Water is the mostly used medium for cellulose hydrolysis. It was also shown that catalyst activity and selectivity are closely determined by the amount of water. When the amount of water is close to the weight of solid catalyst, a maximum yield of glucose is obtained. However, a lower amount of water and longer reaction time (e. g., 24 h) mainly drive the reaction towards the formation of water-soluble |5-1,4-glucans. ILs are efficient for the pretreatment and hydrolysis of lignocellulosic materials, and can dissolve biomass and overcome many of the physical and biochemical barriers for hydrolysis at ambient conditions. [BMIM][Cl] and 3-allyl-1-methylimidazolium chloride {[AMIM][Cl]} can dissolve 10 wt% cellulose at 50-100 °C [85]. Wang et al. [86] conducted a study on the extraction of cellulose from wood chips with [AMIM][Cl] and found that 62 wt% cellulose dissolved in [AMIM][Cl] under mild conditions. So far, water, organic solvents, ILs, and their mixtures are widely used as reaction media.

Zhang et al. [87] demonstrated that high yield of total-reducing-sugars (TRS) (60 %) was achieved when depolymerization of chitosan was performed in ILs in the presence of mineral acids. This might be due to the fact that the reaction medium 1-butyl-3-methylimidazolium bromine {[BMIM][Br]} reinforced the acidity of the mineral acids. A physical barrier for hydrolysis disappeared through the formation of a solution with [BMIM][Br]. With good solubility to dissolve cellulose, IL is a good medium for directly catalytic hydrolysis of cellulose with high efficiency. ILs can promote the dissolution and dispersion of cellulose molecules, leading to the complete mixture between cellulose and acidic sites in a homogeneous phase. Relatively, small cations are often efficient in dissolving cellulose. In another work [88], when hydrolysis reaction was carried out in [BMIM][Cl] in the presence of 7 wt% HCl at 100 °C under atmospheric pressure for 60 min, TRS yield was 66 %, 74 %, 81 %, and 68 % for the hydrolysis of corn stalk, rice straw, pine wood, and bagasse, respectively. The high TRS yield is due to the dissociated Cl-1/Br-1 and the electron-rich aromatic system of [BMIM+] weakening the glycosidic linkage to facilitate hydrolysis.

Water addition was found to have a significant impact on the degree of cellulose hydrolysis, because water acts both as a reactant (for producing monosaccharides) and an inhibitor [for producing 5-hydroxymethylfurfural (5-HMF)] in the overall cellulosic conversion. As mentioned above, Qi et al. [75] developed an effective con­version technique for transforming cellulose into 5-HMF via a two-step process. In the first step, high glucose yield (83 %) was obtained from the cellulose hydrolysis by a strong acidic cation-exchange resin in [EMIM][Cl] with gradual addition of water. In their study, cellulose firstly dissolved in [EMIM][Cl]. Then a certain amount of water and the cation-exchange resin was added. Hydrolysis reaction started when the system was heated to 110 °C. Compared with one-time addition of water, glu­cose yield was improved by adding water to [EMIM][Cl] system during reaction. In the hydrolysis step, increasing the amount of water could increase the yield of monosaccharides, and the maximum yield of 25 % (monosaccharides + 5-HMF) was obtained when H2O/cellulose molar ratio was 10 [89]. However, excessive water addition caused cellulose to precipitate from IL.