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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Bifunctional Sulfonated MoO3−ZrO2 Binary Oxide Catalysts for the One-Step Synthesis of 2,5-Diformylfuran from Fructose Jun Zhao, Anjali Jayakumar, and Jong-Min Lee* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459 Singapore ABSTRACT: Sulfonated MoO3−ZrO2 binary oxides (MZS) with different Mo/Zr ratios were synthesized and applied as bifunctional catalysts for the simple one-pot transformation of fructose to 2,5-diformylfuran (DFF). The presence of Brønsted acid sites and the molybdenum oxide species in the catalysts is responsible for the high efficiency and good activity of the catalysts. The former contributes to a high yield of 5-hydroxymethylfurfural (HMF) in the fructose dehydration, and the latter has the role of catalyzing the selective aerobic oxidation of the resulted HMF into DFF. In optimized reaction conditions, DFF yield of 74% with fructose of 100% can be achieved in a one-step reaction. The catalyst can be separated, simply regenerated, and reused without any significant loss in activity, indicating its great potential for the industrial mass production of DFF from fructose. KEYWORDS: Fructose, 5-Hydroxymethylfurfural, 2,5-Diformylfuran, Zirconia, Biomass, Molybdenum

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INTRODUCTION The exhaustion of conventional and nonrenewable sources of energy like petroleum and gas is leading to environmental deterioration, and this is scaling up at an alarming rate. Biomass, as a huge renewable source of energy, is anticipated to play a vital role for a sustainable supply of fuels and chemicals.1 Thus, a tremendous amount of effort and energy has been put forth in the research and development of methods for the transformation of carbohydrates to valuable chemicals.2 A common strategy is to convert carbohydrates to a platform chemical first, and then, the corresponding derivatives can be obtained through a series of further reactions. Acid-catalyzed dehydration of carbohydrates provides an easy way for the chemical synthesis of one such chemical, 5-hydroxymethylfurfural (HMF). 2,5-Diformylfuran (DFF) is one of the HMF oxidation products which has received particular attention because of its symmetrical and unsaturated structure.3 It can be applied in the synthesis of fungicides,4 macrocyclic ligands,5 and furan resins and novel polymers.6 DFF could be synthesized with the help of oxidants such as NaClO, BaMnO4, trimethylammonium chlorochromate, Pb(OAc)4, pyridinium chlorochromate, pyridine, and 2,2,6,6tetramethylpiperidine-1-oxide by the selective oxidation of hydroxymethyl group on HMF.7−12 Since these oxidants could cause environmental problems, and the separation process of the oxidants would have high energy cost, air or molecular oxygen becomes a better choice as the oxidant in the view of green chemistry. Co/Mn/Zr/Br-based catalysts were used with air being the oxidant, and the HMF was oxidized to DFF.13 Various metal-modified vanadylphosphate and SiO2-supported vanadium catalysts were applied in the production of DFF from HMF in a biphasic reaction system.14,15 Selective aerobic © XXXX American Chemical Society

oxidation using Cu- and V-based catalysts was tested in the conversion of HMF to DFF.16 V-containing polymeric catalysts and V2O5 are known to have good performance in the presence of oxygen or air (oxidant) in oxidizing HMF to DFF.17 Noblemetal-supported catalysts, such as Pt-ZrO2, Pt/SiO2, and Pt/C, have been widely used and researched as catalysts for the HMF oxidation.18−20 Within the optimized reaction conditions, HMF can be proficiently oxidized to DFF using the above catalysts; thus researchers tried to synthesize DFF from fructose that can be converted to HMF through dehydration. In this way, the heavy costs incurred in the separation and purification of HMF can be avoided, and fructose is a more economical starting material than HMF. Since the generation of HMF from fructose using a dehydration reaction requires an acidic catalyst, and the conversion of HMF to DFF is an aerobic oxidation reaction, the catalyst systems have to meet the demands for these two-step reactions. Therefore, a series of combined catalyst systems containing acidic catalysts and redox catalysts have been developed for the reactions. Solid acids, such as Hform cation-exchange resin, Amberlyst-15, and Fe3O4−SBA− SO3H, were applied as the dehydration reaction catalyst for the chemical conversion of fructose to HMF, cooperated with supported metals or metal oxides (VOHPO4, Ru/HT, SBA-15biimidazole-Ru, and K-OMS-2) applied as the catalyst for selective aerobic oxidation of HMF to DFF.21−23 During the reaction, the redox catalysts were introduced into the reaction system when most of the fructose had been converted to HMF; otherwise the coexistence of the acidic catalysts and redox Received: August 3, 2017 Revised: December 6, 2017

