Asphaltenes Precipitated by a Two-Step Precipitation Procedure. 2

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Asphaltenes Precipitated by a Two-Step Precipitation Procedure. 2. Physical and Chemical Characteristics Martin Fossen,*,† Harald Kallevik,‡ Kenneth D. Knudsen,§ and Johan Sj€oblom|| †

SINTEF Petroleumsforskning AS, NO-7031 Trondheim, Norway Research and Development Centre, Statoil ASA, NO-7005 Trondheim, Norway § Institute for Energy Technology, NO-2007 Kjeller, Norway Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway

)

‡

ABSTRACT: In a previous study (10.1021/ef060311g), a two-step precipitation procedure for asphaltenes from three crude oils (WA, NS-A, and NS-B) was reported. Crude oils were diluted 3:1 with n-pentane, and precipitated asphaltenes were filtrated off (first fraction). A second fraction consisting of asphaltenes still present in the crude oil was precipitated by further dilution of 18:1 npentane/crude oil. In the previous work, interfacial tension, aggregation size, and onset of precipitation were investigated and shown. In the current work, elemental analysis indicated that the first fractions contain relatively more heteroatoms than the second fractions and whole asphaltenes. Fourier transform infrared (FTIR) spectroscopy, proton and carbon nuclear magnetic resonance (NMR) spectroscopy, and NMRdistortionless enhancement by polarization transfer (DEPT) indicated that the less soluble fractions WA and NS-B were more aromatic and had a more polar aromatic core and a larger aromatic core consisting of more rings. Furthermore, there were indications that the more soluble fractions contained more branched aliphatic side chains with a larger degree of hydroxylic and carboxylic groups. Laser desorption ionizationmass spectroscopy (LDIMS) molecular-weight determination indicated that the less soluble asphaltene samples had a higher average molecular weight compared to the more soluble fractions and the whole asphaltenes. The results could help explain the differences in interfacial tension and solvent properties that were reported previously.

1. INTRODUCTION Asphaltenes are a solubility class consisting of compounds in crude oils that precipitate upon the addition of n-alkanes. Often a n-alkane/crude oil ratio of 40:1 is used to ensure complete precipitation of the whole asphaltene fraction from the crude oils.1 During oil recovery, asphaltenes are known to precipitate upon pressure reductions, changes in the temperature conditions, and oil-phase composition.25 The aggregation behavior followed by precipitation of the asphaltenes has been debated for decades, and the processes involved are still not fully understood. Normally, the elemental composition of asphaltenes from different sources is fairly similar to the H/C ratio around 1.15 and heteroatom contents varying from a few percentages to as much as 10% for sulfur in some oils.5 Asphaltenes consist of tens of thousands of different molecules with a high degree of polydispersity in both size and structure. Determination of their molar mass is difficult and has been and still is a subject of debate. Values of the molar mass of asphaltenes have been reported in the range of some hundreds up to several millions grams per mole depending upon the techniques used.6,7 According to recent studies,8 the average molecular weight (MW), as determined by fluorescence depolarization (FD) is 750 g/mol. This number is disputed by others, who argue that the FD technique is not capable of detecting the full breadth of the molecular-mass distribution and, thus, cannot detect some of the heavier materials within the asphaltene samples.911 Another study uses 3501500 g/mol as the average MW range.12 In the current r 2011 American Chemical Society

study, the average MW measured was in the range of 445 840 g/mol. Vapor pressure osmometry (VPO), gel permeation chromatography (GPC), and size-exclusion chromatography (SEC), using 1-methyl-2-pyrrolidinone or N-methyl pyrrolidone (NMP) and tetrahydrofuran (THF) have been used for the determination of the average MW of polyaromatic hydrocarbons, vacuum residues, and asphaltenes.9,10,1315 The MWs reported have been in the order of up to 1 million g/mol obtained using GPC.13 It has been argued that the large MWs are due to aggregation in NMP.1619 In one study,19 it was pointed out that the apparent MW of alkyl-substituted hexabenzocoronenes and asphaltenes increased at an increasing concentration and that nuclear magnetic resonance (NMR) measurements confirmed this. With other techniques, such as diffusion measurements and ionization techniques, the MW of asphaltenes has been determined to be in the range of some hundreds to above 1000 g/mol.7,20 Hunt (referenced in the study by Sheu21) obtained average MWs of ∼500800 g/mol for heptane-precipitated asphaltenes, while for asphaltenes precipitated with pentane, the MWs were determined to be in the range of 200650 g/mol. Bansal et al.,22 who based their work on the study by Christopher et al.,23 obtained results indicating larger asphaltene molecules with MWs of 1500 and 2500. Received: March 10, 2011 Revised: July 1, 2011 Published: July 14, 2011 3552

dx.doi.org/10.1021/ef200373v | Energy Fuels 2011, 25, 3552–3567

Energy & Fuels The debate around the structure of asphaltenes is closely linked to the debate on the MW and whether they are of polymeric, cross-linked ring systems24,25 or monomeric7 nature. Recent studies using FD has supported the view that asphaltenes are monomeric compounds that self-aggregate at very low concentrations, even in solvents known to dissolve asphaltenes.7,17,26 Again, some911 are sceptical to the use of ultraviolet (UV) fluorescence and argue that aromatic units connected with flexible polymethylene bridges and even bridges with restricted mobility readily undergo intramolecular energy-transfer processes, so that the rotation detected is not necessarily the rotation of the whole asphaltene molecule. One other important point is that not all species in the asphaltene fraction would necessarily absorb in the spectral range under study.9 Mass spectrometry (MS) techniques have been extensively used for molecular-weight distributions (MWDs) of asphaltenes and structural characterization.9,21,27,28 Despite this, mass spectrometric determination of asphaltene MWDs and, in particular, laser desorption ionization (LDI)-based MS remain controversial.7 One reason for this is that different measurements have resulted in quite different MWs, with some in agreement with other ionization techniques27,28 and some not.10 The LDIMS data of asphaltenes can be sorted into two groups, namely, those yielding the most likely MWs for the asphaltene monomer within 4001000 g/mol,21,28,29 consistent with a single fused-ring structure for the monomers, and those yielding MWs of 17002000 g/mol or greater.16,30 Furthermore, the inconsistency of the LDIMS results is due to the self-aggregation of asphaltenes, and Hortal et al.27 have recently systematically looked into the concentration dependency and the energy density of the laser radiation used in LDIMS measurements. It has been claimed16 that the reason for the discrepancies in the LDIMS results are that the MWDs have been measured at low laser powers, while others20,27 have suggested that a lower laser power should be used in LDIMS measurements. In this work, LDIMS with low laser power was used based on the conclusions obtained by Hortal et al.27 NMR spectroscopic techniques have been reported in numerous studies on asphaltenes to determine the number of polycondensed aromatic rings,31 aromatic carbon fraction, average number of carbons per alkyl side chain, average substitution of aromatic carbons, number of substituent rings, and degree of condensation.3,4,32 Average structural parameters of asphaltenes based on proton and carbon NMR have also been determined in several studies.31,33,34 Furthermore, the distortionless enhancement by polarization transfer (DEPT) sequence is often used in addition to 13C.23,3538 In the DEPT sequence, the carbon atoms are excited indirectly by polarization transfer from the hydrogen atoms to which they are coupled. As a result, DEPT does not give signals from quaternary carbons. On the other hand, the carbon relaxation time bottleneck is overcome, leading to great improvements in the signal-to-noise ratio for the same amount of recording time. Using 135° pulses (DEPT-135), the carbons related to CH2 are phased downward, resulting in an easy way to determine the shifts of these carbons.39,40 On the other hand, it complicates the integration and demands more care in the selection of shift value ranges used for the integration. The shifts are not changed compared to the 13C NMR spectra, and the spectra are thus comparable, although the DEPT spectral intensities will have different ratios depending upon the nucleus and its environment studied. Detailed quantification of the molecules can be performed by proper investigation and integration of the peaks and peak ranges in both carbon and DEPT spectra.37 The

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Table 1. Percent Weight (wt %) of Asphaltenes for the First Fraction (3:1), Second Fraction (18:1), and the Asphaltenes Precipitated by the Standard Procedure of Excess (40:1) nPentane (Whole Fraction)a oil

wt % first wt % second sum wt % first and wt % whole fraction (3:1) fraction (18:1) second fractions fraction (40:1)

