Ionic Liquid-Mediated Formation of 5-Hydroxymethylfurfural—A

Chem. Rev. 2011, 111, 397–417

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Ionic Liquid-Mediated Formation of 5-HydroxymethylfurfuralsA Promising Biomass-Derived Building Block Małgorzata E. Zakrzewska,† Ewa Bogel-Łukasik,† and Rafał Bogel-Łukasik*,†,‡ REQUIMTE, Departamento de Quı´mica, Faculdade de Cieˆncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal, and Laborato´rio Nacional de Energia e Geologia, I.P., Unit of Bioenergy, Estrada do Pac¸o do Lumiar 22, 1649-038 Lisboa, Portugal Received June 2, 2010

Contents 1. 2. 3. 4.

Introduction Dehydration of Carbohydrates Dehydration of Monosaccharides to 5-HMF in ILs Dehydration of Oligo- and Polysaccharides to 5-HMF in ILs 5. Influence of Reaction Conditions on Dehydration Efficiency 5.1. Temperature 5.2. Reaction Time 5.3. Catalyst Loading 5.4. Carbohydrate Loading 5.5. Water Content 5.6. Extraction of a Final Product 5.7. Recycling and Reuse of Chemicals 6. Perspectives 7. Conclusions 8. Acknowledgments 9. References

397 399 401 409 413 413 413 413 413 414 414 414 415 416 416 416

1. Introduction The reduction of fossil fuels dependence in a framework of shifts in oil prices and geopolitical instability1 is one of the major interests of the current world. It can be achieved by using lignocellulosic biomass. However, there is also growing concern about its overall sustainability, especially regarding land use change, intensified use of agricultural inputs, and possible limitations on food security. Furthermore, the global energy demand is projected to grow over 50% by 2030. This will have an additional impact on the climate and, hence, on our planet. The recent United Nations Framework Convention on Climate Change in Copenhagen, Denmark, has ratified the Kyoto Protocol and is intended to reduce global emissions by at least 20% by 2020 and by 50%-60% by 2050 relative to the emission level in 2006.2 To achieve these ambitious goals in the near future, the next generation of chemicals and fuels from the biorefinery of lignocellulosic biomass has to be used sustainably, since the competition for raw materials between the food and energy industries prevents further (significant) increase of the current first-generation biofuels already on the market. Biomass, especially that which exists in the form of nonedible lignocellulosic materials such as grasses, woods * Fax: +351217163636. Telephone: +351210924600ext 4224. E-mail: [email protected]. † Universidade Nova de Lisboa. ‡ Laborato´rio Nacional de Energia e Geologia.

Małgorzata Ewa Zakrzewska received her two M.Sc. B.Sc. degrees in Environmental Protection Technology and in Biotechnology from the Gdan´sk University of Technology, Poland. Currently, at REQUIMTE, Universidade Nova de Lisboa, she has been gaining experience in highpressure work under the supervision of Doctor Rafał Bogel-Łukasik and Professor Manuel Nunes da Ponte. Her research is focused on the application of supercritical CO2 in reaction and extraction.

(hard and soft), and crop residues (corn stover, wheat straw, sugar cane, bagasse, etc.), serves as renewable feedstock and could be considered as an alternative source of the chemicals and energy currently derived from petroleum. There are a number of technological breakthroughs necessary to reach a mature and cost-effective commercial technology for biomass utilization. Cost reductions in biological and chemical conversion are to be found in the improvement of individual process steps, far-reaching integration, the development of new efficient methods of carbohydrate conversions by alternative solvents or by robust microbial cell fermentation and by integration of all residues (e.g., spent lignins) and wastewaters into a one-pot process. Lignocellulosic biomass is composed of cellulose, hemicellulose, and lignin. The compositions of these materials vary, and their structures are very complex. Biomass requires many hydrolytic technologies and biological as well as chemical pretreatments to be reduced in size and have its physical structure opened.3 Various methods such as acid hydrolysis, hydrothermal or alkaline treatments, organosolv, solid (super)acids, ionic liquids, or subcritical or supercritical fluids can be employed.4 Carbohydrates constitute up to 75% of the annual production of biomass, estimated at 170 × 109 tons.5 Carbohydrates are an abundant, diverse, and reusable source of carbon. They find many industrial applications in such diverse areas as the chemistry, fermentation, petroleum production, food, paper, and pharmaceutical industries.6 Unfortunately, the

10.1021/cr100171a  2011 American Chemical Society Published on Web 10/25/2010

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Ewa Bogel-Łukasik earned a M.Sc. B.Sc. in Chemical Technology from Warsaw University of Technology. She has obtained a doctoral award for her Ph.D. thesis on the synthesis and physicochemical properties of ionic liquids. After completing her Ph.D. dissertation in 2004 under the supervision of Professor Urszula Doman´ska, she joined Institute of Experimental Biology and Technology, Portugal (2004-2006), where she worked as a Marie Curie Experienced Researcher in the SUPERGREENCHEM Network and a postdoctoral researcher in The Interreg III-B Network Supermat. She has worked with Professor Kenneth Seddon and Doctor Martyn Earle in QUILL Laboratories, U.K., as a Marie Curie Training Site Researcher and with Professor Gerd Brunner in Technische Universita¨t Hamburg-Harburg, Laboratory of Thermal and Separation Processes, Germany, as a Marie Curie Experienced Researcher. Currently, she is a researcher of REQUIMTE. In 1995 she was awarded the Contest organized by American Peace Corps. She has worked in PKN Orlen—Polish Oil Concern. Her current research interest is aimed at the application of supercritical CO2 and ionic liquids in reactions (in particular hydrogenation, oxidation, and telomerization). She has published over 20 papers and 1 patent.

