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A comprehensive characterization of asphaltenes by FTICR MS precipitated under different n-alkanes solvents Marcos Albieri Pudenzi, Jandyson Machado Santos, Alberto Wisniewski Jr, and Marcos Nogueira Eberlin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02262 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018
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A comprehensive characterization of asphaltenes
2
by FT-ICR MS precipitated under different n-
3
alkanes solvents
4
Marcos Albieri Pudenzi*‡, Jandyson Machado Santos‡, Alberto Wisniewski Jr.†, Marcos
5
Nogueira Eberlin‡
6
‡
7
UNICAMP, Campinas, São Paulo, 13083-970, Brazil
8
†
9
000, Brazil
ThoMSon Mass Spectrometry Laboratory, Institute of Chemistry, University of Campinas,
Federal University of Sergipe, Department of Chemistry, São Cristóvão, Sergipe, 49100-
10 11
ABSTRACT
12
Asphaltenes are still problematic fractions and their composition is not fully unveiled as it
13
relates to the original crude oil and the precipitation method. In this work, the composition of
14
asphaltenes precipitated with n-heptane and n-pentane from ten crude oils from the Sergipe-
15
Alagoas Basin were assessed by Ultra-High Resolution Fourier Transform Ion Cyclotron
16
Resonance Mass Spectrometry (FT-ICR MS) using five ionization procedures (ESI(+) with
17
formic acid, ESI(-) with ammonium hydroxide, ESI(-) with tetramethyl ammonium
18
hydroxide, and APPI(+) and APPI(-)) in combination with a 7.2T LTQ FT Ultra Thermo
19
Fisher spectrometer. The purpose was to evaluate compositional differences between C5- and
20
C7-asphaltenes according to the adopted ionization methodologies. Asphaltenes were
21
compared as a function of heteroatomic composition and aromaticity. Chemometrics
22
strategies were employed to evaluate the heteroatom classes of higher variation among the
23
groups. Ions that were shared by the asphaltenes and the crude oils, and between the
24
ionization methods were also evaluated for similarity. Results showed that n-heptane
25
asphaltenes are more aromatics than n-pentane asphaltenes; which in turn were found to be
26
more acidic and to display a higher heteroatomic complexity. Principal component analysis
27
(PCA) and the evaluation of shared ions showed that APPI(±) is able to detected asphaltene
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1
ions directly from the whole crude oil. It was also found that the use of different ionization
2
methods with and without additives is fundamental to obtain more comprehensive chemical
3
profiles in terms of classes and their constituents for crude oils and their asphaltenes, and that
4
the specific n-alkane used to precipitate asphaltenes leads to different chemical profiles.
5 6
INTRODUCTION
7
The high dependency of fossil fuels has increased the use of heavy crude oils in the
8
production of fuels and petrochemical derivatives, but the problems caused by the heavier
9
fractions of petroleum, which these oils contain on greater amounts than light oils, are
10
becoming critical.[1] Those heavy fractions can cause the formation of emulsions, clogging
11
and deposition, which affects directly the oil production chain. Analyzing these heavy
12
fractions at the molecular level is not a trivial task. As they show low volatility, high
13
tendency of aggregation and high molecular mass (>450 Da), are not amenable to detection
14
by standard gas chromatography methodologies.[2,3] Nevertheless, ultra-high resolution mass
15
spectrometry has been successfully used for the analysis of either whole heavy crude oils[4-6]
16
or their fractions[7-8] through a petroleomics approach.
17
Petroleomics can be defined as the comprehensive analysis at the molecular level of crude
18
oils, fractions and derivatives to relate chemical composition to physical and chemical
19
properties.[1,9] This field of research has mainly relied on ultra-high resolution mass
20
spectrometry (UHRMS), especially on Fourier transform ion cyclotron resonance mass
21
spectrometry (FT-ICR MS), with eventually other analytical techniques being used to add
22
more comprehensiveness to the field, such as GCxGC-MS[2,3], Orbitrap-MS [12,13]
[10,11]
and ion
23
mobility MS.
