асфальтены

Fuel 324 (2022) 124525
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Fuel
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Full Length Article
In-depth characterization of light, medium and heavy oil asphaltenes as
well as asphaltenes subfractions
Marziyeh Salehzadeh a, b, Maen M. Husein b, *, Cyrus Ghotbi a, *, Bahram Dabir c,
Vahid Taghikhani a, d
a
Department of Chemical & Petroleum Engineering, Sharif University of Technology, Tehran, Iran
Department of Chemical & Petroleum Engineering, University of Calgary, Calgary, Canada
Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran
d
Department of Chemical and Biomolecular Engineering, Rice University, Houston, USA
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Asphaltenes
Fractionation
Spectroscopic technique
Elemental analysis
Solubility
Association
Asphaltenes, and their related issues, have been the focus of many literature investigations. However, in-depth
analysis of asphaltenes structure and its relation to asphaltenes stability has been considered by fewer studies. In
this research, extensive analysis of the structure of asphaltenes extracted from light, medium, and heavy oils is
provided, together with analysis of three subfractions of the medium oil asphaltene having the least, interme­
diate, and highest solubilities. To this end, elemental analysis, EDX, mass spectroscopy, FTIR, NMR, XRD, and
SEM results were collected. Higher hydrogen content and hydrogen/carbon atomic ratio, lower aromatic nature
and olefinic entities were observed in heavy oil asphaltenes as well as the most soluble subfraction. Moreover,
the molecular structure of heavy oil asphaltenes and the most soluble subfraction displayed broader mass to
charge (m/z) distribution range, longer alkyl side chains as well as higher number of stacked aromatic sheets
within their nanoclusters. Asphaltenes hydrogens in α position followed similar trend as their heteroatom con­
tent, which was higher for the less soluble subfraction and medium oil asphaltenes. Generally, heavy oil
asphaltenes possessed a molecular structure that was closer to the most soluble asphaltenes subfraction, whereas
light oil asphaltenes possessed a molecular structure similar to the least soluble subfraction. Asphaltenes pre­
cipitation/destabilization was attributed to primarily higher attractive π-π interactions coupled with lower steric
hindrance.
1. Introduction
Asphaltenes are the most polar and complex components of crude oil
[1,2]. While soluble in aromatic solvents, asphaltenes tend to precipitate
with the addition of different types/amounts of n-alkanes to crude oil.
Such solubility attributes follow from the highly polydisperse nature of
asphaltenes molecules [3]. Over the past decades, a tremendous effort
had been invested to characterize, analyze, and determine the properties
Abbreviations: A3000-2750, C–H stretching area of aliphatic; A3150-3000, C–H stretching area of aromatic; APCI, Atmospheric pressure chemical ionization; C aliphatic,
Aliphatic carbon; C aromatic, Aromatic carbon; CA-Caliph, Substituted aromatic carbons; CAH, Aromatic carbon atoms bonded to hydrogen atoms; CAHβX, Aromatic carbons bonded
to β hydrogens to heteroatom; CAP3, External peripheral aromatic carbon atoms at the junction of two fused rings; Cau, Carbons number per aromatic unit; CAX, Aromatic
carbons attached to heteroatoms; CDCl3, Deutero-chloroform; CHNS, Carbon-hydrogen–nitrogen-sulfur; CY, Internal triple bridgehead aromatic carbons; dm, Distance between
two aromatic sheets; dr, Distance between saturate chains; EDX, Energy-dispersive X-ray spectroscopy; F1, Least soluble subfraction; F2, Intermediately soluble subfraction; F3,
Most soluble subfraction; fa, Aromaticity; FTIR, Fourier transform infrared; GPC, Gel permeation chromatography; H, Heavy crude oil; Hal, Aliphatic hydrogen; Halα, Aliphatic
hydrogen in α-position; Halβ, Aliphatic hydrogen in β-position; Halγ, Aliphatic hydrogen in γ-position; Har, Aromatic hydrogen; Hol, Hydrogen attached to olefinic carbon; I1376, CCH2/C-CH3 symmetric bending; I14555, C-CH2/C-CH3 asymmetric bending; I1600, Aromatic C=C stretching; I2924, Aliphatic CH2 stretching asymmetric; I2957, Aliphatic CH3
stretching asymmetric; I3050, Aromatic C–H stretching; IAr, Hydrogen aromaticity index; L, Light crude oil; La, Average diameter of aromatic sheets; Lc, Average height of
aromatic cluster perpendicular to plane of sheets; M, Medium crude oil; m/z, Mass to charge; Me, Number of aromatic sheets in a cluster; MS, Mass sectrometry; MW, Molecular
weight; NMR, Nuclear magnetic resonance; PAH, Polycyclic aromatic hydrocarbon; Ra, Number of aromatic rings for each aromatic sheet; SEC, Size exclusion chromatography;
SEM, Scanning electron microscopy; VPO, Vapor pressure osmometry; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction.
* Corresponding authors.
E-mail addresses: mhusein@ucalgary.ca (M.M. Husein), ghotbi@sharif.edu (C. Ghotbi).
https://doi.org/10.1016/j.fuel.2022.124525
Received 13 March 2022; Received in revised form 25 April 2022; Accepted 4 May 2022
Available online 10 May 2022
0016-2361/Crown Copyright © 2022 Published by Elsevier Ltd. All rights reserved.
M. Salehzadeh et al.
Fuel 324 (2022) 124525
of asphaltenes molecules with the aid of various techniques [4-12].
Nevertheless, these studies have been challenged by the complexity of
asphaltenes molecules, their source- and extraction proceduredependent properties, presence of impurities and asphaltenes selfaggregation [13].
Asphaltenes precipitation and deposition upstream and downstream
of oil industry decreases well productivity, alters rock wettability, re­
stricts/clogs production facilities and flow lines, stabilizes emulsions,
and fouls catalysts and equipment [1,14,15]. On the other hand, the
recent success of functional carbon materials derived from asphaltenes
in gas adsorption [16], electrodes manufacturing [17-19], water puri­
fication, and other fields [20] depicts a bright potential for asphaltenes
applications. Subsequently, elucidating the chemical structure of
asphaltenes molecules and its role in asphaltenes surface activity, pre­
cipitation and deposition is crucial to prescribing effective remedies and
maximizing the beneficial use of asphaltenes.
