Effect of Oxidation on the Chemistry of Asphalt and its Fractions

<strong>Effect</strong> <strong>of</strong> <strong>Oxidation</strong> on the Chemistry <strong>of</strong> Asphalt and its Fractions
ABSTRACT
Mohammad Nahid Siddiqui
Department <strong>of</strong> Chemistry
King Fahd University <strong>of</strong> Petroleum & Minerals
Dhahran 31261, Saudi Arabia
E-mail: mnahid@kfupm.edu.sa
The composition <strong>of</strong> asphalt is very important as it plays a vital role in determining its
performance-related properties. The main problem in studying asphalt composition is its
chemical complexity; however, the characterization <strong>of</strong> asphalt can better be achieved by
separating into different fractions. In this work, the chemical properties <strong>of</strong> commercial grade
asphalt procured from Al-Ahmadi refinery, Kuwait was evaluated. The rolling thin film oven
(RTFO), termed as short-term aging, and pressurized aging vessel (PAV), termed as long-term
aging, tests were used to simulate the laboratory aging <strong>of</strong> the asphalt. The asphalt was
fractionated using normal alkane chiefly into two fractions such as insoluble asphaltenes and
soluble maltenes. The maltenes were further fractionated into polar aromatics, naphthene
aromatics, and saturates. Different spectroscopic and analytical techniques were used to
investigate the effect <strong>of</strong> oxidation on the chemical composition and structure <strong>of</strong> asphaltenes.
Nuclear magnetic resonance and infrared combined have provided very useful information
concerning the changes in the chemical composition <strong>of</strong> asphaltenes occurred during the oxidation
<strong>of</strong> asphalt.
INTRODUCTION
Asphalt is a black viscous liquid or solid obtained by refinery processes from petroleum.
Asphalts are manufactured by the following processes: (1) atmospheric and vacuum distillation
<strong>of</strong> crude oils, (2) oxidation or air blowing <strong>of</strong> residues from distillation <strong>of</strong> crude oils, (3)
deasphalting <strong>of</strong> petroleum residues <strong>of</strong> lubricating oil and asphalt origin, and (4) blending hard
propane asphalt from deasphalting unit with resins and oils (extracts). The chemical composition

<strong>of</strong> asphalt is complex and varies considerably dependent upon feedstock and method <strong>of</strong>
manufacture. The most widely accepted concept <strong>of</strong> the constitution is that asphalt is made up <strong>of</strong>
three major components: asphaltenes, resins and oils. The consistency <strong>of</strong> asphalt can vary almost
infinitely by the variations in the proportions <strong>of</strong> the components. Relatively high asphaltenes
content is generally desirable. The absence <strong>of</strong> sufficient asphaltenes tends to result in an asphalt
<strong>of</strong> oily character and poor quality [1].
Asphaltenes are a dark brown to black material containing large fused aromatic rings bearing
long aliphatic substituents and saturated paraffins as straight chain and branched compounds
along with metals and heteroatoms as part <strong>of</strong> a ring system. The asphaltenes have been
considered to be repeating units <strong>of</strong> similar composition with the major difference being mainly in
the aromaticity [2].
Nuclear Magnetic Resonance (NMR) spectroscopy provides both statistical and structural
information about the distribution <strong>of</strong> carbon and hydrogen atoms present in different chemical
environments <strong>of</strong> the complex petroleum matrix. 1 H NMR has been employed to estimate the
aromatic ring distribution in heavy oils and bitumen assuming equal degree <strong>of</strong> substitution i.e.
equal number <strong>of</strong> substituents on aromatic rings. The use <strong>of</strong> 13C NMR spectroscopy has been
applied extensively for the characterization <strong>of</strong> crudes and their fraction [3]. 13 C NMR is
employed to make a clear distinction between aliphatic and aromatic hydrocarbons. By using 1 H
and 13 C NMR one can obtain information in terms <strong>of</strong> average structural parameters <strong>of</strong> asphalt
and asphaltenes such as percentages <strong>of</strong> aromatic carbons, aliphatic carbons, bridged carbons,
methyl carbons, ring carbons, naphthenic carbons, paraffinic chain lengths and other parameters
<strong>of</strong> such nature in conjunction with elemental analysis data [4]. Important average structural
parameters like total C/H ratio; Csat/Hsat ratio, aromaticity and compactness index (fc) <strong>of</strong> the
aromatic part <strong>of</strong> the asphaltenes has also been derived. The aromaticity (fa) is defined as the
ratio <strong>of</strong> aromatic carbons to the total carbon content estimated respectively from 13 C and 1 H-
NMR spectra, and is explained due to non-accountability <strong>of</strong> quaternary aromatic carbons in 1 H-
NMR spectra.
2

