Catalytic Conversion of Fructose and 5-Hydroxymethylfurfural into 2,5

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Catalytic conversion of fructose and 5-hydroxymethylfurfural into 2,5-diformylfuran over SBA-15 supported ruthenium catalysts Fan Wang, Liang Jiang, Junmei Wang, and Zehui Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01148 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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Catalytic conversion of fructose and 5-hydroxymethylfurfural into 2,5-diformylfuran over SBA-15 supported ruthenium catalysts Fan Wang, Liang Jiang, Junmei Wang, Zehui Zhang*

Key Laboratory of Catalysis and Material Sciences of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Minyuan Road 182, Wuhan, 430074, China. ABSTRACT: In this study, the mesporous SBA-15 was prepared and used to graft biimidazole groups, which were used to anchor Ru3+ to give rise to a new ruthenium catalyst (SBA-15-Biimidazole-Ru). The structure of the SBA-15-Biimidazole-Ru catalyst was well characterized and used for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF). Several important parameters were studied and found that the reaction solvent and oxygen pressure showed a crucial role in the activity of the as-prepared SBA-15-Biimidazole-Ru catalyst. HMF conversion of 96.9% and DFF yield of 88.7% were achieved after 11 h at 110 oC under 15 bar oxygen pressure. One important characteristic of the SBA-15-Biimidazole-Ru catalyst is that it was very stable without loss of its catalytic activity during the recycling experiments, which was due to the strong interaction between the Ru3+ and the Biimidazole groups on the surface of SBA-15. Finally, the direct conversion of fructose into DFF was also performed by the use of Amberlyst 15 and SBA-15-Biimidazole-Ru as two binary catalysts. The dehydration of fructose

1

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over Amberlyst 15 catalyst and the subsequent oxidation of HMF over SBA-15-Biimidazole-Ru catalyst afforded DFF with an overall yield of 72.4%. KEYWORDS: Oxidation, 5-Hydroxymethylfurfural, 2,5-diformylfuran, supported Ru catalysts, sustainable chemistry

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INTRODUCTION Currently, the transformation of renewable resource into fuels and chemicals has emerged as an important research area due to the fact that the none-renewable fossil resources are gradually decreasing.1,

2

Biomass is a renewable source of great

abundance in the earth, and is the only carbon-containing renewable resources. Thus, biomass can serve as a promising alternative to the none-renewable fossil resource to supply chemicals and fuels for the world.3 In recent years, tremendous research interest has been attracted for the development of various methods for the production of chemicals and fuels from biomass.4, 5 Carbohydrates represent the major component of biomass. Catalytic dehydration of

carbohydrates

can

generate

two

important

furans

(furfural

and

5-hydroxymethylfurfural). Among them, 5-hydroxymethylfurfural (HMF) has been identified as one of the top 12 important platform chemicals by the US Department of Energy.6 It is a powerful intermediate linking the biomass based carbohydrate chemistry with conventional petroleum based industrial chemical technology.7 Hundreds of commodity chemicals and fuels can be obtained from HMF via different kinds of chemical reactions.8 HMF and its derivatives could potentially replace voluminously consumed petroleum-based building blocks, which are currently used to make plastics and fine chemicals. For example, 2,5-furandicarboxylic acid (FDCA), derived from the oxidation of HMF, can potentially replace terephthalic, alternative to petroleum-based terephthalic acid for the synthesis of PET (polyehyleneterephthalate) and PBT (polybutylenetere- phthalate) plastics.9 3


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Catalytic oxidation of HMF can generate several kinds of furan chemicals (Fig. 1). Besides the above mentioned FDCA, 2,5-diformylfuran (DFF) has also been found to be widely used in many fields such as the synthesis of pharmaceuticals, furanic polymers, antifungal agents, furan-urea resin.10-13 Synthesis of DFF is attractive and challenging. In early, the oxidation of HMF into DFF was performed by the use of classical oxidants, such as such as NaOCl and pyridinium chlorochromate.14-16 However, those methods demonstrated some distinct drawbacks such as the release of hazardous wastes and the high cost of the oxidants. To overcome the drawback of the use of expensive oxidants with high toxicity, catalytic oxidation of HMF with molecular oxygen seems to be promising, as air is cheap and environmental-friendly. To date, homogeneous catalytic system such as Co/Mn/Br,17 and Cu(NO3)2/VOSO4)18 and heterogeneous transition metal-based catalysts mainly including

ruthenium,19-22

vanadium,23–25 manganese,26, 27 have been studied for the aerobic oxidation of HMF into DFF. However, most of these catalysts suffer from low activities, high catalyst-to-substrate ratios and difficult recyclability because of the leaching of active species. For example, H-beta zeolite supported V2O5 catalyst (V2O5/H-beta) afforded 84% conversion of HMF and 82% yield of DFF under 10 bar oxygen pressure at 125 o

