**6. Ionic liquid pretreatment**

yield from 32.9% to 60.9%. The hot water hemicellulose extraction step allowed the conver‐ sion of 83.7% of xylan to xylose and favored cellulose hydrolysis [93]. Rape straw extrusion

The high sugar recovery due to extrusion pretreatment is related to fibrillation, the increase in surface area [79-81, 86, 87] and pore size [103], which facilitate the access of enzymes to cellulose. Some authors have reported that the crystallinity, which confers resistance to en‐ zymatic hydrolysis, was not significantly reduced in extruded biomass [79, 102] and there‐ fore was not related to the increase in biomass digestibility. Moreover, an increase of 82% in the crystallinity of soybean hulls by thermomechanical pretreatment, using a twin-screw ex‐ truder was reported [82]; as there was no change in material composition, crystallization of the amorphous structure during thermomechanical extrusion was suggested. Some re‐ searchers have also noted the crystallinization of cellulose in the presence of moisture and heat, as has been observed for wood pretreated by steam explosion [104], cotton linter and wood treated in aqueous media after ball milling [105] and hemp cellulose treated by wet ball milling [106]. In accordance with the aforementioned, some researchers suggested that the opening of the cell wall structure at a microscopic scale is sufficient for enzymatic sac‐ charification, regardless of the cellulose crystallinity index [79]. Furthermore, the combina‐ tion of thermomechanical and/or chemical pretreatments can deconstruct the hemicelluloses chains and/or remove part of the hemicelluloses and lignin, facilitating biomass digestibility

The twin-screw extruder is highly efficiency for pulverization by applying high shearing forces and shows adaptability to different processes, such as chemical, high-pressure appli‐ cations and explosion pretreatments (steam or other solvents) [79-81, 92, 108, 109]. The proc‐ ess is easy to operate and the extrusion process allows the continuous pretreatment of large amounts of biomass with high throughputs, which is advantageous in comparison to batch procedures for the industrial setting. Extrusion compares well to pretreatment technologies that have as drawbacks the batch processing mode, low solids loading or the use of large amounts of water, as already mentioned. Extrusion allows temperature control and does not require washing and conditioning steps, as required with diluted acid, alkali or ionic liquid pretreatments and does not produce effluent; thus there is no effluent disposal cost, no sol‐ ids loss and no significant safety issues [86]. In comparison to other mechanical pretreat‐ ments, the extrusion process is normally less energy intensive than the milling pretreatment options. If extrusion is combined with chemical pretreatment, due to its effective ability of kneading and mixing, the process requires less chemical loadings and thus less residual ef‐ fluents are formed; the combination of extrusion with chemical pretreatment can further re‐ duce energy consumption as it is economically suitable for large-scale operation. Furthermore, extrusion does not produce fermentation inhibitors, such as furfural and hy‐ droxyl methylfurfural; nevertheless, low concentrations of acetic acid have been reported [84-87]. However, the extrusion pretreatment of lignocellulosic biomass requires the use of additives to increase the flow ability inside the barrel and avoid the accumulation, burning

pretreatment with 3.5% sulfuric acid at 165 °C obtained a glucose yield of 70.9% [94].

64 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

[82, 102, 107].

*5.2.1. Advantages and disadvantages of extrusion pretreatment*

Ionic liquids (ILs) can be defined as salts that melt below 100 °C and are composed ex‐ clusively of ions. The first report of a room temperature IL dates back to 1914 [110] and did not prompt any significant interest at that time. It was in the 1980s that these chemi‐ cals have come under intense worldwide attention due to the implications for their use as solvents [111, 112]. The fact that many ILs can be liquid at room temperature and, in general, present a negligible vapor pressure has justified the attention that this group of chemicals has received. They have also been suggested as candidates to substitute for low-boiling-point solvents, such as toluene, diethyl ether and methanol. In addition, ILs are versatile materials and often called designer solvents because their physical and chemical properties can be tuned to meet a specific purpose by preparing new ILs with different combinations of ions [113].

