1

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 Julio.Facelli@utah.edu

2

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

3

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

4

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

5

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

6

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

7

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.

8

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.

9

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.

10

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.

11

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

12

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

13

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

15

Conformation 7

Conformation 8 (mc)

16

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

19

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.

20

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.

21

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

23

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