Ionic liquids were reported as early as 1914 when ethylammonium nitrate was shown to melt at 12 ° C [41]. In recent years the study of these compounds has experienced a resurgence. An ionic liquid is defined as a chemical compound that exists as an organic anion and a cation and has a melting point below 100 °C. There are many different kinds of ILs, and many more are being developed (Fig. 8.3). The most common forms are based on dialkylimidazolium, tetraalkylammonium, alkylpyridinium, or tetraalkylphosphonium cations coupled with an inorganic

Common Anions

MeSO,- CFoCOO’ CFECOO

Fig. 8.3 Common IL anions and cations anion [42]. Due to their ionic character, ILs have essentially no vapor pressure. While it has been reported that some ILs can be distilled under the right conditions [43], in general, the vapor pressure is low enough to be neglected. Because ILs are composed of discrete anions and cations, the solvent properties, such as viscosity, melting point, and miscibility with other solvents can be tuned through the right combination and design of each ion. The ability to design ILs to specific substrates, chemistries, and situations is important, because ILs are increasingly being looked to as a medium for applications, including biomass processing [44].

Additionally, most ILs display good stability under a wide range of chemical, thermal, and electrochemical conditions. Some ILs have reported thermal stability of over 300 °C, although the stability is highly dependent on the identity of the IL’s constituent ions [45]. There has been research to show that the long term thermal stability of some ILs is significantly less than that indicated by standard thermogra­vimetric analysis techniques [46]. This may be important in the development of biomass processing techniques in ILs, because one of the main advantages of ILs is the potential for the essentially complete recycling of solvents. ILs are also gener­ally assumed to have good chemical stability. In many cases, strongly acidic, reducing, or oxidizing agents can be used without degradation of the ILs [47, 48].

There are, however, some exceptions to this rule [49]. Dialkylimidazolium based ILs have a mildly acidic hydrogen that undergoes hydrogen exchange in aqueous media and can even deprotonate to form a reactive carbene under basic conditions [49, 50]. Some ILs, such as halide, acetate, or formate based ILs, can form volatile acids (such as hydrochloric, acetic, or formic acid) [43, 51]. Additionally, some ILs can be designed to be reactive with the addition of acidic moieties or metal centers [47, 52, 53]. ILs have also been looked to as media for novel electrochemistry, as some of them have a wide window of electrochemical stability [54].

ILs have found a place in a number of catalytic reactions. In many cases, the solvent properties of ILs increase reaction rate and selectivity [55—57]. Addition­ally, post reaction separations are often made easier due to immiscibility of products with the IL phase, such as in the case of esterification in acid ILs [58, 59]. ILs also give the ability to distill volatile products and reuse the IL [60]. The coordination of ILs to metal centers has also been shown to increase the activity and recyclability of some metal catalysts [48, 52]. While ILs are often designed around specific solvent properties, some ILs are designed to work as a combined solvent and catalyst. A common method for this is to attach an acid group to the end of an alkyl chain on the cation, which has been used to depolymerize cellulose and to catalyze esterification reactions [53, 61, 62].

Recently, some ILs have been shown to be able to either partially or completely dissolve cellulose, lignin, or lignocellulosic biomass. Imidazolium-based ILs seem to be especially well suited for this application. The most common solvents that are used to dissolve biomass are alkylimidazolium chlorides, acetates, and formates, although others have been investigated and used in biomass chemistry [21, 63, 64]. This property of ILs has been exploited in the production of novel materials, such as cellulosic aerogels and films in addition to being used as a solvent for catalysis of lignocellulose [65—67].

The property of these ILs that enables them to effectively dissolve biomass is their ability to function as a hydrogen bond acceptor while only having a limited ability to act as a hydrogen bond donor. In general, it is the ability of the anion to form hydrogen bonds with the hydroxyl groups of the cellulose, disrupting the hydrogen bond crosslinking of the polysaccharide, that makes these ILs effective at solubilizing biomass [22, 68] (Fig. 8.4). Dissolution of glucose in 1,3-dimethylimidazolium chloride was studied through computer modeling to analyze the IL/saccharide interactions further. This work demonstrated the almost exclusive coordination of the chloride anion to saccharides with only minimal contributions from hydrogen bonding and van der Waal forces from the imidazolium cation [69]. The dissolution process first swells the cellulose and, in the case of lignocellulose, extracts the lignin

[70] . Some ILs have even been specifically designed to dissolve carbohydrates without denaturing enzymes to allow for homogenous enzymatic catalysis ofbiomass

[71] . Work has been done to investigate ILs using solvatochromic dyes to probe the hydrogen bonding acidity and basicity, the polarity, and dispersion forces in various ILs [72]. These properties are part of what makes ILs an attractive solvent for the processing of biomass.