Environmentally Friendly Oxidation of Biomass Derived 5

Bhaskar SarmahBiswarup SatpatiRajendra Srivastava. ACS Omega 2018 3 (7), ..... U. Chinna Rajesh , Divya Divya , Diwan S. Rawat. RSC Adv. 2014 4 (78), ...
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Environmentally Friendly Oxidation of Biomass Derived 5‑Hydroxymethylfurfural into 2,5-Diformylfuran Catalyzed by Magnetic Separation of Ruthenium Catalyst Shuguo Wang, Zehui Zhang,* Bing Liu, and Jinlin Li Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan, 430074, People’s Republic of China ABSTRACT: In this study, aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) was studied over a magnetic catalyst [Fe3O4@SiO2-NH2-Ru(III)]. Various reaction parameters were optimized for the oxidation of HMF into DFF. A high DFF of 86.4% and HMF conversion of 99.3% were obtained after 4 h at 120 °C. More importantly, the catalyst also showed high catalytic activity in air, and high HMF conversion of 99.7% and DFF yield of 86.8% were obtained after 16 h. The high catalytic performance of Fe3O4@SiO2-NH2-Ru(III) in air makes this method much more convenient and economical. Moreover, the procedure of the catalyst recycle was simple as the Fe3O4@SiO2-NH2-Ru(III) catalyst could be readily recovered from the reaction mixture by a permanent magnet. The catalyst could be reused several times without significant loss of its catalytic activity.

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INTRODUCTION With the diminishing of fossil fuels as well as the global warming caused by the emission of greenhouse gases, there is a growing interest in the production of fuels and chemicals from renewable resources. Biomass is the only carbon-containing renewable resource, which serves as the sole and valuable renewable resource to provide liquid fuels and bulk chemicals.1−3 In this context, much effort has been devoted to the conversion of biomass into organic chemicals and liquid fuels.4−6 5-Hydroxymethylfurfural (HMF), which is generated by the dehydration of C6-based carbohydrates, is considered to be a key platform molecule. It can be used as a versatile precursor for the production of high-energy-content fuels, chemicals, and polymers.7,8 Therefore, the synthesis of HMF from biomass has been extensively studied in various catalytic systems in recent years.9−13 Currently, there is a direction in biorefineries to produce valuable chemicals through the treatment of HMF. As shown in Scheme 1, oxidation of HMF can generate some important chemicals such as 2,5-furandicarboxylic acid (FDCA) and 2,5-diformylfuran (DFF).14−17 For instance, FDCA can be

used as a substitute for terephthalic, isophthalic, and adipic acids in the synthesis of furan-containing polymers. DFF as another important oxidation product of HMF is also considered as a potential chemical intermediate. It can be used as a precursor for the production of furanic polymers,18 pharmaceuticals,19 antifungal agents,20 and renewable furan− urea resin.21 In this context, significant attention has also been paid to the synthesis of DFF from HMF. However, as shown in Scheme 1, several furan compounds such as 5-formyl-2furancarboxylic acid (FFCA), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), FDCA, and DFF can be produced during the oxidation of HMF. Therefore, the selective oxidation of HMF to DFF is still challenging. In early reports, the oxidation of HMF into DFF was carried out in the presence of traditional oxidants such as NaOCl and pyridinium chlorochromate.22−24 However, those methods have some distinct drawbacks such as the release of toxic waste and the high cost of oxidants. At present, it is recognized that the oxidation process using molecular oxygen as a terminal oxidant is much more economical and environmentally friendly. It has been reported that catalytic oxidation of HMF by homogeneous metal bromide catalysts (MBr2, M = Co(II), Mn(II), Zr(II)) at 70 bar oxygen pressure gave 99.7% HMF conversion with 61% DFF selectivity.25 However, recycling of homogeneous catalysts is difficult. Thus the use of heterogeneous catalysts is really accepted as a sustainable biorefinery process. Sádaba et al. recently reported that the aerobic oxidation of HMF catalyzed by zeolite supported vanadium could give rise to high DFF selectivity of 99% and HMF conversion of 84% under 10 bar oxygen pressure.26 Ruthenium catalysts showed high catalytic activity in the aerobic oxidation of alcohols under mild

Scheme 1. Schematic Illustration of the Oxidation Products during the Oxidation of HMF

