5 Commercial Aspects of Biomass Deconstruction with Ionic Liquids
95
and intra-hydrogen-bond network and creating new ones with the IL [43]. The disturbance of the hydrogen-bond network in cellulose reduces cellulose crystallinity
significantly [25]. Cellulose was recovered using a mixture of acetone and water (1:1
v/v), which is capable of dissolving lignin while simultaneously acting as an antisolvent for the cellulose-rich material. Following these findings, Rogers and co-workers
[72] in 2009 reported another major milestone in the field when they reported the
use of 1-ethyl-3-methylimidazolium acetate [C 2 C 1 im][Ace] as a better solvent to
dissolve wood than [C 4 C 1 im][Cl]. The complete dissolution of pine wood was successful at mild conditions and only required mild grinding of the wood chips, which
translates to a lower energy requirement. Also, the use of [C 2 C 1 im][Ace] offered
several advantages due to its attractive properties, such as lower toxicity, lower corrosivity, lower melting point, and potentially higher biodegradability, compared to
chloride-based ILs. The study also revealed the significant crystallinity reduction of
the regenerated cellulose material compared to the original feedstock. By comparing
XRD patterns, it was evident that the regenerated cellulose was transformed from its
natural state in the untreated biomass (cellulose I) to a revised structure of reduced
inter-chain interactions and more thermodynamically stable (cellulose II) [73] Since
then, many studies investigating the effect of ionic liquid pretreatment on cellulose
crystallinity have been published [26, 73–75].
The amorphous regenerated cellulose provides more surface area making enzymatic hydrolysis easier and significantly faster, which is a key success factor for a
pretreatment process that targets bioethanol production [27, 73]. The IL anion plays
the main role in determining cellulose dissolution ability with the cation having a
minor effect [48, 49]. ILs with hydrogen-bonding basic anions, such as chloride [16,
76] and carboxylate (e.g., formate, acetate, phosphate, and lactate) [72, 77, 78], were
shown to form strong hydrogen bonds with cellulose. Among several tested ILs, 1ethyl-3-methylimidazolium acetate [C 2 C 1 im][Ace] has been the IL predominantly
used in dissolution biomass pretreatment due to its high cellulose-dissolving capability, non-toxic nature, as well as its ability to dissolve a wide variety of biomass
feedstocks (e.g., Miscanthus giganteus, switchgrass, willow, oak, and pine). Singh
et al. [27] studied the dynamic solubilization mechanism of switchgrass subjected to
pretreatment with [C 2 C 1 im][Ace] using the auto-fluorescence of plant cell walls.
They showed that, at first, the IL swelled the secondary cell walls, which resulted in
a complete disruption of the cell wall structure after a treatment time of 2 h at 120 °C.
Fig. 5.3 shows the dissolution phases of switchgrass stem using [C 2 C 1 im][Ace]
starting from plant cell disintegration in the first phase and separation to complete
dissolution in the last phase [27].
However, facile extraction of the lignin was achieved. The close association of
the anion hydrogen-bond basicity and cellulose dissolution was confirmed by NMR
studies [76] and molecular dynamics studies [79]. The empirical solvatochromic
Kamlet-Taft parameters were also used to correlate the anion hydrogen-bond basicity
with the IL cellulose-dissolution ability. Kamlet-Taft parameters include: solvent
hydrogen-bond acidity denoted as α, hydrogen-bond basicity denoted as β, and
π * for solvent polarizability. For ILs, β was found to be mainly influenced by the
anion [80]. ILs that are capable of dissolving cellulose exhibited high hydrogen-bond
95
and intra-hydrogen-bond network and creating new ones with the IL [43]. The disturbance of the hydrogen-bond network in cellulose reduces cellulose crystallinity
significantly [25]. Cellulose was recovered using a mixture of acetone and water (1:1
v/v), which is capable of dissolving lignin while simultaneously acting as an antisolvent for the cellulose-rich material. Following these findings, Rogers and co-workers
[72] in 2009 reported another major milestone in the field when they reported the
use of 1-ethyl-3-methylimidazolium acetate [C 2 C 1 im][Ace] as a better solvent to
dissolve wood than [C 4 C 1 im][Cl]. The complete dissolution of pine wood was successful at mild conditions and only required mild grinding of the wood chips, which
translates to a lower energy requirement. Also, the use of [C 2 C 1 im][Ace] offered
several advantages due to its attractive properties, such as lower toxicity, lower corrosivity, lower melting point, and potentially higher biodegradability, compared to
chloride-based ILs. The study also revealed the significant crystallinity reduction of
the regenerated cellulose material compared to the original feedstock. By comparing
XRD patterns, it was evident that the regenerated cellulose was transformed from its
natural state in the untreated biomass (cellulose I) to a revised structure of reduced
inter-chain interactions and more thermodynamically stable (cellulose II) [73] Since
then, many studies investigating the effect of ionic liquid pretreatment on cellulose
crystallinity have been published [26, 73–75].
The amorphous regenerated cellulose provides more surface area making enzymatic hydrolysis easier and significantly faster, which is a key success factor for a
pretreatment process that targets bioethanol production [27, 73]. The IL anion plays
the main role in determining cellulose dissolution ability with the cation having a
minor effect [48, 49]. ILs with hydrogen-bonding basic anions, such as chloride [16,
76] and carboxylate (e.g., formate, acetate, phosphate, and lactate) [72, 77, 78], were
shown to form strong hydrogen bonds with cellulose. Among several tested ILs, 1ethyl-3-methylimidazolium acetate [C 2 C 1 im][Ace] has been the IL predominantly
used in dissolution biomass pretreatment due to its high cellulose-dissolving capability, non-toxic nature, as well as its ability to dissolve a wide variety of biomass
feedstocks (e.g., Miscanthus giganteus, switchgrass, willow, oak, and pine). Singh
et al. [27] studied the dynamic solubilization mechanism of switchgrass subjected to
pretreatment with [C 2 C 1 im][Ace] using the auto-fluorescence of plant cell walls.
They showed that, at first, the IL swelled the secondary cell walls, which resulted in
a complete disruption of the cell wall structure after a treatment time of 2 h at 120 °C.
Fig. 5.3 shows the dissolution phases of switchgrass stem using [C 2 C 1 im][Ace]
starting from plant cell disintegration in the first phase and separation to complete
dissolution in the last phase [27].
However, facile extraction of the lignin was achieved. The close association of
the anion hydrogen-bond basicity and cellulose dissolution was confirmed by NMR
studies [76] and molecular dynamics studies [79]. The empirical solvatochromic
Kamlet-Taft parameters were also used to correlate the anion hydrogen-bond basicity
with the IL cellulose-dissolution ability. Kamlet-Taft parameters include: solvent
hydrogen-bond acidity denoted as α, hydrogen-bond basicity denoted as β, and
π * for solvent polarizability. For ILs, β was found to be mainly influenced by the
anion [80]. ILs that are capable of dissolving cellulose exhibited high hydrogen-bond