Energy Fuels 2009, 23, 5556–5563 Published on Web 10/02/2009
: DOI:10.1021/ef900596y
Explaining the Flocculation of Hassi Messaoud Asphaltenes in Terms of Structural Characteristics of Monomers and Aggregates M. Daaou,†,‡ D. Bendedouch,† Y. Bouhadda,† L. Vernex-Loset,^ A. Modaressi,§ and M. Rogalski*,§ † LCPM, D epartement de chimie, Facult e des Sciences Universit e d’Oran (Es-senia), Oran 31000, Algeria, ‡Department of industrial chemistry, Faculty of sciences, University of sciences and technology of Oran, P.O. Box 1505 el m’naouer, Oran 31000, Algeria, §LCME, ^LSMCL, and University of Metz, 1, bd Arago, 57070 Metz, France
Received June 11, 2009. Revised Manuscript Received September 11, 2009
X-ray diffraction, liquid 1H and solid 13C nuclear magnetic resonance (NMR), Raman, Fourier transform infrared (FTIR), laser desorption/ionization-time of flight (LDI-TOF) mass and fluorescence spectroscopy have been used to determine structural properties of asphaltenes obtained from Hassi Messaoud crude oil. While liquid 1H- and 13C-NMR, IR, and LDI-TOF mass and fluorescence spectroscopy allow the determination of the chemical composition and structure of single asphaltene molecules, X-ray diffraction and Raman spectroscopy make it possible to asses parameters characterizing small asphaltene clusters. The proposed characterization of petroleum asphaltenes affords information that leads to better understanding of asphaltene flocculation conditions.
and of asphaltenic clusters is necessary. This knowledge may be assessed using different analytical techniques.5-16 1H nuclear magnetic resonance (NMR) results associated with infrared (IR), gas permeation chromatography (GPC), or vapor pressure osmometry (VPO) suggested that an asphaltene molecule contains aromatic sheets formed with fused aromatic-naphthenic rings.17-21 Alkyl side chains with an average length between 4 and 6 carbon atoms are bonded to the aromatic sheets and bridge them. The simultaneous use of 1 H- and 13C-NMR allowed a more accurate determination of the average structural parameters such as the aromatic carbon ratio, the average number of alkyl side chains, and the degree of peripheral aromatic carbon substitution.22,23 Other techniques were used to obtain more specific information. Wong and Yen24 used electron spin resonance (ESR) to examine the ability of microwave power to dissociate the petroleum asphaltene macrostructure. Leon et al.25 employed
1. Introduction Asphaltenes constitute the most complex fraction of crude oil. They are conventionally defined as a fraction of the crude oil that is insoluble in n-alkanes but soluble in toluene.1 Because of their natural tendency to form aggregates that may flocculate and precipitate, asphaltenes cause severe problems in oil production, transportation, and refining. The propensity of asphaltenes to flocculate is a complex function of the crude oil composition. Algerian crude oil is very unstable with respect to the flocculation despite the low content of asphaltenes. Therefore, no simple theory relating the flocculation with the concentration of asphaltenes, resins, or aromatics is generally valid. It is believed that the probable flocculation mechanism should take into account the structural features of asphaltene molecules leading to self-assembly2 and the entropic interactions between aliphatic chains.3,4 The structure of aggregates depends on the chemical composition of monomeric asphaltenes containing polynuclear aromatic moieties surrounded by aliphatic chains and heteroatoms (sulphur, nitrogen, oxygen as well as traces of metals like nickel, iron, and vanadium). When the mechanism of the flocculation is addressed, the knowledge of the chemical composition and structure of single asphaltene molecules
(10) Buenrostro-Gonzalez, E.; Andersen, S. I.; Garcia-Martinez, J. A.; Lira-Galeana, C. Energy Fuels 2002, 16, 732. (11) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquın, G.; Garcıa, J. A.; Tenorio, E.; Torres, A. Energy Fuels 2002, 16, 1121. (12) Ibrahim, Y. A.; Abdelhameed, M. A.; Al-sahhaf, T. A.; Fahim, M. A. Pet. Sci. Technol. 2003, 12, 825. (13) Khan, M. A.; Ahmed, I.; Ishaq, M.; Shakirullah, M.; Jan, M. T.; Tehman, E.; Behader, A. Fuel Process. Technol. 2003, 85, 63. (14) Avid, B.; Sato, S.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 1792. (15) Acevedo, S.; Gutierrez, L. B.; Negrin, G.; Pereira, J. C.; Delolme, B. M. F.; Dessalces, G.; Broseta, D. Energy Fuels 2005, 19, 1548. (16) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20, 1227. (17) Yen, T. F.; Wu, W. H.; Chilingar, G. V. Energy Sources 1984, 7 (3), 203. (18) Yen, T. F.; Wu, W. H.; Chilingar, G. V. Energy Sources 1984, 7 (3), 275. (19) Speight, J. G. Fuel 1970, 49, 76. (20) Jacobs, F. S.; Filby, R. H. Fuel 1983, 62, 1186. (21) Speight, J. G. Prepr. Am. Chem. Soc., Div. Pet. Chem. 1986, 31 (4), 818. (22) Dickinson, E. M. Fuel 1980, 59, 290. (23) Dereppe, J.-M.; Moreaux, C.; Castex, H. Fuel 1978, 57, 435. (24) Wong, G. K.; Yen, T. F. J. Pet. Sci. Eng. 2000, 28, 55. (25) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6.