majority of them are stored in the form of cellulose or hemicelluloses and require technologically advanced pretreatments, as was already pointed out above. These are the main hurdles that significantly hinder their reformation to industrial applications. Cheaper and less energy-intensive biorefinery processes must be developed to make them competitive with petroleum-based resources. Traditional petrochemical industry feedstocks are based on seven hydrocarbon-based platform chemicals (toluene, benzene, xylene, 1,3-butadiene, propylene, ethene, methane).7 In a short period of time, a similar approach wherein a small number of carbohydrate or oil- or lignin-based chemicals would be used as simple intermediates in traditional chemical processing could be applied to chemicals derived from biomass.8 Several reports have identified a number of building block chemicals produced from biomass primary products (proteins, oils, lignin, hemicellulose, cellulose, starch), with 5-hydroxymethylfurfural (5-HMF) being one such example, as shown in Figure 1. According to the Top Value Added Chemicals from Biomass report by the U.S. Department of Energy, 5-HMF is one of the top building block chemicals obtained from biomass.8,9 It can be used to synthesize a broad range of value added compounds currently derived from petroleum.10 5-HMF has been called “a sleeping giant”,11 “petrochemical readily accessible from regrowing resources”,12 and “a key substance between carbohydrate chemistry and mineral oil-based industrial organic chemistry”.13 A five-membered ring compound is an interesting raw material due to its high reactivity and multifunctionality. It is a complex primary aromatic alcohol, an aldehyde, and a furan ring system. It has already been used in the production of resins,14 but it has much

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Rafał Bogel-Łukasik graduated from Warsaw University of Technology, Poland, with a M.Sc. B.Sc. (2002). He received his Ph.D. (2007) in Chemical Engineering under the supervision of Professors Susana Barreiros and Manuel Nunes da Ponte from New University of Lisbon, Portugal. Since 2009 he has held a position of an Assistant Researcher of the Unit of Bioenergy in National Laboratory for Energy and Geology (LNEG, I. P.), Portugal. He is an Invited Researcher of REQUIMTE, The Associated Laboratory for Green Chemistry, Clean Technologies and Processes. He has received a Socrates Fellowship in University of Rostock in Germany (2002) and a Marie Curie Fellowship in The Queen’s University Ionic Liquid Laboratories Research Centre, U.K. (2003), and in the Institute of Experimental Biology and Technology, Portugal (2005-2007), in the frame of SUPERGREENCHEM Marie Curie Research Training Network. In 2008 he was awarded The International Association of Chemical Thermodynamics Junior Award for Excellence in Thermodynamics. His scientific interest is focused on the application of ionic liquids and supercritical fluids in green and sustainable chemistry, bioenergy, and biomass refinery research areas. He has published more than 25 scientific publications and 1 patent application.

greater potential as an intermediate for other 2,5-disubstituted furan derivatives. Such derivatives can be subsequently used in the production of fine chemicals, pharmaceuticals, polymers (polyesters, polyamides, or polyurethanes), solvents, or liquid transportation fuels.15-19 Employing various types of reactions, 5-HMF can be converted into many versatile compounds (Figure 2). Several comprehensive reviews describing the chemistry and applications of 5-HMF and its derivatives have been presented.7,20-23 Through oxidation, 5-HMF can be transformed to 5-hydroxymethyl furoic acid (HFCA), 2,5-furandicarboxaldehyde (FDC), or 2,5-furandicarboxylic acid (FDCA). Hydrogenation can yield hydroxymethyl tetrahydrofurfural (HMTHFA), 2,5-dihydroxymethyl furan, or 2,5-dihydroxymethyl tetrahydrofuran. The aldol condensation leads to the production of C7-C15 liquid alkanes, while rehydration gives levulinic (LEVA) and formic (FA) acids. Despite the versatile application profile of 5-HMF-derived intermediate chemicals, 5-HMF is not yet produced on an industrial scale, mainly because of the high production costs.20,23,24 The formation of 5-HMF occurs by triple acid catalyzed dehydration of hexoses or their precursors (Figure 3). Up until now, different feedstocks have been used to produce 5-HMF. Examples include mono- (fructose25-46or glucose25-36,47-50), di- (maltose,30 sucrose,25,30,36,42,51,52 or cellobiose25,30,53), or more complex, high-molecular-weight poly- (inulin,25,29,30,36,54 starch,25,30 or cellulose28,30,34,47,53,55-57) saccharides or raw lignocellulosic biomass.28,53,56,58,59 The strategy of direct use of the lignocellulosic biomass for the large-scale production of 5-HMF and its derivatives would be ideal. It could remove a major barrier for the development

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Figure 1. Proposed biomass-derived platform chemicals. The italic font emphasizes the group of biomass primary products which are the main source of biomass-derived platform chemicals.