Fractionation is commonly practiced for enhancing asphaltenes
characterization. By splitting the asphaltenes fraction into subfractions,
the complexity of the sample is reduced, resulting in higher resolution,
rather than relying on broad average values. Considerable research has
been conducted on asphaltene fractionation using various approaches,
including binary solvent mixtures that differ in solvent type, tempera­
ture, contact time and sample/solvent ratio [6,21-31].
A linear relationship between the amount of precipitated asphaltenes
and toluene/n-heptane volumetric ratio was reported by Trejo et al.
[28], which was at odds with the findings of Tojima et al. [24]. Another
investigation on toluene/n-heptane binary solvent showed that the most
highly condensed polynuclear aromatics were detected in the lowest
soluble asphaltenes subfraction [24]. Asphaltenes fractionation using a
binary solvent of tetrahydrofuran/n-hexane showed that the subfraction
more responsible for the high viscosity of heavy oils displayed a lower
H/C ratio and a higher content of metals, aromaticity, heteroatoms, and
condensed aromatic rings [22]. Asphaltenes fractionation using a binary
solvent of methylene chloride/pentane suggested that the highest po­
larity subfraction contains more metals and dissolves slower than the
lowest polarity subfraction [27]. Moreover, using two n-pentane/crude
oil (v/v) ratios showed that the higher volume ratio corresponded to
more interfacially active asphaltenes which displayed smaller aggregate
sizes and more branched aliphatic side chains with a higher content of
hydroxylic and carboxylic groups [6,32].
The analytical techniques utilized to characterize asphaltenes have
been mostly defined by the analysis objective, which was mostly focused
on chemical structure and elemental analysis [1]. Various techniques
such as mass spectrometry (MS), size exclusion chromatography (SEC),
nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR),
X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray
diffraction (XRD), and many others have all been used separately, or in
tandem, for this purpose [9,11,20,33]. Spectroscopic techniques are
powerful tools to characterize complex compounds. In particular, NMR
spectroscopy has been successful in demonstrating a variety of structural
motifs within asphaltenes molecules, such as the average number of
aromatic rings and the alkyl chain lengths of an average molecule
[4,7,10,22,34-39]. Nevertheless, the average parameters obtained from
NMR, elemental analysis and MW results are controversial, since they do
not capture the molecular distribution and might not accurately reflect
the true chemical nature of asphaltenes [40].
A review of the asphaltenes characterization literature reveals that,
despite the large volume of investigations, no studies have considered
detailed characterization of asphaltenes derived from crude oils with
appreciable difference in API gravity. In the current work, asphaltenes
extracted from light, medium and heavy oils were subjected to detailed
characterization, including elemental analysis, energy-dispersive X-ray
spectroscopy (EDX), MS, FTIR NMR, XRD and scanning electron mi­
croscopy (SEM). These measurements were used to explore asphaltenes
MW distribution, functional groups, aromaticity and carbon and
hydrogen distribution in the molecular structure, nanocluster features
(including the distance between two aromatic sheets/aliphatic chains
and the average number of aromatic sheets in a molecule and their size)
as well as the morphology of asphaltenes aggregates. Previously, these
techniques were used separately [8,9,36,41] or in a limited number of
combinations [4,6,11,42]. In addition to the origin of the asphaltenes,
the asphaltenes of the medium crude were fractionated into least,
intermediately and most soluble subfractions and subjected to the same
analyses. This characterization was used to correlate asphaltene char­
acteristics to their solubility. More accurate information, e.g. MW dis­
tribution, is expected when comparing properties of asphaltenes
subfractions to those of their parent asphaltenes or different origin
asphaltenes. Unlike some literature reports [6,40], procedures to more
carefully isolate asphaltenes subfractions have been developed in this
study. We believe that the structural differences of asphaltenes from
diverse crude oil sources as well as solubility-based subfractions provide
deeper insight on asphaltenes properties. This in-depth understanding of
structural differences is required to comprehend and predict the mech­
anisms of asphaltenes precipitation/destabilization behavior on a large
scale.
2. Experimental procedure
2.1. Materials and methods
Asphaltenes were extracted from two crude oil samples (light and
medium) from south west of Iran as well as Athabasca bitumen (Canada)
using ASTM standard methodology D6560-12 with slight variation [4345]. Briefly, 40-fold n-heptane (≥99%, Macron, Canada) was added to
the oil sample and the mixture was refluxed for 1 h. Then, the mixture
was kept in a dark place for 24 h. The precipitated asphaltenes were
filtered using a 2.5 μm filter paper (Whatman grade 42). The filter paper
containing precipitated asphaltenes was placed in a Soxhlet and refluxed
in n-heptane to remove the maltenes from the precipitate. Reflux
continued until the n-heptane was clear. Asphaltenes were then
extracted from the filter with refluxing toluene (99.5%, VWR, Canada).
The toluene was left to evaporate and the asphaltenes were left to dry at
room temperature. The API gravity and the asphaltenes content of the
light crude oil (L) were 32 and 0.5 wt%, respectively, whereas the API
gravity and asphaltenes content of the medium crude oil (M) were 26.9
and 5.5%. The bitumen sample (designated as H, for heavy) had an API
gravity of 9.6 and included 11 wt% asphaltenes.
The asphaltenes obtained from the medium crude oil (M) were
fractionated into three subfractions using a procedure described in
Fig. 1. The medium crude oil asphaltenes were designated as parent
asphaltenes with respect to their subfractions. To fractionate the parent
asphaltenes, 1 g of the asphaltenes was dissolved in 67 mL toluene and
stirred for 24 h before being centrifuged for 20 min at 12000 rpm to
remove potential contaminants. The first subfraction, F1, was prepared
by adding n-heptane to the asphaltenes solution in toluene to make a 50/
50 (v/v) n-heptane/toluene volume ratio. F1 subfraction was precipi­
tated upon stirring the mixture for 24 h and centrifuging for 20 min at
12000 rpm. The second subfraction, F2, was precipitated by adding nheptane to the centrifuged 50/50 (v/v) n-heptane/toluene mixture to
achieve a 75/25 (v/v) n-heptane/toluene mixture, then centrifuging the
resultant mixture for 20 min at 12000 rpm. The asphaltenes remaining
in the 75/25n-heptane/toluene mixture were designated as F3 sub­
fraction, and were subsequently recovered by evaporating the n-hep­
tane/toluene solvent at room temperature. The mass proportions of
subfractions F1, F2, and F3 were 53.4 wt%, 31.3 wt%, and 13.7 wt% of
parent the asphaltene, respectively.
