Nano Ce–Mo Composite Oxides as Effective

Mar 29, 2017 - Then, the precipitate was cooled to room temperature and washed thoroughly with absolute alcohol and finally calcined at 600 °C for 2 ...
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Research Article pubs.acs.org/journal/ascecg

3D Flower-like Micro/Nano Ce−Mo Composite Oxides as Effective Bifunctional Catalysts for One-Pot Conversion of Fructose to 2,5Diformylfuran Zhenzhen Yang,† Wei Qi,*,†,‡,§ Rongxin Su,†,‡,§ and Zhimin He† †

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, 135 Yaguan Road, Jinnan District, Tianjin 300072, P. R. China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 92 Weijin Road, Nankai District, Tianjin 300072, P. R. China § Tianjin Key Laboratory of Membrane Science and Desalination Technology, 135 Yaguan Road, Jinnan District, Tianjin, 300072, China S Supporting Information *

ABSTRACT: Three-dimensional flower-like Ce−Mo micro/ nano composite oxides with different Ce/Mo atomic ratios (fCe10‑xMoxOδ) were synthesized as effective bifunctional catalysts for one-pot conversion of fructose to 2,5-diformylfuran (DFF) via dehydration and aerobic oxidation. The catalytic activities of f-Ce10‑xMoxOδ depended on their structure and physicochemical properties, which were closely related to their elementary compositions. The f-Ce9Mo1Oδ, which possesses polymeric octahedral MoOz species, the highest Oα ratio, BET surface area, and acidity density, exhibited the highest catalytic activity for DFF production. High DFF yields of 94% and 74% were obtained from f-Ce9Mo1Oδ catalyzed HMF oxidation and “one-pot” fructose conversion, respectively. Furthermore, f-Ce9Mo1Oδ is easily prepared, has low cost, and is ecofriendly, which has potential application in industrial production of DFF from fructose. KEYWORDS: 5-Hydroxymethylfurfural, 2,5-Diformylfuran, Fructose, Ce−Mo composite oxides, f-Ce9Mo1Oδ

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The “one-pot” conversion of fructose to DFF involves two steps: fructose dehydration and subsequently HMF oxidation, which performs on acid and redox catalytic sites, respectively. One feasible strategy for conversion of fructose to DFF was combining the acid and redox catalysts by stepwise addition, such as the previously reported H-form cation exchanger resin and V-based compound,23 Amberlyst-15 and Ru/HT,24 Fe3O4−SBA−SO3H and K−OMS−2,25 Fe3O4−RGO−SO3H and ZnFe1.65Ru0.35O426 catalytic systems. However, the stepwise addition of catalysts complicated a potential implementation for industrial production. From this point of view, bifunctional catalysts that integrate the acid and redox functions were preferable. The reported bifunctional catalysts including Mo-27 or Mo−V-containing Keggin heteropolyacid,28,29 proton- and vanadium-containing graphitic carbon nitride (V-g-C 3N 4 (H+)),30 and GO31 exhibited considerable activities for “onepot” conversion of fructose to DFF. Regrettably, in the preparation of these bifunctional catalysts, toxic V-based compounds or strong corrosive acids were inevitably used,

INTRODUCTION In recent years, synthesis of high-value added chemicals from renewable carbon-containing biomass resources has become a research hotspot, as fossil resources have been depleted drastically and energy consumption has increased rapidly.1,2 2,5-Diformylfuran (DFF), which can be obtained from carbohydrate-based biomass by using 5-hydroxymethylfurfural (HMF) as a “bridge” molecule, has potential applications in pharmaceuticals,3 antifungal agents,4 macrocyclic ligands,5 and polymeric materials.6 So, effective synthesis of DFF has recently attracted much attention. DFF was commonly synthesized from selective oxidation of pure HMF by using stoichiometric oxidants7,8 or diverse catalysts including homogeneous metal salts9−11 and heterogeneous metal or metal oxide catalysts.12−22 Among them, V-,12−14 Mn-,15,16 Ru-,17−20 and Co-based21,22 metal catalysts showed considerable catalytic activities for the oxidation of HMF to DFF in various catalytic systems. However, currently the high energy cost for isolation and purification of HMF in its production limited large-scale synthesis of DFF from pure HMF. With regard to the cost and green chemistry, fructose was an ideal alternative raw material for DFF production by a “one-pot” method. © 2017 American Chemical Society

Received: January 17, 2017 Revised: March 11, 2017 Published: March 29, 2017 4179

