Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis.

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Thermo- and Photocatalytic Activation of CO2 in Ionic Liquids Nanodomains.

Ionic liquids (ILs) are considered to be potential material devices for CO2 capturing and conversion to energy-adducts. They form a cage (confined-space) around the catalyst providing an ionic nano-container environment which serves as physical-chemical barrier that selectively controls the diffusion of reactants, intermediates, and products to the catalytic active sites via their hydrophobicity and contact ion pairs. Hence, the electronic properties of the catalysts in ILs can be tuned by the proper choice of the IL-cations and anions that strongly influence the residence time/diffusion of the reactants, intermediates, and products in the nano-environment. On the other hand, ILs provide driving force towards photocatalytic redox process to increase the CO2 photoreduction. By combining ILs with the semiconductor, unique solid semiconductor-liquid commodities are generated that can lower the CO2 activation energy barrier by modulating the electronic properties of the semiconductor surface. This mini-review provides a brief overview of the recent advances in IL assisted thermal conversion of CO2 to hydrocarbons, formic acid, methanol, dimethyl carbonate, and cyclic carbonates as well as its photo-conversion to solar fuels.

Cerium Oxide Nanoparticles Inside Carbon Nanoreactors for Selective Allylic Oxidation of Cyclohexene.

The confinement of cerium oxide (CeO2) nanoparticles within hollow carbon nanostructures has been achieved and harnessed to control the oxidation of cyclohexene. Graphitized carbon nanofibers (GNF) have been used as the nanoscale tubular host and filled by sublimation of the Ce(tmhd)4 complex (where tmhd = tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)) into the internal cavity, followed by a subsequent thermal decomposition to yield the hybrid nanostructure CeO2@GNF, where nanoparticles are preferentially immobilized at the internal graphitic step-edges of the GNF. Control over the size of the CeO2 nanoparticles has been demonstrated within the range of about 4-9 nm by varying the mass ratio of the Ce(tmhd)4 precursor to GNF during the synthesis. CeO2@GNF was effective in promoting the allylic oxidation of cyclohexene in high yield with time-dependent control of product selectivity at a comparatively low loading of CeO2 of 0.13 mol %. Unlike many of the reports to date where ceria catalyzes such organic transformations, we found the encapsulated CeO2 to play the key role of radical initiator due to the presence of Ce3+ included in the structure, with the nanotube acting as both a host, preserving the high performance of the CeO2 nanoparticles anchored at the GNF step-edges over multiple uses, and an electron reservoir, maintaining the balance of Ce3+ and Ce4+ centers. Spatial confinement effects ensure excellent stability and recyclability of CeO2@GNF nanoreactors.

Treatment and characterization of biomass of soybean and rice hulls using ionic liquids for the liberation of fermentable sugars.

We investigated the changes in the physical structure of cellulose recovered from soybean and rice hulls treated with the ionic liquids 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) and 1-butyl-3-methylimidazolium acetate ([bmim][Ac]). The characterization was carried out by a combination of thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Regenerated cellulose from soybean hull showed loss of crystallinity and high structural disruption caused by both ionic liquid treatments as compared to the untreated material. In contrast, rice hull presented only a small structural disruption when treated with [bmim][Ac] and was practically unaffected by [bmim][Cl], showing that this biomass residue is recalcitrance towards physico-chemical treatments, possibly as a consequence of its high composition content in silica. These results suggest the use of soybean hull as a substrate to be treated with ionic liquids in the preparation of lignocellulosic hydrolysates to be used in second-generation ethanol production, whereas other methods should be considered to treat rice hull biomass.

Tunneling effects in confined gold nanoparticle hydrogenation catalysts.

Clean surface gold nanoparticles (AuNPs) of ∼6.6 nm that were confined in ionic liquid (IL) cages of hybrid γ-alumina (γ-Al2O3) displayed hydrogenation pathways in the reduction of trans-cinnamaldehyde distinct from those imprinted directly onto γ-Al2O3. Hydrogen activation proceeded via homolytic activation in IL-encapsulated AuNPs and via heterolytic cleavage for IL-free supported AuNPs. Higher negative apparent entropy (ΔSapp) values were obtained for the IL-confined AuNPs compared to the non-hybrid catalyst (Au/γ-Al2O3), suggesting a decrease in the number of microstates induced by the nano-confined environment. High kinetic isotope effect (KIE) values (kH/kD = 2.5-2.9 at 273 K) and Arrhenius convex curves were observed. Furthermore, differences of 5.6 and 6.2 kJ mol-1 between the apparent activation energies of the deuteration and hydrogenation reactions (E-E) associated with pre-exponential factor ratios (AD/AH) of 4.6 and 5.1 provided strong evidence of the possible involvement of a tunneling pathway in the case of the confined AuNPs.