Various analyses were performed on the asphaltenes samples in
order to provide detailed description of the different molecular struc­
tures of the asphaltenes. The elemental composition of asphaltenes L, M
and H as well as the M asphaltenes subfractions was identified using
Elementar UNICUBE (Langenselbold, Germany). Elemental analysis
measured the carbon-hydrogen–nitrogen-sulfur (CHNS) content of each
2
M. Salehzadeh et al.
Fuel 324 (2022) 124525
Fig. 1. Schematic representation of medium crude oil asphaltenes fractionation procedure.
asphaltenes. The oxygen content was calculated by subtracting the
weight percentage of CHNS from 100%. Scanning electron microscope
(SEM) photographs coupled with energy-dispersive X-ray spectroscopy
(EDX) (Phenom Pro X, Phenom-World BV, Netherlands) were collected
to further characterize the structure and elemental composition (except
for hydrogen content [46]) of the asphaltenes samples.
To determine the asphaltenes functional groups and structural pa­
rameters, an Agilent Cary 630 Fourier-transform infrared (FTIR) spec­
trometer (Agilent, Santa Clara, CA, USA) was used, which does not
require any sample preparation for FTIR measurements and does not
require the addition of materials such as KBr. In order to ensure repre­
sentative samples, our asphaltenes samples were thoroughly homoge­
nized prior to FTIR measurements. Each sample was scanned 128 times
with wavelength ranging from 4000 to 400 cm− 1 with a 4 cm− 1
resolution.
In order to unveil the structural characteristics of asphaltenes ag­
gregates, the X–ray fingerprints of the different asphaltenes were
recorded.
Rigaku Ultima III X-ray diffractometer (Rigaku Co.
Ltd.,
Tokyo, Japan) with Cu Kα radiation operating at 40 kV and 44 mA was
used. The samples were loaded in a Rigaku zero background sample
holder model 906,163 (10 mm × 0.2 mm Well Si510) and the scans were
◦
◦
◦
taken in the range of 2θ between 3 to 90 degrees with a 0.05 step and
counting time of 1.0 degrees per min.
Additionally, mass spectroscopy of the asphaltenes was recorded on
Agilent Q-TOF 6520 ESI (Agilent Technologies, Santa Clara, CA, USA)
and positive ionization. Asphaltenes were dissolved in toluene, then
introduced to the atmospheric pressure chemical ionization (APCI)
source via direct infusion while fragmentor voltage was set to 80 V and
APCI source was set at 200 ◦ C.
Nuclear magnetic resonance (NMR) spectra, a strong tool for exam­
ining the chemical nature of asphaltenes and their fragments [7], were
acquired using a 600 MHz Bruker NMR with a broad band probe,
operating at 150 MHz for 13C and 600 MHz for 1H. In each experiment,
samples were prepared by dissolving 20 mg of asphaltenes in 1 mL of
deutero-chloroform (CDCl3) in 5-mm tubes. The C-NMR were carried
out with 15,000 scans, a sweep width of 36000 Hz, flip angle of 90◦ , 3 s
for relaxation delay, acquisition time of 0.36 s, udeft as a pulse program
and Waltz-16 as a decoupling program. The H-NMR spectra were the
result of 100 scans, a flip angle of 30◦ , a relaxation delay of 1.0 s, an
acquisition time of 3.4 s and sweep width of 9600 Hz.
3. Results
3.1. Elemental analysis
Table 1 shows the CHNS content of the L, M and H asphaltenes as
well as the subfractions of the parent M asphaltenes, including F1, F2 and
F3. Between the L and H asphaltenes, the H/C atomic ratio increased
from 0.97 to 1.22, suggesting significant changes in the molecular
structure and a more aromatic character of L asphaltenes. Although the
issue of asphaltenes deposition and well obstruction is more severe in
light oil fields than in medium oil fields, the elemental analysis of L and
M asphaltenes is not significantly different, except for the carbon and
oxygen content.
When comparing M asphaltenes subfractions, the elemental analysis
in Table 1 shows that F1, i.e. least soluble, has the highest heteroatoms
(N + S + O) content of 11.13 wt% and the heteroatoms content
decreased to 9.32 wt% for the most soluble, F3, subfraction. Moreover,
the hydrogen content and H/C atomic ratio are higher for the most
soluble subfractions, F3, whereas the N and S content is lower. The O
content is highest for F3 subfraction, without a specific trend.
EDX analysis was also used to provide elemental analysis (excluding
hydrogen) of the asphaltenes. Literature investigations suggest that
CHNS elemental analysis results are more reliable than EDX results
within the reported range [42], despite the fact that EDX provides in­
formation on more elements, including metals [33,42,47]. It is noted
that EDX is a surface technique, where the electron beam allows char­
acterizing the first micrometers of the surface depth [48,49]. Table S1
reveals that C, O, N, and S are the most abundant elements, with trace
amounts of Ni, V, Ti, Fe, Na, Mg, Ca, Si, Cl, and Al in most of the
asphaltenes samples.
Table 1
Elemental composition (wt%) and H/C atomic ratio of L, M and H asphaltenes as
well as M asphaltenes subfractions, including F1, F2 and F3.
C
H
N
S
O*
Heteroatoms
H/C
*
3
L
M
H
M (Parent Asphaltene)
F1
F2
F3
83.40
6.80
1.20
7.22
1.40
9.81
0.97
81.95
6.88
1.17
7.16
2.85
11.17
1.00
81.32
8.31
1.26
8.15
0.98
10.38
1.22
82.03
6.85
1.23
7.44
2.47
11.13
0.99
82.19
7.43
1.19
6.89
2.31
10.38
1.08
81.89
8.80
0.92
5.20
3.21
9.32
1.28
Calculated in Section 2.1.
M. Salehzadeh et al.
Fuel 324 (2022) 124525
While elemental composition of asphaltenes is key to describing the
asphaltenes molecular structure, more analyses are needed to properly
describe the structure.