*To whom correspondence should be addressed. E-mail: rogalski@ univ-metz.fr. (1) Knight, S. A. Chem. Ind. 1920, 196. (2) Yang, X.; Hamza, H.; Czarnecki, J. Energy Fuels 2004, 18, 770. (3) Mahmoud, R.; Gierycz, P.; Solimando, R.; Rogalski, M. Energy Fuels 2005, 19, 2474. (4) Stachowiak, C.; Viguie, J. P.; Grolier, J.P. E.; Rogalski, M. Langmuir 2005, 21, 4824. (5) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225. (6) Shirokoff, J. W.; Siddiqui, M. N.; Ali, M. F. Energy Fuels 1997, 11, 561–565. (7) Pekerar, S.; Lehmann, T.; Mendez, B.; Acevedo, S. Energy Fuels 1999, 13, 305. (8) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (9) Bouhadda, Y.; Bendedouch, D.; Sheu, E.; Krallafa, A. Energy Fuels 2000, 14, 845. r 2009 American Chemical Society
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Figure 1. IR DRIFT spectrum of Haoud El-Hamra asphaltenes. K-M represents Kubelka-Munk units. Table 1. Chemical Composition of HH Asphaltenes element % wt a
C
H
N
S
Oa
H/C
84.96
6.71
0.59
0.49
7.25
0.95
Table 2. Infrared Spectral Range Assignments for HH Asphaltenes: (w) Weak, (s) Strong, (m) Medium 2800-3000 cm-1 C;H (stretching) 1600-1800 cm-1 CdO 1590-1620 cm-1 CdC (aromatic stretching) 1375-1450 cm-1 (C;H deformation) ∼1000 cm-1 SdO, C;S, C;O, and/or C;N 730-900 cm-1 C;H aromatic 730-700 cm-1 nCH2; n g 4
Determined from the mass balance.
elemental analysis, VPO, and NMR to explore a possible relationships existing between structural parameters of asphaltenes, their self-aggregation, and the instability of crude oils. Joshi et al.26 studied properties of the asphaltene precipitate using optical scattering techniques. Shirokoff et al.6 reported the structural characterization of four Saudi Arabian crudeoil-derived asphaltenes by X-ray diffraction, NMR, and highperformance gas permeation chromatography (HP-GPC). Ancheyta et al.11 used vapor pressure osmometry, proton and carbon nuclear magnetic resonance, and elemental analysis for characterizing Mexican asphaltenes petroleum. Siskin et al.16 determined structural properties of Canadian asphaltenes applying solid state carbon nuclear magnetic resonance and X-ray photoelectron spectroscopy methods. Recently Bouhadda et al.27 characterized Algerian asphaltenes using X-ray diffraction and Raman spectroscopy. In a previous work,28 we characterized the asphaltenes fraction obtained from the tank storage deposit of Hassi Messaoud crude oil. These asphaltenes, instable with respect to flocculation, displayed high polarity due to the high heteroatom’s content and the average length of aliphatic chains longer than usually observed. More recently, Trejo et al.29 studied the morphology
2800 (s), 2900 (s) 1700 (w) 1600 (m) 1450 (s), 1380 (s) 1025 (w) 875 (s), 800 (m) 725 (w)
of asphaltenes obtained from pure and hydroprocessed Maya crude oil by scanning and transmission electron microscopy (SEM and TEM) and showed that the removal of alkyl chains during hydroprocessing induces a rearrangement of solid asphaltenes favoring stacking of aromatic sheets. All these results show that the flocculation of asphaltenes depends as well of the chemical properties of the crude oil (oil’s origin) as well of the modifications of the crude occurring during extraction processes.30 In this study, we are looking for appropriate experimental methods allowing the characterization of asphaltenes with respect to flocculation processes. At first, the chemical composition and structure of single asphaltenes were determined. Next, the aggregation of asphaltenes was dealt with. The aggregation starts with the formation of small clusters of asphaltene molecules. The further association of theses clusters leads to the asphaltene precipitation. We believe that the formation of small aggregates is a fundamental stage of formation of the asphaltene flocks. The mechanism of the asphaltene flocculation was for the first time addressed by Pfeiffer and Saal31 who considered the asphaltic crude oil as a colloidal solution. Further developments by Yen32 suggested a
(26) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden, J. Energy Fuels 2001, 15, 976. (27) Bouhadda, Y.; Bormann, D.; Sheu, E.; Bendedouch, D.; Krallafa, A.; Daaou, M. Fuel 2007, 86, 1855. (28) Daaou, M.; Modarressi, A.; Bendedouch, D.; Bouhadda, Y.; Krier, G.; Rogalski, M. Energy Fuels 2008, 22, 3134. (29) Trejo, F.; Ancheyta, J.; Rana, M S. Energy Fuels 2009, 23, 429.