of a sustainable 5-HMF platform. Conversely, current methods presented in the literature are focused on monosaccharides, which allows the investigation and understanding of process mechanisms. Fructose was chosen from among the ketohexoses for this investigation, which showed the process to be highly selective and efficient. Employment of aldohexoses such as glucose and mannose24,27,60 resulted in low yields of 5-HMF, which can be attributed to their stable ring structures. This stability leads to a lower degree of the enolization that determines the rate of 5-HMF formation. Nevertheless, the utilization of cheap and abundantly available glucose to generate valuable compounds would be very beneficial. Therefore, a strong incentive exists for the development of processes that can efficiently convert cellulosic materials into simple monosaccharides such as glucose. This review summarizes current achievements in the synthesis of 5-HMF using ionic liquids as published in scientific periodicals up to the middle of July 2010. Moreover, it points out obstacles described in the studies presented and discusses perspectives in the application of ionic liquids to the conversion of biomass-derived carbohydrates.

2. Dehydration of Carbohydrates The dehydration of carbohydrates has been studied in many catalytic systems and reaction media. A variety of catalysts such as mineral and organic acids,11,24,36,46,74,78,85 salts,31,50,64,70,76 or solid acid catalysts such as ion-exchange resins36,43,44,46,66 or zeolites75 have been investigated. Dehydration was demonstrated in many solvents including water,61-65 organic solvents (dimethylsulfoxide (DMSO),66-68 dimethylformamide (DMFA),69,70 poly(glycol ether),71 acetonitrile (ACN)72), mixed systems,15,22,36,44,46,73-75 and more advanced reaction media such as sub- or supercritical solvents and their mixtures (water,76-84 acetone, methanol, or acetic acid11,85) or ionic liquids.25-32,34,37-43,47-49,53-58,86

The dehydration reaction is accompanied by a series of side-reactions which strongly influence the efficiency of the whole process. The most important side-reactions are rehydration of 5-HMF into levulinic and formic acids, and either polymerization of 5-HMF or cross-polymerization of 5-HMF and carbohydrate (Figure 2). Polymeric compounds commonly denoted as humins consume initial carbohydrate, leading to an increase in viscosity and thus causing mass transfer limitations. The literature reports that aqueous and nonaqueous processes may lead to the formation of as many as 37 products.65 The aqueous processes are favored from an ecological point of view, but unfortunately, the selectivity to 5-HMF in water is usually low. Water is abundant and nonhazardous and is a proper solvent for monosaccharides and the product. However, water promotes the formation of undesired side-products, especially levulinic and formic acids, in a much higher level than do organic media.65 The rehydration of 5-HMF can be suppressed in nonaqueous systems. For example, DMSO diminishes rehydration to give the highest yields of 5-HMF.66,68 Nevertheless, polymeric substances still form and decreased reaction yield remains a problem. Moreover, there are difficulties in separation, and the possible toxic sulfur-containing side-products from the decomposition of DMSO limit its application on a larger scale.23 Using a mixed system composed of water and water miscible organics improves dehydration by shifting the equilibrium and suppressing of 5-HMF hydrolysis.65 The commonly employed organic modifiers include DMSO,36 acetone,44,85 polyethylene glycol (PEG),65 poly(1-vinyl-2pyrrolidinone) (PVP),46 or 1-methyl-2-pyrrolidinone (NMP).46 One disadvantage of this approach is the high energy demand of the required isolation procedures. To overcome this problem, another modification of the aqueous phase system was invented.45 It involves introduction of any solvent immiscible with water to create an aqueous biphasic reaction system as shown in Figure 4. An organic phase was intended to continuously extract from the aqueous phase the

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Figure 2. Chemistry and applications of 5-HMF and its derivatives (solid arrow, direct transformation; broken arrow, multistep reaction; 5-HMF, 5-hydroxymethylfurfuran; LEVA, levulinic acid; LEVE, levulinic ester; FA, formic acid; HFCA, 5-hydroxymethylfuroic acid; FDC, 2,5-furandicarboxyaldehyde; FDCA, 2,5-furandicarboxylic acid; DHMF, 2,5-di(hydroxymethyl)furan; DHM-THF, 2,5-di(hydroxymethyl)tetrahydrofuran; HMTHFA, 5-(hydroxymethyl)tetrahydrofuran-2-carbaldehyde; 1, 2-(hydroxy(5-(hydroxymethyl)tetrahydrofuran-2yl)methyl)-5-(hydroxymethyl)tetrahydrofuran-2-carbaldehyde; 2, (E)-4-(5-(hydroxymethyl)furan-2-yl)but-3-en-2-one; 3, (1E,4E)-1,5-bis(5(hydroxymethyl)furan-2-yl)penta-1,4-dien-3-one; 4, tetrahydrofurfuryl alcohol; 5, 2,5-dimethyltetrahydrofuran; 6, furan; 7, 2-hydroxymethyl5-vinylfuran; 8, furfuryl alcohol; 9, 2,5-di(aminomethyl)furan; 10, 2-methyl tetrahydrofuran; 11, 2,5-dimethylfuran; 12, 2-methylfuran).

Figure 3. Formation of 5-HMF in acidic medium.

