Ionic biphasic mixture

Because of the great importance of liquid-liquid biphasic catalysis for ionic liquids, all of Section 5.3 is dedicated to specific aspects relating to this mode of reaction, with special emphasis on practical, technical, and engineering needs. Finally, Section 5.4 summarizes a very interesting recent development for biphasic catalysis with ionic liquids, in the form of the use of ionic liquid/compressed CO2 biphasic mixtures in transition metal catalysis. 

The first application involving a catalytic reaction in an ionic liquid and a subsequent extraction step with SCCO2 was reported by Jessop et al. in 2001 [9]. These authors described two different asymmetric hydrogenation reactions using [Ru(OAc)2(tolBINAP)] as catalyst dissolved in the ionic liquid [BMIM][PFg]. In the asymmetric hydrogenation of tiglic acid (Scheme 5.4-1), the reaction was carried out in a [BMIM][PF6]/water biphasic mixture with excellent yield and selectivity. When the reaction was complete, the product was isolated by SCCO2 extraction without contamination either by catalyst or by ionic liquid. 

Catalytic Conversion of CO2 in an Ionic Liquid/scC02 Biphasic Mixture... 

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. 

Sellin MF, Webb PB, Cole-Hamilton DJ (2001) Continuous flow homogeneous catalysis hydroformulation of alkenes in supercritical fluid-ionic liquid biphasic mixtures. Chem Commun 8 781-782... 

Fig. 8.10 (a) A biphasic mixture of the ionic liquid 1-butyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)amide containing 1.6 M AICI3 at room temperature, (b) The biphasic mixture becomes monophasic at 80°C [89],... 

The ionic liquid [EMIMJTFSA shows, at room temperature, biphasic behavior on addition of AICI3. AICI3 dissolves well in [EMIM] TFSA up to a concentration of about 2.5 mol L-1, then a biphasic mixture is obtained on further addition of AICI3. The upper phase of the mixture AICI3/[EMIM] TFSA is clear and colorless while the lower one is pale and more viscous. On further addition of AICI3 the viscosity of the lower phase increases. It is worth noting that Al can only be electrodeposited from the upper phase, the clear one, at AICI3 concentrations > 5 mol L 1. Furthermore, after a few days a precipitate which contains A1(TFSA)3 forms as a third phase. 

Nanocrystalline aluminum can be made in the employed ionic liquid without additives, see Chapter 8. The SEM micrograph of Figure 12.9 shows the surface morphology of a deposited aluminum layer obtained potentiostatically on mild steel at —0.75 V (vs. Al) for 2 h in the upper phase ofthe biphasic mixture [Pyi TfiN M AICI3 at 100 °C. Prior to Al electrodeposition, the electrode was anodically polarized at a potential of 1V (vs. Al) for 2 min. The deposited layer is dense, shining and adherent to the substrate with crystallites in the nanosize regime. 

Most widely used are N,N -dialkyhrnidazohum salts, since they are easily prepared. Ionic liquids have been used as solvents for numerous reactions. Their physical and chemical properties vary with the combination of cation and anion. This allows a degree of tuning of their properties. Since they are highly polar solvents, ionic liquids can dissolve many inorganic salts and transition metal complexes, and often form biphasic mixtures with non-polar organic solvents. Thus, organic products can be extracted from ionic liquids, while ionic transition metal catalysts are immobilized. Volatile products can be easily distilled off from ionic liquids, since the latter show no volatility [17]. 

Based on their work on supercritical CO2 (see also Scheme 23), Reetz and Leitner introduced a technologically new and interesting continuous flow process for enzymatic reactions [65]. The group designed a protocol for enzymatic reactions, namely the lipase-catalyzed acylation (CAL B) of octan-1 -ol by vinyl acetate in ionic liquids (l-butyl-3-methylimidazolium bis(trifluo-romethanesulfonimide) [BMIM] [BTA]) using supercritical CO2 as the mobile phase (Scheme 25). The alcohol is pumped through the biphasic system and the products are obtained in solvent-free form in a cold trap. The enzyme/ionic liquid mixture can be recycled in batchwise or continuous flow operations. 

The hydroxylation of benzene to give phenol was carried out using a biphasic mixture of hydrophobic ionic liquids such as [OMIM][BF4] or [OMIM][Pp6] and water/hydrogen peroxide [283]. A range of transition metal dodecylsulfate catalysts were employed with the iron giving the best results. Conversions of benzene to phenol of 54% were obtained and the ionic liquid/catalyst combination could be... 

While unmodified xanthene ligands (Fig. 5.3-13(a)) show highly preferential solubility in the organic phase in the biphasic mixture l-octene/[BMIM][PF6], even at room temperature, the application of the guanidinium-modified xanthene ligand (Fig. 5.313(b)) resulted in excellent immobilization of the Rh catalyst in the ionic liquid. 

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