WA

0.8

0.9

1.7

2.0

NS-A

1.0

0.8

1.8

1.8

NS-B

0.4

0.5

0.9

1.6

The first and second fractions did not sum up to the whole, as shown for the WA and NS-B asphaltenes. While for the NS-A, the fractions did sum up to the whole fraction. a

aromaticity of asphaltenes is determined from the ratio of the aromatic/aliphatic regions in the 13C NMR spectra. Another structural characterization technique frequently used in the study of asphaltenes is infrared (IR) spectroscopy. Fourier transform infrared (FTIR) spectroscopy permits the determination of functional groups, such as hydroxyls, carbonyls, and adjacent aromatic CH groups, found in coals, crude oils, and asphaltenes.31,41,42 In our previous work (10.1021/ef060311g),43 asphaltenes were precipitated in a two-step precipitation process, where the least soluble asphaltenes were precipitated at a low dilution ratio (3:1) of n-pentane, while the more soluble asphaltene fraction was precipitated from the same sample at further dilution. Thus, the more soluble (second fraction) did not contain the least soluble asphaltenes. The reason for choosing this procedure was the assumption that, within the whole asphaltene fraction, there are still solubility fractions that may have quite different functionalities. Experiments on fractionated asphaltenes are often performed, where the least soluble fraction is still present in all of the subfractions.44 In this work, this has been avoided by including the precipitation and separation in the same procedure. In the present work, the asphaltene samples obtained from the two-step precipitation procedure (10.1021/ef060311g)43 have been studied with regard to MW and structure. From the previous work, assumptions were made that the less soluble fractions (first fractions) would have higher MWs and/or are more aromatic and polar compared to the more soluble fractions (second fractions). Furthermore, it was suggested that the second fraction would contain relatively longer alkyl groups, explaining the higher solubility upon the addition of n-pentane to the crude oils. In this work, we conclude that the main differences between the less and more soluble asphaltenes are the size (MW and radius of gyration), the aromaticity, and the relative polarity. The first fraction shows larger values for these three parameters. In addition, there are indications that the more soluble asphaltenes have more branched alkyl side chains and a higher degree of hydroxyl and carboxyl groups connected to the aliphatic part.

2. MATERIALS AND METHODS 2.1. Materials. Three crude oils [one from West Africa (WA) and two from the North Sea (NS-A and NS-B)] were used. More details on the oils are found in part 1 (10.1021/ef060311g).43 In short, the crude oils were diluted with n-pentane in a ratio of 3:1 with subsequent filtration after 24 h and further dilution of the crude oil to a ratio of 18:1 n-pentane/crude oil followed by filtration after another 24 h. The amount of asphaltene obtained for each sample was weighted (Table 1). 3553

dx.doi.org/10.1021/ef200373v |Energy Fuels 2011, 25, 3552–3567

Energy & Fuels Whole asphaltenes were also precipitated from the crude oils by direct dilution of 40:1 n-pentane/crude oil. Total weight percent of the first and second fractions do not sum up to the amount of the whole asphaltenes for the WA and NS-B crude oils. This was probably because not the entire asphaltene fraction was precipitated at the dilution ratio of 18:1. The solvent used in proton NMR was chloroform-d, 99.8 atom %, from Sigma-Aldrich, and the internal standard was dichloromethane. Chloroform-d was also used as the solvent for the 13C and DEPT-135 measurements, with 1,4-dioxane pro analysi (pa) from Merck as an internal standard and chrome(III) acetylacetonate, 98% pa quality, from Merck-Schuchardt as a relaxation agent. 2.2. Elemental Composition (C, H, N, O, and S). The elemental analysis was performed by SGS Vernolab (France) using the American Society for Testing and Materials (ASTM) D5291 standard. Weight percentages of the carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S) atoms were determined. Experimental errors were (0.20.3 wt % for C and (0.2 wt % for H, N, O, and S. 2.3. LDIMS. LDIMS experiments were performed in a customized apparatus equipped with an ion source coupled to a time-of-flight (TOF) mass spectrometer. Laser radiation (266 nm) is focused on the sample plate in pulses of 7 ns duration with an energy of 20 μJ. Further details on the apparatus working conditions are given elsewhere.27 The asphaltene samples for the LDIMS experiments were prepared with the dried-droplet method by depositing 20 μL volumes of 50 mg/L asphaltenes in toluene solution onto a flat stainless-steel sample plate and letting it dry in air. The samples had the visual appearance of a uniform pale-brown asphaltene film on the plate. The analysis was similar to one that was used in a previous asphaltene study.27 The recorded spectra have been fitted with empirical distributions built with a log-normal functionality (eq 1) " # N ln W ln W0 2 exp 0:5 ð1Þ f ðWÞ ¼ W σ where the variable W represents the MW and W0 and σ are fit parameters controlling the position of the maximum and width of the MWD f(W), respectively, and N is a normalization constant. Number averages of MWs and related quantities are obtained from integration of the probability distribution f(W). Furthermore, the reason for choosing a low laser power was that the aggregation process in the desorption plume increases with the laser pulse energy, meaning that the values obtained for high energies (80100 μJ) are most likely due to asphaltene aggregates rather than monomeric species.27 This necessitates the use of low energy densities (20 μJ) and diluted samples (50 mg/L) to define Waveraged for the asphaltenes. 2.4. FTIR Spectroscopy. The FTIR measurements were performed to identify and quantify functional groups and a comparison to proton and carbon NMR measurements. IR spectroscopy measurement was performed on asphaltene powder using the Tensor 27 from Bruker Optics (Stockholm, Sweden), a KBr beamsplitter, and MKII Golden Gate ATR unit with a Michelson interference gauge and a N2cooled MCT detector. The software used was the OPUS version 4.2 (build 4.2.37) from Bruker Optics GmbH. The IR spectra were transformed with a BlackmanHarris three-term apodization function. The IR spectra were manually baseline-corrected using the software by drawing straight lines between 4000 and 1800 cm1 and 1800 and 610 cm1. These points were chosen to avoid negative absorbance values and to obtain the same baseline for all spectra for comparison. Average molecular parameters were calculated on the basis of the FTIR spectra of the asphaltene samples. Wavenumbers used for the determination of the structural functional groups (Tables 68) from the asphaltene fractions analyzed have been reported elsewhere in relation to the characterization of crude oils and asphaltenes.31,41,42,45,46 The

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Table 2. Descriptions of the Chemical Groups That Are Likely To Be Incorporated in the Shift Ranges Used in the Evaluation of the DEPT-135 Spectra39,47 ppm range

description

178100

aromatic carbon

178150

heterosubstituted aromatics (OH, OR, CHO, COOR, COR, NH2, NR2, and NO2)

150138

esters, and acids heterosubstituted aromatics

138110

aromatic hydrocarbons

(pyridines, thiophenes, and furans) 7037.7

carbons R to oxygen- and nitrogen-containing groups and aliphatic and cyclic structure, which may contain carboxylic acids or amines

37.722.5

CH2

37.736.7 32.531.5

carbons R to a branch at an alkyl chain CH2 on long-chain alkyls R to CH3

30.828.3 22.8522.63

CH2 on long-chain alkyls iso-alkyl

22.510

CH3 (methyl)

20.519.3

iso-alkyl

22.517.0

methyl attached to aromatic atoms

1710 14.413.9

methyl in alkyl chains containing more than two carbon atoms terminal methyl

values reported are based on the absorbance at a specific wavenumber, the integral between two wavenumbers, or the relative value between certain wavenumbers. Polar functional groups containing oxygen are CO (1100 cm1) from secondary alcohols and ether functional groups, CO (1175 cm1) from tertiary alcohols, CdO (1710 cm1) from carbonyl and cyclic ketones, CO (1215 cm1) phenol compounds, and OH (3300 cm1) bonded groups. In addition, organic esters, aldehydes, and ketones absorb light with wavenumbers in the range of 17001750 cm1. Nitrogen groups are CdN (1648 cm1) and CN (1260 cm1) from amides and amines and NH (3435) groups, which overlap with OH groups. Sulfur groups are SdO (10301040 cm1) from sulfoxides. 2.5. 1H, 13C, and DEPT-135 NMR. Proton NMR spectra were recorded at room temperature on a Bruker Avance DPX 300 MHz with a tube diameter of 5 mm. Proton (1H) measurements were performed on solutions containing 16 mg of asphaltenes in 0.7 mL of CDCl3, with 0.025 M CH2Cl2 as an internal standard. The acquisition time (Aq) was 2.65 s, and sweep width was 20.56 kHz. Free induction decay (FID) was Fourier-transformed using an exponential broadening of 1 Hz. The normalized proton spectra were corrected for the ratio between the weight of asphaltenes and the amount of internal standard. Carbon (13C) and DEPT NMR spectra were recorded at room temperature on a Bruker Avance DPX 400 MHz. Carbon (13C) and DEPT-135 measurements on the subfractions were performed on near saturated solutions of asphaltenes in a CDCl3 solution containing 0.16 M 1,4-dioxane as an internal standard and 0.03 M chrome(III) acetylacetonate as a relaxation agent. The whole asphaltenes were dissolved in CDCl3 containing 0.013 M 1,4-dioxane and 0.037 M Cr(AcAc)3. For the carbon spectra, the Aq was 0.40 s, the relaxation delay (d1) was 10, and 12 000 scans were taken to improve the signal-tonoise ratio. For DEPT-135, the pulse sequence had the same Aq, d1 was 10 s, and the number of scans was 4800. For whole asphaltenes, a shorter d1 (2 s) was used on the DEPT-135 sequences, giving the opportunity to 3554


