accelerating voltage 8 kV). Sample was introduced through a direct insertion probe which was heated from 50 to 400 °C at 30 °C/min to evaporate the sample. The magnet was scanned from m/z 70 to 800 at a rate of five s per decade. The interscan delay was 0.5 s. The results were calculated using the PCMASPEC program. Wide-Angle X-ray Diffraction Spectrometry (WAXD). The WAXD experiments were made at room temperature using a horizontal Geigerflex diffractometer on a Rigaku RU-200B rotating Cu anode at a power of 4 kW and using Ni-filtered Cu KR radiation (λ ) 1.542 Å). The diffractometer was equipped with a scintillation counter, and the width of the divergence, receiving, and scattering slit were 0.2°, 0.15 mm, and 0.5°, respectively. Measurements on 2 mm thick compacted powders were conducted in the transmission mode covering diffraction angles (2θ) between 10° and 34°. The data were accumulated for 1 s at angular intervals of 2θ ) 0.01°. For the determination of crystallinity, the Marquardt-Levenberg nonlinear least-squares curve-fitting algorithm as implemented in Microcal Origin (version 6.0, Microcal Software, Inc., Northampton, MA) was used.

Results and Discussion Wax Separation. It is well-known that bitumen is a complex material, which is composed of a great number of molecules varying widely in structure, polarity, and molecular weight. For (15) Lu, X.; Langton, M.; Olofsson, P.; Redelius, P. Wax morphology in bitumen. J. Mater. Sci. 2005, 40, 1893-1900.

Characterization of Waxes Isolated from Bitumens

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Table 2. Wax Contents in Bitumens Determined by Different Methods wax content (%)

b

bitumen

DIN methoda

SEC methoda

DSCb

A B C D E F

1.1 1.3 1.5 1.2 1.7 3.4

2.3 5.9 c c 6.7 c

2.4 4.2 4.1 4.1 6.2 7.4

a See the section in the Results and Discussion entitled Wax Separation. Direct measurements as described in ref 15. c Not measured.

Table 3. Thermal Characteristics of the Waxes Isolated Using the SEC Method crystallization upon cooling

melting upon heating

bitumen for wax isolation

peak temp °C

enthalpy J/g

peak temp °C

enthalpy J/g

A Ba Ca D Ea Fa

52 36 46 57 55 40

60.7 71.9 73.3 39.4 100.2 63.6

62 41 56 64 63 49

64.3 70.9 73.8 39.4 101.2 65.1

Figure 1. DSC thermograms of the waxes isolated from bitumens A and B using the SEC method (cooling and heating scans at 10 °C/ min).

a The waxes obtained from these bitumens display shoulder peaks in the cooling and heating scans.

such a system, it is very difficult to chemically characterize certain species without fractionation. Due to the high viscosity of bitumen, it is not possible to separate the crystallizing material (wax) at lower temperature. If a solvent is used to decrease the viscosity there is usually considerable coprecipitation of polar molecules which makes isolation of the wax almost impossible. Instead, a two-step procedure, preparation of a fraction for dewaxing followed by precipitation of wax at a low temperature, is normally used. Current methods differ mainly in the first step, which may be based on different separation mechanisms; the precipitation procedures followed are in general similar although different solvents can be selected. Assuming that bitumen wax consists mainly of saturated hydrocarbons, the first step is selected to isolate a fraction consisting of only saturated molecules from the bitumen. Two examples are isolation of the saturated fraction by silica gel chromatography16 and EN 12606-2 (based on AFNOR NF T 66-015),17 where the wax is determined after the extraction of asphaltenes with petroleum spirit and the extraction of most aromatic components with oleum, respectively. However, it has been suspected that bitumen wax also may contain polar molecules, which might be removed in the separation processes above. Thus, more selective separation procedures have been used, such as SEC and ion-exchange chromatography (IEC).10,11 Those procedures retain slightly polar molecules that might crystallize at the low-temperature precipitation of the wax. Another example is EN 12606-1,13 in which wax present in bitumen is determined in a distillate obtained by a distillation process.13 This method may not reflect the true nature of the wax in the bitumen. Since the distillation is taking place at very high temperature (up to over 500 °C), there is an obvious risk that cracking might create molecules which were not originally present in the bitumen. Thus, those aromatics or cycloalkanes substituted with long-chain alkanes (16) Gawel, I.; Czechowski, F. Wax content of bitumens and its composition. Analytik 1998, October, 507-509. (17) Bitumen and bituminous binderssdetermination of the paraffin wax contentspart 2: method by extraction; European Standard EN 12606-2; 1999.

