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Analytical Information on the Asphaltenes from a Few StandardCharacterization TechniquesA. Peksoza; S. K. Akaya; Y. Kayab; H. Ovalioglua; G. Kaynaka; A. Yalcinera

a Department of Physics, Sciences and Arts Faculty, Uludag University, Gorukle-Bursa, Turkey b

Department of Chemistry, Sciences and Arts Faculty, Uludag University, Gorukle-Bursa, Turkey

Online publication date: 19 May 2011

To cite this Article Peksoz, A. , Akay, S. K. , Kaya, Y. , Ovalioglu, H. , Kaynak, G. and Yalciner, A.(2011) 'AnalyticalInformation on the Asphaltenes from a Few Standard Characterization Techniques', Energy Sources, Part A: Recovery,Utilization, and Environmental Effects, 33: 15, 1474 — 1481To link to this Article: DOI: 10.1080/15567030903397909URL: http://dx.doi.org/10.1080/15567030903397909

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Energy Sources, Part A, 33:1474–1481, 2011

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DOI: 10.1080/15567030903397909

Analytical Information on the Asphaltenes from a

Few Standard Characterization Techniques

A. PEKSOZ,1 S. K. AKAY,1 Y. KAYA,2 H. OVALIOGLU,1

G. KAYNAK,1 and A. YALCINER1

1Department of Physics, Sciences and Arts Faculty, Uludag University,

Gorukle-Bursa, Turkey2Department of Chemistry, Sciences and Arts Faculty, Uludag University,

Gorukle-Bursa, Turkey

Abstract The asphaltene has been obtained from asphalt cement with penetrationgrade 60, which is extracted from crude Libya petroleum. The elemental composition

and some structural properties of the asphaltene samples are determined by variousmethods, such as SEM, infrared spectroscopy, and X-ray analysis. Some experiments

are performed on 1H dynamic nuclear polarization (DNP) in a number of asphaltenesuspensions in organic solvents. The frequency dependence of the DNP enhancement

under conditions of weak EPR saturation in a low magnetic field is obtained andinterpreted as the EPR line shape of some free radicals present in the samples.

Keywords asphaltene, dynamic nuclear polarization, EPR, FTIR, SEM

1. Introduction

Asphaltenes are defined operationally as the fraction of hydrocarbon fuels insoluble in

a straight chain alkane (e.g., n-heptane), and soluble in a light aromatic (e.g., toluene)

(Toulhoat et al., 1994). Asphaltenes are obtained by the treatment of petroleum, residua,

heavy oil, or bitumen with a low-boiling liquid hydrocarbon. Asphaltenes are dark brown

to black, friable solids and their solubility varies in a large scale from benzene derivatives

to n-alkanes. Asphaltenes consist of condensed polynuclear aromatic ring systems at the

center bearing alkyl side chains with hetero elements (i.e., nitrogen, oxygen, and sulphur)

(Hausser and Stehlik, 1968). Depending on their origin, they can be quite polar and

polarizable (Kallevik et al., 2000).

Asphaltenes play a prominent role in the petroleum industry. Despite the impact

of asphaltenes in many technological and economic spheres, some of their fundamental

molecular properties have remained unresolved. Studies of asphaltene solutions, particu-

larly with aromatic solvents, such as toluene, could give important information regarding

the concentration at which the asphaltene aggregation begins and the manner of interac-

Address correspondence to Dr. Ahmet Peksoz, Physics Department, Sciences and Arts Faculty,Uludag University, Gorukle Campus, Bursa, 16059, Turkey. E-mail: peksoz@uludag.edu.tr

1474

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Asphaltenes From a Few Standard Characterization Techniques 1475

tion of asphaltenes with other crude oil components, such as resins and aromatics. The

association of asphaltenes has been the subject of several studies, basically focusing on

the apparent micellization of asphaltenes in solutions (Evdokimov et al., 2003).

Dynamic nuclear polarization (DNP) is a well-known double resonance technique of

magnetic resonance. In this technique, the nuclear magnetic resonance (NMR) is observed

during simultaneous irradiation of the electron spin resonance (EPR). In the Overhauser

effect (OE) type DNP for free radical solutions, if the EPR of the paramagnetic solute

is saturated, important changes in the intensity of the NMR signal of the solvent occur

(Hausser and Stehlik, 1968; Overhauser, 1953).