Dealing with supramolecular structure for ionic liquids: a DOSY NMR approach.

Diffusion-ordered spectroscopy (DOSY) is arguably a powerful method for the NMR analysis of ionic liquids, since the self-diffusion coefficients for cations and anions can be measured straightforwardly. In this work, the dynamic-structural behaviour of imidazolium ionic liquids containing different anions has been investigated by experimental measurements of direct 1H diffusion coefficients in chloroform and water solutions. The influence of ion structure has been tested by using six IL salts formed by the association of different cations (1-n-butyl-3-methylimidazolium, 1,2,3-trimethylimidazolium and tetra-n-butylammonium) with different anion structures (prolinate, acetate and o-trifluoromehtylobenzoate). The influence of IL concentration (from 0.01 to 0.5 mol L-1) was also evaluated for BMI·Pro. The contact ion pairs (or aggregates) are maintained in both chloroform and water within the range of concentrations investigated. In the particular case of 1,2,3-trimethylimidazolium imidazolate (TMI·Im) containing confined water in DMSO the maintenance of the contact ion pairs depends on the water content which may even disrupt the IL supramolecular structure.

Correspondence on "Preorganization and Cooperation for Highly Efficient and Reversible Capture of Low-Concentration CO2 by Ionic Liquids".

The preorganization and cooperation mechanism of imide-based ionic liquids reported in a recent Communication was evocated to rationalize the extremely high gravimetric CO2 capture displayed by these fluids. An analysis of the reported spectroscopic evidences together with additional experiments led to the proposition of an alternative, simpler, and feasible mechanism involving the formation of bicarbonate.

Imprinted Naked Pt Nanoparticles on N-Doped Carbon Supports: A Synergistic Effect between Catalyst and Support.

A synergistic effect resulting from the interaction of small (2.4-3.1 nm) naked Pt nanoparticles (NPs) imprinted on N-doped carbon supports is evidenced by structural, electronic and electrochemical characterization. The size and distribution of the sputtered Pt NPs are found to be related to the nature of the support because Pt NPs are preferentially located at Ngraphitic sites. In addition, Rutherford backscattering shows that a deeper penetration of the Pt NPs is obtained in the N-doped carbon support with larger pore diameters. The ligand effect of the N-doped carbon supports is found to occur by electron donation from Npyrrolic and Ngraphitic sites to the Pt NPs and the electron acceptor behavior of the C=Npyridinic sites. The carbon matrix acquires a basic characteristic (electron-richer, metallic behavior) capable of interacting with metallic NPs akin to a bimetallic-like system. The imprinted Pt NPs are active catalysts for oxidation, although displaying poor catalytic activity for reduction reactions. The catalyst N-doped carbon supports play an important role in the overall catalytic process, rather than only acting as a simple active phase carrier.

Cation-Anion-CO2 Interactions in Imidazolium-Based Ionic Liquid Sorbents.

A series of functionalized N-alkylimidazolium based ionic liquids (ImILs) were designed, through anion (carboxylates and halogenated) and cation (N-alkyl side chains) structural modifications, and studied as potential sorbents for CO2 . The sorption capacities of as prepared bare ImILs could be enhanced from 0.20 to 0.60 molar fraction by variation of cation-anion-CO2 and IL-CO2 -water interaction. By combining NMR spectroscopy with molecular dynamics simulations, a good description of interactions between ImIL and CO2 can be obtained. Three types of CO2 sorption modes have been evidenced depending on the structure of the ImIL ion pair: Physisorption, formation of bicarbonate, and covalent interaction through the nucleophilic addition of CO2 to the cation or anion. The highest CO2 sorption capacity was observed with the ImIL containing the 1-n-butyl-3-methylimidazolium cation associated with the carboxylate anions (succinate and malonate). This study provides helpful clues for better understanding the structure-activity relationship of this class of materials and the ion pair influence on CO2 capture.

Intermolecular hydrogen bonds in water@IL supramolecular complexes.

The role of small amounts of water in ionic liquids (ILs), namely, 1-n-butyl-2,3-dimethylimidazolium imidazolate (BMMI·Im), 2-methylimidazolate (BMMI·MeIm), and pyrazolate (BMMI·Pyr), is examined using NMR spectroscopy and density functional theory (DFT) calculations. The nuclear Overhauser effect (NOE) indicates that a water molecule is trapped inside the ionic network, keeping the ion pair in contact through strong H-bonds involving the hydrogen atoms of water and the nitrogen atoms of the IL anions to give a guest@host supramolecular structure. The formation of the H2O@IL pair complex with different ILs combined with the strong hydrogen bond strength within the complex is responsible for the selective H/D exchange reactions at the imidazolium C2-Me and ketone Cα positions.

Supramolecular interaction of non-racemic benzimidazolium based ion pairs with chiral substrates.