(30) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzadeh, K. Energy Fuels 2002, 16, 462. (31) Pfeiffer, J.P.; Saal, R.N. J. Phys. Chem. 1940, 44, 139. (32) Yen, T.F. Energy Sources 1974, 1, 447.
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Figure 2. LDI-TOF mass spectrum of HH asphaltenes.
Figure 3. Liquid state 1H-NMR spectrum of HH asphaltene.
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evaporation of the solvent asphaltenes were washed with 10 ml of n-heptane, until the solvent was colorless. 2.2. Materials and Methods. 2.2.1. Elemental Analysis. Elemental chemical composition of the asphaltene fraction was obtained using the Thesmoquest, CE instrument, elemental analyzer. This instrument allows the determination of the carbon, hydrogen, sulfur, and nitrogen content. 2.2.2. Diffusive Reflection Infrared Fourier Transform (DRIFT) Spectroscopy. Diffusive reflection infrared spectra were recorded using a Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer) in Kubelka and Munk mode for the diffuse reflectance with a spectral resolution of 4 cm-1 in the 700-4000 cm-1 spectral domain. 2.2.3. Mass Spectroscopy. Molecular weight was determined by mass spectroscopy (Reflex VI Bruker Daltonics) using a LDI-TOF technique in reflector mode and optimized nitrogen laser (λ = 337 nm) energy at 80% of the full power. The asphaltene fraction was dissolved in toluene at a concentration of 0.001 g/L. 2.2.4. Liquid State 1H-NMR Spectroscopy. 1H NMR spectra were obtained with a 300 MHz Bruker 300 spectrometer. Chemical shifts (δ) reported here was relative to the tetramethylsilane (TMS) used as internal standard. All spectra were recorded in deuteriochloroform (CDCl3). 2.2.5. Solid State 13C-NMR Spectroscopy. Experiments were carried out using a Bruker solid-state NMR spectrometer with 1H and 13C Larmor frequencies of 400 and 100 MHz, respectively, with a 4 mm OD zirconia rotors span in with a double resonance magic angle spinning (MAS) probe. The spectra were referenced to tetramethyl silane. All experiments were conducted at a spinning rate of 14 kHz. The recycling delay was equal to 120 s. All crosspolarization (CP) experiments used a 1H 90 pulse with a contact time of 1 ms. Both the 1H spin-lock and decoupling frequencies were fixed at 25 kHz. Also experiments were performed under a standard Hartmann-Hahn match (νrf = 100 kHz). Simple and variable CP contact time experiments ranging from 0 to 1.5 ms were used to determine the optimum contact time. 2.2.6. X-ray diffraction (XRD). XRD measurements were made with an XRD diffractometer CGR Theta 60 with Cu KR1 radiation (λ = 1.542 A˚), operating at 40 kV and 30 mA, in the 7-100 scan range diffraction angle. A scan rate of 0.01/s and a count time of 10 s/step were used. A 0.03 g HH asphaltene sample was mounted on a dimpled silica plate sample holder. 2.2.7. Raman Spectroscopy. The micro-Raman experiments were conducted on a DILOR XY 800 Raman spectrophotometer equipped with a charge coupled device (CCD) camera and a Spectra Physics laser tube (Model 2018) with an excitation wavelength of 514.5 nm. All spectra were recorded at room temperature in the microconfocal retrodiffusion configuration in the range from 1000 to 1800 cm-1 which covers the entire firstorder region.36 Prior to the measurements, a small quantity of mortared sample was deposited on a slide and the surface was manually smoothed under microscope. A visually acceptable flat surface was used as the focal point for a laser impact/excitation. A series of spectra acquisitions in this area was then performed in order to explore the response of the sample and to optimize the experimental conditions. To improve the statistics, several spectra were collected from different points of the flat area. The final spectrum was retained as an average of these data. 2.2.8. Fluorescence Spectroscopy. The fluorescence emission spectra of samples containing 5 mg/L of asphaltenes in toluene were measured with Jobin-Yvon Horiba FluoroMax-3 spectrometer in the range from 300 to 800 nm with an excitation wavelength of 265 nm.
