Ionic Liquid (IL) Pretreatment
This is a relatively new physicochemical pretreatment technique available for cellulosic ethanol production from biomass. Ionic liquids (ILs) are used as biomass solvents in this process [134-136]. Ionic liquids are organic salts with low melting points and these compounds are well known for their physical properties such as high polarities, high thermal stabilities, and negligible vapor pressure [137]. In 2002, Rogers et al. first reported [138] the use of 1-”butyl-3-methylimidazolium (BMIM) salts, which are room temperature ionic liquids for the dissolution of cellulose. They showed that high molecular weight pulp cellulose (DP~1000) slowly dissolves (5-10g/100g ionic liquid) in [BMIM]+ ionic liquids with Cl-, Br — and SCN- anions when heated to about 100°C, yielding viscous solutions. Furthermore, they found that cellulose can be reprecipitated by adding an anti-solvent like water or alcohol to cellulose — ionic liquid solution. Later other research groups extended this discovery to raw biomass forms showing that dry biomass powders such as sawdust can also be dissolved in certain ionic liquids. Most importantly, the regenerated biomass showed lower crystallinity than original samples and facile hydrolysis when exposed to cellulase enzymes.
The most widely used ILs for biomass pretreatment-fractionations are 1-allyl-3-methylimidazonium chloride (AMIMCl), 1-”butyl-3-methylimidazonium chloride (BMIMCl), and 1-”butyl- 3-methylimidazonium acetate (BMIMOAc) (Figure 5.9). These imidazolium ionic liquids can be used effectively in most cases as a non-derivatizing solvent for the dissolution of cellulose at temperatures below 100°C. A possible dissolution mechanism suggests that cations and anions in these ionic liquids compete with hydrogen bonding in lignocellulosic components, thus disrupting
Figure 5.9 Ionic liquids 1-allyl-3-methylimidazonium chloride (AMIMCl), 1-”butyl-3-methylimidazomum chloride (BMIMCl), and 1-”butyl-3- methylimidazonium acetate (BMIMOAc) commonly used in ionic liquid pretreatments.
its three-dimensional network. The non-hydrated anion like Cl — in AMIMCl or BMIMCl is believed to be very effective in breaking the extensive hydrogen-bonding network present, thus bringing a much quicker dissolution and dissolving a higher concentration of cellulose than the traditional solvent systems. The biomass can be regenerated and can be recovered by the addition of an anti-solvent such as water, ethanol or acetone [134, 137]. The solvent can be recovered and reused by various methods such as pervaporation, reverse osmosis, salting out and ionic exchange. In general ILs are considered to be environmentally friendly, single component solvents for cellulose with potential applications in cellulose fractionation and dissolution in the integrated biorefinery concept [137].
Ionic liquid (IL) pretreatment is effective on a variety of biomass forms and some recent applications are in rice straw [139, 140], wheat straw [141], cassava pulp [140], pine and spruce wood [142], Japanese cedar wood [143], and forest and agricultural residues
[144] . However, currently available ionic liquids are very expensive, and nearly 100% recycling of the ionic liquid is essential in an industrial process. Therefore, further developments in efficient recovery methods are vital for any large-scale application of ionic liquid-based pretreatment methods.
Traditionally, cellulose of biomass is dissolved in the ionic liquid under anhydrous conditions, however, Zhang and coworkers have recently studied the possibility of using aqueous solution of ionic liquid with added acid for pretreatment with encouraging results
[145] . According to their claim, a new biomass pretreatment process has been developed using acidified ionic liquid (IL) solutions containing 10-30% water. Pretreatment of sugarcane bagasse at 130°C for 30 min by aqueous 1-”butyl-3-methylimidazolium chloride (BMIMCl) solution containing 1.2% HCl resulted in a glucan digestibility of 94-100% after 72 h of enzymatic hydrolysis. They found that HCl is a more effective catalyst than H2SO4 or FeCl3 [145] in the aqueous ionic liquid medium. Furthermore, glucan digestibility was increased by increasing acid concentration from 0.4% to 1.2% and reaction temperature from 90 to 130°C. The pretreatment medium appears to be recyclable, as the glucan digestibility of solid residue obtained with the acidified BMIMCl solution that was reused three times was greater than 97%. The addition of water to ILs for pretreatment could significantly reduce IL solvent costs and allow for increased biomass loadings, making pretreatment by ILs a more cost-effective proposition.