Wet-ionic liquid

Water in an ionic liquid may be a problem for some applications, but not for others. However, one should in all cases know the approximate amount of water present in the ionic liquid used. Moreover, one should be aware of the fact that water in the ionic liquid may not be inert and, furthermore, that the presence of water can have significant influence on the physicochemical properties of the ionic liquid, on its stability (some wet ionic liquids may undergo hydrolysis with formation of protic impurities), and on the reactivity of catalysts dissolved in the ionic liquid. 

Screening of the reaction media showed that, with the use of an ionic liquid/ water mixture (a so-called wet ionic liquid ), recycling of the catalyst could be improved compared to the reaction in ionic liquid without co-solvent. The com-... 

AU ILs, either hydrophobic or hydrophilic, are hygroscopic and quickly absorb a significant quantity of water when exposed to air. Their properties, such as viscosity, solubility, polarity, conductivity, and electrochemical characteristics [48] (see Section 5.2.1.2), are significantly dependent on the concentration of water. Wet ionic liquids cannot be considered as homogeneous solvents but rather as nano-... 

In the presence of chlorides (as an impurity in an ionic liquid) or of fluorides (which are formed as a result of hydrolysis of tetrafluoroborate anion in a wet ionic liquid) the passive layer is destroyed and pitting corrosion develops. This is why the first plateau discussed above for dry and chloride-free BMIBF4 (Fig. 3) almost disappears in the presence of water and chlorides when the corrosion inhibitor is absent (Fig. 4). In contrast, in the presence of MBT the first plateau in wet and chloride-containing BMIBF4 (Fig. 4) is almost as wide as in dry and chloride-free BMIBF4 (Fig. 3). Apparently the MBT layer protects the surface from chloride- and fluoride attack. 

In a number of general properties, such as viscosity and thermal conductivity, melts differ little from solutions. Their surface tensions are two to three times higher than those of aqueous solutions. This leads to poorer wetting of many solids, including important electrode materials such as carbon and graphite, by the ionic liquids. 

Pioneering work in fibroin wet spinning can be traced back to 1930s. After that, little work has been done until the late 1980s, when more research was done to investigate the spinning dope systems, and structure and properties of the artificial fibroin silk. The composition of the dope is very important to the properties of the final fiber. Several kinds of solvents, such as LiBr—EtOH, Ca(NOo,)2—MeOH, formic acid, HFIP, hexafluoro acetone (HFA), and so on, are used to prepare the spinning dope (Table 4). Very recently, an ionic liquid was used as dope solvent (Phillips et al., 2005). 

In contrast to dead-end microfiltration, which could also be used to remove solids from spent electrolytes producing (after addition of a solvent and at elevated temperatures) an ionic liquid as residue, the residue in the extractive regeneration is wet sludge only. 

The easiest impregnation technique for the immobilisation of ionic liquids is called the method of incipient wetness . This method basically consists of the slow addition of the ionic liquid to the support material, a period of stirring to guarantee homogenisation, and... 

The method of incipient wetness has one major drawback, which is the destruction of structured support materials. Immobilisation on zeolites or MCM-41 leads to the decomposition of the carrier when a highly acidic ionic liquid is added directly. The reason for this seem to be superacidic properties of HCI in ionic liquids. Especially the MCM-41 materials, which are mesoporous aluminosilicates with a very high surface area, are interesting carrier materials. Alternatives to the method of incipient wetness have been developed and will be presented elsewhere. 

Highly Lewis-acidic chloroaluminate ionic liquids (ILs) are well known to be both versatile solvents and effective catalysts for Friedel-Crafts reactions [1,2]. Tailoring the physical and chemical properties of the ILs to the needs of a specific reaction allows for a high diversity of applications [3,4]. We could show that immobilising these ILs on inorganic supports yields very active catalysts for alkylation reactions. The immobilisation of ionic liquids leads to novel Lewis-acidic catalysts (NLACs). The methods presented include the method of incipient wetness (method 1, further on called NLAC I), which has been introduced in detail by Hoelderich et al. f5], but focus of this presentation lies on the methods 2 (NLAC II) and 3 (NLAC III). 

Different methods for the immobilisation of ionic liquids have been tested. These can be divided into two different categories a) via the inorganic anion (method of incipient wetness and b) via the organic cation, here a methyl-alkyl-imidazolium cation. The method of incipient wetness was presented by Hoelderich [5], we concentrate on the immobilisation of ionic liquids by the methods of grafting or by synthesising organically modified MCM 41 materials (Sol gel method) (seeFigure 1). 

Method 1 is known as the method of incipient wetness, because the ionic liquid is added to the support until the mixture starts to lose the appearance of an dry powder. This is the most simple of the presented methods, allowing the immobilisation of high amounts of chloroaluminate liquids on any given silica support. Unfortunately, during the immobilisation step HC1 is created which leads to a decomposition of zeolites and MCM 41 type supports. This problem could be overcome by a modification of the immobilisation method. The supported ILs synthesised this way show a high catalytic activity in Friedel-Crafts reactions. 

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). 

In summary, the few initial studies conducted so r on the inter cial structure between ionic liquids and solid sur ces clearly suggest - in a more general context - that confinement of ionic liquids on solid sur ces definitely induces some distinctive, structural control of the molecular kyering of the ionic liquid and of the ionic hquid distribution (i.e. wetting ability). However, further investigations are surely needed to elucidate these effects in the context of supported ionic liquid catalysis in more detail. 

Table 5.4), prepared from reduction of Pd(II) salts with potassium graphite. The results suggested that this catalyst was not very active. However, some years later Jikei and Kakimoto [73] prepared a more active Pd/CGr based on a smaller crystallite size. In 2002, Kohler et al. [74] studied a variety of Pd/C catalysts with different properhes (Pd dispersion, oxidation state, water content, conditions of catalysts preparation etc.) in the Heck reaction of aryl bromides with olefins (entry 4, Table 5.4). The authors pointed out the hypothesis that the leached Pd from the support is the active species and the solid Pd/C catalyst acts as a reservoir that delivers catalytically active Pd species into solution. All catalysts were obtained by wet impregnation (5% Pd loading). The Heck reaction can also be conducted in ionic liquids through promotion by microwave irradiation. Moreover the reaction of iodobenzene with methylacrylate in NMP was reported to be accelerated by ultrasound [75]. The ionic liquid containing the catalyst system was used five consecutive times with only a slight loss of activity (entry 5, Table 5.4) [76]. Perosa [77] reported the addition of a phase transfer catalyst to an ionic liquid as a method to accelerate the C-C coupling reaction. As far as we know, only by using ionic liquids has Pd on carbon been recovered and reused with success.

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