Room ionic liquid

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

T. Welton, Room temperature ionic liquids. Solvents for synthesis and catalysis, Chem Rev 99 2071-2083 1999. C.M. Gordon, New developments in catalysis using ionic liquids, Appl. CatalA General 222 101-117 2001. 

Room-temperature ionic liquids, salts with A,A-dialkylimidazolium cations in synthesis and catalysis 99CRV2071. 

There are many synonyms used for ionic liquids, which can complicate a literature search. Molten salts is the most common and most broadly applied term for ionic compounds in the liquid state. Unfortunately, the term ionic liquid was also used to mean molten salt long before there was much literature on low-melting salts. It may seem that the difference between ionic liquids and molten salts is just a matter of degree (literally) however the practical differences are sufficient to justify a separately identified niche for the salts that are liquid around room temperature. That is, in practice the ionic liquids may usually be handled like ordinary solvents. There are also some fundamental features of ionic liquids, such as strong... 

We had no good way to predict if they would be liquid, but we were lucky that many were. The class of cations that were the most attractive candidates was that of the dialkylimidazolium salts, and our particular favorite was l-ethyl-3-methylimid-azolium [EMIM]. [EMIMJCl mixed with AICI3 made ionic liquids with melting temperatures below room temperature over a wide range of compositions [8]. We determined chemical and physical properties once again, and demonstrated some new battery concepts based on this well behaved new electrolyte. We and others also tried some organic reactions, such as Eriedel-Crafts chemistry, and found the ionic liquids to be excellent both as solvents and as catalysts [9]. It appeared to act like acetonitrile, except that is was totally ionic and nonvolatile. 

What constitutes an ionic liquid, as distinct from a molten salt It is generally accepted that ionic liquids have relatively low melting points, ideally below ambient temperature [1, 2]. The distinction is arbitrarily based on the salt exhibiting liquidity at or below a given temperature, often conveniently taken to be 100 °C. However, it is clear from observation that the principle distinction between the materials of interest today as ionic liquids (and more as specifically room-temperature ionic liquids) and conventional molten salts is that ionic liquids contain organic cations rather than inorganic ones. This allows a convenient differentiation without concern that some molten salts may have lower melting points than some ionic liquids . 

However, ionic liquids containing other classes of organic cations are known. Room-temperature ionic liquids containing organic cations including quaternary ammonium, phosphonium, pyridinium, and - in particular - imidazolium salts are currently available in combination with a variety of anions (Figure 3.1-1 provides some common examples) and have been studied for applications in electrochemistry [7, 8] and in synthesis [9-11]. 

A wide variety of physical properties are important in the evaluation of ionic liquids (ILs) for potential use in industrial processes. These include pure component properties such as density, isothermal compressibility, volume expansivity, viscosity, heat capacity, and thermal conductivity. However, a wide variety of mixture properties are also important, the most vital of these being the phase behavior of ionic liquids with other compounds. Knowledge of the phase behavior of ionic liquids with gases, liquids, and solids is necessary to assess the feasibility of their use for reactions, separations, and materials processing. Even from the limited data currently available, it is clear that the cation, the substituents on the cation, and the anion can be chosen to enhance or suppress the solubility of ionic liquids in other compounds and the solubility of other compounds in the ionic liquids. For instance, an increase in allcyl chain length decreases the mutual solubility with water, but some anions ([BFJ , for example) can increase mutual solubility with water (compared to [PFg] , for instance) [1-3]. While many mixture properties and many types of phase behavior are important, we focus here on the solubility of gases in room temperature IFs. 

The early history of ionic liquid research was dominated by their application as electrochemical solvents. One of the first recognized uses of ionic liquids was as a solvent system for the room-temperature electrodeposition of aluminium [1]. In addition, much of the initial development of ionic liquids was focused on their use as electrolytes for battery and capacitor applications. Electrochemical studies in the ionic liquids have until recently been dominated by work in the room-temperature haloaluminate molten salts. This work has been extensively reviewed [2-9]. Development of non-haloaluminate ionic liquids over the past ten years has resulted in an explosion of research in these systems. However, recent reviews have provided only a cursory look at the application of these new ionic liquids as electrochemical solvents [10, 11]. 

Table 3.6-2 The room-temperature electrochemical potential windows for binary and ternary chloroaluminate and related ionic liquids.

