Energy & Fuels 1995,9, 225-230
225
Structural Characterization of Asphaltenes of Different Origins V. Calemma, P. Iwanski, M. Nali, R. Scotti, and L. Montanari" ENIRICERCHE S.P.A., 20097 San Donato Milanese, Italy Received August 1, 1994. Revised Manuscript Received November 29, 1994@
Asphaltenes obtained by precipitation with n-heptane from crude oils of different geological origin has been analyzed by a great number of spectroscopic techniques in order t o show the main molecular features which differentiate them. The most important differences found are (1)aromatic carbon and heteroatom content, (2) alkyl side-chain length, and (3)molecular weight. Smaller molecular weight asphaltenes are more aromatic, contain less heteroatoms, particularly S, and have shorter aliphatic side chains. Asphaltenes were characterized in detail through the multiplet selective NMR techniques (GASPE) t o obtain individual CH, (n = 0-3) quantitative subspectra: no evidence of quaternary aliphatic carbon signal was found while the aliphatic CHd CH ratio seems to be correlated with aromaticity. The degree of condensation and/or substitution of aromatic rings was evaluated by applying solid-state 13CNMR, FT-IR, and EPR. The results of our studies show that asphaltenes with different aromaticity seem to be similar in their aromatic rings condensation and/or substitution degrees. The estimated value of the average number of condensed aromatic rings is nearly 7.
Introduction Studies of petroleum asphaltenes have rapidly increased during the past years because of the production of heavier crude oils. Asphaltenes are a problem for their tendency to flocculate and precipitate during both oil production and oil refinery. In the first case they cause a dramatic reduction in oil flow or even blockage during production while in the second case they cause severe drawbacks during the processing of heavy ends such as the tendency to form coke, catalyst deactivation, and p~isioning.l-~ The need for a more efficient exploitation of heavy feedstocks had led to an increased interest in elucidating the molecular structure of asphaltenes. The reason for this is the belief that a deeper knowledge of chemical structure of this class of substances is the key for understanding their behavior in thermal and catalytic processes. By d e f i n i t i ~ n ,as~ phaltenes are a solubility class; that is, they are the insoluble part of petroleum after addition of n-alkane solvents in volume ratio at least l(oil):40(solvent). They are dark brown to black friable solids with no definite melting point. The molecular nature of petroleum asphaltenes has been the subject of numerous investigations,5-11 and they appear to be very complex
* Author to whom correspondence should be addressed. @Abstractpublished i n Advance ACS Abstracts, January 1, 1995. (1)Dickakian, G.; Seay, S. Oil Gas J . 1988, 85, 47-50. (2) Cornahan, N. F.; Quintero, L.; Pfund, D. M.; Fulton, J. L.; Smith, R. D.; Capel, M.; Leontaris, K. Langmuir 1993, 9, 2035-2044. (3) Sheu, E. Y.; Detar, M. M., Storm, D. A,; De Canio, S. J. Fuel 1992, 71, 299-302. (4)Annual Book of ASTM Standards; American Society for Testing and Materials, Philadelpia, Part 24, Standard No. D-2006, 1978. (5) Gillet, S.; Rubini, P., Delpuech, J. J., Escalier, J . C.; Velentin, P. Fuel 1981, 60, 226-230. (6) Knight, S. A. Chem. Ind. 1967, 11, 1920-1923. (7) Dickinson, E. M. Fuel 1980,59, 290-294. (8) Dereppe, J. M.; Moreax, C.; Castex, H. Fuel 1978,57,435-441. (9) Strausz, 0.P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992,71,13551363.
