Catalysts in ionic liquids

1 The active species is known to be ionic in organic conventional solvents. 

Not only cationic, but also anionic, species can be retained without addition of specially designed ligands. The anionic active [HPt(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. 

Certain amines, when linked to TPPTS, form ionic solvents liquid at quite low temperature. Bahrman [34] used these ionic liquids as both ligands and solvents for the Rh catalyst for the hydroformylation of alkenes. In this otherwise interesting approach, however, the ligand/rhodium ratio, which influences the selectivity of the reaction, is difficult to control. 

As well as phosphorus ligands, heterocyclic carbene ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to 

The ease of formation of the carbene depends on the nudeophUicity of the anion associated with the imidazohum. For example when Pd(OAc)2 is heated in the presence of [BMIM](Br], the formation of a mixture of Pd imidazolyhdene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [35]. The highest activity and stability of palladium is observed in the ionic hquid [BMIM][Br]. Carbene complexes 

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Activation of a transition metal catalyst in ionic liquids... 

Methods of Analysis of Transition Metal Catalysts in Ionic Liquids... 

Many transition metal-catalyzed reactions have already been studied in ionic liquids. In several cases, significant differences in activity and selectivity from their counterparts in conventional organic media have been observed (see Section 5.2.4). However, almost all attempts so far to explain the special reactivity of catalysts in ionic liquids have been based on product analysis. Even if it is correct to argue that a catalyst is more active because it produces more product, this is not the type of explanation that can help in the development of a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. Clearly, much more spectroscopic and analytical work is needed to provide better understanding of the nature of an active catalytic species in ionic liquids and to explain some of the observed ionic liquid effects on a rational, molecular level. 

In general, most of the methods used to analyze the chemical nature of the ionic liquid itself, as described in Chapter 4, should also be applicable, in some more sophisticated form, to study the nature of a catalyst dissolved in the ionic liquid. For attempts to apply spectroscopic methods to the analysis of active catalysts in ionic liquids, however, it is important to consider three aspects a) as with catalysis in conventional media, the lifetime of the catalytically active species will be very short, making it difficult to observe, b) in a realistic catalytic scenario the concentration of the catalyst in the ionic liquid will be very low, and c) the presence and concentration of the substrate will influence the catalyst/ionic liquid interaction. These three concerns alone clearly show that an ionic liquid/substrate/catalyst system is quite complex and may be not easy to study by spectroscopic methods. 

As early as 1990, Chauvin and his co-workers from IFP published their first results on the biphasic, Ni-catalyzed dimerization of propene in ionic liquids of the [BMIM]Cl/AlCl3/AlEtCl2 type [4]. In the following years the nickel-catalyzed oligomerization of short-chain alkenes in chloroaluminate melts became one of the most intensively investigated applications of transition metal catalysts in ionic liquids to date. 

When the same [NiI (NHC)2] complexes are employed as alkene dimerisation catalysts in ionic liquid (IL) solvent [l-butyl-3-methylimidazolium chloride, AICI3, A-methylpyrrole (0.45 0.55 0.1)] rather than toluene, the catalysts were found to be highly active, with no evidence of decomposition. Furthermore, product distributions for each of the catalyst systems studied was surprisingly similar, indicating a common active species may have been formed in each case. It was proposed that reductive elimination of the NHC-Ni did indeed occur, as outlined in Scheme 13.8, however, the IL solvent oxidatively adds to the Ni(0) thus formed to yield a new Ni-NHC complex, 15, stabilised by the IL solvent, and able to effectively catalyse the dimerisation process (Scheme 13.9) [25-27],... 

Application of Metal Nanoparticle Catalysts in Ionic Liquids for Energy- and Environment-Related Systems... 

A rather new concept for biphasic reactions with ionic liquids is the supported ionic liquid phase (SILP) concept [115]. The SILP catalyst consists of a dissolved homogeneous catalyst in ionic liquid, which covers a highly porous support material (Fig. 41.13). Based on the surface area of the solid support and the amount of the ionic liquid medium, an average ionic liquid layer thickness of between 2 and 10 A can be estimated. This means that the mass transfer limitations in the fluid/ionic liquid system are greatly reduced. Furthermore, the amount of ionic liquid required in these systems is very small, and the reaction can be carried in classical fixed-bed reactors. 

Many organometallic catalysts are soluble in ionic liquids, especially including ionic compounds. Neutral species, such as Wilkinson s catalyst, are also soluble to some extent in ionic liquids (169). There are numerous examples illustrating the dispersion and isolation of organometallic catalysts in ionic liquids a list of examples is given in a recent review (/). 

The strong acidity of the proton at the C2 position of a [AMIM] ion has been well recognized 183). This cation can react with palladium complexes to form inactive l,3-dialkylimidazol-2-ylidene palladium complexes 200), as confirmed in a study of the conventional Pd(OAc)2/PPh3/base catalyst in ionic liquids for the telomerization of butadiene with methanol at 85°C 201). 

Cinnamic acid and its derivatives were reduced in high yields under mild conditions using ammonium formate as hydrogen donor, in the presence of palladium(II) as catalyst in ionic liquids.294... 

The recycling potential of hydroformylation catalysts in ionic liquids is highly dependent on the way the product is isolated. In terms of catalyst lifetime, simple decantation is certainly the method of choice. However at an industrial level, distillation is the most common separation technique and evaluation of catalyst recyclability under somewhat more stressful conditions has been determined/451... 

Phase separation Immiscibility of ionic liquid with product/ extract, immobilisation of catalyst in ionic liquid phase BASIL [108, 109, 193-195], Difasol [82, 111, 112] Easy removal, low energy requirement Loss of catalyst to product phase if not well immobilised, contamination of product... 

Shen HY, Judeh ZMA, Ching CB et al (2004) Comparative studies on alkylation of phenol with rert-butyl alcohol in the presence of hquid or solid acid catalysts in ionic liquids. J Mol Catal A Chem 212 301-308... 

Wolfson A, Wuyts S, De Vos DE, Vankelecom IFJ, Jacobs PA (2002) Aerobic oxidation of alcohols with ruthenium catalysts in ionic liquids. Tetrahedron Lett 43 8107-8110... 

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