Bimetallic PdAu nanoparticles as hydrogenation catalysts in imidazolium ionic liquids

Metallic and bimetallic PdAu nanoparticles were solubilized in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid (IL) by a phase-transfer method using poly(vinylpyrrolidone) (PVP) as a stabilizer. Nanoparticles were characterized by UV–vis spectroscopy and transmission electron microscopy. The bimetallic PdAu nanoparticles in the IL-phase were examined as catalysts for hydrogenation reactions; both the activity and selectivity of the hydrogenation reactions could be tuned by varying the composition of the bimetallic nanoparticles, with maximum activities seen at 1:3 Au:Pd ratios. These nanoparticles/IL catalysts were recycled and then reused for further catalytic reactions with minimal loss in activity.     

The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids

The controlled decomposition of an Ru(0) organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF(6)), tetrafluoroborate (BMI.BF(4)) or trifluoromethane sulfonate (BMI.CF(3)SO(3)) ionic liquids with H(2) represents a simple and efficient method for the generation of Ru(0) nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of approximately 57 nm that contain dispersed Ru(0) nanoparticles with diameters of 2.6+/-0.4 nm. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75 degrees C). The ternary diagram (benzene/cyclohexene/BMI.PF(6)) indicated a maximum of 1 % cyclohexene concentration in BMI.PF(6), which is attained with 4 % benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in BMI.PF(6). Selectivities of up to 39 % in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2 % in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles.     

Recent progress in biphasic catalytic systems using ionic liquids

Catalytic reactions in two liquid phases containing ionic liquids (ILs), in which organic reactions proceed in the ILs phase and products are extracted to the other liquid phase, are efficient and environmentally benign. This short review briefly reports the development of catalytic application in biphasic systems containing ILs in the last two years. Recent progress for the functionalization of ILs themselves and combinations of homogeneous and heterogeneous catalysts with ILs are described. Prospects and future challenges are also addressed.     

Tandem catalytic condensation and hydrogenation processes in ionic liquids

A domino reaction composed of a Knoevenagel condensation combined with a simultaneous catalytic hydrogenation is reported in an ionic liquid solvent under mild conditions (298-363 K and 300 kPa). No interference between the catalysts (Pd/C and amine acetate salt) of the two diverse steps was monitored. The product could be neatly extracted by diethyl ether and the solvent containing the catalysts could be recycled and reused five times without any loss in activity or selectivity. The same methodology in a common organic solvent such as DMA resulted in significant competing parallel hydrogenation of the aldehyde to alcohol.     

Olefin metathesis in room temperature ionic liquids using imidazolium-tagged ruthenium complexes

New recyclable imidazolium-tagged ruthenium catalysts have been developed to perform olefin metathesis in room temperature ionic liquids (RTILs). High level of recyclability combined with a high reactivity were obtained in the ring-closing metathesis (RCM) of a variety of di- or tri-substituted and/or oxygen-containing dienes. Extremely low residual ruthenium levels were detected in the RCM products (average of 7.3 ppm per run). Several examples of olefin cross-metathesis (CM) have been also studied.     

Porous texture of silica aerogels made with ionic liquids as gelation catalysts

The effect of four ionic liquids on the porous texture of silica aerogels synthesized from mixed tetramethoxysilane and methyltrimethoxysilane and dried by the CO₂ supercritical method, was studied. Two of these ionic liquids were composed of BF₄ ⁻ anions while the other two included Cl⁻ anions. The synthesis of gels from ionic liquids did not require another acidic catalyst for silica hydrolysis, nor a basic catalyst for silica condensation. These aerogels were compared with traditional aerogels made according to a double step catalysis, which first involved hydrolysis with HCl followed by condensation with pH 9 Tris HCl buffer. Gel mass analysis and thermogravimetric data showed that, when the initial molar of ionic liquid to Si was 1.58, only ~2% (by mass) of the initial ionic liquids consisting of BF₄ ⁻ anions and ~10% (by mass) of ionic liquids containing Cl⁻ anions, remained in the aerogels after supercritical drying. Moreover, X-ray diffraction confirmed that in ionic liquids based on BF₄ ⁻ anions, evaporation of the volatile components before supercritical CO₂ drying led to the formation of regularly ordered mesopores.     

Protic metal-containing ionic liquids as catalysts: Cooperative effects between anion and cation

HCo(CO)₄ is known to be the active species in the cobalt-catalyzed hydroformylation reaction. Although it is known that the anion [Co(CO)₄]⁻ is catalytically inactive, some cobalt carbonyl-containing ionic liquids are surprisingly able to catalyze hydroformylation reactions. However, only ionic liquids with protic cations demonstrate activity, whilst aprotic cations such as BMIM⁺ result in a completely inactive compound. The four applied cobalt-containing ionic liquids differ only by the cation component. Their different performance in catalytic activity allows the presumption of cooperative effects between the cation and the anion. These fundamental influences of the cation on the hydroformylation kinetics give hints for the reaction mechanism of biphasic hydroformylation reactions as well as on the reaction pathways of the conventional hydroformylation reaction under different reaction conditions.     

