Enhanced Formation of 5-Hydroxymethylfurfural from Glucose Using a

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Kinetics, Catalysis, and Reaction Engineering

Enhanced Formation of 5-Hydroxymethylfurfural from Glucose Using a Silica-supported Phosphate and Iron Phosphate Heterogeneous Catalyst Fangmin Huang, Yuwen Su, Zhouyang Long, Guojian Chen, and Yue Yao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01531 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Industrial & Engineering Chemistry Research

Enhanced Formation of 5-Hydroxymethylfurfural from Glucose Using a Silica-supported Phosphate and Iron Phosphate Heterogeneous Catalyst Fangmin Huang,† Yuwen Su,† Zhouyang Long,*,†Guojian Chen,† and Yue Yao*,‡ †

School of Chemistry and Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry

for Functional Materials, Jiangsu Normal University, Xuzhou 221116, People’s Republic of China. ‡

College of Chemical Engineering and Materials Science, Tianjin University of Science and

Technology, Tianjin 300457, People’s Republic of China. * Corresponding authors, E-mail: [email protected] (Z. Y. Long); [email protected] (Y. Yao)

1

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Industrial & Engineering Chemistry Research 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

ABSTRACT: The production of 5-hydroxymethylfurfural (HMF) from carbohydrates is scientifically valuable but technologically challenging. The silica-supported phosphate and ironic phosphate heterogeneous catalyst H3PO4-SiO2-FePO4(0.15) was prepared and used to catalyze the conversion of glucose to 5-hydroxymethylfurfural (HMF). The morphologies, acidic properties and surface chemical state of the H3PO4-SiO2-FePO4(0.15) were characterized via XRD, FTIR, Py-FTIR, SEM, XPS and NH3-TPD. The H3PO4-SiO2-FePO4(0.15) gave a high yield of HMF (76.3%) from glucose under 433 K for 100 min. The H3PO4-SiO2-FePO4(0.15) can be used for four runs without significant decrease of catalytic activity. The influence of the reaction conditions on the HMF yields is investigated and the kinetic analysis was also carried for the present catalytic system. Keywords: Iron phosphate; Phosphoric acid; Silica; Glucose; 5-Hydroxymethylfurfural

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1. INTRODUCTION Nowadays, human beings are confronted with the problems of petroleum resources diminishing and environmental pollution.1 An effective method to relieve these problems is to use biomass as the feedstock for the production of chemicals or transportation fuels.2,3 The conversion of biomass to 5-hydroxymethylfurfural (HMF) has received much attention due to the extensive applications of HMF for producing furanic polyesters, polyamides, and polyurethanes analogous.4-9 To date, many catalytic systems have been reported for the conversion of glucose to HMF. Among them, using homogeneous catalysts is followed by complex catalyst separation process.10-14 Due to the advantage of easy recovery, various heterogeneous catalysts have been exploited for this reaction, such as mesoporous aluminium-doped MCM-41 silica,9 zeolite,15,16 amorphous Cr2O3, SnO2 and SrO,7 modified tin oxide17 and ZrO2,18,19 etc. Nevertheless, complex synthetic steps for the catalysts preparation, using expensive ionic liquid as the solvent and requiring harsh reaction conditions are usually involved in these reported heterogeneous catalytic systems,20,21 which limits their further application. Given the above, highly efficient heterogeneous catalytic systems with the merits of environment-friendliness and cost-effectiveness are still urgently desired for this reaction. Here we report a highly efficient heterogeneous catalyst H3PO4-SiO2-FePO4 for the conversion of glucose to HMF. The H3PO4-SiO2-FePO4 is prepared by loading H3PO4 and FePO4 on the support SiO2 with proper proportion. The reaction conditions are optimized and the catalyst reusability is studied. Under the optimized conditions, high HMF yield and superior catalyst reusability are observed. A dual-acid catalytic process is described based on the reaction results, the characterization analysis and preciously reported works. 2. EXPERIMENTAL PROCEDURE 2.1 Materials and Methods All chemicals were of analytical grade and used as received. X-ray diffraction (XRD) of the catalyst H3PO4-SiO2-FePO4 were recorded with Cu K (D2 PHASER) radiation (λ=0.1541 nm) using a PANalyticalXpert Pro instrument (BRUKER) at 298 K with a silicium mono-crystal 3