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DOI: 10.1021/acssuschemeng.7b02671 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

cm−1 with a resolution of 4 cm−1 using a standard KBr disk technique. The element composition of the catalyst was estimated by inductively coupled plasma (ICP). An X-ray photoelectron spectroscopy (XPS) experiment was carried out on a KRATOS AXIS DLD spectrometer. Catalytic Reactions. Dehydration of Fructose to HMF. The fructose dehydration reaction was conducted in a three-necked and round-bottomed flask connected with a reflux condenser to avoid DMSO evaporation. The reaction was heated in a thermostatic oil bath with magnetic stirring and maintained at the reaction temperature for a specific reaction time. Fructose (200 mg), catalyst (10 mg), and DMSO (5 mL) were used in the reaction. The mixture after reaction was filtered and diluted with deionized (DI) water prior to the analysis by HPLC. Oxidation of HMF to DFF with Oxygen as Oxidant. The aerobic oxidation of HMF was conducted as follows: a 25 mL three-necked flask equipped with a reflux condenser and magnetic stirring was used as the reactor; HMF (63 mg), catalyst (10 mg), and DMSO (5 mL) were added to the flask. The reaction was then heated in a thermostatic oil bath to the target temperature and was performed under a constant oxygen flow (20 mL min−1). The mixture after reaction was diluted with DI water, filtered, and then analyzed by HPLC. One-Pot Synthesis of DFF from Fructose. The direct conversion of fructose to DFF was carried out in a 25 mL flask connected with a condenser to prevent the evaporation of solvent. A 200 mg portion of fructose, 10 mg of catalyst, and 5 mL of DMSO were used in the reaction. The reaction was heated in an oil bath, and oxygen was fed into the reaction solution at a flow rate of 20 mL min−1. The mixture after reaction was diluted with DI water, filtered, and then analyzed by HPLC. Products Analysis. The resulted solution after reaction was diluted with water, and filtered using PTFF filters, then the liquid samples were analyzed on an Agilent 1260 HPLC with a Biorad aminex 87H column. The mobile phase was constituted of 5 mM H2SO4 solution at 0.6 mL/min. During the measurement, the temperature of the column should maintained at 60 °C. The quantification of the reaction products was based on an external standard calibration curve method. The calibration curves were obtained by the measurement of pure compound.

catalysts in the beginning of the reaction could significantly lead to a decrease of DFF yield, because of the fructose decomposition by the redox catalysts in the presence of air or oxygen, resulting in a reaction that is one-pot but two-step. Recently, bifunctional catalysts have been prepared and studied extensively in the one-pot direct approach production of DFF from fructose. A 53% DFF yield was attained from fructose after 24 h of reaction at 140 °C with graphene oxide as the catalyst.24 Graphitic carbon nitride (g-C3N4(H+) and V-gC3N4) containing protons and vanadium successfully gave a DFF yield of 45% for the reaction of fructose to DFF.25 Cesium salts of molybdovanadophosphoric heteropolyacids and MoKeggin heteropolyacids were synthesized and used for the direct conversion of fructose to DFF with a DFF yield of 39% and 69%, respectively, under the optimized reactions.26,27 DFF yield can reach 45% in 12 h by using Ce−Mo composite oxides as catalysts.28 Similar catalytic systems (sulfonated MOFderived Fe3O4, Nanobelt α-CuV2O6, WO3HO−VO(salten)− SiO2@Fe3O4) were also reported to be effective for the direct conversion of fructose to DFF.29−31 These catalysts showed better performance when N2 was served as a shielding gas to protect fructose from oxidation during the dehydration process, and oxygen or air was fed into the reaction system after the exhaustion of fructose. However, switching the inert gas to oxygen to maintain the relatively high yield of DFF from fructose is not a real one-step method. Thus, we report that a combination of sulfonate zirconia (ZS) and molybdenum oxide can be adopted in the one-step production of DFF with fructose as a starting material. As bifunctional catalysts, the sulfated zirconia-supported molybdenum oxide (MZS) showed excellent performance with the oxidant being oxygen during the continuous producing of DFF from fructose. Under optimized reaction conditions, the DFF yield as high as 74% can be achieved. Moreover, the used catalysts can be regenerated conveniently and maintain the activity during the recycling test.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