5-HMF immediately upon its formation. Consequently, the formation of side-products would be reduced and the selectivity of 5-HMF in a single reactor would be enhanced. Many extracting agents (methyl isobutyl ketone,30,36,37,46 ethyl acetate,32,37,38,40 diethyl ether,26,42 dibutyl ether,86 toluene,30,37 tetrahydrofuran,33,37 dichloromethane,48 or acetone73,87,88) and their modifiers (2-butanol46,89 or 1-butanol74) have been investigated. Using the salting out effect with, for example, sodium chloride led to improvement of the reaction selectivity by modifying the solvent properties without affecting the chemistry of the process.74 Nevertheless, the partitioning of 5-HMF between the organic and aqueous phases remained relatively poor. Large amounts of organic solvent were required to purify the diluted 5-HMF product, and this again led to large energy expenditures. An alternative to organic solvents could be the use of sub- or supercritical water. Unfortunately, this strategy does not solve the problems

Figure 4. Biphasic reaction system.

Ionic Liquid-Mediated Formation of 5-Hydroxymethylfurfural

connected with undesired side-products and unsatisfactory yields.77,81-83 Better results were obtained from the reaction performed under supercritical conditions in a mixed system composed of water and low boiling organic solvents. Promising results were achieved with a mixture of acetone and water in a nine to one volume ratio.11,85 In the past few years, interesting results have been presented on the dehydration of carbohydrates in ionic liquids (ILs)26,30,31,38,40-43 (Tables 1-4). The literature shows that ILs can serve as good solvents for various carbohydrates.90 They facilitate more green applications in reactions and separations due to their unique properties such as negligible vapor pressure91 and comparative thermal stability.92,93 The very low vapor pressure of ILs reduces the risk of exposure, which is a clear advantage over the use of the classical volatile solvents. However, it is important to emphasize that basic results in the environmental risk assessment of ILs are still scarce.94 Therefore, ILs need to be treated with the same caution as any other chemical for which there is limited data about toxicity and biodegradability.95 Nevertheless, ILs are considered as more environmentally friendly than their hazardous volatile organic counterparts. Moreover, these compounds are composed solely of ions with countless combinations of anions and cations. This results in IL properties such as hydrophobicity,96 polarity,97 acidity,98 and miscibility with other solvents being highly tunable.99-102 Additionally, ILs can combine with liquid-liquid or supercritical CO2 extraction processes.103,104 The list of cations and anions constituting the ILs considered in this review is presented in Figures 5 and 6.

3. Dehydration of Monosaccharides to 5-HMF in ILs The first work that applied molten salts to the dehydration of monosaccharides dates back to the year 1983.105 Fructose was converted to 5-HMF with 70% yield in the presence of pyridinium chloride. The corresponding result for glucose was only a 5% yield. This early work showed that ILs can play an important role in the field of 5-HMF formation. However, researchers have only recently revisited using ILs in the dehydration of hexoses into 5-HMF. After 20 years, the next approach was made by Lansalot-Matras et al.,43 who investigated the acid-catalyzed dehydration of fructose in 1-butyl3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) with DMSO as a cosolvent. They demonstrated the positive effect of ILs as solvents. In pure DMSO under catalyst-free conditions, only trace amounts of 5-HMF were found even after 44 h. Upon adding [bmim][BF4], a yield of 27% was obtained after 15 h and a yield of 36% after 32 h. Addition of the catalyst increased the yield even more. It reached 87% within 24 h when Amberlyst-15 sulfonic ionexchange resin catalyst was used and almost 70% when p-toluene sulfonic acid was used. For the [bmim][PF6]/ DMSO solvent system, yields increased to 80% and to 75% for the Amberlyst-15 sulfonic resin and the p-toluene sulfonic acid, respectively (Table 1). All of the above-mentioned reactions were performed at 80 °C. The temperature required for the dehydration is an important parameter, since one of the goals in the conversion of carbohydrates into value added compounds is decreasing the operational temperature. The traditional synthetic methods have required temperatures ranging from 100 to 300 °C. When applied at the industrial scale, a decrease in the

