successfully degraded at 400 °C temperature into phenol,
catechol, m,p-cresol and o-cresol in 11.75, 26.18, 6.98 and
11.21 wt% yield, respectively. At 350 °C temperature, the
yield of above chemicals was changed significantly to 3.31,
30.55, 8.76 and 3.65 wt%, respectively. Furthermore, the
qualitative measurements for the production of
2,4-di-tert-butyl phenol and 1,3-di-tert-butyl benzene as the
key chemical compounds from hydrothermal treatment of
lignin using gas and mass spectroscopies were also performed (Quitain et al. 2003). Oasmaa and Jin evaluated
hydrothermal degradation of lignin in the presence of catalysts and alkali and found that catalysts like K 2 CO 3 , NaOH
and Ca(OH) 2 enhance yield of liquid product, while alkali
facilitates the bond breakings in lignin and enhances the
formation of formic or acetic acid (Oasmaa and Johansson
1993; Jin et al. 2012). Alkali lignin polymer was successfully converted through hydrothermal decomposition under
alkaline conditions into guaiacol (11.23 wt%), catechol
(11.21 wt%), phenol (4.21 wt%) and cresol (7.00 wt%) at
temperatures approximately 280, 380, 390 and 390 °C,
respectively (Pinkowska et al. 2012).
Natural oils and fats containing triglycerides and fatty
acids are potential renewable feedstocks for synthesis of
fuels and commodity chemicals. Selective hydrothermal
deoxygenation of saturated and unsaturated triglycerides to
C n−1 hydrocarbons was carried out by using Pd/C catalyst
(Hollak et al. 2014). Deoxygenation was performed in HCW
at 250 °C temperature, which resulted in the formation of
linear paraffin and olefins as the main products. Catalytic
hydrothermolysis at 21 MPa pressure and temperatures
varying from 450 to 475 °C using zinc acetate as catalyst
was performed using water as a solvent (Li et al. 2010). The
resulting reaction produced non-ester biofuels in 40–52%
yield. The factors like temperature, pressure, heating oil rate,
catalyst and oil to water ratio were identified as main factors
in catalytic hydrothermolysis reaction to control the selectivity of the product.
Amino acids, the important precursor of all kinds of
proteins, have high market value due to their great demand
in food, pharmaceutical and cosmetic industries. Previously,
a large number of efforts have been made to explore
hydrothermal techniques for the synthesis of amino acids
from protein-rich biomass feedstocks (Quitain et al. 2001,
2006; Rogalinski et al. 2005). Earlier, hydrothermal treatment of shrimp shells was performed at different pressure
and temperature for production of different amino acids
(Quitain et al. 2001). Amino acids were produced in highest
yield at temperature of 250 °C for 60 min residence time that
was 2.5 times longer in comparison to that produced at 90 °
C. The amount of alanine and glycine amino acids first
raised with enhancing temperature to 250 °C and declined
afterward. To enhance recovery of tyrosine amino acid
through the degradation of silk protein, microwave-assisted
hydrothermal technique was employed (Quitain et al. 2006).
Addition of alkali and acid to the reaction mixture significantly enhanced the yield and NaOH favored the hydrolysis
of protein. Bovine serum albumin protein was treated in
continuous-flow reactor at different temperature and residence time for production of amino acids (Rogalinski et al.
2005). At subcritical conditions, the highest yield of amino
acid was formed at 290 °C temperature for 65 s residence
time, while 310 °C was the optimum temperature for 30 s
residence time.
Thermal degradation of a variety of terpene derivatives at
different residence time temperatures was studied (McGraw
et al. 1999). The percent degradation of the terpenes at
heating temperature of 120 °C was 100% for R-terpinene in
4 h, 38% for camphene in 72 h, 50% for limonene in 24 h
and 36% for Δ
3 -carene in 72 h. Later, hydrothermal degradation of pinene, camphor, carvacrol, limonene and
citronellol terpenes was studied in subcritical water (Yang
et al. 2007). Among all terpenes, pinene and limonene
showed highest degradation of 25–31% at 100–150 °C after
30 min heating, which reached to 64% at 250 °C temperature. However, the camphor, carvacrol and citronellol terpenes showed lower degradation (10%) and better stability at
subcritical conditions.
3.2 Reaction Mechanism and Potential Reaction
Pathways for HTL
Hydrothermal liquefaction is a complicated process due to
involvement of various kinds of chemical reactions. Some
work has been done to predict yield of HTL bio-crude using
both model compounds and kinetic modeling based on
chemical composition of biomass (Biller and Ross 2011;
Teri et al. 2014; Leow et al. 2015; Sheng et al. 2018; Hietala
et al. 2016, 2017; Li et al. 2017; Déniel et al. 2017; Sheehan
and Savage et al. 2017). Transformation of biomass feedstocks during the HTL process is a combined reaction of
hydrolysis of biopolymers, degradation of feedstock and
several other secondary processes involving the synthesis of
hydrolyzed products (Yang et al. 2018; Arturi et al. 2016).
Hydrolysis is the first degradation step, resulting in monomers and oligomers intermediates. Monosaccharides can be
obtained from cellulose and hemicellulose, while
methoxyphenol derivatives can be obtained from lignin,
lipid, protein fatty acids and amino acids, respectively
(Déniel et al. 2016, 2017; Peterson et al. 2008; Sasaki et al.
2000; Mok and Antal 1992; Garrote et al. 1999; Gao et al.
