modeling of asphaltenes: assessment of sensitivity of 13c
TRANSCRIPT
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Modeling of Asphaltenes: Assessment of Sensitivity of 13C SSNMR to Molecular
Structure
Shyam Badu,† Ian S.O. Pimienta,†,& Anita M. Orendt,† Ronald J. Pugmire,‡
and
Julio C. Facelli†,#,*
†Center for High Performance Computing and ‡Departments of Chemical and Fuels
Engineering and #Biomedical Informatics, University of Utah, Salt Lake City, UT 84112
and & Department of Chemistry & Physics, Troy University, Troy, Al 36082
Abstract
This paper presents calculations of 13C SSNMR spectra of model asphaltenes. The
overall goal of this work is to assess how valuable 13C SSNMR studies of asphaltenes can
be in guiding the development of representative 3D (three dimensional) models of
asphaltenes. The calculations were done using 3D models based on previously published
2D (two dimensional) models. The calculated spectra show overall agreement with the
existing data and the results show that the 13C SSNMR spectra of model asphaltenes are
quite sensitive to both the 2D and the 3D structures, indicating that this property can be
used to guide further model development.
* Corresponding author at:
THE UNIVERSITY OF UTAH 155 S 1452 E RM 405 SALT LAKE CITY UT 84112-0190 e-mail [email protected]
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Introduction
Asphaltenes are an important constituent of many oils and one of the major
components in heavy oils sources for liquid hydrocarbon fuels. With light oil sources
rapidly depleting, it is necessary to use alternative sources from heavy oil reservoirs.
Refining these heavy oil resources presents new challenges and the study of the detailed
molecular composition and structure of asphaltenes is becoming of great importance for
optimizing the entire refinery process.1
At present, the structure of asphaltenes is not fully understood and only a few
validated 3D (three dimensional) representative structures of asphaltenes have been
published in the literature.2 Three dimensional models representative of asphaltene
structures are a necessary requirement to use advance computational modeling techniques
to address asphaltene chemistry and aggregation during extraction, transportation and
refinement.
Atomistic modeling is routinely used in many industries (pharmaceutical,
polymers, coatings, explosives, membrane proteins, etc.) to gain insight into the
properties of materials.3 Using 3D molecular models it becomes possible to calculate
molecular properties that can be correlated with experimental data obtained from solid
and liquid state 13C nuclear magnetic resonance (NMR) spectroscopy, atomic pair-wise
distribution functions, thermo gravimetric analysis (TGA) data on pyrolysis kinetics,
small angle X-ray scattering (SAXS) and ion cyclotron resonance mass spectroscopy
(ICR-MS), etc. Calculated properties that are sensitive to the 3D models and the
underlying 2D chemical models used in their construction can be used as guideposts in
the validation of the proposed models. The application of these techniques to the
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elucidation of asphaltene structures has been limited to a few applications reported in
Ref. 2. Among them it is noteworthy to mention the recent work by Ruiz-Morales and
Mullins to understand UV spectra and fluorescence of asphaltenes.4-10
Solid state NMR (SSNMR) has been used to characterize complex materials such
as coals,11-16 but there are very few applications of SSNMR to asphaltenes. The effective
use of 13C SSNMR in the characterization of asphaltenes must be predicated on the
intrinsic sensitivity of 13C SSNMR spectra to the structure of these materials, a fact that
has not yet been established in the literature. In this paper we calculate 13C SSNMR
spectra of model asphaltene structures with the overall goal of assessing how valuable
13C SSNMR studies of asphaltenes can be in guiding the development of representative
3D models of asphaltenes. We accomplish this by analyzing the sensitivity of the 13C
SSNMR spectra to: i) the 2D chemical model used to develop the 3D model, ii) the
different thermally accessible 3D conformations for the same 2D model and iii) the
aggregation of asphaltenes.
Methods
To test the sensitivity of 13C SSNMR spectra to the 2D chemical model, Siskin’s
2D models for six different asphaltenes (Fig. 1),17 namely Campana, Mid-Continent US,
San Joaquin Valley, Loydminster Wainwright, Maya, and Heavy Canadian, were used to
build 3D models using the molecular builder tool in the HyperChem suite of programs.18
Each of these models was then optimized using GAMESS19 at the restricted Hartree-Fock
(RHF/STO-3G)20 level of theory to generate the initial 3D models. In each of the
asphaltene structures the rotation about the bonds of the flexible bridging group
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connecting the aromatic unit with the aliphatic unit was explored. For the six model
asphaltene structures we performed multiple optimizations from different initial
structures to determine structures as close to the global minima as possible.
