species, such as Fomes fomentarius, Irpex lacteus, Trametes
versicolor, and Bjerkandera adusta have been explored in
the recent past (Kumar and Sharma 2017; Kumar et al.
2009). Commonly used white-rot fungi species for lignocellulose pretreatment includes Cyathus stercolerus, Ceriporia lacerata, Pleurotusostreaus sp., Ceriporiopsis
subvermispora, Phanerochaete chrysosporium, and Pycnoporus cinnarbarinus (Salakkam et al. 2019; Kumar and
Sharma 2017). Taha et al. have reported a 20-fold increase in
the hydrolysis rate during the biological pretreatment of
straw waste using fungal consortium (2015). Cui et al.
studied the effect of Viscozyme L (20 mL), with a concentration (0.25–4% v/v), pH (3.0–7.0), enzymolysis time (1–
5 h), and temperatures (35–55 °C) for pretreating 1 g of dry
poplar leaves and reported highest hydrogen production of
44.92 mL at 2% VL concentration with pH 4, time 3 h, and
temperature 50 °C (Cui et al. 2010). Even though the biological pretreatment of lignocellulose is fascinating, it is not
suitable for large-scale applications since the rate of
hydrolysis is very slow (Sun and Cheng 2002).
2.5 Combined Pretreatments
The biological pretreatment in a combination of different
convention pretreatments offers a promising approach that
can outperform the individual approach (Ren et al. 2016;
Kumar and Sharma 2017; Sindhu et al. 2016; Ummalyma
et al. 2019). Wang et al. have reported a 2.6-fold increase
in the glucose yield after combining the biological and
liquid hot water approach, in comparison to the liquid hot
water method alone (Wang et al. 2012). Similarly, Yu et al.
have reported the combination of biological pretreatment
with the physical or chemical method for the efficient
removal of lignin from rice husk (2009). On an industrial
scale, the combined approach of sulfur catalyzed
steam-explosion method is widely used for the pretreatment
of lignocellulose (Ren et al. 2009, 2016). This pretreatment
resulted in a high yield of fermentable sugar from hemicellulose while enhancing the cellulose accessibility to
enzymes for further conversion (Ren et al. 2009). However,
the method has few disadvantages, such as the requirement
of acid removal or neutralization before fermentation,
destruction of a fraction of xylan portion, and it generates
few compounds that can inhibit the activity of enzyme and
microorganism while affecting the overall productivity of
the process (Ren et al. 2009; Palmqvist and Hahn-Hägerdal
2000).
3 Hydrolysis of Lignocellulose Material
In hydrogen production, pretreatment given to lignocellulosic
material is generally followed by hydrolysis, a method to
transform cellulose and hemicellulose into sugars (Cheng
et al. 2011). Hydrolysis of lignocellulosic biomass is also
termed as saccharification. In a typical hydrolysis reaction, a
molecule of water ruptures one or more chemical bonds.
Generally, reducing sugars can be obtained by employing hydrolysis to cellulose, and this sugar can be further
subjected to fermentation to obtain ethanol. However, pretreatment has to be given to the biomass prior to hydrolysis.
The purpose of pretreatment is to offer an enhancement in the
efficiency of saccharification of lignocellulosic residue and,
ultimately, the process of H 2 production. This can be
achieved by disrupting the lignocellulose crystallinity,
removing lignin, reducing the cellulose’s degree of polymerization, and increasing the available area of biomass to
microorganisms for further fermentation (Chundawat et al.
2010; Ye and Berson 2014). As discussed in Sect. 2, physical, chemical, physicochemical, and biological pretreatments
are commonly used before employing the lignocellulose
biomass to the hydrolysis (Ravindran and Jaiswal 2016).
After pretreatment, hydrolysis must be employed to obtain a
high yield of sugars. Chemicals (acidic or alkaline solutions)
and enzymes or microorganisms usually perform the
hydrolysis reaction. Enzymatic hydrolysis offers various
benefits over acid hydrolysis, including higher glucose yields
and lower equipment costs (Cara et al. 2007). Hemicellulase
and cellulase enzymes can hydrolyze hemicellulose and
cellulose associated with lignocellulosic residues, respectively. Several fungal strains secrete cellulases, such
as Penicillium echinulatum, Trichoderma reesei, Aspergillus
fumigatus, Penicillium
purpurogenum, and Aspergillus
niger and employed in hydrolysis (Soni and Soni 2010).
Exo-glucanases, endo-glucanases, and a few other enzymes
secreted by these strains can act synergistically. Clostridium sp., Caldicellulosiruptor sp., and Shigella sp. are
anaerobic microorganisms that have also been found to
demonstrate hydrolysis of lignocellulosic biomass (Wang
et al. 2011). Further, the cellulosome (a large, extracellular
enzyme complex) has shown high hydrolysis efficiency.
Similarly, several bacterial strains also have been explored in
the field. The bacteria isolated from anaerobic cellulose
provide bioaugmentation, producing hydrogen, and reducing
the processing time while improving the product formation
efficiency (Ren et al. 2016). Hydrogen production from lignocellulosic residues can be classified based on the mode of
Bioconversion of Lignocellulosic Residues into Hydrogen
65
versicolor, and Bjerkandera adusta have been explored in
the recent past (Kumar and Sharma 2017; Kumar et al.
