One-Step Approach to 2,5-Diformylfuran from Fructose over

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One-Step Approach to 2,5-Diformylfuran from Fructose over Molybdenum Oxides Supported on Carbon Sphere Chunmei Zhou, Jun Zhao, Haolin Sun, Yu Song, Xiaoyue Wan, Hongfei Lin, and Yanhui Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03470 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Sustainable Chemistry & Engineering

One-Step Approach to 2,5-Diformylfuran from Fructose over Molybdenum Oxides Supported on Carbon Sphere Chunmei Zhou†#, Jun Zhao‡#, Haolin sun†, Yu Song†, Xiaoyue Wan†, Hongfei Lin§* and Yanhui Yang†* Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials,

†

Nanjing Tech University, Nanjing 211816, China. School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore, 637459, Singapore

‡

Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA.

§

*Y. Yang (E-mail: [email protected]) *H. Lin (E-mail: [email protected])

ABSTRACT: An effective one-pot process of synthesizing 2,5-diformylfuran (DFF) directly from fructose was accomplished over carbon sphere (CS) supported molybdenum oxides (MoOx/CS) catalysts. The MoOx/CS catalyst was firstly prepared by a glucose hydrothermal carbonization method and subsequently annealed under different atmospheres. The annealing treatment under air at 275 oC afforded abundant mesopores over the CS and exposed more active sites. Kinetics studies suggested that 5-hydroxymethylfurfural (HMF) was the key intermediate, acid and oxide sites were active toward the dehydration of fructose and aerobic oxidation of HMF to DFF, respectively. A relatively faster dehydration compared to oxidation rate was critical for the dehydration of fructose to HMF instead of decomposing fructose under oxidative conditions. Under the optimized reaction conditions, near 78% yield of DFF at 100% conversion of fructose was obtained in dimethylsulfoxide under atmospheric pressure of oxygen at 160 oC within 2 h. KEYWORDS: Fructose, 2,5-diformylfuran, Molybdenum oxides, Dehydration, Aerobic oxidation

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INTRODUCTION

Biomass has recently attracted increasing interests on the production of chemicals and energy in a sustainable manner1-5. Carbohydrates are suggested as the most abundantly available resources among the biomass to produce various chemicals6-9. As one of the chemical building blocks derived from renewable carbohydrate, 5-hydroxymethylfurfural (HMF) acts as a link between the biomass chemistry and petro-chemistry10-12. Selectively oxidizing the hydroxyl group of HMF can lead to the formation of 2,5-diformylfuran (DFF), an important monomer due to its symmetrical and unsaturated structure13, which has been considered as a potential intermediate for fungicides14, pharmaceuticals15, heterocyclic16, and macrocyclic ligands17. Up to now, the primary synthetic route of DFF from carbohydrates has to go through two separate steps, the formation of HMF and its transformation to form DFF. As for the first step, HMF has been massively synthesized from cellulose-derived sugars (such as glucose and fructose) or cellulose itself over acid catalysts. The second step of selectively oxidizing of HMF to DFF had also made great progress. For instance, stoichiometric oxidants such as NaOCl, BaMnO4, pyridinium chloro chromate, and 2,2,6,6-tetramethylpiperidine-1-oxide have been attempted to prepare DFF from HMF18-20. Compared to these oxidants, molecular oxygen as a mild oxidant was found easier to obtain high DFF selectivity in addition to its low cost and environment-friendly features. Several homogeneous and heterogeneous catalysts have been developed for the aerobic oxidation of HMF using either oxygen or air. Among them, the homogeneous catalysts, such as Mn-salen21, Co/Mn/Zr/Br22 and Cu(NO3)2/VOSO423 showed moderate to excellent (61-98%) DFF yields, yet suffered the difficulties on catalyst recycle and catalyst/product separation. The heterogeneous catalysts, vanadium-based24-26, manganese-based catalysts27-29 and some noble metals30-33 have been reported lately. For example, nearly 85% DFF