C after 3 h.23 However, V2O5 leached into the reaction solution seriously. Through

the hot-filter experiments, 45% of the total activity was from dissolved V2O5. Therefore, it is still desirable to design new catalytic systems for the effective oxidation of HMF into DFF.

4


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O

O

O

MA O O

OH [O]

O

O

O

O

[O]

DFF

HMF

[O]

[O]

O O

O

O OH

[O]

FFCA

O HO

OH

FDCA

O O HO

OH

HFCA

Fig. 1 The possible reaction routes for the oxidation of HMF In recent years, Ru based catalysts have emerged as effective metal catalysts for the oxidation of HMF into DFF with high conversion and selectivity.19-22 For the Ru catalysts, the active Ru species were the supported Ru nanoparticles. Ru nanoparticles easily suffered from the loss of its catalytic activity due to the leach of active species into the reaction solution or the aggregation of nanoparticles. For example, Antonyraj and co-workers reported that full conversion of HMF and 97% selectivity of DFF were achieved at 130 oC and 2.8 bar O2 after 4 h over Ru/γ-Al2O3,19 but the conversion decreased over five consecutive recycling steps. Nie et al. reported that the hydrothermal treatment of the spent Ru/C catalyst was essential to keep its catalytic activity.20 Besides the supported Ru nanoparticles, supported Ru complex with the active species as Ru3+ were also reported for the oxidation of HMF into DFF. For example, Chen

et

al.

reported

that

the

ruthenium

complex

immobilized

on

poly(4-vinylpyridine)-functionalized carbon-nanotube could produce DFF with a yield of 94% at 120 oC under 2 MPa O2. Nevertheless, this catalytic system still loss it catalytic activity due to the leach of Ru3+. In addition, the use of high expensive carbon nanotube and the tedious procedure of the catalyst preparation which makes this catalytic system less attractive for the oxidation of HMF. Herein, SBA-15 as the 5


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common and cheap support was used to graft the diimidazole linker for immobilzation of the Ru3+ for the selective oxidation of HMF into DFF. EXPERIMENTAL SECTION Materials RuCl3·xH2O (38.0-42.0 wt.% Ru), imidazole, sodium hydroxide, dimethylsulfoxide, 1,2-dibromoethane and 3-chloropropyltriethoxysilane (Cl-PTES) were purchased from Aladdin Chemicals Co. Ltd. (Beijing, China). Tetraethoxysilane (TEOS, 99.5%) and P123 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2,5-Diformylfuran was purchased from the J&K Chemical Co. Ltd., (Beijing, China). 5-Hydroxymethylfurfural (98%) was supplied by Beijing Chemicals Co. Ltd. (Beijing, China). Fructose was purchased from Sanland-Chem International Inc. (Xiamen, 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, USA). Preparation of SBA-15 SBA-15 was prepared according to the known procedure.28 In a typical synthesis, P123 (2.0 g) was firstly dissolved in 2 M HCl solution (70 mL) at 35 oC. To this solution, TEOS (4.2 g) was added dropwise with a magnetic stirring. Then the mixture was kept at 35 oC under a stirring for 24 h. The synthesis gel was kept at 100 o

C for another 24 h under static conditions. After cooling to room temperature, the

slurry was filtered, washed thoroughly with distilled water until the pH was 7, and

6


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thereafter dried in air at 100 oC for 12 h. The polymer P123 was removed by calcination in air at 500 oC for 6 h. Synthesis of 1-(2-(1H-imidazol-1-yl)ethyl)-1H-imidazole N