DOI: 10.1021/acssuschemeng.7b00175 ACS Sustainable Chem. Eng. 2017, 5, 4179−4187

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ACS Sustainable Chemistry & Engineering

Jarell-Ash; USA). Brunauer−Emmett−Teller (BET) surface areas were measured on a Micromeritics ASAP 2020 (Micromeritics; USA) using nitrogen adsorption at 77 K. The Raman spectra were measured using an inVia reflex microscope (Renishaw; England) equipped with an aircooled charge-coupled device (CCD) camera. The temperatureprogrammed desorption (TPD) of NH3 was performed on a chemisorption analyzer under a 10% NH3/He gas flow (50 mL/ min) at a rate of 10 o/min up to 550 °C. Aerobic Oxidation of HMF to DFF. In a typical procedure, HMF (126 mg, 1 mmol), catalyst (6 mol % Mo), and DMSO (4 mL) were added into a glass tube equipped with a total reflux condenser to avoid solvent evaporation. The reaction was performed under a constant O2 flow (10 mL min−1) and maintained at the reaction temperature for a specific time under vigorous stirring. Finally, the reaction was quickly terminated by cooling the reactor to room temperature in an ice bath, and the sample was taken from the mixture for product analysis. Fructose Dehydration to HMF. In a typical procedure, fructose (180 mg), catalyst, and DMSO (4 mL) were added into a glass tube equipped with a total reflux condenser to avoid solvent evaporation. The reaction was performed under a constant N2 flow (10 mL min−1) and maintained at the reaction temperature for a specific time under vigorous stirring. Finally, the reaction was quickly terminated by cooling the reactor to room temperature in an ice bath, and the sample was taken from the mixture for product analysis. “One-Pot, One-Step” Synthesis of DFF from Fructose. In a typical procedure, fructose (180 mg), f-Ce9Mo1Oδ (6 mol % Mo), and DMSO (4 mL) were added into a glass tube equipped with a total reflux condenser to avoid solvent evaporation. Then, the reaction mixture was maintained at 120 °C under a constant O2 flow (10 mL min−1). Finally, the reaction was quickly terminated by cooling the reactor to room temperature in an ice bath, and the sample was taken from the mixture for product analysis. “One-Pot, Two-Step” Synthesis of DFF from Fructose. In a typical procedure, fructose (180 mg), f-Ce9Mo1Oδ (6 mol % Mo), and DMSO (4 mL) were added into a glass tube equipped with a total reflux condenser to avoid solvent evaporation. The dehydration reaction was first performed under a constant N2 flow (10 mL min−1) at 120 °C for 2 h, and the oxidation reaction was then triggered by exchanging the N2 flow with O2 flow (10 mL min−1). Finally, the reaction was quickly terminated by cooling the reactor to room temperature in an ice bath, and the sample was taken from the mixture for product analysis. Recycling Experiment. In a typical procedure, after being separated from the reaction mixture by filtration, the catalyst was washed with H2O several times and dried at 80 °C overnight. Then, the catalyst was reused for the next cycle under the optimum reaction conditions. Analytical Methods. After the reaction, the mixture was diluted to 100 mL with deionized water and filtered using PTFF filters (0.22 μm). The liquid products were analyzed by HPLC (Agilent 1260 Infinity) with an instrument equipped with a refractive index (RI) detector and Aminex HPx-87H Ion Exclusion column (7.8 mm × 300 mm) using a dilute H2SO4 solution (0.004 M) as the eluent at a flow rate of 0.6 mL/min. The hexose conversion, HMF, DFF, levulinic acid (LA) and formic acid (FA) yields were calculated on the basis of external standard curves constructed with authentic standards.

