Computational Investigations Complement Experiment for a System of Non-Covalently Bound Asphaltene Model Compounds

The nanoaggregation of asphaltenes is an important and poorly-understood field at the juncture of petrochemistry, analytical chemistry, and computational chemistry. As- phaltene precipitation from crude oils and bitumens, and subsequent deposition in and on equipment causes plugged or constricted pipelines, coking and fouling on heaters and heat exchangers, and inactivation of catalysts, to enormous economic and environ- mental expense. However, the mechanisms behind the aggregation and precipitation of asphaltenes are still poorly understood. In this paper, the aggregation of asphal- tene model compounds has been explored using a combination of density functional tight-binding (DFTB) and density functional theory (DFT), in a manner that revisits a prior experimental study. The model compounds investigated include a porphyrin with an acidic side chain, and a three-island archipelago compound with pyridine as the central island, and pyrene for the outer islands. The possible stoichiometries and conformations for complexes were explored and compared to the experimental results. Our results show that there are four possible complexes involving these two model compounds with large (K>1000) equilibrium constants of formation, which will exist in competition with each other. We find that both hydrogen bonding and π−π stacking are important to this aggregation. On the other hand, neither water-mediated aggrega- tion nor coordination to open porphyrin sites was found to be significant, despite some suggestions in the literature that these might be important. The multiple possible stoichiometries of complexes confound some of the analysis done in the experimental paper, as Job plots assume that only one complex is present. Gibbs free energies of association were determined for various complexes, with and without microhydration, at the ωB97X-V/def2-QZVPP//ωB97X-D4/def2-SVP level of theory. We also briefly explore some of the factors influencing the change in NMR chemical shift for select nuclei measured in the experimental paper.