Figure 4. Solid state 13C-NMR spectrum of HH asphaltenes. Dashed lines correspond to the Gaussian fit.
gradual mechanism of asphaltene aggregation. Recent results published in the literature gave a deeper insight in the structure of asphaltene aggregates33,34 and elucidated the role played by other components of the oil (resins, paraffins, and the mineral matter) on the formation and composition of asphaltene nanoparticles.35 The comprehension of the aggregation process depends of the knowledge of the mechanism of formation of elementary aggregates. This problem was particularly addressed in the present paper. We have studied the composition and the structure of asphaltenes obtained from an Algerian crude oil from the Houd Elhamra well of the Hassi Messaoud oil field. The oil production from this field is continuously perturbed by deposition of asphaltenes. In the recent paper,28 we described results of characterization of asphaltenes obtained from the storage tank deposit sedimented spontaneously during the transport of the Houd Elhamra oil. This oil, d420 = 0.8304 g/cm3, contained 0.6 % w/w of asphaltenes and 6.5 % w/w of resins. The total acidity number was 0.28 mg KOH/g. We looked for relationship existing between the chemical and structural characteristics of asphaltenes and the stability of the crude oil with respect to the flocculation of asphaltenes. The spectroscopic methods such as Fourier transform infrared (FTIR), laser desorption/ ionization-time of flight (LDI-TOF) mass, 1H-NMR, solidstate 13C-NMR, X-ray diffraction, Raman, and fluorescence spectroscopy were used to elucidate the problem of the structure of elementary asphaltene aggregates in relation to the flocculation process. 2. Experimental Section 2.1. Extraction of Asphaltenes. Asphaltenes used in this study were obtained from the Haoudh El-Hamra (HH) crude oil from the Hassi Messaoud field. An n-heptane/crude oil mixture at 40:1 volume:mass ratio was gently shaken over 24 h at ambient temperature to precipitate the total asphaltenes present in the oil. Then, precipitated asphaltenes were filtered through a 0.45μm pore size paper filter and dissolved in toluene. After (33) Sharma, A.; Mullins, O. Petroleomics and Characterization of Asphaltene Aggregates Using Small Angle Scattering. Asphaltenes, Heavy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 8. (34) Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. Energy Fuels 2007, 21, 2785. (35) Zhao, B.; Shaw, J. M. Energy Fuels 2007, 21, 2795.
3. Results and Discussion 3.1. Elemental Analysis. Results of the asphaltene elemental analysis are presented in Table 1. The value of the atomic (36) Tuinstra, F; Koenig, J.L. J. Chem. Phys. 1970, 53, 1126.
