Ionic liquids are a relatively new class of solvents that have recently gained popularity as environmentally friendly alternatives to organic solvents. An ionic liquid is a salt composed of anions and cations that are poorly co-ordi­nated and which has a melting point typically below 100°C. Ionic liquids are also referred to as liquid electrolytes, ionic melts, liquid salts, ionic glasses, and the like. There are thousands of substances that fall into this category. Ionic liquids have been demonstrated as very efficient solvents in the fields of hydrogenation, esterification, nanomaterial synthesis, biocatalysis, and selective extraction of aromatics [55, 56].

The development of a novel biomass pretreatment technology using ionic liquids has only recently been initiated. The first demonstration of an ionic liquid as a cellulose solvent under relatively mild processing conditions was reported in 2002 by Swatloski [56]. In experiments using a range of anions and 1-butyl-3-methylimidazolium cations, some ionic liquids were able to completely dissolve microcrystalline cellulose, and the cellulose was recovered through the addition of an antisolvent such as water or eth­anol. Moreover, the recovered product could be regenerated into a wide range of shapes and morphologies. The most effective cellulose solvents were the ionic liquids that contain chloride anions. An important finding associated with this novel pretreatment method is that enzymes can more efficiently hydrolyze into glucose the amorphous cellulose produced by ionic liquids than the microcrystalline cellulose found in lignocellulose naturally [55, 57].

Ionic liquids are an exciting area of new scientific discovery and inher­ently possess many processing merits in lignocellulose pretreatment. More in-depth R&D work needs to be conducted, however, before a commercially viable process can be fully developed and exploited.