Gutowsky et al. (1958) showed that the asphaltenes exhibit paramagnetism. They

performed EPR experiments in crude petroleum and found g value (i.e., Landé factor)

to be 2:0030 ˙ 0:0005. The unpaired electrons were responsible for the paramagnetism

and they were delocalized on the incomplete carbon bonds of the condensed aromatic

structure of the asphaltene particles. Poindexter (1959) examined first OE in asphalt

solutions in the field of 1.86 mT. Abragam (1961) gives the principles of DNP in his

master work. Kramer et al. (1965) worked on the molecular motion and relaxation in free

radical solutions of benzene, toluene, and some ethers as studied by DNP, and Hausser

and Stehlik (1968) collect all valuable knowledge about DNP in liquids.

The theory of the OE, in low magnetic fields, is available in the literature (Poindexter,

1972; Yalçiner, 1981; Peksoz et al., 2008, 2009).

The aim of this article is to analyze chemical composition and molecular structure

of the asphaltene extracted from asphalt cement with penetration grade 60. The present

study also reports experimental EPR spectra for suspensions consisting of pure and mixed

chlorobenzene and pyridine and asphaltene in a low magnetic field at room temperature.

The goal of the DNP experiments is to provide new information to the EPR oximetry

supported by the OE in low magnetic fields.

2. Experimental Methods

2.1. Preparation of the Asphaltene

The asphaltene was extracted from asphalt cement with penetration grade 60, which

is extracted from crude Libya petroleum. The asphalt cement was taken from Tüpras

Refinery in Izmit-Turkey. The asphalt cement was dissolved in 10 times excess volume

of benzene, precipitated in a further 10 times excess volume of petroleum ether with a

boiling range of 40–60ıC (i.e., 1 part asphalt cement, 10 parts benzene, and 100 parts

petroleum ether) (Poindexter, 1972). The resultant precipitate was collected by filtration

and dried. The asphaltene so obtained was dark brown semi-solids. The percentage of

asphaltene in the asphalt cement was 30% by weight.

2.2. Preparation of the Samples

The suspensions were prepared in the chlorobenzene, pyridine, and their mixtures. The

solvents were taken from Fluka and were 99% and over in purity. The samples were

prepared about 7.0 cc and in concentration, 7.0 kg-m�3. The samples in Pyrex tubes

were degassed by using at least five freeze-pump-thaw cycles with liquid nitrogen at

about 10�3 Pa and sealed. The samples appeared essentially identical to the eye; there

was no visible evidence of rapid flocculation.

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1476 A. Peksoz et al.

2.3. Characterization of the Asphaltene

FTIR spectra were recorded on a THERMO NICOLET FT-IR-6700 spectrometer in

transmittance mode; the ATR technique was used over the wave number region of 400–

4,000 cm�1. The surface morphology of the asphaltene was performed in a Carl Zeiss

EVO 40 type SEM (Carl Zeiss NTS Limited Company, Cambridge, UK) operated at

20 kV. The elemental analysis was performed in a Bruker AXS microanalysis energy

dispersive X-ray (EDX) analysis operated at 10 keV with an XFlash 4010 detector.

Before the analysis, asphaltene particles were fixed on the specimen holder with an

aluminum tape and mounted on an aluminum specimen holder. The dynamic nuclear

polarization experiments were performed in a double resonance NMR spectrometer

with a low magnetic field of 1.44 mT (Akay and Yalciner, 1995). This spectrometer,

which operates with the continuous-wave technique, has a resonance frequency 61.2

kHz (crystal oscillator) for protons and 40.3 MHz for electrons. The observations in

this spectrometer are concerned with pure and double resonance NMR signals that have

P0 and Pz signal intensities, respectively. The signals are detected by the amplitude

modulation technique and using Q-meter detection; they are then amplified in a low

frequency, narrow-band amplifier that is connected to a phase-sensitive detector and an

X-Y recorder. The spectrometer also has an automatic temperature control system using

the liquid nitrogen vapor or heated air flow. The experiments were performed at the room

temperature .23 ˙ 2/ıC.

3. Results and Discussion

3.1. Infrared Spectroscopy

Pérez-Hernández et al. (2003) worked the microstructure, molecular structure, and ele-

mental composition of asphaltene precipitated from vacuum residue using solvent mix-

ture. Chemical structures of asphaltenes and resins are complex and similar. Figure 1

presents the infrared spectrum of the asphaltene that has very strong bands corresponding

to methyl groups (–CH3) and methylene groups (–CH2) stretching and bending vibrations

at 2,916.4 and 2,847.9 cm�1 , and 1,450 and 1,370 cm�1. The broad peak at 1,593.1

cm�1 can be assigned to the aromatic CDC stretch. The absorption peak at 1,021.4 cm�1

corresponds to a sulfoxide functional group (C2SDO) (Pérez-Hernández et al., 2003). A

peak at 803.7 cm�1 is related with aromatic C–H out-of-plane deformation of a single

adjacent hydrogen atom and a peak at 721 cm�1 corresponds to an alkyl chain longer

than 4 methylene units (Pérez-Hernández et al., 2003).

