Conversion of fructose into furfural or 5-hydroxymethylfurfural over HY zeolites selectively in γ-butyrolactone

Accepted Manuscript Title: Conversion of fructose into furfural or 5-hydroxymethylfurfural over HY zeolites selectively in ␥-butyrolactone Authors: Liqin Wang, Heqin Guo, Qilong Xie, Jungang Wang, Bo Hou, Litao Jia, Jinglei Cui, Debao Li PII: DOI: Reference:

S0926-860X(18)30619-7 https://doi.org/10.1016/j.apcata.2018.12.023 APCATA 16927

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

14 September 2018 30 November 2018 19 December 2018

Please cite this article as: Wang L, Guo H, Xie Q, Wang J, Hou B, Jia L, Cui J, Li D, Conversion of fructose into furfural or 5-hydroxymethylfurfural over HY zeolites selectively in ␥-butyrolactone, Applied Catalysis A, General (2018), https://doi.org/10.1016/j.apcata.2018.12.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Conversion of fructose into furfural or 5-hydroxymethylfurfural over HY zeolites selectively in γ-butyrolactone Liqin Wang

a, b

, Heqin Guo a, Qilong Xie a, Jungang Wang a, Bo Hou a, Litao Jia a,

Jinglei Cui c, *, Debao Li a, * a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001,PR China

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b University of Chinese Academy of Sciences, Beijing, 100049, PR China c State Environmental Protection Key Laboratory of Efficient Utilization Technology

of Coal Waste Resources, Institute of Resources and Environmental Engineering,

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Shanxi University, Taiyuan, 030006, PR China * Corresponding author. Tel: +86-351-4040087

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E-mail: [email protected] (Jinglei Cui); [email protected] (Debao Li)

Highlights

Brønsted acid sites density in HY play vital role in products distribution in

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

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ED

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A

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Graphical abstract

fructose conversion in GBL-water solvent.



The maximum furfural yield was 37.8% over HY-3, while the maximum HMF

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yield was 69.2% over HY-1.



This work can help to understand the synergy between the solvent and acid sites in the process for furfural from fructose.

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Abstract Furfural is one of the most valuable biomass platform compounds which is typically prepared from hemicellulose. Conversion of cellulose and its derived hexoses, which are the most abundant resource in nature, to furfural, is a big challenge. The amount of the reactive form (fructofuranose) which was beneficial to the formation of furfural and HMF, was increased when the solvent transformed from

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water to γ-butyrolactone (GBL)-water. The GBL promoted the adsorption of fructose

on HY. The transfer of fructose from the solution to the channels of HY was enhanced

with the introducing of GBL in solvent. The HY zeolite with apertures of 7.4 Å was

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found to promote the formation of acyclic fructose from cyclic fructose (8.6 Å) in the synergy with GBL. The Brønsted acid sites in the channels of HY favored the

selective cleavage of the C-C bond in acyclic fructose to xylose, and promoted the

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following dehydration of xylose to furfural, simultaneously. The furfural yield was

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increased while the 5-hydroxymethylfurfural (HMF) yield was decreased with the decrease of the Brønsted acid sites density. The maximum furfural yield was 37.8%

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over HY-3, while the maximum HMF yield was 69.2% over HY-1 in the conversion

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of fructose in GBL-water solvent. In addition, the etherification of HMF was the main factor for the low HMF yield and carbon balance in fructose conversion in GBL-water

Keywords:

HY

ED

solvent. zeolites,

Furfural,

5-Hydroxymethylfurfural,

Fructose,

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cleavage of carbon-carbon bond

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1. Introduction

With the excess exploitation and utilization of fossil fuels, energy crisis,

climate change and environmental pollution are increasingly serious[1]. The

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development of clean renewable energy is an efficient way to solve these problems. The biofuel, which can be obtained from renewable and abundant biomass, seems to be an ideal solution[2]. 5-Hydroxymethylfurfural (HMF) and furfural, which were used as feedstocks for high-valued chemicals, are the major plant-derived platforms chemicals obtained from the conversion of C6 and C5 sugars[2]. Generally, HMF can be achieved through the dehydration of hexoses and cellulose. HMF is the raw material for the preparation of 2,5-furan 2

dicarboxylic acid, 2,5-diformylfuran, 2,5-dimethylfuran, 1,6-hexanediol and levulinic acid[3]. Recently, a great deal of efforts have been devoted to develop efficient solid acid catalysts for HMF production in lab scale process. The relatively high production cost for HMF is the major obstacle to realize its commercialization, which resulted in a low output of HMF[4]. Furfural is one of the most valuable biomass-derived chemicals, which can supplement the fossil energy for preparing resins, lubricants, adhesives and plastics[5].

