7.4 Chemical Pre-treatment Methods
195
Organosolv pre-treatment processes employ sulfur-free reagents and operate at
mild conditions and produce sulfur-free lignin and high-purity cellulose/pulp products [29]. Thus, the organosolv pre-treatment has many advantages such as high efficiency, mild operating conditions, easy solvent recovery and recycling, and relatively
high purity of the biomass fractionation products. However, the inevitable loss and
flammability of organic solvents are the main obstacles for industrial applications of
these processes [11, 18, 20]. Whereas, with the development of biorefinery, where the
fractionation products (i.e., cellulose and lignin) could be valorized for the production of various high-value bioproducts, e.g., sodium carboxymethyl cellulose (CMC)
[6], phenol–formaldehyde adhesives [7], epoxy resins [8], and polyurethane foams
[9], organosolv fractionation offers immense opportunities for resource utilization
of agricultural/forestry residues.
7.4.5 Fractionation Using Ionic Liquids
The application of ionic liquids (ILs) for pre-treatment of lignocellulosic biomass
has attracted a lot of attention in the last decade. Ionic liquids are salts existing
as liquids at room temperature, typically composed of a small anion and a large
organic cation. Their unique properties, including a heterogeneous structure, low
melting points (<100 °C), negligible vapor pressure, high polarities and high thermal
stabilities, good solubility for some organics and excellent recyclability [20, 26, 27],
make them green solvents for many reactions. Interestingly, ionic liquids have ability
to dissolve a wide variety of biomass types including biomass component polymers
[20, 26]. When pre-treating lignocellulosic biomass in an IL [26], the biomass could
dissolve in the IL at atmospheric pressure and 90–130 °C at a residence time of
1–24 h. After the pre-treatment, the cellulosic component is precipitated out by the
addition of water and washed prior to the enzymatic hydrolysis.
Various ILs have been used for fractionation of lignocellulosic biomass, and the
cations of these ILs normally include imidazolium, pyridinium, aliphatic ammonium, alkylated phosphonium, and sulfonium ions, and the anions can be either
organic or inorganic anions [18]. In the past decade, dissolution of biopolymers
such as cellulose in ionic liquids has demonstrated a great potential of using
ionic liquids as solvents [52]. A variety of ionic liquids such as 1-butyl-3-methyland 1-allyl-3-methylimidazaolium, chloride ([C 4 mim]Cl and [Amim]Cl), 1-ethyl3-methylimidazolium acetate ([C 2 mim]Ac), 1,3-dialkylimidazolium formates, and
1-ethyl-3-methylimidazolium phosphate, have shown capability of dissolving cellulose. The role of ionic liquids in the fractionation of lignocellulosic biomass is
believed to be related to the formation of hydrogen bonding between the IL and the
lignocellulosic components, hence disrupting the biomass structure [20, 27]. They
break the extensive hydrogen bond network in cellulose and assist lignin solubilization through the π-π interactions of the ionic liquid cation with the aromatic ring of
lignin [23, 53]. In a typical fractionation process with ionic liquids, as displayed in
Fig. 7.7, biomass is first mixed with the ionic liquid and heated to the desired temper-
195
Organosolv pre-treatment processes employ sulfur-free reagents and operate at
mild conditions and produce sulfur-free lignin and high-purity cellulose/pulp products [29]. Thus, the organosolv pre-treatment has many advantages such as high efficiency, mild operating conditions, easy solvent recovery and recycling, and relatively
high purity of the biomass fractionation products. However, the inevitable loss and
flammability of organic solvents are the main obstacles for industrial applications of
these processes [11, 18, 20]. Whereas, with the development of biorefinery, where the
fractionation products (i.e., cellulose and lignin) could be valorized for the production of various high-value bioproducts, e.g., sodium carboxymethyl cellulose (CMC)
[6], phenol–formaldehyde adhesives [7], epoxy resins [8], and polyurethane foams
[9], organosolv fractionation offers immense opportunities for resource utilization
of agricultural/forestry residues.
7.4.5 Fractionation Using Ionic Liquids
The application of ionic liquids (ILs) for pre-treatment of lignocellulosic biomass
has attracted a lot of attention in the last decade. Ionic liquids are salts existing
as liquids at room temperature, typically composed of a small anion and a large
organic cation. Their unique properties, including a heterogeneous structure, low
melting points (<100 °C), negligible vapor pressure, high polarities and high thermal
stabilities, good solubility for some organics and excellent recyclability [20, 26, 27],
make them green solvents for many reactions. Interestingly, ionic liquids have ability
to dissolve a wide variety of biomass types including biomass component polymers
[20, 26]. When pre-treating lignocellulosic biomass in an IL [26], the biomass could
dissolve in the IL at atmospheric pressure and 90–130 °C at a residence time of
1–24 h. After the pre-treatment, the cellulosic component is precipitated out by the
addition of water and washed prior to the enzymatic hydrolysis.
Various ILs have been used for fractionation of lignocellulosic biomass, and the
cations of these ILs normally include imidazolium, pyridinium, aliphatic ammonium, alkylated phosphonium, and sulfonium ions, and the anions can be either
organic or inorganic anions [18]. In the past decade, dissolution of biopolymers
such as cellulose in ionic liquids has demonstrated a great potential of using
ionic liquids as solvents [52]. A variety of ionic liquids such as 1-butyl-3-methyland 1-allyl-3-methylimidazaolium, chloride ([C 4 mim]Cl and [Amim]Cl), 1-ethyl3-methylimidazolium acetate ([C 2 mim]Ac), 1,3-dialkylimidazolium formates, and
1-ethyl-3-methylimidazolium phosphate, have shown capability of dissolving cellulose. The role of ionic liquids in the fractionation of lignocellulosic biomass is
believed to be related to the formation of hydrogen bonding between the IL and the
lignocellulosic components, hence disrupting the biomass structure [20, 27]. They
break the extensive hydrogen bond network in cellulose and assist lignin solubilization through the π-π interactions of the ionic liquid cation with the aromatic ring of
lignin [23, 53]. In a typical fractionation process with ionic liquids, as displayed in
Fig. 7.7, biomass is first mixed with the ionic liquid and heated to the desired temper-