Chemical pretreatments utilize different chemical agents such as ozone, acids, alkalis, peroxide, and organic solvents (Table 4.4). The ozonolysis can be used

to degrade lignin and hemicellulose in many lignocellulosic materials. For the case of poplar sawdust pretreated with ozone, the enzymatic hydrolysis yield is increased from 0 to 57% while the lignin content is reduced from 29 to 8%. Despite its advantages, this method requires high amounts of ozone for an effec­tive pretreatment making the process quite expensive (Sun and Cheng, 2002).

Inorganic acids, especially sulfuric and hydrochloric acids, are the most used agents for biomass pretreatment using acid catalysts. These acids are toxic, dan­gerous, and require reactors resistant to corrosion. Moreover, if concentrated acids are employed, their recycling should be implemented by economic con­siderations. The pretreatment using dilute acids, especially sulfuric acid, has been developed in a successful way to process different lignocellulosic materials whereby high reaction rates can be attained and the subsequent cellulose hydroly­sis can be significantly improved. Nevertheless, the dilute-acid pretreatment costs are usually high related to those of steam explosion or AFEX process (Sanchez and Cardona, 2008; Sun and Cheng, 2002) . The pretreatment of corn stover at pilot plant scale has been studied using dilute sulfuric acid (0.5 to 1.4% p/v) in a continuous reactor for processing 1 ton/day of feedstock (Schell et al., 2003). In this case, high solids load (20%) was utilized unlike those reported in the open literature. Xylose yield reached 77% at 190°C. The digestibility of the pretreated material was evaluated by a simultaneous saccharification and fermentation (SSF) process attaining values of 87%.

Like steam explosion, dilute-acid pretreatment can be combined with other pretreatment methods to carry out a two-stage process. In particular, lignin removal can be greater if the biomass is treated with a dilute acid in the first stage followed by the addition of a concentrated acid plus ethanol in the second stage. In this way, the biomass fractionation can be accomplished, i. e., the separation of its three main components:

1. Sugars generated from hemicellulose hydrolysis that remain in the liquid fraction after the first stage,

2. Cellulose with a higher susceptibility to the enzymatic attack that remains in the solid fraction, and

3. Oligomers of lignin that result from the combined action of the con­centrated acid and ethanol and that are solubilized in the second stage (delignification) releasing the cellulosic fiber.

These oligomers can be precipitated after biomass fractionation (Papatheofanous et al., 1995). Another variant of the two-stage dilute-acid pretreatment consists of conducting the hemicellulose hydrolysis at 140°C during 15 min in a first stage to reduce the formation of furans and carboxylic acids, and then increasing the tem­perature to 190°C for 10 min to make the cellulose more accessible to the cellulase attack (Saha et al., 2005a, 2005b; Sanchez and Cardona, 2008). If dilute-acid pre­treatment is performed at a lower temperature (121°C), the degradation of sugars into furans (furfural and hydroxymethylfurfural), which may have an inhibitory effect on the fermentation, can be prevented, but sugar yields are reduced.

TABLE 4.4 Chemical Methods for Pretreatment of Lignocellulosic Biomass for Ethanol Production
examples of Pretreated Materials Poplar sawdust Pine Bagasse, wheat straw, cotton straw, green hay, peanut Poplar wood Bagasse, corn stover, wheat straw, rye straw, rice hulls Switchgrass, Bermuda grass
Methods ProcedureMgents Ozonolysis Ozone, room temperature, and pressure	Remarks No inhibitors formation Further cellulose conversion can be >57% Lignin degradation pH neutralization is required that generates gypsum as a residue 80-100% hemicellulose hydrolysis, 75-90% xylose recovery Cellulose depolymerization occurs at certain degree High temperature favors further cellulose hydrolysis Lignin is not solubilized, but it is redistributed Acid recovery is required Residence time greater compared to dilute-acid hydrolysis Peracetic acid provokes lignin oxidation	References Sun and Cheng (2002)
Dilute-acid 0.75-5% H2SO4, HCl, or HNO3, hydrolysis p~1 MPa; continuous process for low solids loads (5-10 wt.% dry substrate/mixture): T = 160-200°C; batch process for high solids loads (10^0 wt.% dry substrate/mixture): T = 120-160°C	Esteghlalian et al. (1997); Hamelinck et al. (2005); Lynd et al. (2002); Martinez et al. (2000); Rodriguez-Chong et al. (2004); Saha et al. (2005a, 2005b); Schell et al. (2003); Sun and Cheng (2002); Sun and Cheng (2005); Wooley et al. (1999)
Concentrated-acid 10-30% H2SO4, 170-190°C, hydrolysis 1:1.6 solid-liquid ratio 21-60% peracetic acid, silo-type system	Poplar sawdust Bagasse	Cuzens and Miller (1997); Teixeira et al. (1999a, 1999b)

