In general, chemical pretreatments show a high degree of selectivity for the biomass component they degrade; they also involve relatively harsh reaction conditions, which may not be ideal in a biorefinery scheme due to the possible production of toxic substances and their possible effects on downstream biological processing (FitzPatrick et al., 2010). Degradation of lignin has been observed in most chemical pretreatments, and particularly in dilute-acid and lime pretreatments (Samuel et al., 2011).

Acid treatments solubilize the hemicellulose, and by this, make the cellulose more accessible. The main reac­tion that occurs during acid pretreatment is the hydroly­sis of hemicellulose, especially xylan as glucomannan is relatively acid stable. The condensation and precipitation of solubilized lignin components is an unwanted reac­tion, as it decreases digestibility (Hendriks and Zeeman,

2009) . Dilute-acid pretreatment is considered as one of the promising pretreatment methods despite its high — energy (steam or electricity) requirements and/or corrosion-resistant high-pressure reactors, and extensive washing, which increases the cost (Isroi et al., 2011). On the other hand, pretreatments with strong acids for the ethanol production is not an attractive option, because there is a risk of formation of inhibiting compounds (Hendriks and Zeeman, 2009). Other weak organic acids such as lactic acid and phosphoric acid have been also investigated (Monauari et al., 2011).

During alkaline pretreatment the first reactions taking place are solvation and saponification. This causes a swollen state of the biomass and makes it more accessible for enzymes and bacteria. At "strong" alkali concentra­tions, dissolution, "peeling" of end groups, alkaline hy­drolysis, degradation and decomposition of dissolved polysaccharides can take place. Alkali extraction can also cause solubilization, redistribution and condensation of lignin and modifications in the crystalline state of the cellulose (Hendriks and Zeeman, 2009).

ILs are generally defined as salts that melt at or below 100 °C, providing liquids exclusively composed of ions. Simple inorganic salts (e. g. NaCl) melt at very high tem­peratures (803 °C), rendering unfeasible their routine use as solvents for organic chemical processing (Tadesse and Luque, 2011). ILs have been termed green solvents due to their negligible vapor pressure (Patel and Lee, 2012). The application of ILs to biomass valorization and pretreatment recently started to attract a great deal of attention because they are capable of disrupting the hydrogen bonds between different polysaccharide chains, thus decreasing the compactness of cellulose and making the carbohydrate fraction more susceptible to hydrolysis (Tadesse and Luque, 2011). Additionally, the recovering and the recycling of ILs has been pro­posed for decreasing the cost of the pretreatment process (Tadesse and Luque, 2011). However, cost and energy­intensive recycling of the solvents are major constraints preventing ILs from commercial viability (Fu and Mazza, 2011). Another drawback of ILs is the fact that cellulases are inactivated even at low concentrations of ILs (Wang et al., 2011a). The ILs pretreatment has been tested using coadjuvant metal or acid catalysts to obtain higher conversion and/or yields of intermediates (Tadesse and Luque, 2011).

Organosolv pretreatment is the process to extract lignin from lignocellulosic feedstocks with organic sol­vents or their aqueous solutions. This process is similar to that used in industrial paper-making processes but the degree of delignification for pretreatment is not demanded to be as high as that of pulping. Generally, organosolv processes are conducted at high temperatures

(100—250 °C) using low boiling point solvents (methanol and ethanol), high boiling point alcohols (ethylene glycol, glycerol, tetrahydrofurfuryl alcohol) and other classes of organic compounds including ethers, ketones, phenols, organic acids, and dimethyl sulfoxide (Agbor et al., 2011). This pretreatment removes extensive lignin and nearly complete hemicellulose, enhancing the enzymatic digestibility as a consequence of the increase in accessible surface area and pore volume (Agbor et al., 2011; Zhao et al., 2009). The organosolv pretreatment is more expen­sive at present than the leading pretreatment processes; however, organosolv can provide some valuable by­products that might lead it to be a promising pretreat­ment for biorefining lignocellulosic feedstock in the future (Zhao et al., 2009).

The advantages of organosolv pretreatment includes, organic solvents are always easy to recover by distilla­tion and recycled for pretreatment; the chemical recov­ery in organosolv pulping processes can isolate lignin as a solid material and carbohydrates as syrup, both of which show promise as chemical feedstocks. However, there are inherent drawbacks to the organosolv pretreat­ment, such as air and water pollution, the pretreated solids always need to be washed with organic solvent before water washing in order to avoid the reprecipita­tion of dissolved lignin, which leads to cumbersome washing arrangements (Zhao et al., 2009).