Theoretical insight into the catalytic effect of transition metal ions on the aquathermal degradation of heavy oil: A DFT study of cyclohexyl phenyl amine cleavage

Abstract

The recovery of oil deposits strongly depends on the flow characteristics. The degree of oil extraction can be substantially increased using the steam injection method, which can be further enhanced by catalytic upgrading of heavy oil fractions under conditions of aquathermolysis directly in the reservoir with the purpose to reduce viscosity. Transition metal ions, such as Fe(II), Co(II), Ni(II), and Cu(II), are capable of catalyzing the aquathermolysis reactions, but it is essential to better understand the mechanisms and kinetics of the catalytic aquathermolysis for effective selection and molecular design of catalysts for different types of heavy oil. In continuation of our previous theoretical efforts to clarify the catalytic mechanism and kinetics of the aquathermal cleavage of ether and sulfide heteroatomic linkages in heavy oil with use of quantum chemical tools, the thermochemical and kinetic parameters of catalytic aquathermolysis of cyclohexyl phenyl amine in the presence of transition metal ions are calculated by the DFT method based on the B3LYP, ωB97X-D, and M06-2X functionals. An analysis of the thermochemistry and kinetics of the aquathermolysis of model compound N-cyclohexyl aniline shows that the most probable mechanism of the aquathermal degradation proceeds through the heterolytic cleavage of the C–N bond with formation of cyclohexyl carbocation. According to the DFT calculation results, hydroxonium ions and transition metal ions, such as Fe(II), Co(II), and Ni(II), can substantially reduce the kinetic energy barrier of C–N bond breakage and, thereby, catalyze the aquathermal degradation of the model amine compound. However, the copper(II) ion stands out from other transition metals under study by stronger coordination bonds with amines, which leads to the self-poisoning of this catalyst by a reaction product, namely, aniline that is formed as a result of the aquathermolysis reaction. The catalytic effects of the hydroxonium ion and the iron(II), cobalt(II), and nickel(II) ions are comparable to each other and can be arranged in the following sequence in decreasing order of their catalytic strength: Ni2+ ≈ Co2+ > H3O+ > Fe2+.