N-Na+

+

Br

Br

N

N

N

N

+ NaBr

1

Fig. 2. Synthesis of 1-(2-(1H-imidazol-1-yl)ethyl)-1H-imidazole (compound 1) The

schematic

illustration

of

the

synthesis

of

1-(2-(1H-imidazol-1-yl)ethyl)-1H-imidazole is shown in Fig. 2. The reaction of imidazole (2.72 g, 40 mmol) and sodium hydroxide (1.60 g, 40 mmol) were first performed in 10 mL of dimethylsulfoxide at 60 oC for 1 h to give the salt. After cooling down the reaction mixture to room temperature, 1,2-dibromoethane (3.76 g, 20 mmol) was added and stirred at room temperature for 24 h. The resulting mixture was poured into 200 mL of ice cold water and the precipitated product was filtered, washed

with

cold

water

(3

×

50

mL),

and

dried

to

give

1-(2-(1H-imidazol-1-yl)ethyl)-1H-imidazole as a white powder with a yield of 95% Modification SBA-15 with 3-Chloropropanyl Group SBA-15 (1.0 g) was suspended in 50 mL of toluene, and Cl-PTES (1.0 g) was added. The mixture was stirred at reflux temperature for 2 days in a nitrogen atmosphere. Then the solid phase was separated, washed with toluene and ethanol, giving rise to 3-chloropropanyl functionalized SBA-15 (abbreviated as SBA-15-Cl). 1-(2-(1H-imidazol-1-yl)ethyl)-1H-imidazole (1.0 g) was added to the suspension of the SBA-15-Cl (1.0 g) in 50 mL of dry toluene, and then the mixture was refluxed for

7


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48 h under nitrogen atmosphere. The solid was then filtered and washed thoroughly with toluene, followed by ethanol in a Soxhlet extractor. After drying under high vacuum, the solid was obtained with gray color, which was abbreviated as SBA-15-Biimidazole. Preparation of the Ru Catalyst The SBA-15-Biimidazole support (1.0 g) was added to 40 mL of an aqueous ruthenium chloride solution (5×10-3 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 SBA-15-Biimidazole support. The catalyst was then collected from the solution by a permanent magnet, washed twice with

distilled

water

and

acetone

and

dried

in

vacuum

(denoted

as

SBA-15-Biimidazole support-Ru). Catalyst Characterization Transmission electron microscope (TEM) images were obtained using an FEI Tecnai G2-20 instrument. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.3. The mesoporous pore size distributions were derived from desorption branches of isotherms by using BJH model. The sample powder were firstly dispersed in ethanol and dropped onto copper grids for observation. X-ray powder diffraction (XRD) patterns of samples were determined with a Bruker advanced D8 powder diffractometer (Cu Kα). All XRD patterns were collected in the 2θ range of 0–5◦ with a scanning rate of 0.016◦/s. X-ray photoelectron spectroscopy (XPS) was conducted 8


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on a Thermo VG scientific ESCA MultiLab-2000 spectrometer with a monochromatized Al Kα source (1486.6 eV) at 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 C1s (284.6 eV) peak of the contamination carbon as an internal standard. FT-IR measurements were recorded on a Nicolet NEXUS-6700 FTIR spectrometer with a spectral resolution of 4 cm-1 in the wave number range of 500-4000 cm-1. Thermogravimetric analysis (TGA) was performed with a thermogravimetric analyzer (NETZSCH TG209) at a heating rate of 10 K min-1. The Ru content in the catalyst was quantitatively determined by inductively coupled atomic emission spectrometer (ICP-AES) on an IRIS Intrepid II XSP instrument (Thermo Electron Corporation). Firstly, the catalyst was calcinated in the air at 600 oC to burn the organic biimidazole in the catalyst, and Ru3+ was transformed to its oxides. Then, the resulting powder was treated with HNO3 (20 wt.%) to leach the Ru3+ in the solution, and SBA was removed by centrifugation. Typical Procedure for the Oxidation of HMF In a typical run, the oxidation of HMF was conducted in an autoclave equipped with pressure control system. 50 mg of the SBA-15-Biimidazole-Ru catalyst and 0.5 mmol of HMF was added to 8 ml of p-Chlorotoluene in the autoclave with a magnetic bar. Then the autoclave was flushed with oxygen for five times to completely remove the air. After being sealed, the autoclave was charged with O2 until 15 bar, then it was heated to 110 oC with a magnetic stirrer for 12 h. After reaction, the products were analyzed by HPLC. 9


