Leveraging π-Conjugation in Benzimidazolium Ionic Liquids for Selective Extraction of Bicyclic Aromatic Hydrocarbons: A Combined Experimental and Computational Study

Selective separation of high-value bicyclic aromatic hydrocarbons (BAHs) from complex hydrocarbon mixtures is essential for enhancing the product value and utilization efficiency of the energy industry. Although COSMO-RS facilitates high-throughput screening of ionic liquids (ILs), the absence of a robust structure–performance relationship (SPR) limits the molecular-level design for separating weakly polar, structurally similar hydrocarbons. In this work, 1-ethyl-3-methylbenzimidazolium bis(trifluoromethylsulfonyl)imide ([EMBzIM][TF2N]) was identified as a promising extractant via COSMO-RS solubility predictions. Its extraction performance was systematically evaluated using liquid–liquid equilibrium experiments with both a representative four-component model oil and real hydrocarbon samples. The results indicate that [EMBzIM][TF2N] exhibits superior performance, achieving the highest distribution coefficient (0.5051) and selectivity (204.70) for aromatics at an aromatic concentration of 10 wt %. Furthermore, it demonstrated a stable extraction efficiency and facile regeneration over five consecutive cycles. Molecular polarity index calculations, interaction energy analysis, visualization of weak intermolecular interactions, and 1H NMR characterizations were employed to elucidate the molecular-level mechanism for this performance. The van der Waals interactions, significantly enhanced by the extended π-conjugation of the benzimidazolium cation, predominantly govern the extraction of aromatics. This study establishes a rational design strategy for ILs, integrating a COSMO-RS thermodynamic prediction with chemically interpretable SPR to achieve efficient separation of BAHs from hydrocarbon streams.