8.2.1 Pretreatment of Biomass

The production of ethanol through fermentation is already a common process for the utilization of biomass resources [39]. While simple sugars and starches can be easily used in this process, feed stocks of cellulose and hemicellulose would provide a source of sugars that would not compete with food and could be produced in otherwise unused land area. The conversion of these polysaccharides into simple sugars adds a significant challenge compared to the use of corn or sugarcane feed stocks [20]. In order for cellulosic biomass to be used in ethanol production, the feedstock must be pretreated and saccharified to provide a substrate suitable to the ethanol producing yeast. Pretreatment is a key step in a number of catalytic biomass processes [9], so while the pretreatment step itself may not be catalytic, it is important to an understanding of processing of biomass in ILs.

Because the structure of biomass, especially the presence of lignin, inhibits the saccharification of structural carbohydrates, pretreatment is needed to open up the structure of plant matter (Fig. 8.5). While a number of methods, such as steam explosion, dilute acid treatment, ammonia explosion, and milling have been explored, the unique solvent properties of ILs have garnered significant attention as a pretreatment option [9, 73]. What makes ILs promising for the catalytic treatment of biomass, namely their ability to make homogeneous solutions of lignocellulose, is also what make ILs a good medium for pretreatment. Because much of the recalcitrance of biomass to saccharification comes from the structure of the cell wall and the presence of lignin, when the structure is disrupted through dissolution in ILs, the carbohydrates are made available for enzymatic attack [70].

The general procedure for IL pretreatment of biomass is to dissolve or swell the biomass with an IL solvent. After treatment at a given temperature for a given time, an antisolvent, such as water, ethanol, or an acetone/water mixture, is added

Pretreatment

Decreased Cellulose Crystallinity

Fig. 8.5 Schematic of the role of pretreatment in the conversion of biomass to fuel (Adapted with permission from [73]. Copyright 2009 American Chemical Society) to precipitate the biomass and wash away the IL (see Fig. 8.6). The biomass is then dried and saccharified through enzymatic or chemical methods. This process builds off of the work by Rogers in which lignocellulose was fractionated into a cellulose and a lignin rich phase through dissolution in 1-ethyl-3-methylimi — dazolium acetate (EMIMAc). In this work, an acetone/water solution was added to precipitate the cellulose while keeping the lignin in solution. Evaporation of the acetone precipitates the lignin after the cellulose had been filtered from the solution [74].

This method was applied to pretreatment using a variety of ILs by a number of different researchers. Lynam conducted a study to measure the effect of a few different ILs on the composition and structure of lignocellulose in which ground rice hulls pretreated with 1-ethyl-3-methylimidazolium acetate, 1-allyl-3- methylimidazolium chloride (AMIMCl), or 1-hexyl-3-methylimidazolium chloride (HMIMCl) and either ethanol or water as antisolvents for cellulose precipitation. In this study, the EMIMAc was able to completely remove the lignin and significant amounts of hemicellulose from the rice hulls, while the other ILs removed less lignin but more hemicellulose [75]. Lee et al. worked with EMIMAc and other ILs to treat wood flour for enzymatic saccharification. By removing the lignin and reducing the cellulose crystallinity, the IL treated wood flour was 95 % digestible to cellulase enzymes from Trichoderma viride [76]. It has been demonstrated that IL pretreatment works even at a biomass loading well above the solubility limit of the biomass in the IL [77]. Other ILs, such as 1-ethyl-3-methylimidazolium diethyl phosphate, alkyloxyalkyl substituted imidazolium acetate, or alkyloxyalkyl substituted ammonium acetate have also been investigated [78, 79]. The use of

ILs as a pretreatment strategy has been compared to a more common method, namely pretreatment with dilute acid. In this study, it was found that IL pretreated samples produced a higher yield of monosaccharides and in a shorter time than samples pretreated with dilute acid [14]. Some studies have employed variations on dissolution and washing, such as the addition of an ammonia treatment step [80] or combining the pretreatment and saccharification into one step with aqueous ILs and enzymes [78, 81, 82].

One of the more difficult challenges facing pretreatment of biomass with ILs or saccharification in ILs is the separation of the ILs and carbohydrates after the treatment is completed. Ideally, the products will be insoluble in the IL or be precipitated with an antisolvent. These options may not be sufficient, such as when monosaccharides must be extracted from the IL. Brennan and coworkers developed a liquid-liquid extraction procedure for the removal of sugars from an IL phase using organic soluble boronic acids that have an affinity for sugars [83]. Another method relies on the use of kosmotropic, or water-structuring, salts to induce a biphasic system with water and an IL [84]. This effect has been used to separate and reuse ILs after pretreatment of biomass [85].