Ionic liquids are strong solvents. They are able to dissolve the components of LB at ambient to moderate temperatures. Furthermore, ionic liquids are highly tunable through the selection of anion and cation. Beyond toxicity and corrosivity, other considerations affecting the selection of an ionic liquid include price, availability, water tolerance, biodegradability, and phys­ical properties such as viscosity, melting point, dipolar­ity and hydrogen bond basicity. An effective wood dissolution is possible when both the ionic liquid and conditions are properly identified and employed.

The most significant consideration for practical large-scale operations is the toxicity of the ionic liquid to be used. For example, 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) is a good solvent to use on cellulose as it is only moderately toxic compared to that of 1-ethyl-3-methylimidazolium chloride ([EMIM] [Cl]) (Swatloski et al., 2004; Wu et al., 2004).

Corrosivity of the selected ionic liquid is also impor­tant. It plays a large role in the economics of a commercial operation. One can minimize corrosivity by selecting an ionic liquid that is halogen free. Good choices include

1- ethyl-3-methylimidazolium acetate ([EMIM][OAc]) (Liebert, 2010) and 1,3-dimethylimidazolium-dimethyl- phosphate ([MMIM][(MeO)2PO2]) (Zavrel et al., 2009).

Balancing the physical properties and operational conditions is important to obtaining the most ideal disso­lution of LB. For example, if the viscosity of an ionic liquid is high, it may be necessary to operate the pretreat­ment at a high temperature to obtain a practical dissolu­tion. As a result, the reactions may become unstable and may give rise to undesirable reactions and by-products. A solution to this problem is to reduce the viscosity of the ionic liquid by combining it with a cosolvent. A good viscosity-reducing cosolvent is polyethylene glycol (Willauer et al., 2000).

The dissolution rate is inversely proportional to wood chip sizes. For example, ball-milled wood powder produces a higher dissolution rate than does sawdust. The dissolution rate for TMP fibers is higher than that for sawdust, which is much greater than that of wood chips (Kilpelainen et al., 2007; Sun et al., 2009; Zavrel et al., 2009).

In addition to particle size, dissolution efficiency is also highly sensitive to the water content. Water attenu­ates the dissolution effectiveness of an ionic liquid. Studies have shown that storing wood chips at warm temperatures, e. g. 50°C or 90°C, reduces the water con­tent of the wood and thus improves the pretreatment effectiveness (Kilpelainen et al., 2007; Sun et al., 2009). Reducing the water content improves the dissolution power of an ionic liquid regardless of the type of wood being treated. However, if the wood becomes too dry, the wood composition may change unfavorably. Determining the precise water content of LB is quite difficult and is complicated due to the diversity of envi­ronmental conditions of the regions from which the wood studied grew. Variables such as humidity and var­iances in species present a challenge when comparing literature on the subject (Wang et al., 2012).

The type of LB, dissolution time, temperature and ionic liquid to wood ratio, are all factors that contribute to the dissolution power of an ionic liquid. That said, those ionic liquids that were effective at dissolving both lignin and cellulose were also excellent at overall LB dissolution. One of the best solvents for wood chips is the combination of 1-allyl-3-methylimidazolium chlo­ride ([AMIM][Cl]) and [EMIM][OAc]. Ionic liquids derived from polycyclic amidine bases have been shown to dissolve aspen wood chips completely (D’Andola et al., 2008). The ionic liquids used in this study were 1,8-diazabicyclo[5,4,0] undec-7-enium salt, and 1,8-diazabicyclo[5,4,0] undec-7-enium chloride [HDBU] [Cl] (D’Andola et al., 2008). It has been observed that [AMIM][Cl] can effectively dissolve both hardwood and softwood wood chips. However, the same solvent only partially dissolved Norway spruce (Kilpelainen et al., 2007). The efficiency of [AMIM][Cl] in dissolution of wood is due to the presence of p-electrons both in the alkenyl chain as well as in the imidazolium ring. Possible p—p interactions may occur between the aro­matic part of lignin and the ionic liquid (Hunter and Sanders, 1990; Kilpelainen et al., 2007). The highest sol­ubility of maple wood powder was achieved using [AMIM][Cl] and [BMIM][Cl] (Lee et al., 2009).

[EMIM][OAc] can completely treat three types of wood chips. It is used to treat spruce, beech and chestnut. However, it only partially dissolves silver fir (Abies alba) wood chips (Zavrel et al., 2009). A plausible explanation for this difference is that silver fir contains more cellulose (50.3%) and lignin (27.7%) than the other wood species (Kuznetsov et al., 2002). When comparing the dissolution effectiveness of [EMIM][OAc] to [BMIM][Cl] and control­ling for species, wood chip size, and temperature one can obtain a 3.6-fold increase in dissolution effectiveness us­ing [EMIM][OAc] vs [BMIM][Cl]. In this case, southern yellow pine wood chips were treated at 110 °C. This

3.6- fold increase in dissolution effectiveness is

attributable to the basicity of the acetate anion, higher than that of the chloride anion. Thus, [EMIM][OAc] is stronger at breaking the intramolecular hydrogen bonds (Fort et al., 2007; Sun et al., 2009).

An opportunity for improvement in using ionic liq­uids is better recovery of solubilized cellulosic materials and lignin. A significant drawback is that much of the hemicellulose is washed away during the recovery pro­cess. Figure 27.5 illustrates the process.

Following pretreatment with an ionic liquid, an enzy­matic hydrolysis pretreatment is applied to produce the sugars for downstream fermentation. This pretreatment can recover as much as 90% of the cellulose for enzy­matic hydrolysis. While cellulose is recovered at a high rate, the hemicelluloses are not as they are washed away. As Figure 27.5 illustrates, the ionic liquids are recovered and recycled for reuse. Even so, the problems of price, toxicity, and the lost hemicellulose persist, which inhibit wide adoption in industrial scale operations.