Received: Revised: Accepted: Published: © 2014 American Chemical Society

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constant analyzer pass energy of 25 eV. The binding energy was estimated to be accurate within 0.2 eV. All binding energies (BEs) were corrected referencing to the C 1s (284.6 eV) peak of the contamination carbon as an internal standard. The content of Ru was determined by inductively coupled plasma atomic emission spectroscopy on an IRIS Intrepid II XSP instrument (Thermo Electron Corp.). Magnetization measurement was performed by using a physical property measurement system (PPMS-9T) with VSM option from Quantum Design. Applied magnetic fields H between −30 and 30 kOe and temperature 300 K were used in the experiments. General Procedures for the Aerobic Oxidation of HMF to DFF. Aerobic oxidation of HMF was carried out in a 25 mL round-bottom flask, which was coupled with a reflux condenser and capped with a balloon. Typically, HMF (0.8 mmol, 100 mg) was added into toluene (7 mL), and the mixture was stirred vigorously to give a clear solution. Then Fe3O4@SiO2NH2-Ru(III) (100 mg) was added and flushed with pure oxygen at a flow rate of 20 mL min−1. The reaction was carried out at a given reaction time at 110 °C. After reaction, the Fe3O4@SiO2-NH2-Ru(III) catalyst was separated from the reaction mixture by a permanent magnet and products were analyzed by HPLC. Analytical Methods. Analyses of HMF and DFF were conducted on a VARIAN ProStar 210 HPLC system. Samples were separated by a reversed-phase C18 column (200 × 4.6 mm) with a detection wavelength of 280 nm. The mobile phase was constituted of acetonitrile and 0.1 wt % acetic acid aqueous solution (v:v = 30:70) at 1.0 mL min−1. The column oven temperature was kept at 25 °C. The contents of HMF and DFF in samples were calculated by the external standard calibration curve method, which were constructed based on the pure compounds.

conditions. Hydrotalcite-supported ruthenium catalyst (Ru/ HT) showed high catalytic activity for the activation of molecular oxygen in the oxidation of HMF with 92% DFF yield.27 However, recycling of heterogeneous catalysts by common methods such as filtration and centrifugation is tedious and time-consuming. Recently, the use of magnetic catalysts has attracted considerable attention.28 The magnetic catalysts can be readily separated from the reaction mixture by a permanent magnet, which is considered to be an environmentally benign separation approach. Herein, we pay particular attention to the immobilization of ruthenium on a functionalized silica-coated magnetic support with great catalytic and separable properties. The magnetic supported Ru(III) catalyst showed a good catalytic performance in the aerobic oxidation of HMF into DFF.

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MATERIALS AND METHODS Materials. RuCl3·xH2O (38.0−42.0 wt % Ru) and 3aminopropyltriethoxysilane (APTES) were purchased from Aladdin Chemicals Co. Ltd. (Beijing, China). FeSO4·7H2O (99.5%), FeCl3·6H2O (99.5%), and tetraethoxysilane (TEOS, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2,5-Diformylfuran was purchased from the J&K Chemical Co. Ltd. (Beijing, China). 5Hydroxymethylfurfural (98%) was supplied by Beijing Chemicals Co. Ltd. (Beijing, China). All the solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and freshly distilled before use. Acetonitrile (HPLC grade) was purchased from Tedia Co. (Fairfield, OH, USA). Catalyst Synthesis. First, silica-coated Fe3O4 nanoparticles (Fe3O4@SiO2) were used as a magnetic support, which was prepared and characterized as described in our previous work.29 Briefly, Fe3O4 nanoparticles were prepared by the coprecipitation of ferric and ferrous salts under a strong alkaline solution. Then, Fe3O4 was coated with a silica layer using TEOS as silica source to form Fe3O4@SiO2 under alkaline conditions. Fe3O4@ SiO2 was further modified with amino groups by the reaction of Fe3O4@SiO2 with 3-aminopropyltriethoxysilane (APTES) in dry toluene under N2 atmosphere at reflux temperature for 24 h. After reaction, the amino-functionalized solid (abbreviated as Fe3O4@SiO2-NH2) was separated by a permanent magnet and washed with toluene to remove the unreacted APTES. Then Fe3O4@SiO2-NH2 was dried in a vacuum at 80 °C overnight. Finally, the catalyst was prepared as follows: Fe3O4@SiO2NH2 (1.0 g) was added to 40 mL of an aqueous ruthenium chloride solution (3.61 × 10−3 mol/L). The mixture was treated under ultrasonic irradiation for 0.5 h at room temperature. The black solution turned colorless, which indicated that Ru(III) was completely anchored on Fe3O4@ SiO2-NH2. The catalyst was then collected from the solution by a permanent magnet, washed twice with distilled water and acetone, and dried in a vacuum (denoted as Fe3O4@SiO2-NH2Ru(III)). Catalyst Characterization. Fourier transform infrared (FT-IR) measurements were recorded on a Nicolet NEXUS6700 FTIR spectrometer with a spectral resolution of 4 cm−1 in the wavenumber range 500−4000 cm−1. X-ray powder diffraction (XRD) patterns of samples were determined with a Bruker Advance D8 powder diffractometer (Cu Kα). The scan ranges were 10−80° with 0.016° steps, respectively. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo VG Scientific ESCA MultiLab-2000 spectrometer with a monochromatized Al Kα source (1486.6 eV) at a