which do not conform to the current subject of environment protection. Thus, highly effective, easily prepared, and ecofriendly bifunctional catalysts are still urgently needed in the field of “one-pot” conversion of fructose to DFF. Recently, metal-oxide catalysts were important because of their low cost, good thermal stability, and easy preparation. For example, bimetallic Ce−Mo composite oxides were widely investigated in the field of heterogeneous catalysis as acid or redox catalysts.32−34 However, to our best knowledge, there has been no report on the “one-pot” conversion of fructose to DFF using such a bifunctional Ce−Mo composite oxide. On the other hand, catalytic activity greatly depends on the catalyst’s structure properties such as morphology, surface area, and interaction strength between components. 3D micro/nano materials are composed of nanosized building blocks, while the total size is on the micrometer scale. Such a hierarchical structure with the cooperation of microstructure and nanostructure can effectively inhibit aggregation, thus retaining high activity and ease of separation or recycling compared to that of common nanoparticles. On the basis of the above reasons, we herein report 3D flower-like micro/nano Ce−Mo composite oxides with different Ce/Mo atomic ratios (f-Ce10‑xMoxOδ) as catalysts for “onepot” conversion of fructose to DFF via dehydration and aerobic oxidation. The f-Ce10‑xMoxOδ catalysts were synthesized through an ethylene−glycol-mediated process, and their structure and physicochemical properties were well characterized. Step-wise studies demonstrated the activities of fCe10‑xMoxOδ catalysts depended closely on their acidity density and structure properties in fructose dehydration and HMF oxidation, respectively. Here f-Ce9Mo1Oδ, which possesses polymeric octahedral MoOz species, the highest Oα ratio, BET surface area, and acidity density, exhibited the highest catalytic activity for DFF production.

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EXPERIMENTAL SECTION

Materials. Dimethyl sulfoxide (DMSO) and ethylene glycol (EG) were purchased from Tianjin Guangfu Chemical Reagent Co., Ltd. Ammonium heptamolybdate ((NH4)6Mo7O24·6H2O), cerium nitrate (Ce(NO3)3·6H2O), urea, tetrabutylammonium bromide (TBAB), fructose, and HMF were purchased from Aladdin Reagent Co., Ltd. DFF was purchased from TCI Development Co., Ltd. Preparation of 3D Flower-like Micro/Nano Ce−Mo Composite Oxide. In a typical procedure, 0.036 mol of Ce(NO3)3·6H2O, 0.004 mol of (NH4)6Mo7O24·6H2O, 0.019 mol of TBAB, and 0.037 mol of urea were dissolved to 150 mL of EG in a round-bottomed flask. The flask was heated in an oil bath to 180 °C and allowed to equilibrate for 2 h under magnetic stirring to form a precipitate. Then, the precipitate was cooled to room temperature and washed thoroughly with absolute alcohol and finally calcined at 600 °C for 2 h to obtain the 3D flower-like Ce−Mo composite oxide, denoted as f-Ce9Mo1Oδ. By varying the molar ratio of Ce(NO3)3·6H2O/ (NH4)6Mo7O24·6H2O to 1/0, 9.5/0.5, 8/2, and 7/3, different 3D flower-like Ce−Mo composite oxides were prepared, denoted as fCeO2, f-Ce9.5Mo0.5Oδ, f-Ce8Mo2Oδ, and f-Ce7Mo3Oδ, respectively. Catalyst Characterization. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed using an S-4800 microscope (HITACHI; Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed using a JEM-2100F (JEOL; Japan) operated at 200 kV. The X-ray diffraction (XRD) spectra were obtained using a Rigaku D/Max-2500 X-ray diffractometer (Rigaku; Japan) with Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) data were collected using a PHI-5000 versa probe (Ulvac-Phi; Japan) with Al Kα radiation. The inductively coupled plasma mass spectrometry (ICPMS) data were obtained from an ICP-9000 spectrometer (Thermo

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RESULTS AND DISCUSSION Catalyst Characterization. A scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) was used to analyze the morphologies, element compositions, and distributions of the f-Ce10‑xMoxOδ samples. The SEM images of f-Ce9 Mo 1 O δ illustrated that the construction units were micrometer-scale 3D flower-like structures, which were constituted by multiple layers of nanopetals (Figure 1a and b). The EDS mapping images showed the homogeneous distributions of Ce, Mo, and O elements in the overall 3D flower-like hierarchical structure 4180

DOI: 10.1021/acssuschemeng.7b00175 ACS Sustainable Chem. Eng. 2017, 5, 4179−4187

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Figure 2. TEM images (a and b), high-magnification TEM image (c), and selected area electron diffraction pattern (d) of f-Ce9Mo1Oδ. Figure 1. Low-magnification SEM image (a), high-magnification SEM images of f-Ce9Mo1Oδ (b and c), and EDS element mapping of Ce (d), Mo (e), and O (f) from image c.

