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.

Tunable Aryl Alkyl Ionic Liquid Supported Synthesis of Platinum Nanoparticles and Their Catalytic Activity in the Hydrogen Evolution Reaction and in Hydrosilylation

Tunable aryl alkyl ionic liquids (TAAILs) are ionic liquids (ILs) with a 1-aryl-3-alkylimidazolium cation having differently substituted aryl groups. Herein, nine TAAILs with the bis(trifluoromethylsulfonyl)imide anion are utilized in combination with and without ethylene glycol (EG) as reaction media for the rapid microwave synthesis of platinum nanoparticles (Pt-NPs). TAAILs allow the synthesis of small NPs and are efficient solvents for microwave absorption. Transmission electron microscopy (TEM) shows that small primary NPs with sizes of 2 nm to 5 nm are obtained in TAAILs and EG/TAAIL mixtures. The Pt-NPs feature excellent activity as electrocatalysts in the hydrogen evolution reaction (HER) under acidic conditions, with an overpotential at a current density of 10 mA cm^-2 as low as 32 mV vs the reversible hydrogen electrode (RHE), which is significantly lower than the standard Pt/C 20% with 42 mV. Pt-NPs obtained in TAAILs also achieved quantitative conversion in the hydrosilylation reaction of phenylacetylene with triethylsilane after just 5 min at 200 °C.

Sweet, Sugar-Coated Hierarchical Platinum Nanostructures for Easy Support, Heterogenization and Separation

Metal nanoparticles are increasingly gaining interest in the field of heterogeneous catalysis. Here, we present a novel strategy for synthesizing sugar-coated platinum nanostructures (SC-Pt-NS) from the carbohydrates sucrose and D(-)-fructose. In the synthesis from a mixture of H2PtCl6·6H2O, the carbohydrate in an ionic liquid (IL) yielded primary particles of a homogeneous average size of ~10 nm, which were aggregated to hierarchical Pt nanostructures of ~40–65 nm and surrounded or supported by the carbohydrate. These sugar-coated platinum nanostructures present a facile way to support and heterogenize nanoparticles, avoid leaching and enable easier separation and handling. The catalytic activity of the SC-Pt-NS was shown in the hydrosilylation test reaction of phenylacetylene with triethylsilane, where very high turnover frequency (TOF) values of up to 87,200 h−1 could be achieved, while the platinum metal leaching into the product was very low.

Platinum-Catalyzed Alkene Hydrosilylation: Solvent-Free Process Development from Batch to a Membrane-Integrated Continuous Process

The integration of a membrane separation protocol with the platinum-catalyzed hydrosilylation of olefins is investigated. The catalytic reaction is first optimized in batch where [Pt(IPr*)(dms)Cl₂ ] (IPr*=1,3-bis[2,6-bis(diphenylmethyl)-4-methylphenyl]imidazol-2-ylidene, dms=dimethyl sulfide) demonstrates superior activity compared to the less sterically encumbered [Pt(SIPr)(dms)Cl₂ ] (SIPr=1,3-bis(2,6-diisopropylphenyl)imidazolidine) congener. Filtration conditions are identified in membrane screening experiments. Hydrosilylation of 1-octene catalyzed by [Pt(IPr*)(dms)Cl₂ ] is conducted in continuous mode and the platinum catalyst is separated efficiently over the commercially available Borsig oNF-2 membrane, all under solvent-free conditions. An advantage of this process is that both reaction and separation are coupled in a single step. Moreover, at the end of the process the intact catalyst was recovered in 80 % yield as an off-white solid without any further purification.

Selective hydrosilylation of allyl chloride with trichlorosilane

The transition-metal-catalysed hydrosilylation reaction of alkenes is one of the most important catalytic reactions in the silicon industry. In this field, intensive studies have been thus far performed in the development of base-metal catalysts due to increased emphasis on environmental sustainability. However, one big drawback remains to be overcome in this field: the limited functional group compatibility of the currently available Pt hydrosilylation catalysts in the silicon industry. This is a serious issue in the production of trichloro(3-chloropropyl)silane, which is industrially synthesized on the order of several thousand tons per year as a key intermediate to access various silane coupling agents. In the present study, an efficient hydrosilylation reaction of allyl chloride with trichlorosilane is achieved using the Rh(I) catalyst [RhCl(dppbz^F)]₂ (dppbz^F = 1,2-bis(diphenylphosphino)-3,4,5,6-tetrafluorobenzene) to selectively form trichloro(3-chloropropyl)silane. The catalyst enables drastically improved efficiency (turnover number, TON, 140,000) and selectivity (>99%) to be achieved compared to conventional Pt catalysts.