Alkaline Dilute NaOH, 24 h, 60°C; Reactor costs are lower compared Hardwood Hamelinck et al. (2005); Hari hydrolysis Ca(OH)2, 4 h, 120°C; it can be complemented by adding H2O2 (0.5-2.15 vol.%) at lower temperature (35°C) to acid pretreatment >50% hemicellulose hydrolysis, 60-75% xylose recovery Low inhibitors formation Cellulose swelling Further cellulose conversion can be >65% 24-55% lignin removal for hardwood, lower for softwood Bagasse, corn stover, straws with low lignin content (10-18%), cane leaves Krishna et al. (1998); Kaar and Holtzapple (2000); Lynd et al. (2002); Rivers and Emert (1988); Saha and Cotta (2006); Sun and Cheng (2002); Teixeira et al. (1999b) Oxidative delignification Peroxidase and 2% H2O2, 20°C, 8 h Almost total solubilization of hemicellulose Further cellulose conversion can be 95% 50% lignin solubilization Bagasse Sun and Cheng (2002) Wet oxidation 1.2 MPa oxygen pressure, 195°C, 15 min; addition of water and small amounts of Na2CO3 or H2SO4 Solubilization of major part of hemicellulose Inhibitors formation Lignin degradation Corn stover, wheat straw Bjerre eta al. (1996); Varga et al. (2004) Organosolv Organic solvents (methanol, Solvent recovery required Poplar wood Lynd et al. (2002); Pan et al. process ethanol, acetone, ethylene glycol, triethylene glycol) or their mixture with 1% of H2SO4 or HCl; 185-198°C, 30-60 min, pH = 2.0-3.4 Almost total hydrolysis of hemicellulose, high yield of xylose Almost total lignin solubilization and breakdown of internal lignin and hemicellulose bonds Mixed softwood (spruce, pine, Douglas fir) (2005); Rezzoug and Capart (1996); Sun and Cheng (2002) Source: Adapted from Sanchez, O. J., and C. A. Cardona. 2008. Bioresource Technology 99:5270-5295. Elsevier Ltd.

Process Synthesis for Fuel Ethanol Production

Pretreatment, using concentrated acids, for fuel ethanol production also has been proposed. Arkenol, Inc. (USA) has reported the development of a fuel ethanol production process from cane bagasse through pretreatment with concentrated sulfuric acid, which has been patented (Farone and Cuzens, 1996). This technology requires the retrofitting of sugar mills in order to pro­duce ethanol and improve energetic indexes of this kind of processes (Cuzens and Miller, 1997; Sanchez and Cardona, 2008). This company is evaluating Zymomonas bacteria for use in its concentrated-acid process (Mielenz, 2001). An alternative approach has been tested by Teixeira et al. (1999a, 1999b), which employed a silo-type system that introduced the feedstock (bagasse or hybrid poplar) into plastic bags to which a peracetic acid solution was added with a concentration range from 0 to 60% (by weight). To enhance the process effi­ciency, sodium or ammonium hydroxide was added before the acid treatment, which allowed the use of lower amounts of peracetic acid. Cellulose conversion of pretreated material reached 93.1% in 120 h using 21% acid concentration or in 24 h using 60% acid concentration. This system requires low energy because the process is carried out at room temperature. Other methods involve the conversion of both cellulose and hemicellulose into fermentable sugars, which eliminates the necessity of adding cellulases, but the operation condi­tions are far from economically viable (Iranmahboob et al., 2002). In addition, there exists an additional problem related to the oxidation of glucose, which is obtained because of the high acid concentration and relatively prolonged times for biomass heating.

Alkaline pretreatment is based on the effects of the addition of dilute bases on the biomass: increase of internal surface by swelling, decrease of polymeriza­tion degree and crystallinity, destruction of links between lignin and other poly­mers, and breakdown of lignin. The effectiveness of this method depends on the lignin content of the biomass (Sun and Cheng, 2002). This type of pretreatment has been applied to corn stover obtaining 60 to 80% delignification efficiency employing 2.5 to 20% ammonium at 170°C for 1 h (Sun and Cheng, 2002), as well as to sugarcane bagasse and rice straw (Rivers and Emert, 1988). The addi­tion of an alkali can be combined with the addition of hydrogen peroxide as reported by Hari Krishna et al. (1998). Lignin degradation can also be carried out using the peroxidase enzyme in the presence of hydrogen peroxide through a process called oxidative delignification (see Table 4.4). Another pretreatment method involving lignin degradation is wet oxidation that is based on the addi­tion of oxygen and water at high temperatures and pressure leading to the open­ing of crystalline cellulose and the breakdown of lignin into simpler compounds, such as CO2, water, and carboxylic acids (Bjerre et al., 1996). Lignin oxidation can also be carried out with KMnO4, although with low cellulose conversions (below 50% for rice straw and cane bagasse; Rivers and Emert, 1988). In gen­eral, the utilization of bases like sodium hydroxide or solvents like ethanol or methanol (organosolv process) allows the dissolution of lignin, but their costs are so high that these methods are not competitive for large-scale plants (Lynd et al., 1999).