245
H 2 SO 4 reacts with lignocelluloses to give methyl glycoside. A mixture of methanol
in water and ethanol in water was tested to be suitable for liquefaction of
lignocelluloses [11]. Comparatively, the solvolysis of cellulose (in water or alcohol
or mixture) is difficult than hemicelluloses and lignin due to its chemical structure.
The typical solvents used for this solvolysis are acetone, ethylene glycol, toluene,
etc. The solvolysis method is also used for lignin extraction along with other
pretreatment methods such as acid/alkaline hydrolysis and reductive or oxidative
catalytic fractionation. Non-catalytic solvolysis of lignocellulosic biomass is
possible but requires longer reaction time and results in low product yield.
2.1.3 Hydrothermal Liquefaction of Lignocellulosic Biomass
The objective of the liquefaction process is to maximize the production of C 1 –C 6
compounds over xylose and glucose. Compared with the catalytic hydrolysis
process, the hydrothermal process targets the deep degradation of lignocellulosic
biomass that involves isomerization and dehydration of glucose. The final
distribution of the product depends on the type of catalyst used. An advantage of the
hydrothermal treatment is that it does not get affected by the presence of water in
biomass, whereas other processes require pre-drying of biomass. Numerous halide,
sulfate, and alkaline catalysts are used for the hydrothermal liquefaction process
such as CrCl 3 , FeCl 3 , CuCl 3 , NiSO 4 , CoSO 4 , KOH, Ca(OH) 2 , etc. The use of halide/
sulfate catalysts promotes the formation of levulinic acid, furans, and formic acid,
whereas alkaline catalysts give bio-oil and phenolic compounds as major products.
The hydrothermal reaction is typically carried out at ~500 K in order to convert
cellulose completely. In the case of bio-oil, higher reaction temperature (>500 K)
and longer reaction times lead to the formation of secondary products, thereby
decreasing the bio-oil yield. The separation of the products produced from catalytic
hydrothermal liquefaction requires multiple extraction steps with different types of
solvents due to the complex mixture of liquid, gases, and tar. At present, it is
uneconomical to produce bio-oil by hydrothermal liquefaction when compared with
petro-diesel and gasoline. Hydrothermal liquefaction is energy-intensive and timeconsuming [9]. Therefore, innovative separation methods are required for the
separation of products obtained from hydrothermal liquefaction. Comparatively, the
fast pyrolysis method discussed in the following section is more promising.
2.1.4 Catalytic Pyrolysis
Pyrolysis is a high-temperature decomposition process carried out in the absence of
oxygen and can be used to convert lignocellulosic biomass to bio-oil in the absence
of oxygen. Pyrolysis of biomass results in three products, namely, oil, gas, and solid
char. Pyrolysis operating parameters such as residence time and heating rate can be
tuned to get the maximum of oil or gas or char. Furthermore, pyrolysis can be
classified as slow pyrolysis (heating rate <20 °C/min, residence time >15 min), fast
Sustainability of the Catalytic Process for Biomass Conversion: Recent Trends and…
H 2 SO 4 reacts with lignocelluloses to give methyl glycoside. A mixture of methanol
in water and ethanol in water was tested to be suitable for liquefaction of
lignocelluloses [11]. Comparatively, the solvolysis of cellulose (in water or alcohol
or mixture) is difficult than hemicelluloses and lignin due to its chemical structure.
The typical solvents used for this solvolysis are acetone, ethylene glycol, toluene,
etc. The solvolysis method is also used for lignin extraction along with other
pretreatment methods such as acid/alkaline hydrolysis and reductive or oxidative
catalytic fractionation. Non-catalytic solvolysis of lignocellulosic biomass is
possible but requires longer reaction time and results in low product yield.
2.1.3 Hydrothermal Liquefaction of Lignocellulosic Biomass
The objective of the liquefaction process is to maximize the production of C 1 –C 6
compounds over xylose and glucose. Compared with the catalytic hydrolysis
process, the hydrothermal process targets the deep degradation of lignocellulosic
biomass that involves isomerization and dehydration of glucose. The final
distribution of the product depends on the type of catalyst used. An advantage of the
hydrothermal treatment is that it does not get affected by the presence of water in
biomass, whereas other processes require pre-drying of biomass. Numerous halide,
sulfate, and alkaline catalysts are used for the hydrothermal liquefaction process
such as CrCl 3 , FeCl 3 , CuCl 3 , NiSO 4 , CoSO 4 , KOH, Ca(OH) 2 , etc. The use of halide/
sulfate catalysts promotes the formation of levulinic acid, furans, and formic acid,
whereas alkaline catalysts give bio-oil and phenolic compounds as major products.
The hydrothermal reaction is typically carried out at ~500 K in order to convert
cellulose completely. In the case of bio-oil, higher reaction temperature (>500 K)
and longer reaction times lead to the formation of secondary products, thereby
decreasing the bio-oil yield. The separation of the products produced from catalytic
hydrothermal liquefaction requires multiple extraction steps with different types of
solvents due to the complex mixture of liquid, gases, and tar. At present, it is
uneconomical to produce bio-oil by hydrothermal liquefaction when compared with
petro-diesel and gasoline. Hydrothermal liquefaction is energy-intensive and timeconsuming [9]. Therefore, innovative separation methods are required for the
separation of products obtained from hydrothermal liquefaction. Comparatively, the
fast pyrolysis method discussed in the following section is more promising.
2.1.4 Catalytic Pyrolysis
Pyrolysis is a high-temperature decomposition process carried out in the absence of
oxygen and can be used to convert lignocellulosic biomass to bio-oil in the absence
of oxygen. Pyrolysis of biomass results in three products, namely, oil, gas, and solid
char. Pyrolysis operating parameters such as residence time and heating rate can be
tuned to get the maximum of oil or gas or char. Furthermore, pyrolysis can be
classified as slow pyrolysis (heating rate <20 °C/min, residence time >15 min), fast
Sustainability of the Catalytic Process for Biomass Conversion: Recent Trends and…