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Table 3. Elemental Composition, by Percent Weight, of the Asphaltene Samplesa sample

carbon

hydrogen

nitrogen

sulfur

oxygen

WA whole

87.2

8.3

1.6

1.2

1.8

WA first WA second

85.5 86.3

9.1 8.6

1.6 1.5

2.3 2.2

1.6 1.5

NS-A whole

87.1

8.1

1.0

2.0

1.7

NS-A first

86.8

8.2

1.1

2.0

1.9

NS-A second

87.3

7.9

1.0

2.0

1.8

NS-B whole

85.5

8.3

1.4

2.2

2.7

NS-B first

83.2

8.4

1.5

3.6

3.2

NS-B second

83.7

8.6

1.4

3.3

3.1

Table 5. Number Averaged MW (Waveraged) Values in g/mol of the Asphaltenes Obtained by LDIMS Using a Laser Pulse with an Energy of 20 μJ fraction

WA

NS-A

NS-B

first fraction (3:1)

825

840

470

second fraction (18:1)

720

745

460

whole fraction (40:1)

500

700

445

a

The values were normalized from the raw data by dividing the value for each element by the sum of all of the elements.

Table 4. Relative Compositions of the Elements to the Amount of Carbon Atoms Obtained by Dividing the Values of the Elements in Table 3 by the Values of the Carbons in Table 3a sample WA whole WA first

H/C

N/C

S/C

O/C

(N + S + O)/C

100

100

100

100

100

9.5

1.8

1.4

2.0

5.2

10.6

1.8

2.6

1.8

6.3

WA second

9.9

1.7

2.5

1.7

5.9

NS-A whole

9.3

1.2

2.3

2.0

5.4 5.7

NS-A first

9.5

1.2

2.3

2.2

NS-A second

9.1

1.2

2.3

2.0

5.5

NS-B whole

9.7

1.7

2.5

3.1

7.3

NS-B first

10.1

1.8

4.4

3.9

10.0

NS-B second

10.3

1.7

3.9

3.7

9.2

a

In addition, the relative total heteroatom content [(N + O + S)/C] to the carbon content was calculated.

increase the number of scans. The number of scans was then 5 times that of the DEPT measurements on the subfraction. The FID was Fouriertransformed using an exponential broadening of 10 Hz. The normalized 13 C and DEPT-135 spectra were corrected for the ratio between the weight of asphaltenes and the amount of internal standard, 1,4-dioxane. The shift ranges for the 1H and 13C are presented in Tables A1A6 in the Appendix, while the shift ranges with descriptions for DEPT-135 spectra are presented below (Table 2).

3. RESULTS 3.1. Elemental Analysis of the Composition of C, H, N, O, and S in the Asphaltene Samples. From the results of the

weight percent of the C, H, N, O, and S atoms in the asphaltene samples (Table 3) the values of hydrogen and heteroatoms (N, O, and S) relative to the carbon content were calculated (Table 4). First of all, the relative differences in C, H, N, O, and S contents were small and within the experimental error of the measurements. However, for all three oils, the first fractions contained 0.1 wt % more N and O with a relative difference in the order of 510% (Table 3). For the amount of sulfur, WA 1 contained 0.1% more than WA 2, NS-A 1 had equal, and NS-B contained 0.3% more than the second fractions, respectively. Furthermore, the total heteroatom/carbon ratios [(N + S + O)/C]

Figure 1. Absorbance spectra from FTIR measurements on powder asphaltene samples of WA whole, WA 1, and WA 2. The spectra were corrected for peaks because of CO2 and H2O (atmospheric correction) and were baseline-corrected using three points (4000, 1800, and 600 cm1).

(Table 4) were higher for the first frations, while the whole fractions had the lowest relative heteroatom content for the values obtained. The differences between the first and second fractions varied from 0.2 wt % for the NS-A samples to 0.8 wt % for NS-B. 3.2. LDIMS. The number-averaged MW (Waveraged) was higher for the first fraction for all three crude oils (Table 5). Waveraged varies from 470 to 840 g/mol for the first fractions (3:1), from 460 to 745 g/mol for the second fractions (18:1), and from 445 to 700 g/mol for the whole asphaltenes (40:1). The whole fractions obtained values that were lower than the two subfractions. 3.3. FTIR Spectroscopy. Because IR spectroscopy has relatively low sensitivity, very small differences between the samples may be difficult to detect (Figure 1). Moreover, asphaltene solubility fractions from the same crude oils will usually also be quite similar with regard to structure, leading to relatively small differences in the determined values between the fractions. Nevertheless, there were some systematic differences between the three asphaltene solubility fractions for the three crude oils. The results are presented below in three sections dealing with the aromatic, aliphatic, and polar functionalities, respectively. 3.3.1. Absorption Bands Related to the Aromatic Part of Asphaltenes. Absorption bands related to the aromatic parts of the asphaltenes are presented below (Table 6). From the relative value between CdC (1600 cm1) and aliphatic CH2 (2921.2 cm1), there were indications that the first fraction was more aromatic compared to the second fraction for WA and NS-B. WA 1 had a value (16.2) that was 9% larger than the value for WA 2 (14.8) and NS-B 1 was 33% larger than NS-B 2. Furthermore, the value for the aromatic/aliphatic ratio was for the whole fractions between the values of the two subfractions. 3555


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Table 6. Description of the Chemical Groups or Relations for the Absorption Bands Related to Aromatics in the FTIR Spectraa description oop CH deformation vibrations

absorption

WA

WA

band (cm1)

W

1

750875

WA NS-A NS-A NS-A NS-B NS-B NS-B 2

W

1

2

W

1

2

634.7 629.1 644.5 576.0 477.1 561.7 594.3 484.0 544.5

of aromatic compounds 1,3-disubstituted aromatic compounds

865

6.6

6.4

6.4

5.6

4.5

5.3

5.7

4.6

4.9

1,4-substituted aromatic compounds

810

6.0

6.0

6.2

5.3

4.5

5.3

5.4

4.5

5.3

5.6

5.7

6.0

5.9

5.1

5.8

5.8

4.8

5.9

1,2-disubstituted aromatic compounds

750

ratio of intensities of aromatic CH oop

S[1H/4H) = (I915 I852)/ (I760 I730)

deformation with one adjacent proton to that of four adjacent protons CdC stretching vibrations of aromatic rings/aliphatic CH2 a

187.5 173.6 172.2 165.4 142.5 156.9 177.6 168.4 145.5

1600/2921.2

16.2

16.2

14.8

12.4

11.6

12.4

17.0

20.0

13.4

In addition, the results are presented as the absorption values obtained multiplied by 100 for convenience.