The XRD patterns of the MZS catalysts with different molybdenum oxide loadings are shown in Figure 1. There are no obvious peaks for MoO3, indicating the good dispersion of MoO3 on ZrO2.32 For the sample of 5-MZS, the ZrO2 shows both tetragonal phase and monoclinic phase. The intensity of all the peaks becomes weaker and finally disappears with the

Materials. HMF, DFF, and ammonium hydroxide were purchased from Alpha. Fructose, zirconium oxynitrate, ammonium molybdate, sulfuric acid, and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich. No further purification was conducted before the used of these chemicals. Catalysts Preparation. In a typical process, ammonia hydroxide was added into the aqueous solution of ZrO(NO3)2 at ambient temperature with stirring to adjust the pH value of the solution to 9. The obtained white slurry was washed thoroughly after stirring for 5 h, and then dried in a 65 °C oven overnight for the formation of Zr(OH)4. Thereafter the Zr(OH)4 powders were impregnated with ammonium heptamolybdate solutions at different concentrations, followed by dropping addition of sulfuric acid to 0.1 M under stirring and drying at 90 °C overnight. Then, the products were calcinated in a muffle furnace at 550 °C for 6 h. The resulting mixed oxides were denoted by the weight percentage of MoO3. For example, 5-MZS indicates that the MoO3 loading is 5 wt %, and 10-MZS has a MoO3 loading of 10 wt %. For comparison, ZrO2 was prepared from Zr(OH)4 by simple calcination at 550 °C for 6 h in air. Moreover, sulfonate zirconia (ZS) was prepared following the above procedures just without the addition of ammonium heptamolybdate. Catalyst Characterizations. X-ray diffraction (XRD) patterns of the catalysts were collected on a Bruker Advance 8 X-ray diffractometer, employing Cu Kα radiation. Diffractograms were recorded at a step width of 0.02° with 2θ in the range 10−80°. Fourier transform infrared (FTIR) spectra were carried out using a Digilab FTS 3100 instrument at room temperature over the range 400−2000

Figure 1. XRD patterns of sulfonated MoO3−ZrO2 binary oxide catalysts. △, monoclinic phase of ZrO2; ○, tetragonal phase of ZrO2; ∗, Zr(MoO4)2 phase. B


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ACS Sustainable Chemistry & Engineering increase of MoO3 loading, indicating that the strong interaction between ZrO2 and MoO3 impedes the transformation of amorphous ZrO2 from tetragonal phase ZrO2.33 The FTIR spectra of the as-synthesized catalysts are shown in Figure 2. The bands between 800 and 400 cm−1 were

Figure 4. Fructose dehydration to HMF over sulfonated MoO3−ZrO2 binary oxide catalysts. Reaction conditions: fructose, 200 mg; DMSO, 5 mL; catalyst, 10 mg; 110 °C; 2 h.