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Meanwhile, furfural is the feedstock for producing high value-added chemicals,

such as furfuryl alcohol, furan, maleic acid and 2-methyltetrahydrofuran[2].

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The furfural market was persistently increasing, which driven by the growing

global demand for renewable biomass-based chemicals. The process for furfural in the industry is the hydrolysis of hemicelluloses to C5 sugars, following by the dehydration of C5 sugars to furfural using mineral acids as

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catalysts[6]. This technology cannot convert cellulose and its derived C6 sugars

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to furfural, and result in the waste of cellulose and its derived C6 sugars which

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account for the most abundant of biomass resources[7]. Therefore, conversion

efficient utilization of biomass.

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of cellulose and its derived C6 sugars to furfural are significant for highly

The key in the conversion of cellulose and C6 sugars to furfural (C5) is the

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selective cleavage of the C-C bond. Typically, hexose is dehydrated to produce HMF instead of cleaving the C-C bond to yield furfural[8-10]. However, some

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researchers have found that furfural could also be obtained from C6 sugars. Luijkx et al. reported that a low yield of 6.7% for furfural could be achieved in the hydrothermal condition when the feedstock was cellobiose[11]. Jae et al.

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found that the production of furfural through the fast pyrolysis of glucose with a yield of 40% was achieved in the presence of ZK-5[12]. Guerbuez et al. found that the solvent γ-valerolactone (GVL) combined with H-mordenite zeolite

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promote the formation of furfural from glucose with a yield of 37%[13]. Recently,

furfural

obtained

from

fructose

in

the

presence

of

γ-butyrolactone(GBL)-water solvent and H-Beta zeolites was also studied[14]. Zhang et al. reported that the Sn-Beta was an effective catalyst in the preparation of furfural from glucose[15]. The researchers have proposed that the formation of acyclic fructose with the help of shape-selective zeolites was the key process for the selective cleavage of the C-C bond in fructose[14]. 3

Typically, the C-C bond cleavage of acyclic fructose obey the retro-adol mechnism[16], and acid sites favor this process[17]. The zeolites, which had suitable pore structure and appropriate acid sites amount, may favor the formation of furfural in the conversion of fructose. Moreover, there is no yet a conclusion about active sites in catalyst which was essential for the selective C-C bond cleavage of fructose. Clearly grasping the active sites for the production of furfural or HMF from fructose would help to understand these

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processes.

Solvent also play a vital role in the formation of furfural from hexoses. Cui

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et al. found that the synergy between Hβ zeolites and GBL-water solvent promoted the formation of pentoses through the selective C-C bond cleavage of acyclic hexoses, and furfural was obtained through the dehydration of pentoses[14]. Recently, Wang et al. reported that the coordinated state of

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framework aluminum in zeolites was affected by solvent, which play critical

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roles in the selectivity of furfural in the conversion of hexose[18]. In conclusion,

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the solvent might affect the solubility of substrates, mass transfer, the interaction between substrates and products, the stabilization of transition state

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and the solvated degree of catalysts in the conversion of sugars[2]. Beyond that, the composition of fructose tautomers was altered with the change of solvent,

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as reported by Kimura[19]. The formation of acyclic fructose may be promoted with the help of solvent effect. Moreover, there is not yet a conclusion about the

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solvent effect on the composition of fructose tautomers, which may affect the selectivity of furfural in the conversion of fructose. In this work, HY zeolites with different density of Brønsted acid sites were

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used as catalysts in fructose conversion. Infrared spectroscopy of adsorbed pyridine was used to confirm the amount of Brønsted and Lewis acid sites in HY zeolites. Infrared spectroscopy of adsorbed 2,6-bis(tert-butyl)pyridine

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(DTBPy) was also introduced to measure the amount of Brønsted acid sites in HY zeolites. 13C NMR spectroscopy was applied to investigate the composition of fructose conformers in different solvent. Fructose adsorption test was performed to investigate the solvent effect. The soluble oligomers which formed in the conversion of fructose in GBL-water solvent over HY-3 were analyzed by electrospray ionization mass spectrometry (ESI-MS). A correlation between the furfural (or HMF) yield with the density of Brønsted acid sites in 4

HY zeolites was established. The possible pathways in the conversion of fructose over HY-1 and HY-3 were also proposed. 2. Materials and methods 2.1. Materials

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D-glucose (99%), D-fructose (99%), xylose (98%), arabinose (98%), furfural (99%), HMF (99%), formic acid (99%), levulinic acid (99%), n-pentanoic acid (99%),

1,4-dioxane (99%), γ-valerolactone (GVL) (98%), 1,4-dioxane (99%) and

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tetramethylene sulfone (98%) were purchased from Aladdin. γ-Butyrolactone (GBL)

(99.5%) was purchased from Hangzhou Youer biological technology Co., Ltd.. All above reagents were used without further purification. HY-1 was purchased from The

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Catalyst Plant of Nankai University. HY-2 and HY-3 were supplied by Alfa Aesar.