Table 7. Description of the Chemical Groups or Relations for the Absorption Bands Related to Aliphatics in the FTIR Spectraa absorption band (cm1)

description

dissymmetric CH3 deformation vibrations 1453.35 symmetric vibrations 1375.5

28.5 20.4

26.9 19.6

28.5 20.3

26.8 17.7

24.5 16.0

25.5 17.2

27.6 17.7

21.0 14.8

26.6 17.7

R(CH2/CH3) = nCH3/nCH2

K I2927/I2957

240.7

259.6

252.5

252.8

287.7

258.6

229.2

240.2

237.4

average number of carbons per

nIR = R[CH2/CH3] n[CH3]

147.2

132.1

147.3

152.5

138.3

142.1

141.8

100.0

138.1

+ n[CH3]

alkyl side chain (nIR)

a

WA W WA 1 WA 2 NS-A W NS-A 1 NS-A 2 NS-B W NS-B 1 NS-B 2

CH2/CH3

1455/1376

138.0

137.1

140.6

151.6

152.3

147.7

154.1

139.7

149.8

R(CH2/CH3) molar ratio

(I2921.2/I2852)

131.5

132.9

133.5

132.7

130.8

134.2

133.5

139.6

137.1


Table 8. Description of the Chemical Groups or Relations for the Absorption Bands Related to Polar Functional Groups in the FTIR Spectraa absorption band (cm1)

description

WA W WA 1 WA 2 NS-A W NS-A 1 NS-A 2 NS-B W NS-B 1 NS-B 2

CdO stretching vibrations of carbonyl 1710

2.1

1.9

2.2

3.8

3.1

3.6

4.9

3.6

4.3 9.2

functional group of cyclic ketones CN group of amides and amines

1260, no peak was present

12.1

10.9

10.7

9.9

8.4

9.2

11.9

9.1

CdN stretching vibrations

1648.42

4.6

4.5

4.9

4.6

5.0

4.2

7.2

5.5

5.6

CO stretching vibrations (secondary alcohols or

1100, no peak was present

5.7

4.6

4.4

4.8

3.8

4.1

6.1

4.7

4.0

60.2

49.1

50.4

59.6

47.6

53.6

76.3

57.4

50.9

0.9

1.1

1.3

1.3

0.6

1.9

0.1

2.1

2.4

17.2

43.5

15.0

52.9

65.1

ether functional groups) SdO stretching vibrations of sulfoxides 10301040

a

organic esters, aldehydes, and ketones

1740

organic esters, aldehydes, and ketones

17501725

17.2

30.4

34.8

30.8

cyclic ketone

17251700

47.0

46.5

55.0

88.6

69.6

84.7

110.5

83.9

103.2

OH and NH groups relative

3435/3050

88.8

88.9

74.3

87.4

166.7

70.4

85.6

76.0

58.8

to the aromatic fraction empirical index of carbonyl abundance

CdO = (I1700 + I1650)/ (I1700 + I1650 + I1600)

36.3

31.7

34.8

47.8

45.9

46.1

51.2

44.4

47.7


For NS-A, the FTIR results indicated that the second fraction was slightly more aromatic than the first fraction. The absorbances at 750, 810, and 865 cm1 are a measure of the substitution on aromatic compounds. First of all, the absorbance values were very similar for all samples, indicating that there was no large differences. Second, one can see for the 1,4-substituted (750 cm1) and 1,2-disubstituted (810 cm1) aromatics that the second fractions all have higher values compared to the first

fractions, with percent differences from 3 to 19%. For 1,3disubstituted (865 cm1) aromatic rings, the second fraction had the highest value for NS-A and NS-B, while the first and second fractions were equal for the WA samples. For this substitution also, the whole fraction was more substituted than the subfractions. The out-of-plane (oop) CH deformation vibrations of aromatic compounds was larger by 1, 15, and 11% for the second fraction compared to the first fraction for the WA, 3556


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Table 9. Correlations Used To Calculate the Average Structural Parameters Based on 1H and 13C NMR, MWs, and Elemental Composition34 property

formula

total number of H atoms per molecule, #H

(MW %H)/100; %H from elemental analysis

total number of C atoms per molecule, #C total number of S atoms per molecule, #S

(MW %C)/1200; %C from elemental analysis (MW %S)/3200; %S from elemental analysis

total number of O atoms per molecule, #O

(MW %O)/1600; %0 from elemental analysis

total number of N atoms per molecule, #N

(MW %N)/1400; %N from elemental analysis

total number of aliphatic H per molecule, #H_al

TI%_aliph_H #H/100

total number of aromatic H per molecule, #H_ar

TI%_ar_H #H/100

total number of aliphatic H in R position, #H_R

TI%_aliph_H_R #H/100

total number of aliphatic H in β position, #H_β

TI%_aliph_H_β #H/100

total number of aliphatic H in γ position, #H_γ total number of aliphatic C per molecule, #C_al

TI%_aliph_H_γ #H/100 TI%_aliph_C #C/100

total number of aliphatic C in CH3 groups per molecule, #C_al;CH3

TI%_CH3 #C/100

total n-alkyl carbons, #C_al;n-alkyl

[(TI%_CH2 in aliphatic chain) + (5 TI%_terminal CH3) #C]/100

average number of C atoms on chain, n

TI%_aliph_H/TI_R-H

total number of aromatic carbons per molecule, #C_ar

TI%_ar_C #C/100

total number of tertiary aromatic C per molecule, #C_ar;t

TI%_ar_H #H/100

total number of quaternary aromatic carbons per molecule, #C_ar;q

#C_ar #C_ar;t

H/C atomic ratio for aliphatic component total number of substituted aromatic carbons per molecule, #C_ar;sub

#H_al/#C_al TI%_aliph_RH #H/(100 H/C ratio for aliphatic part)

total number of heteroatom-substituted aromatic carbons per molecule, #C_ar;X

TI%_heteroatom-substituted aromatic C #C/100

total number of bridged aromatic carbons per molecule, #C_ar;b

#C_ar #C_ar;t #C_ar;sub #C_ar;X

total number of nonbridged aromatic carbons per molecule, #C_ar;nb

#C_ar #C_ar;b

aromaticity, fa

TI%_aromatic/100

degree of substitution of aromatic carbons, σ

#C_ar;sub/(#C_ar;sub + #C_ar;t)

degree of condensation of aromatic carbons, y

#C_ar,b/#C_ar

average C/H weight ratio of alkyl groups, fc total number of aromatic rings per molecule, #R_ar

[(TI%_aliph_C %C)]/[(TI%_aliph_H %H)] 1 + (#C_ar #C_ar;nb)/2

branchiness index, BI

#H_γ/#H_β

R

(n 1) ((0.250(BI + 4.12) 1)/2)

total number of naphtenic rings per molecule, #R_na

#C_ar;sub R

total number of naphtenic carbons per molecule, #C_na

3.5 R_na

NS-A, and NS-B samples, respectively. The CH oop will be influenced by the degree of condensation of the aromatic carbons. For the ratio of intensities of aromatic CH oop deformation with one adjacent proton to that of four adjacent protons, the whole fraction have the largest values, while the subfractions are relatively similar. 3.3.2. Absorption Bands Related to the Aliphatic Parts of Asphaltenes. Absorption bands related to the aliphatic parts of the asphaltenes are presented above (Table 7). For the molar ratio of CH2/CH3 obtained by the ratio between the absorbance at 2923 (2921.2) and 2852 cm1, as well as the ratio between 1455 and 1376 cm1, the values were practically equal. This indicates that there was no or little difference in the length of the alkyl side chains. Similar results were obtained for the relative ratio between 2927 and 2957 cm1, a measure of the relative ratio of CH3/CH2. The exception here was for the NS-A samples, where the first fraction had a value 10% larger than the second fraction, indicating that the latter sample contained, on average, slightly longer alkyl groups. For the dissymmetric and symmetric vibrations of CH3, represented by the wavenumbers 1453.35 and 1375.5 cm1 respectively, it seemed that the second fraction contained a larger portion of these groups with variations of excess 320%. These larger amounts may be due to a higher degree of branching and

substitution on the aromatic core of the asphaltene molecules. Furthermore, the whole fractions contained as much or higher values for the branching as the second fractions for all samples. The values of nIR (relative chain length of the alkyl chains attached to the aromatic ring structure, i.e., the average number of carbons per alkyl side chain41) were the only indications that the second fractions may contain compounds with longer alkyl side chains. The nIR was 11 and 28% larger for the second fractions of WA and NS-B asphaltenes, respectively, compared to the first fractions. 3.3.3. Absorption Bands Related to the Polar Functional Groups of Asphaltenes. Absorption bands related to the polar functional groups of asphaltenes are presented above (Table 8). The absorbance from the CdO stretching vibrations of carbonyl functional groups of cyclic ketones is weak but distinct and is larger for the second fraction by 14% for WA and NS-A asphaltenes and 16% for NS-B asphaltenes. Furthermore, the empirical index of carbonyl groups also indicates that the second fraction was richer in this functional group, although to a lesser extent. Moreover, the whole fraction seemed to contain compounds with an even higher content of these functional groups. From the wavenumber of 1740 cm1 and by integration over the wavenumber range of 17251750 cm1, it was found that the second fraction probably contained compounds with larger amounts of 3557