2009). Commonly used white-rot fungi species for lignocellulose pretreatment includes Cyathus stercolerus, Ceriporia lacerata, Pleurotusostreaus sp., Ceriporiopsis
subvermispora, Phanerochaete chrysosporium, and Pycnoporus cinnarbarinus (Salakkam et al. 2019; Kumar and
Sharma 2017). Taha et al. have reported a 20-fold increase in
the hydrolysis rate during the biological pretreatment of
straw waste using fungal consortium (2015). Cui et al.
studied the effect of Viscozyme L (20 mL), with a concentration (0.25–4% v/v), pH (3.0–7.0), enzymolysis time (1–
5 h), and temperatures (35–55 °C) for pretreating 1 g of dry
poplar leaves and reported highest hydrogen production of
44.92 mL at 2% VL concentration with pH 4, time 3 h, and
temperature 50 °C (Cui et al. 2010). Even though the biological pretreatment of lignocellulose is fascinating, it is not
suitable for large-scale applications since the rate of
hydrolysis is very slow (Sun and Cheng 2002).
2.5 Combined Pretreatments
The biological pretreatment in a combination of different
convention pretreatments offers a promising approach that
can outperform the individual approach (Ren et al. 2016;
Kumar and Sharma 2017; Sindhu et al. 2016; Ummalyma
et al. 2019). Wang et al. have reported a 2.6-fold increase
in the glucose yield after combining the biological and
liquid hot water approach, in comparison to the liquid hot
water method alone (Wang et al. 2012). Similarly, Yu et al.
have reported the combination of biological pretreatment
with the physical or chemical method for the efficient
removal of lignin from rice husk (2009). On an industrial
scale, the combined approach of sulfur catalyzed
steam-explosion method is widely used for the pretreatment
of lignocellulose (Ren et al. 2009, 2016). This pretreatment
resulted in a high yield of fermentable sugar from hemicellulose while enhancing the cellulose accessibility to
enzymes for further conversion (Ren et al. 2009). However,
the method has few disadvantages, such as the requirement
of acid removal or neutralization before fermentation,
destruction of a fraction of xylan portion, and it generates
few compounds that can inhibit the activity of enzyme and
microorganism while affecting the overall productivity of
the process (Ren et al. 2009; Palmqvist and Hahn-Hägerdal
2000).
3 Hydrolysis of Lignocellulose Material
In hydrogen production, pretreatment given to lignocellulosic
material is generally followed by hydrolysis, a method to
transform cellulose and hemicellulose into sugars (Cheng
et al. 2011). Hydrolysis of lignocellulosic biomass is also
termed as saccharification. In a typical hydrolysis reaction, a
molecule of water ruptures one or more chemical bonds.
Generally, reducing sugars can be obtained by employing hydrolysis to cellulose, and this sugar can be further
subjected to fermentation to obtain ethanol. However, pretreatment has to be given to the biomass prior to hydrolysis.
The purpose of pretreatment is to offer an enhancement in the
efficiency of saccharification of lignocellulosic residue and,
ultimately, the process of H 2 production. This can be
achieved by disrupting the lignocellulose crystallinity,
removing lignin, reducing the cellulose’s degree of polymerization, and increasing the available area of biomass to
microorganisms for further fermentation (Chundawat et al.
2010; Ye and Berson 2014). As discussed in Sect. 2, physical, chemical, physicochemical, and biological pretreatments
are commonly used before employing the lignocellulose
biomass to the hydrolysis (Ravindran and Jaiswal 2016).
After pretreatment, hydrolysis must be employed to obtain a
high yield of sugars. Chemicals (acidic or alkaline solutions)
and enzymes or microorganisms usually perform the
hydrolysis reaction. Enzymatic hydrolysis offers various
benefits over acid hydrolysis, including higher glucose yields
and lower equipment costs (Cara et al. 2007). Hemicellulase
and cellulase enzymes can hydrolyze hemicellulose and
cellulose associated with lignocellulosic residues, respectively. Several fungal strains secrete cellulases, such
as Penicillium echinulatum, Trichoderma reesei, Aspergillus
fumigatus, Penicillium
purpurogenum, and Aspergillus
niger and employed in hydrolysis (Soni and Soni 2010).
Exo-glucanases, endo-glucanases, and a few other enzymes
secreted by these strains can act synergistically. Clostridium sp., Caldicellulosiruptor sp., and Shigella sp. are
anaerobic microorganisms that have also been found to
demonstrate hydrolysis of lignocellulosic biomass (Wang
et al. 2011). Further, the cellulosome (a large, extracellular
enzyme complex) has shown high hydrolysis efficiency.
Similarly, several bacterial strains also have been explored in
the field. The bacteria isolated from anaerobic cellulose
provide bioaugmentation, producing hydrogen, and reducing
the processing time while improving the product formation
efficiency (Ren et al. 2016). Hydrogen production from lignocellulosic residues can be classified based on the mode of
Bioconversion of Lignocellulosic Residues into Hydrogen
65