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Table 3. Assignments of (a) 1H and (b) 13C Chemical Shifts in HH Asphaltene NMR Spectra (a) H bonding
chemical shift range (ppm from TMS)
Hγ (hydrogen-to-aromatic ring in position γ) Hβ (hydrogen-to-aromatic ring in position β) HR (hydrogen-to-aromatic ring in position R) Hal (aliphatic hydrogen) Har (aromatic hydrogen) HT (total hydrogen)
0.5-1.0 1.0-2.0 2-4.5 0.5-4.14 7.12-7.64 0.5-7.64
integration
%H
1.47 3.30 1.00 5.78 1.45 7.23
13.83 45.70 20.36 79.89 20.11 100
(b) C bonding
chemical shift range (ppm from TMS)
integrations
%C
methyl (CH3) methyl (CH3) methylene in simple aliphatic CH2 complex aliphatic CH2, CH, C complex aliphatic CH2, CH, C aromatic (C-H, C-R) aromatic (C-H, C-R) aromatic (C-H, C-R) Cal (aliphatic carbon) Car (aromatic carbon)
14.8 21.3 30.0 37.7 48.7 116.4 127.8 138.2 10-60 110-160
8.5 10.0 7.5 7.5 7.0 4.5 36.5 18.5 40.5 59.5
8.5 10.0 7.5 7.5 7.0 4.5 36.5 18.5 40.5 59.5
Table 4. Expressions Allowing Calculation of the Average Structural Parameters5,7,39,40 properties
equationsa
total number of C atoms per molecule, CT total number of H atoms per molecule, HT total number of aliphatic carbons per molecule, Cal total number of aromatic carbons per molecule, Car aromaticity factor, fa average number of carbons per alkyl side chain, n peripheral aromatic carbons number, Cp substituted aromatic carbons number, Csub unsubstituted aromatic carbons number, Cus percent of substitution of peripheral aromatic carbon, As total number of aromatic rings per molecule, Ra number of substituent rings, r shape factor of aromatic sheet, Φ total number of naphtenic rings per molecule, Rna total number of naphtenic carbons per molecule, Cna
CT = % C MW/1200 HT = % H MW/100 (% Cal CT)/100 (% Car CT)/100 fa = Car/(Car þ Cal) n = % (HR þ Hβ þ Hγ)/% HR Cp = HT(% Har þ % HR/2) Csub = Cal/n Cus = Cp - Csub As = 100(Csub/Cus þ Csub) Ra = ((Car - Cp)/2) þ 1 r = [0.250(Hγ/Hβ þ 4.12) - 1][(n - 1)/2] Φ = Cp/Car Rna = rCsub Cna = 3.5Rna
a % C and % H are the weight percent of carbon and hydrogen obtained with the elemental analysis of asphaltene samples, and MW is the molecular weight determined using LDI-TOF mass spectroscopy.
assignments are given in Table 2. The spectral image of HH asphaltenes is similar to asphaltene spectra published in the literature.5,37 The aliphatic stretching bands appear at around 2800 and 2900 cm-1, and the deformation bands at 1450 and 1380 cm-1. The largest intensity is observed at 1600 cm-1 and is attributed to aromatic CdC bonds associated to polyaromatic groups. The region between 900 and 700 cm-1 shows three bands at 875, 800, and 725 cm-1 which correspond respectively to 1CH aromatic moiety, 2CH aromatic moiety, and methylene groups in a long aliphatic chain (nCH2, n g 4). In the 1800-1640 cm-1 spectral range where carbonyl group (CdO) vibrations occur, a well-defined peak around 1700 cm-1 is observed suggesting the presence of ketone or aldehyde functionality. These results confirm the presence of significant amounts of oxygen in asphaltenes. The low intensity of the peak at 1025 cm-1 is due to a C;O, C;S, or C;N bond. 3.3. Molecular-Weight Distribution by LDI-TOF Mass Spectroscopy. The mass spectroscopy experiment was performed with a highly dilute asphaltene solution (0.001 g/L
Table 5. Average Molecular Parameters of HH Asphaltene Molecule Derived from the Combined 1H- and 13C-NMR Analysis parameters
value
CT HT Cal Car fa n Cp Csub Cus As Ra r Rna Cna
38.9 36.9 15.8 23.2 0.59 3.9 11.2 4.04 7.13 36.2 7.0 0.15 0.62 2.17
ratio H/C being 0.95 indicates a high aromaticity of the HH asphaltenes.17 The nitrogen and sulphur content corresponds to usual values observed with Algerian crude oils. The oxygen content estimated from the mass balance is significantly larger than the content of other heteroatoms. 3.2. FTIR Spectroscopy. The baseline corrected, normalized, and Kubelka-Munk transformed diffuse reflectance Fourier transform (DRIFT) spectrum of HH asphaltenes is shown in Figure 1, and the corresponding major band
(37) Midttum, Ø. A physical-Chemical and chemometric study of interfacially active components in crude oils. Ph.D. thesis, University of Bergen, Norway, 1999; 180.
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Figure 5. XRD spectrum of HH asphaltene.