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Analytic Methods Analysis of the content of HMF and DFF were performed on a ProStar 210 HPLC system coupled with a UV detector. Samples were separated by a reversed-phase C18 column (200 × 4.6 mm) with a detection wavelength of 280 nm. The mobile phase was composed of acetonitrile and 0.1 wt.% acetic acid aqueous solution at the volume ratio of 30:70, and the flow rate was set at 1.0 mL/min. The content of HMF and DFF in samples were obtained directly by interpolation from calibration curves. When using HMF as starting material, DFF yield and HMF conversion are defined as follows: HMF conversion = (1-moles of HMF/moles of starting HMF)×100% DFF yield = moles of DFF/moles of starting HMF×100% When using fructose as the starting material, DFF yield and HMF yield were calculated according to the following equations: HMF yield = moles of HMF/moles of starting fructose ×100% DFF yield = moles of DFF/moles of starting fructose ×100% Recycling of Catalyst After reaction, the SBA-15-Biimidazole-Ru catalyst was recovered by centrifugation, washed three times with ethanol, and dried at 60 oC overnight in a vacuum oven. The spent catalyst was reused for the next cycle under the same reaction conditions. RESULTS AND DISCUSSION Synthesis and Characterization of the Catalyst

10


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OH OH

O

Cl(CH2)3Si(OEt)3

O

Toluene, Reflux OH SBA-15

RuCl3

N Si

O

Si OEt

N

N

O O

Toluene, Reflux

OH SBA-15-Cl

O

N

Cl

OEt

N+ Cl-

N

N

N+ Cl-

Si OEt

N

N

N

OH SBA-15-Biimidazole

N

3+

Ru

OH SBA-15-Biimidazole-Ru

Fig. 3 Schematic illustration for the synthesis of SBA-Im-Ru catalyst The SBA-15-Biimidazole-Ru catalyst was prepared according to the procedure shown in Fig. 3. SBA-15 reacted with 3-chloropropyltriethoxysilane in toluene at reflux temperature,

giving

SBA-15-Cl.

Then

1-(2-(1H-imidazol-1-yl)ethyl)-1H-imidazole

SBA-15-Cl to

further

produce

the

reacted

with

diimidazole

functionalized SBA-15. The SBA-15-Biimidazole-Ru catalyst was prepared by the strong coordination ability of imidazole group with Ru3+. During the reaction process, the black solution became clear in a short time, suggesting

that the

SBA-15-Biimidazole material showed a high ability to anchor the Ru3+. The loading of ruthenium in the catalyst SBA-15-Biimidazole-Ru was determined to be 2.0 wt.% by ICP-AES. N2 adsorption-desorption technology was used to characterized the texture structure of the SBA-15 and the SBA-15-Biimidazole-Ru catalyst. BET surface area of SBA is 875 m2 g-1, while that was only 491 m2 g-1 for the SBA-15-Biimidazole-Ru catalyst, and the corresponding volume also decreased from 1.41 cm3 g-1 to 0.793 cm3 g-1. These results suggested that the surface of the mesoporous SBA-15 was modified by functionalized groups. TEM image of the SBA-15-Biimidazole-Ru is shown in Fig. 4. It shows the image of channels and framework, demonstrating a highly ordered pore structure. TEM image indicated the structural feature of SBA-15 was remained 11


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after the chemical modifications. However, it is clearly observed that the surface of SBA-15 was modified a lay of the Ru-imidazole complex, as indicated by the dark area of the TEM image.

Fig. 4 TEM image of the SBA-15-Biimidazole-Ru catalyst. The low-angle XRD pattern of SBA-15-Biimidazole-Ru is depicted in Fig. 5. The strong peak observed at 2θ = 0.9o is the characteristic peak of SBA-15, which is assigned to (100) diffraction peak. The characteristic diffraction peak indicates that the typical two-dimensional hexagonal symmetry of SBA-15 was present in the SBA-15-Biimidazole-Ru catalyst,29 suggesting that the mesoporous structure of the catalysts remained without change after the modification, which is consistent with the TEM result. In addition, there was no other peaks present in the high-angle XRD patterns with 2θ=10o-80 o of the prepared catalyst, which suggested that no crystalline Ru species were observed in the catalyst.

