Selective CO₂ Hydrogenation to Formic Acid with Multifunctional Ionic Liquids

The development of simple, cost-effective, and sustainable methods to transform CO₂ into feedstock chemicals is essential to reduce the dependence of the chemical industry on fossil fuels. Here, we report the selective and efficient catalytic hydrogenation of CO₂ to formic acid (FA) using a synergistic combination of an ionic liquid (IL) with basic anions and relatively simple catalysts derived from the precursor [Ru₃(CO)₁₂]. Very high TON (17000) and TOF values have been observed, and FA solutions with concentrations of up to 1.2 M have been produced. In this system, the imidazolium-based IL associated with the acetate anion acts as a precursor for the formation of the catalytically active Ru–H species, as a catalyst stabilizer, and as an acid buffer, shifting the equilibrium toward free formic acid. Moreover, the IL acts as an entropic driver (via augmentation of the number of microstates), lowering the entropic contribution imposed by the IL surrounding the catalytically active sites. The favorable thermodynamic conditions enable the reaction to proceed efficiently at low pressures, and furthermore the immobilization of the IL onto a solid support facilitates the separation of FA at the end of the reaction

Ru-Catalyzed Estragole Isomerization under Homogeneous and Ionic Liquid Biphasic Conditions

The isomerization of estragole to <i>trans</i>-anethole is an important reaction and is industrially performed using an excess of NaOH or KOH in ethanol at high temperatures with very low selectivity. Simple Ru-based transition-metal complexes, under homogeneous, ionic liquid (IL)-supported (biphasic) and “solventless” conditions, can be used for this reaction. The selectivity of this reaction is more sensitive to the solvent/support used than the ligands associated with the metal catalyst. Thus, under the optimized reaction conditions, 100% conversion can be achieved in the estragole isomerization, using as little as 4 × 10^–3 mol % (40 ppm) of [RuHCl­(CO)­(PPh₃)₃] in toluene, reflecting a total turnover number (TON) of 25 000 and turnover frequencies (TOFs) of up to 500 min^–1 at 80 °C. Using a dimeric Ru precursor, [RuCl­(μ-Cl)­(η³:η³-C₁₀H₁₆)]₂, in ethanol associated with P­(OEt)₃, a TON of 10 000 and a TOF of 125 min^–1 are obtained with 100% conversion and 99% selectivity. These two Ru catalytic systems can be transposed to biphasic IL systems by using ionic-tagged P-ligands such as 1-(3-(diphenylphosphanyl)­propyl)-2,3-dimethylimidazolium bis­(trifluoromethanesulfonyl)­imide immobilized in 1-(3-hydroxypropyl)-2,3-dimethylimidazolium bis­(trifluoromethanesulfonyl) imide with up to 99% selectivity and almost complete estragole conversion. However, the reaction is much slower than that performed under solventless or homogeneous conditions. The use of ionic-tagged ligands significantly reduces the Ru leaching to the organic phase, compared to that in reactions performed under homogeneous conditions, where the catalytic system loses catalytic performance after the second recycling. Detailed kinetic investigations of the reaction catalyzed by [RuHCl­(CO)­(PPh₃)₃] indicate that a simplified kinetic model (a monomolecular reversible first-order reaction) is adequate for fitting the homogeneous reaction at 80 °C and under biphasic conditions. However, the kinetics of the reaction are better described if all of the elementary steps are taken into consideration, especially at 40 °C

Surface Composition/Organization of Ionic Liquids with Au Nanoparticles Revealed by High-Sensitivity Low-Energy Ion Scattering

High-sensitivity low-energy ion scattering (HS-LEIS) analysis was used to elucidate the outermost layer of both functionalized and non-functionalized imidazolium ionic liquids (ILs). The IL outermost layer is composed of all atoms of both cations and anions. The HS-LEIS analyses also allow for quantitative measurement of the thickness of IL overlayers on Au nanoparticles prepared by sputter deposition, which was shown to be a monolayer of ions, as predicted by density functional theory calculations