To test the sensitivity of 13C NMR spectra to the conformation of the 3D
molecular model seven additional conformations of the Mid-Continent US asphaltene
were generated using molecular mechanics method, starting with the 3D model of the
Mid-Continent US asphaltene obtained previously. The MM+ force field21 as
implemented in the program Hyperchem18 was used, with the 3D model of a single unit
of the Mid-Continent US asphaltene in a cubical box of dimension 30Å×30Å×30Å; the
model was annealed at different physical conditions by varying the temperature and
simulation times. This process was repeated as necessary in order to obtain seven
different configurations.
To test the effect of stacking and aggregation of asphaltenes, 3D three-unit
models of the Mid-Continent US asphaltene were built by stacking the units in parallel,
antiparallel and inverted configurations, starting with the original single unit 3D model.
These configurations were optimized by using the functional PBE1PBE22 with the 6-
311G basis set23 and the Gaussian09 program.24 Additionally, a larger cluster of 11 units
of single mc asphaltene was assembled in such way that the units were close enough to
allow interaction between them. To eliminate any obvious geometry flaws this cluster
was partially optimized for few steps using the STO-3G basis set23 with the PBE1PBE
functional in Gaussian 09.
Calculation of the NMR chemical shielding was performed for all the 3D models
developed here (pdb files with their molecular structures are available in the
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supplemental material) using the Gaussian09 program suite24 at the DFT/GIAO level of
theory, with the PBE1PBE functional and the 6-311G basis set (4-31G basis set for the 11
unit model). When using structures previously optimized only with the MM+ force field,
a local optimization at the STO-3G level was performed before the NMR calculation to
correct for systematic errors due to the C-H bond distances.25, 26 The calculated chemical
shielding values were converted to chemical shift values on the tetramethylsilane (TMS)
scale using to the shielding calculation of methane at the same level of theory, 199.3 ppm
(200.52 ppm for the 4-31G calculations in the 11 unit model), adjusted by -7 ppm (shift
of dilute methane on TMS scale).27 A Gaussian broadening of 2 ppm for aliphatic and 7
ppm for aromatic was added to the chemical shifts to obtain the calculated spectrum.
To compare the experimental and calculated SSNMR spectra the maximum
intensity of the calculated spectra was adjusted to coincide with the one for the
experimental one. To estimate the quality of the agreement the correlation coefficient
between the intensities of calculated and experimental spectra was calculated over all the
4096 points in the spectra. The values of the correlation coefficients are given in the
figures below.
Results and Discussion
Fig. 2 depicts the 3D models obtained from all the 2D chemical structures of
asphaltenes proposed by Siskin et al.17 The figures correspond to the final structures
obtained after several optimization attempts and correspond to representative structures
of the global minima. However, as discussed below they can be considered neither the
global minimum nor the only possible 3D structure. Fig. 3 depicts the six 13C SSNMR
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spectra corresponding to the 3D models from Fig. 2. It is apparent from the figure that
the 13C SSNMR spectra of the model is quite sensitive to the 2D chemical model used for
the construction of the 3D model, supporting the notion that the comparison of these
spectra with experimental ones can be of use to guide the refinement of the chemical
group make up of the 2D models.
