60
4 Effective Biomass Valorization Procedures Using Ultrasound …
4.2.1.1 Acoustic Cavitation in the Presence of Alternative Solvents
Lignin has been identified as one of the major obstacles to the efficient enzymatic
hydrolysis. The use of ionic liquids (ILs) in biomass fractionation proved an effective
mean of disrupting the intricate network of non-covalent interactions of lignin. It has
been reported that the imidazolium cations of ILs may interact with the aromatic
rings of lignin, via π–π interactions, to promote its dissolution. In particular, the
relatively small 1-hexyl-3-methylimidazolium [HMIM]
+ cation was found efficient
in fractionating lignocellulosic biomass. Moreover, ILs containing an anion with
strong hydrogen-bond capability also shows good solubility for lignin (Mäki-Arvela
et al. 2010).
In this regard, imidazolium-based ionic liquids displaying a large variety of anions
(chloride, sulphate, acetate, phosphate, methanesulfonate, p-toluenesulfonate) have
been investigated by Zhang et al. (2015). Corn stover was chosen as the starting
material, due to its complex structure. Its fractionation into cellulose, hemicellulose
and lignin was successfully performed in ILs at 70 °C after 3 h of US irradiation
(400 W). Higher yields of lignin (S-G-H type) were isolated in the presence of
[HMIM]OAc and [HMIM]Cl (60.48% of the original lignin). Achievements were
imputed to both the hydrogen-bond capability and the acidity of the engaged ILs
Moreover, the same study revealed that the use of a small anion was preferable as it
was able to diffuse into the lignocellulosic matrix more quickly. The use of a chloride
anion, a small-sized and strong hydrogen-bond acceptor, with a small-sized cation
([HMIM]
+ ) showed an effective pretreatment solvent for the dissolution of lignin.
The enzymatic hydrolysis of recovered cellulose was performed, affording a high
yield (97.77%) of reducing sugars. These ILs have shown a great potential for the
preparation of biofuels.
Besides the ionic liquids commonly used for lignocellulosic biomass pretreatment,
recent years have been witness to even greater interest in bio-based deep eutectic solvents (NaDES). In fact, these mixtures have been reported to be more biodegradable
and bio-renewable than the even more expensive imidazolium ILs commonly used
for pretreatment. In this context, Ninomiya et al. (2013) have reported, for the first
time, that the US-assisted pretreatment of powdered bamboo biomass in the presence of choline acetate (ChOAc) was able to dramatically improve the subsequent
saccharification yields. When thermally pretreated at 110 °C for 60 min in ChOAc,
saccharification yield was approximatively 55%, whereas it reaches 92% after the
US pretreatment (24 kHz and 35 W power) in the same solvent at 25 °C for 60 min.
Moreover, the reported analyses showed that the cellulose crystallinity of pretreated
bamboo powder was lower after ultrasonic pretreatment in ChOAc than after thermal
pretreatment (Fig. 4.5).
In order to examine the capabilities of ChOAc in the pretreatment of different
lignocellulosic materials, Ninomiya et al. (2015) used bagasse powder that was US
pretreated in the presence of ChOAc for subsequent enzymatic hydrolysis. The mixture was sonicated for 60 min at 24 kHz and an emission power of 35 W using
an ultrasonic sonotrode. After ChOAc pretreatment, cellulose and hemicellulose
saccharification percentages were higher, with an enzymatic reaction time of 48 h,
4 Effective Biomass Valorization Procedures Using Ultrasound …
4.2.1.1 Acoustic Cavitation in the Presence of Alternative Solvents
Lignin has been identified as one of the major obstacles to the efficient enzymatic
hydrolysis. The use of ionic liquids (ILs) in biomass fractionation proved an effective
mean of disrupting the intricate network of non-covalent interactions of lignin. It has
been reported that the imidazolium cations of ILs may interact with the aromatic
rings of lignin, via π–π interactions, to promote its dissolution. In particular, the
relatively small 1-hexyl-3-methylimidazolium [HMIM]
+ cation was found efficient
in fractionating lignocellulosic biomass. Moreover, ILs containing an anion with
strong hydrogen-bond capability also shows good solubility for lignin (Mäki-Arvela
et al. 2010).
In this regard, imidazolium-based ionic liquids displaying a large variety of anions
(chloride, sulphate, acetate, phosphate, methanesulfonate, p-toluenesulfonate) have
been investigated by Zhang et al. (2015). Corn stover was chosen as the starting
material, due to its complex structure. Its fractionation into cellulose, hemicellulose
and lignin was successfully performed in ILs at 70 °C after 3 h of US irradiation
(400 W). Higher yields of lignin (S-G-H type) were isolated in the presence of
[HMIM]OAc and [HMIM]Cl (60.48% of the original lignin). Achievements were
imputed to both the hydrogen-bond capability and the acidity of the engaged ILs
Moreover, the same study revealed that the use of a small anion was preferable as it
was able to diffuse into the lignocellulosic matrix more quickly. The use of a chloride
anion, a small-sized and strong hydrogen-bond acceptor, with a small-sized cation
([HMIM]
+ ) showed an effective pretreatment solvent for the dissolution of lignin.
The enzymatic hydrolysis of recovered cellulose was performed, affording a high
yield (97.77%) of reducing sugars. These ILs have shown a great potential for the
preparation of biofuels.
Besides the ionic liquids commonly used for lignocellulosic biomass pretreatment,
recent years have been witness to even greater interest in bio-based deep eutectic solvents (NaDES). In fact, these mixtures have been reported to be more biodegradable
and bio-renewable than the even more expensive imidazolium ILs commonly used
for pretreatment. In this context, Ninomiya et al. (2013) have reported, for the first
time, that the US-assisted pretreatment of powdered bamboo biomass in the presence of choline acetate (ChOAc) was able to dramatically improve the subsequent
saccharification yields. When thermally pretreated at 110 °C for 60 min in ChOAc,
saccharification yield was approximatively 55%, whereas it reaches 92% after the
US pretreatment (24 kHz and 35 W power) in the same solvent at 25 °C for 60 min.
Moreover, the reported analyses showed that the cellulose crystallinity of pretreated
bamboo powder was lower after ultrasonic pretreatment in ChOAc than after thermal
pretreatment (Fig. 4.5).
In order to examine the capabilities of ChOAc in the pretreatment of different
lignocellulosic materials, Ninomiya et al. (2015) used bagasse powder that was US
pretreated in the presence of ChOAc for subsequent enzymatic hydrolysis. The mixture was sonicated for 60 min at 24 kHz and an emission power of 35 W using
an ultrasonic sonotrode. After ChOAc pretreatment, cellulose and hemicellulose
saccharification percentages were higher, with an enzymatic reaction time of 48 h,