How to select ionic liquids as extracting agents systematically: a special case study for extractive denitrification processes

Extractive denitrification (EDN) of shale oil using ionic liquids (ILs) as the extracting agent has good industrial prospects. In such processes, ILs with higher selectivity to N-compounds and lower solubility in shale oil are desired to improve the EDN efficiency, and reduce the loss of ILs and the contamination of shale oil. In the present study, we employed COSMO-RS to calculate the selectivity of 70 ILs to the typical N-compounds (pyridine, quinoline and indole). The influence of the IL structural characteristics, composition of shale oil and properties of N-compounds are investigated from a micro-level view with the σ-surface and σ-profile. The selectivity strongly depends on anionic species and it is greatly influenced by hydrogen bonding (HB) and π–π interaction between N-compounds and ILs. ILs composed of [H2PO4]− and [MeSO3]− with larger HB donor energy show higher selectivity to the basic N-compounds, while ILs composed of [Ac]− with larger π-electron cloud density show higher selectivity to the non-basic N-compounds. Anions with stronger polarity have lower solubility in shale oil. Moreover, experimental determinations of EDN indicated that [C4py][H2PO4]/[C4mim][H2PO4] and [C2mim][Ac]/[C2py][Ac] have good EDN performance for quinoline/pyridine with efficiency of 100% and for indole with efficiency of 91%, respectively. This work presents a theoretical basis to design and select ILs having higher selectivity for N-compounds and lower solubility in shale oil for use in denitrification.