A series of novel benzimidazolium-based non-racemic ionic liquids (ILs) was synthesized from low-cost chiral terpenoid alcohols and fully characterized by the use of a wide variety of techniques, such as DSC, ESI-MS, ATR FT-IR, polarimetry as well as 1H and 13C NMR spectroscopy. The ILs were investigated as chiral shift agents for the chiral recognition of racemic mixtures of Mosher's acid potassium salt by 19F NMR spectroscopy, leading to high splitting values of the CF3 signal. Supramolecular interactions between salt and H-C2 of chiral benzimidazolium cation are responsible for the chiral recognition, as was demonstrated by experimental evidences. Indeed, the enantiomeric excess value of enantioenriched substrates depends mainly on the strength of the contact ion pairs.

Is the formation of N-heterocyclic carbenes (NHCs) a feasible mechanism for the distillation of imidazolium ionic liquids?

We describe the synthesis of two tetrachloroindate ionic liquids used as probes to study the involvement of NHCs (N-heterocyclic carbenes) in the distillation of imidazolium derivatives. Atmospheric-pressure chemical ionization mass spectrometry (APCI-MS), electrospray ionization mass spectrometry (ESI-MS), atmospheric-pressure thermal desorption ion mass spectrometry (APTDI-MS) and laser-induced acoustic desorption (LIAD) were used to depict the possibility of the involvement of NHCs during the distillation process. Each type of imidazolium derivative showed a particular mechanism of distillation, pointing firmly to the dependence of both the cation and the anion natures to distil as ion pairs or NHCs. Ionic liquid 1-n-butyl-3-methylimidazolium tetrachloroindate (1a) exhibited a preference to distil as ion pairs, whereas 3,3'-(ethane-1,2-diyl)bis(1-methyimidazolium)bis-tetrachloroindate (1b) may react with the Lewis acid anion, affording a bidentate NHC complex to distil. Thermodynamics, quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses of the ionic liquid 1a were also conducted and helped understand the preference for ion pairs instead of NHCs. The performed theoretical calculations did not forwent the possibility of NHC formation; however, they clearly indicated the high stability of the anions (Lewis acids in nature) and also indicated that the possible reaction between NHC and the anion is not favoured. The calculated thermodynamic values were in accordance with the features observed by MS and indicated ion pairs as the feasible species for the distillation of imidazolium-based ionic liquids.

Tuning the structure and magnetic behavior of Ni-Ir-based nanoparticles in ionic liquids.

We report on a simple preparation of extremely small diameter (ca. 2 nm) Ni-Ir-based NPs using Ni(COD)2 and [Ir(COD)OCH3]2 in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm·NTf2). The prepared NPs had either core-shell-like or alloy-like structures with the presence of Ni,Ir-oxides, depending on the synthetic approach. X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and magnetic measurements are combined to describe the influence of nanostructure on the magnetic behavior of these nanosystems. The present findings reveal that the alloy NPs display a disordered magnetic state, similar to a spin glass (SG)-like system (Tf = 7.2 K). Core-shell NPs are formed by a magnetically blocked/unblocked core with a magnetically disordered shell as deduced from the two magnetic responses peaking at TB = 75 K and Tf = 5.8 K. Coupling at the core-shell interface leads to an exchange bias revealed at low temperature as horizontal shifts in the hysteresis loops of 0.12 kOe at 2 K.

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 (pKa 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.

Pristine Ta3 N5 Nanotubes: Trap-Driven High External Biasing Perspective in Semiconductor/Electrolyte Interfaces.

Ta3 N5 is a promising photoelectrode for solar hydrogen production; however, to date pristine Ta3 N5 electrodes without loading co-catalysts have presented limited photoelectrochemical (PEC) performance. In particular, large external biasing has been required to run water oxidation, the origin of which is investigated herein. Ta3 N5 nanotubes (NTs) prepared by nitridation were characterized by a wide range of techniques. The bandgap was confirmed by a novel PEC technique. Nondestructive synchrotron-excited XPS has shown the presence of reduced Ta species deeper in the Ta3 N5 surface. Lower photocurrent and transient spikes that were intense at lower applied biasing were observed under water oxidation; however, spikes were inhibited in the presence of a sacrificial agent and photocurrent was improved even at low biasing. It was observed for the first time that the lower PEC performance under water oxidation can be attributed to the presence of interband trapping states associated with pristine Ta3 N5 NTs/electrolyte junction. These states correspond to the structural defects in Ta3 N5 , devastate PEC performance, and present the necessity to apply higher biasing. The key to circumvent them is to use a sacrificial agent in the electrolyte or to load a suitable co-catalyst to avoid hole accumulation under water oxidation, thereby improving the phootocurrent. The findings on the interband states could also provide guidance for the investigation of PEC properties of new types of semiconducting devices.

