(214u) Asphaltene Structural Models Validated By Density Functional Theory

Asphaltenes – a major component of heavy petroleum – are a complexe mixture of mainly high molecular weight organic compounds, containing polyaromatic moieties tethered with aliphatic side chains. Asphaltenes are defined as the fraction of petroleum soluble in aromatic solvents (e.g., toluene) and insoluble in paraffinic ones (e.g., n-heptane).

Asphaltenes cause serious problems of precipitation during oil production and poisoning of catalyst during refining. These problems are particularly serious in countries like Brazil, where major oil reserves lie deep under the ocean, and Canada, where they are extracted from Oil Sands.

The chemical complexity of the asphaltene fraction of petroleum makes the elucidation of the structures of asphaltene molecules very difficult. Several analytical methods (e.g., NMR, XRD) have been employed to characterize asphaltene fractions from different petroleum samples.

Since the molecular structure of asphaltenes remains elusive, models that represent average asphaltene fractions, such as quantitative molecular representations [1] and synthetic model compounds [2,3] have been developed and employed to study the behavior and properties of asphaltenes.

In previous studies, we calculated the association energy of representative structures of asphaltenes, using molecular dynamics simulations in the presence of a solvent [4].

The objective of the current project is to validate a representative model of asphaltenes developed based on a correlation between experimental NMR and computational results on the association behavior of a series of reference model compounds [4].

The calculations have been performed using the ωB97X-D density functional with an augmented damped R^-6 dispersion correction term and the basis set 6-31G(d,p), as implemented in the Gaussian 09 computational chemistry software suite [5].

The reference model compounds are hexabenzocoronene (HBC), hexa(n-hexyl)-HBC (C6-HBC), and pyrene (P). Experimental results [2,6] indicate that HBC or C6-HBC molecules are associated even in polar solvents, whereas pyrene do not associate. The energy of dimerization of the reference models will be compared with our previous models (A, B, C, and D) [4]. The dimerization energy is used as an association parameter.

The dimerization energy values calculated for HBC and pyrene show that these asphaltene models are associated in a vacuum. We intend to correlate the experimental and computational results and define a dimerization energy limit so as to predict the occurrence of dimerization.

The correlation shows that structures with a gain in dimerization energy over 20 kcal/mol are associated according to the experiment, whereas those with lower dimerization energy values are either weakly associated or do not associate.

References:

[1] Boek E.S. et al. (2009) Energy Fuels, 23 (3), 1209–1219.

[2] Tan X. et al. (2008) Energy & Fuels 22, 715-720.

[3] Costa L.M. et al. Phys. Chem. Chem. Phys. 2012, 14, 3922–3934;

[4] Oliveira, J.S.C. et al. (2010) Poster Presentation at the 11th International Conference on Petroleum Phase Behaviour and Fouling, PetroPhase, New York, NY, USA.

[5] Frisch M.J. et al. (2009) Gaussian 09, Revision A.1.

[6] Akbarzadeh K. et al. (2005) Energy & Fuels 19, 1268-1271.