sample holder at step size of 0.017o. The intensity (Miller indices) as a function of 2θ was measured while the angle range was 5-40o. FTIR spectrum (4000-500 cm-1) was recorded by a Bruker Vertex 80 V FTIR vacuum spectrometer (Ettlingen, Germany) with a resolution of 2 cm-1 and 32 scans per sample. Field emission scanning electron microscope (FESEM; Hitachi SU8010, accelerated voltage: 15 kV) was used to study the morphology of the catalysts. Temperature-programmed desorption of ammonia (NH3-TPD) was conducted using a QuantachromeChem BET Pulsar TPD/TPR to measure the acidities of catalysts. Firstly, the sample (0.2 g) pre-treatment at 823 K for 30 min was placed in a U-shaped tube reactor with He. Subsequently, the sample adsorbed 10 vol. % NH3/He at 323 K with saturation, then using He to eliminate the physisorbed ammonia. Finally, the NH3-TPD analysis was carried out by heating the sample from 323 K at a heating rate of 10 K·min-1. The evolved ammonia was analyzed by a TCD. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250Xi spectrometer with a monochromated Al Ka radiation source (hv=1486.6 eV) and a multichannel detector. All spectra were recorded by using an aperture slot of diameter 500 µm, and survey spectra were recorded with pass energy of 100 eV while high-resolution spectra with a pass energy of 30 eV. The binding energies were calibrated by the adventitious C 1s peak at 284.8 eV as reference. The experiment error was given within ± 0.1 eV; the HPLC-MS analysis was carried using Thermo LC-MS via ESI ionization method. Pyridine Fourier-transform infrared (Py-FTIR) experiments were performed using a USA PE Frontier FT-IR Spectrometer. The concentrations of the Brønsted and Lewis acidic sites on the samples were determined using the FT-IR spectra of adsorbed pyridine; the sample was inserted into a measurement cell with KBr windows that was connected to a vacuum apparatus. The sample was treated at 623 K under vacuum for 2 h and was subsequently cooled to room temperature to collect the background spectra. The pyridine adsorption was performed by equilibrating the sample for 30 min at room temperature. To calculate the weak acidic sites and strong acidic sites, the IR spectra for the samples were recorded after degassing for 60 min at 473 K. Fe content in the liquid phase was analyzed by Agilent 7900 inductively coupled plasma (ICP) spectrometer-mass spectrometry 4


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(MS). 2.2 Catalyst Preparation The H3PO4-SiO2-FePO4 is prepared via two steps as shown in Scheme 1: Firstly, the H3PO4 reacts with the OH groups on the surface of SiO2 via dehydration, and the H3PO4-SiO2 forms at this step, which introduces the Brønsted acid sites on the support SiO2.22-24 Secondly, the FeCl3·6H2O is added to generate the Lewis acid sites of FePO4 on the support SiO2. For example, the target catalyst H3PO4-SiO2-FePO4(0.15) is prepared as follows: 2 g silica gel (60-100 mesh) is put into an aqueous H3PO4 solution (30 wt%, 30 mL); The suspension solution is stirred for 3 h under 2000 rpm. Then 0.15 g FeCl3·6H2O is added into the above suspension solution; The pH value of the suspension solution is adjusted to 7 using NaOH solution (5 wt%). The suspension is filtered; the solid is washed for three times with water and dried at 333 K in an oven. The obtained sample is denoted as H3PO4-SiO2-FePO4(0.15). The control catalysts of H3PO4-SiO2-FePO4(0.11) and H3PO4-SiO2-FePO4(0.19) are prepared by the same method, with the added amount of FeCl3·6H2O being 0.11 g and 0.19 g.

Scheme 1. Two steps for the H3PO4-SiO2-FePO4 preparation. H3PO4-alumina-FePO4(0.15) and H3PO4-carbon-FePO4(0.15) are prepared with the same method as H3PO4-SiO2-FePO4(0.15) using acidic alumina or activated carbon instead of SiO2. Unsupported FePO4 is prepared as the same way as H3PO4-SiO2-FePO4 without adding SiO2. H3PO4-SiO2 is prepared as: 2 g silica gel is put in H3PO4 solution (30 wt%), and the mixture is stirred for 3 h under 2000 rpm; The pH value of the suspension solution is adjusted to 7 using NaOH solution (5 wt%). Then the suspension is filtered, and the solid is collected and dried under 5