Figure 6. Gaussian and Lorentzian fit of XRD spectrum of HH asphaltene. Table 6. Parameters Obtained from the XRD Analysis41-43 parameter
definition
average distance between two aliphatic chains or saturated rings obtained from the γ-band position average distance between aromatic sheets obtained respectively from the 002 band position cluster diameter obtained from the full width at half-maximum (FWHM) of the 10 band average diameter of aromatic sheets obtained from the full width at half-maximum (FWHM) of the 002 band average number of aromatic sheets per stack
dγ(A˚) = 5λ/(8 sin θ) dm(A˚) = λ/(2 sin θ) Lc(A˚) = 0.9 λ/(FWHM cos θ) La(A˚) = 1.84λ/(FWHM cos θ) ns = (Lc/dm) þ 1
The general pattern of obtained spectra is very similar to results published in the literature.15,38 These last observations make it possible to advance the assumption that the representative asphaltenic elementary structure has a mass from approximately 550 uma and that dimers (1100 uma.) are also present in considerable quantity. Another possible interpretation of this result is to consider that the monomer molar mass corresponds to 1100 uma and that the monomer can easily be broken in two smaller (550 uma) moieties. 3.4. Nuclear Magnetic Resonance Spectroscopy. The 1Hand 13C-NMR spectra of asphaltenes are shown in Figures 3
Table 7. Crystallite Structure Parameters of HH Asphaltenes parameters value (A˚)
dγ
dm
Lc
La
ns
4.55
3.53
20.92
10.20
6.92
toluene) to reduce aggregation of asphaltenes. Figure 2 corresponding to HH asphaltene LDI-TOF spectra shows a bimodal distribution with two maxima of ion abundance: the former is found at around m/z 550 and the second at 1100. (38) Rizzi, A.; Cosmina, P.; Flego, C.; Montanari, L.; Seraglia, R.; Traldi, P. J. Mass Spectrom. 2006, 41, 1232.
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Figure 7. Baseline corrected Raman spectrum of HH asphaltenes fitted with Gaussian/Lorentzian hybrid function based on three peaks (D, G, and G00 ).
Figure 8. Fluorescence spectrum in emission mode of HH asphaltenes.
respectively 7 and 0.62. The values of As (∼36 % ) and of Φ (0.48) indicate a high ratio of condensation of aromatic rings. 3.5. X-ray Diffraction. The XRD pattern of asphaltenes is shown in Figure 5 and displays a well-defined γ band at around 18 (2θ) and two sharp peaks in the region 20-23 and the 10 band at 43. The graphene band (002 band) which appears around 25 was not well-defined because it is screened by the sharp peaks. By application of Gaussian or Lorentzian fit to the whole spectrum, the 002 band appears at 25 (see Figure 6). The spaced sharp peaks observed in the X-ray diffraction spectrum were attributed to the apparent presence of crystalline long chain n-paraffins which coprecipitated with asphaltenes. The presence of long chain n-alkanes was also observed in IR spectrum of asphaltenes at 725 cm-1, Figure 1, where splitting of the methylene peak indicates the formation of ordered arrangements of long alkyl chains. The small peak of the IR intensity indicates that the paraffin content in asphaltenes is low as was observed already by Andersen et al.43,44 Table 6 defines structural parameters of asphaltene clusters according to the XRD data analysis proposed by Yen.45 The values of structural parameters corresponding to HH asphaltenes are reported in Table 7. The average layer distance between aromatic sheets (3.53 A˚), the average interchains layer distance (4.55 A˚) as well as the number of aromatic sheets in a stacked cluster (7.0 A˚), and the average diameter (10.20 A˚) are of the same order as the values reported in the literature.45,46 3.6. Raman Spectroscopy. Aromatic sheet diameter was also determined by Raman spectroscopy through accurate determination of G and D band integrated intensities. An estimate of the aromatic sheet diameter La was calculated using the Tuinstra and Koening36 equation: ´ ¼ 44I G =I D La ðeÞ ð1Þ