12


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Intensitity (a. u.)

Page 13 of 28

1

2

3

4

5

2θ

Fig. 5. XRD pattern of the synthesis of SBA-15-Biimidazole-Ru catalyst The SBA-15-Biimidazole-Ru catalyst is further characterized by FT-IR technology (Fig. 6). As shown in Fig. 6, the strong peak in the FT-IR spectra of the SBA-15-Biimidazole-Ru catalyst around 1000 to 1200 cm-1 is assigned to the anti-symmetric Si-O-Si stretching vibration. Bands around 2920 cm-1 and 2878 cm-1 , which were assigned to the aliphatic CH vibration, are clearly observed in the SBA-15-Biimidazole-Ru catalyst, indicating the Cl-PTES has been successfully immobilized on the surface of SBA-15.30 The bands at 3436 cm-1 and 1630 cm-1 were attributed to the -OH vibration of the SBA-15. Two weak peaks at 3164 and 3110 cm-1 were assigned to the stretching vibration of the aromatic C-H of imidazolium ring, and the band at 622 cm-1 was attributed to the bending vibration of the C-H bond in imidazole. In addition, the peaks at 1566 cm-1 and 1460 cm-1 were assigned to the vibration of C=C and C=N bonds in the imidazole ring, respectively.31 These peaks confirmed that the biimidazolium groups were successfully grafted on the surface of SBA-15

by

the

reaction

of

SBA-15-Cl

with

the

1-(2-(1H-imidazol-1-yl)ethyl)-1H-imidazole. 13


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Transmittance (a.u)

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4000

3500

3000

2500

2000

1500

1000

500

Wavenumber(cm-1)

Fig. 6 FT-IR of the SBA-15-Biimidazole-Ru catalyst The surface composition of SBA-15-Biimidazole-Ru catalyst was analyzed by XPS technology. As shown in Fig. 7a, these peaks corresponding to carbon, oxygen, nitrogen, silicon, chloride and ruthenium are observed in the survey scan of XPS spectrum of the SBA-15-Biimidazole-Ru catalyst. The presence of N 1s peak once again confirmed that the imidazole groups were successfully grafted on the surface of the SBA-15 support. In addition, it is worth noting that the intensity of Cl 1s peak was too weak to be observed. The molar ratio of Cl- to Ru3+ is 3 in precursor of RuCl3, but the atomic ratio of Ru/Cl was 15 by XPS, Ru3+ was coodirnetated with the nitrogen atom due to the high affinity of the lone pair electrons in nitrogen to Ru3+. The high resolution XPS spectrum of Ru3p is shown in Fig. 7 b. Two peaks with the binding energies at 461.9 and 484.2 eV were corresponded to the Ru 3p3/2 and Ru 3p1/2, respectively, which indicated that the oxidation state of the ruthenium species is +III.32 The XPS results strongly indicated that Ru3+ was actually coordinated to SBA-15-Biimidazole.

14


Page 15 of 28

(a)

Intensity (a.u.)

O1s

C1s N1s Ru3p

Si2s Si2p O2s

Cl 2p 1000

800

600

400

200

0

Binding Energy (eV)

(b)

Ru3p 3/2

Ru3p 1/2

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

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490

480

470

460

Binding Energy (eV)

Fig. 7 XPS of the SBA-15-Biimidazole-Ru catalyst. (a) Survey scan of the catalyst; (b) Ru3p region Thermogravimetric (TG) analysis is important method for the study of organic-inorganic hybrid materials. Therefore, in order to get more information about the SBA-15-Biimidazole-Ru catalyst, both the support SBA-15-Biimidazole and the catalyst SBA-15-Biimidazole were subjected to be treated in N2 atmosphere. As shown in Fig. 8, SBA-15-Biimidazole started to decompose at 150 oC, while the SBA-15-Biimidazole-Ru catalyst started to decompose at higher temperature of 170 o

C. The reason should be that the interaction of imidazole groups with Ru3+ enhanced

the stability of the catalyst. The initial weight loss at lowest temperature for the two 15