After the sulfonation of ZrO2, the fructose conversion and HMF yield significantly increase from 58% to 65% and 22% to 54%, respectively, demonstrating the strong effect of the acidity of the catalysts. The doping of molybdenum oxide can further increase the fructose conversion and HMF yield as high as 88% and 66%, respectively. However, the molybdenum oxide loading amount has different effects on the dehydration of fructose to HMF, especially the HMF yield. The highest yield of HMF (66%) was achieved by 5-MZS; then, the HMF yield decreases gradually with the further addition of molybdenum oxide loading. This should be attributed to the dual function of the molybdenum oxide. In fact, on one hand molybdenum oxide is a Lewis acid, and on the other hand it is a redox catalyst. The former plays a positive role in fructose dehydration to HMF, while the latter has an adverse effect due to the undesired fructose degradation and subsequent transformation of HMF to HMF-derived chemicals. The performance of the as-synthesized catalysts on selective oxidation of HMF to DFF with oxygen as the only oxidant is depicted in Figure 5. Typically, DMSO was used as the solvent to form 0.1 M HMF solution with 10 mg of catalyst. The concentration of each component was measured by HPLC. The molybdenum oxide loading has a profound effect on the oxidation of HMF to DFF. When ZS was used as the catalyst, HMF conversion of 11% was obtained with a DFF yield of lower than 4%. The loading of 5 wt % molybdenum oxide leads to a significant increase of HMF conversion to 49% and DFF yield to 41%. Additionally, HMF fructose and DFF yield

Figure 2. FTIR spectra of sulfonated MoO3−ZrO2 binary oxide catalysts.

assigned to ZrO2.34 The peaks at 995 cm−1 were attributed to the symmetric vibrations of S−O, and peaks at 1053, 1073,1158, and 1242 cm−1 were due to the asymmetric vibrations of O−S−O bonds.35 The bending vibration of physical adopted water associated with the metal oxide showed the peak around 1630 cm−1. The peak at 870 cm−1 was attributed to the stretching mode of Mo−O−Mo of surface MoO3. The high-resolution XPS spectra of the MZS are shown in Figure 3. The spin−orbit doublet peaks at around 183.2 and 185.7 eV correspond to Zr 3d5/2 and 3d3/2 electrons, respectively, in Figure 3a, suggesting that Zr atoms in the MZS composites have the +4 oxidation state, Zr4+ from ZrO2.36 In Figure 3b, the two peaks at 233 and 236 eV, respectively, corresponding to Mo 3d5/2 and Mo 3d3/2 suggest that the Mo species is MoO3. In addition, there are two small peaks at 231.2 and 234.4 eV, indicating that a tiny amount of Mo with a lower oxidation state (between VI and IV) is also formed.37 The as-synthesized catalysts were initially examined on the dehydration of fructose to HMF, as shown in Figure 4. Typically, DMSO was chosen as the solvent dissolving with 200 mg of fructose and 10 mg of catalyst for the reaction. The concentration of each component was measured by HPLC.

Figure 3. XPS spectra of (a) Zr 3d, and (b) Mo 3d of sulfonated MoO3−ZrO2 binary oxide catalyst. C


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yield when the MoO3 loading increases from 5 to 10 wt %. However, this is not the case with the higher molybdenum oxide loading where the decrease of HMF does not lead to the corresponding increase of DFF yield. It is possible that the high molybdenum oxide loading reinforces the transformation of DFF to other byproducts.25 To achieve the highest DFF yield, it is obvious that 10-MZS is the best. For an investigation into the effect of reaction temperature on the transformation efficiency of fructose to DFF, an experiment was conducted under different temperatures, and the yield of HMF/DFF was collected after 8.5 h. As shown in Figure 7, generally HMF yield decreases with the rise of the

Figure 5. Aerobic oxidation of HMF to DFF over sulfonated MoO3− ZrO2 binary oxide catalysts. Reaction conditions: HMF, 63 mg; DMSO, 5 mL; catalyst, 10 mg; O2 = 20 mL min−1; 140 °C; 17 h.

increase gradually with the addition of molybdenum oxide loading, reaching as high as 99% and 86%, respectively, under 30-MZS as illustrated in Figure 5. However, it can be seen that the increasing rates of HMF conversion and DFF yield are smaller than the incremental rate of molybdenum oxide loading. For example, instead of a double increase of DFF yield, the increase of molybdenum oxide loading from 10 to 20 wt % only led to a 42% increment (from 55% to 78%). The performance of the as-synthesized catalysts in the dehydration of fructose to HMF and aerobic selective oxidation of HMF to DFF suggests that MZS would be a promising candidate to achieve the direct synthesis of DFF from fructose. Therefore, in addition to the aforementioned analysis and discussion on the two-step reaction, the effects of molybdenum oxide loading on conversion of fructose and HMF/DFF yield were evaluated, as shown in Figure 6. Overall, it shows that