ILs have become increasingly trendy over the past few years in the biomass field due to the ability of some members of this class of chemicals to dissolve a wide variety of bio‐ mass types. ILs have been reported for the pretreatment of cellulose [114] and lignocellu‐ losic materials, such as rice straw [115], sugarcane bagasse [116, 117], wheat straw [118], switchgrass [119], *Miscanthus* [120] and wood [121, 122, 123], among others. However, this concept is not new since in 1934 a patent claimed that certain organic salts were ca‐ pable of dissolving cellulose and alter its reactivity [124]; nevertheless, at that time this publication did not generate any important reaction in the scientific community. In 2002, a research group from the University of Alabama investigated new compounds, now known as ILs, based on the concept of cellulose dissolution by a molten salt described by Graenacher in 1934. As result, they found that the IL 1-methyl-3-butyl imidazolium chloride ([Bmim][Cl]) could dissolve up to a 10% solution of cellulose by stirring cellu‐ lose with the IL while heating (100 °C). When heating was performed in a microwave oven, the dissolution achieved was up to 25% (wt%) [125]. Their pioneer work has now been cited over 1000 times and is considered a breakthrough that has set the basis for a novel concept for lignocellulosic biomass pretreatment.

Based on the concept of cellulose dissolution described by Swatloski and co-workers [124] and lately by another work that has shown that [Bmim][Cl] was also able to partially dis‐ solve wood [126], many research groups have described processes of biomass pretreatment with ILs; most of these studies document the complete or partial dissolution of lignocellu‐ lose under heating conditions followed by precipitation with water as an antisolvent. The aim of this procedure is to recover a pretreated part of the biomass that is highly susceptible to enzymatic attack. After IL pretreatment, the biomass native structure is altered in the re‐ covered material in such a manner that the reconstructed cellulose is essentially amorphous compared to highly crystalline untreated cellulose [127].

The mechanism for IL cellulose dissolution has been investigated by applying different ana‐ lytical methods. In one study, nuclear magnetic ressonance (NMR) relaxation measurements on [Bmim][Cl] confirmed that chloride ions form hydrogen bonds with the cellulose hydrox‐ yl group in a stoichiometric 1:1 ratio [128]. This interaction causes the break of intermolecu‐ lar and intramolecular hydrogen bonding between cellulose fibrils, which ultimately leads to cellulose dissolution. Additionally, depending on the type of IL, an efficient extraction of lignin can be facilitated by the cellulose dissolution process, as more lignin can be exposed to the solvent [121, 126].

The reduction of cellulose crystallinity is usually reported as a main effect of IL pretreat‐ ment. However, in pretreatment of sugarcane bagasse catalyzed by acid, using HCl-[Bmim] [Cl] [132] or H2SO4-1-butyl-3-methylimidazolium methylsulfate ([Bmim][MeSO4]) systems [133], cellulose crystallinity remained unaltered even though a significant increase of cellu‐ lose digestibility was achieved. The authors suggested that digestibility increase was due to the highly effective and simultaneous removal of xylan and lignin that facilitated cellulose enzymatic saccharification efficiency. Table 2 presents the experimental conditions for the

> **Solid loading (%)**

[Emim][Ac] - 120 30/120 5 15 2.5 95.3/98.2 - [116] [Emim][Ac] - 120 30 5 15 1.0 87.0 - [134] [Emim][Ac] - 145 15 14 30 2.0 - 69.7 [131] [Emim][Ac] - 140 continuous 25 15 2.5 90.3 - [95] [Bmim][Cl] - 140/150 90 5 15 a 1.0 62.0/100 [117] [Bmim][Cl] - 120 120 5 15 2.5 38.6 - [116] [Bmim][Cl] - 130 120 10 20 a 2.0 29.5 - [132] [Bmim][Cl] H2SO4/HCl 130 30 10 20 a 2.0 93.5/94.5 - [132] [Bmim][Cl] NH4OH-H2O2 100 60 3 20 2.0 - 90.0 [135] [Amim][Cl] - 120 120 5 15 2.5 43.3 - [116] [Amim][Cl] NH4OH-H2O2 100 60 3 20 2.0 - 91.4 [135] [Bmim][MeSO4] - 125/150 120 10 60 1 79.0/100 [133] [Bmim][MeSO4] H2SO4 100 120 10 60 1 74.0 [133] [Mmim][DEP] - 120 120 5 15 2.5 61.9 - [116]