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2.2. The preparation of the adsorbed fructose in HY zeolites for IR

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0.75 g fructose and 0.1 g HY-3 zeolite were added into 14.25 g water,

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GBL/water (mass ratio: 1:1), or GBL/water (mass ratio: 4:1) solvent. Then the mixture was agitated for 4 h at 25°C. The HY-3 was obtained through centrifuging,

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washing with the solvent for several times, and drying at 25°C for 48 h.

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2.3. Catalyst characterization

The lattice structures of the catalysts were measured on a Bruker Advanced

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X-ray Solutions/D8-Advance working with a Cu Kα1 radiation (λ = 0.15418 nm, 50 kV, 35 mA) source with a scanning speed of 1.2°/s in the range from 3° to 85° (2θ). The TG test of the used catalyst was performed on a SetaramTGA-92 analyzer with an air flow of 50 mL/min and a heating rate of 10°C/min. Before the test, the spent

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catalyst (HY-3-C) was dried in vacuum at 100°C for 12 h. The amounts of Si and Al in HY zeolites were calculated by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Nitrogen adsorption and desorption isotherms were measured at -196°C using a Micromeritics ASAP2020HD88 instrument after the HY zeolites were degassed at 300°C for 12 h under vacuum.

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Infrared spectroscopy of adsorbed pyridine (Py-IR) was introduced to measure the amounts of Brønsted and Lewis acid sites in HY zeolites using a Nicolet Magna 550 spectrophotometer. Infrared spectroscopy of adsorbed DTBPy was also used to measure the amount of Brønsted acid sites in HY zeolites using a Nicolet Magna 550 spectrophotometer. The self-supporting wafers of 20 mg/m2 were degassed at 450°C for 1 h under vacuum (10-3 Pa) and then exposured to pyridine or DTBPy for 30 minutes followed by outgassing at 150°C. The integrated area of adsorption bands at

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ca. 1540 and 1450 cm-1 were used to confirm the amount of the Brønsted and Lewis

acid sites, respectively[20]. The peak at 1616 cm-1 in DTBPy-IR spectrum was

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attributed to Brønsted acid sites in HY[21]. Infrared spectroscopy of HY-3 which had adsorbed fructose was analyzed using a Nicolet Magna 550 spectrophotometer. The acid amounts in HY zeolites were determined by NH3-TPD recording on a fixed-bed equipped with mass spectra[22]. The HY zeolite (100 mg) was pretreated at

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500°C for 1 h under Ar flow of 30 ml/min and then cooled to 120°C. The HY zeolite

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was exposed to NH3 until saturation. The weakly physisorbed NH3 on the surface of

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HY zeolite was removed through flushing with Ar flow at 120°C for 1 h. The desorption of NH3 was recorded on a Shanghai GC-920 equipped with a thermal

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conductivity detector (TCD) using a linear heating rate of 10°C/min from 120°C to 600°C. The quantitative analysis of the acid amount was determined by the area of the

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desorption peaks. The tautomers of fructose in GBL-water or water were measured by 13

C-NMR on a Bruker 400 MHz spectrometer[23]. The internal calibration and the

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locking of magnetic field were achieved through tetramethylsilane in the D2O solution within a sealed capillary which was put into the NMR tube. The 1-13C-labelled fructose was dissolved in water or GBL-water with a ratio of 4:1 to form the 0.10 M

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solution. 1-13C-labelled fructose solutions were injected into the NMR tubes for the subsequent analyses. ESI-MS (LCMS-2020, Shimadzu) was applied to analyze liquid oligomers . The operating parameters of the instrument: capillary voltage 4.5 kV; dry

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heater, 150 °C; N2 flow, 4.0 L/min. 2.4. Catalytic tests The catalytic experiments were carried out in a 50 mL magnetically stirred stainless steel autoclave. In a typical reaction, sugar (0.75 g), GBL-water (14.25 g) and a certain amount of catalyst were charged into the batch reactor. Before the 6

reaction, the reactor was flushed with N2 (99.95%) for three times and then pressurized to 2 MPa at room temperature. The reactor was heated to the desired temperature from 25°C for 30 minute. After the reaction, the reactor was immersed in an ice bath and cooled to room temperature rapidly. The liquid products were obtained by the rapid centrifugation of the mixture of products and catalyst. The spent catalyst whichseparated from the reaction solution was washing with GBL-water and water for several times, and then dried at 110°C for 12 h before the next recycling test.