The most informative and abundant part for each sample is displayed in
Fig. 2. Asphaltenes L, M, and H all have distinct mass to charge (m/z)
distribution, with the highest intensity peaks at m/z = 207.1, 391.26,
and 198.1, respectively. The m/z distribution for asphaltenes L is nar­
rower when compared with asphaltenes M and H, while asphaltenes H
shows a notable peak at m/z = 922.
The distribution of relative abundance peaks of M asphaltenes sub­
fractions varied, with common most abundance peak at m/z = 391.26.
Among the different subfractions, the most soluble subfraction, F3,
revealed broader distribution, from m/z = 108 to 1325.9 with signifi­
cant relative abundance, while the number of intense peaks for sub­
fractions F2 and F1 dropped as well as their spectrum spanning.
Moreover, in subfraction F2, the abundance of m/z = 666.4 is very sig­
nificant, while this peak displays almost the same intensity with m/z =
279.1 in subfraction F1.
The average MW of asphaltenes from APCI mass spectroscopy were
obtained using equation (E1) [41]. The average was derived by taking
into account all peaks up to m/z ~ 1330, since there was no noticeable
peak beyond this one. The outcome of MW calculation is included in
Table 2.
3.2. Mass spectroscopy
Investigating MW distribution for each asphaltenes provides valu­
able data, particularly in combination with the acquired relative
weight/atomic ratios of various elements in the asphaltenes structure.
Asphaltenes MW measurements are most often performed using vapor
pressure osmometry (VPO), size exclusion chromatography (SEC) and
mass spectrometry (MS) [50]. The MW obtained by VPO are in the range
of several thousand amu [51], whereas those derived from SEC (or gel
permeation chromatography (GPC)) yield values in the neighborhood of
10,000 amu [51]. Nevertheless, aggregation of asphaltenes molecules is
problematic and may impact measurements obtained from VPO [50].
On the other hand, GPC measurements suffer from the possibility of
exceeding asphaltenes critical micelle concentration and the uncertainty
in determining asphaltenes MW, which are based on comparing with
known standards [51]. MS analysis of asphaltenes MW has also proven
to be challenging [41]. For example, the abundance of low MW fractions
tends to conceal the less abundant higher MW fractions [52]. Although
ionization techniques do not fully unfold the complex nature of mole­
cules such as asphaltenes, APCI has been reported as one of the more
suitable ionization methods, since it displays less ionization bias and is
capable to access wide range of classes of compounds and carbon
number distribution [12,41].
The mass spectra for L, M and H asphaltenes as well as M asphaltenes
subfractions are shown in Fig. 2. The m/z detection range was
100–3165, and the acquired abundance intensity was in the order of 107
for asphaltenes M and its subfractions, and 105 for asphaltenes L and H.
Table 2
Average molecular weight determined for L, M and H asphaltenes as well as M
asphaltenes subfractions, including F1, F2 and F3.
L
MW
Avg Mw
302.4
M
409.5
H
376.3
M (Parent Asphaltene)
F1
F2
F3
404.3
477.3
485.9
Fig. 2. Mass spectroscopy of asphaltenes (a) L; (b) M; and (c) H as well as M asphaltenes subfractions (d) F1; (e) F2; and (f) F3.
4
M. Salehzadeh et al.
Fuel 324 (2022) 124525
∑ (m)
*area(allpeaks)
z
MW = ∑
area(allpeaks)
3.3. FTIR
(E1)
FTIR spectra for L, M and H asphaltenes as well as asphaltenes M
subfractions F1, F2 and F3 are shown in Fig. 3. The raw results of the
samples are collected under the same conditions without linear slope
and offset baseline. The interpretation and calculations were performed
without any baseline correction or smoothing, which in turn may in­
fluence the results. The FTIR spectra were normalized based on the C–H
stretching of CH2 group. The areas of certain regions under the curve
and the absorbance at specific wavenumbers were determined using
OMNIC software. Table 3 lists the FTIR results reported based on ratios
of integrated areas and/or intensities within a single spectrum.
The ratio of the area under the FTIR curve of C − H stretching mode
for the aliphatic (3000–2750 cm− 1) and the aromatic (3150–3000 cm− 1)
groups is calculated and denoted as (A3000− 2750 /A3150− 3000 ) [53] in
Table 3. H and F3 asphaltenes show the highest ratios of aliphatic to
aromatic structures among the different asphaltenes and asphaltenes M
subfractions. The ratio A3000− 2750 /A3150− 3000 reveals ~ 2 times increase
between asphaltenes L and H, and a ~ 4 time increase between
asphaltenes subfractions F1 and F3. Accordingly, light oil asphaltenes are
The average MW increased with increasing subfraction solubility and
the MW of the parent medium crude oil asphaltenes is within the range
of its subfractions. In addition, M asphaltenes showed higher value of
MW among the different origin asphaltenes, whereas L asphaltenes had
the lowest MW.
The elemental analysis and MW distribution are crucial to under­
standing asphaltenes molecular structure. These characterization tech­
niques, however, do not provide detailed information on the distribution
of different elements within the structure of asphaltenes, including the
number of carbon atoms in the polycyclic aromatic hydrocarbon (PAH)
part versus the aliphatic part. Moreover, elemental analysis and MW
distribution do not provide precise analysis of the different functional
groups on asphaltenes molecules. FTIR, H-NMR, and C-NMR analyses
have been carried out in an attempt to unveil this information.
Fig. 3. FTIR results of (a) asphaltenes L, M and H; (b) subfractions of asphaltenes M, including F1, F2 and F3.
5
M. Salehzadeh et al.
Fuel 324 (2022) 124525
more aromatic than heavy oil asphaltenes, and the least soluble
asphaltenes subfractions are more aromatic. Two other parameters for
assessing asphaltenes aromaticity can be extracted from the FTIR
spectra, the hydrogen aromaticity index (IAr = I3050 / (I3050 + I2924)) [54]
and the intensity ratio between the C = C aromatic stretch and the
aliphatic CH2 stretching (I1600 / I2924) [6]. Higher values of IAr and I1600
/ I2924 suggest more aromatic structure in the asphaltene molecule.