Infrared spectroscopy provides both supplementary and complimentary data to NMR
spectroscopy to enable fairly detailed information on the distribution <strong>of</strong> CHn groups and widely
used to identify the functional groups in asphalt and their sub-fractions. The characteristic
frequencies <strong>of</strong> functional groups which include free and hydrogen bonded OH and NH groups,
carbonyls, carboxylic acids, pyridines, pyrroles, ether linkages and some others are well
documented in the literature [5,6]. It has also been used for quantitative determination <strong>of</strong> various
functional groups like phenol, pyrrole, carboxyl, ketone, amide and sulfoxide in Vacuum
Residue and their solvent extracted fraction [7]. In other studies, methods are proposed for the
quantitative estimation <strong>of</strong> the compounds absorbing in the carbonyl region in bitumen samples
[8] and determination <strong>of</strong> sulfur compound types (i.e. sulfoxides, sulfides and thiopenes) in
asphaltenes by IR spectroscopy [9,10].
Asphalt Sample Collection
EXPERIMENTAL
Asphalt sample were collected from Al-Ahmadi refinery, Kuwait (KW), which supplies all <strong>of</strong> the
asphalt cement needed for construction in the state <strong>of</strong> Kuwait. A Ratawi-Burgan crude oil mix is
used to produce 750-1000 tons <strong>of</strong> asphalt per day using vacuum distillation and air blowing to
produce the required asphalt grade.
Fractionation by Precipitation Method
The separation method used to divide asphalt into operationally defined fractions was developed
by Corbett and later adopted as a standard procedure by ASTM (ASTM D-4124) [11]. The
scheme and the names given to the separated fractions obtained from this procedure are
described in Figure 1. This method involves precipitating an asphaltene fraction by mixing
whole asphalt in n-heptane solvent. The heptane soluble (maltenes) are then separated by
adsorption on alumina and subsequent desorption with solvents <strong>of</strong> increasing polarity (toluene,
methanol/toluene, and trichloroethylene).
3

MALTENES
Adsorption / Desorption
Chromatography
on Alumina
n-Heptane
Toulene
ASPHALT
n-Heptane
Methanol-Toluene
Trichloroethylene
Precipitation
ASPHALTENES
SATURATES
NAPHTHENE
AROMATICS
POLAR
AROMATICS
Figure 1: Adsorption\desorption chromatography (ASTM D-4124)
Infrared Spectroscopy
Infrared spectra were recorded on a Perkin Elmer Model 1610 infrared spectrophotometer loaded
with Infrared Data Manager (IRDM) s<strong>of</strong>tware. Solutions for obtaining the IR spectra <strong>of</strong>
4

