of sugars from both cellulose and hemicellulosic components
(Lau and Dale 2009; Wyman et al. 2005). The glycosidic
bonds between the sugar monomers are broken during the LC
hydrolysis using chemicals or enzymes or a combination of
both. Since the costs of combined chemical and enzymatic
pretreatment is the potential drawback to enzymatic hydrolysis (Binder and Raines 2010) so, the development of highly
active catalysts for the selective catalytic conversion of plant
biomass to desired products remains a formidable challenge
(Nosrati-Ghods et al. 2018). After hydrolysis, the most
abundant sugars present in the LC hydrolysates are glucose
(C6 sugar) and xylose (C5 sugar) (Singla et al. 2012; Lachke
2002).
5 Pretreatment of Lignocellulosic Feedstock
The use of pretreatment methods depends on the fraction of
lignin, hemicellulose, and cellulose in feedstock. Delignification is necessary prior to hydrolysis of hemicellulosic and
cellulosic component (Kumar et al. 2009). Acid mediated
hydrolysis of hemicellulosic fraction under controlled
conditions results in the production of sugars, which are
highly value-added compounds for biorefinery. Since
hemicelluloses are amorphous and branched as compare to
cellulose, therefore, organic acids with lower acid strength
can more efficiently hydrolyze hemicellulose than mineral
acids (Bhaumik and Dhepe 2016; Fanta et al. 1984). The
activation energy of hydrolysis of hemicellulose to sugars
varies from 50 to 199 kJ mol
−1 that depends on the linkages
present in hemicellulose and its source (Mäki-Arvela et al.
2011). Table 4 describes the yield of xylose from biomass
through different pretreatment processes.
5.1 Use of Hydrogen Peroxide (H 2 O 2 )
One strategy to remove lignin in LC biomass is to use H 2 O 2
as an oxidizing agent (Mosier et al. 2005). The use of H 2 O 2
generates carboxylic acids which cause problems in later
stages of biomass processing, hence they need to be neutralized or removed (Azzam 1989). H 2 O 2 pretreatment also
results in the loss of significant amount of hemicelluloses
(Bhaumik and Dhepe 2016).
Table 2 Distribution of different
hemicellulosic components in
wood (Belgacem and Gandini
2008; Lundqvist et al. 2002;
Willför et al. 2005a, b, 2008)
Hemicellulose
Softwood (%)
Hardwood (%)
Arabinogalactans
1–15
0.1–1
Arabinomethylglucuronoxylans
15–30
0.1–1
Galactoglucomannans
60–70
0.1–1
Glucomannans
1–5
1 –5
Methylglucuronoxylans
5–15
80–90
Other galactans
0.1–1
0.1–1
Pectins
1–5
1 –5
Table 3 Linkages present in
various types of hemicelluloses
(Bhaumik and Dhepe 2016)
Type
Components
Linkages
Galactoglucomannan
Mannose, galactose, glucose, acetyl
substitution
1, 4-, 1, 6Softwood hemicellulose
Arabinogalactan
Glucuronic acid, galactose, arabinose
1, 3-, 1, 6-,
1, 4-, 1, 5Hardwood hemicellulose
Glucomannan
Mannose, glucose
1, 4Arabino-4-Omethylglucuronoxylan
Xylose, glucuronic acid, arabinose, methyl
substitution
1, 4-, 1, 2-,
1, 3O-Acetyl-4-O-methylglucuronoxylan
Glucuronic acid, xylose, other substitution
(acetyl, methyl)
1, 4-, 1, 2Arabinoglucuronoxylan/Arabinoxylan
Arabinose, xylose, glucuronic acid, other
substitution (acetyl, coumaroyl, feruloyl,
methyl)
1, 4-, 1, 3-,
1, 2-, 1, 2/3Biomass to Xylose
249
(Lau and Dale 2009; Wyman et al. 2005). The glycosidic
bonds between the sugar monomers are broken during the LC
hydrolysis using chemicals or enzymes or a combination of
both. Since the costs of combined chemical and enzymatic
pretreatment is the potential drawback to enzymatic hydrolysis (Binder and Raines 2010) so, the development of highly
active catalysts for the selective catalytic conversion of plant
biomass to desired products remains a formidable challenge
(Nosrati-Ghods et al. 2018). After hydrolysis, the most
abundant sugars present in the LC hydrolysates are glucose
(C6 sugar) and xylose (C5 sugar) (Singla et al. 2012; Lachke
2002).
5 Pretreatment of Lignocellulosic Feedstock
The use of pretreatment methods depends on the fraction of
lignin, hemicellulose, and cellulose in feedstock. Delignification is necessary prior to hydrolysis of hemicellulosic and
cellulosic component (Kumar et al. 2009). Acid mediated
hydrolysis of hemicellulosic fraction under controlled
conditions results in the production of sugars, which are
highly value-added compounds for biorefinery. Since
hemicelluloses are amorphous and branched as compare to
cellulose, therefore, organic acids with lower acid strength
can more efficiently hydrolyze hemicellulose than mineral
acids (Bhaumik and Dhepe 2016; Fanta et al. 1984). The
activation energy of hydrolysis of hemicellulose to sugars
varies from 50 to 199 kJ mol
−1 that depends on the linkages
present in hemicellulose and its source (Mäki-Arvela et al.
2011). Table 4 describes the yield of xylose from biomass
through different pretreatment processes.
5.1 Use of Hydrogen Peroxide (H 2 O 2 )
One strategy to remove lignin in LC biomass is to use H 2 O 2
as an oxidizing agent (Mosier et al. 2005). The use of H 2 O 2
generates carboxylic acids which cause problems in later
stages of biomass processing, hence they need to be neutralized or removed (Azzam 1989). H 2 O 2 pretreatment also
results in the loss of significant amount of hemicelluloses
(Bhaumik and Dhepe 2016).
Table 2 Distribution of different
hemicellulosic components in
wood (Belgacem and Gandini
2008; Lundqvist et al. 2002;
Willför et al. 2005a, b, 2008)
Hemicellulose
Softwood (%)
Hardwood (%)
Arabinogalactans
1–15
0.1–1
Arabinomethylglucuronoxylans
15–30
0.1–1
Galactoglucomannans
60–70
0.1–1
Glucomannans
1–5
1 –5
Methylglucuronoxylans
5–15
80–90
Other galactans
0.1–1
0.1–1
Pectins
1–5
1 –5
Table 3 Linkages present in
various types of hemicelluloses
(Bhaumik and Dhepe 2016)
Type
Components
Linkages
Galactoglucomannan
Mannose, galactose, glucose, acetyl
substitution
1, 4-, 1, 6Softwood hemicellulose
Arabinogalactan
Glucuronic acid, galactose, arabinose
1, 3-, 1, 6-,
1, 4-, 1, 5Hardwood hemicellulose
Glucomannan
Mannose, glucose
1, 4Arabino-4-Omethylglucuronoxylan
Xylose, glucuronic acid, arabinose, methyl
substitution
1, 4-, 1, 2-,
1, 3O-Acetyl-4-O-methylglucuronoxylan
Glucuronic acid, xylose, other substitution
(acetyl, methyl)
1, 4-, 1, 2Arabinoglucuronoxylan/Arabinoxylan
Arabinose, xylose, glucuronic acid, other
substitution (acetyl, coumaroyl, feruloyl,
methyl)
1, 4-, 1, 3-,
1, 2-, 1, 2/3Biomass to Xylose
249