The Effect of Imidazolium Ionic Liquid on the Dehydration of Fructose to 5‐Hydroxymethylfurfural, and a Room Temperature Catalytic System

Abstract

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...

Synthesis of 5‐(Hydroxymethyl)furfural in Ionic Liquids: Paving the Way to Renewable Chemicals

“…[54] The yield was enhanced further when using continuous extraction with THF. In a very recent publication, Lai and Zhang [47] argued that the catalytic effect was present even in the absence of metals. Adding concentrated aqueous HCl to a mixture of [BMIm]Cl and fructose afforded a low-viscosity liquid at ambient temperature which, after 24 h, resulted in an HMF yield of 72 %.…”

“…[44,45] In addition, there have been examples in which addition of a Brønsted acidic to a non-acidic IL markedly promoted the reaction rate of fructose dehydration. [46][47][48] The use of polyimidazolium salts (PIMS) that were acidified with HCl as catalysts gave yields of 79-84 % of HMF from fructose in [BMIm]Cl at 80-100 8C and could be reused 11 times without any detected deactivation. [49] Even though the dehydration of fructose to HMF has been dominated by imidazolium-based ILs, there are some noteworthy exceptions.…”

Conversion of fructose into 5-HMF catalyzed by GeCl4 in DMSO and [Bmim]Cl system at room temperature

“…It was clearly proved that [Bmim]Cl provided a superior reaction environment than organic solvents for the dehydration of hexoses. It was reported that imidazolium molecule may take the role of activating the reactants and stabilizing the products of each step through the hydrogen bonding interaction (Lai & Zhang, 2010). It is reported that the C-2 hydrogen of imidazolium ion exhibits acidic character, and it can catalyzed the dehydration of fructose at high temperature (Zhao et al, 2007).…”

5‐Hydroxymethylfurfural Based Polymers

“…Ionic liquid -ethyl acetate bi phase system was introduced by Hu [24] for the easy separation of the HMF. By using DFT calculations Lai [25] demonstrated that ionic liquid solvent "switches" the dehydration from thermodynamically unfavorable into a thermodynamically favorable reaction.…”

“…14 Synthesis of poly(ethylene-2,5-furandicarboxylate) (PEF) (26) using frans-esterification polymerization of diester diol(25).…”