aromatic molecules surrounded and linked by aliphatic chains and heteroatoms. A great variety of analytical techniques have been employed to investigate asphaltene molecular structure. Particularly, 'H and 13CNMR provide reliable average molecular parameter^,^-^ such as aromatic carbon fraction (fa), average number of carbon per alkyl side chains (n),and the average percent of substitution of aromatic carbon (A&. IR permits the determination of the relative abundances of some functional groups, chiefly hydroxyl, carbonyl, and adjacent aromatic CH groups.12 Moreover, nondestructive molecular techniques, such as solid-state NMR and EPR,13,14have been used to evaluate the degree of condensation and of substitution of aromatic rings. More recently, the use of oxidizing agents capable of selectively oxidizing aromatic carbondo and experiments of flashpyrolysis coupled with GC-MS15 has led to a clearer picture of chemical structure of asphaltenes. However, the latter methods concern only a part of asphaltenes (oxidized or pyrolyzed ones) and several assumptions are to be made t o draw conclusions. Asphaltene molecular weights have been determined by applying gel permeation chromatography (GPC). The determination of molecular weight of asphaltenes is very difficult because of both the lack of suitable standard compounds for calibration and their tendency t o associ(10) Mojelsky, T. W.; Ignasiak, T. M.; Frakman, Z.; McIntyre, D. D.; Lown, E. M.; Montgomery, D. S.; Strausz, 0. P. Energy Fuels 1992, 5, 83-96. (11)Mitra-Kirtley, S.; Mullins, 0. C.; van Elp, J.; George, S. J.;Chen, J., Cramer, S. P. J.Am. Chem. SOC.1993, 115, 252-258. (12)Benkhedda, Z.; Landais, P.; Kister, J . ; Dereppe, J . M.; Monthioux, M. Energy Fuels 1992, 6 , 166-172. (13) Sethi, N. K.; Pugmire, R. J.; Facelli, J . C.; Grant, D. M. Anal. Chem. 1988,60, 1574-1579. (14) Requejo, A. G.; Gray, N. R.; Freund, M.; Thomann, M.; Melchior, M. T.; Gebhard, L. A.; Bernardo, M.; Pietroski, C. F.; Msu, C. S.; Energy Fuels 1992, 6, 203-214. (15)Speight, J. G.; Moschopedis, S. E. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1979,24,910-923.
0887-0624/95/2509-0225$09.00/0 0 1995 American Chemical Society
Calemma et al.
226 Energy & Fuels, Vol. 9, No. 2, 1995
ate, so that values are dependent on the dilution and temperature used during elution from the chromatographic column.16 We have studied the best experimental condition to evaluate the molecular weight (that is, the greatest dilution compatible with analytical instrumental sensitivity). However, the molecular weights found are not absolute values and they can be only used to compare different asphaltenes.
Experimental Section Asphaltenes used in this work were isolated from crude oil Villafortuna (VIL) and Gaggiano (GA);residues 350+ "C Brent (BREI, Gela (GE), and Safaniya (SAFA); residues 550+ "C Arabian Light (AL) and Belayin (BEL). Asphaltenes were precipitated with an excess of n-heptane (40:1), sonicated for 20 min, and then filtered. Asphaltenes were then purified by Soxhlet extraction with n-heptane for 8 h and dried under vacuum a t 100 "C until constant weight. Liquid-state 'H and I3C NMR were carried out on a Bruker AMX-300 spectrometer operating a t 'H resonance frequency of 300 MHz and I3C resonance frequency of 75.47 MHz. 