**Table 2.** Sugarcane bagasse IL pretreatment parameters and corresponding data for enzymatic saccharification and

Many reports can be found for the pretreatment of wood biomass with ILs. Initial studies have focused on the use of ILs to dissolve lignocellulosic biomass aiming its fractionation [126, 136]. Moreover, the possibility to perform the derivatization of wood components *in situ* using the biomass IL solution was considered an interesting approach to reduce the number of steps to produce derivatives, such as acylated cellulose from raw materials [137]. More recently there have been reports on the enzymatic digestibility of recovered wood bio‐ mass after IL dissolution. Over 90% cellulose hydrolysis was obtained after *Pinus radiata* pre‐ treatment with [Emim][Ac] at 120 °C for 180 min, using a 5% solid loading during pretreatment [138]. The authors demonstrated that the IL pretreatment induced composi‐ tional and structural changes in the wood, including extraction and deacetylation of the

**Enzyme dosage (FPU/g)**

**Substrate loading (%)**

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

**Glucose yield (%)**

**Reducing sugars yield (%)**

http://dx.doi.org/10.5772/53378

**Ref.**

67

pretreatment of sugarcane bagasse with ILs.

**Temperature (°C)**

**Time (min)**

**Pretreatment association**

Enzyme dosage per gram of cellulose

**IL**

a

sugar yields

Different combinations of anion and cation compositions have been examined for biomass pretreatment, as the dissolution of biomass components is highly affected by the nature of the IL. In general, in order to dissolve cellulose, the anion of the IL must be a good hydrogen bond acceptor [123, 129]. The most promising anions have been shown to be chlorides, ace‐ tates, formates and phosphates. It has also been demonstrated that cations play a role in cel‐ lulose solubility as the imidazolium cation, whose electron-rich aromatic π system interacts with cellulose hydroxyl oxygen atoms via nonbonding and π electrons, prevents the cross‐ linking of cellulose molecules. In general, the most appropriate cations for cellulose dissolu‐ tion are based on methylimidazolium and methylpyridinium cores, with allyl-, ethyl-, or butyl-side chains [130].

Considering the IL pretreatment of sugarcane bagasse, the IL 1-ethyl-3-methylimidazolium acetate ([Emim][Ac]) has been selected among six ILs studied as the best choice for pretreat‐ ment (120 min, 120 °C and 5% solid loading) as it was possible to reach a glucose yield of 98.2% after 48 h of enzymatic hydrolysis of 2.5% pretreated bagasse loading using commer‐ cial enzymes at a dosage of 15 FPU/g bagasse [116]. The authors suggested that the resulting pretreated biomass was highly digestible due to its amorphous-like structure, the high abili‐ ty of [Emim][Ac] to extract lignin and the increased specific surface area (SSA) of 131.8 m2 /g compared to an SSA of 1.4 m2 /g measured for untreated bagasse. In another study, yields of 69.7% of reducing sugars were obtained for the enzymatic hydrolysis (30 FPU/g substrate) of 2% bagasse loading pretreated with [Emim][Ac] for 15 min at 145 °C, using a 14% solid load‐ ing during pretreatment [131]. Since high-solid loading during pretreatment was applied, [Emim][Ac] was ineffective in bagasse delignification, even though it was able to reduce the biomass crystallinity.