yield was obtained over VOx/TiO226 at a relatively high catalyst/substrate ratio (2:1 wt./wt.). Noble metals, such as Ru-based catalysts32-33 afforded high DFF yield (92-99%), regardless of their high cost. Recently, some manganese-based such as OMS-2 catalysts24-25 have been documented to be efficient, selective and recyclable catalysts and obtained over 99% yield of DFF. Nonetheless, to obtain DFF with a high yield directly from carbohydrate in a one-step route still remains challenging. The synthesis of HMF from carbohydrate is an acid-catalyzed reaction and the conversion of HMF to DFF is selective oxidation, the direct transformation of carbohydrate to DFF in a one-pot process has to rely on the combination of dual acidic and oxidative catalytic functions. For instance, H-form cation-exchanged resin and vanadium-based catalysts25, Amberlyst-15 and Ru/HT34, CrCl3·6H2O/NaBr and NaVO3·2H2O35, Fe3O4-SBA-SO3H and K-OMS-227, Amberlyst-15 and polymer supported IBX amide36 have been investigated as potential bifunctional catalysts for the one-pot production of DFF from fructose, in which the acid sites promoted the dehydration of fructose to HMF while the metal oxide-derived catalysts catalyzed the aerobic oxidation of HMF to DFF. In these catalytic reactions, the oxidation catalysts have to be added into the reaction after the fructose has been converted to HMF in the presence of acidic catalysts, resulting in a one-pot but two-step method transforming fructose to DFF. Recently, a few catalysts have been reported to show good performance in one-pot and one-step synthesis of DFF from fructose. Typically, a proton- and vanadium-containing graphitic carbon nitride bifunctional catalyst combined g-C3N4(H+) and V-g-C3N4 together and successfully afforded the direct synthesis of DFF from fructose with DFF yield of 45% under 1 MPa of oxygen37. Several acidic cesium salts of molybdovanadophosphoric/phosphomolybdic heteropolyacid such as CsMVP-HPA and Csx-H3-xPMo12 were synthesized and attempted as the catalysts for the direct transformation of fructose to DFF, and DFF yield of 60%-69% was obtained under the optimum conditions38-39.

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In our previous work, carbon sphere (CS) based solid acid was developed and over 99% yield of HMF from fructose dehydration was accomplished40, meanwhile molybdenum oxides was found to have moderate activity for HMF oxidation41-42. However, separate CS and molybdenum oxides are difficult to accomplish both fructose dehydration and HMF oxidation in one pot. Sulfonated MoO3-ZrO2 binary oxides41 and protonated MoO3 composited nitrogen-doped carbon (Mo-HNC)42 were used in the one-step fructose transformation reaction. Although near 75% DFF yield can be achieved, long reaction time (9~10 h) is needed, besides the preparation of these two catalysts are relatively complicated. Expecting to combine the excellent dehydration activity of CS and the moderate oxidation activity of MoOx, a MoOx supported on CS bifunctional catalyst was prepared by a simple hydrothermal method, and used on the synthesis of DFF from fructose in one-pot and one-step approach. In this study, a synergic effect of acid sites and exposed oxide sites on MoOx/CS was proved to promote the formation of DFF directly from fructose.

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EXPERIMENTAL SECTION

Materials and catalyst synthesis. All the HMF and DFF used in this work are from Alfa aesar. Glucose, phosphomolybdic acid, fructose and dimethyl sulfoxide (DMSO) are from Sigma-Aldrich. The carbon microspheres with molybdenum oxides encapsulated were prepared by a classic hydrothermal carbonization procedure, in which glucose and phosphomolybdic acid were used as the carbon and MoOx precursors, respectively. Typically, glucose and phosphomolybdic acid were dissolved in 30 mL of DI water with different molar ratios of carbohydrate to metal (6:1, 8:1, 12:1 and 24:1). The solution was kept stirring for 6 h until the color turned from yellow to green. Afterward, the solution was poured into a 40 mL Teflon-lined autoclave and place into a constant temperature oven of 180 oC for 12 h. The solid product was collected by filtering and washing thoroughly with DI water and ethanol and then dried in vacuum overnight. The as-synthesized sample was denoted as MoOx/CS. The MoOx/CS sample was then transferred into a muffle furnace open to air and annealed at 275 oC for 3 h. The resulted sample was marked as MoOx/CS-air. The MoOx/CS sample was also annealed under nitrogen in a tube furnace at 750 oC for 3 h, the sample was donated as MoOx/CS-N2. For comparison, an Al2O3 supported MoOx sample (MoOx/Al2O3) was also prepared by a traditional impregnation method with similar Mo loading to MoOx/CS-air. Characterization. SEM and TEM images of the samples were obtained from a field emission scanning electron microscope (JOEL JSM 6701F) and a transmission electron microscopy (JEM-2100Plus), respectively. XRD patterns of the samples were obtained from a Bruker Advance 8 X-ray diffractometer with a Ni filtered Cu Kα radiation (at λ=0.154nm) and with a resolution of 0.02°. FTIR spectra of the samples were recorded on a Digilab FTS 3100 FTIR using a standard KBr disk technique with a resolution of 4 cm-1. Raman spectrum of the catalysts were obtained from a Renishaw 1000 Raman spectrometer with a He-Ne laser of 633 nm excitation, of which the detection depth is about 150 nm. BET surface areas of different catalysts were measured by an Autosorb-6B instrument, through a conventional liquid N2 adsorption method. Elemental compositions of different samples were detected by a Vario EL III CHNS Elemental Analyzer. The molybdenum metal loading (wt.%) was determined by an ICP (inductively coupled plasma) measurement (AA6800, Shimadzu). The NH3-TPD and H2-TPD profiles of different catalysts