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Table 10. Average Structural Data Obtained by NMR of the Whole and Fractionated Asphaltenes of the WA Crude Oil property

WA

WA

WA

whole

first

second

total H

41

76

63

aromatic H

5

10

8

aliphatic H

36

66

53

aliphatic H in the R position

11

20

16

aliphatic H in the β position

20

36

29

aliphatic H in the γ position

6

10

8

total carbon aromatic carbon

36 16

60 39

53 25

tertiary aromatic carbon

5

10

8

quarternary aromatic carbon

11

28

17

substituted aromatic carbon

5.9

6.3

8.1

bridged aromatic carbon

5

19

8

nonbridged aromatic carbon

11

20

17

aliphatic carbon

20

21

27

naphtenic carbon n-alkyl carbon

2.6 1.5

2.6 1.6

3.4 2.3

aliphatic carbon in CH3 group

4.3

4.6

6.0

average number of C atoms on chains, n

3.4

3.4

3.4

total number of aromatic rings per molecule

3.5

10.5

5.0

total number of naphtenic rings per molecule

0.75

0.75

0.96


0.31

0.29

0.28

aromaticity, fa

0.45

0.65

0.48

degree of substitution of aromatic carbon, σ degree of condensation of aromatic carbon, γ

0.52 0.31

0.38 0.49

0.50 0.32

average C/H weight ratio of alkyl groups, fc

6.6

3.8

6.2

total sulfur

0.18

0.59

0.50

total nitrogen

0.55

0.94

0.77

total oxygen

0.55

0.83

0.68

empirical formula

C36H41N0.6O0.5S0.2

C60H76N0.9O0.8S0.6

C53H63N0.8O0.7S0.5

formula MW (g/mol)

496

841

737

the organic esters, aldehydes, and ketones (including cyclic ketone). Quantitatively, the second fractions had values of 14, 65, and 15% higher than the first fractions for the WA, NS-A, and NS-B samples, respectively. There were no trends in the relative values of the CN and CdN functional groups with regard to the first and second fractions. On the other hand, it seemed that the first fraction contained more of the NH and OH groups relative to the aromatic fraction, as shown by taking the ratio of the absorbance at 3435 cm1 to that of 3050 cm1. The relative values here were 16, 58, and 22% compared to the second fractions for the WA, NS-A, and NS-B samples, respectively. For sulfur groups of sulfoxides (10301040 cm1), there were distinct peaks in the spectra and there were differences, although not systematic, between the first and second fractions. On the other hand, the whole fractions had higher peak values for the sulfoxide (SdO) functional groups, than the first and second fractions. 3.4. 1H and 13C NMR. Average structural parameters of the asphaltene samples from the crude oils WA (Table 10), NS-A (Table 11), and NS-B (Table 12) were calculated from correlations (Table 9). In the application of these correlations, information was needed on the elemental compositions and MWs in addition to the information obtained from the proton and carbon NMR spectra (see Tables A1A6 in the Appendix) of the

samples. The spectra from the proton, carbon, and DEPT-135 measurements showed differences in both the aliphatic and aromatic regions between the asphaltene samples from the same crude oils, as indicated by the figures of the spectra obtained for the samples from the WA crude oil (Figures 24). The results from the NMR measurements are presented in three sections, which treat the aromatic, aliphatic, and polar relationships separately. 3.4.1. Aromatic Part of the NMR Spectra. Tables 1012 show the results obtained using the equations of correlation. The aromaticity is higher for the first fractions for WA (0.65 versus 0.48) and NS-B (0.45 versus 0.39) asphaltenes, while the first and second fractions had equal aromaticity for the NS-A asphaltenes (0.51). There was no trend in the values of fa (aromaticity) for the whole asphaltenes relative to fa of the fractions. The relative amounts of quarternary carbons were 0.68, 0.72, and 0.68 for the WA whole, WA 1, and WA 2 samples, 0.68, 0.67, and 0.61 for the NS-B whole, NS-B 1, and NS-B 2 samples, and 0.67, 0.65, and 0.70 for NS-A whole, NS-A 1, and NS-A 2 samples. These values are to some degree a measure of the condensation of the aromatic core, although the degree of condensation of aromatic carbons (γ) was calculated in a slightly different manner (Table 9). For the WA asphaltenes, the first fraction obtained the highest value (0.49), followed by WA 2 (0.32) and WA whole (0.31). For the NS-A samples, the second fraction obtain the highest degree of 3558


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Table 11. Average Structural Data Obtained by NMR of the Whole and Fractionated Asphaltenes of the NS-B Crude Oil property

NS-B first

NS-B whole

NS-B second

total H

57

68

58

aromatic H

7

9

8

aliphatic H

50

60

51


14

16

14


28

34

29


8

9

8

total carbon

51

60

54

aromatic carbon tertiary aromatic carbon

25 7

27 9

21 8


17

18

13


7.2

8.8

8.9


8

8

3


16

19

18

aliphatic carbon

26

33

33

naphtenic carbon

3.4

4.2

4.1

n-alkyl carbon aliphatic carbon in CH3 group

2.5 5.7

2.9 7.8

2.6 6.9 3.7


3.7

3.8


5.2

4.8

2.7


0.97

1.19

1.19


0.29

0.27

0.28

aromaticity, fa

0.48

0.45

0.39

degree of substitution of aromatic carbon, σ

0.49

0.49

0.53

degree of condensation of aromatic carbon, γ average C/H weight ratio of alkyl groups, fc

0.34 6.3

0.29 6.7

0.16 7.8

total sulfur

0.43

0.51

0.45

total nitrogen

0.51

0.64

0.55

total oxygen

0.76

0.98

0.82

empirical formula

C51H57N0.5O0.8S0.4

C60H68N0.6O1.0S0.5

C54H58N0.5O0.8S0.5

formula MW (g/mol)

702

828

742

condensation (0.33), followed by NS-A whole (0.29) and NS-A 1 (0.22). For the NS-B asphaltenes, the order was NS-B whole (0.34), NS-B 1 (0.29), and NS-B 2 (0.16). Of the number of aromatic rings per molecule, the interest was mainly the relative values between the first and second fractions of asphaltenes, and here, the first fraction of the WA and NS-B asphaltenes obtained twice the number of aromatic rings (10.5 and 4.8) compared to the second fractions (5.0 and 2.7). For NS-A, on the other hand, the second fractions seem to have a larger ring structure (3.8 versus 2.8). The degree of substitution is a measure of the number of aromatic carbons that have a methyl or a chain that substitutes the proton. For WA, the order in the degree of substitution is WA whole (0.52), WA 2 (0.50), and WA 1 (0.38), and for NS-A and NS-B, the values were almost equal (0.49, 0.47, and 0.46 and 0.49, 0.49, and 0.53, respectively). Of the heteroatoms S, N, and O, the first fractions contained slightly more on average than the second fractions, with the whole fractions as the samples having the lowest content of heteroatoms. According to the results of the integration of the DEPT-135 spectra (Table 13), the first fractions contained more aromatic carbon, as seen from the integration over 178100 and 138110 ppm. The reason for this was that the first fraction contained more of the quarternary carbons, which were not detected when using the DEPT-135 sequence. Furthermore,

there were no trends in the aromatic carbons substituted with heteroatoms (178150 and 150138 ppm), although there were differences. For example, WA 1 seemed to have higher signals for these shift values than WA 2. NS-A 1 has a much higher value (0.43) than NS-A 2 (0.06) for the shift range of 178150. 3.4.2. Aliphatic Part of the NMR Spectra. The aliphatic hydrogen in the γ position gives an indication of the amount of alkyl side chains with more than three carbons (Tables 1012). For WA whole, the value of the ratio between hydrogens in the γ position to the total amount of hydrogen was 0.16, and for WA 1 and WA 2, these values were 0.15 (dividing 10 by 66 and 8 by 53). These relative values for the whole, first, and second fractions of asphaltenes were 0.16, 0.15, and 0.16 for NS-A and 0.16, 0.14, and 0.17 for NS-B, respectively, indicating that the length of the aliphatic chains were more or less equal. For all samples, the average number of carbon atoms on chains was close and varied from 3.2 to 3.9. The degree of branching indicates the amount of branches on the alkyl side chains, and this value was practically equal for all of the samples. The only indication that there were some differences in the aliphatic part of the asphaltene molecules was the larger values obtained for the second fractions of WA and NS-B for the average C/H weight ratio of alkyl groups (fc). When it came to the naphtenic part of the asphaltenes, the total number of rings per molecule was around 1. For WA, the values are 0.75, 3559