Whilst having some disadvantages such as the insensitivity to low mass
24
compounds and traditional biomarkers and strong difficulties to perform quantitative
25
determinations of individual components, the highest resolution and mass accuracy provided
26
by FT-ICR MS make it the most commonly employed mass analyzer for petroleomics
27
assessments. The ionization sources, however, have varied considerably and different types
28
of oil constituents from a wide range of polarities can be focused depending of the chosen
29
technique. Electrospray ionization (ESI)[14] and atmospheric pressure photoionization
30
(APPI)[15] have been the most commonly employed, since they preferentially ionize polar and
31
mid polar compounds, respectively. Analyses that employ ESI and APPI FT-ICR MS result,
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therefore, on comprehensive and detailed data on the mid to high polar heteroatom
2
composition of oils that have been shown to enable deeply understanding of crude oil
3
properties.[16,17]
4
The ultra-high resolution and high mass accuracy m/z values obtained by FT-ICR MS enable
5
attribution of molecular formulae to ions with high confidence levels, and from these
6
molecular formulae it is possible to visualize class distribution via selective plots. One of the
7
most important of such plots is the class chart, which sums the relative abundances of
8
compounds having the same heteroatom numbers in order to facilitate comparisons between
9
the heteroatomic content of different crude oils. Another very important plot is the one that
10
relates carbon number (Cn) versus double bond equivalents (DBE) as:
11
DBE= C - (H/2) + (N/2) + 1
12
where C=number of carbons, H=number of hydrogens and N=number of nitrogen atoms in
13
the molecular formula
14
DBE vs Cn plots allow organizing constituents of the same heteroatomic class as a function
15
of carbon number and unsaturation/cyclization levels. Thus, it is possible to track how the
16
aromaticity and cyclization level of the oil vary in terms of molecular weight as measured by
17
Cn. Such trends have been shown to be directly related and to provide reliable information on
18
a diverse number of geochemical properties.[6,18]
19
In the petroleomics field, many different applications have been explored, but the analysis of
20
asphaltenes is still a great challenge, both on the analytical and on data interpretation
21
fronts.[19-21] Asphaltenes are defined as the n-alkane insoluble and toluene soluble fraction of
22
crude oils. Their definition is, therefore, not directly related to their chemical composition,
23
but simply to their solubility. Due to a complex chemical composition, aggregation tendency,
24
low solubility and high boiling points, the challenges to characterize asphaltenes at the
25
molecular level are great and their actual composition is still a matter of debate.[19-21] It is
26
generally accepted, however, that asphaltenes are highly aromatic compounds, with several
27
heteroatoms in their composition and few and small alkyl chains.[19-21] [19, 21, 22]
Asphaltenes from
, and eventually other alkanes
[23-25]
28
crude oils are often precipitated using n-heptane
29
but the chemical composition of these precipitates have never been compared via FT-ICR MS
30
petroleomic approaches. Gu and collaborators[23], using mostly physicochemical assays and
31
bulk spectroscopy analysis, have compared asphaltenes precipitated by using three different
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1
n-alkanes, concluding on highest aromaticity and MW for n-heptane precipitation when
2
compared to propane and n-pentane. For that work, however, the authors did not characterize
3
the asphaltenes at the molecular level of high-resolution mass spectrometry techniques.