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dehydration temperature could result in a significant reduction of the energy consumption and material replacement costs. It was found that with ionic liquids (ILs) the temperature of the reaction can be decreased to less than 100 °C. Some groups have demonstrated that the dehydration reaction using ILs and cosolvents can even be run at room temperature. Qi et al.39 studied 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) and different cosolvents such as DMSO, acetone, methanol, ethanol, ethyl acetate, and supercritical carbon dioxide (Table 1). They found that it was possible to perform reactions at 25 °C after predissolving the starting material in an IL. In a typical experiment, fructose was first solubilized in [bmim][Cl] at 80 °C for 20 min. Subsequently, the mixture was cooled down to room temperature, forming a gel-like solution with a very high viscosity. The catalyst (Amberlyst-15 sulfonic ion-exchange resin) together with a cosolvent were employed to allow the stirring, which facilitates the reaction. The viscosities were reduced from an estimated value of 6800 mPa s to values of around 2000 mPa s, with the best results being obtained for acetone (1850 mPa s) and ethyl acetate (1930 mPa s). Such significant reductions in viscosity allowed reactions to be processed at room temperatures, and 5-HMF was obtained in a 78%-82% yield. Another example of the dehydration of carbohydrates in ILs at low (below 50 °C) temperature was presented by Zhang and co-workers.37 This group selected a system composed of [bmim][Cl] and tungsten chloride, though a variety of metal salts were screened as well. Many of the metal chlorides tested showed high activities in the conversion of fructose at 80 °C, but only some of them proved to be active at 50 °C. Among the chloride salts of ruthenium(III), titanium(IV), zirconium(IV), and tungsten(IV) or tungsten(VI), the last one seemed to be the most promising, giving an 5-HMF yield as high as 63%. Moreover, tungsten(VI) was also active at 30 °C (53% yield of 5-HMF after 4 h) as well as at 22 °C (42% yield of 5-HMF after 4 h). The authors were encouraged by these results and examined the possibility of a continuous 5-HMF production process. They designed a biphasic system composed of ILs and a modifier, tetrahydrofuran (THF).33 Using the solvents mentioned and tungsten(VI) chloride as a catalyst, they were able to obtain a 5-HMF yield of 72%. This achievement was crucial, since it offered the first efficient catalytic system for the IL-mediated conversion of fructose to 5-HMF at room temperature. Zhao et al.31 were the first who found that metal halides in 1-alkyl-3-methylimidazolium chlorides are effective catalysts for conversion of carbohydrates into 5-HMF. They described the use of many metal halides and different ILs ([bmim][Cl], 1-ethyl-3-methylimidazolium ([emim][Cl]), and 1-octyl-3-methylimidazolium ([omim][Cl])). Such systems led to conversions of fructose and the more demanding glucose to 5-HMF with yields of 83% (RhCl3, PtCl2) and 70% (CrCl2), respectively, at 80 °C for 3 h of reaction. However, chromium(II) chloride (CrCl2) in [emim][Cl] was uniquely effective. Unfortunately, the exact catalytic mechanism for the exceptional effectiveness of CrCl2 remained unclear, but the authors offered some insights into the mechanism. Previously the literature had mentioned that the dehydration of hexoses can occur through two possible pathways.24,81,106,107 The first of them consisted of a series of cyclic furan intermediates. The second open-chain pathway included formation of an enediol as an intermediate in the isomerization of glucose to fructose. Zhao et al.31

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Figure 5. Cations of ILs referred to in this review: (a) the most common cations of ILs, (b) choline cations of ILs obtained by mixing of choline chloride based IL with the molecular donors (presented along the arrows).

proposed that a complex between CrCl2 and the IL interacts with the open chain of glucose, facilitating isomerization to fructose and a direct conversion to 5-HMF, as presented in Figure 7. Furthermore, the authors suggested that mutarotation of the R-anomer of glucose to the β-anomer is a key step of the reaction. Using 1H NMR studies, they confirmed that the chromium chloride species (CrCl3-) catalyzes the proton transfer by forming hydrogen bonds with the hydroxyl groups of the carbohydrate. The enolate intermediate created enabled conversion of aldoses (glucose) into ketoses (fructose), which were subsequently dehydrated to 5-HMF.

Additional investigations into the influence of halides on the formation of 5-HMF from carbohydrates have been performed by Binder et al.28 In a series of reactions with N,N-dimethylacetamide (DMA) as a solvent and fructose as a starting material, these authors demonstrated a weak adjunctive effect of the ion-paired halide ions on the reaction. Several ILs, to wit, ([emim][Cl], 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim][CF3SO3])), were applied only as additives, improving the yield of 5-HMF as shown in Table 1. The addition of [emim][Cl] to both the

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Figure 6. Anions of ILs referred to in this review.

systems DMA-LiCl-CuCl or DMA-H2SO4 increased the 5-HMF yield from around 70% to more than 80%. The trifluoromethanesulfonate and tetrafluoroborate salts of [emim] without LiCl afforded a modest 5-HMF yield of 48% and 59%, respectively, without LiCl. When the lithium salt was introduced to the solution, the yield was raised to 71% in both cases. However, distinct differences were found in the ability of various halide ions to mediate formation of 5-HMF from fructose in DMA containing sulphuric acid. Among the halides, fluoride ions were ineffective due to their low nucleophilicity and high basicity. On the other hand, bromide and iodide ions, which tend to be less ion-paired than fluoride or chloride ions, enabled an exceptionally high (up to 93%) 5-HMF yield in DMA. Binder et al.28 proposed two variations on the mechanism previously suggested by Zhao et al.31 (Figure 8). In one of them, halide ion (X-) attacked the fructofuranosyl cation previously formed from fructose to afford a 2-deoxy-2-halo intermediate. Such an intermediate was less prone to side reactions or reversion to fructose but easily lost HX to give an enol. A second option assumed that a halide ion acted as a base, creating the enol merely by deprotonation of C-1. According to the literature, the first postulated mechanism is more probable and depends on the fact that the fructofuranosyl cation tends to react with compounds that have functional groups such as hydroxyl, amino,107 or even fluoride.108 Thus, it is reasonable to expect that the fructofuranosyl cation undergoes attacks by chloride, bromide, or iodide. The bromide and iodide are better leaving groups and nucleophiles than chloride, and for this reason they were more effective as ionic additives. A marked halide effect was also observed in the case of the conversion of glucose to 5-HMF. Without chromium salts the 5-HMF yield was negligible. Addition of chromium(II) chloride to pure DMA or DMA combined with lithium chloride afforded yields between 47% and 60%, which could be further improved up to 69% by the introduction of [emim][Cl]. Although the addition of iodide salts to the chromium chloride reaction did not change the 5-HMF yield significantly, the use of bromide salts increased the yield of 5-HMF to 80%. Researchers expected that the yield of the product in reactions utilizing chromium would correlate with metal coordination. In their opinion, halide additives must balance between two roles: (1) serving as ligands for the chromium cation; (2) facilitating a selective conversion of fructose.

