Theoretical elucidation of the dual role of [HMIm]BF4 ionic liquid as catalyst and extractant in the oxidative desulfurization of dibenzothiophene

While the oxidative desulfurization (ODS) of aromatic sulfur compounds, such as dibenzothiophene (DBT), in ionic liquids (ILs) has attracted great attention, the role of ILs is still not well elucidated at the molecular level. Focusing on a model system, i.e., the ODS of DBT using H2O2 as oxidant in 1-methylimidazolium tetrafluoroborate ([HMIm]BF4) IL, this work presents a theoretical elucidation of the dual role of IL as a catalyst and an extractant by performing density functional theory calculations. It is found that as a catalyst, both the cation and anion of the IL play important roles in the oxidation process of DBT to dibenzothiophene sulfone (DBTO2): they act respectively, as the donor of one intermolecular hydrogen bond and the acceptor of another intermolecular hydrogen bond to promote the O-O and O-H cleavages of H2O2 and to stabilize transition state structures. With the assistance of the IL, the barriers of two elementary steps involved in the transformation from DBT to DBTO2 are reduced to 13.1 and 23.8 kcal mol(-1) from 32.1 and 37.8 kcal mol(-1), respectively. On the other hand, as an extractant both the cation and anion of IL show stronger interactions with DBTO2 than with DBT, which is attributed to the larger polarity of the former than the latter. Furthermore, calculated interaction energies of the cation-molecule complexes are larger than those of the anion-molecule complexes, indicating that the cation plays a more important role in the extraction and separation process of aromatic sulfur compound from fuels. (C) 2014 Elsevier B.V. All rights reserved.