4
Crude oils from the Sergipe-Alagoas Basin (SEAL) were selected due to their unique
5
intrinsic characteristics[4], such as high amounts of C7-asphaltenes (10%), aromatics (20%),
6
high acidity (overall TAN>5 mg KOH g-1) and variable API gravity (16-30 °API (Table
7
1)).[4] These unique set of characteristics make these oils interesting samples to FT-ICR MS
8
petroleomic investigation since: a) the range of API gravity suggests crude oils with high
9
content of polar, heavier compounds that are more easily detected with atmospheric pressure
10
ionization techniques, especially ESI; b) the high level of acidity suggests high concentration
11
of oxygenated compounds such as phenols and naphthenic acids, which are easily detected by
12
ESI(-) and APPI(+) FT-ICR MS and have been shown to function as geochemistry markers
13
for oil migration and maturation,[25,26] c) the high aromaticity makes the samples prompt to
14
APPI(±) ionization; d) the large amount of asphaltenes aids on getting precipitates with a
15
more comprehensive content of chemicals. Here, the composition of asphaltenes obtained by
16
precipitation with n-heptane and n-pentane are assessed by FT-ICR MS using ESI(+) with
17
formic acid, ESI(-) with ammonium hydroxide, ESI(-) with tetramethylammonium
18
hydroxide, APPI(+), and APPI(-) as ionization methods for a more comprehensive analysis of
19
the effects produced by n-alkane precipitants on asphaltene composition.
20 21
EXPERIMENTAL SECTION
22
Samples. Ten crude oil samples from different wells, which represent the total area of
23
the Sergipe Block at the Sergipe-Alagoas Basin were kindly provided by Petroleo Brasileiro
24
S.A. (Petrobras). They were received in glass vials from the Sergipe-Alagoas Exploration and
25
Production Operations Unit (UO-SEAL) and were labeled as UO-SEAL01-10, which will be
26
shortened to UO01-10 in this work (Table 1).
27
Asphaltene precipitation. Asphaltene precipitation was performed through a
28
modification of ASTM D3279-12 methodology.[27] Briefly, 40 mL of the n-alkane of choice
29
(namely n-pentane or n-heptane) was added stepwise (4×10 mL) to 250 mg of the crude oil
30
sample to obtain the corresponding C5-asphaltenes (Asp_Pent) or C7-asphaltenes (Asp_Hept).
31
The oil-solvent mixtures were shortly stirred, and centrifugated during 5 min at 2500 rpm
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rotation frequency in an FANEM Excelsa® (II) 206BL centrifuge, obtaining the asphaltenes
2
by precipitation. The precipitated asphaltenes was washed three more times in the same
3
condition described above to eliminate possible contaminants and maltenes residues. The
4
asphaltenes were dried for 6 hours on stove and had their masses determined gravimetrically.
5
All solvents were used as received. The mass of the asphaltenes and API gravity of each oil
6
are shown on Table 1. Sample UO01 presented a higher Asp_Hept content when compared to
7
Asp_Pent, opposite the trend of the other samples. The °API seems to be not related with
8
asphaltene content in this case[4], probably due to the oil source material and/or following
9
alteration, as reported by Evdokimov.[28,29]
10 11
Table 1. Amount of precipitated asphaltenes and API gravity for each crude oil. The content
12
of asphaltenes are shown in m/m percentage relative to the whole crude oil mass SAMPLE NAME
C5-Asphaltenes,
C7-Asphaltenes,
(% m/m)
(% m/m)
°API
UO01
27.0
12.85
18.64
UO02
30.0
6.00
2.47
UO03
24.0
12.87
10.22
UO04
16.4
13.14
7.11
UO05
27.0
8.53
3.93
UO06
23.5
11.80
5.07
UO07
22.4
11.92
7.58
UO08
23.4
12.78
5.54
UO09
22.7
12.23
4.52
UO10
23.3
15.16
5.83
13 14
Petroleomics analysis by FT-ICR MS. MS analysis was performed using a 7.2T
15
LTQ FT Ultra mass spectrometer (Thermo Scientific, Bremen, Germany) equipped with
16
IonMax ESI and APPI ionization sources operated on both positive and negative ion modes.
17
For ESI analysis 1 mg of the crude oil or asphaltenes was diluted on 1 mL of a 1:1(v/v).