Probably either the large size or low electronegativity of iodide was responsible for its low effectiveness as a ligand. On the other hand, bromide offered the optimal balance of nucleophilicity and coordinating ability to facilitate the remarkable transformation of glucose into 5-HMF. The promising results obtained by Zhao et al.31 for the IL-mediated direct conversion of glucose to 5-HMF encouraged other research groups to search for new solutions. Yong et al.26 studied the production of 5-HMF from fructose and glucose in [bmim][Cl], but using chromium(II) chloride catalyst modified with N-heterocyclic carbenes (NHCs) (Figure 9). These organic ligands showed great flexibility, since the catalytic activity could be modified by varying the stereochemical and electronic properties of the NHCs. Contrary to the previously described CrCl2/[bmim][Cl] system, Yong et al. showed that there was no difference in the catalytic activities between chromium(II) and chromium(III) chlorides. It was found that catalytic activity is closely related to the stereochemical properties of the NHC ligands. The catalysts with the most bulky ligands provided the highest yields regardless of the saturation level. For the 1,3-bis(2,6-diisopropylphenyl)imidazolylidene ligand, the 5-HMF yields reached 96% from fructose and 81% from glucose. It was suggested that sterically crowded complexes protect the Cr center from reacting with [bmim][Cl] and therefore provide the highest catalytic efficiency. Zhang and co-workers37 also modified tungsten salts with the same 1,3bis(2,6-diisopropylphenyl)imidazolylidene ligand but obtained thereby only slightly higher yields of 5-HMF. Zhao and co-workers also obtained the best results for the conversion of glucose into 5-HMF (91%).47 They worked with the same system composed of chromium salt and [bmim][Cl], but under microwave irradiation. Their results indicated that the oxidation level of the metal was not a determinant factor in the conversion of carbohydrates with microwave heating. What is more, chromium(III) chloride was more easily solubilized in [bmim][Cl] under microwave irradiation than was chromium(II) chloride under conventional heating. Moreover, microwaves allowed a remarkable reduction in the time required (from hours to minutes). There are no other reports on catalysts as efficient as chromium chloride for a direct IL-mediated conversion of glucose to 5-HMF. Han and co-workers25 investigated a system composed of tin(IV) chloride and [emim][BF4], which

amount

0.045 g 0.072 g 0.072 g 0.072 g 0.072 g 0.036 g 1g 1g 1g 0.1 g 0.1 g 0.1 g 0.1 g 0.1 g 0.1 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.1 g 0.1 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.072 g 0.072 g 0.2 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.036 g 0.036 g 0.12 mol 0.091 g 0.091 g 0.091 g 0.091 g

substrate

fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose

[bmim][BF4] [bmim][BF4]/DMSO [bmim][BF4]/DMSO [bmim][BF4]/DMSO [bmim][BF4]/DMSO [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl]/acetone [bmim][Cl]/DMSO [bmim][Cl]/AcOEt [bmim][Cl]/EtOH [bmim][Cl]/MeOH [bmim][Cl]/scCO2 [bmim][PF6]/DMSO [bmim][PF6]/DMSO [emim][BF4] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][HSO4] [emim][HSO4] [hmim][Cl] ChoCl/citric acid · H2Og ChoCl/citric acid · H2Og ChoCl/citric acid · H2Og ChoCl/citric acid · H2Og

solvent 0.5 mL 0.5 mL/0.3 mL 0.5 mL/0.3 mL 0.5 mL/0.3 mL 0.5 mL/0.3 mL 0.3 mL 20 g 20 g 20 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 1g 1g 5.73 mmol/0.862 mmol 5.73 mmol/0.862 mmol 5.73 mmol/0.862 mmol 5.73 mmol/0.862 mmol 5.73 mmol/0.862 mmol 5.73 mmol/25 mL 0.5 mL/0.3 mL 0.5 mL/0.3 mL 1g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.3 mL 0.3 mL 0.6 mol 6h 6h 6h 6h

amount

Table 1. IL-Mediated Production of 5-HMF from Monosaccharidesa

6 6 6 6

Amberlyst 15 resin PTSA SnCl4 · 5H2O RhCl3 PtCl2 CrCl2 CrCl3

% % % %

143 mg 0.04 mmol 10 mol %f

resin resin resin resin resin resin resin

mol mol mol mol

40 µL 40 µL 10% molc 10% molc 10% molc 10% molc 10% molc 10% molc 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g

H2SO4 H2SO4 WCl6 WCl6 WCl6 WCl6 WCl6 Ipr-WCl6 1/CrCl2d 2/CrCl2d 3/CrCl2d 4/CrCl2d 5/CrCl2d 6/CrCl2d 7/CrCl2d 8/CrCl2d 4/CrCl3d 5/CrCl3d 6/CrCl3d 7/CrCl3d Amberlyst Amberlyst Amberlyst Amberlyst Amberlyst Amberlyst Amberlyst 15 15 15 15 15 15 15