Some studies have combined other pretreatment strategies to IL pretreatment of sugar‐ cane bagasse to reduce the pretreatment time and increase the efficiency. In an HCl-catal‐ ized pretreatment process in IL aqueous solutions, optimum conditions for the sugarcane bagasse pretreatment was obtained at 130 °C, 30 min, using a water:[Bmim][Cl]:HCl sol‐ ution (%) of 20:78.8:1.2. Cellulose digestibility yields corresponding to 94.5% were ob‐ tained after 24 h saccharification of 2% glucan loading, using commercial enzymes at a dosage of 20 FPU/g glucan; the pretreatment for 120 min using solely [Bmim][Cl] result‐ ed in 29.5% cellulose conversion [132]. Other reports of sugarcane bagasse pretreatment using [Bmim][Cl] have also reported low glucose yields of 38.6% (120 °C, 120 min) [116] and 62% (140 °C, 90 min) [117].

The reduction of cellulose crystallinity is usually reported as a main effect of IL pretreat‐ ment. However, in pretreatment of sugarcane bagasse catalyzed by acid, using HCl-[Bmim] [Cl] [132] or H2SO4-1-butyl-3-methylimidazolium methylsulfate ([Bmim][MeSO4]) systems [133], cellulose crystallinity remained unaltered even though a significant increase of cellu‐ lose digestibility was achieved. The authors suggested that digestibility increase was due to the highly effective and simultaneous removal of xylan and lignin that facilitated cellulose enzymatic saccharification efficiency. Table 2 presents the experimental conditions for the pretreatment of sugarcane bagasse with ILs.


a Enzyme dosage per gram of cellulose

/g

The mechanism for IL cellulose dissolution has been investigated by applying different ana‐ lytical methods. In one study, nuclear magnetic ressonance (NMR) relaxation measurements on [Bmim][Cl] confirmed that chloride ions form hydrogen bonds with the cellulose hydrox‐ yl group in a stoichiometric 1:1 ratio [128]. This interaction causes the break of intermolecu‐ lar and intramolecular hydrogen bonding between cellulose fibrils, which ultimately leads to cellulose dissolution. Additionally, depending on the type of IL, an efficient extraction of lignin can be facilitated by the cellulose dissolution process, as more lignin can be exposed

66 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

Different combinations of anion and cation compositions have been examined for biomass pretreatment, as the dissolution of biomass components is highly affected by the nature of the IL. In general, in order to dissolve cellulose, the anion of the IL must be a good hydrogen bond acceptor [123, 129]. The most promising anions have been shown to be chlorides, ace‐ tates, formates and phosphates. It has also been demonstrated that cations play a role in cel‐ lulose solubility as the imidazolium cation, whose electron-rich aromatic π system interacts with cellulose hydroxyl oxygen atoms via nonbonding and π electrons, prevents the cross‐ linking of cellulose molecules. In general, the most appropriate cations for cellulose dissolu‐ tion are based on methylimidazolium and methylpyridinium cores, with allyl-, ethyl-, or

Considering the IL pretreatment of sugarcane bagasse, the IL 1-ethyl-3-methylimidazolium acetate ([Emim][Ac]) has been selected among six ILs studied as the best choice for pretreat‐ ment (120 min, 120 °C and 5% solid loading) as it was possible to reach a glucose yield of 98.2% after 48 h of enzymatic hydrolysis of 2.5% pretreated bagasse loading using commer‐ cial enzymes at a dosage of 15 FPU/g bagasse [116]. The authors suggested that the resulting pretreated biomass was highly digestible due to its amorphous-like structure, the high abili‐ ty of [Emim][Ac] to extract lignin and the increased specific surface area (SSA) of 131.8 m2

69.7% of reducing sugars were obtained for the enzymatic hydrolysis (30 FPU/g substrate) of 2% bagasse loading pretreated with [Emim][Ac] for 15 min at 145 °C, using a 14% solid load‐ ing during pretreatment [131]. Since high-solid loading during pretreatment was applied, [Emim][Ac] was ineffective in bagasse delignification, even though it was able to reduce the