Table 8. Raman Spectrum Analysis (Gaussian/Lorentzian) of Asphaltenes Using Three Peaks (La = 10.44 A˚) peaks
intensity
full width half maximum
Gaussian character
integrated intensity
1331.34 1566.23 1596.63
5.25 1.87 5.09
179.61 121.84 41.67
0.34 1.00 0.95
2316 549.6 518.1
band nature D G G00
and 4, respectively. The hydrogen and carbon contents in different chemical environments obtained from respectively 1 H- and 13C-NMR spectra are given in Table 3. These results show the relative abundance of aromatic and aliphatic structures. Corresponding results for proton and carbon content are respectively 20.11% and 59.5% for aromatic and 79.89% and 40.5% for aliphatic atoms. The spectrum obtained from the variable amplitude CPMAS experiment is divided into two regions. The first one in the range 0-90 ppm corresponds to the aliphatic carbons while the second ranging from 100 to 160 ppm corresponds to the aromatic carbon resonance. The aliphatic range was fitted with five Gaussian peaks at 14.8, 21.3, 30, 37.7, and 48.7 ppm, while the aromatic range was fitted with three Gaussian peaks centred at 116, 127, and 138 ppm. The VACP-NMR spectrum indicates the presence of alkyl carbon substituted with oxygen and nitrogen functional groups (at 48.7 ppm) in agreement with the DRIFT analysis. A combination of 1H and 13C-NMR data from the pointed out spectral 1H and 13C ranges allowed the determination of several average structural parameters, such as the aromatic carbon fraction (fa), the average number of carbons per alkyl side chain (n), the percentage of peripheral aromatic carbon substitution (As), and the number of substituent rings (r), as outlined by Calemma et al.5 and others.39-42 Equations used for calculating these parameters are given in Table 4. Results listed in Table 5 suggest that the HH asphaltene molecule has an aromaticity of 60 % and that the average number of carbon atoms in alkyl side chains is four carbons. The average number of aromatic and naphthenic rings was
Tuinstra and Koening showed that the G and D band integrated intensities IG and ID are sensitive to the microcrystalline planar crystal size La. The integrated intensities
(39) Mathiew, N.; Abaasa, N.; Daniel, T.; Michael, T. K. Chem. Ing. Sci. 1994, 49 (24A), 4153. (40) Trejo, F.; Ancheyta, J.; Centeno, G.; Marroquın, G. Catal. Today 2005, 109, 178. (41) Trejoa, F.; Centeno, G.; Ancheyta, J. Fuel 2004, 83, 2169. (42) Michael, G.; Al-Siri, M.; Khan, Z. H.; Ali, F. A. Energy Fuels 2005, 19, 1598.
(43) Gonzalez, E. B.; Andersen, S. I.; Garcia-Martinez, J.A.; Lira -Galeana, C. Energy Fuels 2002, 16, 732. (44) Andersen, S. I.; Jensen, J.O.; Speight, J.G. Energy Fuels 2005, 19, 2371. (45) Erdman, J G.; Pollak, S. S.; Yen, T.F. Anal. Chem. 1961, 33, 1587. (46) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39, 1847.
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Table 9. Comparison of Structural Parameters of Asphaltenes from Different Originsa ref
origin
H/C
E%
fa
n
La (A˚)
Lc (A˚)
FOb
PW 27,48 28,49 39
Haoudh El- Hamra well of Hassi Messaoud field DP45 deposit of Hassi Messaoud field Hassi Messaoud oil storage tank deposit vacuum residue of Kuwaitian petroleum
0.95 1.05 1.28 1.28
8.3 8.8 18.1 5.1
0.60 0.69 0.48 0.46
7.0 5.0 4.4 10
10.2 17.6 11.5 8.9
20.9 28.2 23.6 14.2
0.55 0.52 0.32 ndc
a
PW = asphaltenes studied in the present work. b FO = flocculation onset value: n-heptane volume fraction. c nd = not determined in this reference.
Ellingsen et al.49 and Morales and Mullins.50 Moreover, this value agrees with NMR results.
