Efficient Catalytic System for the Selective Production of 5‐Hydroxymethylfurfural from Glucose and Fructose

Abstract

The diminishing fossil fuel reserves and global warming effects have become major concerns, and the search for sustainable, alternative energy is of critical importance. [1,2] Biofuels are highly attractive as they are the only sustainable source of liquid fuels currently available [3] however, the replacement of a petroleum feedstock by biomass is limited by the lack of highly efficient methods to selectively convert carbohydrates into chemical compounds for biofuel production.[4] A practical catalytic process that can transform the abundant biomass into versatile chemicals would also provide the chemical industry with renewable feedstocks.[5] Recently, efforts have been devoted to the conversion of biomass into 5-hydroxymethylfurfural (HMF), a versatile and key intermediate in biofuel chemistry and the petroleum industry. [6] HMF and its 2,5-disubstituted furan derivatives can replace key petroleum-based building blocks. [7] There are currently a number of catalysts that are active in the dehydration of sugars to form HMF. However, most of them also promote side reactions that form undesired byproducts, and rehydrate HMF to form levulinic acid and formic acid. Thus, these catalysts are often limited to simple sugar feedstocks, such as fructose. [4, 6,8] Recent reports illustrate the use of 1-H-3-methyl imidazolium chloride (HMIM + Cl À ) as a solvent and an acid catalyst to efficiently convert fructose into HMF with approximately a 90 % yield.[9] However, such a system has not been shown to convert glucose, which is a more stable and abundant sugar source, into HMF. Dumesic and co-workers have developed a two-phase (aqueous/organic) system for the separation and stabilization of the HMF product.[6a,b] Zhang and co-workers have reported a metal chloride/ionic liquid system that gives moderate to good HMF yields for both fructose (83 % with Pt or Rh chloride, 65 % with CrCl 2 ) and glucose (a record high of 68 % with CrCl 2 ).[10] Herein, we report a new catalyst system that efficiently converts both fructose and glucose into HMF in good to excellent yields (81-96 %).Zhang and co-workers showed that transition metals were good catalysts for the transformation of sugars into HMF. [10] We selected NHC/metal (NHC = N-heterocyclic carbene) complexes as catalysts for the sugar dehydration reaction. [11] These ligands offer a great deal of flexibility as the catalytic activity can be modified by varying the stereo and electronic properties of the NHCs. The conversions of fructose and glucose into HMF were tested using 1-butyl-3-methyl imidazolium chloride (BMIM + Cl À ) with different catalysts (Scheme 1). The NHC-metal complexes were pre-generated by heating a mixture of imidazolium salts, potassium tertbutoxide (KOtBu), and metal chlorides in N,N-dimethylformamide (DMF) for several hours before adding to the reaction system. In a typical reaction protocol, 100 mg of sugar was mixed with 1 g of BMIM + Cl À and 2-9 mol % of preprepared NHC/Cr catalyst. The reaction mixture was kept at 100 8C for 6 hours. HMF was ext...

Bifunctional Solid Catalysts for the Selective Conversion of Fructose to 5-Hydroxymethylfurfural

“…Chemical shifts were referenced to tetrakis(trimethylsilyl)silane. 13 C CP/MAS spectra were obtained using a 90°pulse length of 3.7 ls, a contact time of 2 ms, and high power proton decoupling during detection. Typically, 25,000 scans were acquired at a spinning rate of 10 kHz.…”

“…The solid was collected by filtration and washed with hexanes (3 9 50 mL), ethyl acetate (3 9 50 mL), ethereal HCl (50 mL, 0.50 M), then fresh methanol (3 9 50 mL). The modified silica was dried at 100°C under vacuum (0.1 mTorr) for 12 h. 13 …”

“…Two signals for the methylene carbons bonded directly to the thioether S were observed, at 30.83 and 30.63 ppm. The signature 13 C resonance of the sultone ring (69.9 ppm, CO) was not detected, implying that it had reacted completely. The characteristic C b signal of 1-butanethiol, at ca.…”

“…Although the phenylsulfonic acid-functionalized polystyrene resin Amberlyst 15 was previously used to produce HMF [13], it is stable only up to 120°C. Since selectivity for HMF increases with temperature, we chose to conduct our dehydration reactions at 180°C.…”

“…HMF can be obtained by the acid-catalyzed dehydration of fructose, glucose, sucrose and even cellulose [7,8,[12][13][14]. Recently, a tandem process combining an isomerase enzyme and an acid dehydration catalyst was employed to produce HMF from glucose [15].…”