'H NMR spectra were obtained as CDC13 solution with a pulse width of 3.5 ps (30" flip angle), recycle delay of 2s, data point of 8K, tube diameter of 5 mm, spectral width of 18 ppm, and a t least 200 scans. I3C NMR spectra were obtained by applying an inversegated decoupling technique to suppress NOE effect. Chromium acetylacetonate (Cr(acac)3)(0.01 M in the final solution) was added to assure complete nuclear magnetic moment relaxation between pulses. These conditions are necessary to have quantitative I3C NMR signals. Operating conditions were as follows: pulse width 2.7 ps (30" flip angle), data point 8K, tube diameter 5 mm, solvent CDC13, spectral width 250 ppm, recycle delay 2 s, nearly 20 000 scans. Solid-state I3C-NMR spectra were acquired on a Bruker CXP-300 operating a t a I3C resonance frequency of 75.47 MHz with pulse width of 2 ps (30" flip angle) and with the highpower proton decoupler gated on during acquisition and off between pulses. A delay time of 4 s was applied between pulses. The spectral fitting was carried out by matching a properly phased experimental spectrum with the simulated one (TENSOR software, Bruker). The gated spin echo (GASPE) I3C NMR technique used was that proposed by Cookson and Smith'' with the average scalar coupling constant JCHof 125 Hz for aliphatic carbon. Spectra integration was carried out on C, CH2, and CH CH3 subspectra. Fourier transform IR (FT-IR) spectra were recorded on a DIGILAB FTS-15E spectrometer in absorbance mode. Each spectrum resulted from the accumulation of 1000 scans with a spectral resolution of 4 cm-' in the 4000-600 cm-I spectral domain. Samples were prepared by mixing with spectroscopic grade KBr t o obtain a 0.5% (wiw) a s p h a l t e n e m r mixture. Spectra were acquired relative t o a pure KBr reference and analysis was focused on three regions of the spectrum: 28003200, 1500--1800, and 650-950 cm-'. A deconvolution technique, using the Nelder-Head algorithm, was applied to evaluate the relative intensities of overlapped IR bands. EPR spectra were recorded on a Varian E 112 spectrometer operating a t 9.2 GHz microwave frequency and magnetic field modulation of 100 KHz. Asphaltene radical densities were calculated by comparison with Varian Strong Pitch signal as standard with an error of %15%. DPPH (g = 2.0036) was used as g marker, and the microwave frequency was accurately read with a HP 5350 B frequency counter. EPR resonant cavity was a Varian E-231 cavity with operation mode TE102, equipped
+
(16) Payzant, J. D.; Lown, E. M.; Strausz, 0. P., Energy Fuels 1991, 5,445-453. (17) Cookson, D. J.; Smith, B. E. Anal. Chem. 1985, 57, 864-871.
w m
5
10
0
!j
I
/ ' " " ' " " ' ' ~ ' ~ " 1 " ' ' ' ' " ' 1 ~ ~ ' ' " " ~ l ' ~ " '
Don
200
150
LOO
50
0
Figure 1. Typical 'H (top) and I3C NMR (bottom) spectra of asphaltenes for which the different integration domains are shown. with a home-built temperature controller operating in a temperature range from -150 to 150 "C. Asphaltenes were introduced in 4 mm EPR tube and then evacuated for a t least 1 h under rotating pump (nearly 10+ mbar) vacuum before measurement. The GPC experiments were performed using a Waters 600 E pump for liquid chromatography equipped with a differential refractometer detector Waters 410. A set of six Waters uStyrage1 columns (300 mm x 7.8 mm i.d.1was used: 2 x 100 A, 2 x 500 A, 1 x 1000 A, 1 x 10000 A. The sample concentration was 0.1% w/w, and the solvent flow rate was 1 mumin. The calibration curve was obtained using two vanadyl porphyrins (MW 543 and 679 Da) and two fractions of a polycarbonate of bisphenol A (MW 3650 and 5230 Da). The GPC data were acquired and processed with a NEC APC IV Power Mate1 personal computer and Waters Maxima 825 software.