333 K for 8 h in a dry oven. 2.3 Typical procedure for the catalytic conversion of glucose into HMF In a typical run, an aqueous solution consists of 1 mL acetone and 5 mL water, 0.25 g glucose and 0.2 g H3PO4-SiO2-FePO4(0.15) are added to a sealed 25 mL thick-walled glass reactor (Load = 4%; Load = mglucose: (mglucose + msolvent), in which mglucose is the quantity of glucose and msolvent is the quantity of the solvent.). The reaction mixture is stirred at 500 rpm at various temperatures for a given reaction time. After reaching the target reaction time, the reaction is quenched by introducing the reactor into a cooled water bath. The sample is filtered and further employed for product analysis. For catalyst recycling tests, the H3PO4-SiO2-FePO4(0.15) is separated from the reaction mixture by filtration and washed. The recycled H3PO4-SiO2-FePO4(0.15) is dried in a vacuum oven at 333 K for 12 h and used for the reuse test. 2.4 Determination of the products The products and byproducts in the reaction system are determined using Thermo LC-MS via ESI ionization method. The HMF concentration is determined using the High-performance Liquid Chromatography (HPLC, Agilent 1200) using a UV detector and a column (Zorbax SB-C18) maintained at a column temperature of 303 K, using water-methanol mixture(15/85; v/v) as the mobile phase at a flow rate of 0.4 mL·min-1. The glucose conversion and the yield of HMF are calculated according to the following formulas based on the glucose concentration determined with a refractive index detector and an Carbomix H-NP column maintained at a column temperature of 328 K, using H2SO4 solution (25 mmol·L-1) as the mobile phase at a flow rate of 0.6 mL·min-1. The CG and CP correspond to the molar concentration of glucose and product (HMF), respectively, and the subscripts 0 and t correspond to t=0 and reaction time t, respectively. The glucose conversion: XG= [(CG-0-CG-t) / CG-0] (1) The yield of HMF: YP= [(CP-t-CP-0) / CG-0] (2) 6


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3. RESULTS AND DISCUSSION 3.1 Catalysts characterization The target catalyst H3PO4-SiO2-FePO4(0.15) and its control catalysts are characterized by XRD, FT-IR, SEM, XPS, NH3-TPD and Py-FTIR. Figure 1 shows that the H3PO4-SiO2-FePO4(0.15) and the support SiO2 give similar XRD spectra of a broad peak, indicating they are amorphous. The XRD pattern of the unsupported FePO4 gives no peak (Figure 1), which indicates that the unsupported FePO4 employed in this experiment is also amorphous. The SEM images (Figure 2 and Figure S1 in Supporting Information) shows that the target catalyst H3PO4-SiO2-FePO4(0.15), the support SiO2, the control catalyst of H3PO4-SiO2 and the unsupported FePO4 exhibit bulk morphology with micrometer size. The elemental mapping analysis reveals the uniform distribution of the two elements of Fe and P on the surface of the H3PO4-SiO2-FePO4(0.15). This indicates that the H3PO4 and FePO4 are highly dispersed on the surface of the H3PO4-SiO2-FePO4 (0.15), rather than a simple mechanical mixture.

H3PO4-SiO2-FePO4(0.15)

SiO2 Unsupported FePO4

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


10

20

30

40

50

2 Theta (degree) Figure 1. The XRD patterns of the support SiO2, the H3PO4-SiO2-FePO4(0.15) and the unsupported FePO4.

7



Figure 2. The element mapping of Fe and P on the H3PO4-SiO2-FePO4(0.15).

