Direct conversion of carbohydrates into 5-ethoxymethylfurfural (EMF) and 5-hydroxymethylfurfural (HMF) catalysed by MoO2Cl2(H2O)2 Pereira, Juliana Centro de Química Estrutural, Instituto Superior Técnico, Av. Rovisco Pais, Lisboa Portugal Abstract This work reports a one-pot synthesis of 5-ethoxymethylfurfural (EMF) from fructose catalysed by different dioxo- molybdenum complexes using different solvents and temperatures. This method has the advantage of using an efficient, economical, environmental catalyst with an easy preparation. The best result obtained for EMF (53%) was observed using a mixture of ethanol/THF (5:2) catalysed by MoO2Cl2(H2O)2 (10 mol%) at 120ºC, after 17 h. The possible use of MoO2Cl2(H2O)2 in more than one catalytic cycle was also studied and it was verified that the yield of EMF was reduced 2.5 times on the fourth cycle. The conversion of fructose into EMF in a large scale (10 mmol) was also investigated and enabled a good yield of EMF. The synthesis of EMF from other carbohydrates such as inulin and sucrose, was also explored giving EMF with 40% and 23% yield, respectively. The dehydration of fructose into HMF was achieved in good yield (75%) from fructose using MoO2Cl2(H2O)2 in DMSO at 120ºC after 30 minutes. Keywords: Biomass, carbohydrates, 5-ethoxymethylfurfural (EMF), 5-hidroximethylfurfural (HMF), dioxo-molybdenum complexes. 1. Introduction Due to environmental problems and the decreasing of fossil reserves, the necessity to develop successful methods to produce chemical and fuels from renewable feedstocks emerged. An alternative for non-renewable resources is the use of biomass, renewable carbon sources, where the major constituents are carbohydrates.[1, 2] These compounds are a promising alternative since they are relatively economic, environmentally friendly and can be found in great amount on Earth.[3] The conversion of biomass to organic chemicals and liquid fuels using an efficient catalyst is one of the most important processes in biorefinary.[1, 3, 4] To produce chemicals and fuels, like HMF, EMF or other derivatives from biomass, a catalyst capable of promoting the dehydration of monosaccharides is essential, such as homogeneous mineral acid, Brønsted acidic ionic liquids (IL), Lewis acidic metal halides and recyclable heterogeneous catalysts.[5] The successive transformation of HMF into other value added chemicals, such as promising next generation polyester building block monomers (2,5-furandicarboxylic acid (FDCA), 2,5-bis(hydroxymethyl)furan (BHMF), and 2,5- bis(hydroxymethyl)tetrahydrofuran (BHMTF)) and potential biofuel candidates (2,5-dimethylfuran (2,5-DMF), 5- ethoxymethylfurfural (EMF), ethyl levulinate (EL) and γ-valerolactone (γVL)) (Figure 1) has also been explored using HMF as a starting substrate or directly from biomass in a one-pot process.[6] O O HO OH HO OH OH OH BHMF BHMTF O HO OH O O O O O n O HO HO OH OH OH Cellulose Hydrolysis FDCA O Pretreatment O HO Hydrolysis OH OH O O 2,5-DMF O O Isomerization OH Dehydration O H OEt HO OH HO H OH OH OH OH O Glucose Frutose EMF HMF 2-MF Hydrolysis OH HO O O O HO O HO OH OH O HO EtO OH OH Sucrose O O EL LA O O γVL! Figure 1. Production of liquid fuels and other chemicals from biomass.[5] 1 The application of HMF is indeed versatile and high HMF yields were already achieved. However, HMF presents a high cost and this fact is a limitation to sustainable use as a potential replacement for petroleum resources.[2, 5] The most abundant and cheapest monosaccharide is the glucose, a hexose. This carbohydrate is being investigated on the transformation into chemicals and biofuels.[2, 7] The use of glucose to afford HMF or other 2,5-disubstituted furan derivatives has an increased difficulty because it requires a bifunctional catalyst, allowing the isomerization of glucose to fructose and later dehydration of fructose (Figure 2). In general, fructose is much more reactive and selective toward HMF than glucose.[6] OH HO OH O HO OH HO O O O + O + + O HO H H O H OH OH OH HO H HO -H O OH OH 2 -H2O -H2O OH OH OH Glucose Fructose HMF Figure 2. Isomerization of glucose and dehydration of fructose into HMF.[6] Solvents also play an important role in the improvement of HMF and EMF yields. Easy separation and purity make the process environmentally and economically competitive.