Evaluating the hybridization effect of [BMIM][PF6] ionic liquid and PEG-based DES on extractive oxidative desulfurization: From statistical optimization to antagonistic behaviors

The interactions between ionic liquids (ILs) and deep eutectic solvents (DESs) in dual-solvent systems remain poorly understood, despite their extensive, separate applications in extractive-oxidative desulfurization (EODS). This study investigated the synergistic/antagonistic effects of a system consisting of [BMIM][PF 6 ] IL and PEG200/TsOH-based DES on dibenzothiophene removal from a model fuel. Using response surface methodology-central composite design, the effects of four key variables, including IL/DES volumetric ratio, V S /V F ratio, oxidant-to‑sulfur molar ratio (n(H 2 O 2 )/n(S)), and reaction time, were modeled. The results showed that pure DES achieved a maximum efficiency of 97.33% under the optimal conditions (V S /V F = 0.547, n(H 2 O 2 )/n(S) = 4.7, and time = 1 h), while pure IL recorded an efficiency of 93.4% (V S /V F = 0.6, n(H 2 O 2 )/n(S) = 5, and time = 2.5 h). Findings reveal nonlinear and antagonistic behavior, with the IL/DES volume ratio identified as the dominant parameter. Mixing these two solvents triggered a strong antagonistic effect, such that at a volume ratio of IL/DES = 0.67 (optimum condition: V S /V F = 0.2, n(H 2 O 2 )/n(S) = 4.89, and time = 2.29 h), the efficiency dropped to 37.1%. This ~60% performance reduction is visible in graphical analyses as a saddle-shaped depression in the desulfurization percentage versus IL/DES-V S /V F ratio plot, indicating the perturbation and rearrangement of the DES hydrogen-bonding environment upon IL incorporation. By demonstrating the reduction effects in hybrid systems, this study emphasizes the necessity of precise control of solvent-solvent interactions in the design of multicomponent systems.