244
upgrading. An integrated processing approach with a multifunctional catalyst will
be beneficial for biomass conversion.
2.1.1 Catalytic Hydrolysis-Acid/Alkaline Catalysts
The biomass must be pretreated to improve the product yield in downstream processing. The pretreatment changes the biomass structure that can be achieved either
by physical, chemical, and thermal processing or sometimes by a combination of
these treatment methods. The aim of pretreatment is to decrystallize cellulose, separate hemicellulose, and break the lignin seal. During hydrolysis, the cellulosic component of lignocellulosic biomass is converted to glucose and xylose as the main
products. However, due to its crystalline form and hydrogen bonding, the hydrolysis
of cellulose is significantly difficult and requires the use of strong liquid acid as a
catalyst [9]. This process yields reducing sugars, which is furthermore converted to
useful chemicals. Among the different liquid acids, mineral acids (H 2 SO 4 and HCl),
as well as organic acids (carboxylic acid, p-toluenesulfonic acid), can be used as a
catalyst. Dilute acids are used to avoid corrosion of process equipment and also the
degradation of glucose and xylose to products such as 5-HMF. Dilute H 2 SO 4 has
been used commercially for the production of furfural from cellulosic biomass. It is
also used for the hydrolysis of hemicelluloses to sugar products such as xylose. The
limited solubility of cellulose in water is a major issue; hence, solvents like ethanol
are commonly used. The effect of reaction conditions such as temperature and acid
concentration was investigated with four timber species (aspen, balsam fir, basswood, and red maple) and switchgrass. They were subjected to hydrolysis in the
presence of dilute H 2 SO 4 in an identical reaction condition (temperature, 160–190 °C;
H 2 SO 4 concentration, 0.25–1% (w/v), particle size (28––10/20 mesh)). The maximum product yield was 94% for xylose and 13.6% for glucose. Both temperature
and acid concentration have a strong influence on the reactions [10]. Various solid
catalysts used for hydrolysis are discussed in subsequent sections of this chapter.
In alkaline hydrolysis, lower temperatures and pressures are utilized compared to
acid hydrolysis, but the treatment time runs in a few hours to a few days as against
minutes or seconds for the acid catalyst. The treatment effectiveness largely depends
on the lignin content of the biomass material. Sodium, calcium, potassium, and
ammonium hydroxides are the commonly used alkaline agents, of which sodium
hydroxide is widely used. The enzymatic hydrolysis of cellulose is one of the reactions influenced by alkali pretreatment [10].
2.1.2 Catalytic Solvolysis of Lignocellulosic Biomass
In solvolysis, an organic molecule modifies the chemical and physical properties of
cellulose so that it can be in contact with the catalyst for a longer time. The organic
molecule acts both as solvent and reactant, and therefore it has a significant effect
on the product formation. For example, solvent methanol in the presence of dilute
R. Bhoi et al.
upgrading. An integrated processing approach with a multifunctional catalyst will
be beneficial for biomass conversion.
2.1.1 Catalytic Hydrolysis-Acid/Alkaline Catalysts
The biomass must be pretreated to improve the product yield in downstream processing. The pretreatment changes the biomass structure that can be achieved either
by physical, chemical, and thermal processing or sometimes by a combination of
these treatment methods. The aim of pretreatment is to decrystallize cellulose, separate hemicellulose, and break the lignin seal. During hydrolysis, the cellulosic component of lignocellulosic biomass is converted to glucose and xylose as the main
products. However, due to its crystalline form and hydrogen bonding, the hydrolysis
of cellulose is significantly difficult and requires the use of strong liquid acid as a
catalyst [9]. This process yields reducing sugars, which is furthermore converted to
useful chemicals. Among the different liquid acids, mineral acids (H 2 SO 4 and HCl),
as well as organic acids (carboxylic acid, p-toluenesulfonic acid), can be used as a
catalyst. Dilute acids are used to avoid corrosion of process equipment and also the
degradation of glucose and xylose to products such as 5-HMF. Dilute H 2 SO 4 has
been used commercially for the production of furfural from cellulosic biomass. It is
also used for the hydrolysis of hemicelluloses to sugar products such as xylose. The
limited solubility of cellulose in water is a major issue; hence, solvents like ethanol
are commonly used. The effect of reaction conditions such as temperature and acid
concentration was investigated with four timber species (aspen, balsam fir, basswood, and red maple) and switchgrass. They were subjected to hydrolysis in the
presence of dilute H 2 SO 4 in an identical reaction condition (temperature, 160–190 °C;
H 2 SO 4 concentration, 0.25–1% (w/v), particle size (28––10/20 mesh)). The maximum product yield was 94% for xylose and 13.6% for glucose. Both temperature
and acid concentration have a strong influence on the reactions [10]. Various solid
catalysts used for hydrolysis are discussed in subsequent sections of this chapter.
In alkaline hydrolysis, lower temperatures and pressures are utilized compared to
acid hydrolysis, but the treatment time runs in a few hours to a few days as against
minutes or seconds for the acid catalyst. The treatment effectiveness largely depends
on the lignin content of the biomass material. Sodium, calcium, potassium, and
ammonium hydroxides are the commonly used alkaline agents, of which sodium
hydroxide is widely used. The enzymatic hydrolysis of cellulose is one of the reactions influenced by alkali pretreatment [10].
2.1.2 Catalytic Solvolysis of Lignocellulosic Biomass
In solvolysis, an organic molecule modifies the chemical and physical properties of
cellulose so that it can be in contact with the catalyst for a longer time. The organic
molecule acts both as solvent and reactant, and therefore it has a significant effect
on the product formation. For example, solvent methanol in the presence of dilute
R. Bhoi et al.