The conductivity of ionic liquids often exhibits classical linear Arrhenius behavior above room temperature. However, as the temperatures of these ionic liquids approach their glass transition temperatures (T s), the conductivity displays signif-... 

The room temperature conductivity data for a wide variety of ionic liquids are listed in Tables 3.6-3, 3.6-4, and 3.6-5. These tables are organized by the general type of ionic liquid. Table 3.6-3 contains data for imidazolium-based non-haloaluminate alkylimidazolium ionic liquids. Table 3.6-4 data for the haloaluminate ionic liquids, and Table 3.6-5 data for other types of ionic liquids. There are multiple listings for several of the ionic liquids in Tables 3.6-3-3.6-5. These represent measurements by different researchers and have been included to help emphasize the significant vari-... 

In general, isotopic exchange is both expensive and difficult. In the case of many room-temperature ionic liquids, however, the manufacture of deuterated ionic liquids is relatively easily achievable. For example, the general synthesis of l-allcyl-3-methylimidazolium salts is shown in Scheme 4.1-1 [2]. This methodology allows maximum flexibility in the deuteration on the imidazolium cation that is, it can be either ring or side chain deuteration or both. 

Neutron diffraction has been used extensively to study a range of ionic liquid systems however, many of these investigations have focussed on high-temperature materials such as NaCl, studied by Enderby and co-workers [3]. A number of liquid systems with relatively low melting points have been reported, and this section summarizes some of the flndings of these studies. Many of the salts studied melt above 100 °C, and so are not room-temperature ionic liquids, but the same principles apply to the study of these materials as to the lower melting point salts. 

Bowron et al. [11] have performed neutron diffraction experiments on 1,3-dimethylimidazolium chloride ([MMIM]C1) in order to model the imidazolium room-temperature ionic liquids. The total structure factors, E(Q), for five 1,3-dimethylimidazolium chloride melts - fully probated, fully deuterated, a 1 1 fully deuterated/fully probated mixture, ring deuterated only, and side chain deuterated only - were measured. Figure 4.1-4 shows the probability distribution of chloride around a central imidazolium cation as determined by modeling of the neutron data. 

So far, there have been few published simulation studies of room-temperature ionic liquids, although a number of groups have started programs in this area. Simulations of molecular liquids have been common for thirty years and have proven important in clarifying our understanding of molecular motion, local stmcture and thermodynamics of neat liquids, solutions and more complex systems at the molecular level [1 ]. There have also been many simulations of molten salts with atomic ions [5]. Room-temperature ionic liquids have polyatomic ions and so combine properties of both molecular liquids and simple molten salts. 

These reactions occur with similar rates to those carried out in dipolar aprotic solvents such as DMF or DMSO. An advantage of using the room-temperature ionic liquid for this reaction is that the lower reaction temperatures result in higher selec-tivities for substitution on the oxygen or nitrogen atoms. The by-product (sodium or potassium halide) of the reaction can be extracted with water and the ionic liquid recycled. 

Beckmann rearrangements of several ketoximes were performed in room-temperature ionic liquids based on l,3-dialkylimida2olium or alkylpyridinium salts containing phosphorus compounds (such as PCI5) by Deng and Peng [59] (Scheme 5.1-31, BP = 1-butylpyridinium). Turnover numbers of up to 6.6 were observed, but the authors did not mention whether the ionic liquid could be reused. 

Scheme 5.1-62 The acetylation of benzene in a room-temperature ionic liquid.

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

As early as 1972 Parshall described the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate melts [1]. [NEt4][SnCl3], the ionic liquid used for these investigations, has a melting point of 78 °C. Recently, platinum-catalyzed hydroformylation in the room-temperature chlorostannate ionic liquid [BMIM]Cl/SnCl2 was studied in the author s group. The hydroformylation of 1-octene was carried out with remarkable n/iso selectivities (Scheme 5.2-13) [66]. 

While unmodified xanthene ligands (compound a in Figure 5.2-4) show highly preferential solubility in the organic phase in the biphasic l-octene/[BMIM][PFg] mixture even at room temperature, the application of the guanidinium-modified xanthene ligand (compound b in Figure 5.2-4) resulted in excellent immobilization of the Rh-catalyst in the ionic liquid. 

Unfortunately, investigations with ionic liquids containing high amounts of AlEtCl2 showed several limitations, including the reductive effect of the alkylaluminium affecting the temperature stability of the nickel catalyst. At very high alkylaluminium concentrations, precipitation of black metallic nickel was observed even at room temperature. 

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