Results and Discussion IH and 13C NMR spectra of C oil asphaltenes (as general examples) are shown in Figure 1 in which the main attributions of different spectral zones are emphasized. The choice of the different integration domains for IH NMR spectra has been largely discussed in the literature. Because of the overlap of signals in IH NMR spectra, we divide the spectrum only into four regions: -1.01.0 ppm, y+ CH3 (Ha); 1.0-2.0 ppm, p+ CH3, CH2, CH (Hp); 2.0-4.0 ppm, a CH3, CH2, CH (Hy);6.5-9.0 ppm, aromatic CH. (Har).The 13C NMR spectra have been divided into two different integration domains: 10-65 ppm, aliphatic C (Call; 100-170 ppm, aromatic C (Car). The principal molecular parameters from NMR spectra are the aromatic carbon fraction (fa),average number of carbons per alkyl side chain (n), and the percent substitution of aromatic rings (A,) according to
Characterization of Asphaltenes of Different Origins
Energy & Fuels, Vol. 9,No. 2, 1995 227
Table 1. Molecular Parameters of Seven Asphaltenes of Different Origins asphaltene Gela Belayim Safaniya Arabian Light Brent Gaggiano Villafortuna
fa
n
As (%)
Mw
0.48 0.49 0.51 0.53 0.58 0.60 0.69
6.1 5.3 6.5 4.8 3.2 4.6 2.7
48 40 49 44 40 35 36
3100 2170 2870 2301 2360 2140 1900
Table 2. Analytical Data for Seven Asphaltenes of Different Origins H C N S 0 asphaltene (% wlw) (% WIW) (S WIW) (% WIW) (% WIW) Gela 79.7 6.9 1.1 10.8 1.5 Belayim 83.9 8.1 5.3 1.9 0.8 Safaniya 82.4 7.7 1.0 7.7 1.2 Arabian Light 1.0 84.1 7.0 7.1 0.7 Brent 1.1 86.9 7.4 2.1 2.6 Gaggiano 82.8 6.8 1.4 2.9 6.2 Villafortuna 6.1 1.0 90.3 1.9 0.8 Table 3. Infrared Spectral Region Assignments v , cm-1
3100-3640 3000-3100 2780-3000 1640-1800 1620-1590 915-852 760-730
structural feature 0-H, N-H stretch aromatic C-H stretch aliphatic C-H stretch carbonyl C-0 stretch aromatic C=C stretch arom CH out-of-plane deformation ( 1 adj H) arom CH out-of-plane deformation (4 adj H)
Dickinson's equations:6 n=
H,+Hp+Hv 1 0 0 ~ ~ ~ ; A,=Ha c,
with ClS = (C x C,l)/n = percent substituted aromatic carbon; C1 = Cp 4"= percent nonbridge aromatic carbon; Clu = 12 Ha, x H = percent unsubstituted aromatic carbon; C and H are respectively % carbon and % hydrogen from elemental analysis data. These parameters and GPC molecular weights of the seven different origin asphaltenes are listed in Table 1. The elemental analysis results for the different asphaltenes are tabulated in Table 2. From top t o bottom, the atomic WC ratios and sulfur (S) contents lower while nitrogen (N) contents appear very similar for the seven asphaltenes. The oxygen ( 0 )content is calculated by difference; therefore, caution is needed in the interpretation of any trend for the differnt asphaltenes. A typical FTIR spectrum of asphaltenes is shown in Figure 2 and the assignment of the different bands is reported in Table 3. From the spectra of 20 model compounds (normal and isoalkanes and alkylaromatics), a linear correlation between the molar ratio of CHdCH3 and the ratio between band intensities at 2927 and 2957 cm-' was found on the basis of the following relationship:
+
in which k = 1.243 is simply the linear correlation coefficient of the plot n(CH2)/n(CH3) vs Z(2927cm-l)l I(2957cm-l) (linear correlation coefficient = 0.996).
t 4000
I
3500
I
I
3000
2500 2000 Uavenumbeis
I
I
IS00
1000
Figure 2. Typical FT-IRtransmission spectrum of asphaltenes (GA).