The elemental composition of the H3PO4-SiO2-FePO4(0.15) was further examined by XPS determination. The results listed in Table S1 show that the atomic contents of Si, O, P and Fe are 24.7%, 65.0%, 6.4% and 3.9%, respectively. Accordingly, the ratio of P to Fe in the H3PO4-SiO2-FePO4(0.15) is about 1.6:1. Considering that the ratio of P to Fe in FePO4 molecule is 1:1, the molar ratio of the H3PO4 to FePO4 in the H3PO4-SiO2-FePO4(0.15) is reasonably estimated to be around 0.6:1. It is noted that in the catalyst preparation, 0.15 g of FeCl3·6H2O is added to 2 g of silica gel, thus the theoretical value for the Si/Fe atomic ratio in the H3PO4-SiO2-FePO4(0.15) is about 60; while the Si/Fe atomic ratio value of 38.3 is obtained by the EDS analysis (Figure S2, Table S2) and 6.3 obtained by the XPS analysis. Considering that the units of spatial resolution using EDS for the specimen are microns and XPS only provides information about the surface of a sample (2-3 nm), it reasonably to propose that there might be an enrichment of the FePO4 species on the surface of the support SiO2. To determine whether the FePO4 species are mainly on the surface or in the pores of the support SiO2, the textural properties of the support SiO2 and the target catalyst H3PO4-SiO2-FePO4(0.15) were studied by N2 adsorption-desorption isotherm test (Figure 3A). The BET (Brunauer-Emmett-Teller) surface area for the SiO2 is 335.8 m2/g with the most probable pore size centered at 9.8 nm (Figure 3B); while the BET surface area for the H3PO4-SiO2-FePO4(0.15) is 246.1 m2/g with the most probable pore size centered at 10.0 nm (Figure 3B). These results suggest that after loading FePO4 on the support SiO2, the BET surface area decreases but the probable pore size hardly changes, implying that most of the FePO4 are loaded on the SiO2 surface rather than in its pores, which is in accordance with the above discussion about the XPS analysis. 8


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Figure 3. (A) N2 adsorption-desorption isotherm and (B) BJH pore size distribution of the support SiO2 and the target catalyst H3PO4-SiO2-FePO4(0.15).

SiO2

Transmittance (T%)

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


H3PO4-SiO2-FePO4(0.15) 1078

949

FePO4

2000

547 900~1200 1500

1000

500 -1

Wavenumber(cm

)

Figure 4. The FTIR spectrum of the support SiO2, the H3PO4-SiO2-FePO4(0.15) and FePO4.

Figure 4 gives the FTIR spectra of the support SiO2, FePO4 and H3PO4-SiO2-FePO4(0.15). Compared with the spectrum of support SiO2, the spectrum of the H3PO4-SiO2-FePO4(0.15) gives three new bands at 547 cm-1, 949 cm-1 and 1078 cm-1. The band at 949 cm-1 is within the range of 900 and 1200 cm-1 which exists as a broad peak in the spectrum of FePO4 and is assigned to the P-O vibrations of PO43- polyanion.25 The band at 547 cm-1 for is also observed in the spectrum of FePO4 which corresponds to the O-P-O antisymmetric bending band according to the previously reported works.26,27 The 1078 cm-1 band of H3PO4-SiO2-FePO4(0.15) is attributed to the vibration of Fe-P-O,26,27 which further confirms the load of FePO4 on the support SiO2.

9



360

320

TCD signal

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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SiO2 0.19g 0.15g 0.11g

280

240

200

100

200

300

400

500

600

o

Temperature ( C) Figure 5. The NH3-TPD spectrum of the catalyst H3PO4-SiO2-FePO4 with different FePO4 loading amount.

Table 1. Acidity data deduced from Py-FTIR analysis.

Catalyst

CB+L (µmol/g)

CB (µmol/g)

CL (µmol/g)

CB/CL


58.33

19.56

38.77

0.50


59.11

26.81

32.30

0.83


59.01

30.79

28.22

1.09

The acid strength of the support SiO2 and the H3PO4-SiO2-FePO4 catalysts with different loading amount of FePO4 is determined by NH3-TPD. As shown in Figure 5, the peak from 120 oC to 300 oC corresponds to the hydroxyl groups on the surface of the support SiO2.9 The desorption temperature

of

the

three

H3PO4-SiO2-FePO4

catalysts,

H3PO4-SiO2-FePO4(0.11),

H3PO4-SiO2-FePO4(0.15) and H3PO4-SiO2-FePO4(0.19), is between 220 oC and 380 oC, which indicates that their acidity is stronger than the support SiO2. Then the acid categories of the three H3PO4-SiO2-FePO4 catalysts are tested by the Py-FTIR analysis. As shown in Figure 6, the absorbance peaks at about 1538 cm-1 and 1445 cm-1 in the desorption spectrum under 200 oC indicate the existence of the Brønsted and Lewis acid sites.9 The Brønsted acid sites are due to the presence of H3PO4 on the surface of the H3PO4-SiO2-FePO4 catalysts, and the Lewis acid sites may be resulted from the existence of FePO4 on them.24 Therefore, the H3PO4-SiO2-FePO4 catalysts simultaneously contain the Brønsted and Lewis acid sites and thus are regarded to be the dual-acid 10


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catalysts. As shown in Table 1, the total concentration of the Brønsted acid sites to Lewis acid sites in

the

three

catalysts

of

H3PO4-SiO2-FePO4(0.11),


and

H3PO4-SiO2-FePO4(0.19) are 58.33 µmol/g, 59.11 µmol/g and 59.01 µmol/g, respectively, indicating that they have similar acid sites concentration. Moreover, for the target catalyst H3PO4-SiO2-FePO4(0.15), it was found that the concentration ratio of Brønsted acid sites to Lewis acid sites (CB/CL) was 0.83, which is moderate when compared with the other two catalysts of H3PO4-SiO2-FePO4(0.11) and H3PO4-SiO2-FePO4(0.19).