Figure 7. Effect of reaction temperature on 10-MZS-catalyzed onestep conversion of fructose to DFF. Reaction conditions: fructose, 200 mg; DMSO, 5 mL; catalyst, 10 mg; O2 = 20 mL min−1.

reaction temperature, while the yield of DFF increases accordingly. This result is consistent with our expectation that a higher temperature can accelerate the aerobic oxidation of HMF to DFF. However, the DFF yield slightly decreases from 70% to 67% when the reaction temperature increases from 150 to 160 °C. This may be attributed to the fact that DFF is not stable under the high temperature, and it may be converted to other byproducts.28 The above findings suggests that 150 °C is the best temperature for the synthesis of DFF from fructose directly. Figure 8 illustrates the reaction process of one-pot and onestep synthesis of DFF directly from fructose. Specially, the

Figure 6. Direct production of DFF from fructose over sulfonated MoO3−ZrO2 binary oxide catalysts. Reaction conditions: fructose, 200 mg; DMSO, 5 mL; catalyst, 10 mg; O2 = 20 mL min−1; 150 °C; 8.5 h.

fructose conversion reaches 100% under all the cases, while the sum of yield of HMF and DFF slightly decreases with the addition of molybdenum oxide loading. In particular, the amount of HMF decreases dramatically from 28% to less than 1%. The DFF yield reaches the highest value (70%) when 10MZS was used as the catalyst, and then marginally decreases to 63% when the MoO3 loading increases to 30 wt %. As expected, the increase of DFF yield comes with the decrease of HMF

Figure 8. Time course of one-pot and one-step transformation of fructose to DFF using 10-MZS. Reaction conditions: fructose, 200 mg; DMSO, 5 mL; catalyst, 10 mg; 150 °C; O2 = 20 mL min−1. D


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Scheme 1. Bifunctional Catalytic Mechanism for (a) Dehydration of Fructose to HMF over Brønsted Acid Sites of MZS and (b) Oxidation of HMF to DFF over Molybdenum Oxide Species of MZS

Figure 9. Recycling experiment for one-step conversion of fructose to DFF with catalyst (A) reused directly without calcination and (B) regenerated by calcination at 550 °C. Reaction conditions: fructose, 200 mg; DMSO, 5 mL; catalyst, 10 mg; 150 °C; O2 = 20 mL min−1; 8.5 h.

followed by washing and drying in an oven at 65 °C overnight before the next use, as plotted in Figure 9A. Overall, the catalyst maintains its activity for the conversion of fructose, giving steady fructose conversion of 100% during all the recycle runs. However, the composition of products changes significantly though HMF and DFF remain the main two products. It can be obviously seen that the proportion of HMF increases while that of DFF decreases accordingly with the increase of cycle number. The color of the reused catalysts becomes dark after the recycling test, suggesting that the carbonaceous deposit on the catalytically oxidative sites results in the low efficiency during the conversion of HMF to DFF. To eliminate the negative effect of the carbon deposit, another group of recycling test experiments were conducted following the same procedures except the addition of a calcination step for the recovery of the catalysts (Figure 9B). The recycled catalyst was calcined in air at 550 °C for 5 h to remove the carbon component before the next run. It can be clearly seen that the performance of the catalyst regenerated from the calcination is obviously different from that of the catalyst without the calcination treatment. Only a slight decrease of DFF yield is observed in the first two runs; then, the DFF yield remains stable, and no significant activity loss appears during the recycling test.

