Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Cation structure-dependence of the induced free charge density gradient in imidazolium and pyrrolidinium ionic liquids

We report on the structure-dependence and magnitude of the induced free charge density gradient (ρ_f) seen in room-temperature ionic liquids (RTILs) with imidazolium and pyrrolidinium cations. We characterize the spatially-resolved rotational diffusion dynamics of a trace-level cationic chromophore to characterize ρ_f in three different pyrrolidinium RTILs and two imidazolium RTILs. Our data show that the magnitude of ρ_f depends primarily on the alkyl chain length of RTIL cation and the persistence length of ρ_f is independent of RTILs' cation structure. These findings collectively suggest that mesoscopic structure in RTILs plays a significant role in allowing charge density gradients to form.

Ionic liquid surfactants as multitasking micellar catalysts for epoxidations in water

Surface-active imidazolium tungstate ILs are multitask epoxidation catalysts in water using H₂O₂ as oxidant. The micelles solubilize olefins in the aqueous phase and catalyze the epoxidation. Organophosphonic acids greatly increase the reaction rate.

Determination of the Critical Micelle Concentration of Imidazolium Ionic Liquids in Aqueous Hydrogen Peroxide

The critical micelle concentrations (CMCs) of several imidazolium-based ionic liquids (ILs) in aqueous H₂O₂ (50 wt % in H₂O) were determined by tensiometry, conductometry, and the rate of catalytic epoxidation of cis-cyclooctene. CMC values in aqueous H₂O₂ were significantly lower compared to values in pure water. In both H₂O₂ solution and water, the CMC of all ILs decreases with an increasing alkyl chain length and increases with a rising temperature. The degree of micelle ionization of 1-methyl-3-octylimidazolium tetrafluoroborate ([OMIM][BF₄]) was calculated by conductometry in a temperature range of 22-70 °C.

Surface-Active Ionic Liquids in Catalytic Water Splitting

We report the application of surface-active ionic liquids as ligands and optional reaction media in iridium-catalyzed water oxidations. Three novel catalysts with N,N-dialkylimidazolidin-2-ylidene ligands based on amphiphilic imidazolium ionic liquids were synthesized and characterized. Excellent turn-over frequencies of up to 0.92s−1 were obtained in catalytic water splitting, and activity was maintained for five consecutive catalytic cycles, with an overall turn-over number of 8967. The addition of external surface-active ionic liquid showed unexpected behaviour, because strongly enhanced initial reaction rates were observed.