asphaltenes were prepared by dissolving 30-50 mg <strong>of</strong> sample in 1.00 ml <strong>of</strong> CCl4 solvent
(spectroscopic grade). All IR spectra were obtained using a 0.1-mm path length sample NaCl
cell. Spectra were recorded using the following settings; number <strong>of</strong> scans 4; gain 1; apodization
weak; and resolution 4. Salt plates and windows <strong>of</strong> sealed cells were <strong>of</strong> sodium chloride.
Nuclear Magnetic Resonance Spectroscopy
Solutions for the measurement <strong>of</strong> 13C NMR spectra were prepared by dissolving 1 g <strong>of</strong>
asphaltenes in 2 ml <strong>of</strong> deuteriated-chlor<strong>of</strong>orm containing 10 mg ferric acetonyl acetonate,
Fe(acac)3, as the relaxation agent. Solutions for the measurement <strong>of</strong> 1H NMR spectra were
prepared, in a similar way, as described for 13C spectra with the exception that no relaxation
reagent was used. 1H NMR spectra were recorded on a Varian XL-200 Pulse Fourier Transform
(PFT) spectrometer operating at 200 MHz using 5 mm sample tubes. 1H NMR spectra were
obtained and the experimental conditions were: spectrum width, 2800 Hz; data points, 15680;
pulse width, 3.5 ms (45°); pulse delay, zero; and number <strong>of</strong> transients, 64.
RESULTS AND DISCUSSION
The percent fractions obtained for fresh asphalt from Kuwait (KW) refinery along with their
aged samples are given Table 1. Weight percent <strong>of</strong> asphaltene were found to increase from
RTFO to PAV aging tests. The maltene fraction from the fresh and aged samples <strong>of</strong> asphalt from
Kuwait (KW) refinery was also separated to their generic fractions namely saturates naphthene
aromatics and polar aromatics. The results are shown in Tables 1. There was a distinct change
in the composition <strong>of</strong> KW asphalt on aging as shown by a decrease in naphthene aromatics and
an increase in polar aromatics. A slight increase in saturates was also noticed. Similar changes
in Arabian asphalt composition following RTFO and PAV aging were observed by Siddiqui et
al. [12] It was concluded that the naphthene aromatics converted in part to polar aromatics and
which later turned to asphaltenes on RTFO and PAV aging tests. Gaestel Index (Ic) was used to
reflect the relationship between the RTFO and PAV aging and colloidal structure <strong>of</strong> asphalt.
Gaestel has defined a colloidal instability index (Ic) as a ratio <strong>of</strong> the sum <strong>of</strong> the amount
5

asphaltenes and saturates to the sum <strong>of</strong> the amounts in polar aromatics and naphthene aromatics.
The Ic values significantly increased in asphalt after PAV tests. This indicates that colloidal
changes occur due to the sequential transformation <strong>of</strong> naphthene to polar aromatic to asphaltenes.
Sample
KW - F
KW - R1
KW - R4
KW - P1
KW - P4
Table 1: Percent Composition <strong>of</strong> Kuwait (KW) Asphalt’s Fractions
Asphaltenes
17.97
19.93
24.59
24.98
29.29
Saturates
6.75
7.36
7.87
7.94
8.07
Infrared Spectroscopy <strong>of</strong> Asphaltenes
Naphthene
Aromatics
44.36
38.01
32.28
30.85
23.57
Polar
Aromatics
30.75
33.58
35.21
36.25
37.71
Total
(%)
99.83
98.88
99.95
100.02
98.64
Gaestel
Index
IR spectroscopy was used to study the distribution <strong>of</strong> functional group types present in the
asphaltene fraction <strong>of</strong> KW asphalt. IR proved to be a very useful technique in analyzing
structural changes in asphaltene fractions following RTFO and PAV tests <strong>of</strong> KW asphalt. The
most prominent IR vibrations <strong>of</strong> particular interest were those for C-H, C=O, and S=O modes <strong>of</strong>
vibrations and the area <strong>of</strong> the peaks were determined using the baseline method. The IR spectra
displayed a distinct and very important C=O stretch absorption at 1698 and 1704 cm-1 due to
carbonyl and/or carboxyl groups. The area <strong>of</strong> the carbonyl absorption was calculated between
1752-1653 cm -1 , this covering the region containing the absorption peaks for carboxylic acids,
ketones and anhydrides. Ketones and anhydrides form on oxidative aging and carboxylic acids
occurs naturally in asphalt but increase on oxidative aging. Together, these three functional
groups are the most significant chemical functionalities which are an integral part <strong>of</strong> large
asphalt molecules and which can be related to oxidative aging [13]. The main reason behind
considering the total carbonyl area rather than each characteristic area is that the IR spectra in the
0.33
0.38
0.48
0.49
0.61
6

