Ionic liquids biphasic reactions

Figure 7.5. Concentration of gaseous feedstock 1 in a gas-ionic liquid biphasic reaction for slow (A) and fast (B) chemical reaction - mass transfer resistance on the gas side is neglected...

When either the organic solvent or the ionic liquid is used as pure solvent, proper control over the water content, or rather the water activity, is of crucial importance, as a minimum amount is necessary to maintain the enzyme s activity. For ionic liquids, a reaction can be operated at constant water activity by use of the same methods as established for organic solvents [17]. [BMIM][PFg] or [BMIM][(CF3S02)2N], for example, may be used as pure solvents and in biphasic systems. Water-miscible ionic liquids, such as [BMIM][BF4] or [MMIM][MeS04], can be used in the second case. 

In contrast, we intend to demonstrate the principle aspects of catalyst recycling and regeneration using the ionic liquid methodology. These aspects will be explored in more detail for the example of Rh-catalysed hydroformylation (see Section 7.2). First, however, we will briefly introduce important general facts concerning transition metal catalysis in ionic liquids (see Section 7.1.2). This will be followed by a consideration of liquid-liquid biphasic reactions in these media from an engineering point of view (see Section 7.1.3). 

The few examples where SILP catalysis has been tested so far showed highly encouraging results. It is very likely that other applications where ionic catalyst solutions were tested in liquid-liquid biphasic reactions could be reinvestigated under SILP conditions. If very high catalyst stability over time can be realised or simple catalyst regeneration protocols can be developed than SILP catalysis can be expected to make its way into industrial processes. 

Liquid-liquid biphasic reaction was carried out in a 70-mL autoclave at a stirring speed of 1600 rpm with a catalyst charge of c(Rh) = 0.018x10"5 mol in 4mL ionic liquid. 

An excellent demonstration of the tunability of ionic liquids for catalysis is provided by an investigation of the dimerization of 1-butene (235). A Ni(cod)(hfacac) catalyst (Scheme 23) was evaluated for the selective dimerization of 1-butene after it was dissolved in various chloroaluminate ionic liquids. Earlier work on this reaction with the same catalyst in toluene led to the observations of low activity and difficult catalyst separation. In ionic liquids of varying acidity, little catalytic activity was found. However, a remarkable activity was achieved by adding a weak buffer base to an acidic ionic liquid. The reaction took place in a biphasic reaction mode with facile catalyst separation and catalyst recycling. A high selectivity to the dimer product was obtained because of a fast extraction of the Cg product from the ionic liquid phase, with the minimization of consecutive reaction to give trimers. Among a number of weak base buffers, a chinoline was chosen. The catalyst performance was compared with that in toluene. The catalyitc TOF at 90°C in toluene was... 

Certain solvents are immiscible at low temperature, but on heating, form a single phase. This allows reactions to be conducted in single phase under homogeneous conditions followed by separation using normal biphasic extraction methods.4 The technique was first recognized in fluorous-organic biphasic catalysis, and has more recently been applied to ionic liquid-aqueous reactions (see below). 

The cationic rhodium complex [Rh(nbd)(PPh3)2]+ (nbd = norbornadiene), previously described by Osborn as a catalyst precursor for the hydrogenation of alkenes and dialkenes when dissolved, e.g., in acetone [28], has proved to be active in dialkylimidazolium PF, CuCI, or SbFf, salts [29]. Due to the very low solubility of alkanes in ionic liquids, the reaction is truly biphasic. The rhodium loss in the organic phase is under the limit of detection. The catalyst can be used repeatedly. In this case, the presence of coordinating anions, such as traces of chloride, inhibits the catalyst activity. Halide contaminants may arise from unreacted starting material used in the preparation of NAILs or from the possible decomposition of the halogenated anion (e.g., PF ). Purification and quality control of ionic liquids are then crucial [30]. 

For the oxidation of alkanes Li et al. used iodobenzene diacetate [PhI(OAc)2] as oxygen source. An electron-deficient manganeseporphyrin catalyst was immobilized in [BMIM][PF6] and tested in liquid-liquid biphasic reaction mode with CH2CI2 as organic phase [154]. They found the catalyst more active in the [BMIM][PF6]/CH2Cl2 system than in neat CH2Q2- The increase in activity was attributed by them to the higher polarity of the ionic liquid. 

