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

In recent years, the dominant organizing role of non-covalent pi-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 pi-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 pi-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 pi-stacking configuration is much preferred to the axial coordination. Energy decomposition analysis verifies the significant contribution of non-covalent forces in stabilizing the pi-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 pi-stacking interactions. This matter includes the pi-stacking interactions of asphaltene-asphaltene. Isosurfaces of deformation density (Delta rho) provide better insights into the pi-stacking preference. NOCV deformation densities are delocalized over the entire complex in the pi-stacking conformer, leading to the multi-centric charge transfer zone; Delta rho isosurfaces of axial coordination are mostly localized on the limited centers involved in chemical bonding.