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Table 12. Average Structural Data Obtained by NMR of the Whole and Fractionated Asphaltenes of the NS-A Crude Oil property

NS-A first

NS-A whole

NS-A second

total H

37

40

40

aromatic H

5

6

5

aliphatic H

32

35

35


9

11

9


17

19

20


5

5

6

total carbon

31

33

32

aromatic carbon tertiary aromatic carbon

15 5

17 6

17 5


10

11

12


4.8

5.1

4.1


4

4

6


11

13

11

aliphatic carbon

17

16

16

naphtenic carbon

2.2

2.0

2.3

n-alkyl carbon aliphatic carbon in CH3 group

1.6 3.8

1.4 4.2

1.3 3.6 3.9


3.4

3.2


3.2

2.8

3.8


0.63

0.58

0.65


0.31

0.29

0.32

aromaticity, fa

0.47

0.51

0.51

degree of substitution of aromatic carbon, σ

0.49

0.47

0.46

degree of condensation of aromatic carbon, γ average C/H weight ratio of alkyl groups, fc

0.29 6.2

0.22 5.6

0.34 5.4

total sulfur

0.30

0.54

0.47

total nitrogen

0.45

0.50

0.46

total oxygen

0.73

0.97

0.89

empirical formula

C31H37N0.4O0.7S0.3

C33H40N0.5O1.0S0.5

C32H40N0.5O0.9S0.5

formula MW (g/mol)

435

475

461

0.75, and 0.96, for NS-A, they were 0.63, 0.58, and 0.65, and for NS-B, they were 0.97, 1.19, and 1.19 for the whole asphaltenes and first and second fractions, respectively. The amount of CH2 (37.722.5 ppm) was equal for NS-A 1 and 2 (6.82 and 6.63) and lower for WA 1 (5.19) compared to WA 2 (6.10) and NS-B 1 (6.55) compared to NS-B 2 (8.21). It was also clear that the second fractions had a higher amount of carbons R to a branch (37.736.7 ppm) and also CH2 on long-chain alkyls R to CH3. On the other hand, the difference was not pronounced between the two subfractions with regard to CH2 on long-chain alkyls, represented by the shift range of 30.8 28.3 ppm, in general indicating that there was not a big difference in the length of the alkyl side chains. The differences in the shift values representing the iso-alkyl at 22.8522.63 and 20.519.3 ppm were small, and there were no trends detected within the samples. It seems that the second fractions contain more methyl groups in general (22.510 pm), with methyl attached to aromatic atoms (22.517 ppm) and methyls in alkyl chains containing more than two carbon atoms (1710 ppm). Furthermore, the second fractions had slightly larger values for terminal CH3 (14.413.9), possibly indicating a larger degree of branching. 3.4.3. Polar Functionalities from the NMR Spectra. The first fractions contained more of the heteroatoms (S, N, and O), as seen from the percentage of total heteroatom-substituted aromatic carbons, where the values were 5.3, 2.2, and 7.2 for the first

fractions of WA, NS-A, and NS-B compared to 1.9, 1.3, and 6.6 for the second fractions (see Tables A1A6 in the Appendix). The 7037.5 ppm range (which is a broad peak with its largest intensity in the range of 4060 ppm) may be due to carbons R to oxygen- or nitrogen-containing groups and aliphatic and cyclic structures containing carboxylic acids or amines.47 For the shift range of 7037.7 ppm, which accounts for carbons R to O- and N-containing groups and aliphatic and cyclic structures that contain carboxylic or amine groups, the second fractions of WA and NS-B asphaltenes had much larger values (1.38 and 0.26) than the first fractions (0.50 and 0.07).

4. DISCUSSION Asphaltenes, as a solubility class, are not uniform in their functionality.39 With asphaltenes being a mixture of tens of thousands of different, although relatively similar compounds, attempts of identification of MW, structure, or functional groups will normally provide average values. For example, a value of 3 for the average number of carbons in an alkyl chain could indicate either a uniform distribution of chain length of about 3 or a bimodal distribution with high concentrations of very short and much longer chains.39 Moreover, this average structure may not at all represent the active compounds. Furthermore, asphaltenes obtained by solvent precipitation may not represent the asphaltene 3560


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Figure 2. Spectra obtained for WA asphaltenes from the 13C NMR measurements. There were differences in both the shape and intensities of the spectra between the samples.

fraction precipitated during pressure depletion.48 Below, the connection between the solvent properties and interfacial tensions measured in the previous work (10.1021/ef060311g)43 and the structural relationships from the present work will be discussed. Although differences detected are small, they potentially indicate that the samples contain compounds with extremes in either way of the properties, such as size, aromaticity, functional groups, and chain length. These extremes may be the

reason for the differences in functionality; however, hidden inside the samples, they are difficult to identify. It is therefore of importance to develop methods for extracting the active components and performing the necessary analysis on them to be able to understand the behavior of asphaltenes better than what is currently done. 4.1. Size and MW. The number-averaged MW (Waveraged) (Table 5) was higher for the first fraction for all three crude oils, 3561


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Figure 3. Spectra obtained for WA asphaltenes from the 1H NMR measurements. There were differences in both the shape and intensities of the spectra between the samples.

indicating that the two-step precipitation process lead to a fractionation of the asphaltenes in terms of different MWs. Furthermore, the results contributed to the assumption of the MW being a possible co-explanation for the onset of precipitation. Although the MWs reported for the asphaltenes were somewhat low, the average MW is still in the range of what has been reported for asphaltenes by LDIMS experiments.21,28,29 It is important to point out that it was the differences in the MW between the less and more soluble asphaltenes that was of most interest. In comparison of the MWs with the radius of gyration (RG) determined by small-angle neutron scattering (SANS) (10.1021/ef060311g),43 the relative values showed that the first fractions formed aggregates with larger RG values compared to the second fractions for WA and NS-B, 30 versus 25 Å

and 26 versus 21 Å, respectively. The fact that the NS-A asphaltenes formed aggregates that were so large that they were outside the detection limit of the instrument (approximately 700 Å) fits well with the fact that these were the fractions from the samples where the first and second fractions added up to the whole fraction (Table 1), indicating that they aggregated more easily. This may also explain why this sample behaved differently in many of the parameters investigated compared to the WA and NS-B asphaltenes. The conclusion was therefore that the less soluble asphaltenes in this study were larger in MW and, thus, formed larger aggregates. 4.2. Aromaticity and Substitution on the Aromatic Core. A larger number of rings in the aromatic ring structures lead to an increase in the ππ* interactions between aromatic sheets and a 3562


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Figure 4. Spectra obtained for WA asphaltenes from the DEPT-135 NMR measurements. There were differences in both the shape and intensities of the spectra between the samples.

greater propensity for stacking, aggregation, flocculation, and potentially precipitation. From the NMR analysis, for two of the asphaltene samples, WA and NS-B, the first fractions had higher aromaticity compared to the second fraction, while for NS-A, the aromaticity was equal (Tables 1012). These results were supported by the FTIR measurements, where it was also shown that the relative values between 1600 and 2921.2 cm1, indicating the aromaticity, were larger for the first fractions of WA and NS-B and relatively similar for NS-A compared to the second fractions (Table 6). The degree of substitution on aromatic rings is important for the solubility. FTIR measurements indicated that the second fractions had slightly higher degrees of substitution for 1,2-disubstituted, with values relative to the first fractions in the order of 319%. Condensation of the aromatic rings will

have the opposite effect of substitution; thus, a higher degree of condensation will reduce the solubility in alkanes. NMR indicated that the first fractions of WA and NS-B had larger values, while NS-A had a lower value than the second fractions (Tables 1012). The same relative values were obtained from FTIR measurements by looking at the CH oop, which was larger for the second fraction compared to the first fraction by 1, 15, and 11% for WA, NS-A, and NS-B samples, respectively (Table 6). For WA and NS-B, the degree of condensation can be used as an explanation for the lower solubility of the first fractions and also for the larger RG obtained by the SANS measurements reported previously (10.1021/ef060311g).43 It should also be mentioned that, from the elemental analysis, the H/C ratio was higher for the first fractions than the whole 3563