Some studies have combined other pretreatment strategies to IL pretreatment of sugar‐ cane bagasse to reduce the pretreatment time and increase the efficiency. In an HCl-catal‐ ized pretreatment process in IL aqueous solutions, optimum conditions for the sugarcane bagasse pretreatment was obtained at 130 °C, 30 min, using a water:[Bmim][Cl]:HCl sol‐ ution (%) of 20:78.8:1.2. Cellulose digestibility yields corresponding to 94.5% were ob‐ tained after 24 h saccharification of 2% glucan loading, using commercial enzymes at a dosage of 20 FPU/g glucan; the pretreatment for 120 min using solely [Bmim][Cl] result‐ ed in 29.5% cellulose conversion [132]. Other reports of sugarcane bagasse pretreatment using [Bmim][Cl] have also reported low glucose yields of 38.6% (120 °C, 120 min) [116]

/g measured for untreated bagasse. In another study, yields of

to the solvent [121, 126].

butyl-side chains [130].

compared to an SSA of 1.4 m2

and 62% (140 °C, 90 min) [117].

biomass crystallinity.

**Table 2.** Sugarcane bagasse IL pretreatment parameters and corresponding data for enzymatic saccharification and sugar yields

Many reports can be found for the pretreatment of wood biomass with ILs. Initial studies have focused on the use of ILs to dissolve lignocellulosic biomass aiming its fractionation [126, 136]. Moreover, the possibility to perform the derivatization of wood components *in situ* using the biomass IL solution was considered an interesting approach to reduce the number of steps to produce derivatives, such as acylated cellulose from raw materials [137]. More recently there have been reports on the enzymatic digestibility of recovered wood bio‐ mass after IL dissolution. Over 90% cellulose hydrolysis was obtained after *Pinus radiata* pre‐ treatment with [Emim][Ac] at 120 °C for 180 min, using a 5% solid loading during pretreatment [138]. The authors demonstrated that the IL pretreatment induced composi‐ tional and structural changes in the wood, including extraction and deacetylation of the hemicellulose fraction and loss of lignin ether linkages. The cellulose crystallinity was al‐ tered, prompting the suggestion that cellulose I was transformed, to some extent, to cellu‐ lose II. However, in contrast to an earlier report for the pretreatment of maple wood flour with [Emim][Ac] in which up to 80% of delignification was achieved [121], no significant de‐ lignification of *P. radiata* was observed. The glucose saccharification yields obtained for the maple wood flour pretreated at 130 °C for 90 min reached 95%. In contrast to the high yields obtained after wood pretreatment with [Emim][Ac], the use of [Emim][Cl] was shown to be ineffective, as only 30% of total sugars were obtained after saccharification of pretreated eu‐ calyptus at 150 °C for 60 min [139].

**6.1. Advantages and disadvantages of IL pretreatment**

source of an IL-based biorefinery [141].

looking for new enzymes that are stable in ILs [144, 145].

lectivity toward biomass processing.

ILs are able to disrupt the plant cell wall structure by the solubilization of its main compo‐ nents. This class of salts is also able to alter cellulose crystallinity and structure, rendering the amorphous cellulose prone to high rates and yields from enzymatic saccharification. In‐ deed, this combination of effects generates a pretreated material that can be easily hydro‐ lyzed into monomeric sugars when compared to other pretreatment technologies, also rendering the enzymatic attack faster as the initial hydrolysis rate is greatly increased [116, 119]. In order to achieve high cellulose conversion yields (>80%) using other pretreatment processes, enzymatic saccharification times of 48–72 h are generally reported. However, in the case of IL pretreatment of bagasse and also some woody types, those yields can be ob‐ tained in less than 24 h with enzymatic hydrolysis. Nevertheless ILs are still too expensive to be used for biomass pretreatment at the industrial scale; however, the possibility of recov‐ ering the extracted lignin opens up the possibility for producing high-value products in ad‐ dition to ethanol, which would favor the economics of a biorefinery based on IL biomass pretreatment. Indeed, modeling studies have shown that selling lignin can effectively lower the minimum selling price of ethanol to the point where lignin becomes the main revenue