Table 3 shows that IAr and I1600 / I2924 are higher for asphaltenes L and
subfraction F1. In addition, the condensation degree, which is propor­
tional to the ratio of aromatic C–H stretching to aromatic C = C in­
tensities (I3050 / I1600) [11], suggests greater extent of condensation in L
asphaltenes and the least soluble, F1, subfraction.
The intensity ratios of CH2 to CH3 stretching asymmetric modes
(I2924 / I2957) [6,55] and CH2/CH3 asymmetric to symmetric bending
modes (I14555 / I1376) [53] can be used to determine differences in the
aliphatic part of the asphaltenes molecules. Table 3 shows that
Table 3
Relative parameters applied for comparing the aromatic and aliphatic parts of
the asphaltenes L, M and H and sub-fractions of asphaltenes M.
A3000− 2750
A3150− 3000
IAr
I1600
I2924
I3050
I1600
nCH2
mCH3
I1455
I1376
L
M
H
M (Parent Asphaltene)
12.079
13.855
23.67
0.403
0.846
0.398
0.839
0.289
0.59
0.389
0.837
0.338
0.699
0.196
0.449
0.798
0.787
0.686
0.76
0.731
0.543
1.234
1.256
1.51
1.299
1.411
1.64
1.015
1.002
1.1
1.013
1.026
1.102
F1
F2
F3
12.72
22.2
46.381
Fig. 4. H-NMR results of (a) asphaltenes L and H; (b) asphaltenes M and its subfractions, including F1, F2 and F3.
6
M. Salehzadeh et al.
Fuel 324 (2022) 124525
asphaltenes H and subfraction F3 display higher CH2/CH3 ratios, sug­
gesting that there is more CH2 in the aliphatic part, implying longer alkyl
side chains in their molecular structure.
Despite the fact that H and F3 asphaltenes showed similarity in
their molecular structures, including aromaticity and alkyl side chains,
they display differences in some functional groups. The broad peak at
1030 cm− 1 is attributed to sulfoxides (S = O) [8], which is stronger for
asphaltenes H, whereas this peak appeared weaker in subfraction F3, as
shown in Fig. 3. Sulfoxide groups are highly polar and induce a variety
of hydrogen bond interactions [56]. Such bonding was believed to
provide strong building blocks for supramolecular structures in crystal
engineering [57]. Sulfoxides are also considered as key factors in
asphaltene adsorption at the oil–water interface, hence, W/O emulsion
stability [56]. Another surface-active functional group, carbonyl groups
recognizable at ~ 1700 cm− 1, appeared to be very significant in sub­
fraction F3, while asphaltenes H seems to have no peak in this region.
3.5. C-NMR
C-NMR analysis is limited to M asphaltenes and their subfractions F1,
F2 and F3, in order to maintain manageable volume of data and com­
putations. Fig. 5 depicts the C-NMR spectra of M asphaltenes and their
subfractions, and Table 5 summarizes the results for defined chemical
shifts. Various chemical shifts have been used in evaluating the C-NMR
spectrum according to the literature [7,10,58,64]. However, owing to a
lack of consensus, the most recently recommended chemical shift ranges
[10,58,64,65] were evaluated, and the most relevant ones were picked
based on the spectra of our asphaltenes samples. In line with the results
of FTIR and H-NMR measurements, C-NMR spectrum shows an
ascending trend in the aliphatic portion of the molecular structure of
subfractions with higher solubility. Moreover, asphaltenes aromaticity
reveal that the highest and lowest values of 0.58 and 0.31 correspond to
F1 and F3 subfractions, respectively. In addition, asphaltenes M, as the
parent asphaltenes, have an aromaticity of 0.4, which corresponds to
approximately the average of the different subfractions. In general, as
the subfractions become more soluble, the ratios of total CH3, branched
CH3, terminal isobutyl CH3, and the naphthenic parts in asphaltenes
molecular structures increase, whereas the ratio of terminal CH3 and
CH3 bonded to PAH remain constant for all subfractions. In the chemical
structure of the less soluble subfractions, the olefinic carbons and the
substituted aromatic carbons (CA-Caliph) occupy larger portions. The ar­
omatic carbons attached to heteroatoms (CAX) and the aromatic carbons
bonded to β hydrogens to heteroatom (CAHβX) reveal a similar trend. The
ratio of aromatic carbons associated with heteroatoms decreases with
increasing the solubility of the subfractions. This result is consistent with
the heteroatom content from the CHNS test, which revealed lower het­
eroatom content for the more soluble asphaltenes subfractions.
There is overlapping of chemical shifts in the aromatic carbon signals
in the 120.07 – 130.95 ppm range. Within this overlap, three types of
carbons exist: aromatic carbon atoms bonded to hydrogen atoms (CAH),
internal triple bridgehead aromatic carbons (CY), and external periph­
eral aromatic carbon atoms at the junction of two fused rings (CAP3).
Andrews et al. [38] and Ruiz-Morales et al. [64] used the quantitative
distortionless enhancement by polarization transfer (DEPT) experiment
to calculate the percentage of protonated aromatic carbon (CAH) in re­
gions where signals overlapped, then calculate CY and CAP3. In this
study, CAH was determined using the H-NMR spectra. The aromatic part
of the H-NMR spectrum was integrated to show the hydrogens attached
to aromatic carbons. We assumed that the same number of carbons were
associated with these hydrogens as CAH. Following the subtraction of
protonated carbons from the total carbons in the region containing the
sum of the three carbons CAH, CY and CAP3, 116.53 – 134.76 ppm, CY and
CAP3 are calculated based on their integral percentage in this region.
Elemental composition, m/z distribution and detailed molecular
structure information of the asphaltenes provide profound insight into
how asphaltenes structures differ within a given asphaltenes sample or
different asphaltenes sources. The structure, size, and appearance of
asphaltenes aggregates were further studied using XRD and SEM tech­
niques to describe asphaltenes structure at a different level.