(Figure 1c−f). The EDS quantitative analysis showed that the Ce/Mo molar ratio in the as-synthesized f-Ce9Mo1Oδ was 9/ 0.95 (Figure S1), which was quite in accordance with the ICPMS result (9/0.93) shown in Table S1. When the Ce/Mo molar ratio in the precursors was varied to 1/0, 9.5/0.5, 8/2, and 7/3, the 3D flower-like hierarchical structure could also be formed (Figure S2). In addition, the Ce/Mo molar ratios in the f-Ce10‑xMoxOδ samples were obtained by both EDS and ICPMS analyses, and the results are shown in Table S1. The TEM images of f-Ce9Mo1Oδ are shown in Figure 2, which further illustrated that the micrometer-scale 3D flowerlike structure consisted of many interconnected nanoparticles (Figure 2a and b). The results verified that f-Ce9Mo1Oδ was a type of micro/nano material, possessing the advantages of both microstructure and nanostructure. From the high-magnification TEM image and the selected area electron diffraction analysis of f-Ce9Mo1Oδ (Figure 2c and d), the lattice fringes were clearly visible with spacings of 0.31, 0.28, 0.19, and 0.17, which were agreement with spacings of the (111), (200), (220), and (311) planes of CeO2. The XRD patterns of the f-Ce10‑xMoxOδ catalysts are shown in Figure 3. All the peaks can be attributed to the cubic fluorite phase CeO2 (PDF# 34-0394), and no diffraction peaks attributed to the MoO3 phase can be found. Compared to pure CeO2, there was no obvious shift at the positions of CeO2 diffraction peaks in f-Ce10‑xMoxOδ catalysts. However, the diffraction peaks broadened, and their intensities decreased with an increase in the Mo/Ce atomic ratio, suggesting that molybdena species were in amorphous states or highly dispersed on the surface of CeO2. The Raman spectra of f-Ce10‑xMoxOδ catalysts are shown in Figure 4. The main peak at 462 cm−1 was attributed to the F2g mode of oxygen atoms in the cubic phase CeO2, and the peaks at 259 and 598 cm−1 were attributed to a second-order transverse acoustic mode and a defect-induced mode of CeO2,

Figure 3. XRD patterns of f-Ce10‑xMoxOδ catalysts.

Figure 4. Raman spectra of the f-Ce10‑xMoxOδ catalysts: (a) f-CeO2, (b) f-Ce9.5Mo0.5Oδ, (c) f-Ce9Mo1Oδ, (d) f-Ce8Mo2Oδ, and (e) fCe7Mo3Oδ.

respectively. No characteristic bands of crystalline MoO3 (665, 818, and 995 cm−1) were observed for all the f-Ce10‑xMoxOδ samples, indicating that molybdena species were well dispersed on the surface of CeO2. These results were quite consistent with XRD analysis. For f-Ce10‑xMoxOδ (x = 0.5, 1, 2, and 3), the band at approximately 800 cm−1 was attributed to the 4181

DOI: 10.1021/acssuschemeng.7b00175 ACS Sustainable Chem. Eng. 2017, 5, 4179−4187

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ACS Sustainable Chemistry & Engineering stretching vibration of Mo−O−X (X = Ce or Mo) of the surface molybdena species, and the frequencies keep stay unchanged. The bands at 902−970 cm−1 were attributed to the stretching vibration of terminal Mo = O of the surface molybdena species. The frequencies of terminal Mo = O were quite different in f-Ce10‑xMoxOδ (x = 0.5, 1, 2, and 3), indicating different structures of molybdena species formed on the surface of these catalysts. According to previous reports,34−36 for fCe9.5Mo0.5Oδ that has a low Mo/Ce atomic ratio, the Raman band at 920 cm−1 indicated isolated tetrahedral molybdena species (MoOy) formed on the surface of CeO2. By decreasing the Ce/Mo atomic ratio to 9/1 (f-Ce9Mo1Oδ), the band of terminal Mo = O shifted to 965 cm−1, indicating that polymeric octahedral molybdena species (MoOz) were formed. Further decreasing the Ce/Mo atomic ratio to 8/2 (f-Ce8Mo2Oδ) and 7/3 (f-Ce7Mo3Oδ), the band of terminal Mo = O detected at 945 cm−1 and a new band appeared at 327 cm−1 were attributed to the cerium molybdena compond (Ce2Mo4O15) with a molybdenum atom in an octahedral position.32 The Raman results indicated that the structures of molybdena species depended closely on the elementary composition of fCe10‑xMoxOδ. Oxygen species play an important role in oxidation reactions. From the XPS spectra of O 1s for the f-Ce10‑xMoxOδ catalysts (Figure 5), the O 1s peaks can be fitted into two peaks. The