HMF conversion =

DFF yield =

mol of converted HMF × 100% mol of starting HMF

mol of DFF × 100% mol of starting HMF

Recycling of Catalyst. After reaction, the Fe3O4@SiO2NH2-Ru(III) catalyst was separated from the reaction mixture by a permanent magnet, washed three times with ethanol, and dried at 60 °C overnight in a vacuum oven. The spent catalyst was reused for the next cycle under the same reaction conditions.

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RESULTS AND DISCUSSION Catalyst Synthesis and Characterization. Scheme 2 illustrates the procedures of the catalyst preparation. Fe3O4@ SiO2 was prepared and characterized as described in our previous work.29 Then Fe3O4@SiO2 was modified with 3aminopropyl groups by the reaction with 3-aminopropyltriethoxysilane (APTES) in dry toluene under N2 at reflux for 24 h. The Fe3O4@SiO2-NH2-Ru(III) catalyst was prepared by the uptake of Ru3+ ions from RuCl3 aqueous solution using aminofunctionalized groups. During the reaction process, the black solution gradually became clear, which indicated that ruthenium(III) was anchored by Fe3O4@SiO2-NH2. The loading of ruthenium in the catalyst Fe3O4@SiO2-NH2-Ru(III) was determined to be 1.4 wt %. Under the same synthesis conditions for Fe3O4@SiO2-NH2-Ru(III), the nonfunctional5821

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Scheme 2. Schematic Illustration of the Preparation of Magnetically Recoverable Fe3O4@SiO2-NH2-Ru(III)

ized support Fe3O4@SiO2 was also treated with RuCl3 solution and it was found that the Ru content was 0.3 wt %. XRD diffraction patterns of Fe3O4@SiO2-NH2 and Fe3O4@ SiO2-NH2-Ru(III) are shown in Figure 1. There was no distinct

Figure 2. XPS spectra of the samples. (a) Survey scan of Fe3O4@SiO2NH2-Ru; (b) Ru 3p region.

SiO2 was successfully modified with 3-aminopropyl groups. Peaks belonging to Ru 3d and C 1s are overlapped around 280.4 eV.33 The O 1s peak with binding energy of 532.6 eV should be attributed to SiO2 rather than to RuO2, as in this case the O 1s would appear at a lower energy.34 The XPS spectrum of Ru 3p shows the binding energies of Ru 3p3/2 and Ru 3p1/2 core levels at 462.2 and 485.4 eV, respectively, which indicate that the oxidation state of the ruthenium species is +III (Figure 2b).35 It should be pointed out that there was no peak for chlorine, which suggests that chlorine was completely removed during the immobilization of the Ru(III) on Fe3O4@SiO2-NH2. These results strongly indicated that Ru(III) was actually coordinated to Fe3O4@SiO2-NH2. Fe3O4@SiO2-NH2 and Fe3O4@SiO2-NH2-Ru(III) were further characterized by FT-IR technology. As shown in Figure 3, there is no distinct difference between Fe3O4@SiO2-NH2 and Fe3O4@SiO2-NH2-Ru(III). A broad band around 1630 cm−1 is observed in the spectra of both Fe3O4@SiO2-NH2 and Fe3O4@SiO2-NH2-Ru(III), which should be attributed to physically adsorbed water and the vibration of O−H bands in the silanol group.36 The spectra of the two samples contain the typical Si−O−Si bands around 1000−1200 cm−1, which is assigned to the antisymmetric Si−O−Si stretching vibration. A band at 582 cm−1 is also observed in the two samples, which is attributed to Fe−O vibrations.37 Bands around 2920 cm−1 associated with the aliphatic CH vibration are also observed in the two samples, indicating the presence of the anchoring APTES on the silica surface. Moreover, the absorption band at 1558 cm−1, which almost overlapped with the bending vibration of the adsorbed H2O, was attributed to the NH bending

