Solving problems: An isopropyl alcohol‐mediated reaction system for the production of 5‐hydroxymethylfurfural (HMF) from fructose reaches a yield of up to 87 %. The solvent can be easily recycled by evaporation, giving the HMF product. The system avoids the use of large amounts of organic solvent, has a minimal environmental impact, and offers a new route to large‐scale economically viable processes.

Biomass-derived carbohydrates are promising as both carbonbased alternative energy source and sustainable feedstock for the chemical industry. [1,2] Recently, many efforts have been devoted towards converting biomass into 5-hydroxymethylfurfural (HMF), a versatile and key intermediate in biofuel chemistry and the petrochemical industry. [3,4] Acidic catalysts have been employed for the production of HMF from d-fructose in water, aprotic solvents (e.g., DMSO), and biphasic systems. [5][6][7] However, at elevated temperatures the yields of the target product have been low owing to side reactions, such as HMF decomposition and oligomerization. [3,8] Imidazolium-based ionic liquids have been applied for this conversion in a number of recent reports, with acids or metal salts as catalysts. [9][10][11][12] Throughout these studies, it was generally accepted that ionic liquids may stabilize HMF in the reaction mixture and increase the reaction selectivity; [10] however, the detailed role of the imidazolium ionic liquid in the sugar dehydration process remained to be studied. Current protocols conduct the sugar dehydration reaction at relatively high temperatures (80-120 8C), [9][10][11][12] which not only results in significant energy consumption but may also have other negative effects, such as more side reactions and a shorter catalyst lifetime. Fructose dehydration was recently demonstrated at 50 8C and at room temperature in a 1-butyl-3-methylimidazolium chloride (BMIMCl) solvent system, using WCl 6[13] and solid acidic resin [14] catalysts, respectively. In an effort to find more cost-effective methods for the large-scale synthesis of HMF, we report a recyclable HCl-BMIMCl catalytic system for converting fructose into HMF at room temperature. DFT calculations were carried out in order to better understand the role of the imidazolium ionic liquid in this process. A study of a metal chloride-BMIMCl system (i.e., TiCl 4 -IL) has shown that the metal cation may in fact not be the actual catalyst during the reaction (Scheme 1).[13] It is known that early transition-metal chloride salts (such as TiCl 4 ) easily react with alcohols (e.g. fructose) to form metal alkoxides and HCl. If the metal cation is the active agent in the catalytic cycle, metal alkoxides should also be able to catalyze the fructose dehydration reaction; however, this is not the case. Metal alkoxides were found to be inactive towards fructose dehydration (Table 1, entries 2 and 4). We therefore reasoned that the real catalyst must be the acid (HCl), with metal chloride salts serving as source. To confirm this hypothesis, the same amounts of HCl (compared to those generated from metal chlorides) were used as catalyst for fructose dehydration. As the results show, HCl indeed promoted the reaction and gave even better results than metal chloride systems at low temperatures.[14] Encouraged by this result, the room-temperature performance of the HCl-BMIMCl system for fructose dehydration was further studied. With solid acid catalyst systems, BMIMCl neede...