Adsorption of Bitumen Model Compounds on Kaolinite in Liquid and Supercritical Carbon Dioxide Solvents: A Study by Periodic Density Functional Theory and Molecular Theory of Solvation

Abstract

The geometry of phenanthridine, benzothiophene, tetralin, and naphthalene representative of the heterocyclic, naphthenic, and aromatic components of bitumen adsorbed on kaolinite is optimized using density functional theory and periodic boundary conditions in gas phase. These bitumen model compounds preferentially adsorb on the aluminum hydroxide surface of kaolinite with energy decreasing in the order phenanthridine > naphthalene > tetralin ∼ benzothiophene. The adsorption of phenanthridine is strengthened by hydrogen bonding between the pyridinic N atom and an axial hydroxyl group of kaolinite, while the rest of the molecules adsorb through van der Waals interactions. The mechanism of solvation in CO₂ and the effect of liquid and supercritical CO₂ on the adsorption thermodynamics are studied using the three-dimensional reference interaction site model theory with the closure approximation of Kovalenko and Hirata (3D-RISM-KH) molecular theory of solvation at 293–333 K and 10–30 MPa. The CO₂ solvent interacts with the aluminum hydroxide surface of kaolinite by hydrogen bonding, with the pyridinic N atom of phenanthridine by electrostatic interactions, and with the rest of the bitumen model compounds by hydrophobic interactions, as inferred from the 3D site density distribution functions of CO₂. The molecule–kaolinite potentials of mean force in CO₂ show that the adsorption of naphthalene and tetralin on kaolinite is substantially weakened as the pressure is increased and the temperature is decreased. Benzothiophene adsorption is the least sensitive to CO₂ temperature and pressure changes. In liquid CO₂ at 30 MPa and 293 K, the hydrocarbon molecules are weakly adsorbed and can be desorbed by CO₂, while the heterocycles would remain adsorbed, suggesting an approach for extraction of deasphalted bitumen from oil sands. While the most favorable thermodynamic conditions for desorption are in liquid CO₂, the kinetic barrier for desorption is the most sensitive to small changes in the temperature and pressure in supercritical CO₂, indicating that supercritical conditions are important for desorption rate control. These results suggest that the investigated bitumen components can be selectively desorbed from kaolinite by controlling the temperature and pressure of the CO₂ solvent and agree with experimental reports on heavy oil recovery. These insights are valuable for the development of improved techniques for extraction of bitumen from oil sands and deasphalting of bitumen using liquid and supercritical CO₂