were often determined by fitting curve of the Raman spectra. All details concerning the analysis of Raman spectra of asphaltenes can be found in our previous paper.27 In the present study, we applied a three-peak fitting procedure (D, A, and G) with a combination of Gaussian/Lorenzian functions. The presence of shoulders in the Raman spectrum was neglected for the three-peak fitting. Figure 7 presents the HH asphaltenes Raman spectrum. The starting peak positions were chosen at 1330, 1560, and 1600 cm-1, that roughly corresponded to D, A, and G modes.27,33,47,48 The spectral data were corrected for incoherent vibrations and the fluorescence contribution. Then, three peaks were resolved, and the corresponding parameters are given in Table 8. These parameters were used to estimate the dimension of the aromatic sheet of the asphaltene molecule according to the method proposed by Tunistra and Koening.36 An average diameter of the aromatic sheet (La) estimated in this way was of 10.4 A˚ that is close to the value found by X-ray analysis (10.20 A˚). We observe that the La value of asphaltenes studied in the present work is significantly lower than diameters found previously27 with the asphaltenes obtained from the Algerian crude oil extracted from different wells (DP45 La = 14.85 A˚ and HM3077 La = 16.90 A˚). La reflects the condensation ratio of the aromatic rings in the elementary sheet. We conclude that this ratio may differ in a significant way with samples collected from different wells of the same oil field. It will be interesting to investigate the relationship between this parameter and the flocculation propensity of asphaltenes. 3.7. Fluorescence Spectroscopy. The fluorescence spectroscopy was used to estimate the ratio of condensation of aromatic rings in the asphalthenic sheet. Ellingsen et al.49 showed that the wavelength of the emitted light moved towards the red region when the number of fused rings increases. Meanwhile, the benzene light emission is observed at 269 nm, the emissions of naphthalene (two rings), phenanthrene (three rings), and pyrene (four rings) are respectively at 312, 346, and 372 nm, respectively. More condensed compounds (five rings or more) emit at wavelength higher than 400 nm. Morales and Mullins50 used the emission fluorescence with molecular orbital (MO) calculations to demonstrate that polyaromatic hydrocarbons with six, seven, and eight fused aromatic rings display electronic transitions at about 450 nm, the range where asphaltenes fluorescence maxima are observed. Most of these compounds are close to an isolated double-bond sextet carbon ratio of 0.3333. The emission spectra of HH asphaltenes, Figure 8, display an emission intensity peak at 459 nm. This value corresponds to an aromatic sheet containing five to eight condensed rings in agreement with the studies of
4. Conclusion Results obtained with different techniques make it possible to design an average molecule of Hassi Messaud asphaltenes. This molecule is highly aromatic (fa = 0.60) with seven polycondensed aromatic cycles. The aromatic core is substituted with four aliphatic chains with four carbon atoms. The molar mass corresponding to this molecule is about 550 uma. This structure corresponds to the “archipelago” molecular model characterized by large pericondensed ring structures with short alkane chains attached to them.51,52 The molar mass of the monomer of 550 uma concords with the smaller of two masses determined by mass spectroscopy. Consequently, the mass of 1100 uma corresponds to a dimer that confirms a high capacity of asphaltene monomers to aggregate. While the oxygen and nitrogen content was low (only a part of monomers contain these heteroatoms), the percentage in mass of oxygen was about 7% (w/w) which corresponds to two oxygen atoms per asphaltene monomer. The oxygen is present as ketone, aldehyde, or C-O functionality. Nitrogen and sulphur form C-S and C-N bonds. Aggregation of asphaltenes leads to the formation of clusters containing seven sheets with interlayer distances of 3.5 A˚ and an aromatic layer diameter of 10.20 A˚. The cluster diameter is 20.92 A˚. Asphaltenes contain small amounts of long chain, crystalline paraffins that were not eluted with n-heptane during purification of asphaltenes. The structural parameters of HH asphaltene are compared in Table 9 with literature data concerning a vacuum residue of a Kuwaitian petroleum and with a tank storage deposit of a crude oil from Hassi Messaoud field. This comparison shows that the propensity of asphaltenes to flocculate depends on the content of heteroatoms (E %) and on the average diameter of an aromatic sheet, La. Indeed, comparing with Kuwaitian crude, the value of La and E % are significantly higher for Algerian crude that is less stable in respect of flocculation. If we compare the flocculation onset of HH and DP45 asphaltenes (0.55 and 0.52, respectively), we conclude that, at similar contents of heteroatoms (∼8 %), asphaltenes with smaller values of La are more stable. On the other hand, very unstable asphaltenes obtained from the tank storage deposit display a particularly high content of heteroatoms. Concluding, the proposed characterization of petroleum asphaltenes affords information that may be helpful in understanding conditions of asphaltene flocculation. We are working on relationships existing between the propensity to flocculate and parameters determining the structure of asphaltene molecules and clusters.
(47) Sadezky, A; Muckenhuber, H; Grothe, H; Niessner, R; Poschel, U. Carbon 2005, 43 (8), 1731. (48) Jawhari, T.A.; Roid, J.; Casado, J. Carbon 1995, 33, 1561. (49) Ellingsen, G; Fery-Forgues, S. IFP Rev. 1998, 53 (2), 202. (50) Morales, Y.R.; Mullins, O. C. Energy Fuels 2007, 21, 256.
(51) Zhao, S.; Kotlyar, L.S.; Woods, J.R.; Sparks, B.D; Hardacre, K.; Chung, K.H. Fuel 2001, 80, 1155. (52) Maham, Y.; Chodakowski, M.G.; Zhang, X.; Shaw, J.M. Fluid Phase Equilibria 2005, 228-229, 21.
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