variations of fructose conversion, and HMF and DFF yield, are plotted against the reaction time. This reveals that the fructose dehydration goes very quickly, and fructose is almost depleted within 15 min, thus resulting in the instant increase of HMF concentration in the reaction system. After that, the reaction comes to the second stage (aerobic selective oxidation of HMF to DFF). With the progress of the reaction, the HMF decreases gradually from a yield of 75% to trace, accompanying the significant increase of the DFF yield, which reaches 74% after 10 h. After the reaction, the color of the reaction solution was dark brown, and some insoluble humin-like products were observed during the preparation of the sample for HPLC measurement; this may be the reason for the unbalanced carbons. We propose a bifunctional catalytic mechanism for the conversion of fructose to DFF, as shown in Scheme 1. The first step is similar to the mechanism of SO4/ZrO2-promoted dehydration of fructose to HMF reported by Kare Wilson et al.38 It can be seen that the Brønsted acid sites are the active species for the dehydration of HMF to DFF. The second step is similar to the redox mechanism of V-g-C3N4(H+)- or K-OMS2-promoted oxidation of HMF to DFF using molecular oxygen as the oxidant.22,25 The oxidation of HMF to DFF takes place in conjunction with the redox cycle of Mo(VI) and Mo(IV), in which the Mo(VI) is the active sites.39 Recycling of 10-MZS Catalysts. The reusability of the 10MZS was investigated as shown in Figure 9. The catalyst was separated from the reaction solution by filtration after each run,

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CONCLUSION The combination of sulfonate zirconia and molybdenum oxide is successfully employed to synthesize bifunctional catalysts. It E


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(7) Amarasekara, A. S.; Green, D.; McMillan, E. Efficient oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran using Mn(III)−salen catalysts. Catal. Commun. 2008, 9, 286−288. (8) El-Hajj, T.; Martin, J. C.; Descotes, G. Dér ivés de l’hydroxyméthyl-5 furfural. I. Synthése de dérivés du di-et terfuranne. J. Heterocycl. Chem. 1983, 20, 233−235. (9) Van Reijendam, J.; Heeres, G.; Janssen, M. Polyenyl-substituted furans and thiophenes. A study of the electronic spectraI. Tetrahedron 1970, 26, 1291−1301. (10) Cottier, L.; Descotes, G.; Viollet, E.; Lewkowski, J.; Skowroñski, R. Oxidation of 5-hydroxymethylfurfural and derivatives to furanaldehydes with 2, 2, 6, 6-tetramethylpiperidine oxide radical-co-oxidant pairs. J. Heterocycl. Chem. 1995, 32, 927−930. (11) Karimi, B.; Mirzaei, H. M.; Farhangi, E. Fe3O4@ SiO2−TEMPO as a Magnetically Recyclable Catalyst for Highly Selective Aerobic Oxidation of 5-Hydroxymethylfurfural into 2, 5-Diformylfuran under Metal-and Halogen-Free Conditions. ChemCatChem 2014, 6, 758− 762. (12) Mittal, N.; Nisola, G. M.; Malihan, L. B.; Seo, J. G.; Lee, S.-P.; Chung, W.-J. Metal-free mild oxidation of 5-hydroxymethylfurfural to 2, 5-diformylfuran. Korean J. Chem. Eng. 2014, 31, 1362−1367. (13) Partenheimer, W.; Grushin, V. V. Synthesis of 2, 5Diformylfuran and Furan-2, 5-Dicarboxylic Acid by Catalytic AirOxidation of 5-Hydroxymethylfurfural. Unexpectedly Selective Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde with Metal= Bromide Catalysts. Adv. Synth. Catal. 2001, 343, 102−111. (14) Carlini, C.; Patrono, P.; Galletti, A. M. R.; Sbrana, G. Heterogeneous catalysts based on vanadyl phosphate for fructose dehydration to 5-hydroxymethyl-2-furaldehyde. Appl. Catal., A 2004, 275, 111−118. (15) Carlini, C.; Patrono, P.; Galletti, A. M. R.; Sbrana, G.; Zima, V. Selective oxidation of 5-hydroxymethyl-2-furaldehyde to furan-2, 5dicarboxaldehyde by catalytic systems based on vanadyl phosphate. Appl. Catal., A 2005, 289, 197−204. (16) Liao, L.; Liu, Y.; Li, Z.; Zhuang, J.; Zhou, Y.; Chen, S. Catalytic aerobic oxidation of 5-hydroxymethylfurfural into 2, 5-diformylfuran over VO 2+ and Cu 2+ immobilized on amino-functionalized core−shell magnetic Fe3 O4@ SiO 2. RSC Adv. 2016, 6, 94976−94988. (17) Navarro, O. C.; Chornet, S. I. Chemicals from biomass: Aerobic oxidation of 5-hydroxymethyl-2-furaldehyde into diformylfurane catalyzed by immobilized vanadyl-pyridine complexes on polymeric and organofunctionalized mesoporous supports. Top. Catal. 2009, 52, 304−314. (18) da Silva, E. D.; Gonzalez, W. A.; Fraga, M. A. Aqueous-phase oxidation of 5-hydroxymethylfurfural over Pt/ZrO2 catalysts: exploiting the alkalinity of the reaction medium and catalyst basicity. Green Process. Synth. 2016, 5, 353−364. (19) Lilga, M. A.; Hallen, R. T.; Gray, M. Production of oxidized derivatives of 5-hydroxymethylfurfural (HMF). Top. Catal. 2010, 53, 1264−1269. (20) Ait Rass, H.; Essayem, N.; Besson, M. Selective aqueous phase oxidation of 5-hydroxymethylfurfural to 2, 5-furandicarboxylic acid over Pt/C catalysts: influence of the base and effect of bismuth promotion. Green Chem. 2013, 15, 2240−2251. (21) Takagaki, A.; Takahashi, M.; Nishimura, S.; Ebitani, K. One-pot synthesis of 2, 5-diformylfuran from carbohydrate derivatives by sulfonated resin and hydrotalcite-supported ruthenium catalysts. ACS Catal. 2011, 1, 1562−1565. (22) Yang, Z.-Z.; Deng, J.; Pan, T.; Guo, Q.-X.; Fu, Y. A one-pot approach for conversion of fructose to 2, 5-diformylfuran by combination of Fe3 O4-SBA-SO 3 H and K-OMS-2. Green Chem. 2012, 14, 2986−2989. (23) Halliday, G. A.; Young, R. J.; Grushin, V. V. One-Pot, Two-Step, Practical Catalytic Synthesis of 2,5-Diformylfuran from Fructose. Org. Lett. 2003, 5, 2003−2005. (24) Lv, G.; Wang, H.; Yang, Y.; Deng, T.; Chen, C.; Zhu, Y.; Hou, X. Direct synthesis of 2, 5-diformylfuran from fructose with graphene oxide as a bifunctional and metal-free catalyst. Green Chem. 2016, 18, 2302−2307.