Figure 2. DSC thermograms of the waxes isolated from bitumen F using the DIN and SEC methods (cooling and heating scans at 10 °C/ min).

might split to n-alkanes and smaller aromatic or cycloalkane molecules. Likewise, larger n-alkanes might crack to smaller alkanes. In this study, EN 12606-1 (or DIN method) and a method based on preparative size exclusion chromatography (SEC) are used for the wax isolation. The selection of the DIN method is because of its higher popularity. But we believe that a fraction (SEC II) prepared by SEC contains waxes more reliably representing those in original bitumen, as this method is nondestructive. Table 2 shows that different methods yield different amounts of wax, and the lowest values are obtained by the DIN method. As already mentioned, the DIN method involves a destructive step, which may reduce the molecular weight of crystallizing materials. At the same time, the resulting smaller paraffin molecules could be soluble in ether/ethanol and do not crystallize in the solvent at the specified temperature (-20°). These might be reasons for the low wax contents as determined by the DIN method. Differences between the methods as reflected by the wax composition will be discussed later. In fact, all the available methods used for determining wax content in bitumen only give relative results. When a certain separation step is involved, the fraction prepared for wax

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Figure 3. Total ion chromatogram of the GC-MS analysis of the wax isolated from bitumen E using the SEC method.

precipitation may differ significantly. As a consequence, definition of the wax is method related. In this paper, waxes obtained by the DIN method and by the SEC method are called DIN wax and SEC wax, respectively. Thermal Characterization. In the thermal characterization of waxes, DSC is probably the most common method.18-20 The technique measures energy change in the sample under a controlled heating or cooling rate. Curves of heat flow versus temperature provide insight into the thermal characteristics of bitumen waxes, as exemplified in Figure 1. Typical thermal data for the SEC waxes are summarized in Table 3. As indicated, the waxes from different bitumens differ considerably in the peak temperatures of crystallization and melting, as well as in enthalpies. For one and the same wax, the onset of crystallization occurs at a lower temperature than the termination of the melting. The difference is probably caused by supercooling, where the undercooled wax molecules need nucleation sites to start crystallization. It was also observed that the SEC wax from bitumen A melts in a relatively narrow temperature range (35-85 °C) with a single peak, whereas other waxes melt over a wide temperature range, for example, from 5 to 90 °C for the SEC wax from bitumen E as shown in Figure 1. Moreover, the waxes from bitumens B, C, E, and F display shoulder peaks in both cooling and heating scans. Wide temperature ranges of crystallization and melting suggest that the waxes are mixtures of hydrocarbons with different melting points. This will be confirmed later by analyses of the wax compositions. Figure 2 reveals that the waxes obtained by the DIN and SEC methods differ greatly in the thermal characteristics. Besides shifting to higher temperatures for crystallization and melting, the DIN wax exhibits sharper peaks and larger energy changes as compared with the SEC wax. This is attributed to a high content of n-alkanes in the DIN wax. In addition to the sharper peak from n-alkanes, the DIN wax displays another peak, which may be indicative of the existence of other types of crystalline (18) Noel, F.; Corbett, L. W. A study of the crystalline phases in asphalts. J. Inst. Pet. 1970, 56, 261-268. (19) Planche, J. P.; Claudy, P. M.; Letoffe, J. M.; Martin, D. Using thermal analysis methods to better understand asphalt rheology. Thermochim. Acta 1998, 324, 223-227. (20) Michon, L. C.; Netzel, D. A.; Turner, T. F.; Martin, D.; Planche, J. P. A 13C NMR and DSC study of the amorphous and crystalline phases in asphalts. Energy Fuels 1999, 13 (3), 602-610.