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The liquid products were analyzed by HPLC (Agilent 1260) equipped with a Shodex

SC-1821 capillary column (300 mm×8 mm×0.6 μm). The external standard method

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was applied to calculate the concentration of the products. The furfural and 5-HMF were analyzed by a gas chromatograph (GC) equipped with a flame ionization (FID)

detector (Agilent Technologies, Inc.) and an HP-INNOWAX capillary column (30

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m×0.32 mm, 0.5 μm).

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3.1. Characterization of the catalysts

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3. Results and discussion

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The lattice structure of the HY zeolite was characterized by X-ray diffraction (XRD) in the 2 range of 5-50° (Fig. 1). The peaks which centered at 6.3, 10.3, 12.1,

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16.0, 19.0, 20.8, 24.1, and 27.6°, were corresponding to the typical peaks of the "FAU" phases[24], indicating that the three HY zeolites were retained the FAU

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typical texture. The Si/Al ratios of HY zeolites were increased from 4.5 to 51.3 (Table 1). All of the HY zeolites possessed high surface areas (875-928 m2/g) and

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micropore volumes (0.246-0.315 cm3/g). The N2 adsorption-desorption isotherms of

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HY zeolites at 77 K were shown in Fig. S1 (in ESM).

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Fig. 1. The XRD patterns of HY-1, HY-2 and HY-3. Table 1

Si/Ala

HY-1

S (m2/g)b Smicro

4.5

886

790

HY-2

17.8

928

678

HY-3

51.3

875

611

V (cm3/g)c

Vtotal

Vmicro

96

0.408

0.315

250

0.545

0.273

264

0.543

0.246

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a The

SEXT

A

SBET

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Catalyst

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Physicochemical properties of HY zeolites a.

Si/Al molar ratios were measured by ICP-OES; b SBET: BET surface area, Smicro (micropore

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area) and SEXT (external surface area) calculated by the t-plot method. c Vmicro (micropore volume) calculated by the t-plot method.

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For HY zeolites, the amount of acid sites decreased with the increase of the Si/Al ratio, as determined by NH3-TPD (Table 2). The proportion of weak acid sites

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decreased while the proportion of strong acid sites increased with the increase of the Si/Al ratios of HY zeolites (Fig. S2, in ESM). The Py-IR spectra of HY zeolites were shown in Fig. S3 in ESM. As shown in Table 2, both the amounts of Brønsted and Lewis acid sites decreased with the increase of Si/Al ratios of HY zeolites. The ratio

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of the amount of Brønsted acid sites to BET surface area (B/S), which was defined as the density of Brønsted acid sites, decreased with the increase of Si/Al ratio in HY zeolites. The amount of Brønsted acid sites determined by DTBPy-IR decreased with the increase of Si/Al ratio of HY zeolites. Table 2 Acidic Properties of the HY Zeolites. 8

Catalyst

acid amount (mmol/g)a

HY-1

acid amount (mmol/g)b

B/S (μmol/m2)c

BDTBPy (mmol/g)d

0.65

0.15

1.56

total 0.74

L 0.17

B 0.57

HY-2

0.59

0.36

0.09

0.27

0.29

0.05

HY-3

0.28

0.24

0.03

0.22

0.25

0.02

aThe

amount of acid sites determined by NH3-TPD was confirmed by the amounts of ammonia

desorbed at 120-600°C. b The amounts of Brønsted and Lewis acid sites were calculated from the

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Py-IR spectra at 150°C. c The B/S ratio, in which B was the amount of Brønsted acid sites and S was SBET as shown in Table 1. d The amount of Brønsted acid sites (BDTBPy) was obtained from the

3.2. The conversion of fructose with various catalysts

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DTBPy-IR spectra at 150°C[25].

In order to investigate the effect of acid sites in the conversion of fructose to homogeneous

Brønsted

acid

of

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furfural,

1-(4-sulfonic

acid)

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butyl-3-methylimidazolium hydrogen sulfate ([MIMBS][HSO4]), heterogeneous Brønsted acid of Amberlyst-45, homogeneous Lewis acid of Al(NO3)3 and

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heterogeneous Lewis acid of γ-Al2O3 were introduced to the reaction in GBL-water

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solvent (Table 3, entries 1-4). The fructose conversion over Brønsted acid was much higher than that over Lewis acid. The furfural yield obtained over Brønsted acid

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catalyst was higher than that over Lewis acid catalyst, illustrating that the Brønsted acid catalysts was more efficient than Lewis acid catalyst. Meanwhile, the

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homogeneous Brønsted acid catalyst ([MIMBS][HSO4]) revealed lower furfural yield compared with heterogeneous Brønsted acid catalysts (Amberlyst-45). The content of acyclic fructose was rather low when the fructose was dissolved in GBL-water solvent.

