Hydrolysis of cellulose to glucose is virtually an essential step in any practical cellulosic biofuel production via a biological route. However, for a long time, the heterogeneous acidic hydrolysis of cellulose to the production of glucose took the dominant position due to the limit of cellulose solvent. However, the traditional acid hydrolysis of lignocellulose was inefficient and cost-intensive. Considering the full dissolution of cellulose in ILs, it is rational to expect that the dissolution process could break internal and external supramolecular structures among the cellulosic fibers, which will facilitate the interaction between the cellulose and external catalysts and reactants, thus a new hydrolysis behavior of cellulose will be envisioned in ILs.

In 2007, Li et al. first reported the hydrolysis behavior of cellulose in ILs in the presence of mineral acids [58]. The results showed that catalytic amounts of mineral acid were sufficient to stimulate the hydrolysis reaction. For example, when the acid/cellulose mass ratio was set to 0.46, yields of total reducing sugar (TRS) and glucose were 64 and 36%, respectively, after 42 min at 100°C. In fact, excess acid loading in the ILs system was detrimental in terms of sugar yields because side reaction tended to occur which consumed the hydrolysis products. Preliminary kinetic study indicated that the cellulose hydrolysis catalyzed by H2SO4 followed a consecutive first-order reaction sequence, where k1 for TRS formation and k2 for TRS degradation were 0.073 min-1 and 0.007 min-1, respectively. Their further study on the hydrolysis behavior of lignocellulose in ILs demonstrated that hydrochloric acid was also an effective catalyst [59]. TRS yields were up to 66, 74, 81, and 68% for hydrolysis of corn stalk, rice straw, pine wood, and bagasse, respectively, in the presence of only 7 wt% catalyst at 100°C under an atmospheric pressure within 60 min. Under those conditions, the con­stants for k1 and k2 were 0.068 and 0.007 min-1, respectively, for the hydrolysis of corn stalk. Similar work was also done by Li et al. using different woody lignocellulosic materials, and it was found that the acidic pretreatment of woody biomass species (Eucalyptus grandis, Southern pine and Norway spruce) in [Amim] Cl resulted in the near-complete hydrolysis of cellulose, hemicellulose and a significant amount of lignin [60]. Acid-catalyzed conversion of loblolly pine wood was also investigated in [Bmim] Cl and almost identical results were achieved [61].

Toward a better understanding of the acidic hydrolysis behavior of cellulose in ILs, Vanoye et al. investigated the kinetics of the acid-catalyzed hydrolysis of cellobiose in the ILs 1-ethyl-3-methylimidazolium chloride ([Emim]Cl), which was usually studied as a model for general lignocellulosic biomass hydrolysis in ILs systems. The results showed that the rates of the two competitive reactions, polysaccharide hydrolysis, and sugar decomposition, varied with acid strength, and that for acids with an aqueous pKa below approximately zero. It was found that the hydrolysis reaction was significantly faster than the degradation of glucose, thus allowing hydrolysis to be performed with a high selectivity in glucose, which was consisted with the results obtained in Li’s work [62]. It was expected that the higher the degree of polymerization (DP) value of cellulose, the longer the reaction time will be required for a satisfactory glucose yield, while more TRS will be observed with a shorter reaction time in ILs, which implies that cellulose hydro­lysis in ILs catalyzed by mineral acids most likely follows a random hydrolysis mechanism, as observed with the concentrated-acid system [58]. It was proposed that both endoglycosidic and exoglycosidic scissions occur during the hydrolysis process, but the endoglycosidic product, oligoglucoses, is the major one at the initial stage, which was usually observed in traditional heterogeneous hydrolytic systems. Since then, a lot of mineral acids, organic acids, and solid acids have been applied for the homogeneous hydrolysis of cellulose and lignocellulosic materials in ionic liquids. The results have been summarized in Table 3.1 [63, 64].

Among all these significant contributions into the production of monosugars from biomass with the ILs platform, it is worthy of mention that, in 2010, Zhang et al. demonstrated that under relatively mild conditions (<140°C, 1 atm) and in the absence of acid catalysts, such as HCl, H2SO4, the dissolved cellulose in [Emim] Cl could be converted into reducing sugars in up to 97% yield. Their combined study of experimental methods and ab initio calculations demonstrated that the Kw value of water in the mixture was up to three orders of magnitude higher than that of the pure water under ambient conditions. Such high Kw values are typically achievable under high temperature or subcritical conditions, which is responsible for the remarkable performance in the absence of acid catalysts. They hypothesized that the increased [H+] was attributed to the enhanced water auto ionization by ionic liquids. This process will be affected by the electrostatic environment of the solution, the broad dielectric medium of the solvent, and the temperature. Comparative ab initio calculations based on the thermodynamic cycle shows that IL-water mixture exhibits higher concentrations of both [H+] and [OH-] than pure water, thus enabling the acid — and base-catalyzed reactions [70].

