Room temperature ionic liquids (RTILs) were used for the development of new technologies in chemical and biological transformations, separations, and more recently biomass pretreatment. RTILs consist of an organic cation and an organic or inorganic anion. This tremendous variation allows solvent properties to be tailored to specific applications such as biocatalysis, particularly as nonaqueous alternatives to organic solvents. More recently, RTILs have been used as alter­natives for lignocellulosic pretreatment (Mora-Pale et al., 2011). Birch wood was pretreated with N-methylmorpholine-N-oxide (NMMO or NMO) fol­lowed by enzymatic hydrolysis and fermentation to ethanol or digestion to biogas. The pretreatments were carried out with NMMO at 130 °C for 3 h, and the effects of drying after the pretreatment were inves­tigated (Goshadrou et al., 2013). Another interesting process is the use of concentrated phosphoric acid (CPA) in the pretreatment of lignocellulosic biomass (Zhao et al., 2012). After reprecipitation from CPA cellu­lose becomes completely amorphous and contains little lignin and hemicellulose. Further research is needed to evaluate and improve the economics of usage of ionic liquids (ILs), NMMO and CPA for pretreatment of lignocellulosic biomass. Also the integration with subsequent chemocatalytic and enzymatic/fermenta — tive processes such as simultaneous saccharification and fermentation needs further research. Especially, the ability of microorganisms to ferment sugars in the presence of these solvents also needs to be tested to carry out a continuous process. ILs are still very expen­sive and need to be synthesized at a much lower cost and on a much larger scale. Other points of concern are the buildup of inorganics in the ILs introduced with the lignocellulosic biomass (especially a concern with nonwoody lignocellulosic biomass such as straw and bagasse) and chemical modifications of the ILs. So it is rather questionable if the great potential assigned to ILs can be fulfilled for bulk applications such as biomass pretreatment taking into account the aforementioned limitations.

Lignocellulosic biomass pretreatment in RTIL’s is an alternative showing promise, with comparable or supe­rior yields of fermentable sugars, than conventional pre­treatments. The high number of RTILs that can be synthesized allows the design of solvents with specific physicochemical properties that play a critical role inter­acting with lignocellulosic biomass subcomponents. Today, these interaction mechanisms are better under­stood. However, future challenges rely on the ability to make this process economically feasible. This might be achieved by optimizing large-scale pretreatment condi­tions, performing post-pretreatment steps in RTILs, reusing RTILs, recycling the RTILs with reduced energy consumption and enhancing process efficiency, and pro­ducing high-value biobased products and chemicals in addition to ethanol. Moreover, the potential high value of lignin suggests that it might instead be used in the large-scale diversified manufacture of high-value chem­icals, traditionally obtained from petroleum (Mora-Pale et al., 2011).