4.1 General Types of Ionic Liquids
4.1.1 Protic Ionic Liquids (PILs)
PILs are a typical group of ionic liquids that are produced by an easy acid-base
neutralization, for instance, the combination of organic acids and amine bases;
additionally, they are moderately cost-competitive chemicals (Angell et al. 2007).
The interactions in ion pairs of PILs [diethylmethylammonium (dema) and
dimethylpropylammonium (dmpa) based ionic liquids] are stronger (Tsuzuki et al.
2013). Pyridinium, 1 methylimidazolium, pyrrolidinium, and acetate are combined
as the most competent for lignin removal, because of their high iconicity (DocampoÁlvarez et al. 2016, Achinivu 2018).
4.1.2 Aprotic Ionic Liquids
Aprotic ionic liquids are consisting of the combinations of Bronsted acids and bases,
including ethyltrimethylammonium-based ionic liquids. Aprotic ionic liquid-based
ILs are less compared to protic ionic liquids (Tsuzuki et al. 2013). Among them, the
most successful and largely used ILs in native wheat straw pretreatment are 1-ethyl3-methylimidazolium chloride (EMIM-Cl), BMIM-Cl, and 1-ethyl-3methylimidazolium acetate (EMIM-OAc). In different publications, “acronyms” of
ILs are not constantly the same. The number of possible ILs is very bulky; as a result,
only a little of the commonly utilized ionic liquids are present (Fu and Mazza 2011).
4.2 Different Types of ILs for Pretreatment of Wheat Straw
4.2.1 1-Allyl-3-Methylimidazolium Chloride
1-Allyl-3-methylimidazolium chloride (Amim-Cl) has been designed for the dissolution and regeneration of cellulose. Temperature is a significant issue in the
pretreatment involving Amim-Cl. High temperature induces important changes in
the composition of wheat straw (Zhi-guo and Hong-zhang 2012). Amim-Cl is sticky
with reactive side chains; hence, when Amim-Cl was used to treat wheat straw, high
temperature (!150
C) was more efficient than low temperature. It is found that
Amim-Cl can be dissolved by 14.8% weight of cellulose as in dissolved pulp or 8%
weight of cotton linter. However, dissolving Amim-Cl in wild lignocellulose is
comparatively difficult than that of cellulose (Zhang and Chen 2012). The wheat
straw’s compact structure doesn’t allow the ionic liquid to be penetrated much
deeper, which might be the reason that dissolving Amim-Cl in wild lignocellulose
is difficult. The pretreated wheat straw using this IL contained 38.4% cellulose,
24.2%hemicellulose, and 18.3% lignin for 2 h of heating (Zhi-guo and Hong-zhang
2012).
3 Pretreatment of Wheat Straw Using Ionic Liquids for Bioethanol Production. . .
57
4.1.1 Protic Ionic Liquids (PILs)
PILs are a typical group of ionic liquids that are produced by an easy acid-base
neutralization, for instance, the combination of organic acids and amine bases;
additionally, they are moderately cost-competitive chemicals (Angell et al. 2007).
The interactions in ion pairs of PILs [diethylmethylammonium (dema) and
dimethylpropylammonium (dmpa) based ionic liquids] are stronger (Tsuzuki et al.
2013). Pyridinium, 1 methylimidazolium, pyrrolidinium, and acetate are combined
as the most competent for lignin removal, because of their high iconicity (DocampoÁlvarez et al. 2016, Achinivu 2018).
4.1.2 Aprotic Ionic Liquids
Aprotic ionic liquids are consisting of the combinations of Bronsted acids and bases,
including ethyltrimethylammonium-based ionic liquids. Aprotic ionic liquid-based
ILs are less compared to protic ionic liquids (Tsuzuki et al. 2013). Among them, the
most successful and largely used ILs in native wheat straw pretreatment are 1-ethyl3-methylimidazolium chloride (EMIM-Cl), BMIM-Cl, and 1-ethyl-3methylimidazolium acetate (EMIM-OAc). In different publications, “acronyms” of
ILs are not constantly the same. The number of possible ILs is very bulky; as a result,
only a little of the commonly utilized ionic liquids are present (Fu and Mazza 2011).
4.2 Different Types of ILs for Pretreatment of Wheat Straw
4.2.1 1-Allyl-3-Methylimidazolium Chloride
1-Allyl-3-methylimidazolium chloride (Amim-Cl) has been designed for the dissolution and regeneration of cellulose. Temperature is a significant issue in the
pretreatment involving Amim-Cl. High temperature induces important changes in
the composition of wheat straw (Zhi-guo and Hong-zhang 2012). Amim-Cl is sticky
with reactive side chains; hence, when Amim-Cl was used to treat wheat straw, high
temperature (!150
C) was more efficient than low temperature. It is found that
Amim-Cl can be dissolved by 14.8% weight of cellulose as in dissolved pulp or 8%
weight of cotton linter. However, dissolving Amim-Cl in wild lignocellulose is
comparatively difficult than that of cellulose (Zhang and Chen 2012). The wheat
straw’s compact structure doesn’t allow the ionic liquid to be penetrated much
deeper, which might be the reason that dissolving Amim-Cl in wild lignocellulose
is difficult. The pretreated wheat straw using this IL contained 38.4% cellulose,
24.2%hemicellulose, and 18.3% lignin for 2 h of heating (Zhi-guo and Hong-zhang
2012).
3 Pretreatment of Wheat Straw Using Ionic Liquids for Bioethanol Production. . .
57