A wide variety of biomass feedstock/IL combinations has been studied for their potential in biomass pretreatment. Multiple wood species have been studied: poplar [29], spruce [7, 30-34], eucalyptus [31, 32], pine [4, 6, 7, 31, 32, 35-37], maple [25, 38], Metasequoia glyptostroboides [16], red oak [36], common beech [34], cork [39], and Japanese fir [40]. Other biomass feedstocks currently under investigation include grasses, such as switchgrass [41, 42], Miscanthus grasses [26, 43], and agricultural wastes, such as corn stovers [6, 33, 35, 43-45], wheat straw [27] and rice straw [6, 35, 46].

Among the most successful and widely used ILs in native wood pretreatment are the imidazolium-based ILs with the chloride or acetate anion. The ILs 1-allyl-

3- methylimidazolium chloride ([AMIM][Cl]) and [BMIM][Cl] could dissolve
maple wood flour at solubilities above 30 g/IL kg at 80°C under nitrogen atmo­sphere after 24 h [25]. Ball-milled pine powder and spruce sawdust (size 0.1-2 mm) were completely dissolved in [BMIM][Cl] and [AMIM][Cl] at a weight ratio of up to 8% at 80-110°C in 8 h with mechanical stirring [7]. [AMIM][Cl] was able to dissolve completely 5 wt% of spruce, silver fir, beech, chestnut wood chips (particle size 1-2 mm) at 90°C in 12 h, whereas the same wood samples were only partially dissolved in 1-ethyl-3-methylimidazolium chloride ([EMIM][Cl]), [BMIM][Cl], and 1,3-dimethylimidazolium dimethyl — phosphate ([MMIM][Me2PO4]) in the same conditions [34]. The IL 1-ethyl-3- methylimidazolium chloride ([EMIM][Cl]) can partially dissolve wheat straw and pine wood particles (<1 mm, 5 wt%) at 100°C in 24 h, [BMIM][Cl] can only partially dissolve wheat straw, and 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) could dissolve neither [47]. Ground pine, poplar, eucalyptus, and oak were dissolved in [BMIM][Cl] with a 5 wt% solubility at 100°C. After 24 h, about 45 wt% of the cellulosic material was extracted from the native biomass. The extraction rates were higher for softwoods, such as pine and poplar. 13C Nuclear Magnetic Resonance (NMR) confirmed the presence of dissolved polysaccharides in the wood/IL mixture [4].

[EMIM][OAc] dissolved spruce, beech, chestnut completely (5 wt%), but not silver fir [34]. In another study, [EMIM][OAc] could dissolve 5 wt% of red oak (particle size 0.125-0.250 mm) completely in 25 h at 110°C, while it took 46 h to dissolve 5 wt% of southern yellow pine in the same conditions [36]. Pretreatment of maple wood flour with [EMIM][OAc] or 1-butyl-3-methylimi — dazolium acetate ([BMIM][OAc]) at 90°C increased significantly the sugar yield and the amount of extracted lignin [38]. Pretreatment with 1-butyl-3-methyl — imidazolium methyl sulfate ([BMIM][MeSO4]) at the same temperature for the same duration resulted in sugar yields comparable to the untreated wood flour and an amount of extracted lignin lower than with [EMIM][OAc] or [BMIM][OAc]. This was explained by the fact that [BMIM][MeSO4] only delignified the middle lamella and not the primary cell wall and cellulose-rich secondary cell wall. Also, the [EMIM][OAc] or [BMIM][OAc] pretreatment for 12 h reduced the wood fiber diameter from an average of 250 pm in the untreated flour to about 17 pm. Pretreatment with [BMIM][MeSO4] had no effect on the wood fiber diameter [38].

Other combinations of IL/native wood were studied. Dry wood (Metasequoia glyptostroboides, 60 mesh sawdust) was partially dissolved in 1-butyl-3-allylim — idazolium chloride ([BAIM][Cl]) or 1-methyl-3-allylimidazolium chloride ([MAIM][Cl]) at a weight ratio from 4.5:1 to 10.5:1 (60-90°C for 10-40 min) [16]. Phenyl-containing ionic liquids were synthesized to see if the aromatic n-systems would be better at disrupting the strong n-n interactions between aro­matic groups in lignin. Indeed, after the wood was dissolved in 1-benzyl-3- methylimidazolium chloride ([BzMIM][Cl]), the solution was clear, free of any residual lignin [7]. Ball-milled poplar was soluble in 1-allylpyridinium chloride, cyanomethylpyridinium chloride, and pyridinium chloride within 1 h at 60°C with solubilities ranging from 35 to 80 mg/g [29].

In addition to woods, ILs could at least partially dissolve or delignify other feedstocks, including leaves and agricultural wastes. Triticale straw, flax shives, and wheat straw were soluble in [EMIM][OAc] and [BMIM][Cl] [27]. The dis­solution of shredded oil palm fronds in [BMIM][Cl] was studied for temperatures ranging from 60 to 100°C [22]. Lignin was extracted from bagasse using the IL 1-ethyl-3-methylimidazolium alkylbenzenesulfonate at high temperatures (170-190°C) [48]. Rice straw powder (<2 mm) could be dissolved in [BMIM] [Cl], [EMIM][Cl], and [EMIM][OAc] completely in 24 h at 130°C. The amount of regenerated cellulose and glucose after enzymatic hydrolysis was highest for [EMIM][OAc] [46]. Milled corn cob had solubilities above 30 g/kg at 130°C in 1-methyl-3-methylimidazolium dimethylphosphite ([MMIM][DMP]), 1-ethyl-3- methylimidazolium diethylphosphate ([EMIM][DEP]), 2-ethyl-3-methylimidazo — lium dimethylphosphite ([EMIM][DEP]), [BMIM][Cl], and 1-butyl-1-meth- ylpyrrolidinium chloride ([BMPy][Cl]). Pretreatment with chloride ILs resulted in the doubling of reducing sugar yield after enzymatic hydrolysis [44].