Organocatalytic Imidazolium Ionic Liquids H/D Exchange Catalysts

Simple 1,2,3-trialkylimidazolium cation associated with basic anions, such as hydrogen carbonate, prolinate, and imidazolate, is an active catalyst for the H/D exchange reaction of various substrates using CDCl₃ as D source, without the addition of any extra bases or metal. High deuterium incorporation (up to 49%) in acidic C–H bonds of ketone and alkyne substrates (p<i>K</i>_a from 18.7 to 28.8) was found at room temperature. The reaction proceeds through the fast and reversible deuteration of the 2-methyl H of the imidazolium cation followed by D transfer to the substrate. The IL acts as a neutral base catalyst in which the contact ion pair is maintained in the course of the reaction. The basic active site is due to the presence of a remote basic site in the anion namely, OH of bicarbonate, NH of prolinate, and activated water in the imidazolate anion. Detailed kinetic experiments demonstrate that the reaction is first order on the substrate and pseudozero order relative to the ionic liquid, due to the fast reversible reaction involving the deuteration of the ionic liquid by the solvent

Selective Carbon Dioxide Hydrogenation Driven by Ferromagnetic RuFe Nanoparticles in Ionic Liquids

CO₂ is selectively hydrogenated to HCO₂H or hydrocarbons (HCs) by RuFe nanoparticles (NPs) in ionic liquids (ILs) under mild reaction conditions. The generation of HCO₂H occurs in ILs containing basic anions, whereas heavy HCs (up to C₂₁ at 150 °C) are formed in the presence of ILs containing nonbasic anions. Remarkably, high values of TONs (400) and a TOF value of 23.52 h^–1 for formic acid with a molar ratio of 2.03 per BMI·OAc IL were obtained. Moreover, these NPs exhibited outstanding abilities in the formation of long-chain HCs with efficient catalytic activity (12% conversion) in a BMI·NTf₂ hydrophobic IL. The IL forms a cage around the NPs that controls the diffusion/residence time of the substrates, intermediates, and products. The distinct CO₂ hydrogenation pathways (HCO₂H or FT via RWGS) catalyzed by the RuFe alloy are directly related to the basicity and hydrophobicity of the IL ion pair (mainly imposed by the anion) and the composition of the metal alloy. The presence of Fe in the RuFe alloy provides enhanced catalytic performance via a metal dilution effect for the formation of HCO₂H and via a synergistic effect for the generation of heavy HCs

Tunable Ionic Control of Polymeric Films for Inkjet Based 3D Printing

Inkjet printing is a powerful additive manufacturing (AM) technique to generate advanced and complex geometries. However, requirements of low viscosity and surface tension are limiting the range of functional inks available, thus hindering the development of novel applications and devices. Here, we report a method to synthesize materials derived from highly viscous or even solid monomers in a simple, flexible fashion and with the potential to be integrated in the printing process. Polymerizable ionic liquids (PILs) have been employed as a proof of principle due to the broad range of properties available upon fine-tuning of the anion-cation pair and the high viscosity of the monomers. The method consists of the deposition and polymerization of a PIL precursor, followed sequentially by quaternization and anion metathesis of the films. The fine control over the mechanical and superficial properties of inkjet printable polymeric films of neutral and cationic nature by postpolymerization reactions is demonstrated for the first time. A family of different polycationic materials has been generated by modification of cross-linked copolymers of butyl acrylate and vinyl imidazole with liquid solutions of functional reagents. The variation in the mechanical, thermal, and surface properties of the films demonstrates the success of this approach. The same concept has been applied to a modified formulation, designed for optimal inkjet printing. This work will pave the way for a broad range of applications of inkjet printing, with a plethora of anion–cation combinations characteristic of PILs, thus enormously broadening the range of applications available in additive manufacturing

Vapors from Ionic Liquids: Reconciling Simulations with Mass Spectrometric Data

The species involved in the distillation of aprotic ionic liquids are discussed in light of recent simulations and mass spectrometric data obtained by various techniques. New mass spectrometric data collected via laser-induced acoustic desorption and the thermal desorption of ionic liquids are also presented as well as additional DFT calculations. The available evidence of theoretical simulations and mass spectrometric data suggests that the distillation of ionic liquids occurs mainly via neutral ion pairs of composition C_<i>n</i>A_<i>n</i> [C⁺ = cation and A^– = anion], followed by gas-phase dissociation to lower order ion pairs and then dissociation of hot CA to C⁺ and A^–, followed by ion/molecule association events to give [C_<i>n</i>A_<i>n</i>–1]⁺ or [C_<i>n</i>–1A_<i>n</i>]⁻ ions to a degree that depends on the amount of internal energy deposited into the neutral C_<i>n</i>A_<i>n</i> clusters upon evaporation

Influence of the CeO₂ Support on the Reduction Properties of Cu/CeO₂ and Ni/CeO₂ Nanoparticles