3.4. H-NMR
Fig. 4 depicts the H-NMR spectra of asphaltenes L and H. Significant
differences in the chemical shift ranges in aliphatic (0.5 – 4.5 ppm) and
aromatic (6.5 – 9 ppm) regions are observed between the two asphal­
tenes. The aliphatic part is attributed to γ, β and α positioned hydrogens
with chemical shift ranges of 0.5 – 1 ppm, 1 – 1.9 ppm and 1.9 – 4.5 ppm,
respectively [58,59]. The normalized integral results of asphaltenes
derived from light, medium and heavy crude oils in these specified
blocks are presented in Table 4. Although other authors [60] have used
somewhat different chemical shifts, the general trend for the different
asphaltene samples and medium asphaltene subfractions is the same for
the chemical shifts utilized in this work. Table 4 suggests that asphal­
tenes L have more aromatic and less aliphatic components comparing
with asphaltenes M and H. Although the relative value of γ positioned
hydrogens is higher in asphaltenes L, the β positioned hydrogens in L
asphaltenes are less than asphaltenes M and H. For further examination
of the alkyl side chain length, data such as FTIR spectra are necessary.
The FTIR results in Table 3 indicate a lower nCH2/mCH3 ratio for
asphaltenes L, hence these asphaltenes have shorter alkyl side chains
compared with asphaltenes M and H. It is worth noting that the trend in
the α positioned hydrogens and that of the heteroatoms content match.
According to Table 1 the lowest and highest values of α position hy­
drogens are related to asphaltenes L and M, which displayed 9.80 wt%
and 11.17 wt% heteroatoms content, respectively. The olefinic entities
(4.5 – 6.5 ppm) tend to decline for asphaltenes originating in heavier oils
and clearly the asphaltenes structure become more aromatic for
asphaltenes derived from lighter oils.
The H-NMR spectra and normalized integral results in defined blocks
for asphaltenes M and its subfractions are also presented in Fig. 4 and
Table 4. The highest soluble subfraction, F3, contains more aliphatic
hydrogens in γ and β positions and fewer α positioned hydrogens.
Table 4 further shows that the less soluble subfraction, F1, contains more
hydrogens at aromatic and olefinic positions than the more soluble
subfractions.
3.6. XRD
Table 4
H-NMR chemical shift ranges [58,59,61-63] and normalized integral results for
(a) asphaltenes L, M and H; (b) subfractions of M asphaltenes, including F1, F2
and F3.
Hal (0.5-4.5 ppm)
Hal; γ (0.5-1 ppm)
Hal; β (1-1.9 ppm)
Hal; α (1.9-4.5 ppm)
Hol (4.5-6.5 ppm)
Har (6.5-9 ppm)
L
M
H
0.76
0.32
0.39
0.04
0.03
0.21
0.83
0.23
0.44
0.16
0.03
0.15
0.89
0.27
0.49
0.12
0
0.11
The XRD pattern can provide information about the structure and
size of asphaltenes nanoclusters [66]. The XRD fingerprints of the
different asphaltenes and asphaltenes M subfractions are shown in
Fig. 6. The XRD pattern of all asphaltenes shows three superimposed
peaks: one at 2θ ~ 20◦ , which is the γ-band and is attributed to the
aliphatic portion, another at 2θ ~ 25◦ , which is the 002-band and is
attributed to the aromatic portion and a third peak at 2θ ~ 43◦ , which is
the (10)-band and is attributed to the aromatic sheets of asphaltenes.
The XRD patterns in the area under 2θ of 20◦ and 25◦ peaks clearly
shows that asphaltenes L contain more aromatics than asphaltenes M,
and asphaltenes H contains the least aromatics. Inspecting the same 2θ
M (Parent Asphaltene)
F1
F2
F3
0.83
0.18
0.41
0.23
0.05
0.12
0.84
0.17
0.46
0.21
0.04
0.12
0.89
0.33
0.48
0.07
0
0.11
7
M. Salehzadeh et al.
Fuel 324 (2022) 124525
Fig. 5. C-NMR results of sub-fractions and parent asphaltenes.
Table 5
C-NMR chemical shift ranges and normalized integral results for the parent M
asphaltenes and their subfractions.
C aliphatic (10–60 ppm)
C aromatic (110.6–156.2 ppm)
Terminal CH3 (10.5–17 ppm)
Total methyl (10–21.8 ppm)
Branched CH3 (17–21.8 ppm)
CH3 bonded to PAH (21.3 ppm)
Terminal isobutyl CH3; & possible CH2
α to t-CH3 (21.8–24 ppm)
Naphthenic (24–40 ppm)
Olefin (110.6–116.53 ppm)
CAHβX (110.6–116.53 ppm)
Cah (116.53–130.95 ppm)
CY (120–127.78 ppm)
CAp3 (127.78–134.76 ppm)
CA-Caliph (134.7–139.41 ppm)
CAX (139.41–156.2 ppm)
M (Parent
Asphaltene)
F1
F2
F3
0.6
0.4
0.07
0.16
0.08
0.01
0.06
0.42
0.58
0.07
0.15
0.08
0.02
0.04
0.57
0.43
0.07
0.17
0.1
0.02
0.05
0.69
0.31
0.07
0.17
0.11
0.02
0.06
0.41
0.01
0
0.27
0.18
0.11
0.05
0.02
0.3
0.04
0.03
0.23
0.15
0.11
0.07
0.17
0.43
0.01
0
0.26
0.19
0.11
0.06
0.04
0.45
0.01
0
0.18
0.11
0.1
0.04
0.04
lc =
Cau =
λ
2sinθγ
(E1)
dm =
λ
2sinθ002
(E2)
1.84λ
(E3)
la =
ωcosθ10
la + 1.23
0.65
(E4)
(E5)
Ra =
la
2.667
(E6)
Me =
lc
dm
(E7)
fa =
A002
A002 + Aγ
(E8)
where λ is the wavelength of the applied radiation (Cu Kα), θ is the
Bragg angle, ω is the full width of the peaks at half maximum and A is
the area under the corresponding peak.
Table 6 reveals that the average distance of two aromatic sheets (dm)
of the studied asphaltenes are from 3.5 to 3.52 Å, which suggests almost
the same distance between aromatic sheets of all asphaltenes samples.
This range of dm is higher than of a pure graphite structure (dm = 3.4 Å)
[42], which may be a result of steric hindrance imparted by circum­
ferential substituents of the asphaltenes aromatic sheets [67]. Notably a
little higher dm values are obtained for asphaltenes H and the most
soluble subfraction, F3. The distance between the saturates chains (dr) is
higher than that of the aromatic sheets for all asphaltenes samples.