carbonyl region become complicated in aged asphalt because <strong>of</strong> the intense ketone band
appearance. An intense peak at 1032 cm -1 was assigned to the stretching vibration <strong>of</strong> sulfoxide
(S=O), the functional group most easily formed in asphalt on the oxidation <strong>of</strong> sulfide moieties.
The area <strong>of</strong> this band was determined from 1065-1007 cm -1 .
Some parameters calculated from peak areas <strong>of</strong> selected IR bands allowed for a very useful
comparison <strong>of</strong> the spectra <strong>of</strong> fresh and aged samples. The ratios in an equation to determine
weight percent <strong>of</strong> oxygen in C=O types and sulfur in S=O types has been used [9]. An IR
spectrum <strong>of</strong> asphaltene is shown in Figure-2. Parameters and weight percentages <strong>of</strong> sulfur and
oxygen <strong>of</strong> RY asphaltenes obtained by IR spectroscopy are shown in Table 2. In the case <strong>of</strong> KW
asphaltenes, all parameters related to carbonyl group increased on aging. The carbonyl area and
absorption peak was not observed in KW fresh asphaltenes but increased on aging. The weight
percent <strong>of</strong> oxygen calculated as C=O types increased linearly with aging time. These trends
emphasize that there is significant formation <strong>of</strong> functional groups <strong>of</strong> carbonyl type on short term
and long term aging <strong>of</strong> KW asphalt.
Table 2: Parameters <strong>of</strong> KW Asphaltenes Obtained by Infrared
Parameters Ratios by area KW-F KW-R1 KW-R4 KW-P1 KW-P4
C=O/CH3 1698/2954 0.00 0.00 0.00 0.00 1.13
C=O/CH2 1698/2926 0.00 0.00 0.00 0.00 0.28
C=O/C=C 1698/1605 0.00 0.00 0.00 0.00 0.44
S=O/CH3 1032/1376 1.40 1.07 1.41 1.91 2.05
S=O/C=C 1032/1605 0.32 0.21 0.26 0.34 0.39
S=O/CH3 1032/2954 0.73 0.40 0.60 0.79 1.00
S=O/CH2 1032/2926 0.18 0.11 0.15 0.21 0.25
Sulfur (Wt%) SO type 0.69 0.14 0.43 0.57 1.00
Oxygen (Wt%) CO/COOH type 0.00 0.08 0.17 0.19 0.20
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It is needed to clarify at this point that the weight percent <strong>of</strong> total sulfur in asphaltenes decreases
on aging while the weight percent <strong>of</strong> sulfur as sulfoxide type only increases on oxidation. The
formation <strong>of</strong> sulfoxides, as previously noted, is a good indication <strong>of</strong> asphalt aging. Two big
peaks in the region <strong>of</strong> 1273-1070 cm -1 were also observed in KW asphaltenes. These peaks
indicate the presence <strong>of</strong> different sulfur containing compounds such as sulfones, and sulfates
produced due to the oxidation <strong>of</strong> sulfur containing molecules in asphaltenes [6]. The existence <strong>of</strong>
sulfur containing compounds provides a channel for oxygen attack. During aging, sulfur atoms
are readily oxidized to sulfoxide groups. Sulfoxides are formed in preference to ketones, and are
usually precursors to ketone and hence acid formation. The extent to which these groups are
formed on aging influences the rheological properties <strong>of</strong> asphalt [14].
3850
3600
3350
Nuclear Magnetic Resonance Spectroscopy <strong>of</strong> Asphaltenes
31 00
2850
Wa v e numbe r , c m- 1
2600
Figure 2: An infrared spectrum <strong>of</strong> asphaltene
2350
The percent distribution <strong>of</strong> carbon and hydrogen <strong>of</strong> KW asphaltenes obtained from 13C and 1H
NMR spectra are given in Table 3 and Table 4 respectively. The result indicates that the
distribution trend <strong>of</strong> carbons shows that there were no consistent aromatization and
21 00
1 850
1 600
1 350
1 1 00
850
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
8