Under homogeneous conditions, the physical barriers of cellulose (such as crystallinity, morphology, surface area, and other physical features) are not pres­ent. But the recycling of the acidic catalysts is one of the main drawbacks of the conventional acid-catalyzed reaction processes. Separation processes represent more than half of the total investment in equipment for the chemical and fuel industries, while the introduction of heterogeneous catalysis made the catalyst separation easy after the reaction for industrial processes [72]. After the dissolu­tion of cellulose in ionic liquids, different solid acid catalysts have also been investigated for the hydrolysis of cellulose. In 2008, Rinaldi et al. reported that a solid acid (Amberlyst 15 DRY) catalyzed hydrolysis of cellulose and (ligno)cel — lulose in ILs [73, 74]. In these studies, depolymerized cellulose was precipitated and recovered by addition of water to the hydrolytic system, and the DP value was estimated by gel-permeation chromatography. It was found that the size of recovered cellulose fibers became successively smaller over time, resulting in a colloidal dispersion for the material recovered after 5 h. The depolymerization of cellulose proceeded progressively, resulting in the formation of soluble oligosac­charides if the reaction was carried out over a long time. For example, celloo — ligomers consisted of approximately 10 anhydroglucose units (AGU) which were seen after 5 h. The phenomena observed in these studies further supported the proposed hydrolytic pathway in ILs by Li et al. [58]. It was interesting to observe that there was an induction period for the production of glucose, and further titration results of the ILs separated from a suspension of Amberlyst 15DRY in [Bmim]Cl suggested that proton was progressively released into the bulk liquid

a Carbohydrates were hydrolyzed at 1.4—1.5 mol of HCl/g wood acid concentration b The reaction was carried out in aqueous solution c TRS selectivity d N. C not characterized

within an hour upon through an ion-exchange process involving [Bmim]+ of the ionic liquid and H+ species of the solid acid.

The design of solid catalysts, that are suitable for both heterogeneous and homogeneous conversion, is one of the most top challenges for biomass utilization [75]. It was found that the H+ species and reaction media are highly related to their catalytic activity toward the hydrolysis of cellulose. For example, Shimizu et al. developed H3PW12O40 and Sn075PW12O40 for the hydrolysis of lignocellulose, which showed higher TRS yield than conventional H2SO4 in water [66]. Other solid acids, such as Nafion® NR50, sulfonated silica/carbon nanocomposites, have also been studied for the hydrolysis of cellulose in ILs. It was found that the crystalline cellulose was partially loosened and transformed to cellulose II from cellulose I, then to glucose assisted by Nafion® NR50. Afterwards, a catalyst was recycled and the residual (hemi) cellulose solid, which could be hydrolyzed into monosugars by enzymes, was separated by adding antisolvents [67]. Due to the presence of strong, accessible Brpnsted acid sites and the hybrid surface structure of sulfonated silica/carbon nanocomposites, it was found that a 42.5% glucose yield was achieved after three recycles of this catalyst in ILs [69].

Solid acid-catalyzed hydrolysis of cellulose in ILs was greatly promoted by microwave heating. The results showed that H-form zeolites with a lower Si/Al molar ratio and a larger surface area exhibited better performance than that of the sulfated ion-exchanging resin NKC-9. The introduction of microwave irradiation at an appropriate power significantly reduced the reaction time and increased the yields of reducing sugars. A typical hydrolysis reaction with Avicel cellulose produced glucose in around 37% yield within 8 min [68].

Monosugars are intermediates linking the sustainable biomass and clean ener­gies, such as bioethanol and microbial biodiesel. In 2010, Binder et al. first investigated the fermentation potential of sugars produced from cellulose in ILs after separation of ILs by ion-exclusion chromatography. The results showed that adding water gradually to a chloride ionic liquid-containing catalytic HCl led to a nearly 90% yield of glucose from cellulose and 70-80% yield of sugars from untreated corn stover. Ion-exclusion chromatography allowed the recovery of the ILs and delivered sugar feedstocks that support the vigorous growth of ethanol — ogenic microbes. This simple chemical process presents a full pathway from biomass to bio-energy based on the ionic liquids platform, although the devel­opment of more economic technologies for the recovery and separation of the ILs and sugars is still in high demand [65].

Recent work has demonstrated that the recovery of sugars from ILs could be fulfilled by extraction based on the chemical affinity of sugars to boronates such as phenyl boronic acid and naphthalene-2-boronic acid [71]. 90% of mono — and di-saccharides could be extracted up by boronate complexes from aqueous ILs solutions, pure ILs systems, or hydrolysates of corn stove-containing ILs.