92
A. R. Abouelela et al.
Another important application for ILs in biomass processing is their use as a
reaction medium for sugar conversion to 5-hydroxymethyl furfural (HMF) [45]. In
the U.S. Department of Energy report on Top Value-Added Chemicals from Biomass,
HMF was specified as a versatile platform chemical that can be used as a building
block to synthesize numerous polymers and chemicals, such as 2,5-furandicarboxylic
acid, 1,6-hexanediol, adipic acid, levulinic acid, 2-methylfuran, and caprolactone
[46, 47]. The formation reaction of HMF requires the dehydration of hexose sugars
(fructose or glucose) in the presence of a metal salt catalyst. In general, the IL used
as well as the selected catalyst have important roles in the selectivity and yield of the
reactions since the reaction is promoted via the formation of an IL-catalyst complex.
Synthesis of HMF from glucose is more challenging compared to synthesis from
fructose as the reaction proceeds in two steps that require two different catalytic
environments: (1) the isomerization of fructose to glucose which is catalyzed by a
Lewis base followed by (2) dehydration of glucose to 5-HMF which is speculated
to be promoted by Brønsted acids [48]. Conducting the reaction in an aqueous environment usually results in poor yield and selectively due to further dehydration of
the HMF product to levulinic acid with a substantial formation of humins through
side reactions [49]. On the other hand, superior product yield and selectivity were
achieved when common imidazolium-based IL was used as the reaction environment, with 90% conversion achieved when glucose was used as a substrate [48]. The
research field is still in its early stages and several challenges need to be resolved: (1)
HMF product separation from the IL medium, (2) recycling of the IL and catalyst,
(3) increasing glucose substrate loading, and (4) making the system robust enough to
use cellulose as a direct substrate [50, 51]. Biomass pretreatment is one of the major
applications of ionic liquids in biomass processing. In fact, processing and transformation of cellulose and lignin for fuel, chemical, or material production would
first require their effective separation from the biomass polymer matrix, which is the
objective of a pretreatment process. The use of ionic liquids in biomass pretreatment
is discussed in Sect. 5.3.
5.2.2 Protic Ionic Liquids
Protic ionic liquids (PILs), a subclass of ILs, are prepared by a simple stoichiometric
neutralization reaction between a Brønsted acid and a Brønsted base (Scheme 5.1)
that results in an acid–base complex (i.e., the protic IL) [52].
The formation reaction is based on the proton transfer from the acid to the base
which gives a PIL a distinguishable characteristic of having an available proton to
form hydrogen bonds with an anion, other dissolved solutes, and solvents [53].
Scheme 5.1 Protic IL formation reaction from a Brønsted acid and a Brønsted base
A. R. Abouelela et al.
Another important application for ILs in biomass processing is their use as a
reaction medium for sugar conversion to 5-hydroxymethyl furfural (HMF) [45]. In
the U.S. Department of Energy report on Top Value-Added Chemicals from Biomass,
HMF was specified as a versatile platform chemical that can be used as a building
block to synthesize numerous polymers and chemicals, such as 2,5-furandicarboxylic
acid, 1,6-hexanediol, adipic acid, levulinic acid, 2-methylfuran, and caprolactone
[46, 47]. The formation reaction of HMF requires the dehydration of hexose sugars
(fructose or glucose) in the presence of a metal salt catalyst. In general, the IL used
as well as the selected catalyst have important roles in the selectivity and yield of the
reactions since the reaction is promoted via the formation of an IL-catalyst complex.
Synthesis of HMF from glucose is more challenging compared to synthesis from
fructose as the reaction proceeds in two steps that require two different catalytic
environments: (1) the isomerization of fructose to glucose which is catalyzed by a
Lewis base followed by (2) dehydration of glucose to 5-HMF which is speculated
to be promoted by Brønsted acids [48]. Conducting the reaction in an aqueous environment usually results in poor yield and selectively due to further dehydration of
the HMF product to levulinic acid with a substantial formation of humins through
side reactions [49]. On the other hand, superior product yield and selectivity were
achieved when common imidazolium-based IL was used as the reaction environment, with 90% conversion achieved when glucose was used as a substrate [48]. The
research field is still in its early stages and several challenges need to be resolved: (1)
HMF product separation from the IL medium, (2) recycling of the IL and catalyst,
(3) increasing glucose substrate loading, and (4) making the system robust enough to
use cellulose as a direct substrate [50, 51]. Biomass pretreatment is one of the major
applications of ionic liquids in biomass processing. In fact, processing and transformation of cellulose and lignin for fuel, chemical, or material production would
first require their effective separation from the biomass polymer matrix, which is the
objective of a pretreatment process. The use of ionic liquids in biomass pretreatment
is discussed in Sect. 5.3.
5.2.2 Protic Ionic Liquids
Protic ionic liquids (PILs), a subclass of ILs, are prepared by a simple stoichiometric
neutralization reaction between a Brønsted acid and a Brønsted base (Scheme 5.1)
that results in an acid–base complex (i.e., the protic IL) [52].
The formation reaction is based on the proton transfer from the acid to the base
which gives a PIL a distinguishable characteristic of having an available proton to
form hydrogen bonds with an anion, other dissolved solutes, and solvents [53].
Scheme 5.1 Protic IL formation reaction from a Brønsted acid and a Brønsted base