Ionic catalytic solution

Most studies of catalysis in ionic liquids have focused on issues of increased selectivity and particularly the easy separation of product from the catalyst and catalyst recycling via use of a biphase. In some cases, the reaction may occur in a biphase in others, the biphase is only used for product separation. In some special cases, the second phase is exclusively product, due to insolubility of the organic products in the ionic liquid, and is easily separated by decantation, allowing the recovered ionic catalytic solution to be reused. Of course, use of an organic solvent for extraction does reduce some of the potential green benefits of the ionic liquid approach. More recently, SCCO2 has been used to extract the products. Alternatively, volatile products can be separated from the ionic liquid and catalyst by distillation. 

The transition-metal NPs dispersed in imidazolium ILs are active catalysts for the hydrogenation of alkenes, arenes and ketones (Table 6.3). Moreover, Pd(0) NPs are active catalyst precursors for C—C coupling reactions, serving as reservoirs of mononuclear catalytically active species. In most cases, the catalytic reactions are typically multiphase systems in which the NPs dispersed in the ILs form the denser phase and the substrate and product remain in the upper-phase. In these cases the ionic catalytic solution is easily recovered by simple decantation and can be reused several times without any significant loss in catalytic activity. 

Multi-phase catalysis performed in ILs can lead to various phase systems where the catalyst should reside in the IL. Prior to the reaction, and in cases where there are no gaseous reactants, two systems can usually be formed a monophase, that is, the substrates are soluble in the IL and biphasic systems where one or all the substrates reside preferentially in an organic phase. If a gas reactant is involved, biphasic and triphasic systems can be formed. At the end of the reaction, three systems can be formed a monophasic system a biphasic system where the residual substrates are soluble in the ionic catalytic solution and the products reside preferentially in the organic phase and triphasic systems, formed, for example, by ionic catalytic solutions, with an organic phase containing the desired product and a third phase containing the byproducts. In most cases, catalysis performed in ILs involves two-phase systems (before and after catalysis). 

Not only cationic, but also anionic, species can be retained without addition of specially designed ligands. The anionic active [FFPt(SnCl3)4] complex has been isolated from the [NEt4][SnCl3] solvent after hydrogenation of ethylene [27]. The PtCl2 precursor used in this reaction is stabilized by the ionic salt (liquid at the reaction temperature) since no metal deposition occurs at 160 °C and 100 bar. The catalytic solution can be used repeatedly without apparent loss of catalytic activity. 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

The term Supported Ionic Liquid Phase (SILP) catalysis has recently been introduced into the literature to describe the heterogenisation of a homogeneous catalyst system by confining an ionic liquid solution of catalytically active complexes on a solid support [68], In comparison to the conventional liquid-liquid biphasic catalysis in organic-ionic liquid mixtures, the concept of SILP-catalysis offers very efficient use of the ionic liquid. Figure 7.10 exemplifies the concept for the Rh-catalysed hydroformylation. 

Electrochemical reactions are catalytic reactions in which strong electric fields near surfaces are used to alter chemical equilibrium. By applying an external voltage V between two electrodes and creating an electric field in an ionically conducting solution, we can cause current to flow and reaction to occur at electrodes. The Gibbs free energy is altered from its value in the absence of an electric field by the Faraday relation... 

The addition of certain ionic promoters to ruthenium catalytic solutions has been found to dramatically affect the rate and selectivity of CO hydrogenation. Whereas ruthenium solutions do not otherwise produce ethylene glycol as a significant product (except as its derivatives in in reactive solvents),... 

The reactions of a range of aryl, benzylic, and heterocyclic zinc reagents with iodo- and bromoarenes were reported at ambient temperature under biphasic conditions with [C4mmim][PF6] and toluene. The biaryl products were readily isolated by decanting the toluene phase, with yields of 70-92% achieved after several minutes. However, attempts to recycle the catalytic ionic liquid solution resulted in significantly decreased activities. 

The common route to bis(indoyl)methanes is via condensation of indoles with aldehydes or ketones in the presence of either protic or Lewis acids. The reaction has been evaluated in tetrafluoroborate and hexafluorophosphate ionic liquids and of the metal salts tested best results were obtained with In(OTf)3 and FeCl3-6H20. Although In(OTf)3 is somewhat more active, its higher price makes the use of iron(III)chloride more attractive. Furthermore, whereas the activity of In(OTf)3 decreases quickly upon recycling, ionic liquid solutions of FeCl3 remain reasonably active for at least four runs.[62] It was found that in hydrophilic ionic liquids the reaction did not proceed at all, whereas fast conversion was observed with the [PF6] -anion, see Scheme 9.17. As water is produced in the course of the reaction it is possible that elimination of the water from the reaction medium helps to protect the catalyst, however, it cannot be excluded that at least some of the catalytic activity is due to the formation of HF. While the... 

Typically, the reaction is performed in a liquid-liquid biphasic system where the substrates and products (upper phase) are not miscible with the catalyst/ionic liquid solution (lower phase). The SiH-functional polydimethylsiloxane and the olefin are placed in the reaction vessel and heated up to 90 °C. Then the precious metal catalyst (20 ppm) and the ionic liquid (1 %) are added. After complete SiH conversion, the reaction mixture is cooled to room temperature and the products are removed from the reaction mixture by either simple decantation or filtration (in case of non-room-temperature ionic liquids). The recovered catalyst/ionic liquid solution can be reused several times without any significant change in catalytic activity. A treatment or workup of the ionic liquid-catalyst solution after each reaction cycle is not necessary. The metal content of the products was analyzed by ICP-OES (Inductively coupled plasma optical emission spectroscopy) and the chemical identity of the organomodified polydimethylsiloxane was verified by NMR spectroscopy. 

A novel transition metal-catalyzed hydrosilylation process is described. The use of an ionic liquid in this process allows for the immobilization, heterogenization, and recovery of the expensive precious metal catalyst as well as its direct reuse in a subsequent hydrosilylation reaction. From an economic and ecological point of view, this process perfectly fits in the concept of "Sustainable Chemistry". Future research activities will aim at the prolongation of the catalyst life-time. For this, it is necessary to gain a deeper understanding of the catalytically active species in the catalyst/ionic liquid solution. 

Jessop and coworkers investigated the asymmetric hydrogenation of tiglic acid using Ru-tolBINAP as a catalyst in wet [bmim][PFs] [115, 116]. Extraction of the product with SCCO2 from the ionic liquid containing the catalysts provided the extremely pure product from the CO2 effluent, in which neither the ionic liquid nor catalyst was contaminated at all. In this way a conversion of up to 99% and an ee-value of 90% were obtained. The recovered ionic liquid catalytic solution was reused up to four times without any reduction of the conversion and enantioselectivity (Scheme 7.44). 

---