Comparing kinetics and mechanism of adsorption and thermo-oxidative decomposition of Athabasca asphaltenes onto TiO2, ZrO2, and CeO2 nanoparticles

In this study, nC7-asphaltenes adsorption and subsequent catalytic oxidation over three transition metal oxide nanoparticles, namely TiO 2 , ZrO 2 , and CeO 2 were investigated. nC7-asphaltenes studied were obtained from Athabasca vacuum residue using n-heptane. The adsorption equilibrium data of nC7-asphaltenes, dissolved in toluene, fitted very well to the Langmuir adsorption isotherm model. The adsorption behavior of nC7-asphaltenes appeared to change with the type of adsorbent used. It was found that nC7-asphaltenes adsorption at 25 °C is metal oxide specific and the adsorption capacities ranked in the following order: CeO 2 > ZrO 2 > TiO 2 . On the other hand the adsorption affinity followed the following order: CeO 2 > TiO 2 > ZrO 2 . Theoretical calculations of the interaction between the Quinolin-65 (Q-65) (a compound used as a model asphaltene molecule) and some of the metal oxides surfaces were carried out to get more insight into its adsorption behavior. Adsorption energies of Q-65 on the surfaces indicated that there are different adsorption sites on each metal oxide. To confirm the catalytic activity of the selected oxide nanoparticles, the nC7-asphaltenes adsorbed onto the transition metal oxide nanoparticles were subjected to oxidation up to 700 °C in a thermogravimetric analyzer. All tested nanoparticles showed high catalytic activity for post adsorption oxidation. The oxidation kinetics was studied using the Ozawa–Flyn–Wall (OFW) corrected method. The mechanism function of the nC7-asphaltenes oxidation is strongly dependent on the type of nanoparticles. The kinetic oxidation parameters and the transition state thermodynamic functions for the oxidation process of nC7-asphaltenes in the presence and absence of nanoparticles, to the best of our knowledge, are reported for the first time. Among the selected transition metal oxide nanoparticles, CeO 2 appears to have the lowest values of effective activation energy and change in Gibbs free energy of activation (Δ G ‡ ).