Synthesizing of ionic liquids from different chemical pathways.
The term ionic liquids (ILs) has been used to describe salts that melt below 100[degrees]C. ILs are electrolytes forming liquids that consist of cations and anions. ILs extremely low vapour pressure being non-volatile, high stability, highly polar, wide liquid-range, miscible with certain organic solvents and/or water and good solubility of organic and inorganic materials, chemically inert as well. They are reusable, non-inflammable, thermally stable (high thermal stability and liquid range up to about 300[degrees]C), can be designed and tunable solvency (Holbrey & Seddon 1999; Welton 1999; Ngo et al. 2000). ILs are usually characterized by a wide-range electrochemical window of stability, a reasonable ionic conductivity (similar to most non-aqueous electrolytes). ILs are promising environmentally benign, numerous of reaction media which are expected to provide an attractive alternative to conventional volatile organic solvents (VOSs) in current synthetic organic chemistry since their growing use does not lead to air pollution (Yamada et al. 2002).
ILs have been hailed as the alternative to VOSs in chemistry due to their green properties. VOSs nature consists of relatively small molecules so, they have weak intermolecular forces between them, making them highly volatile, they are also flammable and often toxic. Unlike VOSs, ILs do not vaporize into the air, effectively eliminating one of the major routes of environmental contamination (Wasserscheid & Keim 2000), making its operations safer and environmentally acceptable. Among these, ILs have recently classified into five types namely alkylation ILs, metathesis ILs, protic ILs, eutectic ILs and protic eutectic ILs. Nevertheless, during our investigations we have found that the compilations of five types of ILs were never been reported before and comparatively scarce to discuss. Therefore, this paper focused on the types of ILs in order to gain an in-depth understanding of its synthesizing reaction and its application.
Preparation of ILs
The 1-alkylimidazoles and pyridine compounds have proven to be good precursors for the development of alkylation ILs (Wasserscheid & Welton 2003). The chemical structure of common alkylation ILs precursors based on 1-methylimidazole and pyridine compounds is given in Figure 1.
[FIGURE 1 OMITTED]
Preparation of alkylation ILs by use of 1-methylimidazole has been shown in Figure 2. Reaction in particular alkylation is prepared by react 1-methylimidazole with butyl halide in the presence of acetonitrile as catalyst (Dupont et al. 2004).
[FIGURE 2 OMITTED]
ILs are simple charged species that are not simply spheres but have shape and chemical functionality that impose subtle changes in the physical properties. Coulombic interactions are the dominant force between the ions, including dipole, hydrogen-bonding and dispersive forces are also important to the interactions between constituents in the ILs (Tokuda et al. 2004; 2005) however, these forces are weak enough to keep them in the form of liquid at low temperature. Alkylation ILs are micro-biphasic systems composed of polar and non-polar domains (Santos et al. 2007) which special solubility characteristics (Plechkova & Seddon 2008). Alkylation ILs are potential candidates used to dissolve a wide range of organic and inorganic compounds for example, 1-allyl-3-methylimidazolium chloride could be dissolved cellulose up to 39wt.% without derivatization (Wu et al. 2004) and 1-butyl-3-methylimidazolium chloride have been shown to dissolve proteins (Biswas et al. 2006).
The 1-butyl-3-methylimidazolium chloride ILs is the perfect precursor for the preparation of metathesis ILs (Dupont et al. 2004). Metathesis ILs could be prepared by ion exchange reaction between 1-butyl-3-methylimidazolium chloride and potassium salt as shown in Figure 3.
[FIGURE 3 OMITTED]
Based on the structure, the ionic exchange reaction taking place as 1-butyl-3-methylimidazolium ions will replace K+ ions and bind to the negatively charged of anion salt. The potassium salts plays the role as stimulants in ion exchange reaction. The physical and chemical properties of metathesis ILs are similar to those of the alkylation ILs but the range of structure of metathesis ILs is likely to be different from those of alkylation ILs constituted only by discrete anions. Metathesis reaction usually applied to produce hydrophobic and non-water soluble ILs such as 1-butyl-3methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate. It also used to produce low temperature ILs since extinction of strong ion coupling in the compound may suggest the large size of anions compared to alkylation ILs discrete anions. These properties and others are modified by the variation of the ILs anions.
Several researchers have been reported an alternative method to produce ILs through acidic reactions using strong acids as protonator. Proton transfer from a Bronsted acid to a Bronsted base can also form ILs (Yoshizawa et al. 2003). It was recently reported that the protonation of 1-alkylimidazoles by strong acids provided salts, which act as ILs and recognized as protic ILs (Picquet et al. 2003). Some inorganic or organic acids could react directly with N-alkylimidazoles to form a new class of protic IL, which bear an acidic proton on nitrogen of the imidazolium ring. The advantage of protic ILs is the high yield that can be obtained economically with this method. Scheme of synthesized protic ILs has been shown in Figure 4. In protonation the nitrogen constituent phenomenon by strong acid is to neutralize the compound and it created positive and negative ions (Picquet et al. 2003). In addition, melting points of protic ILs are lower owing to the larger size of asymmetry nitrogen cations that providing greater degrees of freedom.
[FIGURE 4 OMITTED]
Protic ILs provided good proton conductivity compared to inorganic media because of their higher mobility. In fact, some protic ILs has been shown to be more conductive than aqueous electrolytes at room temperature (Xu & Angell 2003). Protic ILs have the additional advantage that the proton activity can be adjusted by the choice of Bronsted base and Bronsted acid used in their formation (Byrne & Angell 2008). Protic ILs are currently under intense study in a variety of applications, including fuel cells, where their ability to transport protons between electrodes has been a recent discovery (Belieres et al. 2006). They also have been used as acidic catalysts for the esterification (Fraga-Dubreuil et al. 2002) due to strong acids that have high proton activity.