The only asphaltene for which the 13C SSNMR spectrum is available for
comparison is the Mid-Continent US exemplar.17 Therefore this asphaltene was chosen
for more detailed analysis. As explained in the method section seven different 3D
models were constructed using simulated annealing methodology starting from the Mid-
Continent US 3D structure. These 3D models, including the original one, are depicted in
Fig. 4 and their energies (from single point calculations using the PBE1PBE functional
and the 6-311G basis set) are given in Table 1. Comparison of the energies in the table
and the 3D structures in the figure clearly shows that there is quite a large structural
variation within the energy range considered here, indicating that in actual samples of
asphaltenes numerous conformations may be present. Fig. 5 compares the experimental
13C SSNMR spectrum for a Mid-Continent US sample with the spectra obtained for the
different 3D models and shows that there are significant changes in many spectral
features among the spectra of the different 3D models. All the simulated spectra show
overall agreement with the experimental spectrum, but it is apparent that the calculated
spectra all show more diversity among the chemical shifts as well as more definitive
features that the experimental one, especially in the aliphatic region. The latter of these
can be attributed to conformation averaging in the experimental sample; the effects of
conformational averaging can be approximated on a small scale by averaging the spectra
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of the seven conformations. Fig. 6 presents this comparison; while this averaging
improves the agreement with the experiment, the aliphatic region of the experimental
spectrum is still considerably narrower than the average one, with a high percentage of
aliphatic carbons with chemical shifts in the vicinity of 40 ppm; this point will be
discussed further below. The left shoulder observed in the aromatic region of the
experimental spectra but absent in the calculated ones may be attribute to the lack of
bighead carbons in the mc model.28
Aggregation of asphaltenes has been documented in the literature2 and it may be a
plausible explanation for the disagreement between experimental and simulated spectra in
Fig. 6. As explained in the methods section, three 3D trimer and one 11 mc unit models
of the Mid-Continent US asphaltene structure were analyzed in this work. The trimer
structures are depicted in Fig. 7, their total energies are in Table 1, and their calculated
13C SSNMR spectra are shown in Fig. 8 with the one for the 11 unit model. As in the
case of the monomer 3D models, there is a considerable structural variation observed
among these three trimer structures within a narrow energy range. The simulated spectra
in Fig. 8 show less features than the ones corresponding to single units, and slightly better
agreement with experiment. No significant improvements are observed for the spectra of
the 11 unit model. The average spectrum of the three trimer models is compared with the
experimental spectrum in Fig. 9, showing better agreement with the experiment than the
spectra of the individual; note particularly the break point at higher chemical shift values.
However the simulated spectra all still do not reproduce the dominance of chemical shifts
at about 40 ppm in the aliphatic region observed in the experimental spectrum.
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One factor that could lead to the greater chemical shift diversity in the aliphatic
region observed in the models is that the 2D models were built with all of the aliphatic
carbons intentionally separated from the aromatic cores for illustrative purposes,17 with
methyl groups placed about the aromatic core to account for the proper number of
substituted aromatic carbons. In order to explore the effect of this choice on the
simulated 13C SSNMR spectrum, an alternate model for the Mid-Continent US asphaltene
was built (designated mc2) with the same aromatic core but with the aliphatic carbon
content divided among several substituent locations. This model still fits all of the
criteria presented in the original paper. The new model, shown as both the 2D and 3D
structures in Fig. 10, was optimized and the 13C SSNMR spectrum obtained is shown in
Fig. 11 along with the experimental spectrum and the simulated spectrum from the
original mc model. As can be seen from this figure, the line shape of the aliphatic region
is narrower and has the peak intensity at about 40ppm, resulting in improved agreement
with the experimental spectrum in this region.
It is, however, still surprising that all of the simulated 13C SSNMR spectra are
broader than the experimental Mid-Continent US spectrum due to the fact that the
simulated spectra are based on a single representative chemical unit, whereas the
experimental sample is expected to have a diversity of chemical structures with different
chemical formulas represented. An additional source of uncertainty to consider is the
different experimental conditions for the NMR experiments used to develop the 2D
models and the SSNMR used here for comparison.
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Conclusions
We have developed 3D molecular structures that are representative of the
expected average molecular structures in asphaltenes. These structures are developed
using standard molecular modeling tools using existing 2D chemical models of
asphaltene as starting point. While these structures can be considered as representative,
they are by no means unique. Similar conclusions have been obtained from our studies in
trimer models of asphaltenes. The 13C SSNMR spectra calculated using all the 3D
models developed show great sensitivity to the 2D chemical structure in which they are
based as well as to the 3D molecular conformation and aggregation. In general all the
simulated spectra reproduce the overall features of the existing spectrum for a Mid-
Continent US sample, but average spectrum of multiple plausible conformations appears
to provide a better agreement especially for the average of trimer structures. These
results, however, do confirm our working hypothesis that 13C SSNMR spectra of
asphaltenes can be used as a very effective guide post to developed reliable 3D models of
asphaltenes.