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samples should be attributable to physically absorbed water. Interestingly, it is noted that the weight percentage of absorbed water was the same for the two samples. For the SBA-15-Biimidazole, two distinct decomposition temperature regions were observed. The first region from 150 oC to 367 oC was assigned to the decomposition of the structure of organic groups. In the second region from 367 oC to 685 oC was attributed to the decomposition of organic residues with high molecular weight. For the SBA-15-Biimidazole-Ru catalyst, similar TG curve was observed, but with a terminal temperature lag for each region. This phenomenon can also be explained by the strong interaction of Ru3+ with organic groups that enhanced the catalyst stability, as observed for the delayed starting decomposition temperature. However, it is reported that the TG curves of the organic groups functionalized silica support and its supported nanopartices showed no apparent difference. For example, Wei and co-workers observed that the trend of TG curve of the ionic liquids functionalized silica and Pd nanoparticles in ionic liquids functionalized silica was the same.33 The TG results of the as-prepared SBA-15-Biimidazole-Ru also confirmed that the Ru3+ had a strong interaction with the biimidazole rings. In addition, the weight percentage obtained from TG analysis was very close to the results by ICP.

100

98.07 %

95

Weight (%)

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15.27 %

90 85

82.80 %

80

9.12% 73.68 %

75 70 0

100 200 300 400 500 600 700 800 900

Temperature (oC)

16


Page 17 of 28

100

98.08 %

95

Weight (%)

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

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13.87 %

90

84.21 %

85

8.31 %

80

75.90 % 75 0

200

400

600

800

1000

Temperature (oC)

Fig. 8 TG-DTA patterns of the two samples. (a) SBA-15-Biimidazole, (b) SBA-15-Biimidazole-Ru. Catalytic Oxidation of HMF into DFF in Various Solvents Table 1. The results of the oxidation of HMF into DFF in various solvents.a

Entry

a

HMF

DFF

DFF Selectivity

conversion (%)

yield (%) (%)

Solvent

1

CH3CN

8.9

7.6

85.4

2

CH3CH2OH

9.0

7.1

78.9

4

MIBK

15.1

12.4

82.1

3

DMSO

19.1

11.9

62.3

5

Toluene

31.5

28.4

90.2

6

p-Chlorotoluene

32.9

29.9

90.9

7b

p-Chlorotoluene

0.9

-

-

Reaction conditions: HMF (63 mg, 0.5 mmol) and catalyst (50 mg) were added into

the solvent (8 mL), and the reaction was carried out at 110 °C for 12 h with the flush of O2 at a speed of 20 mL min-1. b

SBA-15-Biimidazole was usded as the catalyst. The activity of the as-prepared SBA-15-Biimidazole-Ru catalyst was evaluated by

oxidation of HMF with molecular oxygen. The solvent effect on the catalytic activity 17


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of the SBA-15-Biimidazole-Ru catalyst was initially studied, and the results are shown in Table 1. Reactions were carried out at 110 oC with oxygen follow rate at 20 ml/min under atmospheric pressure. Low HMF conversions with DFF selectivity around 80% was observed when the oxidation of HMF was carried out in aprotic acetonitrile (MeCN) and protic ethanol with low boiling point (Table 1, Entries 1 &2). Methyl isobutyl ketone (MIBK) afforded similar DFF selectivity with a slight higher DFF yield (Table 1, Entry 3). HMF conversion slightly increased in strong none-aprotic polar solvent with high boiling point such as dimethyl sulfoxide (DMSO) (Table 1, Entry 4), but DMSO gave the lowest DFF selectivity. Among the testing solvents, aromatic solvents were superior to other solvnts in terms of HMF conversion. (Table 1, Entries 5 & 6). Similar results were obtained in toluene and p-chlorotoluene. The best results were achieved in p-chlorotoluene, affording HMF conversion of 32.9% and DFF selectivity of 90.9%. In addition, the oxidation of HMF was also carried out in p-chlorotoluene using SBA-15-Biimidazole. As expected, a neglegible HMF conversion was observed (Table 1, Entry 7). These results indicated that HMF was stable in our reaction system and the active site was the anchored Ru3+. The Effect of Oxygen Pressure on the Oxidation of HMF into DFF Next, the reaction conditions were optimized for the oxidation of HMF by the study of oxygen pressure. Table 2 shows the effect of oxygen pressure on the effect of HMF conversion and DFF selectivity. It is noted that HMF conversion was affected by the oxygen pressure, while DFF selectivity did not changed remarkably with the increase of oxygen pressure. DFF selectivity remained at a stable level around 90% in all cases. 18