Moreover, dr range for asphaltenes derived from light, medium and
heavy oils varies from 4.63 to 4.75 Å, while in the subfractions of
asphaltenes M it varied from 4.56 to 4.65 Å, which can be consider
sufficiently close to each other.
Table 6 shows tight ranges, from 10.06 to 11.31 and from 11.11 to
11.60 Å, for the average diameters of the aromatic sheets (La) of the
different asphaltenes (L, M and H) and asphaltenes M subfractions,
respectively. Asphaltenes H has smaller La, while asphaltenes M display
the largest La. On the other hand, subfraction F1 displays the largest size
of aromatic sheets, La.
Lc values for asphaltenes L, M and H are 25.3, 27.1 and 33.12 Å,
respectively, and for subfractions F1, F2 and F3 are 27.37, 27.62 and
38.07 Å, respectively. Furthermore, the number of stacked aromatic
sheets in a nanocluster of asphaltenes increased from 8.23 in asphaltenes
L to 10.4 for asphaltenes H. The same trend is observed in asphaltenes M
subfractions with Me = 8.83 for F1, and Me = 11.85 for F3 subfraction.
The trend of Lc and Me suggest that by increasing the H/C ratio or
decreasing the aromaticity, the number of stacked aromatic sheets
increased in asphaltenes nanoclusters. On the other hand, the average
number of aromatic rings in the aromatic sheets for all asphaltenes is ~
4, which falls within the range of the literature (4 to 10 fused rings) [68].
interval for asphaltenes M subfractions reveals that the aromaticity of
the parent asphaltenes and their subfraction F1 is higher than sub­
fraction F2, while subfraction F3 displays the lowest aromaticity. When
these data are compared to the H/C ratio of the asphaltenes in Table 1, it
is concluded that a higher H/C ratio corresponds to a lower aromaticity.
The structural parameters of the asphaltenes clusters that can be
quantified through XRD patterns include dr (distance between saturate
chains), dm (distance between two aromatic sheets), Lc (average height
of aromatic cluster perpendicular to plane of sheets), La (average
diameter of aromatic sheets), Cau (carbons number in the aromatic unit),
Ra (number of aromatic rings for each aromatic sheet), M (number of
aromatic sheets in a cluster) and fa (aromaticity) [9,42]. These param­
eters can be calculated using Equations (E1) - (E8) below [9,42]. The
results are presented in Table 6. Additionally, Fig. 7 shows a schematic
structure of a single asphaltenes nanocluster to provide better under­
standing of the different parameters.
dr =
0.9λ
ωcosθ002
8
M. Salehzadeh et al.
Fuel 324 (2022) 124525
Fig. 6. X-ray diffraction pattern of (a) asphaltenes L, M and H; and (b) asphaltenes M subfractions, including F1, F2 and F3.
However, this value does not indicate a major difference among the
different asphaltenes. Also, the number of carbons per aromatic unit
(Cau) obtained between 17.37 and 19.73 for all the asphaltenes. Lastly,
the aromaticity (fa) calculated by XRD pattern is highest for asphaltenes
L and subfraction F1 (0.14 and 0.12, respectively), whereas it is lowest
for asphaltenes H and the most soluble subfraction F3 (0.02 and 0.04,
respectively).
In-depth molecular structure analysis of the asphaltenes coupled
with asphaltenes nanocluster description assists in understanding and
predicting a probable structure of various asphaltenes. The XRD results
for some parameters such as Ra and fa follow the NMR trends. Never­
theless, parameters estimated through NMR would be more accurate and
reliable when it comes to molecular scale.
Although various parameters can influence the appearance and
morphology of asphaltene samples, asphaltenes molecular structure
could be another factor. Figure S1 shows SEM images of asphaltenes L,
M, and H, as well as M asphaltenes subfractions F1, F2, and F3, which
generally shows two types of smooth and porous surfaces.
4. Discussion
From a thermodynamic perspective, asphaltenes solubilization/sta­
bilization versus precipitation/destabilization is an outcome of a bal­
ance among entropic diffusion interactions and intermolecular
interactions among the asphaltenes molecules and the rest of the crude
oil constituents as well as among the asphaltenes molecules themselves.
Attractive intermolecular interactions among asphaltenes molecules and
the rest of crude oil constituents, mostly notably resins [69,70], favor
asphaltenes solubilization/stabilization, whereas attractive intermolec­
ular interactions among asphaltenes molecules favor precipitation/
destabilization. Nevertheless, some literature suggests that asphaltenes
subfractions with less self-association tendency and lower polarity play a
significant role in stabilizing the higher-polarity subfractions [71,72], in
similar fashion to resins. Entropic diffusion interactions always act in
9
M. Salehzadeh et al.
Fuel 324 (2022) 124525
aggregation has been established [71,75,78,79]. M asphaltenes con­
tained just few more stacked aromatic sheets in their nanocluster than L
asphaltenes, which might be attributed to hydrogen bonding. The
aromaticity of asphaltenes decreases monotonically from L to H
asphaltenes, while the length of the pendant groups together with the
fraction of hydrogens in the β position increase by 26.3 % and 25.6 %,
respectively. Accordingly, one may interpret asphaltenes H stability by
the diminishing role of π–π aromatic interaction and the growing role of
steric hindrance. The higher number of stacked aromatic sheets of
asphaltenes H in XRD results might be explained by a strong peak of
sulfoxides in this asphaltenes sample, which can cause a variety of
hydrogen bond interactions.
M asphaltenes were fractionated by dissolving in toluene, then
adding different amounts of n-heptane. Asphaltenes have a range of
solubility parameters close to that of toluene, whereas the solubility
parameter of n-heptane is lower than that of asphaltenes and toluene
[80]. Addition of n-heptane to asphaltenes solution in toluene lowers the
solvent solubility parameter, hence promotes asphaltenes precipitation.
Among M asphaltenes subfractions, F1 portrayed the highest aroma­
ticity, the least branching and the shortest aliphatic chains, suggesting
higher subfraction solubility parameter [81]. Using molecular dynamic
simulation, literature findings showed that asphaltenes with short
lateral chains are the quickest to precipitate, when n-heptane is added
[78]. Subfraction F1 with the highest solubility parameter, displayed the
highest aromaticity, heteroatoms content and shortest pendant groups.
These features promote π–π and other van der Waals interactions,
including Keesom, Debye and London, among asphaltenes molecules.