143 mg 0.04 mmol

Amberlyst 15 resin PTSA

amount 120 mg

catalyst Amberlyst 15 resin

80 80 80 80 80 100 120 120 120 50 50 50 50 50 50 100 100 100 100 100 100 100 100 100 100 100 100 80 120 25 25 25 25 25 35e 80 80 100 120 100 80 80 80 80 100 100 90 80 80 80 80

T (°C) 180 900 1920 1920 1920 30 120 30 120 240 240 240 240 240 240 360 360 360 360 360 360 360 360 360 360 360 360 10 1 360 360 360 360 360 360 1440 1200 180 180 180 180 180 180 180 30 30 45 60 60 60 60

t (min) 52 27 36 87 68 16 78.6 96.0 85.5 63 72 61 59 25 65 65 68 76 89 76 96 93 74 90 77 96 83 83.3 82.2 78.2 78.3 81.1 80.2 82.0 78.7 80 75 62 ∼73 ∼40 ∼83 ∼83 ∼65 ∼69 79 88 92 77.8 85.6 82.1 91.4

5-HMFyield (%)

AcOEtb AcOEti AcOEtj

Tb MIBKb Et2Ob

Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob AcOEtb AcOEtb

THFb MIBKb MIBKb Tb

Tb

separation

43 43 43 43 43 30 27 27 27 37 37 37 37 37 37 26 26 26 26 26 26 26 26 26 26 26 26 38 38 39 39 39 39 39 39 43 43 25 31 31 31 31 31 31 30 30 42 40 40 40 40

ref

404 Chemical Reviews, 2011, Vol. 111, No. 2 Zakrzewska et al.

amount

1g 1g 1g 1g 1g 1g 1g 1g 1g 1g 17.2 wt % 17.2 wt % 0.063 g 0.063 g 0.063 g 0.063 g 0.063 g 0.063 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g 0.0271 g

0.0271 g

0.0271 g

1g 0.036 g 0.036 g 0.036 g 0.036 g 0.1 g 0.1 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g

substrate

fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose

fructose

fructose

glucose glucose glucose glucose glucose glucose glucose glucose glucose glucose glucose glucose glucose glucose glucose glucose

Table 1. Continued

solvent

[bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl]

DMA-LiCl/[emim][BF4]

DMA/LiCl/[emim][OTf]

DMSO DMSO DMSO DMSO ACN CCl4 DMAc EtOH H2O H2O H2O H2O DMSO DMSO DMSO DMSO DMSO DMSO DMA DMA DMA DMA DMA DMA DMA DMA DMA/[emim][BF4] DMA/[emim][Cl] DMA/[emim][OTf] DMA/LiCl DMA/LiCl DMA/LiCl/[emim][Cl]

amount 12 mL 12 mL 12 mL 12 mL 12 mL 12 mL 12 mL 12 mL 12 mL 12 mL Nd Nd 2g 2g 2g 2g 2g 2g 0.203 g 0.203 g 0.203 g 0.203 g 0.203 g 0.203 g 0.203 g 0.203 g DMA-0.203 g, [emim][BF4]-20 wt % DMA-0.203 g, [emim][Cl]-20 wt % DMA-0.203 g, [emim][OTf]-20 wt % LiCl-10 wt %; 0.203 g/0.024 g DMA-0.203 g, LiCl-10 wt % LiCl-10 wt %, [emim][Cl]-40 wt %; IL-nd DMA-0.203 g, LiCl-10 wt %, [emim][OTf]-20 wt % DMA-0.203 g, LiCl-10 wt %, [emim][BF4]-20 wt % 20 g 0.3 mL 0.3 mL 0.3 mL 0.3 mL 1g 1g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g 0.5 g

catalyst

amount

0.04 Mc 0.04 Mc 0.04 Mc 0.006 g 0.006 g 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c 9 mol %c

CrCl3 · 6H2O CrCl3 · 6H2O CrCl3 · 6H2O CrCl3 · 6H2O CrCl3 · 6H2O 1/CrCl2d 2/CrCl2d 3/CrCl2d 4/CrCl2d 5/CrCl2d 6/CrCl2d 7/CrCl2d 8/CrCl2d 4/CrCl3d

120 100 100 100 100 ∼100 ∼100 100 100 100 100 100 100 100 100 100

100

6 mol %f 40 µL

100

%f %f %f %f %f %f %f %f

90 90 90 90 85 85 85 85 90 90 80 80 ∼100 ∼100 ∼100 ∼100 ∼100 ∼100 80 80 100 100 100 100 100 100 100 100 100 120 120 120

T (°C)

6 mol %f

7.5 mol %f 7.5 mol %f 7.5 mol %f 7.5 mol %f 7.5 mol %f 7.5 mol %f 7.5 mol %f 7.5 mol %f 7.5 mol %f 7.5 mol %f 5g 5 g/3 wt %c 0.175 mmol 0.175 mmol 0.17 mmolk 0.29 mmolk 0.22 mmolk 0.54 mmolk 6 mol %/10 wt 6 mol %/10 wt 6 mol %/10 wt 6 mol %/10 wt 6 mol %/10 wt 6 mol %/10 wt 6 mol %/10 wt 6 mol %/10 wt 6 mol %f 6 mol %f 6 mol %f 6 mol %f 6 mol %f 6 mol %f