3.2. Scanning Electron Microscopy and Energy Dispersive

X-ray Analysis

Figure 2 shows the surface morphology of the asphaltene. These images reveal that the

asphaltene has a highly rough surface. The elemental composition of the white rectangular

area in Figure 2a consists mainly of carbon (C), oxygen (O), and sulphur (S) as seen in

Table 1. As the asphaltene was coated with gold (Au) and palladium (Pd) to obtain a good

SEM image, Au and Pd aren’t the elemental composition of asphaltene. The elemental

composition of the asphaltene was C: 78.99, S: 4.78, O: 4.55 wt% as seen in Table 1.

There exist some inorganic particles on the asphaltene surface as seen in Figure 2b. One

of these particles was labeled with a white circular sign in Figure 2b and the results of

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Asphaltenes From a Few Standard Characterization Techniques 1477

Figure 1. Infrared spectrum of the asphaltene.

elemental composition for the particle obtained by EDX analysis were summarized in

Table 2. The elemental composition of the particle was C: 73.70, S: 4.03, O: 14.37 wt% as

seen in Table 2. Oxygen percentage increases sharply in the white pieces, while sulphur

percentage decreases. A similar result is reported in an earlier work (Pérez-Hernández

et al., 2003).

3.3. EPR Spectra

It is important and necessary to obtain the EPR spectrum of free radical to measure the

DNP parameters in a given magnetic field, because each peak in the spectrum should

be separately saturated at or near the maximum. Figure 3 shows the EPR spectra of

suspensions consisting of pure and mixed chlorobenzene and pyridine and asphaltene

Figure 2. (a) SEM image of asphaltene and (b) a different SEM image of the asphaltene. It was

focused to understand white particles on the surface. These particles can be seen by a difference

in contrast as illustrated by the circular marker.

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1478 A. Peksoz et al.

Table 1

EDX analysis results of the asphaltene indicated in Figure 2a

Element

Atomic

number Series

C norm.,

wt%

C atom.,

at%

C 7 K-series 78.99 92.87

Au 79 M-series 8.87 0.64

S 16 K-series 4.78 2.10

Pd 46 L-series 2.80 0.37

O 8 K-series 4.55 4.02

Total 100.00 100.00

extracted from asphalt cement with penetration grade 60 (the OE enhancement factors

versus the EPR frequency) performed at 23ıC. During this experiment, the ratio of f (as

MHz) to Veff (as V) was 2.0; because (i) the inhomogeneous line broadening is larger

than the line width of a single EPR line, and (ii) the local field distribution has a Lorentz

form. Therefore, H1e, the amplitude of the magnetic field produced by the EPR coil

with the frequency �s , must not be too large, otherwise the broadening due to saturation

invalidates condition (i) (Yalçiner, 1978). Each spectrum has the inhomogeneous line

broadening and it can be said that each of the spectra has a single Gaussian line-shape

function, formed by the superposition of several Lorentzians, which has a maximum of

about 42.238 MHz. Best fit function was obtained for the EPR spectra of suspensions

consisting of pure and mixed chlorobenzene and pyridine and the asphaltene in a low

magnetic field of 1.44 mT at room temperature. The fit function is:

y D Ae�.x�x/2=2� 2

; (1)

where y D �ŒPz � Po�=Po, x D f (MHz), x is the peak frequency, and � is the standard

deviation. The peak value of y is x D x. For all samples, obtained Gaussian parameters

are summarized in Table 3. The average peak frequency for the spectra is 42.238 MHz

and peak frequency changed a bit at different solvent mediums as illustrated in Table 3.

