Temperature-responsive ionic liquids (ILs), their fundanmental behaviors, and catalytic applications were introduced, especially the concepts of upper critical solution temperature (UCST) and lower critical solution temperature (LCST). It is described that, during a catalytic reaction, they form a homogeneous mixture with the reactants and products at reaction temperature but separate from them afterward at ambient conditions. It is shown that this behavior offers an effective alternative approach to overcome gas/liquid-solid interface mass transfer limitations in many catalytic transformations. It should be noted that IL-based thermomorphic systems are rarely elaborated until now, especially in the field of catalytic applications. The aim of this article is to provide a comprehensive review about thermomorphic mixtures of an IL with HO and/or organic compounds. Special focus is laid on their temperature dependence concerning UCST and LCST behavior, including systems with conventional ILs, metal-containing ILs, polymerized ILs, as well as the thermomorphic behavior induced via host-guest complexation. A wide range of applications using thermoregulated IL systems in chemical catalytic reactions as well as enzymatic catalysis were also demonstrated in detail. The conclusion is drawn that, due to their highly attractive behavior, thermoregulated ILs have already and will find more applications, not only in catalysis but also in other areas.

The organic carboxylic acid coordinated monomeric peroxoniobate-based ionic liquids (ILs) [TBA]­[NbO­(OH)2(R)] (TBA = tetrabutylammonium; R = lactic acid (LA), glycolic acid (GLY), malic acid (MA)) were prepared and fully characterized by elemental analysis, NMR, IR, Raman, TGA, 93Nb NMR, and HRMS. These IL catalysts exhibited not only high catalytic activity for the epoxidation of olefins under very mild reaction conditions, as the turnover frequency of [TBA]­[NbO­(OH)2(LA)] reached up to 110 h–1, but also satisfactory recyclability in the epoxidation by using only 1 equiv of hydrogen peroxide as an oxidant. Meanwhile, this work revealed that the ILs underwent structural transformation from [NbO­(OH)2(R)]− to [Nb­(O–O)2(R)]− (R = LA, GLY, MA) in the presence of H2O2 by a subsequent activity evaluation, characterization, and first-principles calculations. Moreover, the organic carboxylic acid coordinated monomeric peroxoniobate-based ILs were investigated using density functional theory (DFT) calculations, which identified that [Nb­(O–O)2LA]− was more advantageous than [Nb­(O–O)2(OOH)2]− for the epoxidation of olefins. Due to the coordination between the α-hydroxy acids and the monomeric peroxoniobate anions, the functionalized ILs can efficiently catalyze the epoxidation of a wide range of olefins and allylic alcohols under very mild conditions. Additionally, the effect of solvents on the reaction is illustrated. It was found that methanol can lower the epoxidation barriers by forming a hydrogen bond with a peroxo ligand attached to the niobium center.

This work reports new kinds of monomeric peroxoniobate anion functionalized ionic liquids (ILs) designated as [A+]­[NbO­(O-O)­(OH)2] (A+ = tetrapropylammonium, tetrabutylammonium, or tetrahexylammonium cation), which have been prepared and characterized by elemental analysis, HRMS, NMR, IR, TGA, etc. With hydrogen peroxide as an oxidant, these ILs exhibited excellent catalytic activity and recyclability in the epoxidation of various allylic alcohols under solvent-free and ice bath conditions. Interestingly, subsequent activity tests and catalyst characterization together with first-principles calculations indicated that the parent [NbO­(O-O)­(OH)2]− anion has been oxidized into the anion [Nb­(O-O)2(OOH)2]− in the presence of H2O2, which constitutes the real catalytically active species during the reaction; this anion has higher activity in comparison to the analogous peroxotungstate anion. Moreover, the epoxidation process of the substrate (allylic alcohol) catalyzed by [Nb­(O-O)2(OOH)2]− was explored at the atomic level by virtue of DFT (density functional theory) calculations, identifying that it is more favorable to occur through a hydrogen bond mechanism, in which the peroxo group of [Nb­(O-O)2(OOH)2]− serves as the adsorption site to anchor the substrate OH group by forming a hydrogen bond, while OOH as the active oxygen species attacks the CC bond in substrates to produce the corresponding epoxide. This is the first example of the highly efficient epoxidation of allylic alcohols using a peroxoniobate anion as a catalyst.

No abstract

No abstract

No abstract

We present here a new class of niobium oxoclusters that are stabilized effectively by carboxylate ionic liquids. These functionalized ILs are designated as [TBA][LA], [TBA][PA], and [TBA][HPA] in this work, in which TBA represents tetrabutylammonium and LA, PA, and HPA refer to lactate, propionate, 3‐hydroxypropionate anions, respectively. The as‐synthesized Nb oxoclusters have been characterized by use of elemental analysis, NMR, IR, XRD, TGA, HRTEM. It was found that [TBA][LA]‐stabilized Nb oxoclusters (Nb−OC@[TBA][LA]) are uniformly dispersed with an average particle size of 2–3 nm and afforded exceptionally high catalytic activity for the selective oxidation of various thioethers. The turnover number with Nb−OC@[TBA][LA] catalyst was over 56 000 at catalyst loading as low as 0.0033 mol % (1 ppm). Meantime, the catalyst also showed the high activity for the epoxidation of olefins and allylic alcohols by using only 0.065 mol % of catalyst (50 ppm). The characterization of 93Nb NMR spectra revealed that the Nb oxoclusters underwent structural transformation in the presence of H2O2 but regenerated to their initial state at the end of the reaction. In particular, the highly dispersed Nb oxoclusters can absorb a large amount of polar organic solvents and thus were swollen greatly, which exhibited “pseudo” liquid phase behavior, and enabled the substrate molecules to be highly accessible to the catalytic center of Nb oxocluster units.

A green water‐soluble cyclodextrin polymer has been prepared through cross‐linking of β‐cyclodextrin (β‐CD) and polyethylene glycol diglycidyl ether (PEGDE). The as‐synthesized co‐polymer (PEG‐CD) showed an excellent ability to stabilize and constrain the size of Ru‐Nanoparticles in aqueous phase, as compared with other polymer stabilizer used in this work such as polyvinyl pyrrolidone (PVP−K30) and polyethylene glycol diglycidyl ether (PEGDE). The co‐ploymer PEG‐CD‐stabilizied Ru NPs have been found to be a highly efficient catalyst in the aqueous hydrogenation of levulinic acid into γ‐valerolactone under mild reaction condition. This catalyst system can be easily extended to the selective hydrogenation of other lignocellulose‐derived platform molecule including 5‐hydroxymethylfurfural, furfural, phenol, 2‐methoxy‐4‐methyl phenol and 2‐methoxy‐4‐ethyl phenol, etc. The further characterization analysis indicated that the small‐sized and highly dispersed Ru NPs (1.2‐2.0 nm) embedded in co‐ploymer PEG‐CD matrix were highly stable for the consecutive catalytic recycles. Additionally, the inclusion interaction of substrate molecules and β‐CD units within polymer matrix played a crucial role in the enhancement of catalytic performance.