Figure 6. NH3-TPD profiles for the f-Ce10‑xMoxOδ catalysts in the temperature range of 80−400 °C.

the increase in the weak acid sites in the catalyst. The acidity density is shown in Table S1; when the Ce/Mo atomic ratio decreased from 1/0 to 7/3 gradually, the acidity density first increased and then decreased, with f-Ce9Mo1Oδ possessing the highest acidity density. Table S1 also lists the BET surface areas of the fCe10‑xMoxOδ catalysts. Interestingly, the BET surface area, surface acidity density, and Oα ratio have a similar variation tendency versus the Ce/Mo atomic ratio. These may be influenced by the surface molybdena species, with isolated tetrahedral MoOy species and polymeric octahedral MoOz species enhancing the BET surface area, surface acidity, and O α ratio. Inversely, the cerium molybdena compound (Ce2Mo4O15) weakened these. The physicochemical properties may further affect the catalytic performance of f-Ce10‑xMoxOδ in fructose dehydration and HMF oxidation reactions. Aerobic Oxidation of HMF to DFF. The f-Ce10‑xMoxOδ samples were first evaluated for catalyzing the oxidation of HMF to DFF (Table 1). Compared to the blank experiment in Table 1. Aerobic Oxidation of HMF to DFF over Different Catalystsa

Figure 5. XPS spectra of O 1s for the f-Ce10‑xMoxOδ catalysts.

peaks at 529.0−530.3 eV can be attributed to the lattice oxygen (denoted as Oβ), and the peaks at 531.1−532.3 eV were assigned to the surface chemisorbed oxygen (denoted as Oα). The Oα was reported to be highly active in oxidation reactions because of its higher mobility than Oβ.33 From Table S1, the Oα ratio (refers to Oα/Oα+Oβ) varied with the Ce/Mo atomic ratio in f-Ce10‑xMoxOδ. With the Ce/Mo atomic ratio decreasing from 1/0 to 9.5/0.5 and 9/1, the Oα ratio increased from 34.2% to 44.6% and 48.7%. Further decreasing the Ce/ Mo atomic ratio induced a sharp decrease in the Oα ratio, and the Oα ratios in f-Ce8Mo2Oδ and f-Ce7Mo3Oδ were even lower than that in the pure f-CeO2. The corresponding Ce 3d and Mo 3d spectra of the f-Ce10‑xMoxOδ catalysts are shown in Figure S3, which suggested the presence of Ce3+, Ce4+, and Mo6+ states in f-Ce10‑xMoxOδ. Surface acidity density has been proposed to play a significant role in fructose dehydration. Figure 6 shows the NH3-TPD results of the f-Ce10‑xMoxOδ catalysts. A broad NH3 desorption peak was observed at the range of 100−300 °C for all the f-Ce10‑xMoxOδ catalysts. With the decrease in the Ce/Mo atomic ratio, the peak shifted to a lower temperature, indicating

entry

catalyst

CHMF (%)

YDFF (%)

SDFF (%)

YLA (%)

YFA (%)

1 2b 3 4 5 6 7c 8d

− f-CeO2 f-Ce9.5Mo0.5Oδ f-Ce9Mo1Oδ f-Ce8Mo2Oδ f-Ce7Mo3Oδ f-Ce9Mo1Oδ f-Ce9Mo1Oδ

10 15 46 85 47 33 100 11

5 6 39 80 44 31 94 10

50 40 85 95 94 94 94 91

− 7 4 3 2 2 6 −

− 6 3 3 2 3 6 −

a

Reaction conditions: 1 mmol HMF, 6 mol % Mo in catalyst, 4 mL DMSO, 120 °C, 8 h, 10 mL min−1 O2. b6 mol % mmol CeO2. c Reaction time was 10 h. dIn N2 atmosphere. C = conversion, Y = yield, S = selectivity.

which no catalyst was added, no obvious improvement in DFF yield was observed in the presence of f-CeO2, indicating that fCeO2 had no catalytic activity for the oxidation of HMF to DFF (Table 1, entries 1 and 2). However, varied DFF yields of 39%, 80%, 44%, and 34% were obtained in the catalysis of fCe9.5Mo0.5Oδ, f-Ce9Mo1Oδ, f-Ce8Mo2Oδ, and f-Ce7Mo3Oδ respectively, when the reaction was performed in DMSO, at 4182

DOI: 10.1021/acssuschemeng.7b00175 ACS Sustainable Chem. Eng. 2017, 5, 4179−4187

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Figure 7. Effects of (a) reaction time, (b) temperature, (c) Mo loading, and (d) rate of O2 flow on f-Ce9Mo1Ox-catalyzed oxidation of HMF to DFF. If not specified, the default reaction conditions are as follows: 5-HMF 126 mg, catalyst f-Ce9Mo1Oδ, Mo loading 6 mol %, DMSO 4 mL, O2 10 mL min−1, temperature 120 °C, and reaction time 10 h.