is well-known that sulfonate zirconia is a solid superacid, and molybdenum oxide is widely used as redox catalysts. The bifunctional catalysts possessing both Brønsted/Lewis acidity and oxidability were applied for the one-step approach synthesis of DFF from fructose with molecular oxygen as the oxidant. The acidity and oxidability synergistically contribute to the dehydration of fructose to HMF and also the aerobic selective oxidation of HMF to DFF, respectively. The dehydration reaction results show that the existence of a certain amount of molybdenum oxide species could help to further promote the conversion of fructose to HMF, while excessive molybdenum oxide could also lead to the formation of undesired byproducts due to the direct oxidation of fructose. Though a higher molybdenum oxide loading always provides a higher DFF yield in the reaction of HMF oxidation, the efficiency of molybdenum oxide use decreases. Thus, the balanced 10-MZS exhibits the best catalytic performance for the one-pot and one-step synthesis of DFF. Under the optimized reaction conditions, DFF yield of 74% was obtained with a 100% fructose conversion at 150 °C in 10 h. The catalyst can be recycled and reused without any significant loss of activity, indicating its great potential for industrial and commercial application.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +65-6513-8129. Fax: +65 6794-7553. E-mail: jmlee@ ntu.edu.sg. ORCID

Jong-Min Lee: 0000-0001-6300-0866 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program, as well as the AcRF Tier 1 grant (RG17/16), Ministry of Education, Singapore.

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REFERENCES

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