In Table 6 the molar ratios R are reported and they show a similar trend with the parameters n elaborated from NMR spectra, but for a complete comparison between the two methods it is necessary to estimate an average molecular number of CH3. The interpretation of 13C NMR spectra for asphaltenes is usually limited to the division of spectral region into aromatic and aliphatic chemical shift ranges in order to obtain the fraction of aromatic carbon. To gain insight into CH, (n = 0, 1, 2, 3) assignments and the relative abundances (~cH,) in the alkyl side chains, spectral editing of carbon spectra was accomplished by using pulse methods that select carbons on the basis of the number of directly bonded protons. Assignment of carbon types was achieved using the single-frequency off-resonance decoupling (SFORD) technique. Asphaltenes, however, are complex mixtures yielding spectra that are largely congested. In this case the SFORD technique is of very limited value. Another spectral editing technique that appeared to be more promising for asphaltenes was GASPE, because was successfully used in quantitative determination of percentage abundances of different C group types for liquids derived from petroleum.18 In the case of asphaltenes some typical GASPE 13C NMR subspectra for the different CH, multiplet of the CH3 asphaltenes are shown in Figure 3. The CH subspectra are characterized by some prominent peaks in the 10-30 ppm region due to the different methyl groups, while a broad resonance (confirmed also in a DEFT 90" 13C NMR experiment in which only CH groups are displayed) in the 40-60 ppm range is due to presence of different CH groups. To calculate the CH and CH3 relative intensities, we used a chemical shift cutoff at 30 ppm in the CH CH3 subspectra. All the asphaltenes studied showed no aliphatic quaternary carbon signals. The relative molar abundances of different CH, groups are reported in Table 4. It is interesting to observe the asphaltenes have different CH$CH molar ratios (hereafter referred t o as Q): Q is nearly 1 for GE and BRE, while it is nearly 1.6 for SAFA,AL, and BEL and is more than 3 for GA and VIL. Low Q values are probably due to the presence of cyclic aliphatic structure for which the presence of CH groups does not require two CH3 end groups. High Q values are vice versa index of the relative abundance of CH3 group probably due to a significant contribution
+
+
(18)Cookson, D.J.; Smith, B. E. Fuel 1983,62,34-38.
228 Energy & Fuels, Vol. 9, No. 2, 1995
Calemma et al. C
367
267
167
67
-33
-133 ppm
Figure 4. Experimental (top) and simulated (bottom) NMR static spectra of GA asphaltenes.
Figure 3. Conventional spin echo spectrum (bottom) and saturated CH2, CH + CH3 and C GASPE spectra (as indicated) for GA asphaltenes. Table 4. CH, Abundances Derived from 13C GASPE
NMR asphaltene Gela Belayim Safaniya Arabian Light Brent Gaggiano Villafortuna
C 0 0 0 0 0 0 0
CH 0.20 0.13 0.13 0.14 0.21 0.11 0.11
CH2 0.57 0.65 0.65 0.64 0.58 0.56 0.50
CH3 (CHdCH) (CHdCH3) 0.23 1.15 2.50 0.22 1.69 2.95 0.22 1.69 2.95 0.22 1.57 2.95 0.21 1.00 3.04 0.33 3.00 1.70 0.39 3.54 1.30
of methyl group directly bond to aromatic carbons. Medium Q values may indicate an intermediate situation between the two described. These data seem to evidence a general trend for asphaltenes to have a major contribution of CH3 groups in the alkyl side chains according to their maturation. From GASPE 13CNMR data an average molecular methyl number (n(CH3))can be calculated:
An average number of carbon per alkyl side chain can be now evaluated in the following way:
where nIR is the average number of carbons per alkyl side chain calculated from IR data; R was defined in eq 1; ncH3 is defined in eq 2. In Table 5, 12.1~values are reported and they are in good agreement with the n values obtained from NMR data only. Another important feature in the IR spectra
Table 5. Normalized Intensities of FT-IR Bands (Intensitiedmg of Sample) in the 1800-1600 cm-' Spectral Zone 1695 1665 1656 1647 1620 1600 asDhaltene cm-' cm-I cm-' cm-' cm-' cm-' Gela 0.09 0.06 0.00 0.15 0.08 0.29 Belayim 0.14 0.11 0.00 0.20 0.21 0.64 Safaniya 0.13 0.06 0.00 0.07 0.14 0.46 Arabian Light 0.10 0.04 0.00 0.09 0.12 0.45 Brent 0.09 0.00 0.10 0.00 0.22 0.60 Gaggiano 0.24 0.00 0.10 0.00 0.25 0.56 Villafortuna 0.10 0.00 0.05 0.00 0.23 0.49
of asphaltenes is the low intensity of signals in the region 3600-3000 cm-l. This result indicates very low concentrations of OH and NH groups, which are often considered important for asphaltene aggregation via H bonds.lg Also, IR band intensities of carbonyl groups (1750-1600 cm-l) are generally weak for the asphaltenes. This spectral zone was deconvoluted into four bands centered at 1735 cm-l, esters; 1700 cm-', ketones and aldehydes and carboxylic acids; 1650 cm-l, highly conjugated carbonyls such as quinone-type structure and amides; 1600 cm-l, aromatic -C=C- stretching. The normalized band intensities are reported in Table 5. An empirical index of carbonyl abundances is the following:
The values of (C=O) indexes, which are related to the content of oxygenated groups in the asphaltene molecules, are reported in Table 5. Greater insight into the type of condensation of the asphaltenes have been obtained by using solid-state 13C NMR and FT-IR. Under static conditions, the 13CNMR spectra of asphaltenes (Figure 4 shows a typical example) have line shapes which reflect the chemical shift anisotropy (CSA) of the different molecular carbons.13 CSA is much greater for aromatic carbons than for aliphatic. It is possible t o extract the principal values of the CSA second rank tensor from NMR static spectra. For this purpose, the 13CNMR static spectra of asphalt(19) Moschopedis, S. E.; Speight, J. G. Fuel 1976,55, 187-192.