H3PO4-SiO2-FePO4(0.15) Absorbance (%)

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


H3PO4-SiO2-FePO4(0.11) H3PO4-SiO2-FePO4(0.19)

1560

1520

1480

1440

1400

-1

Wavenumber (cm ) Figure 6. The FTIR spectrum of pyridine adsorption of the H3PO4-SiO2-FePO4(0.15), H3PO4-SiO2-FePO4(0.11) and H3PO4-SiO2-FePO4(0.19).

3.2 Catalytic tests in the conversion of glucose to HMF

H3PO4-SiO2-FePO4(0.15) and several control catalysts are tested for the conversion of glucose to HMF as shown in Table 2. Under the given reaction conditions, blank test gives a low yield of HMF (5.3%) (Table 2, entry1). A similar result is also observed when using SiO2 as the catalyst (Table 2, entry 2), suggesting that the support SiO2 itself is inactive for the reaction. After loading H3PO4 on the support SiO2, the H3PO4-SiO2 gives a higher yield of HMF (39.5%) (Table 2, entry 3). The H3PO4-SiO2-FePO4(0.15), which is prepared by further loading FePO4 onto the H3PO4-SiO2, leads to a much higher yield of HMF (76.3%) (Table 2, entry 4). The mainly existing byproducts are furfural, rather than levulinic acid or formic acid as evidenced by the HPLC-MS (Figure S3). By contrast, the unsupported FePO4 only gives a yield of HMF (35.3%) (Table 2, entry 5). When using 11



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acidic alumina or activated carbon as the support, 11.9% and 22.4% HMF yields are obtained over the H3PO4-alumina-FePO4(0.15) and H3PO4-carbon-FePO4(0.15) catalysts (Table 2, entries 6 and 7), respectively, which are much lower than the one obtained over the H3PO4-SiO2-FePO4(0.15). Hence, the SiO2 is the best support in this work. The loading amount of FePO4 on the support SiO2 is tuned by changing the amount of Fe3+ added, and then the influence of the loading amount of FePO4 on the reaction is also investigated. The H3PO4-SiO2-FePO4(0.11) and H3PO4-SiO2-FePO4(0.19) give 53.7% and 48.7% HMF yields, respectively (Table 2, entries 8 and 9), lower than the one offered by the H3PO4-SiO2-FePO4(0.15). Furthermore, we simply mixed the H3PO4-SiO2 and the unsupported FePO4 together to catalyze the reaction, and only 38.8% yield of HMF is obtained (Table 2, entry 10). This result shows that the H3PO4-SiO2-FePO4(0.15) is better for the glucose conversion than the simple mechanical mixture of the H3PO4-SiO2 and the unsupported FePO4. Besides, the enrichment of the FePO4 species on the surface of the support SiO2 in H3PO4-SiO2-FePO4(0.15) may lead to the facile contact between the reactant and the catalytic sites, which is beneficial to promote the catalytic performance. It is noted that when pure water is used as the solvent, the HMF yield obtained over the H3PO4-SiO2-FePO4(0.15) catalyst is only 49.8% (Table 2, entry 11), lower than the one obtained with the aqueous solution of acetone. It is considered that the product HMF tends to decompose after forms in a monophasic reaction system. The added acetone in the solvent may reduce the formation of the by-product humin by protecting the hexose from degradation and thereby increase the selectivity towards HMF.3,28,29 Table 2. The results for the conversion of glucose to HMF over various catalysts. Entry

Catalyst

XG/%

Yp/%

1

_

19.2

5.3

2a

SiO2

27.6

6.0

3b

H3PO4-SiO2

63.1

39.5

4c


99.9

76.3

5d

Unsupported FePO4

65.4

35.3

12


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6c

H3PO4-alumina-FePO4(0.15)

51.4

11.9

7c

H3PO4-carbon-FePO4(0.15)

58.4

22.4

8c


95.0

53.7

9c


99.7

63.7

10e

H3PO4-SiO2+unsupported FePO4

65.8

38.8

11f


99.8

49.8

Reaction conditions: 0.25 g glucose, water/acetone (5 mL/1 mL), 433 K, 100 min, 500 rpm; a

0.12 g catalyst;

b

0.17 g catalyst;

c

0.20 g catalyst;

d

0.03 g catalyst; e H3PO4-SiO2(0.17 g) + the unsupported

FePO4 (0.03 g).

