Ionic liquid with acidic properties is an important branch in the wide ionic liquid field and the aim of this article is to cover all aspects of these acidic ionic liquids, especially focusing on the developments in the last four years. The structural diversity and synthesis of acidic ionic liquids are discussed in the introduction sections of this review. In addition, an unambiguous classification system for various types of acidic ionic liquids is presented in the introduction. The physical properties including acidity, thermo-physical properties, ionic conductivity, spectroscopy, and computational studies on acidic ionic liquids are covered in the next sections. The final section provides a comprehensive review on applications of acidic ionic liquids in a wide array of fields including catalysis, CO2 fixation, ionogel, electrolyte, fuel-cell, membrane, biomass processing, biodiesel synthesis, desulfurization of gasoline/diesel, metal processing, and metal electrodeposition.

Acidic Ionic Liquids.

Supported ionic liquids (SILs), which refer to ionic liquids (ILs) immobilized on supports, are among the most important derivatives of ILs. The immobilization process of ILs can transfer their desired properties to substrates. Combination of the advantages of ILs with those of support materials will derive novel performances while retaining properties of both moieties. SILs have been widely applied in almost all of fields involving ILs, and have brought about drastic expansion of the ionic liquid area. As green media in organic catalytic reactions, based on utilizing the ability of ILs to stabilize the catalysts, they have many advantages over free ILs, including avoiding the leaching of ILs, reducing their amount, and improving the recoverability and reusability of both themselves and catalysts. This has critical significance from both environmental and economical points of view. As novel functional materials in surface science and material chemistry, SILs are ideal surface modifying agents. They can modify and improve the properties of solids, such as wettability, lubricating property, separation efficiency and electrochemical response. With the achievements in the field of ILs, using magnetic nanoparticles (MNPs) to SILs has drawn increasing attention in catalytic reactions and separation technologies, and achieved substantial progress. The combination of MNPs and ILs renders magnetic SILs, which exhibit the unique properties of ILs as well as facile separation by an external magnetic field. In this article, we focus on imidazolium-based ILs covalently grafted to non-porous and porous inorganic materials. The excellent stability and durability of this kind of SILs offer a great advantage compared with free ILs and IL films physically adsorbed on substrates without covalent bonds. Including examples from our own research, we overview mainly the applications and achievements of covalent-linked SILs in catalytic reactions, surface modification, separation technologies and electrochemistry.

Imidazolium-based ionic liquids grafted on solid surfaces

Immobilization of polyoxometalate-based sulfonated ionic liquids on UiO-66-2COOH metal-organic frameworks for biodiesel production via one-pot transesterification-esterification of acidic vegetable oils

Immobilized polymeric sulfonated ionic liquid on core-shell structured Fe3O4/SiO2 composites: A magnetically recyclable catalyst for simultaneous transesterification and esterifications of low-cost oils to biodiesel

Ionic liquid transported into metal–organic frameworks

Surface-functionalized magnetic nanoparticle supported dual Bronsted acidic ionic liquid was synthesized by immobilizing 1-(propyl-3-sulfonate) imidazolium hydrogen sulfate [SO3H-(CH2)3-HIM][HSO4] onto the surface of Fe3O4@SiO2 using 3-chloropropyltrimethoxysilane as the “linker”. The structure of the catalyst was characterized by XRD, FT-IR, SEM, TEM, and VSM. The results demonstrated that the Fe3O4 particle was well retained in the silica matrix, presenting a core–shell structure with strong magnetic responsiveness. Meanwhile, the ionic liquid was successfully immobilized on the support. The as-prepared catalyst could disperse well in the reactants and showed good performance in esterification of oleic acid with short-chain alcohols and transesterification of soybean oil. Moreover, the catalyst could be easily separated from products by additional magnetic force without obvious mass loss and maintain satisfactory catalytic activity after being recycled eight times.

Shenqin Lu

Papers

Brønsted Acidic Ionic Liquid Modified Magnetic Nanoparticle: An Efficient and Green Catalyst for Biodiesel Production