62
4 Effective Biomass Valorization Procedures Using Ultrasound …
tion of cellulose crystallinity. Anions generally act as hydrogen-bond acceptors and
interact with the hydroxyl groups of cellulose weakening its crystalline structure,
whereas cations interact with lignin through hydrogen bonding and π–π interactions. By contrast, the inhibitory effects of ChOAc on cellulose were significantly
lower than those of [EMIM]OAc, because the cholinium cation was more biocompatible than the imidazolium one. It is therefore suggested that ChOAc is a promising
alternative to [EMIM]OAc for pretreatment and for the in situ saccharification of
lignocellulosic biomass.
4.2.1.2 Hydrodynamic Cavitation as Suitable Biomass Pretreatment
Phenolic hydroxyl groups in lignin dissociate under alkaline conditions forming
a resonance stabilized phenoxy radical, which is the initiator of depolymerization
reactions. An alkaline pretreatment carried out by HC promotes the formation of
hydroxyl and hydroperoxy radicals required for the delignification reaction, thus
intensifying the overall reactions rates. In addition, shock waves formed during HC
are responsible for the breakage of some chemical bonds between lignin and cellulose
in the biomass. The main advantage of HC is that the conditions of high temperature
and pressure necessary for delignification are generated locally by the collapse of
shock waves, while the overall process still remains at ambient conditions, thus
avoiding the requirements for a heating device.
These features make HC a valuable tool for biomass pretreatment; a key step in the
biochemical processes to biofuels. For example, HC has been applied in the alkaline
pretreatment (3.0% NaOH) of reed, used as starting material for the production of
bioethanol (Kim et al. 2015).
HC has also been used to improve the efficiency of the alkaline pretreatment of
sugarcane bagasse (SCB) for ethanol production (Hilares et al. 2016, 2017). A lignin
removal of 60.4% was achieved in only 30–45 min of HC treatment with 97.2%
of enzymatic digestibility. This result was accompanied by the drastic change in
biomass structure provoked by the immensely destructive cavitation.
Finally, the efficiency of the HC and US technologies, with regard to the
polysaccharide degradation of corn stover in flow-through mode, has been compared (Nakashima et al. 2016). The US-SP pretreatment of corn stover led to lower
digestibility than that provided by the HC-SP process, which was more efficient in
terms of both glucose and xylose production. This difference can be ascribed to the
50% greater lignin removal obtained by the HC-SP system. The inhibitor furfural
was not generated in any process, because the pretreatment was conducted under
mild conditions at 30 °C. A comparison of the energy efficiency of the HC- and
US-assisted pretreatment of SCB, reed and corn stover is summarized in Table 4.1.
The hydrodynamic delignification of wheat straw, used for the production of
paper, provided some advantages, such as the reduction of treatment time, energy
consumption (almost 50%) and temperature (ambient temperature). Furthermore,
4 Effective Biomass Valorization Procedures Using Ultrasound …
tion of cellulose crystallinity. Anions generally act as hydrogen-bond acceptors and
interact with the hydroxyl groups of cellulose weakening its crystalline structure,
whereas cations interact with lignin through hydrogen bonding and π–π interactions. By contrast, the inhibitory effects of ChOAc on cellulose were significantly
lower than those of [EMIM]OAc, because the cholinium cation was more biocompatible than the imidazolium one. It is therefore suggested that ChOAc is a promising
alternative to [EMIM]OAc for pretreatment and for the in situ saccharification of
lignocellulosic biomass.
4.2.1.2 Hydrodynamic Cavitation as Suitable Biomass Pretreatment
Phenolic hydroxyl groups in lignin dissociate under alkaline conditions forming
a resonance stabilized phenoxy radical, which is the initiator of depolymerization
reactions. An alkaline pretreatment carried out by HC promotes the formation of
hydroxyl and hydroperoxy radicals required for the delignification reaction, thus
intensifying the overall reactions rates. In addition, shock waves formed during HC
are responsible for the breakage of some chemical bonds between lignin and cellulose
in the biomass. The main advantage of HC is that the conditions of high temperature
and pressure necessary for delignification are generated locally by the collapse of
shock waves, while the overall process still remains at ambient conditions, thus
avoiding the requirements for a heating device.
These features make HC a valuable tool for biomass pretreatment; a key step in the
biochemical processes to biofuels. For example, HC has been applied in the alkaline
pretreatment (3.0% NaOH) of reed, used as starting material for the production of
bioethanol (Kim et al. 2015).
HC has also been used to improve the efficiency of the alkaline pretreatment of
sugarcane bagasse (SCB) for ethanol production (Hilares et al. 2016, 2017). A lignin
removal of 60.4% was achieved in only 30–45 min of HC treatment with 97.2%
of enzymatic digestibility. This result was accompanied by the drastic change in
biomass structure provoked by the immensely destructive cavitation.
Finally, the efficiency of the HC and US technologies, with regard to the
polysaccharide degradation of corn stover in flow-through mode, has been compared (Nakashima et al. 2016). The US-SP pretreatment of corn stover led to lower
digestibility than that provided by the HC-SP process, which was more efficient in
terms of both glucose and xylose production. This difference can be ascribed to the
50% greater lignin removal obtained by the HC-SP system. The inhibitor furfural
was not generated in any process, because the pretreatment was conducted under
mild conditions at 30 °C. A comparison of the energy efficiency of the HC- and
US-assisted pretreatment of SCB, reed and corn stover is summarized in Table 4.1.
The hydrodynamic delignification of wheat straw, used for the production of
paper, provided some advantages, such as the reduction of treatment time, energy
consumption (almost 50%) and temperature (ambient temperature). Furthermore,