Temperature-Responsive Ionic Liquids: Fundamental Behaviors and Catalytic Applications.

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 H2O 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.

A Heterogeneous Metal-Free Catalyst for Hydrogenation: Lewis Acid-Base Pairs Integrated into a Carbon Lattice.

Designing heterogeneous metal-free catalysts for hydrogenation is a long-standing challenge in catalysis. Nanodiamond-based carbon materials were prepared that are surface-doped with electron-rich nitrogen and electron-deficient boron. The two heteroatoms are directly bonded to each other to form unquenched Lewis pairs with infinite π-electron donation from the surrounding graphitic structure. Remarkably, these Lewis pairs can split H2 to form H+ /H- pairs, which subsequently serve as the active species for hydrogenation of different substrates. This unprecedented finding sheds light on the uptake of H2 across carbon-based materials and suggests that dual Lewis acidity-basicity on the carbon surface may be used to heterogeneously activate a variety of small molecules.

Cooperative effects in catalytic hydrogenation regulated by both the cation and anion of an ionic liquid.

The use of transition-metal nanoparticles/ionic liquid (IL) as a thermoregulated and recyclable catalytic system for hydrogenation has been investigated under mild conditions. The functionalized ionic liquid was composed of poly(ethylene glycol)-functionalized alkylimidazolium as the cation and tris(meta-sulfonatophenyl)phosphine ([P(C(6)H(4)-m-SO(3))(3)](3-)) as the anion. Ethyl acetate was chosen as the thermomorphic solvent to avoid the use of toxic organic solvents. Due to a cooperative effect regulated by both the cation and anion of the ionic liquid, the nanocatalysts displayed distinguished temperature-dependent phase behavior and excellent catalytic activity and selectivity, coupled with high stability. In the hydrogenation of α,ß-unsaturated aldehydes, the ionic-liquid-stabilized palladium and rhodium nanoparticles exhibited higher selectivity for the hydrogenation of the C=C bonds than commercially available catalysts (Pd/C and Rh/C). We believe that the anion of the ionic liquid, [P(C(6)H(4)-m-SO(3))(3)](3-), plays a role in changing the surrounding electronic characteristics of the nanoparticles through its coordination capacity, whereas the poly(ethylene glycol)-functionalized alkylimidazolium cation is responsible for the thermomorphic properties of the nanocatalyst in ethyl acetate. The present catalytic systems can be employed for the hydrogenation of a wide range of substrates bearing different functional groups. The catalysts could be easily separated from the products by thermoregulated phase separation and efficiently recycled ten times without significant changes in their catalytic activity.

Synthetic ferripyrophyllite: preparation, characterization and catalytic application.

Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si2O5]2- entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles.

Functionalized poly(ethylene glycol)-stabilized water-soluble palladium nanoparticles: property/activity relationship for the aerobic alcohol oxidation in water.

The preparation, characterization, and catalytic properties of water-soluble palladium nanoparticles stabilized by the functionalized-poly(ethylene glycol) as a protective ligand were demonstrated for aerobic oxidation of alcohols in aqueous phase. UV/vis spectra and X-ray photoelectron spectroscopy (XPS) proved that there was an electronic interaction between the bidentate nitrogen ligand and palladium atoms. Transmission electron microscopy and XPS analysis showed that the particle size and surface properties of the generated palladium nanoparticles can be controlled by varying the amount of protective ligand and the kinds of reducing agents. It was found that both the size and surface properties of palladium nanoparticles played very important roles in affecting catalytic performance. The stabilized metallic palladium nanoparticles were proven to be the active centers for benzyl alcohol oxidation in the present system, and the water-soluble Pd nanocatalysts can also be extended to the selective oxidation of various alcohols.

Ionic liquid-Pluronic P123 mixed micelle stabilized water-soluble Ni nanoparticles for catalytic hydrogenation.

Ionic liquid (1-butyl-2,3-dimethylimidazolium acetate, [BMMIm]OAc)-Pluronic P123 mixed micelle stabilized water-soluble Ni nanoparticles were characterized by UV-vis, XRD, XPS and TEM and then employed for catalytic hydrogenation. It was demonstrated that the mixed-micelle stabilized Ni NPs showed excellent catalytic performance for the selective hydrogenation of CC and nitro compounds in the aqueous phase under very mild reaction conditions, and also the Ni NPs catalysts can be recycled at least for eight times without significant decrease in catalytic activity. The results of characterization revealed that the mixed micelle-stabilized Ni NPs catalysts were highly dispersed in aqueous phases even after five catalytic recycles. In addition, adding ionic liquid ([BMMIm]OAc) can affect the micelle structure of P123 solutions and thus afford an additional steric protection from aggregation of Ni NPs, resulting in enhancing stability and catalytic activity of Ni NPs.

Ionic liquid immobilized nickel(0) nanoparticles as stable and highly efficient catalysts for selective hydrogenation in the aqueous phase.

Nickel nanoparticles (NPs) well-dispersed in the aqueous phase were conveniently prepared by reducing nickel(II) salt with hydrazine in the presence of the functionalized ionic liquid 1-(3-aminopropyl)-2,3-dimethylimidazolium bromide. UV/Vis spectroscopy, elemental analysis, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) show the presence of a weak interaction of the functionalized ionic liquid with Ni(II) and Ni(0) complexes. The face-centered cubic structure of the Ni(0) NPs was confirmed by X-ray diffraction (XRD) characterization. Transmission electron microscopy (TEM) images reveal that smaller Ni(0) particles of approximately 6-7 nm average diameter assemble to give larger, blackberry-shaped particles with an average diameter of around 35 nm. The Ni NPs were employed as highly efficient catalysts for the selective hydrogenation of C=C double bonds in the aqueous phase under mild reaction conditions (40-90 degrees C at 1.0-3.0 MPa), and the Ni(0) nanocatalysts in the aqueous phase are stable enough to be reused at least seven times without significant loss of catalytic activity during subsequent reuse cycles.