2011). The secondary reactions like dehydrogenation,
dehydration, Cannizzaro reaction, retro-aldol condensation,
rearrangements, polymerization and cyclization depend upon
Green and Sustainable Biomass Processing for Fuels and Chemicals
31
catechol, m,p-cresol and o-cresol in 11.75, 26.18, 6.98 and
11.21 wt% yield, respectively. At 350 °C temperature, the
yield of above chemicals was changed significantly to 3.31,
30.55, 8.76 and 3.65 wt%, respectively. Furthermore, the
qualitative measurements for the production of
2,4-di-tert-butyl phenol and 1,3-di-tert-butyl benzene as the
key chemical compounds from hydrothermal treatment of
lignin using gas and mass spectroscopies were also performed (Quitain et al. 2003). Oasmaa and Jin evaluated
hydrothermal degradation of lignin in the presence of catalysts and alkali and found that catalysts like K 2 CO 3 , NaOH
and Ca(OH) 2 enhance yield of liquid product, while alkali
facilitates the bond breakings in lignin and enhances the
formation of formic or acetic acid (Oasmaa and Johansson
1993; Jin et al. 2012). Alkali lignin polymer was successfully converted through hydrothermal decomposition under
alkaline conditions into guaiacol (11.23 wt%), catechol
(11.21 wt%), phenol (4.21 wt%) and cresol (7.00 wt%) at
temperatures approximately 280, 380, 390 and 390 °C,
respectively (Pinkowska et al. 2012).
Natural oils and fats containing triglycerides and fatty
acids are potential renewable feedstocks for synthesis of
fuels and commodity chemicals. Selective hydrothermal
deoxygenation of saturated and unsaturated triglycerides to
C n−1 hydrocarbons was carried out by using Pd/C catalyst
(Hollak et al. 2014). Deoxygenation was performed in HCW
at 250 °C temperature, which resulted in the formation of
linear paraffin and olefins as the main products. Catalytic
hydrothermolysis at 21 MPa pressure and temperatures
varying from 450 to 475 °C using zinc acetate as catalyst
was performed using water as a solvent (Li et al. 2010). The
resulting reaction produced non-ester biofuels in 40–52%
yield. The factors like temperature, pressure, heating oil rate,
catalyst and oil to water ratio were identified as main factors
in catalytic hydrothermolysis reaction to control the selectivity of the product.
Amino acids, the important precursor of all kinds of
proteins, have high market value due to their great demand
in food, pharmaceutical and cosmetic industries. Previously,
a large number of efforts have been made to explore
hydrothermal techniques for the synthesis of amino acids
from protein-rich biomass feedstocks (Quitain et al. 2001,
2006; Rogalinski et al. 2005). Earlier, hydrothermal treatment of shrimp shells was performed at different pressure
and temperature for production of different amino acids
(Quitain et al. 2001). Amino acids were produced in highest
yield at temperature of 250 °C for 60 min residence time that
was 2.5 times longer in comparison to that produced at 90 °
C. The amount of alanine and glycine amino acids first
raised with enhancing temperature to 250 °C and declined
afterward. To enhance recovery of tyrosine amino acid
through the degradation of silk protein, microwave-assisted
hydrothermal technique was employed (Quitain et al. 2006).
Addition of alkali and acid to the reaction mixture significantly enhanced the yield and NaOH favored the hydrolysis
of protein. Bovine serum albumin protein was treated in
continuous-flow reactor at different temperature and residence time for production of amino acids (Rogalinski et al.
2005). At subcritical conditions, the highest yield of amino
acid was formed at 290 °C temperature for 65 s residence
time, while 310 °C was the optimum temperature for 30 s
residence time.
Thermal degradation of a variety of terpene derivatives at
different residence time temperatures was studied (McGraw
et al. 1999). The percent degradation of the terpenes at
heating temperature of 120 °C was 100% for R-terpinene in
4 h, 38% for camphene in 72 h, 50% for limonene in 24 h
and 36% for Δ
3 -carene in 72 h. Later, hydrothermal degradation of pinene, camphor, carvacrol, limonene and
citronellol terpenes was studied in subcritical water (Yang
et al. 2007). Among all terpenes, pinene and limonene
showed highest degradation of 25–31% at 100–150 °C after
30 min heating, which reached to 64% at 250 °C temperature. However, the camphor, carvacrol and citronellol terpenes showed lower degradation (10%) and better stability at
subcritical conditions.
3.2 Reaction Mechanism and Potential Reaction
Pathways for HTL
Hydrothermal liquefaction is a complicated process due to
involvement of various kinds of chemical reactions. Some
work has been done to predict yield of HTL bio-crude using
both model compounds and kinetic modeling based on
chemical composition of biomass (Biller and Ross 2011;
Teri et al. 2014; Leow et al. 2015; Sheng et al. 2018; Hietala
et al. 2016, 2017; Li et al. 2017; Déniel et al. 2017; Sheehan
and Savage et al. 2017). Transformation of biomass feedstocks during the HTL process is a combined reaction of
hydrolysis of biopolymers, degradation of feedstock and
several other secondary processes involving the synthesis of
hydrolyzed products (Yang et al. 2018; Arturi et al. 2016).
Hydrolysis is the first degradation step, resulting in monomers and oligomers intermediates. Monosaccharides can be
obtained from cellulose and hemicellulose, while
methoxyphenol derivatives can be obtained from lignin,
lipid, protein fatty acids and amino acids, respectively
(Déniel et al. 2016, 2017; Peterson et al. 2008; Sasaki et al.
2000; Mok and Antal 1992; Garrote et al. 1999; Gao et al.
2011). The secondary reactions like dehydrogenation,
dehydration, Cannizzaro reaction, retro-aldol condensation,
rearrangements, polymerization and cyclization depend upon
Green and Sustainable Biomass Processing for Fuels and Chemicals
31