Figure 7. The conversion of glucose to HMF over the H3PO4-SiO2-FePO4(0.15).

According to the reaction results, the characterization results and the previously reported works,30,31 the reaction pathway for the conversion of glucose to HMF catalyzed by the dual-acid catalyst H3PO4-SiO2-FePO4(0.15) is described in Figure 7. Firstly, the Lewis acid site FePO4 on the H3PO4-SiO2-FePO4(0.15) catalyzes the conversion of glucose to fructose; then the fructose is dehydrated to the target product HMF over the Brønsted acid site H3PO4 on the H3PO4-SiO2-FePO4(0.15) catalyst. In the absence of FePO4 or H3PO4, the H3PO4-SiO2 or the unsupported

FePO4

gives

much

lower

HMF

yield.

Moreover,

the

target

catalyst

H3PO4-SiO2-FePO4(0.15) which has the CB/CL value of 0.83 offers higher 5-HMF yield than the H3PO4-SiO2-FePO4(0.11) and H3PO4-SiO2-FePO4(0.19) which have lower or higher CB/CL values of 0.50 and 1.09, respectively, although the total concentration of the Brønsted acid sites to Lewis acid sites in the three catalysts are very close as shown in Table 1. This suggests that the moderate 13



CB/CL is beneficial to obtain a high 5-HMF yield. Besides, when the mixture of H3PO4-SiO2 and unsupported FePO4 is used as the catalyst, the HMF yield obtained is still lower than the one offered by the H3PO4-SiO2-FePO4(0.15) where the Brønsted and Lewis acid sites are much closer in distance than the mechanically mixed catalyst, confirming the existence of the synergistic effect between the two acid sites in the H3PO4-SiO2-FePO4(0.15).

Glucose conversion Yield of HMF

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

50

0

1

2

3

4

Reaction runs Figure 8. The variation of the glucose conversion and the yield of HMF with reaction run. Reaction conditions: 0.25 g glucose, water/acetone (5 mL/1 mL), 433 K, 100 min, 500 rpm, 0.2 g catalyst was added for the first run.

When the reaction complete, the solid catalyst H3PO4-SiO2-FePO4(0.15) was isolated by filtration. The filtrate was analysis with ICP to determine the Fe content in it. There is only 2.1 ppm Fe in the filtrate, indicating that the loss of the FePO4 is negligible. This result confirms that the H3PO4-SiO2-FePO4(0.15) really is a heterogeneous catalyst. The reuse performance is important to evaluate the efficiency of heterogeneous catalytic systems according to the principles of green and sustainable chemistry. Here, the reusability of the H3PO4-SiO2-FePO4(0.15) is studied as shown in Figure 8. Although the glucose conversion and the HMF yield slightly decrease during the reusability tests, 93.4% glucose conversion and 68.7% HMF yield are still obtained over the H3PO4-SiO2-FePO4(0.15) even at the fourth run. This result reveals the superior reusability of the H3PO4-SiO2-FePO4(0.15). The deactivation of the catalyst may be due to the formation of undetectable byproduct of humins on the catalyst surface, although the humins is hardly observed 14


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by eyes. Table 3. The comparisons of catalytic behavior of H3PO4-SiO2-FePO4 and the representative catalytic systems reported for the conversion of glucose to HMF

a

Catalyst

Solvent

Load a

T/K

t/min

Yp/%

Ref.