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were recorded on a chemisorption analyzer (Micrometrics Auto Chem 2920) equipped with a quadrupole mass spectrometer, and through the conventional temperature-programmed desorption and reduction approaches, respectively. Catalytic reaction. Reaction of fructose dehydration: 200 mg of fructose, 30 mg of catalyst, and 5 mL of DMSO were added into a three-necked round-bottomed flask (25 mL in volume) with a reflux tube. The flask was heated in an oil bath with sustained stirring. Reaction of HMF oxidation: 63 mg of HMF, 30 mg of catalyst, and 5 mL of DMSO were added into a three-necked round-bottomed flask (25 mL in volume) with a reflux tube. The flask was bubbled into an O2 flow at 20 mL/min, and was simultaneously heated in an oil bath to the set temperature. Reaction of conversing fructose to DFF: 200 mg of fructose, the certain amount of catalyst, and 5 mL of DMSO were added into a three-necked round-bottomed flask (25 mL in volume) with a reflux tube. A 20 mL/min O2 flow was then bubbled into the flask. At the same time, the reaction was conducted at different temperatures (i.e., 100, 120, 140, 160 oC) in an oil bath. Analyses of of products: After reaction, the solid catalyst was filtered out, while the solution with the products was diluted 100 times with DI water. The analyses of fructose, HMF and DFF were conducted by a HPLC instrument (Agilent 1260) with a 300 mm×7.8 mm column (Bio-rad aminex 87H). A 5 mM H2SO4 solution was used as the mobile phase and was injected at 0.6 mL/min. The oven temperature of the column was held at 60 oC.

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RESULTS AND DISCUSSION Characterization of the MoOx/CS samples.

Figure 1 shows the SEM and TEM images of as-synthesized MoOx/CS, MoOx/CS-air and MoOx/CS-N2. From the SEM images of Figure 1 (left entry of 1 and 2), the uniform size of the carbon spheres (CS) can be observed. The average CS size distribution in statistics is around 0.9 μm for MoOx/CS, while slightly decreases to 0.7 μm for both MoOx/CS-air and MoOx/CS-N2. The surface of the CS is smooth for MoOx/CS and MoOx/CS-N2, while moderately rough and grainy for MoOx/CS-air. As for MoOx/CS-N2, gasification and reconstruction of carbon may occur at high annealing temperature (750 oC) in inert N2 atmosphere, resulting in the volumetric shrink of carbon spheres, similar phenomenon was also reported by Gong et al.43. For MoOx/CS-air, as the annealing temperature (275 oC) is insufficiently high to increase the degree of carbonization, the size decrease of carbon spheres may due to the oxidation etching of surface carbon species in active air atmosphere. The etching ruptures the smoothness of the carbon sphere as well as exposes more encapsulated MoOx nanoparticles. As shown in the right entry of Figure 1, MoOx nanoparticles with an average size of around 1-2 nm were found uniformly decorated in carbon fragments of both MoOx/CS and MoOx/CS-N2, while most of those nanoparticles grew up to 10-20 nm and spread around the edge of the carbon fragments for MoOx/CS-air, suggesting that the air annealing not only exposed more MoOx nanoparticles but also agglomerated them into large ones. XRD was carried out to study the crystallographic structure of the catalyst samples. As shown in Figure 2, no MoOx peaks were observed in the patterns for both MoOx/CS and MoOx/CS-N2, while a set of α–MoO3 peaks (well matching with JCPDS # 05-0508) clearly emerged for MoOx/CS-air. The precursor, phosphomolybdic acid, can be reduced by glucose to form MoO2 particles under the hydrothermal