Characterization of Asphaltenes of Different Origins
Energy & Fuels, Vol. 9, No. 2, 1995 229 F value
0-
- - ~-
0,s -
1.
--
I -
-__---__
195
25
2
_.__
~
Anthracene 2-Methylanthracene Dimethylanthracene
1.2-Benzopyrene
1 Graphite Figure 5. F values of the seven different oil asphaltenes (see text) compared with some model compounds.
enes have two main limitations: (1)the aliphatic band covers the aromatic 033 component preventing precise band fitting; and (2) the large line width prevents any precise quantification of the different carbon line shapes and contributions. A qualitative method of gain information about the different aromatic carbon abundances has been derived from the intensities ratio of the NMR band a t 170 and 100 ppm (hereafter called F). The choice comes from the fact that inner aromatic carbons have an axially symmetric tensor with maximum intensity a t nearly 160-190 ppm while protonated and substituted aromatic carbons have an orthorombic symmetric tensor with maximum intensity in the range 140-100 ppm. So, higher F values correspond to larger sized polycondensed aromatic rings. In Figure 5 the F values are reported with some other model compounds. Two main conclusions can be drawn: (1)asphaltenes of different origin show similar average aromatic condensation with the exception of VIL, which seem to be more condensed; (2) the average number of polycondensed aromatic rings is estimated to be between 5 and 7. Since the average number of aromatic rings calcclated from Dickinson's model is generally greater than 20, our results suggest that the aromatic clusters are linked in the average molecule by some heteroatom and aliphatic linkages, as proposed in l i t e r a t ~ r e .Another ~ important molecular parameter can be obtained from the out-of-plane CH aromatic IR bands in the 930-700 cm-l spectral range. The ratio P = (915)/(725)is related both to the degree of condensation and to the degree of substitution.*2 P values for the different asphaltenes are reported in Table 6 and they confirm the NMR results concerning the similarity of the different asphaltenes. In this case the VIL oil asphaltenes have higher P values as expected on the basis of their aromaticity while the higher P value of AL oil asphaltenes may indicate a greater degree of substitution. EPR spectra reveal the presence of organic free radicals: owing to the lack of hyperfine structure, the only structural parameters are g factors, line shapes, and line widths. These parameters are summarized in
Table 6. Molecular Parameters Derived from FT-IR Rands As Described in the Text asphaltene Gela Belayim Safaniya Arabian Light Brent Gaggiano Villafortuna
R(CHdCH3) 3.34 3.21 3.35 3.81 3.07 2.52 2.59
~ I R
6.0 4.9 6.2 5.1 2.8 5.2 3.8
(C=O) 0.44 0.34 0.30 0.29 0.19 0.30 0.18
P(1HI4 H) 1.56 1.63 1.44 2.18 1.43 1.45 2.26
Table 7. EPR Parameters of Seven Asphaltenes of Different Origins Gela Belayim Safaniya Arabian Light Brent Gaggiano Villafortuna
2.0034 2.0031 2.0032 2.0031 2.0029 2.0029 2.0028
5.78 6.31 5.91 6.38 6.07 5.12 6.27
8.05 8.99 8.40 8.76 8.78 7.31 9.26
1.39 1.42 1.42 1.37 1.45 1.43 1.48
1.4 3.2 2.7 2.9 5.3 3.1 7.9
a Peak to peak separation of the EPR derivative peak (gauss). Full width a t half-height in EPR absorption peak (gauss). Lineshape parameter. d Radical density in 10IHspindg.