Piperidinium and Pyrrolidinium Ionic Liquids as Precursors in the Synthesis of New Platinum Catalysts for Hydrosilylation

Six new air-stable anionic platinum complexes were synthesized in simple reactions of piperidinium [BMPip]Cl or pyrrolidinium [BMPyrr]Cl ionic liquids with platinum compounds ([Pt(cod)Cl2] or K2[PtCl6]). All these compounds were subjected to isolation and spectrometric characterization using NMR and ESI-MS techniques. Furthermore, the determination of melting points and thermal stability of the above derivatives was performed with the use of thermogravimetric analysis. The catalytic performance of the synthesized complexes was tested in hydrosilylation of 1-octene and allyl glycidyl ether with 1,1,1,3,5,5,5-heptamethyltrisiloxane. The study has shown that they have high catalytic activity and are insoluble in the reaction medium which enabled them to isolate and reuse them in consecutive catalytic cycles. The most active complex [BMPip]2[PtCl6] makes it possible to conduct at least 10 catalytic runs without losing activity which makes it an attractive alternative not only to commonly used homogeneous catalysts, but also to heterogeneous catalysts for hydrosilylation processes. The activity of the studied catalysts is also affected by the kind of anion and, to some extent, the kind of cation.

Platinum and rhodium complexes ligated by imidazolium-substituted phosphine as efficient and recyclable catalysts for hydrosilylation

Platinum and rhodium complexes ligated by imidazolium substituted phosphine were obtained with high yields and applied as efficient catalysts.

Highly-active, graphene-supported platinum catalyst for the solventless hydrosilylation of olefins

Herein we report the development of the first graphene-supported platinum catalyst that has demonstrated exceptional catalytic activity and stability for hydrosilylation reactions of olefins (TOF 4.8 × 106 h-1, TON = 9.4 × 106). The catalyst also exhibited functional group tolerance over a broad range of industrially relevant substrates with minimal metal leaching. In addition, the catalyst system was successfully translated into a packed bed platform for continuous hydrosilylation reactions.

Optimization and intensification of hydrosilylation reactions using a microreactor system

Utilization of continuous flow chemistry techniques concentrates often on catalytic reactions difficult to be performed for various reasons under typical batch conditions.

The effect of the catalyst and the type of ionic liquid on the hydrosilylation process under batch and continuous reaction conditions

We present a simplified method for performing the hydrosilylation reaction in a heterogeneous catalytic system with the use of ionic liquids and microreactor systems.

The effect of the morpholinium ionic liquid anion on the catalytic activity of Rh (or Pt) complex–ionic liquid systems in hydrosilylation processes

Studies were performed on the catalytic activity for olefin hydrosilylation shown by three rhodium complexes, [{Rh(μ-OSiMe₃)(cod)}₂] (I), [{Rh(μ-Cl)(cod)}₂] (II) and [RhCl(PPh₃)₃] (III), and three platinum complexes, [Pt(PPh₃)₄] (IV), [Pt(PPh₃)₂Cl₂] (V) and PtCl₄ (VI), immobilized in a series of different anion-containing morpholinium ionic liquids. The effect of the kind of anion (its nucleophilic character) on the activity, stability and possibility of a catalytic system with multiple uses in the hydrosilylation process has been established. In the case of the best systems it was possible to reuse the same catalyst sample 10 times almost without any decrease in the activity and a TON value over 99 000 was obtained.

Rhodium and platinum complexes were immobilized in morpholinium ionic liquids, the effect of which on the catalytic activity was determined.

Ionic Liquids as Solvents for Rhodium and Platinum Catalysts Used in Hydrosilylation Reaction

A group of imidazolium and pyridinium based ionic liquids has been synthetized, and their ability to dissolve and activate the catalysts used in hydrosilylation reaction of 1-octane and 1,1,1,3,5,5,5-heptamethyltrisiloxane was investigated. An organometallic catalyst as well as inorganic complexes of platinum and rhodium dissolved in ionic liquids were used, forming liquid solutions not miscible with the substrates or with the products of the reaction. The results show that application of such a simple biphasic catalytic system enables reuse of ionic liquid phase with catalysts in multiple reaction cycles reducing the costs and decreasing the amount of catalyst needed per mole of product.