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preparation conditions44, and some MoO2 particles would further be reduced and carbonized to hexagonal molybdenum after annealing at 750 oC in nitrogen45. The absence of any MoOx or Mo diffraction peak for MoOx/CS and MoOx/CS-N2 was due to the uniform distribution and close encapsulation of most Mo carbides. Obviously, under the surface etching of air annealing atmosphere, more MoO2 particles were exposed and agglomerated and further oxidized to MoO346. Thus, the XRD crystalline peaks of molybdenum oxide only emerged for the MoOx/CS-air sample. The XRD patterns also revealed that amorphous carbon was the main structure for all CS in these three catalysts, showing a broad diffraction peak between 20-30° instead of any graphitic carbon peak47. However, compared with MoOx/CS and MoOx/CS-air, MoOx/CS-N2 showed clear C (002) and C (101) peaks, indicating the rigorous carbonization of MoOx/CS-N2. Raman spectroscopy was employed to measure the disorder degree of carbon in this study. As shown in Figure 3, two typical bands at ~1300 and ~1600 cm-1 were observed for all samples. The peak at ~1300 cm-1 also known as D-band was attributed to the vibration of carbon atoms with dangling bonds, while the G-band (~1600cm-1) corresponded to the in-plane E2g zone-center mode48. The intensity ratios of the D- and G- bands were larger than 0.5, suggesting the low degree of graphitization of the carbon sphere and the presence of high contents of lattice edges and defects in the disordered carbon49-50. Moreover, the ID/IG ratios for both MoOx/CS-air and MoOx/CS-N2 were remarkably higher than that of MoOx/CS, implying that both the etching in air and gasifying reconstruction in N2 enhanced the degree of disorder for CS. Element contents of C, H, O and Mo were measured to further confirm the effects of annealing treatment for MoOx/CS in both air and N2 atmosphere. And the ICP results proved that annealing treatments show negligible effect on the total amount of Mo. As shown in Table 1, the ratios of other elements to Mo were used to indicate the composition change. Among the three catalysts, MoOx/CS showed the highest values of C/Mo, H/Mo and O/Mo, implying the loss of C, H and O in the annealing process for both MoOx/CS-air and MoOx/CS-N2. The lowest C/Mo for MoOx/CS-air confirmed the oxidation etching of surface carbon in air treatment. After deducting the oxygen amount of MoOx (all assumed as MoO2), the ratios of C/(O+H) can be the index of the carbonization degree of the catalysts. The C/(O+H) values were the same for MoOx/CS and MoOx/CS-air, while this value for MoOx/CS-N2 is more than 3 times higher. The remarkably higher carbonization degree for MoOx/CS-N2 confirmed the effect of gasification and reconstitution for carbon in N2 treatment. The loss of surface functional groups such as hydroxyl, carbon-hydrogen, and carbon-oxygen groups during annealing treatments can be observed by FTIR spectroscopy (Figure 4). Generally, all the peaks became less distinct after annealing at 750 oC in nitrogen, indicating the disappearance of the functional groups at high temperature. The broad band centered around 3400 cm-1 was attributed to the -OH groups. The peaks at 2920 and 1380 cm-1 corresponded to the stretching and bending modes of C-H groups. Compared with MoOx/CS, the intensities of these peaks for MoOx/CS-air and MoOx/CS-N2 decreased after annealing, and the intensity of the peak corresponding to C-O group (1000-1300 cm-1) increased after annealing in air. The peaks at 1702 and 1619 cm-1 can be assigned to the C=O vibration of ketones, esters, aldehydes or carboxylic acid and C=C bond of alkenes or aromatic rings51. As the annealing treatments had obviously changed the elemental composition or carbonization degree of the catalysts, their inner structures of the CS were likely to be changed. BET measurements

