3.4 PRETREATMENT METHODS AND MECHANISMS
3.4.2 Chemical and Physicochemical Methods
3.4.2.1 Steam and Other Explosions Explosions constitute combinations of mechanical and chemical pretreatment methods. Explosion pretreatments can be performed with steam (autohydrolysis), SO2, NH3, and CO2. Fundamentally, the explosion pretreatment means heating up lignocellulose slurry under high pres- sure followed by sudden release of the pressure. This causes the fibers to separate, due to explosive decompression. No chemical needs to be added in the procedure, since acetic acid is released from hemicellulose at high temperature, facilitating the process (Taherzadeh and Karimi, 2008; Alvira et al., 2010). Steam explosion is there- fore a very popular method for pretreatment. Steam explosion is usually carried out at 160–260◦C and the operating time is 0.5–20 minutes (Taherzadeh and Karimi, 2008).
Steam explosion pretreatment has nearly reached commercialization; pilot plants are presently operating in Sweden and Canada (Galbe and Zacchi, 2007). Steam explo- sion pretreatment of poplar wood at 210◦C for 4 minutes resulted in 95% cellulose recovery (Taherzadeh and Karimi, 2008).
Supercritical CO2is used for carbon dioxide explosion pretreatment. CO2is cheap, nontoxic, inflammable, and easy to extract after explosion (Taherzadeh and Karimi, 2008). Due to the release of carbon dioxide at high pressure, lignocelluloses are disturbed, which increases the surface area for further hydrolysis. Glucose release was observed to increase with increasing pressure and temperature of the carbon dioxide was applied in supercritical carbon dioxide explosion. However, using subcritical carbon dioxide results in opposite scenario.
The ammonia fiber explosion (AFEX) process uses a combination of temperature (60–100◦C), high pressure, and ammonia. A SEM analysis revealed that the princi- pal target points during the AFEX process are the fibrous regions. However, some intermediates are formed during the process, for example, phenolic acids, aldehy- des (from cleaving lignin–carbohydrates complexes), short-chain oligosaccharides, and organic acids. These compounds act as inhibitors in the subsequent processes.
Figure 3.6 shows the schematic representation of an explosion process. Advantages of the AFEX process usually mentioned are high glucose yield upon enzymatic hydrol- ysis after the pretreatment, comprehensive recovery of the chemical used (ammonia), and no need to wash the materials after the pretreatment since the leftover ammonia can be used as a nutrient for microbial growth (Balan et al., 2009).
3.4.2.2 Dilute and Concentrated Acids Acid pretreatment is probably the most widely used method for pretreating lignocelluloses. It is aimed at dissolving the hemicellulose fraction, hence breaking the bonds between lignin, cellulose, and hemicellulose. The operating temperature usually varies between 30◦C and 220◦C, and the retention time is 5–90 minutes. Hydrochloric, sulfuric, nitric, and phosphorus acids are commonly used for acid-based pretreatments with sulfuric acid dominating (Taherzadeh and Karimi, 2007a). Apart from these acids, fumaric and maleic acids have been tested as well. Organic acids are more effective than sulfuric acid in pretreatment of wheat straw (Alvira et al., 2010). The acid pretreatment can be
Explosive agent
Temperature
control Pressure
control
Heater
Discharge valve
Reactor Lignocellulose before
pretreatment
Lignocellulose after pretreatment
FIGURE 3.6 Schematic representation of an explosion pretreatment process, in which the explosive agents may be steam, ammonia, sulfur dioxide, and carbon dioxide.
performed in two modes: (i) low temperature and high acid concentration and (ii) high temperature and dilute acid.
