Thiophene separation with ionic liquids for desulphurization: A quantum chemical approach

Ionic Liquids (IL's) provide a green alternative for the removal of aromatic sulphur compounds from diesel oil by Liquid–Liquid Extraction (LLE). A total of 24 anions and 11 cations resulting in 264 possible combinations or potential solvents were chosen for this purpose. Continuum solvation models based on the COnductor like Screening MOdel (COSMO) along with its extension to Real Solvents (RS) have been used to directly predict the data. Initially COSMO-RS has been benchmarked in predicting the LLE for 15 reported ternary systems containing thiophene as one of the sulphur derivative. The average root mean square deviation obtained is 8.6%. Thereafter artificial simulated diesel, thiophene and the cation and anion combination was used to predict the capacity (C) and selectivity (S) at infinite dilution. It was found that the lesser size cations have higher selectivity but lower capacity and vice versa. The selectivities varied from: 5.3 to 75.3 and 5.01–35.1 for imidazolium and pyridinium based cations, respectively. For fluorinated anions the thiophene extraction increases with the increase of the van der Waals volume, i.e. [BF4] > [PF6] > [CF3SO3] > [BTA]. Further the Performance Index (PI) values of [MMIM] based cations are higher than that of [OMIM] based IL's. A smaller cation with a sterically shielded large anion gave the highest value of performance index for the removal of thiophene.