[5] High yields of HMF were achieved in the presence of dimethylsulfoxide (DMSO) and dimethylformamide (DMF), methyl isobutyl ketone (MIBK) and ionic liquids (ILs). However, a potential drawback of the ILs solvent mediated process is that ILs are expensive and the separation of HMF from high boiling ILs is energy intensive. Besides the cost factor, ILs is also deactivated by water, which is formed during the dehydration reaction. The high boiling points of DMSO and DMF also pose similar challenges for HMF separation, and therefore these processes are economically unfavorable on a commercial scale. Additionally, high concentrations of oligomeric species, humins, are also form as by-products in organic solvent mediated dehydration reactions. The polar organic solvents have a disadvantage since there is a strong affinity with the hydroxyl group of HMF, resulting in a difficulty to separate both by extraction. One way to solve this problem is the etherification of hydroxyl group in HMF with ethanol, methanol, etc.[2, 8] Ethanol is a green solvent inexpensive and can also be produced from biomass.[9-11] Another advantage of using alcohols as solvents is the supply of humin formation.[9] Recently the etherification of HMF has attracted attention, since the ethers of HMF are a great addition to diesel. In the case of 5-ethoxymethylfurfural (EMF) its own physical and chemical properties make it a remarkable biofuel. EMF has a high energy density of 8.7 kWh/L, similar to that of regular gasoline (8.8 kWh/L), nearly as good as that of diesel (9.7 kWh/L), and significantly higher than that of ethanol (6.1 kWh/L).[2, 8, 9, 11] Currently, there are several methods on the synthesis of EMF. It is undoubted that the etherification of hydroxyl group in HMF with ethanol is the most effective way. However, the practical large-scale synthesis of EMF from HMF is limited due to the high cost of HMF. Using fructose as a start material, since it is inexpensive and renewable, and a different alcohol as a solvent to do the dehydration into HMF following by the etherification to the correspondent ether in one-pot synthesis, saves time, energy and solvent (Figure 3).[12] HO OH O O O -3H O Cat, EtOH OH 2 O O H OH -H2O H OEt OH OH HMF Fructose EMF Figure 3. Conversion of fructose into EMF.[12] Molybdenum-based catalyst has been applied in many organic reactions. In last decade, high valent oxo- molybdenum complexes such as MoO2Cl2 have been proved to be excellent catalysts for the reduction of organic compounds such as aldehydes[13], ketones[13], esters[14], amides[15], imines[16], sulfoxides[17] and N-oxides[17]. The development of environmental friendly process for the synthesis of HMF, EMF and other derivatives from renewables still represents a great challenge.[18] In the present work, the conversion of carbohydrates into HMF or EMF 2 catalysed by several dioxo-molybdenum complexes in water, in polar aprotic solvents or in different alcohols was investigated. 2. Results and discussion Initially, the reaction of fructose was studied with several dioxo-molybdenum catalysts, solvent and temperatures in order to find the best reaction conditions. 2.1. Synthesis of EMF 2.1.1. Effect of catalyst amount The effect of the amount of the catalyst MoO2Cl2(H2O)2 [19] in the conversion of fructose into EMF at 120ºC after 17 h is showed in figure 4. An increase of catalyst amount led to an increase of EMF yield, but also an increase of the by- product EL. The increase of EMF with the increase of the catalyst amount is probably due to the higher number and availability of catalytic active sites.[2] EMF yield reaches a maximum of 50% when the amount of catalyst increases to 30 mol%. As evidenced in figure 4, increasing the amount of MoO2Cl2(H2O)2 allows the increase of EMF and EL yields. 50 45 EMF 40 35 30 Yield (%) 25 20 EL 15 10 10 15 20 25 30 MoO2Cl2(H2O)2 (mol%) Figure 4. The effect of catalyst amount on the conversion of fructose into EMF. Reaction conditions: fructose, 1 mmol; ethanol, 5 mL; 120ºC; 17h. 2.1.2. Effect of catalytic activity of different dioxo-molybdenum complexes Dehydration of fructose was carried out with the dioxo-molybdenum complexes MoO2Cl2(H2O)2, MoO2(acac)2, MoO2Cl2(DMF)2, MoO2Cl2(DMSO)2 and MoO2Cl2(OPPh3)2.The reactions were performed using 30 mol% of these catalysts in ethanol at 120ºC and 150ºC during 17 hours (Figure 5). (a) (b) 60 HMF (%) HMF (%) 60 EMF (%) EMF (%) EL (%) EL (%) 50 50 40 40 30 30 Yield (%) Yield (%) 20 20 10 10 0 0 1 2 3 4 5 1 2 3 4 5 Figure 5. The effect of different molybdenum oxo-complexes on the conversion of fructose into EMF. Reaction conditions: fructose, 0.5 mmol, except for MoO2Cl2(H2O)2, 1 mmol; 1: MoO2Cl2(H2O)2; 2: MoO2Cl2(DMF)2; 3: MoO2Cl2(DMSO)2; 4: MoO2Cl2(OPPh3)2; 5: MoO2(acac)2; 30 mol%; ethanol, 5 mL; 17h at (a) 120ºC and (b) 150ºC. The best result was achieved using MoO2Cl2(H2O)2 at 120ºC, producing 50% yield of EMF and 22% yield of EL. At 150ºC with MoO2Cl2(H2O)2 as catalyst was observed a decrease on EMF yield (30%) and an increase of EL yield (36%), suggesting the conversion of EMF into EL with the increase of temperature.