Alanine-supported protic ionic liquids as efficient catalysts for aldol condensation reactions

A series of novel protic ionic liquids were immobilized on alanine, a cheap readily available amino acid. These short aliphatic chain ionic liquids have a low cost of preparation, a low toxicity, and need simple synthesis/purification processes. Their catalytic activity was tested for citral–acetone and benzaldehyde–acetone condensations, two reactions with an interest for pharmacological as well as flavor and fragrance industry. Good results were obtained in terms of conversion and selectivity; moreover, the catalysts can be recycled and reused for several consecutive cycles without significant loss of activity.     

The preparation of silica entrapped homogeneous hydrogenation catalysts by conventional and ionic liquid mediated sol-gel routes

[RhCl(PPh₃)₃] has been entrapped inside silica matrices by two methods: a conventional sol-gel synthesis in ethanol/water and a new route performed in an ionic liquid. The activity of these heterogenised catalysts has been tested for the hydrogenation of styrene. The catalyst prepared in an ionic liquid was found to be more active and have low Rh leaching.     

An in-depth look at the effect of Lewis acid catalysts on Diels-Alder cycloadditions in ionic liquids

The present work explores in detail the Diels-Alder reaction between cyclopentadiene and a series of dienophiles, performed in an innovative medium such as an ionic liquid. The potential activation of different Lewis acid catalysts and their load effect when used in combination with this solvent have been explored, in order to settle the improvement on rates and selectivities.     

Review of ionic liquids with fluorine-containing anions

Interest in ionic liquids has grown markedly in recent years. The syntheses, characterization and properties of quaternary alkyl-substituted ammonium, imidazolium, triazolium and pyridinium salts with a variety of fluorine-containing anions have been studied in detail; however, ionic liquids with other cations and anions continue to be discovered. This review is limited to the literature from 2000 to 2005 dealing with ionic liquids with fluorine-containing anions.     

New chiral imidazolium ionic liquids: 3D-network of hydrogen bonding

New hydrophobic chiral ionic liquids bearing an imidazolium core have been stereospecifically prepared from the chiral pool; their enantiomeric purity and 3D-network of hydrogen bonding were analysed by NMR and X-ray diffraction, respectively.     

Epoxidation in ionic liquids: A comparison of rhenium(VII) and molybdenum(VI) catalysts

Complexes of the type (dimethyl-bpy)MoO₂Cl₂ and Schiff/Lewis-base complexes of methyltrioxorhenium (MTO), being efficient homogeneous catalysts for the epoxidation of olefins, have been examined with respect to their catalytic performance at 55 and 25 °C in systems employing room temperature ionic liquids (RTILs) of composition [BMIM]NTf₂, [BMIM]PF₆, [BMIM]BF₄ and [C₈MIM]PF₆ as solvents. The performance in the cyclooctene epoxidation was observed to be strongly dependent on the water content of the system and the catalyst solubility in the RTIL. MTO based systems prove to be superior with respect to lower energy consumption, higher stability and higher product yields compared to the investigated Mo(VI) system under the conditions applied.     

From dysfunction to bis-function: on the design and applications of functionalised ionic liquids

Some of the recent developments concerning the synthesis, properties and applications of functionalised ionic liquids are highlighted. Various strategies are presented, including functionalisation of the cation, anion or both cation and anion in the same ionic liquid, leading to what has been termed dual-functionalised ionic liquids. Particular attention is given to the application of functionalised ionic liquids as reaction media, to stabilise nanoparticles/modify surfaces and to generate porous materials.     

Catalytic asymmetric hydrogenation of aromatic ketones in room temperature ionic liquids

Polar bisphosphonic acid-derived Ru(BINAP)(DPEN)Cl₂ precatalysts were synthesized and immobilized in room temperature ionic liquids (RTILs) for asymmetric hydrogenation of aromatic ketones with ee values of up to 98.7%. The performance of the Ru catalysts is highly dependent on the nature of imidazolium ILs. For the imidazolium ILs without acidic protons, both ILs and Ru catalysts were recycled by simple extraction and reused. Such a simple immobilization approach also prevented the leaching of Ru (and Ru catalysts) into the chiral secondary alcohol products, and should prove desirable for the production of pharmaceutical intermediates that are free from metal contaminants.     

Heck reactions of aryl halides in phosphonium salt ionic liquids: library screening and applications

The Heck cross-coupling of aryl iodides and bromides with olefins proceeds in the phosphonium salt ionic liquid trihexyl(tetradecyl)phosphonium chloride (THP-Cl) in excellent yields. Furthermore, it is shown that the counter anion matched to the phosphonium cation exerts a measurable effect on the overall yield.     

Microwave-promoted synthesis of polyhydroxydeoxybenzoins in ionic liquids

A microwave-promoted synthesis of polyhydroxydeoxybenzoins and -phenylpropanones has been developed, using bis{(trifluoromethyl)sulfonyl}amine (HNTf₂) or BF₃·OEt₂ in an ionic liquid solvent.     

Dissolution and regeneration of polybenzimidazoles using ionic liquids

We report here the initial results that polybenzimidazole can be dissolved in and regenerated from, 1-butyl-3-methylimidazolium chloride and other hydrophilic ionic liquids, which may enable the application of ionic liquids as alternatives to environmentally undesirable solvents currently used for dissolution of this polymer.