Acknowledgments
This work has been partially supported by the United States Department of Energy,
National Energy Technology Laboratory Award DE-FE0001243 and by an allocation of
computer time from the Center for High Performance Computing at the University of
Utah.
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Table 1: Calculated (Hartree) and relative (kJ/mol) energies1 for the different
conformations of the single unit and configurations of the trimer stacks for the Mid-
Continent US asphaltene. The structures are depicted in Figs. 4 and 7 and are given as
pdb files in the supplementary material.
Single Asphaltenes
Hartree kJ/mol2
Conformation 1 -3828.54265 0.00 Conformation 2 -3828.51841 63.63 Conformation 3 -3828.51532 71.75 Conformation 4 -3828.53730 14.06 Conformation 5 -3828.52775 39.11 Conformation 6 -3828.52220 53.68 Conformation 7 -3828.53643 16.33
Conformation 8 (mc) -3828.50989 86.01
Trimer Stacks
kJ/mol2 Binding Energy3 (kJ/mol) Parallel Stack 1.55 214.08
Anti-parallel Stack 4.31 211.20 Inverted Stack 0.00 215.69
1Energies are from DFT calculations using the PBE1PBE functional and the 6-311G basis set. 2Energies relative to the lowest energy structure of the group. 3Binding energies are relative to the free monomers.
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Figure 1. 2D chemical structures of asphaltenes used as the starting point for the 3D
models developed in this project. From Siskin et al.17
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Figure 2. Globally optimized 3D structures of the asphaltene 2D chemical structures
from Fig. 1.
Campana
Mid-Continent US
San Joaquin Valley
Loydminster Wainwright
Maya
Heavy Canadian
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Figure 3. Comparison of calculated 13C SSNMR spectra for the six different asphaltene
models1 obtained from the Siskin’s 2D chemical models.
1mc=Mid-Continent US, c=Campana, hc= Heavy Canadian, lw=Lyodminster Wainright,
m= Maya, and sj=San Joaquin Valley.
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Figure 4. The eight 3D conformations of the Mid-Continent US asphaltene obtained by
simulated annealing. Note that Conformation 8 is the original Mid-Content US structure
from Fig. 1.
Conformation 1
Conformation 3
Conformation 5
Conformation 2
Conformation 4
Conformation 6
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Conformation 7
Conformation 8 (mc)
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Figure 5. Comparison of calculated 13C SSNMR spectra for the eight different
conformations of the Mid-Continent US asphaltene model (Fig. 4). The experimental
spectrum from Siskin et al.17 The correlation coefficients with the experimental spectra
are: 0.9013, 0.8455, 0.8498, 0.8164, 0.8138, 0.8474, 0.7949, 0.9099, for models 1-7
and mc, respectively.
17
Figure 6. Average 13C SSNMR spectrum of the eight conformations of a single unit of
Mid-Continent US asphaltene model. Experimental spectrum from Siskin et al.17 The
correlation coefficient with the experimental spectra is: 0.8616.
18
Figure 7. The 3D structures of the three different stacking configurations of three units
of Mid-Continent US asphaltene considered in this study.
Parallel Stack
Inverted Stack
Antiparallel Stack
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Figure 8. 13C SSNMR spectra for the three different stacking configurations of Mid-
Continent US asphaltene considered in this study. Experimental spectrum from Siskin et
al.17 The correlation coefficients with the experimental spectra are: 0.9156, 0.9186,
0.9172, 0.8626, for the parallel, antiparallel, inverted and to 11 unit models, respectively.
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Figure 9. Average 13C SSNMR spectrum for the three different stacking configurations
of three units of Mid-Continent US asphaltene considered in this study. Experimental
spectrum from Siskin et al.17 The correlation coefficient with the experimental spectra is:
0.9181.
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Figure 10. Alternate model (mc2), shown both as 2D and 3D, for Mid-Continent US
asphaltene.
22
Figure 11. 13C SSNMR spectrum for the alternate Mid-Continent US asphaltene (mc2)
from Fig. 10 compared with both the experimental spectrum and the equivalent spectrum
obtained when using the original model (mc).17 Experimental spectrum from
Siskin et al.17 The correlation coefficients with the experimental spectra are: 0.9099,
0.7891, for the mc and mc2 models, respectively
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