Concentrated acid pretreatment is effective in opening up the structure of ligno- celluloses at low temperature. In a study, combining acid and alkaline pretreatments (Chaudhary et al., 2012), lignocelluloses were treated with 30–70% sulfuric acid at 100◦C for 1 hour followed by neutralization with calcium hydroxide resulting in up to 85% recovery of the monomeric sugars in the lignocellulosic materials. In another study (Jafari et al., 2011), pretreatment with 85.9% phosphoric acid resulted in 87–95% enzymatic hydrolysis of the cellulose. Phosphoric acid facilitates removal of most of the lignin. Phosphoric acid reacts with hemicelluloses and lignin, with significant reduction as a result. Concentrated phosphoric acid as pretreatment fur- thermore converts crystalline cellulose into amorphous form (Zhao et al., 2012b).
However, the disadvantage of concentrated acid pretreatment is high investment and maintenance costs. With dilute acid pretreatment almost 100% of hemicelluloses are removed. Lignin is not dissolved as part of the acid pretreatment. However, dilute acid disrupts lignin, leading to increased susceptibility of the cellulose to enzymatic hydrolysis (Wyman, 1996; Yang and Wyman, 2004). During mild acid pretreatment, most of the hemicelluloses are removed (<120◦C). Hemicelluloses are degraded to monosaccharides and the sugars ultimately to HMF and furfural (Zhao et al., 2012b).
Dilute acid pretreatment opens up the lignocelluloses structure as well as enhances the hydrolysis process, if right conditions are employed. It is a very popular method in which a very low concentration of acid (e.g., 0.1–1.0%) is used. Much research has explored the use of acid pretreatments of different lignocellulosic materials, focusing on chemical composition of the materials, operating time, concentration of acid,
temperature, etc., searching for optimal conditions for recovering most of the sugars (Ma et al., 2010; Qi et al., 2010; Chen et al., 2011, 2012; Wei et al., 2011; Thirmal and Dahman, 2012). Two major drawbacks of acid pretreatment are its inability to reduce the crystallinity of cellulose and the formation of inhibitors, such as carboxylic acids, furans, and phenolic compounds, caused by the low pH (Taherzadeh, 1999;
Taherzadeh and Karimi, 2007b). These shortcomings result in lower sugar yields, and a lower subsequent fermentation. Other challenges are equipment corrosion, and high operational and maintenance costs (Alvira et al., 2010).
Dilute acid pretreatment may be combined with other chemical pretreatments (Ma et al., 2010; Chen et al., 2011). For instance, Azzam pretreated bagasse with 0.5%
HCl in a solution of zinc chloride for 10 minutes at 145◦C. The pretreated bagasse was then cooled and precipitated with acetone, resulting in a yield of more than 93%
(Azzam, 1987).
3.4.2.3 Alkali Sodium, potassium, calcium, and ammonium hydroxides are com- monly used for alkaline pretreatment of lignocelluloses. NaOH causes swelling, which increases the internal surface, and decreases the DP of the cellulose. At concen- trations above 10%, and at freezing temperature NaOH completely dissolves cellulose (Mirahmadi et al., 2010), while acting as an effective delignifier agent at a higher temperature. The intermolecular ester bonds, cross-linking xylan hemicelluloses, and lignin are saponified during alkali pretreatment, thus resulting in delignification of the biomass. Cleavage of the intramolecularα-aryl andβ-aryl ether linkages is what essentially contributes to the lignin degradation. Alkali pretreatment also removes acetyl groups and uronic acid from the hemicelluloses. Generally, alkalis are good swelling agents, reducing cellulose crystallinity, which increases the accessibility of cellulose (Zhao et al., 2012b).
Calcium hydroxide or lime, removes amorphous substances such as lignin, from cellulose. The removal of lignin extends the adsorption site for enzymes, hence increasing the accessibility of cellulose. Addition of an oxidant agent (O2or H2O2) improves the lignin removal (Teghammar et al., 2010). Alkaline pretreatment is more effective on agricultural residues, than on woody substances (Taherzadeh and Karimi, 2008). Lime pretreatment can be performed as (i) short-term treatment (up to 6 hours) at 100–160◦C at about 13 bar, with or without oxygen, (ii) long-term treatment (up to 8 weeks) at 55–65◦C at atmospheric pressure, and (iii) simple pretreatment involving, for example, 1 hour in boiling water, with or without oxygen. Higher lignin contents are removed under oxidative conditions, at a pressure ranging from atmospheric to 20 bar (Sierra et al., 2009).