18
methanol:toluene mixture. To improve the ionization efficiency, some additives were added:
19
for positive ion mode, formic acid (HForm) and for negative ion mode ammonium hydroxide
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Another
set
of
samples
for
ESI(-)
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1
(NH4OH).
analysis
was
prepared
using
2
tetramethylammonium hydroxide (TMAH). All additives were added in methanol at 0.1% v/v
3
concentration prior to mixing with samples diluted in toluene. For APPI analysis, samples
4
were diluted only in toluene to reach final concentrations of 1 mg mL-1 for crude oils and of 2
5
mg mL-1 for asphaltenes.
6
Ionization conditions were set as follows: source voltage 3.6 kV for positive ion mode
7
and -3.1 kV for negative ion mode, tube lens voltage at ±160 V, capillary temperature at 280
8
°C and syringe flow of 5 µL min-1 for ESI and 20 µL min-1 for APPI. Spectra were acquired
9
on 200-1000 Da mass range with 400,000@400 resolving power for ESI and 750,000@400
10
for APPI. A hundred scans were averaged for each analysis. Data treatment was carried out
11
through homemade software PetroMS[30] and the principal component analyses (PCA) were
12
conducted using Pirouette 3.11 (Infometrix, Inc., Woodinville, WA) with mean-centered data
13
of relative abundance of heteroatom classes. Several PCAs were performed to investigate the
14
ionization behavior of the crude oils and their asphaltene fractions: one PCA was performed
15
for each of the five ionization strategies and another PCA was performed using the combined
16
data for all ionization strategies. Classes that were not present in some samples were
17
considered to have zero of intensity in those samples for the PCA analysis.
18
The results were first compared inside each ionization strategy, to understand the
19
ionization behavior of the crude oil and their types of asphaltenes, and then overall compared.
20 21
RESULTS AND DISCUSSION
22
ESI(+) FT-ICR MS
23
Figure 1 shows the ESI(+) FT-ICR mass spectra for the UO04 crude oil (a) and its
24
asphaltenes precipitated in n-pentane (c) or n-heptane (e). Attributed ions are marked with
25
color dots, whereas the DBE x Cn plots are shown in (b) and in (d), respectively. It is
26
observed that the MS profile of asphaltenes is drastically different from that of crude oil, with
27
a significant decrease of absolute intensity. as additive in ESI(+) of Asp_Pent provided lower
28
relative intensity for the N1 class and higher relative intensity of oxygen and sulfur
29
containing classes (NxOx, Ox and OxSx) in comparison to the crude oil. The detection of
30
more diverse classes in Asp_Pent can be explained by a signal-enhancement effect produced
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by the extraction of major N1 class compounds during the asphaltene washing. In crude oils,
2
the presence of N1 compounds usually suppress the ESI(+) ionization of NxOx, Ox, and
3
OxSx compounds, and so reducing their absolute intensity. This finding suggests that the N1
4
class compounds found in crude oil are less aromatic when compared to these oxygen and
5
sulfur compounds appearing in Asp_Pent, since N1 compounds are soluble in the n-alkane
6
solvent whereas NxOx, Ox and OxSx compounds are not. This characteristic was confirmed
7
by the DBE vs Cn plots (b) and (d), which show a higher dispersion on Cn for crude oil, in
8
contrast to Asp_Pent plots that show compounds mainly in low Cn regions for each DBE.
9
The remaining N1 compounds that appears in Asp_Pent are also highly aromatic and in less
10
number than those in crude oil. Remarkably, no attributed ions resulted from the Asp_Hept
11
analysis using formic acid in ESI(+) (Figure 1 (e)), indicating a much lower basicity of the
12
compounds that precipitate with n-heptane as compared to n-pentane. Similar trends as those
13
seen in Figure 1 were also observed for all other samples. ESI(+) FT-ICR MS analysis
14
showed, that constituents from both asphaltene display quite low basicity, which is more
15
pronounced when n-heptane is used (Asp_Hept, Figure 1(e)), mainly due to their high
16
aromatic stability as shown by the DBE trends which makes protonation unfavorable.