Figure 1. XRD patterns of Fe3O4@SiO2-NH2 and Fe3O4@SiO2-NH2Ru(III).

difference in the XRD patterns between Fe3O4@SiO2-NH2 and Fe3O4@SiO2-NH2-Ru(III). The diffraction peaks in the two samples were assigned to the planes of the inverse cubic spinel structure Fe3O4, which matched well with the standard Fe3O4 sample (JCPDS File No. 19-0629).30 In addition, XRD results indicated that there were no distinct reflections for Ru(0) or RuO2,31 which indicated that the valence state of Ru did not change during the catalyst preparation process. The surface composition of Fe3O4@SiO2-NH2-Ru was analyzed with XPS technology. As shown in Figure 2a, peaks corresponding to carbon, oxygen, nitrogen, silicon, and ruthenium are present in the survey scan of Fe3O4@SiO2NH2-Ru(III). It was reported that the broadening of the Fe 2p1/2 peak (∼711 eV) and Fe 2p3/2 peak (∼724 eV) on the high energy side was a characteristic of Fe2+ in Fe3O4.32 As shown in Figure 2, there are no peaks with binding energies at ∼711 and ∼724 eV. These results indicate that Fe3O4 nanoparticles were coated with a silica layer, and Fe3O4@ 5822

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Table 1. Effect of Solvents on Aerobic Oxidation of HMF into DFFa entry

solvent

HMF conv (%)

DFF yield (%)

DFF sel (%)

1 2 3 4 5 6 7 8

DMSO DMF MeCN ethanol MIBK toluene p-chlorotoluene trifluorotoluene

23.5 11.7 10.1 9.1 46.7 96.1 97.6 98.8

18.7 9.2 9.3 8.6 40.9 85.9 82.1 80.3

79.6 78.6 92.1 94.5 87.5 89.4 84.1 81.2

a

Reaction conditions: HMF (100 mg, 0.8 mmol) and Fe3O4@SiO2NH2-Ru(III) (100 mg) were added into the solvent (7 mL), and the reaction was carried out at 110 °C for 12 h with the flush of dioxygen at a speed of 20 mL min−1.

Figure 3. FT-IR spectra of the two samples. (a) Fe3O4@SiO2-NH2; (b) Fe3O4@SiO2-NH2-Ru(III).

1, entries 6 and 8). However, the selectivity of DFF slightly decreased in the order toluene, p-chlorotoluene, and trifluorotoluene. It seemed that higher polar aromatic solvents gave lower DFF selectivities. The reason might be that more side reactions such as overoxidation possibly occurred in higher polar solvents. The highest DFF yield of 85.9% with a selectivity of 89.4% was obtained in toluene after 12 h (Table 1, entry 6). In addition, a small amount of HMFCA and FDCA were also detected in toluene, and the yields were 5.4 and 2.1%, respectively. In order to give more insights into the oxidation of HMF into DFF, the time course of the HMF content and DFF yield was recorded during the process of the oxidation of HMF. As shown in Figure 5, the content of HMF gradually decreased, and the yield of DFF gradurally increased. These results indicated that HMF was gradually converted into DFF during the reaction process.

vibration of the NH2 groups. Therefore, FT-IR results also confirmed that propylamine groups were successfully grafted onto the surface of Fe3O4@SiO2. The magnetic property of the catalyst Fe3O4@SiO2-NH2Ru(III) was inverstigated using a physical property measurement system with VSM option. As shown in Figure 4, the

Figure 4. Hysteresis loops for the catalyst Fe3O4@SiO2-NH2-Ru(III) at 300 K.

