Computational and Experimental Investigations of the Role of Water and Alcohols in the Desorption of Heterocyclic Aromatic Compounds from Kaolinite in Toluene

Abstract

Nonaqueous extraction is an attractive alternative to the currently employed warm water process for extraction of bitumen from oil sands, as it could use less energy and water. Hydroxylated cosolvents, such as alcohols, that compete for the adsorptive clay surfaces and help release bitumen components could help improve bitumen recovery. The water naturally present in oil sand also affects oil–mineral interactions. Electronic structure methods and the statistical-mechanical 3D-RISM-KH molecular theory of solvation as well as experimental desorption measurements are employed to study the effects of water and aliphatic alcohol cosolvents in toluene solvent on the desorption of fused pyridinic heterocycles (ArN) from kaolinite. The geometries of phenanthridine and acridine (representative of pyridinic heterocycles of petroleum asphaltenes) adsorbed on the kaolinite clay surface are optimized in periodic boundary conditions using density functional theory. The 3D-RISM-KH method is employed to calculate the solvation free energy and potential of mean force for adsorption of the heterocycles on kaolinite in pure and alcohol-containing toluene. The potentials of mean force show that the adsorption of the fused pyridines on kaolinite is stronger in pure toluene than in toluene mixed with aliphatic alcohol. Analysis of the mechanism of desorption of phenanthridine and acridine from kaolinite in toluene containing alcohol reveals that the alcohol stabilizes both the pyridinic moiety and kaolinite platelet by hydrogen bonding, thus disrupting the ArN···HO–Al(kaolinite) hydrogen bond. A mechanism for retention of toluene on kaolinite is also highlighted. Experimental studies of the desorption of fused pyridines from an ArN–kaolinite aggregate show that in water-saturated toluene the rate of desorption of the phenanthridine from kaolinite is twice as high as that in dry toluene. The experimental and computational results show that water and aliphatic alcohols in toluene help desorb pyridinic heterocycles from kaolinite, a clay mineral abundant in the oil sands. The presented insights are valuable for understanding the molecule-clay interactions in solution and relevant to improving the nonaqueous extraction of bitumen from oil sand.