Figure 4. GC-MS estimation of n-alkanes in the waxes isolated from different bitumens using the SEC method (the wt % shown inside the figure are those for total n-alkanes).

materials or due to solid-solid phase transition of n-alkanes.21 The dominance of n-alkanes in the DIN wax will be evidenced by GC-MS. Chemical Composition and Structural Characterization. Wax composition and structural characteristics are determined by means of HTGC, GC-MS, FIMS, WAXD, and NMR. Figure 3 shows a total ion chromatogram of the GC-MS analysis for the SEC wax from bitumen E, where the pattern is characteristic for n-alkanes. The structure of the n-alkanes was further confirmed by the typical MS pattern from each peak. It was intended to use an internal standard tricosane (n-C23) for quantifying the n-alkanes. The standard was contaminated by n-C21 and n-C25, resulting in higher responses at the corresponding carbon numbers (Figure 3). Instead, content of each n-alkane in the wax samples is estimated by comparing the peak area of n-alkane with the total area under the ion chromatogram. As indicated in Figure 4, the SEC waxes vary largely in the content of n-alkanes smaller than C38, from 2% to 32%, depending on the bitumens. The distribution of n-alkanes is also different between the waxes. Of the five SEC waxes, the one (21) Srivastava, S. P.; Handoo, J.; Agrawal, K. M.; Joshi, G. C. Phasetransition studies on n-alkanes and petroleum-related waxessa review. J. Phys. Chem. Solids 1993, 54 (6), 639-670.

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Figure 5. HTGC chromatogram of the wax isolated from bitumen A using the SEC method.

Figure 6. MS spectrum summarized from the data between n-C34 and n-C35 shown in Figure 3 (the SEC wax from bitumen E).

isolated from bitumen A contains the highest quantity of n-alkanes. This wax also consists of a narrower range of n-alkanes with relatively smaller size as compared with those of the other waxes. It should be noted that the estimation is quite rough, and the temperature limitation in GC-MS leads to certain long-chain n-alkanes being missed. This was confirmed by HTGC analysis, of which an example is shown in Figure 5 for the SEC wax from bitumen A. Again the series of peaks differing in molecular weight by 14 amu reveal a very characteristic pattern for n-alkanes in the wax. The higher temperature in HTGC permits identification of the peaks corresponding to n-C58. A broad hump beneath the n-alkanes suggests the presence of other substances such as isoalkanes, cycloalkanes, and aromatics. The broad hump exceeding the retention time of n-C58 indicates those substances have higher molecular weights. In addition to the characteristic pattern of n-alkanes, small and less resolved peaks appear in Figure 3. By MS analysis, those peaks are identified as mixtures of isoalkanes and cycloalkanes. Figure 6 is a mass spectrum that summarizes the data between n-C34 and n-C35 of Figure 3. Ions at m/z 57, 71,

85, and 99, etc., are typical for alkanes.4 There are also ions coming from alkyl cycloalkanes (m/z ) 97, 111, etc.). Of course, it can be argued that part of the ions typical for isoalkanes could come from cycloalkanes with long saturated alkyl chains. Unfortunately an accurate identification of those substances cannot be made by GC-MS due to the low resolution of the GC peaks. To further determine the wax composition, FIMS was used. In this type of test, the wax sample is evaporated from a direct insertion probe. As gas molecules pass a carbon emitter, they are ionized by electron tunneling. FI is a soft ionization method that tends to produce mass spectra with little or no fragmention content. The obtained mass spectra are usually simple, typically one ionic species per compound. In wax samples, compounds were measured as different saturated hydrocarbons, including alkanes (n-alkanes plus isoalkanes), monocycloalkanes, dicycloalkanes, tricycloalkanes, tetracycloalkanes, pentacycloalkanes, and hexacycloalkanes. The FIMS results obtained for all the SEC waxes are summarized in Table 4. For comparison, contents of n-alkane estimated by GC-MS are also shown in Table 4. Because FIMS analysis was conducted on

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Table 4. Chemical Compositions of the Waxes Isolated Using the SEC Method bitumen samples wax

a

compositiona

A

B

C

D

E

F

normal alkanes (size and wt %)

C18-C38 32

C18-C38 4

C18-C38 6

C18-C38 15

C21-C38 23

C17-C45 2

total paraffins (size and wt %)b

C21-C47 95

C15-C56 44

C18-C52 71

C18-C56 53

C18-C56 50

C15-C64 36

monocycloalkanes (size and wt %)

C24-C44 5

C15-C57 22

C22-C56 16

C17-C57 16

C17-C57 24

C15-C64 28

dicycloalkanes (size and wt %)

0

C15-C57 7

C41-C55 4

C22-C57 7

C19-C56 8

C15-C64 11

others, e.g., tetracycloalkanes, aromatics (wt %)