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Table 13. Intensities Obtained from the DEPT-135 Spectra after Integration over the Shift Values Reported in This Tablea ppm

WA 1

WA 2

WA W

NS-A 1

NS-A 2

178100

3.59

4.10

2.19

4.29

4.55

178150

0.29

0.26

0.15

0.43

0.06

150138

0.23

0.16

0.13

0.25

138110

2.92

3.44

1.89

7037.7

0.50

1.38

5.19

6.10

37.722.5

NS-B 1

NS-B 2

NS-B W

2.09

7.97

9.14

2.13

0.06

1.48

1.53

0.06

0.31

0.08

0.56

0.85

0.06

3.53

4.03

1.87

5.47

6.46

2.06

0.68

1.18

0.77

0.67

0.07

0.26

0.70

3.15

6.82

6.63

3.32

6.55

8.21

3.33

37.736.7

0.29

0.38

0.16

0.21

0.34

0.14

0.26

0.34

0.16

32.531.5 30.828.3

0.38 3.48

0.50 3.30

0.21 1.58

0.41 3.65

0.51 3.49

0.23 1.74

0.43 3.19

0.60 3.74

0.25 1.55

22.8522.63

0.07

0.07

0.05

0.14

0.14

0.06

0.10

0.13

0.05

22.510

2.85

2.96

1.13

1.83

2.72

0.99

2.87

3.44

1.05

20.519.3

0.44

0.58

0.51

0.32

0.43

0.39

0.53

0.47

0.43

22.517

1.33

1.44

0.56

0.61

1.20

0.61

1.25

1.40

0.59

1710

1.51

1.50

0.23

1.25

1.52

0.18

1.61

2.04

0.20

14.413.9

0.39

0.47

0.19

0.48

0.49

0.21

0.46

0.55

0.19

CH2 R to branched relative to CH2 total CH3/CH2 a

NS-A W

5.8 14.8

6.1 14.3

5.1 11.2

3.1 13.2

5.2 13.9

4.2 11.2

4.0 14.4

4.1 14.8

4.7 12.1

The values were multiplied by 1. The “1”, “2”, and “W” designate the first and second fractions and whole asphaltene samples, respectively.

fraction for all samples. The H/C ratio of aromatic compounds often indicates their aromaticity. Nevertheless, the combination of a higher H/C ratio and a higher degree of aromaticity was due to the higher MW and the larger number of total hydrogen and carbon, as given in the tables from the NMR results above (Tables 1012). 4.3. Aliphatic Part: Chain Length and Branching. In the aliphatic part, there are several important structural relationships that affect both solubility and interfacial activity. Amount, length, and branching of the alkyl side chains substituted on the aromatic cores may be factors hindering aggregation of asphaltenes. The second fractions (Table 7) had higher values for dissymmetric and symmetric CH3 vibrations (from 3 to 20% excess), indicating a larger degree of branching. From the DEPT-135 results (Table 13), it was found that the second fractions had higher amounts of carbons R to a branch, indicating a higher degree of branching. These features may lead to increasing solubility in aliphatic solvents from steric hindrance to aggregation and higher interfacial activities because of changes in the hydrophilic lipophilic balance. Furthermore, the relative degree of branching of the alkyl-substituted chains from the DEPT-135 spectra was obtained by dividing the intensity of the 37.736.7 ppm shift range by the intensity of the total amount of CH2 carbons given by the shift intensities from 37.7 to 22.5 ppm. The second fractions obtained values of 6.1, 5.2, and 4.7 compared to 5.8, 3.1, and 4.1 for the first fractions for the WA, NS-A, and NS-B samples, respectively (Table 13), indicating that the second fractions had higher degrees of branching. The peak at 31.9 ppm (32.531.5 ppm) is attributed to CH2 on long-chain alkyls R to CH3, while the peak 29.7 ppm (30.828.3 ppm) is attributed to CH2 on long-chain alkyls. Furthermore, as seen from the table, the second fraction contained more of the terminal methyl (shown by both the 31.9 and 14.1 ppm peaks), which may indicate a larger degree of substitution because of the fact that the relative ratio of terminal methyl (14.1 ppm) to CH2 on long chains (29.7 ppm) (Table 13) was indifferent between the first and second fractions. Only nIR (Table 7) indicated that the second fractions for WA and NS-B contained longer alkyl chains.

For this parameter, the relative values were 11 and 28% larger for the second fraction. The results obtained are not in agreement with the assumptions made in part 1 (10.1021/ef060311g)43 that the second fraction would have longer alkyl chain lengths, which upon the addition of pentane helped to solubilize the molecules and also obstruct aggregation because of steric stabilization.49 Rather, this study (mainly from the DEPT-135 measurements) indicated that the main difference in the aliphatic groups between the less and more soluble fractions was that the degree of branching may be a factor, although polarity and aromaticity must also be taken into account. 4.4. Polarity and Functional Groups. Elemental analysis showed that, for all three oils, the first fractions contained equal or more of the individual heteroatoms and the total relative heteroatom/carbon ratio (N + S + O)/C compared to the second fractions (Tables 3 and 4), although it must be pointed out that the differences are very small and were in some cases within the sensitivity range of the measurements. Therefore, the low relative differences may indicate that, if the polarity is an important factor, it is more due to the type of polar functional groups rather than just the relative amount of the heteroatoms. The results indicated that the first fractions had a larger amount of NH and OH groups relative to the aromatic fraction (Table 8) and, on the basis of the total heteroatomsubstituted aromatic carbons (Tables 1012), that the first fractions also had a more polar aromatic core. On the other hand, the intensity of the peak at 3300 cm1 (Table 8), indicated that the second fractions showed somewhat greater propensity for hydrogen-bond formation, which may indicate a higher degree of carboxylic acid groups or sulfonic acids and may explain the higher interfacial activity. Furthermore, the second fractions obtained higher absorbance values for other frequencies related to oxygen bonds, such as CdO at 1710 cm1 and the ketone groups at 17001725 cm1 (Table 8). These results were supported for WA and NS-B by the findings from the DEPT135 spectra for the integration over the shift range of 7037.7 ppm, which accounts for carbons R to O- and N-containing 3564


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Table A1. Percentage Intensity of Hydrogen in the Whole and Fractionated Asphaltenes from the WA Crude Oil

Table A2. Percentage Intensity of Hydrogen in the Whole and Fractionated Asphaltenes from the NS-A Crude Oil

chemical

WA

WA

WA

chemical

NS-A

NS-A

NS-A

H type

shift (ppm)

whole

first

second

H type

shift (ppm)

whole

first

second

total aromatic hydrogen total aliphatic hydrogen

9.96.5 4.50.5

13.0 88.0

13.34 87.1

12.7 84.5

total aromatic hydrogen total aliphatic hydrogen

9.96.5 4.50.5

13.1 87.7

13.35 87.3

13.66 86.8

total hydrogens on the

4.51.9

25.9

25.9

24.9


4.51.9

24.0

23.2

23.5

1.91.0

49.5

50.5

49.5

1.00.5

14.2

13.7

13.7

R position of an aromatic ring total hydrogen on the

R position of an aromatic ring 1.91.0

47.5

47.5

46.6

total hydrogen on the

β position of an aromatic ring total hydrogen on the

β position of an aromatic ring 1.00.5

14.6

13.6

12.9

total hydrogen on the

γ position of an aromatic ring

groups and cyclic structures containing carboxylic or amine groups (Table 13). Hydrogen-bond formation is often considered a participant in the asphalteneresin interaction.50,51 This asphalteneresin interaction because of OH functional groups may be part of the explanation for the chemical differences between the first and second fractions of the asphaltenes. Potentially, OH functional groups may act as aggregation inhibitors. The second fraction obtained, from both FTIR and DEPT-135 experiments, indications that the alkyl groups contained more hydroxyl (OH) and carboxylic (COOR) groups, which may explain the higher interfacial activity compared to the first fractions. Aquino-Olivos et al.3 found that asphaltene deposits obtained at high pressures were all apparently more polar than standard asphaltenes and seemed to contain less methyl groups. They emphasized that the results contradicted their hypothesis that the first material to precipitate following a FloryHuggins polymer type of approach would be the heaviest material. They propose that maybe it is the polarity rather than the size that is the governing factor in the natural depletion precipitation process. This may indicate that asphaltene deposition during natural depletion and depressurization should not be modeled as the heaviest material but rather as a very polar fraction. Our results partly support that this assumption may be right; however, there may also be other differences in the functional groups determining the solubility and interfacial activity of asphaltenes. 4.5. Evaluation of the PropertyStructure Relationships of the Asphaltene Samples. All of the parameters obtained in the present study are an average of the molecular mixture in the samples. Therefore, the relative differences commented upon in this work may not be the only or main structural traits responsible for the properties determined in the previous work (10.1021/ef060311g).43 For example, in comparison of the aromaticity of the first and second fractions, one finds, according to the assumptions, that, for WA and NS-B, the less soluble fraction had a higher aromaticity, while for NS-A, the aromaticities were equal for the two subfractions. NS-A was also the crude oil where the subfractions added up to the total weight of the whole asphaltenes (Table 1) and also formed very large aggregates according to SANS measurements, indicating that the asphaltenes from this crude oil were different from the two other crude oils. Furthermore, the precipitation procedure using 3:1 and 18:1 may not divide the solubility fractions of asphaltenes in the same manner for different oils. In fact, in the results above, there was a trend that the WA and NS-B asphaltene subfractions followed each other with regard to the relative values in properties, while