revealed that microspores and macropores were found to be emerged after treatment, with the average pore size of 57 nm for MoOx/CS-air, and 183 nm for MoOx/CS-N2. Simultaneously, the surface areas for MoOx/CS-air (28 m2/g) and MoOx/CS-N2 (427 m2/g) were also dramatically higher than that of MoOx/CS (< 1 m2/g). Catalytic performances of the MoOx/CS catalysts The catalytic performances of MoOx/CS, MoOx/CS-air and MoOx/CS-N2 were initially tested towards the reaction of synthesizing DFF directly from fructose. As shown in Figure 5, all the tested catalysts can reach 100% fructose conversion under atmospheric pressure of oxygen at 130 oC within 8 h, with HMF and DFF were the main products. The yield of target product DFF for MoOx/CS-air (35.8%) is higher than those of MoOx/CS (21.3%) and MoOx/CS-N2 (22.0%) under the investigated conditions. The prominent activity of MoOx/CS-air may be due to the relatively high acid density because of its high surface area and low carbonated degree of CS, as well as the strong oxidative capability provided by its larger amount of exposed MoOx. The appearance of the byproduct, HMF, for all these catalysts suggested that all the reactions over these three catalysts followed the same reaction pathway: fructose → HMF → DFF. To further explore the promoting mechanism, the catalytic performances of different catalysts were investigated towards fructose dehydration and HMF oxidation independently. Figure 6a exhibits the catalytic activities of different catalysts for fructose dehydration in oxygen-free environment. With the same amount of catalyst (weight), MoOx/CS-air afforded higher fructose conversion and HMF yield than MoOx/CS and MoOx/CS-N2. For comparing, a CS-air catalyst was prepared using the same method of MoOx/CS-air without adding any Mo species. The performance of CS-air in fructose dehydration was similar to that of MoOx/CS-air, confirming that the influence of MoOx was negligible in this dehydration reaction. The dehydration of fructose to HMF was an acid-catalyzed reaction, the acidity should be the key factor which influenced the catalytic activity. In figure 6c, an Al2O3 supported MoOx catalyst was prepared by a traditional impregnation method to conduct the experiment with the physically mixed CS-air on aerobic transformation of fructose. Compared to single MoOx/Al2O3, the physically mixed catalyst of MoOx/Al2O3 and CS-air obtained a much higher HMF yield, while it exhibited no increase in the yield of DFF. While with MoOx/CS-air, both the yields of HMF and DFF were higher than those with MoOx/Al2O3, which confirmed that there is synergetic effect between the acid sites on the carbon surface and the MoOx oxidation sites of MoOx/CS-air. The surface acidity of these catalysts was measured using NH3-TPD, and the results were shown in Figure 7. A major desorption peak appeared ranging from 250 to 650 oC for MoOx/CS, indicating the rich Bronsted and Lewis acid sites on the surfaces, which were attributed to the various surface functional groups such as hydroxyl, carboxylate groups and the molybdenum oxide species52. This desorption peak disappeared for the samples after annealing, and a new peak at around 220 oC emerged for MoOx/CS-air and MoOx/CS-N2. The peak intensity of MoOx/CS-air is remarkably higher than that of MoOx/CS-N2. Generally, the high-temperature desorption peak was related to medium acid sites and the low-temperature was associated with weak acidity. Therefore, the thermal treatment of MoOx/CS led to the loss of medium acid sites and the formation of weak acid sites due to the change of the surface hydroxyl, carbon-hydrogen, and carbon-oxygen groups, which was in line with the analysis of FTIR results in Figure 4. The total number of

