Unraveling the Mechanism of Extractive Desulfurization to Rationally Design Functional Ionic Liquids and Deep Eutectic Solvents: A Comprehensive Review

Fuel desulfurization is of utmost importance to mitigate multiple environmental issues. Ionic liquids with unique adjustable properties have emerged as highly promising solvents for fuel extractants. Nevertheless, it remains challenging to rationally design ionic liquids in view of the extraction mechanism. This review comprehensively discusses the recent progress of research on ionic liquids in extractive desulfurization. The commonly adopted methods to explore the extraction mechanism are summarized, including characterization and computational methods. In terms of different functional groups, ionic liquids are classified into six categories, including imidazolium-, metal-, amino or phosphonium-, polyether glycol-, pyridinium-based, and protic ionic liquids. According to the extraction mechanisms discussed in detail, emphasis is placed on the role of dominant interactions with sulfur-containing compounds, including pi<middle dot><middle dot><middle dot>pi, S-M complexation, C-H<middle dot><middle dot><middle dot>pi, and hydrogen bonding. This review aims to provide a theoretical foundation for designing functionalized ionic liquids by summarizing recent research on the extractive desulfurization mechanism of ionic liquids. More importantly, it will provide theoretical guidance for designing and utilizing functionalized ionic liquids for targeted extractive desulfurization.