Table 2

EDX analysis results of the inorganic particle’s surface

on the asphaltene indicated in Figure 2b

Element

Atomic

number Series

C norm.,

wt%

C atom.,

at%

C 7 K-series 73.70 84.31

Au 79 M-series 6.86 2.94

S 16 K-series 4.03 0.28

Pd 46 L-series 1.05 0.14

O 8 K-series 14.37 12.34

Total 100.00 100.00

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Asphaltenes From a Few Standard Characterization Techniques 1479

Figure 3. The variation of the �.Pz � Po/=Po with f(MHz) for the samples. The EPR spectra of

the asphaltene (pure and mixed solvent medium: chlorobenzene and pyridine) at a low field of 1.44

mT and the room temperature. The samples are: #, pure pyridine/asphaltene; �, 20% chloroben-

zene C 80% pyridine/asphaltene; M, 40% chlorobenzene C 60% pyridine/asphaltene; , 60%

chlorobenzene C 40% pyridine/asphaltene; ♦, 80% chlorobenzene C 20% pyridine/asphaltene;

and N, pure chlorobenzene/asphaltene. The R2 values are 0.990, 0.989, 0.984, 0.986, 0.978, and

0.990, respectively, which point out the degree of agreement between the Gaussian fit function

and experimental data. For a single point in the graph, the maximum error is about ˙10%.

Each spectrum has a single Gaussian line-shape function, formed by the superposition of several

Lorentzians. The average of peak frequency at these spectra is 42.238 MHz. Increasing pyridine

concentrations in the samples result in the spectra with decreasing peak point.

4. Conclusions

The present study reports microstructure, chemical composition, and molecular structure

of the asphaltene extracted from asphalt cement with penetration grade 60 obtained

from crude petroleum of Libyan origin, and experimental EPR spectra for suspensions

consisting of pure and mixed chlorobenzene and pyridine and asphaltene in a low

magnetic field at the room temperature.

Some inorganic particles have been observed on the surface of the asphaltene.

Because particles include the large oxygen percentage, they may be inorganic parti-

cles. Pérez-Hernández et al. (2003) reports that it cannot be denied that oxygen is

present in other inorganic particles and carbon is present in organic particles coming

from asphaltenes because the interaction between electron beam-inorganic particles and

asphaltenes takes place. Pérez-Hernández et al. (2003) found that S/C: 0.041, O/C: 0.053

for asphaltene surface and S/C: 0.020, O/C: 0.523 for inorganic particles on the asphaltene

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1480 A. Peksoz et al.

Table 3

Gaussian parameters obtained at room temperatures for suspensions consisting of pure

and mixed chlorobenzene and pyridine solvents and asphaltene extracted from asphalt

cement with penetration grade 60 in a low magnetic field of 1.44 mT

Suspending fluid medium

concentration

Asphaltene

concentration,

kg � m�3 a x �

Pure pyridine .C5H5N/ 6.99 2.696 42.114 7.597

20% chlorobenzene C 80% pyridine 7.00 3.469 42.303 7.594

40% chlorobenzene C 60% pyridine 7.04 4.026 42.271 7.951

60% chlorobenzene C 40% pyridine 6.96 4.631 41.782 6.149

80% chlorobenzene C 20% pyridine 7.00 5.176 42.359 7.143

Pure chlorobenzene .C6H5Cl/ 7.00 5.944 42.599 7.779

x is peak frequency, which gives peak point of EPR spectrum; � is standard deviation. � and aare the fit parameters in Eq. (1), which show variation for different experimental data.

surface by EDX analysis. We found S/C: 0.061, O/C: 0.058 for the asphaltene surface

and S/C: 0.055, O/C: 0.195 for inorganic particles on the asphaltene surface by EDX

analysis.

The OE-type dynamic nuclear polarization experiments were performed to study

suspensions of asphaltene (extracted from asphalt cement with penetration grade 60) in

the chlorobenzene, pyridine, and their mixtures at a low magnetic field. The dipole–dipole

interaction is predominant for the intermolecular spin–spin interaction in all suspensions.

The biggest value for the peak point of EPR spectrum was obtained in pure chlorobenzene

solvent medium. The smallest value for the peak point of EPR spectrum was obtained

in pure pyridine solvent medium. This result shows that the colloidal particles of these

suspensions, which involve pyridine, move more slowly, so chlorobenzene is a more

effective solvent than pyridine for the asphaltene. The asphaltene particles in pyridine

solvent media make a slower translational and rotational movement. More information

about asphaltene behavior in the solvent media can be obtained by working in the larger

temperature range and in different magnetic fields.

Acknowledgments

This work was supported by Uludag University, Scientific Research Projects Unit Grant

No.: 2009/44. The authors would like to thank Uludag University for this support and

Tüpras Refinery in Izmit-Turkey for the asphalt cement with penetration grade 60.

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