Synthesis and Characterisation of Fluorescent Carbon Nanodots Produced in Ionic Liquids by Laser Ablation.

Carbon nanodots (C-dots) with an average size of 1.5 and 3.0 nm were produced by laser ablation in different imidazolium ionic liquids (ILs), namely, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4 ), 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI.NTf2 ) and 1-n-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (OMI.NTf2 ). The mean size of the nanoparticles is influenced by the imidazolium alkyl side chain but not by the nature of the anion. However, by varying the anion (BF4 vs. NTf2 ) it was possible to detect a significant modification of the fluorescence properties. The C-dots are much probably stabilised by an electrostatic layer of the IL and this interaction has played an important role with regard to the formation, stabilisation and photoluminescence properties of the nanodots. A tuneable broadband fluorescence emission from the colloidal suspension was observed under ultraviolet/visible excitation with fluorescence lifetimes fitted by a multi-exponential decay with average values around 7 ns.

Interacting Superparamagnetic Iron(II) Oxide Nanoparticles: Synthesis and Characterization in Ionic Liquids.

Interacting superparamagnetic iron(II) oxide nanoparticles (NPs) with sizes of 5.3 ± 1.6 nm were prepared by simple decomposition of [Fe(COT)2] (COT = 1,3,5,7-cyclooctatetraene) with 5 bar of H2 in 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI·NTf2) ionic liquid (IL). The static and dynamic magnetic characterization revealed a superparamagnetic behavior with weak dipolar interactions of these NPs. In situ structural studies by X-ray absorption spectroscopy demonstrated that they consist of nanostructured FeO. This approach is an appropriate method to prepare and stabilize nanostructured FeO particles, where the presence of an IL proved to be fundamental to suppress the aggregation and usual overoxidation of the FeO NPs.

Confined water in imidazolium based ionic liquids: a supramolecular guest@host complex case.

It is well known that the macroscopic physico-chemical properties of ionic liquids (ILs) are influenced by the presence of water that strongly interferes with the supramolecular organization of these fluids. However, little is known about the function of water traces within this confined space and restricted ionic environments, i.e. between cations and anions. Using specially designed ILs namely 1,2,3-trimethyl-1H-imidazol-3-ium imidazol-1-ide (MMMI·Im) and 3-n-butyl-1,2-dimethyl-1H-imidazol-3-ium imidazol-1-ide (BMMI·Im), the structure and function of water have been determined in condensed, solution and gas phases by X-ray diffraction studies, NMR, molecular dynamics simulations (MDS) and DFT calculations. In the solid state the water molecule is trapped inside the ionic network (constituted of contact ion pairs formed by π(+)-π(-) interaction) through strong H-bonds involving the water hydrogens and the nitrogens of two imidazolate anions forming a guest@host supramolecular structure. A similar structural arrangement was corroborated by DFT calculations and MDS. The presence of a guest@host species (H2O@ILpair) is maintained to a great extent even in solution as detected by (1)H-(1)H NOESY-experiments of the ILs dissolved in solvents with low and high dielectric constants. This confined water catalyses the H/D exchange with other substrates containing acidic-H such as chloroform.

Benzene partial hydrogenation: advances and perspectives.

The partial hydrogenation of benzene to cyclohexene is an economically interesting and technically challenging reaction. Over the last four decades, a lot of work has been dedicated to the development of an exploitable process and several approaches have been investigated. However, environmental constraints often represent a limit to their industrial application, making further research in this field necessary. The goal of this review is to highlight the main findings of the different disciplines involved in understanding the governing principles of this reaction from a sustainable chemistry standpoint. Special emphasis is given to ruthenium-catalyzed liquid phase batch hydrogenation of benzene.

Imidazolium salt ion pairs in solution.

The formation, stabilisation and reactivity of contact ion pairs of non-protic imidazolium ionic liquids (ILs) in solution are conceptualized in light of selected experimental evidence as well theoretical calculations reported mainly in the last ten years. Electric conductivity, NMR, ESI-MS and IR data as well as theoretical calculations support not only the formation of contact ion pairs in solution, but also the presence of larger ionic and neutral aggregates even when dissolved in solvents with relatively high dielectric constants, such as acetonitrile and DMSO. The presence of larger imidazolium supramolecular aggregates is favoured at higher salt concentrations in solvents of low dielectric constant for ILs that contain shorter N-alkyl side chains associated with anions of low coordination ability. The stability and reactivity of neutral contact species are also dependent on the nature of the anion, imidazolium substituents, and are more abundant in ILs containing strong coordinating anions, in particular those that can form charge transfer complexes with the imidazolium cation. Finally, some ILs display reactivities as contact ion pairs rather than solvent-separated ions.