A computational study on the steric effects of naphthenic moieties on aggregation interactions of nonconventional petroleum constituents

Abstract

Nonconventional oil is usually heavy and extra heavy crude or light oil that is relatively unstable. This oil contains varying proportions of larger, more aromatic constituents rather than molecules that can be distilled directly into fuels and petrochemicals. We perform density functional theory (ωB97X-D/6-31G(d,p)) calculations to study the contributions of steric effects and dispersion interactions in a series of dimers and trimers of model hydrocarbons containing fused aromatic and cyclohexyl (referred to in the petroleum literature as naphthenic) rings. The aggregation behavior of these molecules is analyzed in terms of the optimized geometry, atomic charges, interaction enthalpy (ΔH), and Gibbs free energy (ΔG 298 ). The ΔH and ΔG 298 values show that all the dimerization and trimerization processes are exothermic, and only a few are spontaneous at 298 K. The naphthenic hydrogen atoms have a key role in the orientation of the monomers in dimer and trimer aggregates. The interaction among naphthenic hydrogen atoms belonging to adjacent monomers causes steric repulsion. The interaction of naphthenic hydrogen atoms with the π-electronic clouds of aromatic rings in adjacent molecules causes attraction. In both cases, the naphthenic hydrogen atoms cause deviation of the monomer from the initial parallel displaced configuration in dimers and trimers. These results reflect the importance of naphthenic rings and their steric interactions in determining the relationship between structures of nonconventional petroleum constituents and their tendency to aggregate and cause fouling.

Molecular Insights into Glass Transition in Condensed Core Asphaltenes

“…As the length of the aliphatic chain increases, the change in specific volume with temperature becomes more prominent. This is again consistent with the conclusions reached by Stoyanov and co-workers using ab initio calculations . These authors suggested that the interaction between molecules with the same number of aromatic rings is stronger for the ones with the largest number of aliphatic groups.…”

“…These results suggest that the glass transition temperature is closely related to the size of the FAR. Using ab initio calculations, Stoyanov and co-workers showed that compounds with a large aromatic cores system have strong aggregation involving π–π stacking . It follows from our previous discussion that the glass transition in asphaltenes is accompanied by breaking of π–π stacking between molecules.…”

Density Functional Theory Study of the Effects of Substituents on the Carbon-13 Nuclear Magnetic Resonance Chemical Shifts of Asphaltene Model Compounds

“…Among the six C aj3 atoms in model compound B, four atoms ( 15 An increase in the number of aromatic rings should lead to stronger aggregation, whereas the presence and arrangement of naphthenic rings tend to weaken the π−π stacking interactions between asphaltene molecules. In model compound A, the C an nuclei are C 27 ,C 26 ,C 20 , and C 16 .…”

“…13 We have also studied the relative contributions of hydrogen-bonding and π−π stacking interactions on asphaltene aggregation in chloroform solvent 14 as well as the contributions of steric effects and dispersion interactions in a series of dimers and trimers of model hydrocarbons containing fused aromatic and cyclohexyl rings. 15 The role of transition metals in the aggregation and visible spectroscopy of asphaltenes was also investigated 16 to address the origin of the black color of bitumen (a very heavy, asphaltene-rich petroleum) and propose approaches to remove transition metals in early stages of heavy oil processing. 17 Studies by Gillet et al 18 and Calemma et al 19 provide important information about the nature of the aromatic rings and the reliability of NMR analysis.…”

Adsorption of Bitumen Model Compounds on Kaolinite in Liquid and Supercritical Carbon Dioxide Solvents: A Study by Periodic Density Functional Theory and Molecular Theory of Solvation

“…38 Such dispersion interactions involving induced dipoles also determine the aggregation of tetralin and other hydrocarbons containing fused aliphatic and aromatic rings. 90 In Figure 1, we present the optimized geometries of the adsorbates on kaolinite. The hydrogen bond between the N atom of phenanthridine and the H atom of the axial OH group of kaolinite is marked with a dashed line.…”