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acid sites on the catalyst surfaces were also measured by an acid-base back neutralization titration method following our previous work40. As listed in the last entry of Table 1, the calculated surface acidity densities are different for MoOx/CS-air (9.4 mmol/g), MoOx/CS (8.6 mmol/g) and MoOx/CS-N2 (5.1 mmol/g). The acidity density sequence for these catalysts is well related to their fructose dehydration activity shown in Figure 6a. Therefore, the best fructose dehydration catalytic activity of MoOx/CS-air was due to its relatively high density of acid sites and the large surface area compared to MoOx/CS-N2 with weak acidity and MoOx/CS with the low BET surface area. The catalytic performances for aerobic oxidation of HMF to DFF over these catalysts were further explored and shown in Figure 6b. Under the same reaction conditions, a HMF conversion of 32.0% and a DFF yield of 30.8% were obtained over MoOx/CS-air, whereas these values were less than 10% when using MoOx/CS or MoOx/CS-N2. There was almost no HMF conversion and a DFF yield of close to zero over the CS-air catalyst, indicating that MoOx was the active site for oxidizing HMF to DFF. Several reasons contributed to the better oxidation activity for MoOx/CS-air. Firstly, the Mo loading for MoOx/CS-air (6.8% wt.) was higher than that of MoOx/CS (2.7% wt.) and MoOx/CS-N2 (4.3% wt.), as listed in Table 1. Secondly, the MoOx exposure for MoOx/CS-air was the highest among these catalysts, which had been proved by the TEM and XRD results. Finally, MoOx/CS-air possessed the highest average molybdenum oxidation state among all the catalysts. As part of MoOx was proved to be oxidized to MoO3, while most MoOx was believed to be MoO2 for MoOx/CS, and even some of MoOx was likely to be reduced to MoC for MoOx/CS-N2. Optimizing reaction conditions for fructose transformation over MoOx/CS-air Further optimization of reaction conditions was carried out over MoOx/CS-air that outperformed other catalysts. Specimens sampled at specified time intervals from the reaction mixture were analyzed to study the time course of the one-pot conversion from fructose to DFF. As shown in Figure 8, dehydration of fructose to HMF was rapid and the HMF yield reached nearly 80% after 1 h with the fructose conversion approaching to 100%. As the reaction proceeded, the yield of HMF decreased and almost disappeared after 30 h. The yield of DFF correspondingly increased from 7.7% at 2 h to 77.8% at 30 h. The rapid dehydration of fructose to HMF was critical, which considerably reduced the contact time of fructose with oxygen and the active sites for selective oxidation, minimizing the formation of by-products from fructose oxidation. Reaction conditions such as temperature, time and ratio of reactant to catalyst were further optimized to achieve a high DFF yield. As shown in Figure 9a, a DFF yield of 57.8% and a HMF yield of 20.2% were obtained after 18 h at 130 oC in DMSO even though the weight ratio of Mo to fructose was only 1%. The effect of MoOx loading for the optimization of the catalyst was also studied. The catalytic performances of MoOx/CS-air with various Mo loading (5.1~15 wt.%) were tested in aerobic transformation of fructose and shown in Figure 9b. As the Mo loading increasing, the compound yield of HMF and DFF as well as the single yield of DFF were increased first and then decreased. When the Mo loading is 6.8wt.%, both the compound yield and the single DFF yield were the highest. Table 2 shows that reaction temperature mainly affected the selective oxidation of HMF to DFF as the dehydration of fructose to HMF was essentially rapid under the investigated temperature range.

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For instance, 96.0% of fructose conversion and 69.6% of HMF yield were attained in 2 h at 110 oC, while the DFF yield was only 1.4% due to the poor oxidation capability at this temperature. When the reaction temperature increased to 150 oC, the HMF yield of 34.3% and the DFF yield of 38.0% were achieved with the fructose conversion of 100% after the same reaction time period. When the reaction temperature further increased to 165 oC, the DFF yield was as high as 77.9% in 2 h with negligible HMF residues. Recyclable stability of the MoOx/CS-air catalyst Catalytic experiments were repeated for five runs to study the reusability of MoOx/CS-air. The spent catalyst was separated from the reaction mixture by hot filtration, washed with ethanol and DI water, followed by drying under vacuum at 45 oC and annealing at 275 oC for 3 h prior to the next run. As shown in Figure 10, the conversion of fructose and the combine yield of HMF and DFF are rather steady during 5 recycle runs, however near 20% DFF yield decline was found at 5th run, compared to 1st run (fresh catalyst). Generally, the catalyst afforded stable performances on the conversion of fructose, however shown clearly inactivation in the reaction of HMF to DFF. The H2-TPD and TEM were used to observe the MoOx accessibility and Mo oxidation states for the fresh and used catalysts. As shown in Figure 11, for fresh and used MoOx/CS-air, the TPR profiles had similar MoOx reduction peak temperature and peak area, and the size and exposure ratio of MoOx particles observed by TEM were also similar, which indicated the oxidation states and accessibility were not obviously changed for the catalysts after reaction. However, only 4.5 wt.% Mo leaching was detected by ICP analysis for the fifth run MoOx/CS-air, which indicated that MoOx/CS-air was a relatively robust catalyst for this reaction.

