198
7 Resource Utilization of Agricultural/Forestry Residues …
ionic liquid-based pre-treatment processes are associated with high cost, difficulty
in recycling and reuse of the ILs [27].
7.4.6 Pre-treatment with Deep Eutectic Solvents (DES)
DESs are a new generation of ionic fluids composed of two or three cheap and safe
components, in which one component is a hydrogen bond acceptor and the other
is a hydrogen bond donor [20]. The melting point of the DESs is much lower than
that of the individual components and they are usually liquids at temperatures below
100 °C. They can be described by the general formula of Cat
+ X
− zY, where Cat
+
could be any ammonium, phosphonium or sulfonium cation, X
− is a Lewis base,
Y is Lewis or Brønsted acid and z is the number of Y molecules that interact with
the anion [18, 27]. DESs are alternatives to ionic liquids, but they have much lower
costs, easier synthesis process, low volatility, low toxicity, good biodegradability
and enzyme compatibility, compared to the traditional ionic liquids [20]. The most
common hydrogen bond acceptor in DESs is choline chloride (ChCl), which is an
inexpensive, biodegradable and non-toxic ammonium salt. In contrast to the ionic
liquids, the DESs can be obtained from non-ionic species [18, 27].
7.5 Physico-chemical Methods
The physico-chemical pre-treatments combine the physical changes and chemical
reactions. In a physico-chemical pre-treatment process, the lignocellulosic biomass
is treated at elevated temperatures and/or pressures with a chemical as a solvent
or catalyst. The pre-treatment leads to the disruption of the biomass structure and
fractionation of biomass components [20].
7.5.1 Steam Explosion (SE)
SE is the most commonly used method for pre-treatment of lignocellulosic biomass
for bio-ethanol production. Steam explosion combines the mechanical forces (pressure drop) and chemical effects (autohydrolysis of acetyl groups of hemicellulose)
[18, 27]. In this process, biomass is treated with high-pressure (0.7–4.8 MPa), saturated steam at a temperature of 160–260 °C for a few seconds or minutes. This lets
the water molecules to penetrate the substrate structure. The system is then rapidly
depressurized which results in the escape of the water molecules and explosion of the
bulk biomass into split fibers [17, 18]. The vapor explosion solubilizes hemicellulose
and improves the accessibility of cellulose. The process also involved autohydrolysis as the hydrolysis of hemicellulose into glucose and xylose releases acetic acid
7 Resource Utilization of Agricultural/Forestry Residues …
ionic liquid-based pre-treatment processes are associated with high cost, difficulty
in recycling and reuse of the ILs [27].
7.4.6 Pre-treatment with Deep Eutectic Solvents (DES)
DESs are a new generation of ionic fluids composed of two or three cheap and safe
components, in which one component is a hydrogen bond acceptor and the other
is a hydrogen bond donor [20]. The melting point of the DESs is much lower than
that of the individual components and they are usually liquids at temperatures below
100 °C. They can be described by the general formula of Cat
+ X
− zY, where Cat
+
could be any ammonium, phosphonium or sulfonium cation, X
− is a Lewis base,
Y is Lewis or Brønsted acid and z is the number of Y molecules that interact with
the anion [18, 27]. DESs are alternatives to ionic liquids, but they have much lower
costs, easier synthesis process, low volatility, low toxicity, good biodegradability
and enzyme compatibility, compared to the traditional ionic liquids [20]. The most
common hydrogen bond acceptor in DESs is choline chloride (ChCl), which is an
inexpensive, biodegradable and non-toxic ammonium salt. In contrast to the ionic
liquids, the DESs can be obtained from non-ionic species [18, 27].
7.5 Physico-chemical Methods
The physico-chemical pre-treatments combine the physical changes and chemical
reactions. In a physico-chemical pre-treatment process, the lignocellulosic biomass
is treated at elevated temperatures and/or pressures with a chemical as a solvent
or catalyst. The pre-treatment leads to the disruption of the biomass structure and
fractionation of biomass components [20].
7.5.1 Steam Explosion (SE)
SE is the most commonly used method for pre-treatment of lignocellulosic biomass
for bio-ethanol production. Steam explosion combines the mechanical forces (pressure drop) and chemical effects (autohydrolysis of acetyl groups of hemicellulose)
[18, 27]. In this process, biomass is treated with high-pressure (0.7–4.8 MPa), saturated steam at a temperature of 160–260 °C for a few seconds or minutes. This lets
the water molecules to penetrate the substrate structure. The system is then rapidly
depressurized which results in the escape of the water molecules and explosion of the
bulk biomass into split fibers [17, 18]. The vapor explosion solubilizes hemicellulose
and improves the accessibility of cellulose. The process also involved autohydrolysis as the hydrolysis of hemicellulose into glucose and xylose releases acetic acid