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

http://dx.doi.org/10.5772/53378

69

There are many challenges to be addressed before ILs can be considered as a real option for biomass pretreatment, including their high cost and the consequent requirement for ionic liquid recovery and recycling, and the high IL loading required for most IL pre‐ treatment processes reported. It has been shown that the reduction in IL loading is more important than increasing the rate of IL recycling [141]. Aiming to tackle IL cost, two re‐ cent works have addressed this issue and were successful in reducing the IL require‐ ment, demonstrating that it is possible to increasing biomass loading up to 33% [95, 140]. Moreover, a continuous pretreatment process using ILs by applying a twin-screw extrud‐ er as a mixing reactor has been developed [95]. Many works have also reported the use of recycled ILs up to 10 cycles without significant loss in pretreatment efficiency [121, 132, 140]; nevertheless the development of energy-efficient recycling methods for ILs for large-scale applications is still an open issue. It is also noteworthy that most studies have performed enzymatic saccharification of IL-treated biomass at low-biomass loadings (<5%). Data on saccharification yields obtained on high-biomass consistency hydrolysis assays (>15%) are also needed to truly evaluate the effectiveness of IL pretreatment on enhancing the enzymatic hydrolysis rate. ILs toxicity to enzymes and fermentative micro‐ organisms must also be addressed as ILs trace residues may negatively affect the per‐ formance of enzymes [142] and inhibit fermentation [143]. Some research groups are now

Despite the current restrictions and the clear need for research and development to pave the way for the industrial use of ILs, ILs have great potential use within the engineering per‐ spective of a biorefinery due to their uncommon and specific chemical features and their se‐

A comparison of the effects of newly synthesized ILs has also been performed for hardwood (barked mixed willow) and softwood (pine sapwood). The ILs 1-butyl-3-methylimidazolium hydrogen sulfate [Bmim][HSO4] and 1-ethyl-3-methylimidazolium methyl sulfate [EMIM] [MeCO2] were mixed to 20% water and used for the pretreatment of both materials [120]. The pretreatment of the softwood sample with those ILs was ineffective as the maximum cellulose-to-glucose conversion achieved was 30%, while the pretreatments of hardwood samples with [Bmim][HSO4] and [EMIM][MeCO2] resulted in glucose yields of over 80% and 60%, respectively. Table 3 presents the experimental conditions for the pretreatment of woody biomass with ILs.


a Enzyme dosage per gram of cellulose; NI – Not informed

**Table 3.** Woody biomass IL pretreatment parameters and corresponding data for enzymatic saccharification and sugars yields

#### **6.1. Advantages and disadvantages of IL pretreatment**

hemicellulose fraction and loss of lignin ether linkages. The cellulose crystallinity was al‐ tered, prompting the suggestion that cellulose I was transformed, to some extent, to cellu‐ lose II. However, in contrast to an earlier report for the pretreatment of maple wood flour with [Emim][Ac] in which up to 80% of delignification was achieved [121], no significant de‐ lignification of *P. radiata* was observed. The glucose saccharification yields obtained for the maple wood flour pretreated at 130 °C for 90 min reached 95%. In contrast to the high yields obtained after wood pretreatment with [Emim][Ac], the use of [Emim][Cl] was shown to be ineffective, as only 30% of total sugars were obtained after saccharification of pretreated eu‐

68 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

A comparison of the effects of newly synthesized ILs has also been performed for hardwood (barked mixed willow) and softwood (pine sapwood). The ILs 1-butyl-3-methylimidazolium hydrogen sulfate [Bmim][HSO4] and 1-ethyl-3-methylimidazolium methyl sulfate [EMIM] [MeCO2] were mixed to 20% water and used for the pretreatment of both materials [120]. The pretreatment of the softwood sample with those ILs was ineffective as the maximum cellulose-to-glucose conversion achieved was 30%, while the pretreatments of hardwood samples with [Bmim][HSO4] and [EMIM][MeCO2] resulted in glucose yields of over 80% and 60%, respectively. Table 3 presents the experimental conditions for the pretreatment of

> **Solid loading (%)**

[Emim][Ac] *Pinus radiata* 120/150 30 5 20 1.5 93/81 [138]

[Emim][Ac] poplar 125 120 33 4.9 1.0 65 [140]

[Bmim][HSO4] Mixed willow 120 120 10 60 a 2 80 [120]