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CONCLUSIONS

In this study, efficiently synthesizing DFF from fructose was achieved over a MoOx/CS-air catalyst. Larger surface area, higher acid density and molybdenum oxide exposure were acquired during an air annealing treatment at 275 oC for MoOx/CS-air. The synergic effect of acid sites and exposed molybdenum oxides was proved to be the promoting mechanism for MoOx/CS-air. Briefly, its high acid density was found benefit to fructose dehydration to form HMF, while the moderate oxidative capability of its exposed MoOx was proved to promote the further oxidization of HMF to DFF. Under optimized conditions, A ~78% yield of DFF was obtained from fructose with MoOx/CS-air as the catalyst and bubbling oxygen as the oxidant in only 2 h at 165 oC.

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AUTHOR INFORMATION

Corresponding authors Y. Yang (E-mail: [email protected]) H. Lin (E-mail: [email protected]) Notes C. Zhou and J. Zhao contributed equally to this work. All the authors declare no competing financial interest.

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ACKNOWLEDGEMENTS 4


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This project is funded by National Natural Science Foundation of China (21503187) and supported by Nanjing Tech University and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials.

5-hydroxymethylfurfural to 2,5-diformylfuran, and synthesis of a fluorescent material. ChemSusChem 2011, 4, 51-54, DOI 10.1002/cssc.201000273. (24) Carlini, C.; Patrono, P.; Galletti, A. M. R.; Sbrana, G.; Zima, V. Selective oxidation of 5-hydroxymethyl-2-furaldehyde to furan-2,5-dicarboxaldehyde by catalytic systems based on vanadyl phosphate. Appl. Catal. A-Gen. 2005, 289, 197-204, DOI 10.1016/j.apcata.2005.05.006. (25) Halliday, G. A.; Young, R. J.; Grushin, V. V. One-Pot, Two-Step, Practical catalytic synthesis of 2,5-diformylfuran from fructose. Organ. Lett. 2003, 5, 2003-2005, DOI 10.1021/ol034572a.

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6


Page 7 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Elemental composition and surface properties of MoOx/CS, MoOx/CS-air and MoOx/CS-N2. Elemental content a and molar ratio Catalysts MoOx/CS MoOx/CS-air MoOx/CS-N2

Mo (wt.%) 2.7 6.8 4.3

C/Mo

H/Mo

O/Mo

C/(Ob+H)

Acidity density c (mmol/g)

188 53 151

64.4 40.9 18.3

177. 8 31.1 33.5

0.8 0.8 3.0

8.6 9.4 5.1

a C,

H and O contents determined by elemental analysis, Mo content determined by ICP. After deducting the oxygen amount of MoOx (all assumed as MoO2). c Determined by an acid-base back neutralization titration method of reference 40. b

7



Page 8 of 20

Table 2. Influence of temperature on the synthesis of HMF and DFF from fructose on MoOx/CS-air Catalyst. Temp. Time Fructose conv. HMF yield DFF yield (oC) (h) (%) (%) (%) 110 2 96.0 69.6 1.4 110 72 100 10.3 50.8 130 18 100 20.2 57.8 150 2 100 34.3 38.0 150 5 100 4.3 66.8 165 2 100 0.8 77.9 Reaction conditions: fructose 200mg, catalyst 30mg, DMSO 5mL, O2 20mL/min.

8


Page 9 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1 SEM and TEM images of MoOx/CS (a,b,c), MoOx/CS-air (d,e,f) and MoOx/CS-N2 (g,h,i).

9



C(002) C(101)

MoOx/CS-N2

*

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 20

*

* MoO3 (JCPDS # 05-0508)

* *

*

*

* *

****

MoOx/CS-air MoOx/CS

10

20

30

40

50

60

70

80

90

2 Theta (degree) Figure 2 XRD patterns of MoOx/CS, MoOx/CS-air and MoOx/CS-N2.

10


Page 11 of 20

MoOx/CS-N2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoOx/CS-air

MoOx/CS

500

1000

1500

2000

2500

-1

Raman Shift (cm ) Figure 3 Raman spectrum of MoOx/CS, MoOx/CS-air and MoOx/CS-N2.