120 °C and under 10 mL min−1 of O2 flow for 8 h (Table 1, entries 3−6). Here f-Ce9Mo1Oδ exhibited the highest catalytic activity for HMF oxidation, and full HMF conversion and high DFF yield of 94% were obtained by prolonging the reaction time to 10 h (Table 1, entry 7). The results on one hand suggested molybdena species in f-Ce10‑xMoxOδ were active sites for HMF oxidation to DFF. On the other hand, it should be noted that the Mo loading in the parallel experiments were equal by varying the amount of f-Ce10‑xMoxOδ samples. So, the differences in catalytic performance were largely due to the different structure of molybdena species in f-Ce10‑xMoxOδ. By correlating the catalytic performance with surface molybdena structures, it can be found that the polymeric octahedral MoOz species that formed in f-Ce9Mo1Ox were most active for HMF oxidation. Surface oxygen species was another important factor and have been reported for starting an oxidation reaction.37 This can be reflected by the fact that a certain amount of DFF can be obtained over f-Ce9Mo1Ox even under a N2 flow of 10 mL min−1 (Table 1, entry 8). On the other hand, from Figure S4, the Oα ratio in the recycled f-Ce9Mo1Ox that has been used under a N2 flow was much lower than that in the fresh fCe9Mo1Ox (16% vs 49%), suggesting that Oα was more active than Oβ in this reaction. Therefore, the highest Oα ratio, as well as the highest BET surface area in f-Ce9Mo1Ox, may also contribute to its highest catalytic activity. From Table 1, LA and FA which were formed in the HMF rehydration reaction were detected as side products, as well as a little amount of another soluble polymer. No overoxidation product was detected. So, HMF rehydration that is a parallel reaction to HMF oxidation here is a competition reaction to the oxidation of HMF to DFF in the catalysis of f-Ce10‑xMoxOδ. Then, the effects of reaction time, temperature, Mo loading, and the rate of O2 flow on the f-Ce9Mo1Oδ-catalyzed oxidation

of HMF to DFF were investigated. From Figure 7a, both the conversion of HMF and the yield of DFF increased with the reaction time in the initial stage, reached the highest at 10 h, and then kept constant. The lack of decrease in DFF yield after 10 h suggested that DFF was not overoxidated to other compounds such as FFCA and FDCA. The selectivity of DFF had no obvious change (93−95%) with the evolution of the reaction time because the main competition reaction (HMF rehydration) was a parallel reaction of HMF oxidation. From Figure 7b, the conversion of HMF and the yield of DFF increased gradually as the reaction temperature increased from 80 to 120 °C, with the selectivity of DFF decreased. This may be because the high temperature enhanced the reaction rate of HMF oxidation and simultaneously the reaction rate of HMF rehydration to a larger degree. From Figure 7c, by increasing the amount of Mo loading from 1 to 8 mol %, the conversion of HMF and the yield of DFF first increased and then kept constant, achieving the highest DFF yield with 6 mol % of Mo loading. The selectivity of DFF had no obvious change, indicating that a higher catalyst amount also promoted the side reaction. From Figure 7d, the rate of O2 flow had a significant effect on the oxidation of HMF to DFF. A higher O2 flow rate improved both the conversion of HMF and the yield of DFF. In addition, the selectivity of DFF also increased as the O2 flow rate increased. The reason may be that the HMF rehydration reaction was facilitated under a low O2 flow rate that was insufficient for HMF oxidation. Dehydration of Fructose to HMF. Fructose dehydration is the first step of the “one-pot” conversion of fructose to DFF. The f-Ce10‑xMoxOδ catalysts were then evaluated as solid acids for catalytic fructose dehydration to HMF by using DMSO as a solvent. As reported previously, the thermolysis of DMSO at high temperature (150−160 °C) can generate strong acids that could catalyze the fructose dehydration to yield HMF.38 From 4183

