5.1.1.2 Chemical Pretreatment Methods
• Dilute acid pretreatment: By using this method of pretreatment we are actually
hydrolyzing the hemicelluloses and it also plays role in making the celluloses
susceptible to degradation by enzymes easily. H 2 SO 4 (strong acids) are used in
dilutions for lignocelluloses. The samples are kept soaked in the acid solution
with the rise of temperature up to 160
C for 10 min. When pretreatment is done
with such strong dilute acids then the reduction in hemicelluloses (xylosyl and
galactosyl) groups can be seen but the contents of lignin will still be higher (Zhou
et al. 2014). In one of the studies, the pretreatment strategy was applied to shells
of Jatropha curcas, where, 70% of cellulose was transformed enzymatically by
using this approach (Martín et al. 2015).
• Alkaline hot water pretreatment: The comparison of pretreatment strategies
shows that the method of alkaline pretreatment is most advantageous as it gives
lower inhibitor generation and low cost for the reactor (Zhuang et al. 2016). This
approach of liquid hot water pretreatment utilizes high pressure while biomass is
kept in auto-ionized water. As a result of auto-ionization and applied pressure,
H 3 O+ ions are generated that result in the hydrolytic disruption of hemicelluloses,
celluloses, and some of the lignin. In one of the studies by Xiao et al. (2014),
pretreatment of bamboo with LHW at 200
C, he received a 3.8-fold increase in
glucose content through hydrolysis y enzymatic methods. But when the temperature was raised to a high level, a drastic decline in the pH of water was observed
and more of the carbohydrates were degraded (Timung et al. 2015). Moreover,
deacetylation of the biomass is introduced by alkali, which eliminates the suppression of acetyl groups on enzymatic hydrolysis (Yang et al. 2019).
• Mild alkaline methods: As only cellulose and hemicellulose can be digested for
ethanol production so we have to eliminate lignin from the biomass. For this
purpose, the best method is the use of mild alkalis. This method produces the least
inhibitors and lesser hemicellulose solubilization; also temperature range will be
reduced for processing.
NaOH and KOH are the best alkalis for this pretreatment but they are costly
which is a serious concern. Some other alkaline solutions for this method are NH 3
and Ca(OH) 2 that can be used in AFEX, ARP (ammonia recycled percolation),
and alkaline pretreatment (Yang and Wyman 2008).
• Organosolv pretreatment: This method involves the addition of a mixture of
organic liquid and water and adding it to lignocellulosic biomass. Usually,
organic liquids are used with water in 1:1, the common organic liquids include:
C 2 H 5 OH (ethanol), CH 3 OH (methanol), and C 3 H 6 O (acetone) (Blanch et al.
2011). The mixture of LG and organic compound is heated so that it may
dissolute lignin and help in giving refined cellulose. Besides the above-mentioned
organic compounds, formic acid and acetic acid can be used as mixtures in 30%
volume by volume, respectively, along with water (Sindhu et al. 2012). In this
method, the resultant dissolved products are lignin and hemicellulose, but cellulose remains in solid form. This yields three classes: residual cellulose, liquid
hemicellulose, and solid lignin (Pan et al. 2006).
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FaizaKausar et al.
• Dilute acid pretreatment: By using this method of pretreatment we are actually
hydrolyzing the hemicelluloses and it also plays role in making the celluloses
susceptible to degradation by enzymes easily. H 2 SO 4 (strong acids) are used in
dilutions for lignocelluloses. The samples are kept soaked in the acid solution
with the rise of temperature up to 160
C for 10 min. When pretreatment is done
with such strong dilute acids then the reduction in hemicelluloses (xylosyl and
galactosyl) groups can be seen but the contents of lignin will still be higher (Zhou
et al. 2014). In one of the studies, the pretreatment strategy was applied to shells
of Jatropha curcas, where, 70% of cellulose was transformed enzymatically by
using this approach (Martín et al. 2015).
• Alkaline hot water pretreatment: The comparison of pretreatment strategies
shows that the method of alkaline pretreatment is most advantageous as it gives
lower inhibitor generation and low cost for the reactor (Zhuang et al. 2016). This
approach of liquid hot water pretreatment utilizes high pressure while biomass is
kept in auto-ionized water. As a result of auto-ionization and applied pressure,
H 3 O+ ions are generated that result in the hydrolytic disruption of hemicelluloses,
celluloses, and some of the lignin. In one of the studies by Xiao et al. (2014),
pretreatment of bamboo with LHW at 200
C, he received a 3.8-fold increase in
glucose content through hydrolysis y enzymatic methods. But when the temperature was raised to a high level, a drastic decline in the pH of water was observed
and more of the carbohydrates were degraded (Timung et al. 2015). Moreover,
deacetylation of the biomass is introduced by alkali, which eliminates the suppression of acetyl groups on enzymatic hydrolysis (Yang et al. 2019).
• Mild alkaline methods: As only cellulose and hemicellulose can be digested for
ethanol production so we have to eliminate lignin from the biomass. For this
purpose, the best method is the use of mild alkalis. This method produces the least
inhibitors and lesser hemicellulose solubilization; also temperature range will be
reduced for processing.
NaOH and KOH are the best alkalis for this pretreatment but they are costly
which is a serious concern. Some other alkaline solutions for this method are NH 3
and Ca(OH) 2 that can be used in AFEX, ARP (ammonia recycled percolation),
and alkaline pretreatment (Yang and Wyman 2008).
• Organosolv pretreatment: This method involves the addition of a mixture of
organic liquid and water and adding it to lignocellulosic biomass. Usually,
organic liquids are used with water in 1:1, the common organic liquids include:
C 2 H 5 OH (ethanol), CH 3 OH (methanol), and C 3 H 6 O (acetone) (Blanch et al.
2011). The mixture of LG and organic compound is heated so that it may
dissolute lignin and help in giving refined cellulose. Besides the above-mentioned
organic compounds, formic acid and acetic acid can be used as mixtures in 30%
volume by volume, respectively, along with water (Sindhu et al. 2012). In this
method, the resultant dissolved products are lignin and hemicellulose, but cellulose remains in solid form. This yields three classes: residual cellulose, liquid
hemicellulose, and solid lignin (Pan et al. 2006).
126
FaizaKausar et al.