γ position of an aromatic ring

Table A3. Percentage Intensity of Hydrogen in the Whole and Fractionated Asphaltenes from the NS-B Crude Oil H type

chemical

NS-B

NS-B

NS-B

shift (ppm)

whole

first

second

total aromatic hydrogen

9.96.5

13.5

14.2

11.9

total aliphatic hydrogen

4.50.5

87.15

86.3

88.4


4.51.9

25.5

26.7

22.8

1.91.0

46.9

46.1

49.7

1.00.5

14.7

13.5

16.0

R position of an aromatic ring total hydrogen on the β position of an aromatic ring total hydrogen on the γ position of an aromatic ring

NS-A was different in the respect that the determined values between the first and second fractions were more equal or more extreme (depending upon the parameter), so that the same conclusions could not be drawn. Nevertheless, the assumptions are that the less soluble fractions, which formed larger aggregates (SANS), were less interfacial-active (from pendant drop measurements), more aromatic, more polar (in the aromatic core), and had a higher average MW. The second fractions had alkyl groups with a higher degree of branching and had more of the hydroxyl and carboxylic groups on the aliphatic parts, which can explain the higher interfacial activity. There is probably no single reason for the precipitation of asphaltene that can be explained by the molecular structure or MW. This work indicates that it is of great interest and importance to fractionate asphaltenes with respect to the less and more soluble asphaltenes. Furthermore, it is not yet determined what the ultimate character of the least soluble asphaltene fraction is and if this fraction is the most harmful with regard to adsorption to surfaces and emulsion stability. The procedure presented here was extended to a four-step precipitation procedure, where it was shown that the interfacial activity was not linearly dependent upon the order of the solubility fraction.52 Therefore, the results in this work do not grasp all of the information available on the properties of the solubility fractions and do not conclude which structural characteristics are determining the properties of the asphaltene samples in this work. However, systematic differences were found, which were supported by several analytical techniques, which would be of interest to study further. It would also be of interest to study 3565


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Table A4. Percentage Intensity of Carbon in the Whole and Fractionated Asphaltenes from the WA Crude Oil chemical C type total aliphatic carbon total methyl carbon total terminal methyl

shift (ppm) 10.070.0 1022.7

WA

WA

WA

whole first second 55.4 11.8

35.3 7.8

52.4 11.4

14.3 (14.2813.97)

0.8

0.5

0.9

19.7 (19.7419.64

0.4

0.3

0.7

28.2 (28.2828.18)

0.2

0.2

0.2

29.7 (29.8529.53)

5.8

3.

6.0

carbon total branched methyl carbon total tertiary (CH) carbon total secondary (CH2) carbon total aromatic carbon

100178

44.7

64.7

47.7

total alkyl-substituted

138150

7.2

9.7

8.2

150178

0.2

5.3

1.9

aromatic carbon total heteroatomsubstituted aromatic carbon

Table A5. Percentage Intensity of Carbon in the Whole and Fractionated Asphaltenes from the NS-A Crude Oil chemical C type total aliphatic carbon

shift (ppm)

NS-B NS-B NS-B whole first

second

10.070.0

51.7

55.2

61.0

1022.7

11.2

13.1

12.9

1.0

0.9

1.0

total branched methyl carbon 19.7 (19.7419.64

0.5

0.4

0.7

total tertiary (CH) carbon

28.2 (28.2828.18)

0.2

0.5

0.4

total secondary (CH2) carbon 29.7 (29.8529.53)

5.8

6.1

5.5

100178 138150

48.3 9.7

44.8 9.5

39.1 7.1

150178

3.1

2.2

1.3

total methyl carbon total terminal methyl carbon

total aromatic carbon total alkyl-substituted

14.3 (14.2813.97)

aromatic carbon total heteroatom-substituted aromatic carbon

Table A6. Percentage Intensity of Carbon in the Whole and Fractionated Asphaltenes from the NS-B Crude Oil chemical C type total aliphatic carbon total methyl carbon total terminal methyl carbon

shift (ppm)

NS-A NS-A NS-A whole first

second

10.070.0

52.5

49.3

48.9

1022.7

12.1

12.7

11.1

1.

0.8

0.8

total branched methyl carbon 19.7 (19.7419.64

0.7

0.3

0.5

total tertiary (CH) carbon

28.2 (28.2828.18)

0.3

0.3

0.3

total secondary (CH2) carbon 29.7 (29.8529.53)

4.9

3.9

4.2

100178 138150

47.5 9.4

50.7 11.4

51.1 11.9

150178

2.6

7.2

6.6

total aromatic carbon total alkyl-substituted

14.3 (14.2813.97)

aromatic carbon total heteroatom-substituted aromatic carbon

and compare the same techniques with asphaltenes obtained from pressure-depletion experiments.

5. CONCLUSION The study includes the characterization of the MW, proton and carbon NMR, FTIR measurements, and elemental analysis of three asphaltene solubility fractions (3:1, 18:1, and 40:1). Previous work showed that the second fractions were more interfacially active, had a different shape of the curve, showing the interfacial activity as a function of time, and formed smaller aggregates (SANS measurements) (10.1021/ef060311g).43 In the present work, it was found that the first fractions of WA and NS-B asphaltenes had higher average MWs, were more aromatic, and had higher contents of heteroatoms attached to the aromatic core. They were also more condensed and contained more of the quaternary carbons in the aromatic region. WA and NS-B asphaltene subfractions followed each other with regard to the relative values in properties, while NS-A was different in the respect that the determined values between the first and second fractions were either more equal or more extreme, so that the same conclusions could not be drawn. Furthermore, the second fractions had a higher degree of branching of the substituted alkyl chain groups and contained a higher amount of hydroxyl (OH) and carboxylic groups. The results thus indicate that the least soluble asphaltenes (first fractions) were larger and more aromatic, while the second fractions (more soluble) were more interfacially active. ’ APPENDIX The appendix contains the results of the proton and carbon NMR experiments and the average structural data calculated (Tables A1A6). ’ AUTHOR INFORMATION Corresponding Author

*Telephone: +47-73591146. E-mail: [email protected].

’ ACKNOWLEDGMENT Thanks to Marcos D. Lobato and Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide (Sevilla, Spain) for performing the LDIMS analysis. The authors are also grateful for the financial contributions from the following partners: Statoil R&D Centre and the members of the Joint Industrial Project on “Particle-Stabilized Emulsions/Heavy Crude Oils”, including Statoil ASA, Vetco Aibel AS, Shell Global Solutions, Aker Kvaerner, BP, Champion Technologies, Chevron Texaco, Norsk Hydro, Petrobras, Total, ENI Technology, and (SINTEF Materials Maersk Oil & Gas. We thank Helene Vralstad and Chemistry) for proofreading an early version of the manuscript. ’ REFERENCES (1) Speight, J. G. Oil Gas Sci. Technol. 2004, 59, 467. (2) Spiecker, P. M.; Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2003, 267, 178. (3) Aquino-Olivos, M. A.; Andersen, S. I.; Lira-Galeana, C. Pet. Sci. Technol. 2003, 21, 1017. (4) Gawel, I.; Bociarska, D.; Biskupski, P. Appl. Catal., A 2005, 295, 89. (5) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (6) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39, 1847. 3566


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Asphaltenes Precipitated by a Two-Step Precipitation Procedure. 2

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