Inedible lignocellulosic biomass is a prime candidate as a starting material for 5-HMF production because it presumably would not compete directly with food sources. The development of efficient routes for converting lignocellulose biomass into 5-HMF is essential for achieving sustainable production of 5-HMF. Many research works focus on the transformation of cellulose, since it is the constituent of biomass that can be used to make 5-HMF. However, cellulose is insoluble in many conventional solvents [54—56]. The main advantage of using ionic liquids as reaction media for biomass conversion is the possibility of ionic liquids to dissolve carbohydrate polymers and subsequently form products in one-pot reactions. The conversion of cellulose to 5-HMF can be thought to involve three chemical processes: hydrolysis, isomerisation and dehydration. Although hydrolysis of cel­lulose in ionic liquids in the presence of mineral acids has been studied in detail [57, 58], the efficient conversion of cellulose into 5-HMF with high yield has not been realized until CrCl2 was found to be active for dehydration of glucose into 5-HMF [41]. Su et al. [59] presented a single-step process for cellulose conversion into 5-HMF by using an ionic liquid solvent system with a pair metal chlorides (CuCl2-CrCl2) catalyst, and obtained a 5-HMF yield of 55 % under relatively mild conditions of 120 °C in 8 h reaction time. After the product 5-HMF was separated from the solvent, the catalytic performance of recovered [EMIM] [Cl] and the catalysts were used in repeated experiments. Under these conditions, cellulose depolymerizes at a rate that is about one order of magnitude higher than when using a homogeneous acid catalysis. Single metal chlorides at the same total loading showed considerably less activity under similar conditions [59]. Binder et al. [44] studied the conversion of cellulose, corn stover, and pine dust in DMA-LiCl — EMIM][Cl] mixture using chromium(II) or chromium(III) chlorides as catalysts and hydrochloric acid as a co-catalyst. The mixed system converted cellulose into 5-HMF with a yield of 54 %. When [EMIM][Cl] was used as solvent and CrCl2-HCl as the catalytic system, results were about the same. These examples show the presence of a catalyst is essential for the reaction system and a 5-HMF yield of only 4 % could be obtained in the absence of the catalyst. For lignocellu — losic feedstocks, 5-HMF yields varied from 16 to 47 %. A 5-HMF yield of 16 % was obtained for corn stover substrate regardless of whether it was subjected to pretreatment of ammonia fiber expansion. Lignin and proteins did not seem to affect the results.

Li et al. [47] studied the conversion of cellulose and raw lignocellulosic biomass with microwave irradiation using CrCl3 as a catalyst in [BMIM][Cl] solvent. Initially the investigation was conducted on transformation of cellulose to 5-HMF using cellulose of various degrees of polymerization (Avicel, Spruce, Sigamcell, R-cellulose). The 5-HMF yields varied from 53 to 62 % for all substrates. The authors proposed that the coordination of cellulose with [CrCl3 + n]n- species is responsible for the partial weakening of the 1,4-glucosidic bonds, thus making cellulose more susceptible to attack by water in the hydrolysis step. The authors observed that the use of microwave heating has significant effect in reducing the reaction time, this was also observed by Qi et al., who examined the conversion of cellulose into 5-HMF in [BMIM][Cl] under microwave irradiation, and a 5-HMF yield of 54 % in 10 min reaction time at 150 °C was obtained [19].

A reaction system that could convert microcrystalline cellulose into 5-HMF under mild conditions was reported by Zhang et al. [60] They designed a green process for the conversion of cellulose into water-soluble reducing carbohydrates with a total yield as high as 97 % in the absence of added acid catalysts with the main point of originality of the process being added water to the ionic liquid — cellulose system. The formed carbohydrates could be transformed into 5-HMF in 89 % yield when CrCl2 was added. Such a high 5-HMF yield of nearly 90 % means that not only glucose, but also other water-soluble reducing sugars were converted to product. The total reducing sugars and 5-HMF yields depend not only on reaction temperature and time but also on the amount of added water. By using ab initio calculations, the authors demonstrated that the favorable results obtained in a catalyst system was probably due to the dissociation constant (Kw) of water that increased with the addition of purified ILs. The increased Kw of water by ILs makes the IL-water mixture exhibits higher concentrations of both [H+] and [OH-] than pure water, thus enabling acid — or base-catalyzed reactions to occur. For example, water in the presence of 15 mol% of the [EMIM][Cl] at 120 °C exhibits Kw values up to three orders of magnitude higher than those of pure water under ambient conditions [60]. This intrinsic property of the IL-water mixtures can not only be used in biomass processing and conversion, but also in organic catalysis, electro­chemistry, or other relative research fields.

Due to the inherent environmental risk of chromium, finding of chromium-free processes for conversion of cellulose into 5-HMF is of great significance for biomass utilization. The transformation of cellulose into 5-HMF involves three chemical processes, that is, hydrolysis of cellulose to glucose catalyzed by acid catalyst, isomerization of glucose into fructose by base catalyst, and dehydration of fructose into 5-HMF [22, 24]. Peng and co-workers synthesized acid-base bifunctionalized mesoporous silica nanoparticles with large pore sizes of around 30 nm (LPMSNs) for cooperative catalysis of one-pot cellulose to 5-HMF conver­sion [61]. They used a grafting method to functionalize LPMSN with an acid (SO3H) or/and a base (NH2) group in order to prepare SO3H, NH2, and both SO3H and NH2 functionalized LPMSN (denoted as LPMSN-SO3H, LPMSN — NH2, and LPMSN-both, respectively). Results show that LPMSN-SO3H and LPMSN-NH2 could increase the yield of 5-HMF converted from the reactions that need acid and base catalysts, respectively. The LPMSN-SO3H catalysts were found to promote for one-pot conversion of cellulose to 5-HMF and gave enhanced 5-HMF yields.