Density functional theory investigation of the effect of axial coordination and annelation on the absorption spectroscopy of nickel(II) and vanadyl porphyrins relevant to bitumen and crude oils

• Published in: Canadian journal of chemistry Vol. 91; no. 9; pp. 872 - 878
• Main Authors: Stoyanov, Stanislav R, Yin, Cindy-Xing, Gray, Murray R, Stryker, Jeffrey M, Gusarov, Sergey, Kovalenko, Andriy
• Format: Journal Article
• Language: English
• Published: Ottawa NRC Research Press 01-09-2013; Canadian Science Publishing NRC Research Press
• Subjects: Absorption spectroscopy; annelation; annélation et coordination axiale; Atomic absorption spectroscopy; axial coordination; bitume; bitumen; Bituminous materials; Crude oil; Density functionals; déplacement de la bande de Soret; Heavy metals; Hydrocarbons; metalloporphyrin; métalloporphyrine; Nickel; Optimization; Petroleum; Porphyrins; Properties; Soret band shift; Spectrum analysis; théorie de la fonctionnelle de la densité dépendant du temps; time-dependent density functional theory; Ultraviolet radiation; Vanadium; Canada
• ISSN: 0008-4042; 1480-3291
• DOI: 10.1139/cjc-2012-0532

The vanadium and nickel components in heavy oils and bitumen are important impurities in catalytic processing and form aggregates with other asphaltene components. Metalloporphyrins are commonly analyzed using the characteristic Soret band in the UV–vis absorption spectrum. However, the Soret band of metalloporphyrins in petroleum is broadened and weaker than expected based on the concentration of Ni and V in heavy oils and the extinction coefficients of isolated porphyrins. We hypothesize that the low intensity and broadening of the Soret band could be due to axial coordination of the metal center or fusion (annelation) of aromatic rings on the porphyrin π-system. This hypothesis is examined using the density functional theory for geometry optimization and time-dependent density functional theory (TD-DFT) for calculation of excited states of nickel(II) and vanadyl porphyrins with axially coordinated ligands and annelated polyaromatic hydrocarbons. Predictions of the excited electronic states performed using the tandem of TD-DFT and conductor-like polarizable continuum model of solvation support this hypothesis and provide insight into the extent of Soret band broadening and intensity decrease due to coordination and annelation. These computational results, validated with respect to visible absorption spectra, are important for understanding asphaltene aggregation and spectroscopic characterization and suggest methods for removal of transition metals from heavy oil.