3.4.2.4 Oxidizing Agents Oxidizing agents such as ozone (Alvira et al., 2010), hydrogen peroxide, and oxygen, can be used for pretreatment of lignocelluloses. This pretreatment is usually combined with other chemical or hydrothermal treatments.
In ozonolysis, degradation of aromatic and olefinic structures involves initial elec- trophilic attack by oxidants. In the oxidation by hydrogen peroxide, these structures
are instead destroyed by a nucleophilic attack of hydrogen peroxide anions (Zhao et al., 2012b).
Ozonolysis pretreatment is an effective method and is specifically used for degra- dation of lignin and partial degradation of hemicelluloses. It is mostly carried out at room temperature. Prominent advantages of ozonolysis are that no inhibitory com- pounds are formed, and also no acids, alkali, or toxic compounds, making it a very attractive choice of pretreatment (Neely, 1984; Vidal and Molinier, 1988; Sun and Cheng, 2002). It is quite expensive to buy a large volume of ozone, which is a chal- lenge to its feasibility. Important parameters to consider in ozonolysis pretreatment are moisture content of sample, particle size, and concentration of ozone.
In wet oxidation pretreatment, the material is treated with water using air or oxygen as oxidizing agents. The operating temperature is usually in the range of 150–200◦C, with residence time, approximately 30 minutes. Temperature, reaction time, and the oxygen pressure are considered to be the crucial parameters in this pretreatment method (Schmidt and Thomsen, 1998). Wet oxidation attacks all fractions of the lignocelluloses. Hemicelluloses are decomposed into monomeric sugars. Lignin is both oxidized and cleaved, and cellulose is degraded to a certain extent. The main reactions in wet oxidation pretreatment comprise formation of acids and oxidative reactions.
3.4.2.5 Organosolvs Using organosolvs is another pretreatment means for lig- nocellulosic feedstock, in which biomass is treated with organic solvents or an aque- ous solution of organosolv usually at 100–250◦C and with or without a catalyst.
A major function of organosolvs is complete removal of lignin and hemicelluloses, resulting in a highly accessible surface area, and large pore volumes (Zhao et al., 2012b). Alcohols, ketones, glycols, organic acids, phenols, esters, and ethers are the different organic solvents explored to date for organosolv pretreatments. Acetone is a widely used organic solvent for pretreatment of lignocelluloses. The pretreatment of D. Don chips with an acetone–water solution for 5 minutes at 195◦C resulted in an ethanol yield of 99.5% of the theoretical (Araque et al., 2008).
Use of the organic solventN-methylmorpholine-N-oxide (NMMO) for pretreat- ment of lignocelluloses has received increasing attention (Zhao et al., 2009). This chemical is able to open up the crystalline structure of cellulose, resulting in more than 99% sugar yield in the subsequent enzymatic hydrolysis (Jeihanipour and Taherzadeh, 2009). In the pretreatment with this organosolv, lignocelluloses are mixed with organic solvents and heated. Part of the lignin and the hemicelluloses are dissolved in the liquid phase, leaving cellulose in the solid phase (Taherzadeh and Karimi, 2008).
A catalyst, for example dilute sulfuric acid, oxalic, salicylic, and acetylsalicylic acid, may be added to improve delignification or to decrease the operating temperature (Taherzadeh and Karimi, 2008; Huijgen et al., 2011). Removal of the solvent after the pretreatment is required as the solvent might inhibit the enzymatic hydrolysis (Sun and Cheng, 2002). Recovery and recycling of the solvent is also necessary when using this pretreatment, or the process will be too expensive (Zhao et al., 2009).
