Ionic Liquids for olefin extractive separation from fluid catalytic cracking naphtha: Thermodynamics and molecular mechanisms

This study investigates the extractive separation of olefins from fluid catalytic cracking (FCC) naphtha using ionic liquids (ILs), from the perspectives of molecular thermodynamics and separation mechanisms. Initially, a representative binary mixture of benzene and 1-hexene was employed to screen various ILs for their separation performance, based on COSMO-RS calculations combined with relevant physicochemical property evaluations. Consequently, 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) was identified as the most promising extractant. Subsequently, the liquid-liquid equilibrium (LLE) behavior of ternary systems involving the selected IL and the benchmark solvent sulfolane (SUL) was quantitatively predicted using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). In this modeling framework, ILs were treated as electrically neutral components due to the significant ion-pair electrostatic interactions in non-aqueous media, thus, the IL-related mixed systems are named as “complex strong electrostatic-hydrogen bonding organic systems”. Finally, quantum chemical (QC) calculations were conducted to elucidate the underlying molecular-level separation mechanism. The results indicate that [EMIM][SCN] exhibits significantly stronger intermolecular interactions with the organic components than SUL, thus accounting for its superior extraction performance. These findings provide valuable insights for the rational design of task-specific ILs for efficient olefin separation in petroleum refining processes.