Higher interaction coupled with low steric hindrance promotes asphal­
tenes association and precipitation. The least soluble subfractions have
been reported to have a greater proclivity to aggregate and form deposits
that are more difficult to remediate [72]. On the other hand, sub­
fractions with longer lateral chains have a stronger interaction with nheptane molecules, but also high steric hindrance, making them more
stable. Lower aromaticity, longer pendant groups and more branching of
the more stable subfraction F3 favor higher steric hindrance with less π–π
interaction. While subfraction F3 was the most stable, its highest oxygen
content and very significant peak of carbonyl groups, which promote
hydrogen bonding, may explain the higher number of stacked aromatic
sheets in this subfraction’s nanocluster. Literature reports suggest that
asphaltenes fractions with less self-association tendency and lower po­
larity (similar to subfraction F3) play a significant role in reducing
nanoaggregation and stabilizing the higher-polarity fractions (similar to
subfraction F1) [71,72]. Accordingly, the presence of subfraction F3 may
in fact improve the stability and reduces precipitation and deposition for
the parent M asphaltenes.
Fig. 7. Schematic structure of one asphaltenes nanocluster [42].
Table 6
Nanocluster structural analysis of asphaltenes L, M and H and asphaltenes M
subfractions, including F1, F2 and F3, based on XRD patterns.
dr
dm
La
Lc
Me
Ra
Cau
fa
L
M
H
4.75
3.5
11.02
25.3
8.23
4.13
18.85
0.14
4.63
3.51
11.31
27.1
8.73
4.24
19.3
0.11
4.68
3.52
10.06
33.12
10.4
3.77
17.37
0.02
M (Parent Asphaltene)
F1
F2
F3
4.65
3.5
11.6
27.37
8.83
4.35
19.73
0.12
4.56
3.5
11.11
27.62
8.88
4.16
18.98
0.10
4.65
3.51
11.11
38.07
11.85
4.17
18.99
0.04
favor of asphaltenes solubilization/stabilization together with the
repulsive steric hindrance among asphaltenes molecules. In this anal­
ysis, precipitation and aggregation are assumed to proceed hand in
hand, away from any kinetic limitations.
Literature investigations suggest that polyaromatic cores, polarity/
heteroatoms and pendant groups are key features governing the inter­
molecular interactions among asphaltenes molecules as well as other
materials and surfaces [73,74]. The characterization results of this study
show that L asphaltenes, i.e. the least stable asphaltenes, possess the
largest aromatic cores with slightly higher metals and nitrogen content,
lower overall content of heteroatoms and shorter aliphatic chains. These
features suggest asphaltenes association majorly driven by attractive π–π
interaction coupled with low steric hindrance. The contribution of
attractive electrostatic, polar interactions [74,75] and hydrogenbonding are likely less influential. Notably, large aromatic cores in­
crease asphaltenes phobic forces toward the increasing alkane fraction
in the mixture. Literature showed that asphaltenes originating from
unstable oil fields generally display more aromaticity and polarity than
stable ones [72,76]. While L asphaltenes are the most aromatic, they
were not the most polar. M asphaltenes have higher polarity owing to
their higher content of heteroatoms. These findings show that aroma­
ticity is more important parameter in asphaltenes association relative to
polarity, which is consistent with the molecular dynamic studies of
Sedghi et al. [75,77]. The main difference in heteroatoms content of L
and M asphaltenes is related to oxygen, since their sulfur and nitrogen
contents are nearly identical. Oxygen typically exists in functional
groups such as phenolic, carboxylic, hydroxylic, and ketonic. The role of
these functional groups in initiating hydrogen bonding and promoting
5. Conclusions
In this study, the molecular structure of asphaltenes derived from
light, medium and heavy crude oils was thoroughly investigated utiliz­
ing CHNS elemental analysis, EDX, MS, FTIR, NMR, XRD, and SEM
techniques. Additionally, solubility-based subfractions from the medium
crude oil asphaltenes were characterized. The hydrogen content and H/
C ratio were higher in the asphaltenes derived from heavy oil and the
most soluble asphaltenes subfraction, these asphaltenes seem to have
higher steric hindrance with less π–π interaction among their asphal­
tenes molecules. Elemental analysis showed high abundance of C, H, N,
S and O with trance values of Ni, V, Ti, Fe, Na, Mg, Ca, Si, Cl, and Al. The
mass spectroscopy revealed a broader m/z distribution for heavy oil
asphaltenes and the most soluble asphaltenes subfraction. Moreover,
while the m/z distribution of the subfractions differed, the most abun­
dant peak was the same for the parent asphaltenes and all its sub­
fractions. The FTIR, NMR, and XRD analyses corporated each other and
confirmed the more aromatic nature of light oil asphaltenes and the least
soluble asphaltenes subfraction. On the other hand, asphaltenes of
heavier oils and the more soluble subfractions exhibited lower aromatic
10
M. Salehzadeh et al.
Fuel 324 (2022) 124525
to aliphatic ratios and longer alkyl side chains. Less π–π interaction
among these asphaltenes and higher steric hindrance promoted
asphaltenes stability. Light oil asphaltenes possessed a molecular
structure that was closer to the least soluble asphaltenes subfraction. On
the other hand, heavy oil asphaltenes were more similar to the most
soluble asphaltenes subfraction. Nevertheless, sulfoxide groups were
abundant in heavy oil asphaltene, whereas carbonyl functional groups
were abundant in the most soluble subfraction. These functional groups
contribute in establishing hydrogen bonding. The olefinic entities fol­
lowed a similar trend to aromaticity. Furthermore, the α position hy­
drogens closely tracked the heteroatom content. The C-NMR test of
asphaltenes subfractions revealed higher ratios of total CH3, branched
CH3, terminal isobutyl CH3, and naphthenic parts in the more soluble
asphaltenes subfractions. On the other hand, the less soluble sub­
fractions displayed larger portions of aromatic carbons that are
substituted, attached to heteroatoms and bonded to hydrogen in β po­
sition to a heteroatom. These features along with higher aromaticity,
heteroatoms content and shorter aliphatic chains promote π–π and other
van der Waals interactions, including Keesom, Debye and London in the
less soluble asphaltenes subfraction, while steric hindrance role
decreased.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.fuel.2022.124525.
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