Encapsulation of the Sulfur Compounds by Cucurbit[7]uril: A Quantum Chemistry Study

Benzothiophene (BT) and dibenzothiophene (DT) are the most important contaminants in the petroleum derivatives responsible for serious environmental and health problems. Therefore, we have investigated the absorption of these compounds for the first time by considering cucurbit[7]uril (CB[7]) as the host molecule and using the theoretical levels of density functional theory//B3LYP-D3/6-31G(d). BT and DT absorbed into CB[7] do not undergo a significant structural change in the CB[7] structure. The energy gap of the S-compounds@CB[7] in water and hexane solvents was approximately 5 eV, and this large value implies that the complexes have high chemical stability. Moreover, the absorption of the BT and DT into CB[7] in the water and hexane solvents is a favorable process, whereas the lowest binding energy was observed between the dibenzothiophene and CB[7] in the DT@CB[7] complex. The solvation enthalpy shows a preferential solvation of the complexes in water than in hexane solvent. This trend is confirmed by the AIM analysis that shows the highest stability for the DT@CB[7] system with the contribution of cooperative hydrogen bonding. The transfer free energy of S-compounds@CB[7] complexes from hexane to water are -66.12 and -59.56 kcal/mol for BT@CB[7] and DT@CB[7], respectively, implying the spontaneous transference of these complexes from hexane to water solvent. Overall, our results show that the cucurbiturils can be a new class of host molecules to be used in the removal of S-compounds from petroleum derivatives. Finally, a schematic flow diagram of the desulfurization process by cucurbiturils was proposed.