The influence of water content on the production of 5-HMF from biomass has many aspects. Firstly, in the dehydration of fructose in ionic liquids, a small amount of water (5 wt%) present in the ionic liquid mixture has a negligible effect on the fructose conversion and 5-HMF yield, and it reduces the viscosity of the ionic liquid system and is beneficial to mass transfer [20, 36]. However, when the water content increases above about 5 wt%, the fructose and 5-HMF yield decrease significantly, which can be ascribed to the loss of catalytic activity due to the lowering of the dielectric constant of the reaction media by the addition of water [41]. Further, the presence of water in the reaction mixture favors the rehydration of 5-HMF to generate undesired products such as levulinic and formic acids. From one point of view, water should be completely avoided and removed from the production process of 5-HMF. On the other hand, water serves as a necessary reactant in the hydrolysis step in the conversion of oligosaccharides, thus an appropriate amount of water is required in the reaction system, although water has all the disadvantages connected to the dehydration reaction, and polysaccharides are no longer soluble in ionic liquids and precipitate from the ionic liquid solution when the water content exceeds a certain level [94, 95]. Therefore, water content should be carefully controlled in the conversion of polysaccharides in ionic liquids.

Considering that the addition of water can reduce the catalytic activity of the reaction system but increase the stability of glucose, Qi et al. developed an efficient two-step process for converting microcrystalline cellulose into 5-HMF with ionic liquids under mild conditions [95]. In the first step, cellulose was efficiently hydrolyzed by a strong acidic cation exchange resin in 1-ethyl-3-methyl imidazolium chloride ([EMIM][Cl]) with gradual addition of water. The addition of water probably allowed a balance to be achieved between glucose stability and cellulose solubility, since too much water will result in the precipitation of the cellulose substrate. Through the water addition technique, Qi et al. reported glucose yields above 80 % for converting microcrystalline cellulose under mild conditions [95]. Based on the high glucose yield, a second step was applied to produce 5-hydroxymethylfurfural by separating the resin from the reaction mixture and adding CrCl3 as catalyst, which lead to a 5-HMF yield of 73 % based on cellulose substrate. The strategy described should be useful for efficient conversion of cellulose into 5-HMF as well as into other biomass-derived chemicals.