5- Hydroxymethylfurfural (HMF) can be produced from poly — and monohexoses and is a valuable platform chemical that can be used to make polymers, fuels, and commodity chemicals (Fig. 8.7). HMF is the product of the dehydration of

6- carbon sugars such as fructose, glucose, and mannose. Because ketoses are furanoses when in their cyclic form, they are much easier to convert into HMF than aldoses. Polymers of six carbon sugars can also be used to produce HMF. The following procedure is generally used in the conventional production of HMF:

(1) hydrolyze polyhexoses into monomers, (2) isomerize aldoses into ketoses, and

(3) use an acid catalyst to dehydrate ketoses into HMF [100]. Once HMF is produced, it can be processed into fuels, resins, solvents, alkanes, fuel additives, or polymers as a replacement for petroleum resources. The utility of HMF as a renewable biomass based platform chemical has led to significant research in the production of HMF in ILs, although the technology has still not matured into an industrial process [101, 102].

Acid catalyzed dehydration of a ketose, such as fructose, is the easiest method for production of HMF. This method has been implemented successfully using ILs as solvents. Lansalot-Matras and Moreau demonstrated up to 80 % yield of HMF from fructose in the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) with added DMSO to solubilize fructose. The advantage of the IL was demonstrated in a reaction without an added catalyst. An HMF yield of 36 % was obtained after 32 h in a DMSO/IL solution while only trace amounts of HMF were detected in pure DMSO after 44 h [103]. Later, the same research group demonstrated the use of the acidic IL 1-H-3-methylimidazolium chloride as a combination solvent and catalyst by producing 92 % yield of HMF from fructose and nearly quantitative amounts of HMF and glucose from sucrose. It was also noted in this article that there was no observed degradation of the HMF after the conclusion of the reaction [104]. Qi and coworkers tested sulfuric acid, HCl, phosphoric acid, acetic acid,

CuCl2, PdCl2, Dowex resin, and Amberlyst 15 as catalysts for production of HMF from fructose in BMIMCl. The Amberlyst catalyst was the best of the catalysts, producing an 83.3 % yield of HMF after only 10 min [105]. Others have taken the idea of acid catalyzed dehydration of fructose in ILs and worked to make it more environmentally friendly by using ILs made from renewable materials. Hu et al. tested a number of ILs and found that choline chloride coupled with citric acid was the most effective system for the fructose to HMF conversion, achieving over 90 % yield [106]. Acid catalyzed dehydration was tested on a number of different 6-carbon sugars by Sievers et al. through the use of added sulfuric acid in BMIMCl. Since glucose and mannose can both be isomerized into fructose, both sugars should be viable feed stocks for HMF production. As has been demonstrated previously, fructose gave high yields of HMF (>90%). Glucose only produced up to 12 % HMF yield while only very small amounts of HMF were detected when mannose was used as a substrate. Additionally, xylose, a 5-carbon sugar, was shown to undergo an analogous reaction to form furfural with up to a 13 % yield [107].

Recently, it was discovered that some metal chlorides can catalyze the conversion of aldoses such as glucose and mannose into HMF in ILs. Zhao et al. demonstrated the conversion of glucose to HMF using CrCl2 in 1-ethyl-3-methylimidazolium chloride with a yield of almost 70 %. Both CrCl2 and CrCl3 were found to be effective in this system, although CrCl2 demonstrated the highest catalytic activity [108]. This discovery has led to many studies investigating metal chloride promoted production of HMF in ILs. Pidko et al. performed work using a combination of experimental techniques and computational modeling to show that CrCl2 and CrCl3 effect the dehydration of glucose into HMF through ring opening and hydrogen shift catalyzed by CrCir ions in solution [109, 110]. Binder et al. worked to elucidate the mechanism further through the use of glucose, mannose, galactose, lactose, tagatose, psicose and sorbose along with isotopic labeling. In this study, it was demonstrated that the chromium catalyst causes a 1,2-hydride shift which leads to a furanose that can be dehydrated [111]. In these studies, the efficacy of the CrCl2/IL system could be enhanced through the use of microwave irradiation and by supporting the CrCl3. The addition of microwave irradiation instead of simple heating in an oil bath allowed HMF to be recovered with up to a 40.2 % yield after only 2.5 min while convention­ally heated reactions obtained 32.5 % yield after 32 min [112]. In addition to CrCl2 and CrCl3, it has been shown that SnCl4 can catalyze the dehydration of sugars, inulin, and starch into HMF in EMIMBF4. In this system, and HMF yield of between 40 and 65 % could be obtained, depending on the substrate [113].

While production of HMF from monosaccharides is a useful technology, by producing HMF directly from lignocellulose, the saccharification step of biomass processing would be removed. Su et al. demonstrated the use of coupled metal chlorides to produce HMF from cellulose in a single step. By using CuCl2 and CrCl2 in EMIMCl, cellulose could be converted to HMF with a yield of 55.4 % that stayed constant over several recycles of the IL/catalyst system. Additionally, the metal chlorides were shown to work in a synergistic manner, with almost no HMF production with either metal chloride on its own [114]. Binder and Raines demon­strated a similar system in which CrCl2 or CrCl3 in a solution of N, N-dimethylformamide (DMA) with LiCl and the IL EMIMCl. With this system, fructose, glucose, cellulose and lignocellulose from corn stover and pine wood could all be converted into HMF. The yields were dependent on conditions and substrate, although even the lignocellulose produced up to a 48 % yield [115]. In a study by Zhang et al. that was previously mentioned in the section on cellulose hydrolysis, the EMIMCl/water mixtures that could be used to hydrolyze cellulose were also demonstrated to be effective at HMF production when a CrCl2 catalyst was added [94]. By incorporating the use of microwave irradiation with the CrCl3/ IL system, glucose and cellulose could be converted to HMF with 90 and 60 % yields, respectively [116]. Expanding the use of microwave treatments, lignocellu — lose from corn stover, rice straw and pine wood could be converted to HMF and furfural with yields of 45-52 and 23-31 %, respectively, with CrCl3 in BMIMCl in 3 min or less [117].