H3PO4-SiO2-FePO4

Water/acetone

4.0%

433

100

76.3

This work

FePO4

Water/THF

2.5%

413

60

22.6

(32)

Sn-Beta/HCl

Water

10.0%

413

120

7.92

(33)

Al-MCM41

Water/MIBK

--

468

150

31.3

(9)

SnPO b

[EMIM]Br

10.0%

393

180

54.9

(34)

Beta-Cal750 c

Water/DMSO

2.4%

453

60

20.2

(35)

TaPO d

Water/MIBK

3.0%

443

60

18.5

(36)

SO4/ZrO2

Water

0.5%

373

360

10.0

(37)

FeCl3

[EMIM]Cl

6.0%

353

180

10.0

(10)

CrCl3

[EMIM]Cl

6.0%

353

180

70.0

(10)

Load = mglucose: (mglucose + msolvent), in which mglucose is the quality of glucose and msolvent is the quality of the

solvent.

b

SnPO is the abbreviation of tin phosphate; c Beta-Cal750: Beta-NH4 was calcined in air at 1023 K to

obtain the proton-form samples; d TaPO is the abbreviation of tantalum phosphate.

The catalytic performance of the H3PO4-SiO2-FePO4(0.15) is compared with some previously reported works as shown in Table 3. The HMF yield of 76.3% obtained over the H3PO4-SiO2-FePO4(0.15) in the present work is much higher than the ones over the reported heterogeneous catalysts, such as FePO4 (22.6%),32 Sn-Beta/HCl (7.92%),33 Al-MCM (31.3%),9 SnPO (54.9%),34 Beta-Cal750 (20.2%),35 TaPO (18.5%)36 and SO4/ZrO2 (10.0%).37 The reaction result here is even better than the one offered by the reported homogeneous catalyst, such as FeCl3 (10.0%) and CrCl3 (70.0%), which implied the expensive ionic liquid as the solvent.10 Besides, compared with the homogeneous catalysts, the H3PO4-SiO2-FePO4(0.15) is conveniently to be recovered and reused, which meets the requirement of green chemistry. 3.3 The influence of reaction temperature and time on the reaction

The influences of reaction temperature and time on the glucose conversion and the HMF yield 15



are investigated through parallel experiments by varying the reaction time from 20 min to 160 min at 393 K, 403 K, 413 K, 423 K and 433 K (Figure 9). The reaction time required to attain a HMF yield of 49.2% is only 40 min at 433 K, while a longer time of 100 min is required to achieve 49.5% yield of HMF at 403 K; by contrast, only 18.5% yield of HMF is achieved after 100 min at 393 K. The HMF yield reaches the maximum value of 54.8% at 403 K for 120 min, 61.1% at 413 K for 120 min, 67.7% at 423 K for 120 min and 76.3% at 433 K for 100 min. At 433 K, the yield of HMF increases rapidly in the initial stage, and the HMF yield of 72.3% is obtained after 80 min. In addition, in all cases, the yield of HMF decreases on further extending the reaction time and it decreases more sharply at higher reaction temperatures with unidentified soluble polymers and humins being observed.32

Figure 9. The influence of the reaction temperature (K) and time on the glucose conversion (a) and the yield of HMF (b). Reaction conditions: 0.25 g glucose, 0.2 g catalyst, water/acetone (5 mL/1 mL), 500 rpm.

3.4 The influence of the catalyst H3PO4-SiO2-FePO4(0.15) dosage on the reaction

The influence of the catalyst H3PO4-SiO2-FePO4(0.15) dosage on the glucose conversion and HMF yield is studied. As shown in Figure 10, with increasing the H3PO4-SiO2-FePO4(0.15) dosage from 0.08 g to 0.2 g, the glucose conversion and the HMF yield increase from 81.8% to 99.9% and from 39.3% to 76.3%, respectively. However, further increasing the H3PO4-SiO2-FePO4(0.15) dosage to 0.24 g results in a remarkable decrease of HMF yield to 58.2%. This may be due to the formation of more byproducts promoted by the excess catalyst.14

16


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Page 17 of 24

Glucose conversion

100

Yield of HMF

80

%

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


60 40 20 0

0.08

0.12

0.16

0.20

0.24

H3PO4-SiO2-FePO4(0.15) dosage (g) Figure 10. The influence of the H3PO4-SiO2-FePO4(0.15) dosage on the reaction. Reaction conditions: 0.25 g glucose, water/acetone (5 mL/1 mL), 433 K, 100 min, 500 rpm.