DOI: 10.1021/acssuschemeng.7b00175 ACS Sustainable Chem. Eng. 2017, 5, 4179−4187

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covered the acid sites of the catalyst. So, 120 °C and 2 h were selected as the optimum reaction temperature and time (Table 2, entry 7). One-Pot Conversion of Fructose to DFF. After stepwise investigating the catalytic activities of f-CexMo10‑xOδ in the oxidation of HMF to DFF and the dehydration of fructose to HMF, f-Ce9Mo1Oδ was evaluated as a bifunctional catalyst for “one-pot” conversion of fructose to DFF under optimum reaction conditions The synthesis of DFF from fructose catalyzed by fCe9Mo1Oδ was first investigated by a “one-pot, one step” method. As shown in Figure 9a, fructose was completely converted to HMF, DFF, and LA with a carbon balance of 56% in 2 h. The intermediate product HMF reached a maximum yield of 40% in 2 h and received full conversion by continuing to be heated at 120 °C for another 10 h. The yield of DFF increased with reaction time, reaching the maximum of 45% in 12 h. Levulinic acid, a main byproduct, was also detected with a yield of 8%. After the reaction, the mixture was heavily colored, and the carbon balance was calculated as 54%. Considering the carbon balance in 2 h (56%), it can be concluded that the low DFF yield obtained in the “one-pot, one step” style was attributed to the formation of humins and the oxidation of fructose in the initial stage, as reported by others.25,31 To inhibit the undesired side reaction of fructose oxidation, a “one-pot, two-step” method was performed under N2 flow for 2 h and subsequent O2 flow for another 10 h. As shown in Figure 9b, fructose was completely converted in 2 h, giving HMF with a maximum yield of 76% under a N2 flow of 10 mL min−1. Then, the HMF oxidation reaction was ignited by exchanging the N2 with O2 flow. The obtained HMF was gradually converted to DFF, achieving a highest DFF yield of 74% after another 10 h, with a small amount of LA. The results indicated that f-Ce9Mo1Oδ could be used as an efficient bifunctional catalyst in synthesis of DFF from fructose through a “one-pot, two-step” method. Catalyst Recycling. Catalyst recycling is of great importance in the field of heterogeneous catalysis. The reusability and stability of f-Ce9Mo1Oδ were first investigated in the oxidation of HMF to DFF. As shown in Figure 10a, the DFF yield remained about 90% in the fifth run, suggesting no obvious decrease in the catalytic activity of f-Ce9Mo1Oδ as a redox catalyst. Then, the stability of f-Ce9Mo1Oδ was studied in one-pot synthesis of DFF from fructose. Unsatisfactorily, there was a little decrease in the DFF yield in “one-pot” fructose conversion after five runs (Figure S6a). The reason could be that the formed humins in fructose dehydration covered the active sites of f-Ce9Mo1Oδ. The study of the recovery performance of f-Ce9Mo1Oδ in the dehydration of fructose to HMF further confirmed this speculation (Figure S6b). Fortunately, the catalytic activity of the used f-Ce9Mo1Oδ can be recovered by calcining at 600 °C for 2 h to remove the absorbed humins. As shown in Figure 10b, DFF yield has no obvious decrease in the five cycling experiments by calcination−regeneration used f-Ce9Mo1Oδ before each cycle. Furthermore, the Ce/Mo ratio and the Mo content in the reused f-Ce9Mo1Oδ were detected by ICP as 9.5/0.47 and 3.21 wt %, which had no obvious difference with the fresh fCe9Mo1Oδ. The above information suggests considerable stability of f-Ce9Mo1Oδ in the reaction system.

Table 2, the blank experiment without any additive acid catalyst in neat DMSO showed that only a 10% fructose conversion and Table 2. Fructose Dehydration to HMF over f-CexMo10‑xOδ Catalystsa entry

catalyst

C (%)

YHMF (%)

SHMF (%)

YLA (%)

YFA (%)

1 2 3 4 5 6 7b

− f-CeO2 f-Ce9.5Mo0.5Oδ f-Ce9Mo1Oδ f-Ce8Mo2Oδ f-Ce7Mo3Oδ f-Ce9Mo1Oδ

10 50 60 66 45 44 100

6 39 48 54 34 33 75

60 78 80 82 76 75 75

− 5 7 8 5 4 13

1 5 6 7 5 3 12

Reaction conditions: fructose 180 mg, catalyst 90 mg, 120 °C, 1 h, N2 10 mL min−1. bReaction time was 2 h. C = conversion, Y = yield, S = selectivity. a