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Energy & Fuels

As far as HMF conversion, it increased dramatically with the increase of the oxygen pressure from 1 bar to 10 bar, and slowly increased from 10 bar to 15 bar. The increase of HMF conversion with an increase of oxygen pressure should be due to the fact that oxygen concentration increased with an increase of oxygen pressure in the range from 1 bar to 15 bar. However, further increasing oxygen pressure, HMF conversion leveled off at 15–20 bar. The highest yield of DFF was achieved in 88.7% with HMF conversion of 96.9% after 12 h under 15 bar oxygen pressure. Compared with the results in Entries 4-6, HMF conversion was not greatly affected by the oxygen pressure, possibly due to the fact that the concentration of oxygen in the reaction solution almost reached constant at high pressure of 10~20 bar. Table 2. The results of the oxidation of HMF into DFF at different oxygen pressure

a

Entry

Oxygen pressure (bar)

HMF conversion (%)

DFF yield (%)

DFF Selectivity (%)

1

1

25.7

23.4

90.9

2

2.5

49.9

45.3

90.8

3

5

84.8

78.1

92.1

4

10

92.1

85.5

92.8

5

15

96.9

88.7

91.5

6

20

97.3

87.9

90.3

Reaction conditions: HMF (63 mg, 0.5 mmol) and catalyst (50 mg) were added into

p-chlorotoluene (8 mL), and the reaction was carried out at 110 °C for 12 h under different oxygen pressure. The effect of Reaction Temperature on the Oxidation of HMF into DFF 19


Energy & Fuels

Finally, the effect of the reaction temperature on the oxidation of HMF was carried out and the results are shown in Fig. 9. Reactions were carried out under 10 bar oxygen pressure at 90 oC, 110 oC and 130 oC, respectively. As shown in Fig. 9, a significant effect of reaction temperature on HMF conversion is observed. HMF conversion greatly increased from 44.1% at 90 oC to 92.1% at 110 oC. Further increasing the reaction temperature to 130 oC, HMF conversion of 100% was achieved after 12 h under 10 bar oxygen pressure. As far as the yield of DFF, it also increased with the increase of the reaction temperature from 90 to 110 oC. However, a slight lower DFF yield of 81.7% was obtained at 130 oC, which indicated that higher reaction temperature accelerated the side reactions during the process of HMF oxidation. Therefore, 110 oC should be an appropriate temperature to achieve high catalytic activity and DFF selectivity.

HMFconversion DFF yield

100

Percentage (%)

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 20 of 28

80 60 40 20 0 90

110

130

Temperature (oC)

Fig. 9 Effect of reaction temperature on the oxidation of HMF. Reaction conditions: HMF (63 mg, 0.5 mmol) and catalyst (50 mg) were added into the p-chlorotoluene (8 mL), and the reaction was carried out at different reaction temperature for 12 h under 10 bar oxygen pressure.

20


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One-pot Conversion of Fructose into DFF Currently, the direct conversion of carbohydrates into HMF derivates is an attractive direction in biomass conversion. Therefore, the one-pot conversion of fructose into DFF was also studied by the combination of the commercial availability Amberlyst 15 and the SBA-15-Biimidazole-Ru catalyst, which were used for the dehydration of fructose into HMF and the subsequent oxidation of HMF into DFF. p-Chlorotoluene was the best solvent for the oxidation of HMF into DFF, but fructose can not disscolve in p-chlorotoluene. Therefore, a two-step method was adopted for the one-pot conversion of fructose into DFF (Fig. 10). Firstly, the dehydration of fructose was carried out in 1 ml of DMSO at 110 oC, affording high HMF yield of 91.1% after 2 h. Then the solid catalyst was recovered from the reaction system, and p-chlorotoluene (7 ml) and the SBA-15-Biimidazole-Ru catalyst were added. The subsequent oxidation reaction was carried out at 110 oC for 12 h under 20 bar oxygen pressure. DFF yield was calculated to be 72.4% from fructose. In addition, FDCA was also produced in a little amount of 5.3%. These results indicated that the activity of the SBA-15-Biimidazole-Ru catalyst for the oxidation of HMF into DFF was not affected by the impurities from the first step of the dehydration of fructose. CH2OH CH 2OH O OHC OH OH Fructose