11



MoOx/CS-N2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 20

MoOx/CS-air

MoOx/CS

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Figure 4 FTIR spectra of MoOx/CS, MoOx/CS-air and MoOx/CS-N2.

12


Page 13 of 20

Fructose conversion 100

Conversion & yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HMF yield DFF yield

80

60

40

20

0

MoOx/CS

MoOx/CS-air

MoOx/CS-N2

Figure 5 Direct transformation of fructose to DFF catalyzed by MoOx/CS, MoOx/CS-air and MoOx/CS-N2. Reaction conditions: fructose 200mg, weight Mo/fructose=1%, DMSO 5mL, O2 20mL/min, 130oC, 8h.

13



Conversion & yeild (%)

100

(a)

Fructose conversion HMF yeild

80 60 40 20 0

MoOx/CS

MoOx/CS-N2

MoOx/CS-air

CS-air

(b)

Conversion & yeild (%)

32 HMF conversion DFF yeild

24 16 8 0

MoOx/CS

MoOx/CS-N2

Fructose conversion

100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 20

MoOx/CS-air HMF yield

CS-air DFF yield

(c)

80 60 40 8 4 0

CS-air

MoOx/Al2O3

MoOx/Al2O3+CS-air

MoOx/CS-air

Figure 6 Kinetic curves of fructose dehydration and HMF oxidation over different catalysts. Reaction conditions: (a) fructose 200mg, catalyst 30mg, DMSO 5mL, 110oC, 2h.(b)HMF 63mg, catalyst 30mg, O2 20mL/min, DMSO 5mL , 140oC, 2h. (c) fructose 200mg, catalyst 30mg each, O2 20mL/min, DMSO 5mL , 130oC, 2h.

14


Page 15 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoOx/CS

MoOx/CS-air

MoOx/CS-N2

Figure 7 NH3-TPD curves of MoOx/CS, MoOx/CS-air and MoOx/CS-N2.

15



100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 20

Fructose conversion HMF yield DFF yield

80 60 40 20 0 0

4

8

12

16

20

24

28

Time (h) Figure 8 Kinetic curves for transformation of fructose over MoOx/CS-air. Reaction conditions: fructose 200mg, CS-Mo-air 30mg (Mo/fructose=1%), DMSO 5mL, O2 20mL/min, 130oC.

16


Page 17 of 20

100

(a)

DFF yield HMF yield

HMF or DFF yield (%)

80 60 40 20 0 75

0.4

1

2

4

Mo/Fructose (wet.%)

(b)

DFF yield HMF yield

70

HMF or DFF yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


65 60 55 50

5.1

6.8

10.2

15.0

Mo loading of MoOx/CS-air (wet.%) Figure 9 Influence of ratio of Mo/fructose (a) and Mo loading (b) on the synthesis of HMF and DFF from fructose over MoOx/CS-air. Reaction conditions: (a) fructose 200mg, DMSO 5mL, O2 20mL/min, 130oC, 18h. (b) fructose 200mg, 30mg catalyst, DMSO 5mL, O2 20mL/min, 130oC, 2h.

17



HMF yield DFF yield

Fructose conversion 100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 20

80 60 40 20 0

1

2

3

4

5

Run number Figure 10 Reusability test of CS-Mo-air. Reaction conditions: fructose 200mg, MoOx/CS-air 30mg, DMSO 5mL, O2 20mL/min, 155oC, 2h.

18


Page 19 of 20

0.06 MoOx/CS-air (fresh) MoOx/CS-air (used)

H2 comsuption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fresh

CS-air

0.04

0.02

used 0.00

-0.02 100

200

300

400

500

600

Temperature (C) Figure 11 TPR profiles and TEM images for fresh and used MoOx/CS-air catalysts.

19



Page 20 of 20

TOC/Abstract Graphic

HO

OH

MoOx/CS-air

O

O

O O

HO HO

OH

Fructose 100% Conversion

DFF 78% Yield One-step

An effective one-step process of synthesizing 2,5-diformylfuran (DFF) directly from fructose was achieved over a porous carbon spheres supported molybdenum oxides catalyst.

ACS Paragon Plus Environment 20

One-Step Approach to 2,5-Diformylfuran from Fructose over

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