Table 7 together with free radical densities. From top to bottom oil asphaltenes g factors tend to decrease and this indicates a diminishing contributions from heterothe~ electron molecular orbital, in accordance a t o m to ~~ with elemental analysis data. In fact, the heteroatoms shift the g values higher toward the free electron value (g, = 2.0023). EPR line widths and the Gaussian/ Lorentzian ratios are similar for the different asphaltenes. This is in accordance with the preceding results showing that the core size of polynuclear aromatic moieties are not very different for the seven asphaltenes. A study on kerogens**showed that the EPR line width monotonically decreases with increasing level of aromaticity (maturation) while the line-shape parameter (defined in Table 7) AH1/2/AHpp (value 1.72 for Lorentzian and 1.18 for Gaussian) had a trend toward a more Lorentzian character with increasing maturity. This trend is true for aromatic molecules of the same nature without different degree of substitution while in the case of asphaltenes, for which aliphatic side chains are present, also the degree of substitution of the aromatic
230 Energy & Fuels, Vol. 9, No. 2, 1995 rings seems to influence the Gaussiafiorentzian lineshape ratios. Electron spin density was correlated with the maturation level in kerogens and this is consistent with both the decrease in the WC ratio and the increase in the aromatic core size of polynuclear aromatic moieties.20 Spin densities for the seven different asphaltenes show a clear trend to increase with aromaticity and also in this case the VIL asphaltenes show a greater degree of condensation compared with the other ones.
Conclusions The multidisciplinary and analytical characterization of different origin asphaltenes provides a better understanding of their analogies and peculiarities. The seven different origin asphaltenes show several significant trends in molecular structure with increasing carbon content, which can be summarized as follows: aromaticity increases; the average length of alkyl side chains decreases; the heteroatom content lowers; the average molecular weights become smaller; and the average aromatic core size increases. Analogous with kerogens,12these structural changes are consistent with the degree of maturation of asphaltenes and the trends can have interesting implications in the asphaltenes tendency to associate in solution. Particularly, more “matured” asphaltenes could have a more pronounced tendency to aggregate through n-n aromatic interactions.21 (20) Bakr, M. Y.; Akiyama, M.; Sanada, Y. Org. Geochem. 1990,15, 595-599.
Calemma et al.
In addition, the average number of polycondensed aromatic clusters seems t o be less than 7 for all asphaltenes. This implies the presence of linkages between aromatic clusters. On the basis of our results, the picture of asphaltene molecules as large aromatic sheets containing more than 100-300 carbon atoms as proposed on X-ray diffraction studies22cannot be supported. Rather, the average asphaltene molecule is better represented by isolated clusters of polycondensed groups consisting of between 5 and 7 rings jointed by aliphatic and heteroatom bridges. The 13CGASPE NMR technique gives clear evidence of the presence of CH groups in the asphaltenes aliphatic chains. The calculated CWCH3 ratios indicate the presence of condensed alicyclic structures which seem to be concentrated in the more aliphatic asphaltenes molecules. Therefore, the CHdCH ratio could be an index of asphaltenes “maturity”.
Acknowledgment. The authors are particularly grateful to the EN1 group for financial support of this work and to Mrs. M. Anelli, Mrs. N. Sommariva, Mr. W. Stringo, and Mr. A Manclossi for their precious technical contributions. EF940155Q (21)Khryashchev, A. N.; Popov, N. A,; Posadov, N. A,; Rozendal, D. A. Pet. Chem. 1991,31,601-604. (22) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967,39,1847-1854.