17
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1 2 3
Figure 1. (a) ESI(+) FT-ICR mass spectra for the UO04 crude oil, and their asphaltenes precipitated in (c) n-pentane and (e) n-heptane using formic acid (ESI_HForm) as additive. DBE versus carbon number plots for the N1 class of (b) crude oil and (d) Asp_Pent.
4 5
ESI(-) FT-ICR MS.
6
For ESI(-), two additives were evaluated separately: NH4OH that favors naphthenic
7
acids (O2 class) ionization or TMAH that favors carbazols (N1 class). Figure 2 shows
8
representative ESI(-) FT ICR mass spectra and plots for the crude oil UO05 sample , and its
9
Asp_Pent and Asp_Hept using NH4OH as additive. The decrease of ion absolute intensities
10
from crude oil to asphaltenes was also observed, but for ESI(-), the Asp_Hept analysis
11
resulted in attributed ions, when no ions could be attributed using ESI(+), (Figure 1(e)).
12
Asp_Pent spectrum displayed overall higher intensity and greater variety of the oxygen and
13
sulfur containing classes compared to crude oil with a subtle decrease of the O2 class. For
14
most samples, the number of O2 class ions increases from crude oil to Asp_Pent and, even
15
so, the relative intensity of this class diminishes. This trend is the result of the lower ESI(-)
16
ionization efficiency of the O2 compounds present in asphaltenes when compared to those in
17
the crude oil.
18
Asp_Hept displays lower variety of classes and lower intensity of the O2 class as compared
19
to Asp_Pent. This trend shows that in the crude oil, most O2 class compounds have low
20
aromaticity, being more soluble in n-heptane as compared to n-pentane.
21
The increase in oxygen and sulfur containing classes (NxOx, Ox and OxSx) from crude oil to
22
asphaltenes was also noted in ESI(-) using TMAH, but in this case, Asp_Hept showed lower
23
complexity of heteroatomic classes when compared to Asp_Pent (Figure S2 in Supporting
24
Information for the UO06 sample). Lower absolute intensities were also found in the
25
asphaltenes when compared to the crude oil. On the other hand, the CH class ionized by ESI(-
26
) in crude oil is absent in both asphaltenes, revealing the low aromaticity and polarity of this
27
CH class, being soluble in both n-alkanes. Concerning the N1 class, the compounds with
28
highest intensity were concentrated on higher DBE for each carbon number when moving
29
from crude oil to Asp_Pent and finally to Asp_Hept. This trend shows the higher aromaticity
30
of Asp_Hept (Figure S3). The O2 class also displayed lower intensity, but did not concentrate
31
on the high-DBE-low-carbon-number regions as the N1 class. This result suggests that highly
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aromatic carboxylic acids are either absent or have low ionization efficiencies in ESI(-) using
2
TMAH as additive (Figure S3).
3
Another important fact is that the trends of the O2 and N1 classes observed in sample UO06
4
were also noticed in the rest of the analyzed samples. This result strongly suggests that
5
asphaltene acidity is mainly due to pyrrolic derivatives rather than carboxylic acids and that
6
the best ESI (-) method for asphaltene analysis is, therefore, to use TMAH as the additive
7
since it favors the ionization of the N1 class. These findings also show that, if present, the
8
carboxylic group can be found on asphaltenes in multifunctional molecules as indicated by
9
the higher abundances of the OxSx and NxOx classes.
10 11 12 13
Figure 2. (a-c) Mass spectra and (d-f) class charts of UO05 for ESI(-) using NH4OH as additive. Crude oil plots are shown on (a) and (d), Asp_Pent on (b) and (e) and Asp_Hept on (c) and (f).
14 15
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APPI(+) FT-ICR MS
1 2
APPI(+) FT-ICR MS provided overall low intensity spectra for asphaltenes (Figure S4), with
3
heteroatomic class composition varying little between crude oil and their asphaltenes. As
4
Figures 3(a-c) show, the major differences were noted for the N1O1, N1O2 and O1 classes,
5
which
6
oil