Step 1

O O

OH

Step 2

O

HMF

O

O

DFF

Fig.10 Direct conversion of fructose into DFF via two-step method. The Stability of the SBA-15-Biimidazole-Ru catalyst

21


Energy & Fuels

80

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

Page 22 of 28

60

40

20

0 1

2

3

4

5

6

Run No. Fig. 11 The results of catalyst recycling experiments. Reaction conditions: HMF (63 mg, 0.5 mmol), catalyst (50 mg), p-chlorotoluene (8 mL), at 110 °C , 10 bar. The recyclability of the SBA-15-Biimidazole-Ru catalyst was investigated because the reuse of the heterogeneous catalyst is an important property over the homogeneous catalysts. The oxidation of HMF was used as the model reaction, and the reaction was carried out at 110 oC for 12 h under 10 bar O2 pressure. After the first run, the catalyst was recovered by centrifugation. Then, the catalyst was washed several times by ethanol, and dried at 50 oC in a vacuum oven. The spent catalyst was reused under identical reaction conditions. The reactivity could be fully restored and no apparent loss in efficiency was observed for up to five runs ( Fig. 11). We found that both the structure of the catalyst and the reaction medium should be attributed to the high stability of the SBA-15-Biimidazole-Ru catalyst. On the one hand, our catalyst did not suffer the change of the morphology and the oxidation state, which was often observed for the supported Ru nanoparticles catalysts. For example, Nie et al. tested Ru on several supports at 110 oC for the oxidation of HMF, and found 22


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Energy & Fuels

that the catalyst had to be reactivated by hydrothermal treatment for 4 h after each reaction cycle.20 On the other hand, the SBA-15-Biimidazole-Ru catalyst did not suffer from the leaching. In fact, no Ru species were detected by ICP in the reaction solution due to the fact that the strong interaction between Ru3+ and the biimidazloe ring. In addition, the reaction solvent p-chlorotoluene did not dissolve Ru3+ or coordinate with the Ru3+, which might also played a role to improve the high stability of the catalyst. For example, a leaching of Ru3+ was observed by Chen and co-workers

when

they

used

Ru

complex

immobilized

on

poly(4-vinylpyridine)-functionalized carbon-nanotube as the heterogeneous catalyst for the oxidation of HMF in the strong polar solvent N,N-dimethylformamide (DMF). CONCLUSIONS In this study, a new SBA-15-Biimidazole-Ru catalyst was successfully prepared by the anchor of the Ru3+ with the biimidazole groups on the surface of SBA-15. The SBA-15-Biimidazole-Ru catalyst showed high activity toward the aerobic oxidation of HMF into DFF, and the active sites were proved to be Ru3+. Several parameters were investigated and high HMF conversion of 96.9% and DFF yield of 88.7% were obtained after 12 h at 110 oC under 15 bar oxygen pressure. Compared with other kinds of Ru based catalysts, the SBA-15-Biimidazole-Ru catalyst showed high stability, and could be reused without the loss of its catalytic activity. More importantly, the transformation of fructose into DFF can also be smoothly proceeded via two steps by the combined use of Amberlyst 15 and the SBA-15-Biimidazole-Ru catalysts, which were used for the dehydration of fructose and the subsequent 23


Energy & Fuels

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oxidation reaction, respectively. A satisfactory DFF yield of 72.4% was achieved from fructose. Due to the excellent performace of the SBA-15-Biimidazole-Ru catalyst, the developed method showed a promising potential in the conversion of abundant carbohydrates into valuable bulk chemicals. ACKNOWLEDGEMENTS The Project was supported by National Natural Science Foundation of China (No. 21203252) AUTHOR INFORMATION Corresponding Author *Tel.: +86-27-67842572. Fax: +86-27-67842572. E-mail: [email protected]

Notes The authors declare no competing financial interest. REFERENCES

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Catalytic Conversion of Fructose and 5-Hydroxymethylfurfural into 2,5

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