[EMIM][MeCO2] Mixed willow 120 120 10 60 a 2 60 [120]

**Table 3.** Woody biomass IL pretreatment parameters and corresponding data for enzymatic saccharification and

**Enzyme dosage (FPU/g)**

125 120 33 4.9 1.0 72 [140]

130 90 5 NI NI 95 [121]

150 60 5 180 5 30 [139]

150 30 5 180 5 40 [139]

120 120 10 60 a 2 30 [120]

120 120 10 60 a 2 25 [120]

**Substrate loading (%)**

**Glucose yield (%)**

**Reference**

calyptus at 150 °C for 60 min [139].

woody biomass with ILs.

[Emim][Ac] Maple wood

[Emim][Ac] Maple wood

[Emim][Cl] *Eucalyptus*

[Emim][Cl] *Nathofagus*

[Bmim][HSO4]

[EMIM][MeCO2]

sugars yields

a

**IL Biomass Temperature**

flour

flour

*globulus*

*pumilo*

Pine sapwood

Pine sapwood

Enzyme dosage per gram of cellulose; NI – Not informed

**(°C)**

**Time (min)** ILs are able to disrupt the plant cell wall structure by the solubilization of its main compo‐ nents. This class of salts is also able to alter cellulose crystallinity and structure, rendering the amorphous cellulose prone to high rates and yields from enzymatic saccharification. In‐ deed, this combination of effects generates a pretreated material that can be easily hydro‐ lyzed into monomeric sugars when compared to other pretreatment technologies, also rendering the enzymatic attack faster as the initial hydrolysis rate is greatly increased [116, 119]. In order to achieve high cellulose conversion yields (>80%) using other pretreatment processes, enzymatic saccharification times of 48–72 h are generally reported. However, in the case of IL pretreatment of bagasse and also some woody types, those yields can be ob‐ tained in less than 24 h with enzymatic hydrolysis. Nevertheless ILs are still too expensive to be used for biomass pretreatment at the industrial scale; however, the possibility of recov‐ ering the extracted lignin opens up the possibility for producing high-value products in ad‐ dition to ethanol, which would favor the economics of a biorefinery based on IL biomass pretreatment. Indeed, modeling studies have shown that selling lignin can effectively lower the minimum selling price of ethanol to the point where lignin becomes the main revenue source of an IL-based biorefinery [141].

There are many challenges to be addressed before ILs can be considered as a real option for biomass pretreatment, including their high cost and the consequent requirement for ionic liquid recovery and recycling, and the high IL loading required for most IL pre‐ treatment processes reported. It has been shown that the reduction in IL loading is more important than increasing the rate of IL recycling [141]. Aiming to tackle IL cost, two re‐ cent works have addressed this issue and were successful in reducing the IL require‐ ment, demonstrating that it is possible to increasing biomass loading up to 33% [95, 140]. Moreover, a continuous pretreatment process using ILs by applying a twin-screw extrud‐ er as a mixing reactor has been developed [95]. Many works have also reported the use of recycled ILs up to 10 cycles without significant loss in pretreatment efficiency [121, 132, 140]; nevertheless the development of energy-efficient recycling methods for ILs for large-scale applications is still an open issue. It is also noteworthy that most studies have performed enzymatic saccharification of IL-treated biomass at low-biomass loadings (<5%). Data on saccharification yields obtained on high-biomass consistency hydrolysis assays (>15%) are also needed to truly evaluate the effectiveness of IL pretreatment on enhancing the enzymatic hydrolysis rate. ILs toxicity to enzymes and fermentative micro‐ organisms must also be addressed as ILs trace residues may negatively affect the per‐ formance of enzymes [142] and inhibit fermentation [143]. Some research groups are now looking for new enzymes that are stable in ILs [144, 145].

Despite the current restrictions and the clear need for research and development to pave the way for the industrial use of ILs, ILs have great potential use within the engineering per‐ spective of a biorefinery due to their uncommon and specific chemical features and their se‐ lectivity toward biomass processing.