H2SO4

H2SO4

H2SO4

[nmp][HSO4] [mim][HSO4] [nmp][CH3SO4] [mim][CH3SO4] [nmp][CH3SO4] [nmp][CH3SO4] [nmp][CH3SO4] [nmp][CH3SO4] [nmp][HSO4] [nmp][CH3SO4] [bmim][CH3SO3] [bmim][CH3SO3] + CH3SO3H [ASBI][Tf] [ASCBI][Tf] IM-SiO2-[ASCBI][Tf] IM-SiO2-[ASBI][Tf] IM-SiO2-SO2Cl IM-SiO2-SO3H H2SO4/KCl H2SO4/LiF H2SO4/LiBr H2SO4/NaBr H2SO4/KBr H2SO4/LiI H2SO4/NaI H2SO4/KI H2SO4 H2SO4 H2SO4 H2SO4 CuCl CuCl

120 240 240 240 240 1 60 360 360 360 360 360 360 360 360 360

120

60

120 120 120 120 120 120 120 120 120 120 120 120 4 4 4 4 4 4 120 120 240 120 120 360 300 300 240 120 120 60 180 90

t (min)

11.9 0 91 79 81 91 17 66 65 62 80 50 81 70 14 78

71

71

69.4 23.6 72.3 25.1 4.2 0.3 20.5 5.9 2.4 2.7 ∼75 ∼85 ∼78 ∼84 70.1 67.2 63.1 60.2 56 0 92 93 92 89 91 92 59 84 48 68 71 83

5-HMFyield (%)

CC CC Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob Et2Ob

Tb Tb MIBKb

AcOEtb AcOEtb AcOEtb AcOEtb AcOEtb AcOEtb AcOEtb AcOEtb AcOEtb AcOEtb

separation

27 30 30 30 30 47 47 26 26 26 26 26 26 26 26 26

28

28

32 32 32 32 32 32 32 32 32 32 29 29 41 41 41 41 41 41 28 28 28 28 28 28 28 28 28 28 28 28 28 28

ref

Ionic Liquid-Mediated Formation of 5-Hydroxymethylfurfural Chemical Reviews, 2011, Vol. 111, No. 2 405

0.0271 g

1g

glucose

mannose

solvent

[bmim][Cl]

DMA/LiCl/[emim][Cl]

[bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][Cl] [emim][BF4] [emim][BF4] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][Cl] [emim][HSO4] [hmim][Cl] DMSO DMSO DMSO DMSO T T T T DMA DMA DMA DMA/[emim][Cl] DMA/LiCl DMA/LiCl/[emim][Cl]

amount 0.5 g 0.5 g 0.5 g 1g 1g 1g 1g 1g 1g 1g 1g 0.5 g 0.5 g 0.5 g 1g 1g 1g 1g 1g 1g 0.3 mL Nd 12 mL 12 mL 12 mL 12 mL Nd Nd Nd Nd 0.203 g 0.203 g 0.203 g DMA-0.203 g, [emim][Cl]-10 wt % DMA-0.203 g, LiCl-10 wt % DMA-0.203 g, LiCl-10 wt %, [emim][Cl]-20 wt % DMA-0.203 g, LiCl-10 wt %, [emim][Cl]-20 wt % 20 g H2SO4

CrCl2

[nmp][CH3SO4] [nmp][HSO4] H2SO4 HCl IM-SBA-15-[smim][Cl]-CrCl2 IM-SBA-15-[smim][Cl]-AlCl3 IM-SBA-15-[smim][Cl]-CuCl2 IM-SBA-15-[smim][Cl]-FeCl3 CrCl2 CrCl2/LiBr CrCl2/LiI CrCl2 CrCl2

100 120

40 µL

100 100 100 140 140 140 140 140 140 100 100 180 100 100 140 140 140 140 140 140 100 90 90 90 90 90 RT RT RT RT 100 100 100 100 100 100

T (°C)

6 mol %l

10 mol %l 10 mol %l 10 mol %l 10 mol %l nd nd nd nd 6 mol %l 6 mol %/10 wt %l 6 mol %/10 wt %l 6 mol %l 6 mol %l

6 mol % 6 mol % 0.056 mmol 0.056 mmol 0.056 mmol 0.056 mmol 0.056 mmol 0.056 mmol

CrCl2 CrCl3 CeCl3 PrCl3 NdCl3 DyCl3 YbCl3 Yb(OTf)3

amount 9 mol %c 9 mol %c 9 mol %c 0.056 mmol 0.056 mmol 0.056 mmol 0.056 mmol 0.056 mmol 0.056 mmol 10 mol %l 15 mol %l

catalyst 5/CrCl3d 6/CrCl3d 7/CrCl3d CeCl3 PrCl3 NdCl3 DyCl3 YbCl3 Yb(OTf)3 SnCl4 · 5H2O SnCl4 · 5H2O

20

360

360 360 360 360 360 360 360 360 360 180 180 180 180 180 360 360 360 360 360 360 30 30 120 120 120 120 ON ON ON ON 240 240 240 360 300 360

t (min)

2.1

62

72 78 81 3 7 8 10 12 24 53 60 ∼3 ∼68 ∼45 3 13 12 10 5 10 3 3 3 2.4 1.4 21.2 22 8 3 0 60 79 54 67 60