Ceria (CeO₂) is being increasingly used as support of metallic nanoparticles in catalysis due to its unique redox properties. Shedding light into the nature of the strong metal support interaction (SMSI) effect in CeO₂-containing catalysts is important since it has a strong influence on the catalytic properties of the system. In this work, Cu/CeO₂ and Ni/CeO₂ nanoparticles are characterized when submitted to a reduction treatment at 500 °C in H₂ atmosphere with a combination of in situ (XAS – X-ray absorption spectroscopy and time-resolved XAS) and ex situ (TEM – transmission electron microscopy and XPS - X-ray photoelectron spectroscopy) techniques. The existence of a capping layer decorating the Ni/CeO₂ nanoparticles after the reduction treatment is shown, representing evidence for the SMSI effect. The kinetics of the SMSI occurrence is elucidated. It is proposed that the electronic factor of the SMSI effect has a strong influence on the reduction properties of the Ni nanoparticles supported on CeO₂, decreasing its reduction temperature if compared to nonsupported Ni nanoparticles. The same phenomenon is not observed for Cu/CeO₂ nanoparticles, where there is no evidence for the SMSI effect, and no changes on the reduction properties between supported and nonsupported Cu nanoparticles are observed

Core–Shell Fe–Pt Nanoparticles in Ionic Liquids: Magnetic and Catalytic Properties

The reaction of Fe­(CO)₅ and Pt₂(dba)₃ in 1-<i>n</i>-butyl-methylimidazolium tetrafluoroborate (BMIm.BF₄), hexafluorophosphate (BMIm.PF₆), and bis­(trifluoromethanesulfonyl)­imide (BMIm.NTf₂) under hydrogen affords stable magnetic colloidal core–shell nanoparticles (NPs). The thickness of the Pt shell layer has a direct correlation with the water stability of the anion and increases in the order of PF₆ > BF₄ > NTf₂, yielding the metal compositions Pt₄Fe₁, Pt₃Fe₂, and Pt₁Fe₁, respectively. Magnetic measurements give evidence of a strongly enhanced Pauli paramagnetism of the Pt shell and a partially disordered iron-oxide core with diminished saturation magnetization. The obtained Pauli paramagnetism of the Pt shell is 2 orders of magnitude higher than that of bulk Pt, owing to symmetry breaking at the surface and interface, resulting in a strong increase in the density of states at the Fermi level, and thus to enhanced Pauli susceptibility. Moreover, these ultrasmall NPs showed efficient catalytic activity for the direct production of selective short-chain hydrocarbons (C₁–C₆) by the Fischer–Tropsch synthesis with efficient conversion (18–34%) and selectivity (69–90%, C₂–C₄). The selectivity and activity were dependent on the Fe-oxides@Pt particle size. The catalytic activity decreased from 34 to 18% as the NP size increased from 1.7 to 2.5 nm at 15 bar and 300 °C

Effect of Oxygen Content on the Photoelectrochemical Activity of Crystallographically Preferred Oriented Porous Ta₃N₅ Nanotubes

Crystallographically preferred oriented porous Ta₃N₅ nanotubes (NTs) were synthesized by thermal nitridation of vertically oriented, thick-walled Ta₂O₅ NTs, strongly adhered to the substrate. The adherence on the substrate and the wall thickness of the Ta₂O₅ NTs were fine-tuned by anodization, thereby helping to preserve their tubular morphology for nitridation at higher temperatures. Samples were studied by scanning electron microscopy, high-resolution electron microscopy, X-ray diffraction, Rietveld refinements, ultraviolet–visible spectrophotometry, X-ray photoelectron spectroscopy, photoluminescence spectra, and electrochemical techniques. Oxygen content in the structure of porous Ta₃N₅ NTs strongly influenced their photoelectrochemical activity. Structural analyses revealed that the nitridation temperature has crystallographically controlled the preferential orientation along the (110) direction, reduced the oxygen content in the crystalline structure and the tubular matrix, and increased the grain size. The preferred oriented porous Ta₃N₅ NTs optimized by the nitridation temperature presented an enhanced photocurrent of 7.4 mA cm^–2 at 1.23 V vs RHE under AM 1.5 (1 Sun) illumination. Hydrogen production was evaluated by gas chromatography, resulting in 32.8 μmol of H₂ in 1 h from the pristine porous Ta₃N₅ NTs. Electrochemical impedance spectroscopy has shown an effect of nitridation temperature on the interfacial charge transport resistance at the semiconductor–​liquid interface; however, the flat band of Ta₃N₅ NTs remained unchanged