isothermal magnetization curve of the catalyst Fe3O4@SiO2NH2-Ru(III) at 300 K displayed a rapid increase with increasing applied magnetic field due to superparamagnetic relaxation. The saturation magnetization reached up to 37 emu/g. Aerobic Oxidation of HMF into DFF in Various Solvents. The catalytic activity of Fe3O4@SiO2-NH2-Ru(III) was evaluated by the aerobic oxidation of HMF into DFF. The aerobic oxidation of HMF was initially carried out in various solvents in order to screen the most suitable reaction media, and the results are shown in Table 1. Both HMF conversion and DFF selectivity were low when reactions were carried out in strong polar solvents with high boiling points such as dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) (Table 1, entries 1 and 2). Aprotic acetonitrile (MeCN) and protic ethanol with low boiling points also gave low HMF conversion, but gave high DFF selectivity (Table 1, entries 3 and 4). Methyl isobutyl ketone (MIBK) gave a medium DFF yield of 40.9% with HMF conversion of 46.7% after 12 h (Table 1, entry 5). Among the testing solvents, aromatic solvents were superior to other solvents in terms of HMF conversion (Table

Figure 5. Time course of product distribution during oxidation of HMF.

Effect of Reaction Temperature on Aerobic Oxidation of HMF into DFF. The effect of reaction temperature on the oxidation of HMF was studied, and reactions were carried out at 70, 90, 110, and 130 °C, respectively. As shown in Figure 6, the reaction temperature showed a remarkable effect on the HMF conversion, DFF selectivity, and DFF yield. The higher the reaction temperature was, the higher the HMF conversion was. HMF conversion was 35.6% after 12 h at 70 °C with a DFF yield of 32.0%. However, HMF conversions increased to 66.0 and 96.1% for reaction temperatures at 90 and 110 °C, respectively, and the corresponding DFF yields were 57.7 and 5823

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HMF. The increase of HMF conversion with an increase of the catalyst amount should be attributed to an increase in the availability and number of catalytically active sites. However, DFF selectivity was almost the same around 87% for all the different catalyst amounts. These results clearly indicated that the catalyst amount had little influence on DFF selectivity. The selectivity of DFF mainly depended on other reaction conditions such as the reaction temperature, the solvent, and oxidants. Oxidation of HMF into DFF Using Various Oxidants. Catalytic oxidation of HMF into DFF was carried out under various oxidation conditions, and the results are shown in Table 2. First, t-BuOOH and H2O2 as the common oxidants were also Figure 6. Aerobic oxidation of HMF into DFF at different reaction temperatures. Reaction conditions: HMF (100 mg, 0.8 mmol) and Fe3O4@SiO2-NH2-Ru(III) (100 mg) were added into toluene (7 mL), and the reaction was carried out at different reaction temperatures for 12 h with the flush of dioxygen at a speed of 20 mL min−1.

Table 2. Catalytic Oxidation of HMF into DFF Using Various Oxidantsa

85.9%. Further increasing the reaction temperature to 130 °C, HMF conversion of 99% was achieved after 8 h, but the DFF yield was only 80.1%, which was a little lower than that obtained at 110 °C. It was noted that the DFF selectivity at 130 °C was lower than those obtained at 70, 90, and 110 °C, respectively. The following two reasons might be helpful in understanding the lowest selectivity at 130 °C. On the one hand, side reactions during the oxidation process such as overoxidation of HMF into FDCA would reduce DFF selectivity. On the other hand, the starting material HMF is not stable at high temperature.38 Effect of Catalyst Loading on Aerobic Oxidation of HMF into DFF. Figure 7 shows the effect of the catalyst

entry

oxidant

time (h)

HMF conv (%)

DFF yield (%)

DFF sel (%)

1 2 3 4b 5c 6c 7d

t-BuOOH H2O2 O2 O2 air air O2

6 12 12 12 12 16 12

100 8.2 96.1 94.7 77.5 99.7 2.1

17.4 5.7 85.9 83.5 68.3 86.8 0

17.4 69.5 89.4 88.2 88.1 87.1 0

a

Reaction conditions: HMF (100 mg, 0.8 mmol) and Fe3O4@SiO2NH2-Ru(III) (150 mg) were added into toluene (7 mL), and the reaction was carried out at 120 °C using various oxidants. bWith oxygen balloon. cIn air. dFe3O4@SiO2-NH2 (150 mg) was used as the catalyst.

