Deciphering structure and aggregation in asphaltenes: hypothesis-driven design and development of synthetic model compounds

Asphaltenes comprise the heaviest and least understood fraction of crude petroleum. The asphaltenes are a diverse and complex mixture of organic and organometallic molecules in which most of the molecular constituents are tightly aggregated into more complicated suprastructures. The bulk properties of asphaltenes arise from a broad range of polycyclic aromatics, heteroatoms, and polar functional groups. Despite much analytical effort, the precise molecular architectures of the material remain unresolved. To understand asphaltene characteristics and reactivity, the field has turned to synthetic model compounds that mirror asphaltene structure, aggregation behavior, and thermal chemistry, including the nucleation of coke. Historically, molecular asphaltene modeling was limited to commercial compounds, offering little illumination and few opportunities for hypothesis-driven research. More recently, however, rational molecular design and modern organic synthesis have started to impact this area. This review provides an overview of commercially available model compounds but is principally focused on the design and synthesis of structurally advanced and appropriately functionalized compounds to mimic the physical and chemical behavior of asphaltenes. Efforts to model asphaltene aggregation are briefly discussed, and a prognosis for the field is offered. A referenced tabulation of the synthetic compounds reported to date is provided.

Non-Covalent π-Stacking Interactions between Asphaltene and Porphyrin in Bitumen

In recent years, the dominant organizing role of non-covalent π-stacking interactions in the association of asphaltenes and porphyrins was criticized and replaced with cooperative forces that are mostly covalent in nature. Here, we show the significant contribution of non-covalent forces in stabilizing the π-stacking of asphaltenes and porphyrins. To understand the binding chemistry of metalloporphyrin-asphaltene, the interaction of nickel octaethylporphyrin with a series of model fragments for asphaltene was studied in two different pathways: axial coordination and π-stacking. Nickel octaethylporphyrin was specifically studied because a main fraction of vanadium and nickel metals in petroleum residues are chelated with porphyrins, and the refining processes in petroleum industries are affected by the significant detrimental impact of these metal compounds. The results of the extended transition state-natural orbital of chemical valence (ETS-NOCV) technique provide strong evidence that the bonding interaction in the π-stacking configuration is much preferred to the axial coordination. Energy decomposition analysis verifies the significant contribution of non-covalent forces in stabilizing the π-stacking of asphaltene-porphyrin, showing that there are other forces driving the formation of asphaltene-porphyrin stacks. Indeed, a non-negligible portion of these stabilizing forces is contributed by strong orbital mixing interactions through charge transfer between active centers; this contribution is mostly overlooked in π-stacking interactions. This matter includes the π-stacking interactions of asphaltene-asphaltene. Isosurfaces of deformation density (Δρ) provide better insights into the π-stacking preference. NOCV deformation densities are delocalized over the entire complex in the π-stacking conformer, leading to the multi-centric charge transfer zone; Δρ isosurfaces of axial coordination are mostly localized on the limited centers involved in chemical bonding.

Reactivity patterns of vanadium(iv/v)-oxo complexes with olefins in the presence of peroxides: a computational study

Vanadium porphyrin complexes are naturally occurring substances found in crude oil and have been shown to have medicinal properties as well. Little is known on their activities with substrates; therefore, we decided to perform a detailed density functional theory study on the properties and reactivities of vanadium(iv)- and vanadium(v)-oxo complexes with a TPPCl8 or 2,3,7,8,12,13,17,18-octachloro-meso-tetraphenylporphyrinato ligand system. In particular, we investigated the reactivity of [VV(O)(TPPCl8)]+ and [VIV(O)(TPPCl8)] with cyclohexene in the presence of H2O2 or HCO4-. The work shows that vanadium(iv)-oxo and vanadium(v)-oxo are sluggish oxidants by themselves and react with olefins slowly. However, in the presence of hydrogen peroxide, these metal-oxo species can be transformed into a side-on vanadium-peroxo complex, which reacts with substrates more efficiently. Particularly with anionic axial ligands, the side-on vanadium-peroxo and vanadium-oxo complexes produced epoxides from cyclohexene via small barrier heights. In addition to olefin epoxidation, we investigated aliphatic hydroxylation mechanisms by the same oxidants and some oxidants show efficient and viable cyclohexene hydroxylation mechanisms. The work implies that vanadium-oxo and vanadium-peroxo complexes can react with double bonds through epoxidation, and under certain conditions also undergo hydroxylation, but the overall reactivity is highly dependent on the equatorial ligand, the local environment and the presence or absence of anionic axial ligands.

A computational scheme of pKa values based on the three-dimensional reference interaction site model self-consistent field theory coupled with the linear fitting correction scheme

A scheme for quantitatively computing the acid dissociation constant, pKa, of hydrated molecules is proposed. It is based on the three-dimensional reference interaction site model self-consistent field (3D-RISM-SCF) theory coupled with the linear fitting correction (LFC) scheme. In LFC/3D-RISM-SCF, pKa values of target molecules are evaluated using the Gibbs energy difference between the protonated and unprotonated states calculated by 3D-RISM-SCF and the parameters fitted by the LFC scheme to the experimental values of training set systems. The pKa values computed by LFC/3D-RISM-SCF show quantitative agreement with the experimental data.