dehydrogenation <strong>of</strong> asphaltenes but isomerization <strong>of</strong> saturated carbons (Csat) may be responsible
for this type <strong>of</strong> distribution. There was dealkylation on aromatic rings following first short term
aging but alkylation took place on further long term aging process. The changes in total saturated
carbon (Csat) contents were found to inconsistent indicating the isomerization <strong>of</strong> saturated
carbons in asphaltenes. In short term aging, substitution <strong>of</strong> hydrogen atoms was preferred while
on long term aging dehydrogenation took place. The aromatic hydrogen (Har) contents <strong>of</strong>
asphaltenes increased on first short term aging only otherwise decreased on further aging. The
relative decrease <strong>of</strong> aromatic hydrogen following aging indicates that the aromatic structures
were getting condensed and substituted. Thus, the increasing ratios <strong>of</strong> aromatic protons <strong>of</strong>
asphaltenes suggest the higher degree <strong>of</strong> aromatic condensations.
Table 3: Percent Distribution <strong>of</strong> Carbon <strong>of</strong> KW Asphaltenes
Obtained by 13C NMR Spectra
TYPE CHEMICAL Percent Distribution <strong>of</strong> Carbon
SHIFT(ppm) KW-F KW-R1 KW-R4 KW-P1 KW-P4
Aliphatic 0-70 Csat 57.5 56.2 59.5 61 57.3
14.1 Cα 3.8 3.6 3.5 3.7 2.5
19.7 CH3-b 4.7 4.7 4.0 4.3 3.8
22.9 Cβ 4.7 5.2 5.2 4.9 5.1
29.7 Cn 12.7 22.9 16.8 18.3 23.6
32. 2 Cγ 9.0 8.9 8.1 7.9 5.1
9.4 9.4 8.1 7.3 9.6
% Straight-chain alkane 30.2 40.6 33.6 34.8 36.3
Average chain length 15.9 22.6 19.2 18.8 29.0
Aromatic 110-160 Car 42.5 43.8 40.5 39 42.7
137-160 Car-alk 14.2 9.9 14.5 14 19.1
9

Table 4: Percent Distribution <strong>of</strong> Hydrogen <strong>of</strong> KW Asphaltenes
Obtained by 1H NMR Spectra
PARAMETERS SAMPLES
KW-F KW-R1 KW-R4 KW-P1 KW-P4
Hsat. 78.9 76.5 81.8 80.0 79.6
Hα 21.1 18.5 23.4 20.0 18.5
Hn 7.0 7.4 9.1 8.0 7.4
Hβ 35.2 35.8 36.4 36.0 37.0
Total Hβ 42.2 43.2 45.5 44.0 44.4
Hγ 15.5 14.8 13.0 16.0 16.7
Har 21.1 23.5 18.2 20.0 20.4
CONCLUSIONS
During oxidation <strong>of</strong> asphalt, aromatization, dehydrogenation and intramolecular hydrogen
bonding <strong>of</strong> polar functional groups increase which lead to formation <strong>of</strong> more asphaltenes [12].
Haley [15] proposed that with free radical reaction taking place during the blowing, more free
radicals will be formed at the higher temperature and more internal cross-linking will occur.
Smith and Schweyer [16,17] measured the air blowing process heat <strong>of</strong> reaction and postulated
that dehydrogenation <strong>of</strong> naphthenic rings was one <strong>of</strong> the main asphalt blowing reactions. The
reactions that occur during thermal and catalytic cracking <strong>of</strong> petroleum residue are quite
complex. The bond scission occurs at certain definite locations in catalytic reactions but
randomly occurs in thermal reaction [18]. There are several possible reactions and mechanisms
following the short-term and long-term aging <strong>of</strong> Kuwait asphalt. Some <strong>of</strong> these reactions might
be condensation with ester formation, polymerization or isomerization, dehydrogenation,
aromatization, and dealkylation as shown in Figure-3.
ACKNOWLEDGEMENT
The facility support provided by the King Fahd University <strong>of</strong> Petroleum and Minerals, Dhahran,
Saudi Arabia is gratefully acknowledged.
10

+
S
S
+
O
*
Figure 3: Possible representation <strong>of</strong> reaction types in a hypothetical asphaltene
structure after RTFO and PAV aging.
S
Aromatization
Dealkylation
Fragmentation
( )
Condensation / Isomerization
Rupture <strong>of</strong> Naphthene Rings
*
+
O
S
(
)
( )
( )
( *
)
11

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