used for the oxidation of HMF. HMF conversion up to 100% was obtained after 6 h using t-BuOOH as the oxidant, but DFF was only obtained in a low yield of 17.4% (Table 2, entry 1). Furthermore, other oxidation products such as HMFCA and FDCA were not detected. One important reason might be that the breakage of the furan ring occurred, due to the strong oxidative ability of t-BuOOH. In contrast to t-BuOOH, the mild oxidant H2O2 could not effectively catalyze the oxidation of HMF, and low DFF yield of 5.7% and HMF conversion of 8.2% were reached after 12 h (Table 2, entry 2). Molecular oxygen as a cheap and clean oxidant was found to be the best oxidant for the oxidation of HMF into DFF (Table 2, entries 3−6). High HMF conversion of 96.1% and DFF yield of 85.9% were obtained after 12 h, when molecular oxygen was flushed at a flow rate of 20 mL min−1 (Table 2, entry 3). Interestingly, similar results were also obtained when the reaction was conducted under an oxygen atmosphere by the use of an oxygen balloon (Table 2, entries 3 and 4). The results indicated that the oxygen concentration in the toluene with the oxygen balloon was almost the same as that with the flush of oxygen. The excellent catalytic performance of Fe3O4@SiO2-NH2Ru(III) under the oxygen balloon encouraged us to conduct the oxidation of HMF in air. As shown in Table 2, the catalyst also showed a good catalytic activity in air, and HMF conversion of 77.5% and DFF yield of 68.3% were obtained after 12 h (Table 2, entry 5). The relatively poor catalytic performance in air should be attributed to the low concentration of molecular oxygen. With further increase of the reaction time to 16 h, DFF could be obtained in a high yield of 86.8% with HMF conversion of 99.7 (Table 2, entry 6). The use of air as the oxidant makes the developed method

Figure 7. Aerobic oxidation of HMF into DFF with different catalyst amounts. Reaction conditions: HMF (100 mg, 0.8 mmol) and a set amount of Fe3O4@SiO2-NH2-Ru(III) were added into toluene (7 mL), and the reaction was carried at110 °C for 4 h with the flush of oxygen at a speed of 20 mL min−1.

amount on the oxidation of HMF into DFF. HMF conversion and DFF yield increased with an increase of the catalyst amount. A DFF yield of 21.6% was obtained after 4 h by the use of 25 mg of Fe3O4@SiO2-NH2-Ru(III). DFF yields increased to 39.8 and 70.1% after 4 h when 50 and 100 mg of the catalyst were used, respectively. With further increase of the catalyst amount to 150 mg, the maximum DFF yield of 86.4% was achieved after 4 h, with HMF conversion of 99.3%. These results clearly indicated that an increase of the catalyst amount resulted in an increase of the aerobic oxidation rate of 5824

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Table 3. Catalytic Oxidation of HMF into DFF Using Various Methods entry

catalyst

1 2 3 4 5 6

Ru/C Ru/γ-alumina Ru/hydrotalcite VO2+/Cu2+ immobilized on sulfonated carbon catalyst V2O5/H-β Fe3O4@SiO2-NH2-Ru(III)

reaction conditions 383 403 393 413 398 393

K, K, K, K, K, K,

2.0 MPa of O2 40 psi O2 1 bar of O2 at 20 mL/min 40 bar of Ar 40 bar of O2 1 bar of O2 at 20 mL/min

HMF conv (%)

DFF yield (%)

ref

100 100 94.8 100 92 96.1

96.0 97.0 92.0 98.0 81 85.9

38, 39 40 27 26 41 this work

product DFF in the residue. The residue mainly contained DFF, HMFCA, and FDCA. As shown in Scheme 1, the structures of DFF, HMFCA, and FDCA are quite different. The polarities of FDCA and HMFCA are much stronger than that of DFF. When the reaction mixture is separated on a silica gel, DFF with the lowest polarity shoud be first washed off by the eluting solvent (such as ethyl acetate) due to the short retention time on silical gel. The reused catalyst and the solvent toluene could be recycled for the next cycle. The recycle of Fe3O4@SiO2-NH2-Ru(III) was conducted, and the aerobic oxidation of HMF into DFF was used as a model reaction. Reactions were carried out at 110 °C with 150 mg of Fe3O4@SiO2-NH2-Ru(III) for 4 h. The reused catalyst was washed by toluene and ethanol to remove the adsorbed products, and dried under vacuum at 80 °C overnight. Then the reused catalyst was used for the next cycle under otherwise identical reaction conditions. As shown in Figure 9, the DFF

