Imidazolium based ionic liquid-phase green catalytic reactions

Abstract

1,3-Dialkyl imidazolium salts are amongst the most versatile family of ionic liquids (ILs) for catalytic processes, acting as a solvents, supports or modifiers given their anisotropic-like structural organisation, as well as their modular physical–chemical properties (thermal chemical stability, basicity, hydrophilicity, viscosity, etc.). Regardless of the catalyst's nature (i.e. transition-metal complexes, nanoparticles, enzymes and classical heterogeneous), the IL forms a barrier surrounding the catalyst and modifies the geometric and electronic properties of the catalyst or serves as a physical–chemical barrier “controlling” the access of the substrates and the eviction of the products. The miscibility of substrates and products, hydrophobicity, ionicity and basicity of the IL can be modulated by the proper choice of the imidazolium alkyl side chain and anions, allowing fine-tuning of the catalytic system, even for straightforward product separation processes. Alternatively, the poly ILs (PILs) and supported IL phases (SILP) on solid carriers also offer the possibility to design greener catalytic tools for various transformations since they facilitate transposing from batch to continuous flow conditions. ILs also enable the fabrication of devices using additive manufacturing to generate advanced and complex geometries. They also provide various methods to separate the products from the catalytic mixture by simple decanting, extraction with supercritical fluids or using external stimuli such as, for example, pressure, temperature, or magnetic induction. This review presents the properties of catalyst/substrates/products in bare, poly and supported ILs that can be used to help readers to select the adequate IL and derivatives for the desired catalytic system.