3.5 Kinetic analysis of the conversion of glucose

The kinetics of glucose conversion in water/acetone with the presence of the H3PO4-SiO2-FePO4(0.15) is studied. The value of ln (1-x) (where x refers to the glucose conversion) is plotted against reaction time t at different temperature (393 K~433 K) aiming to obtain the value of reaction rate constant k (Figure S4). The results in Table 4 show that the value of rate constant k increases with the increase of reaction temperature, indicating that higher temperature accelerates the glucose conversion, which is agreed with the previous report.38 Using the rate constant value, an Arrhenius plot is generated (Figure S5). The kinetic parameters for H3PO4-SiO2-FePO4(0.15) catalyzed glucose conversion to HMF are summarized in Table 5. It is found that the value of activation energy is 32.7 kJ/mol (Table 4, entry 2), which is lower than that of the solid acid Sn-Beta (95.0 kJ/mol) and ordered mesoporous Nb-W oxides (90.2 kJ/mol) for the isomerization of glucose.20,39 The lower activation barrier clearly demonstrates the outstanding activity of H3PO4-SiO2-FePO4(0.15) for glucose conversion. Table 4. The reaction rate constant of the glucose conversion under different reaction temperature 17



Page 18 of 24

Entry

Temperature (K)

k (min-1)

Correlation coefficient

1

393.15

0.0037±2.25×10-4

0.9893

2

403.15

0.0072±6.39×10-4

0.9770

3

413.15

0.0147±1.87×10-3

0.9766

4

423.15

0.0222±2.44×10-3

0.9766

5

433.15

0.0300±2.98×10-3

0.9762

Conditions: 0.2 g H3PO4-SiO2-FePO4(0.15); 500 rpm; water/acetone (5 mL/1 mL).

Table 5. The kinetic parameters for the conversion of glucose Parameter

Value

Reaction order, n

1

Activation energy, Ea (kJ/mol)

32.7

Pre-exponential factor, A (/min)

4.18×107

Correlation coefficient

0.9900

4. CONCLUSION

An effective heterogeneous catalyst H3PO4-SiO2-FePO4(0.15) is prepared for the conversion of glucose to HMF by loading H3PO4 and FePO4 on the support SiO2. High glucose conversion and HMF yield are obtained over this dual-acid catalyst, where the Lewis acid site FePO4 and the Brønsted acid site H3PO4 work synergistically. Furthermore, H3PO4-SiO2-FePO4(0.15) can be reused at least four times without significant loss of the catalytic activity, confirming its potential for further application. ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications web site. AUTHOR INFORMATION Corresponding authors

*E-mail: [email protected] (Z. Y. Long) *E-mail: [email protected] (Y. Yao) 18


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ORCID

Zhouyang Long: 0000-0002-5520-6263 Notes

The authors declare no competing financial interest. ACKNOWLEDGMENTS This research is funded by NSFC (Nos. 21503098, 21603089), PAPD of Jiangsu Higher Education Institutions, Jiangsu Province Science Foundation for Youths (BK20160209), the Youth Foundation of Southeast University ChengXian College (z0003). REFERENCES (1) Catrinck; M. N.; Ribeiro, E. S.; Monteiro, R. S.; Ribas; R. M.; Barbosa, M. H. P.; Teófilo, R. F. Direct conversion of glucose to 5-hydroxymethylfurfural using a mixture of niobic acid and niobium phosphate as a solid acid catalyst. Fuel. 2017, 159, 280-286. (2) Zhang, Y.; Wang, J.; Li, X.; Liu, X.; Xia, Y.; Hu, B.; Lu, G.; Wang, Y. Efficient conversion of glucose to HMF using organo catalysts with dual acidic and basic functionalities-A mechanistic and experimental study. Fuel. 2017, 162, 30-36. (3) Pedersen, A. T.; Ringborg, R.; Grotkjær, T.; Pedersen, S.; Woodley, J. M. Synthesis of 5-hydroxymethylfurfural (HMF) by acid catalyzed dehydration of glucose-fructose mixtures. Chem. Eng. J. 2015, 273, 455-464. (4) Pagán-Torres, Y. J.; Wang, T.; Gallo, J. M. R.; Shanks, B. H.; Dumesic, J. A. Production of 5-hydroxymethylfurfural from glucose using a combination of Lewis and Brønsted acid catalysts in water in a biphasic reactor with an alkylphenol solvent. ACS Catal. 2012, 2, 930-934. (5) Zhang, Y.; Pan, J.; Shen, Y.; Shi, W.; Liu, C.; Yu, L. Brønsted acidic polymer nanotubes with

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Enhanced Formation of 5-Hydroxymethylfurfural from Glucose Using a

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