convenient and economical for the synthesis of DFF from HMF. In addition, the oxidation of HMF was also carried out in the presence of Fe3O4@SiO2-NH2 under otherwise identical conditions. There were no oxidation products when Fe3O4@ SiO2-NH2 was used as the catalyst. The results indicated that the support showed no catalytic activity for the oxidation of HMF. The Ru(III) is the active site in the aerobic oxidation of HMF. Comparison of Our Method with Other Reported Methods. The catalytic performance of Fe3O4@SiO2-NH2Ru(III) was compared with that using other previously reported methods, and the results are shown in Table 3. The heterogeneous Ru catalysts generally showed high HMF conversion and DFF yield (Table 3, entries 1−3), but some cases required high pressure (Table 3, entries 1−2), which was potentially dangerous in practical application. Non-noble based heterogeneous catalysts have also been reported for the oxidation of HMF. The vanadium based catalysts had some distinct drawbacks such as the requirement of high pressure, the need for a cocatalyst (Table 3, entry 4), and the loss of catalytic activity (Table 3, entry 5). Albeit our method showed a little lower DFF yield as compared with some of the abovementioned methods, it demonstrated a unique advantage (Table 3, entry 6). The Fe3O4@SiO2-NH2-Ru(III) catalyst could be readily separated from the reaction mixture by an external magnet. Catalyst Recycling Experiments. Developing a convenient and economical method to reuse the catalyst and the solvent is a very crucial goal in terms of green chemistry. The main advantage of magnetic catalysts is that they can be readily separated from the reaction mixture by an external magnet. As shown in Figure 8, the reaction mixture was muddy after reaction, but it quickly became clear with the assistance of a magnet. Then the catalyst was separated from the reaction mixture by decantation. The solvent toluene was distilled from the reaction mixture under a reduced pressure, leaving the

Figure 9. Recycle experiments of catalyst Fe3O4@SiO2-NH2-Ru(III). Reaction conditions: HMF (100 mg, 0.8 mmol) and Fe3O4@SiO2NH2-Ru(III) (150 mg) were added into toluene (7 mL), and the reaction was carried out at 110 °C for 4 h with the flush of oxygen at a speed of 20 mL min−1.

yield was almost stable (86.4% in the first cycle versus 80.8% in the sixth cycle), which indicated that the catalyst could be reused without significant decrease of catalytic activity. Although nonmagnetic heterogeneous catalysts could also be reused, some heterogeneous catalysts lost their catalytic activity during the recycling experiments, due to the inevitable loss of catalyst mass during the catalyst recycling process by the conventional filtration method.42 Compared with the filtration method, the method for the recovery of the catalyst by a permanent magnet obviously avoids the catalyst loss during the recycling experiments.

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CONCLUSION In summary, a new method was developed in this study for the aerobic oxidation of HMF into DFF. The magnetic Fe3O4@

Figure 8. Schematic illustration of procedures for recycling of catalyst and solvent. 5825

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SiO2-NH2-Ru(III) catalyst showed high catalytic activity for the aerobic oxidation of HMF into DFF in toluene. High DFF yield of 86.4% and HMF conversion of 99.3% were obtained in toluene after 4 h at 120 °C under atmospheric oxygen. More importantly, the oxidation of HMF could occur smoothly in air. High HMF conversion of 99.7% and DFF yield of 86.8% were also obtained in air after 16 h, which is facile and economical. Moreover, the catalyst recycling experiment was simple to operate by a permanent magnet, and it could be reused several times without significant decrease of catalytic activity. The environmentally friendly method described herein should be promising to facilitate cost-effective conversion of biomass derived precursors into valuable chemicals.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-27-67842572. Fax: +86-27-67842572. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the National Natural Science Foundation of China (No. 21203252) and the Special Fund for Basic Scientific Research of Central Colleges, South-Central University for Nationalities (CYZ 10006).

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