Another ILs that is receiving interest is eutectic ILs (usually known as deep eutectic solvents, with special properties composed of a mixture forming a eutectic with a melting point much lower than either of the individual components) (Abbott et al. 2001; 2003; 2004). Best of all, they are easy to make simply take the two solids, mix them together with gentle heating, until they melt, and when they cool they remain liquid. The approach to making eutectic ILs is to start with a simple quaternary ammonium halide. In this case, particularly choline chloride (2-hydroxyethyltrimethylammonium chloride), has currently received widespread attention as a hydrogen bond receiver for complexation reactions (Abbott et al. 2003). The use of choline chloride, as shown in Figure 5, is interesting because this quaternary ammonium salt forms a liquid with complexing agent in a same phase, making eutectic ILs.
[FIGURE 5 OMITTED]
Choline chloride formed hydrogen bonds with hydrogen bond donors (including amides, carboxylic acids, alcohols) or metal chlorides, created a homogeneous liquid with a significant decrease in the melting point (Abbott et al. 2006). In the eutectic ILs phase, it consists of nitrogen cations and anions complex, in these structures, the weak interactions of the nitrogen cations with the anions complex, and the large radius ratios, make possible break away of cation from anion relation, and cation motion quantities can turn out to be greater. In nitrogen cations and anions, complex species the increased stabilization of the liquid state also reduced the kinetics of the crystallization process and in this system leads to the possibility of lowering melting points given low temperature ILs (Abbott et al. 2005).
These eutectic ILs have some superior characters in the preparation procedure exhibited as following; firstly, the preparation procedure of these eutectic mixtures is very simple, only needs to mix two different compounds mechanically and requires no medium; secondly, 100% reaction mass efficiency and zero emission in the synthesis are achieved, and which is relatively environmentally benign. Additionally, substance density and energy density in the preparation process is the lower (Constable et al. 2002; Curzons et al. 2001). Abbott et al. (2005) have shown that eutectic ILs can be successfully employed in electropolishing, electroplating and metal oxide processing. Recently, they have also shown that choline chloride and either urea or ethylene glycol (as hydrogen bond donors) based eutectic ILs can be employed in the electrodeposition of zinc, tin and zinc-tin alloys.
Protic Eutectic ILs
The concept of hybrids protonation and complexation of nitrogen based organic compounds have been utilized in our previous studies to produce ILs named protic eutectic ILs (Shamsuri & Dzulkefly 2010), it similar protic and eutectic ILs have been made using a wide variety of quaternary ammonium salts, most notably imidazolium and choline cations. The protonation of nitrogen based organic compounds have altered its insoluble properties into hydrochloride salts is a common way to make them water and acid-soluble substances. Protonation of nitrogen based organic compound by hydrochloric acid to produce hydrochloride salt has been shown in Figure 6.
[FIGURE 6 OMITTED]
Protic eutectic ILs that is a protic eutectic mixture of hydrochloride salts and complexing agents (hydrogen bond donors) produced via protonation and complexation. The idea was exploited by using hydrochloric acid as proton-rich electrolytes for proton carrying medium. This preparation simply used hydrochloric acid and complexing agent. protic eutectic ILs were prepared through environmentally friendly simple techniques, inexpensive and efficient congruent to protic and eutectic ILs which show similar properties to those of ILs. During our studies, we have found that the protic eutectic ILs were successfully produced by means of aminobenzaldehyde, hydroxymethyl amine and cyclic amide precursors giving low viscous and low melting point ILs (Shamsuri & Dzulkefly 2010). When a complexing agent was added to hydrochloride salt as shown in Figure 7, the melting point rapidly reallocated to lower temperature. Complexation of hydrochloride salt, actually, is driven by relatively strong hydrogen bonding interactions between the chloride ions and the hydrogen constituent of complexing agent. protic eutectic ILs are air and water stable and have the high conductivity needed for electrochemical applications and they also have become increasingly alternative to acidic reaction medium.
[FIGURE 7 OMITTED]
ILs believed as novel chemical agents and widely regarded as a greener alternative to many commonly used solvents, because they are designable, recyclable and nonvolatile. Based on it, ILs have been studied for a wide range of synthetic applications, it have attracted considerable interest for use as non-volatile solvent based electrolytes due to they possess many benefits that those solvent based on VOSs. Several studies indicated the feasibility of using simple chemical reactions to synthesize ILs economically to large-scale processes. Five types of ILs have been described specifically alkylation ILs, metathesis ILs, protic ILs, eutectic ILs and protic eutectic ILs. However, almost limitless varieties of ILs are still to be discovered using the simple synthesis method for the producing of ILs with a wide range of possible applications.
The author is thankful to Professor Dr. Dzulkefly Kuang Abdullah for his valuable discussions on ionic liquids and also the Institute of Bioscience, Universiti Putra Malaysia for providing the materials and facilities.
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Ahmad Adlie Shamsuri and Dzulkefly Kuang Abdullah
Laboratory of Industrial Biotechnology, Institute of Bioscience, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia
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|Author:||Shamsuri, Ahmad Adlie; Abdullah, Dzulkefly Kuang|
|Publication:||International Journal of Applied Chemistry|
|Date:||Jan 1, 2011|
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