Despite all the advantages of this process, one main limitation is the continuous catalyst carry-over by the products, with the need to deactivate it and dispose of wastes. One way to optimize catalyst consumption and waste disposal is to operate the reaction in a biphasic system. The first difliculty was to choose a good solvent. N,N-Dialkylimidazolium chloroaluminate ionic liquids proved to be the best candidates. They are liquid at the reaction temperature, butenes are reasonably soluble in them (Table 5.4-3), and they are poorly miscible with the products (Table 5.4-2, case (a)). The chloroaluminate eSiciently dissolves and stabilizes the nickel catalyst in the ionic medium without the addition of special ligand. The ionic liquid plays the role of both catalyst solvent and co-catalyst. Its Lewis acidity can be adjusted to get the best performance. The catalytically active nickel complex is generated directly in the ionic liquid by reaction of a commercialized tiickel(II) salt, as used in the Dimersol process, with an alkylaluminum chloride derivative. 

Detailed kinetic investigations of the reaction of cumene with propene in [C2mim]Cl—AICI3 (X(AlCl3) = 0.67) were conducted by Joni et al. in a liquid-liquid biphasic reaction mode [24]. Various products (di-, tri- and tetraisopropylbenzene) result from a series of consecutive alkylation reactions. It is necessary to take the solubility of these products into account to fit kinetic models to the data. A conductor-like screening model for real solvent (COSMO-RS) method was used to predict the relative solubilities of the products. Higher alkylated products are less soluble in the reactive ionic liquid phase, leading to an irtproved selectivity for the monoalkylated product. 

As illustrated in Figure 6.3. many liquid-liquid biphasic reactions using active catalysts dissolved in ionic liquids suffer from severe mass transport limitation issues. Thus, only a small part of the ionic liquid takes effectively part in the reaction, a fact that the reaction engineer calls Tow ionic liquid utilisation . 

Catalyst Immobilization in Liquid-Liquid Biphasic Reaction Systems using Fluorous Phases, Supercritical CO2 or Ionic Liquids... 

The appearance of an offset explains why net photochemistry is observed in the IL, in spite of an efficient back reaction. Fig. 4.7. The chalcone should be preferentially solvated by the [BMIM] organic cation rather than water. As Cc is formed by photoisomerization of Ct, it is either readily converted back to Ct (the back isomerization reaction) or to B2 and AH+. Afterwards, AH in the ionic liquid can be preferentially solvated by the anion and probably by the water molecules present on the water-saturated ionic liquid phase. Therefore, the photochemical production of net AH+ in water/ionic liquids biphasic systems for this chalcone could be explained by the existence of a microheterogeneous structure, where a small fraction of the flavyUum cations would be stabilized by hydrogen bonding and electrostatic interactions with the anions into the polar domains that contain water.[54]... 

Singer and Scammells have investigated the y-Mn02 oxidation of codeine methyl ether (CME) to thebaine in the ionic liquid [BMIM][BF4] [63]. The ionic liquid was used in different ways and with mixed results (Scheme 5.1-35). For example, the oxidation of CME in the ionic liquid gave 38 % yield after 120 hours. A similar reaction under biphasic conditions (with diethyl ether) gave a 36 % yield of thebaine. This reaction gave a 25 % yield of thebaine when carried out in tetrahydrofuran... 

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. 

Apart from the activation of a biphasic reaction by extraction of catalyst poisons as described above, an ionic liquid solvent can activate homogeneously dissolved transition metal complexes by chemical interaction. 

The first successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

Chauvin s group described the selective hydrogenation of cyclohexadiene to cyclohexene through making use of the biphasic reaction system [46]. Since the solubility of cyclohexadiene in [BMIM][SbFg] is about five times higher than the solubility of cyclohexene in the same ionic liquid, the latter was obtained in 98 % selectivity at 96 % conversion. 

Finally, a special example of transition metal-catalyzed hydrogenation in which the ionic liquid used does not provide a permanent biphasic reaction system should be mentioned. The hydrogenation of 2-butyne-l,4-diol, reported by Dyson et al., made use of an ionic liquid/water system that underwent a reversible two-... 

---