6% HMF yield were obtained at 120 °C and in 1 h under a N2 flow (Table 2, entry 1). Addition of f-Ce10‑xMoxOδ as an acid catalyst promoted fructose conversion to 44−66% and HMF yield to 33−54% under the same reaction conditions (Table 2, entries 2−6). From Table 2, the catalytic activities of the fCe10‑xMoxOδ samples in terms of fructose conversion and HMF yield follow the order f-Ce9Mo1Oδ > f-Ce9.5Mo0.5Oδ > f-CeO2 > f-Ce8Mo2Oδ > f-Ce7Mo3Oδ. This trend was quite consistent with the NH3 desorption results. More interestingly, a good linear correlation between fructose conversion/HMF yield and the acidity density of f-Ce10‑xMoxOδ were found, as shown in Figure 8. Previously, the Chen group also reported the strong

Figure 8. Conversion of fructose and the yield of HMF at a reaction time of 1 h versus acid density of f-CexMo10‑xOδ.

correlation between the catalytic activity and acid-site density of the solid acid catalyst in the dehydration of fructose to HMF.39,40 To maximize HMF yield, the influences of reaction temperature and time on the dehydration of fructose into 5HMF were then evaluated, as shown in Figure S5. The 5-HMF yield first increased rapidly and then decreased with progression of the reaction time. This is because of the formation of humins after the optimal reaction time. The maximum 5-HMF yield can be obtained by using an appropriate combination of reaction temperature and time, for example, 100 °C and 3 h for 69%, 110 °C and 2.5 h for 73%, 120 °C and 2 h for 75%, and 130 °C and 1.5 h for 70%. It is possible that elevation of the reaction temperature accelerated the dehydration of fructose but simultaneously increased the formation of humins that 4184

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ACS Sustainable Chemistry & Engineering

Figure 9. One-pot synthesis of DFF from fructose catalyzed by bifunctional f-Ce9Mo1Oδ: (a) “one-pot, one-step”method and (b) “one-pot, twostep” method. Reaction conditions: fructose 180 mg, catalyst f-Ce9Mo1Oδ, Mo loading 6 mol %, DMSO 4 mL, 120 °C, N2 10 mL min−1, and O2 10 mL min−1.

Figure 10. Recycling experiments for DFF production from (a) HMF oxidation catalyzed by f-Ce9Mo1Oδ and (b) one-pot fructose conversion catalyzed by calcination-regenerated f-Ce9Mo1Oδ.

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CONCLUSION A series of f-CexMo10‑xOδ samples were synthesized by an ethylene−glycol-mediated process and systematically characterized to obtain their morphologies, structures, aciditiesd and other basic physicochemical properties. The f-CexMo10‑xOδ samples were first evaluated as catalysts in the reactions of HMF to DFF and fructose to HMF. Relationships between the catalytic activities with the structure and physicochemical properties were studied. It was found that f-Ce9Mo1Oδ with polymeric octahedral MoOz species, the highest Oα ratio, BET surface area, and acidity exhibited the highest catalytic activity for both HMF oxidation and fructose dehydration. Then, “onepot” methods have been adopted for direct conversion of fructose to DFF. Under the optimum reaction conditions, a DFF yield of 74% can be obtained from “one-pot, two-step” fructose conversion. Furthermore, f-Ce9Mo1Oδ was easily prepared, has low cost, and is ecofriendly, which has potential application in industrial production of DFF from fructose.

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flow, effect of reaction time and temperature on fructose dehydration to HMF catalyzed by f-Ce9Mo1Oδ, and recycling experiments for f-Ce9Mo1Oδ-catalyzed one-pot conversion of fructose to HMF, and dehydration of fructose to HMF. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 22 27407799. Fax: +86 22 27407599. ORCID

Wei Qi: 0000-0002-7378-1392 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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ASSOCIATED CONTENT

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Tianjin, China (No. 16JCYBJC19600), Natural Science Foundation of China (No. 21621004), Beiyang Young Scholar of Tianjin University (2012), and Program of Introducing Talents of Discipline to Universities of China (No. B06006).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00175. Basic properties of f-Ce10‑xMoxOδ catalysts, EDS results for f-Ce9Mo1Oδ, SEM images of f-CeO2, f-Ce9.5Mo0.5Oδ, f-Ce8Mo2Oδ, and f-Ce7Mo3Oδ, XPS spectra of Ce 3d and Mo 3d for f-Ce10‑xMoxOδ catalysts, XPS spectra of O 1s for recycled f-Ce10‑xMoxOδ that was used under an N2

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