Peracetic acid (PAA) is a powerful oxidizing agent which can be used for pre- treating lignocelluloses. This pretreatment can be accomplished at low temperature
O
OCH3 O CH+ 3
O H
HO
O OCH3
OH
+HO+ -H+ (1)
OH
+HO+ OCH3
- H+ CH3OH
O O
(2) OCH3
OH O+H
O[OH]
OCH3
+HO+
OCH3
O+[O+H]OH
O+[O+H]
O+[O+H]
O OCH3
O [OH]
O OCH3
O [OH]
O - H+
+HO+ (3)
O[OH]
OCH3
HCOH
+HO+
OCH3
HCOH OH
- H+ -HC=O
O[OH]
OCH3
OH
(4)
C=C +HO+ C C - H+ C C OH O
(6) O[OH]
OCH3
CHOH HC
- H O2
O[OH]
OCH3
CH C
+HO+
O[OH]
OCH3
CHOH CO
OCH3
CH C OH
+H O2
, -H+
(5) O
OCH
3
O OC
H3
O OCH
3
HO OCH3
FIGURE 3.7 Mechanisms of organosolv peracetic acid pretreatment (Zhao et al., 2009).
and atmospheric pressure. The mechanism of the PAA pretreatment is most likely that hydroxonium ions (H3O+), present in PAA, attack the lignin electron sites electrophilicly, resulting in the breakdown of lignin. The main reactions of PAA pretreatment are shown in Figure 3.7, and include (i) ring hydroxylation, (ii) oxida- tive demethylation, (iii) oxidative ring opening, (iv) displacement of side chains, (v) cleavage ofβ-aryl ether bonds, and (vi) epoxidation. Lignin is then disordered and dissolved in the liquid phase. Delignification increases the surface area of cellu- lose and, as a consequence, its accessibility (Suchy and Argyropoulos, 2001; Zhao et al., 2009). The effectiveness of various pretreatment methods described above on enzymatic hydrolysis of different lignocellulosic substrates is shown in Table 3.2.
TABLE 3.2 Comparison of Enzymatic Hydrolysis for Different Biomass With and Without Pretreatment
Lignocellulose biomass
Type of pretreatment
Enzymatic hydrolysis without pretreatment
(%)
Enzymatic hydrolysis with
pretreatment
(%) References
Softwood spruce
Alkali (NaOH)
14.1 35.7 Mirahmadi et al.
(2010) Harwood birch Alkali
(NaOH)
6.9 82.3 Mirahmadi et al.
(2010)
Corn stover Dilute acid 15.1 60.6 Lloyd and Wyman
(2005) Corn stover Steam
explosion
60 80 Ohgren et al. (2007)¨
Poplar Organosolv 8 36–41 Chum et al. (1988)
White pine Sulfuric acid 4 40 Grethlein et al. (2004)
3.4.2.6 Ionic Liquids Ionic liquids (ILs) are green solvents, since they contain no toxics or explosives. They are generally salts, existing as liquids below 100◦C, and contain large organic cations and small inorganic anions (Zhu et al., 2006; Alvira et al., 2010). Cations and anions play different significant roles during pretreatment. Cations interact with lignin by hydrogen bonding andπ–πinteractions, while anions act as hydrogen bond receptors, interacting with the hydroxyl group of the cellulose. As a consequence, the crystalline structure of the lignocelluloses is disrupted (Fukaya et al., 2008; Janesko, 2011; Wu et al., 2011), resulting in a very effective pretreatment. ILs convert the crystalline form of cellulose into an amorphous form partially removing hemicelluloses and lignin (Zhao et al., 2012b). After the pretreatment, the ILs can be recovered and reused, and different methods such as evaporation, salting out, pervaporation, ion exchange, and reverse osmosis, can be employed for this purpose (Zhu et al., 2006). However, using ILs for pretreatment is still expensive that provides a hurdle for its industrialization (Zhu et al., 2006; FitzPatrick, 2011; Mora-Pale et al., 2011; Wu et al., 2011). Different ILs and their specific targets for pretreatment action are summarized in Table 3.3.