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The selectively cleaving the C-C bond of acyclic fructose could decrease the content of acyclic fructose, which in turn promoted the equilibrium shifting of fructose tautomers toward acyclic fructose [26, 27]. With the help of the Brønsted acid sites in

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Amberlyst-45, acyclic fructose in the macropores of Amberlyst-45 could selectively cleave C-C bond to produce pentose, and furfural was obtained through the dehydration of pentose. The catalysts, whose pore size was greater than the kinetic diameter of cyclic fructose (8.6 Å)[12], have no limitation for the diffusion of cyclic fructose. The cyclic fructose is preferably dehydrated to produce HMF rather than selectively cleaving the C-C bond to yield furfural. Therefore, Amberlyst-45 with pore size much larger than the kinetic diameter of cyclic fructose, exhibited a low 9

furfural yield. The catalyst with pore size less than the kinetic diameter of fructose, would be beneficial for the diffusion of acyclic fructose which was the key process for furfural from fructose. Therefore, HY zeolite with apertures of 7.4 Å [12] was introduced in the process for furfural from fructose (Table 3, entries 5-7). With the increase of Si/Al ratio, the furfural yield increased from 7.7% to 37.8%, while the HMF yield reduced from 69.2% to 21.4%. The slightly reduce of fructose conversion

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might be related to the decrease of the amount of acid sites in HY zeolites. Table 3 The conversion of fructose over various catalysts a. Catalyst

Conv. %

1b

[MIMBS][HSO4]

100

5.7

29.2

39.6

2b

Al(NO3)3

88.2

2.6

8.5

0.5

3b

Amberlyst-45

100

13.6

33.9

22.8

4b

γ-Al2O3

85.4

2.5

4.4

5

HY-1

99.8

7.7

69.2

6

HY-2

98.8

28.6

7c

HY-3

98.5

37.8

0.4

0

4.2

2.7

0

0.1

0.1

0

0.1

2.3

3.0

0

1.0

0.5

0.4

0

39.1

0

0.7

0.3

0.5

1.7

0.7

0.2

0.5

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0.1

A

LA+FA Glucose Xylose Arabinose

21.4

g HY zeolites; 13.125 g GBL, 1.125 g water, 0.75 g fructose; 150°C, 50 minutes, 2 MPa N2. b

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a 0.1

Furfural HMF

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Yield mol%

Entry

50 mg of [MIMBS][HSO4]), Al(NO3)3, Amberlyst-45, and γ-Al2O3. c Carbon balance: 56.5%; LA:

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Levulinic acid; FA: formic acid.

The relation between the density of Brønsted acid sites (B/S) and furfural (or

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HMF) yield was investigated (Fig. 2). With the increase of the density of Brønsted acid site, the furfural yield decreased while the HMF yield increased. In order to

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further understand this result, the amount of Brønsted acid sites on HY external surface was investigated. Generally, the DTBPy molecule could interact with the Brønsted acid sites on the external surface and in the pore mouth region of HY totally.

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A limited number of DTBPy molecule could interact with Brønsted acid sites in the channel of HY[21]. The amount of Brønsted acid sites calculated from DTBPy-IR spectra were 0.15, 0.05 and 0.02 mmol/g for HY-1, HY-2 and HY-3, respectively (Table 2). Pyridine molecules could interact with Brønsted acid sites on the external and internal surface of HY. The amount of Brønsted acid sites calculated from Py-IR spectra were 0.57, 0.27 and 0.22 mmol/g for HY-1, HY-2 and HY-3, respectively (Table 2). The differences between the amounts of Brønsted acid sites calculated 10

from DTBPy-IR and Py-IR spectra could reflect that the amount of Brønsted acid sites on the external surface decreased with the increase of Si/Al ratios. The external surface areas, which calculated through t-Plot method, were 93.6, 250.2 and 264.1 m2/g for HY-1, HY-2 and HY-3, respectively (Table 1). The HY zeolites, which had the lower amount of Brønsted acid sites located at external surface, exhibited higher external surface areas. Those results indicating that Brønsted acid sites density of the

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external surface decreased with the increase of the Si/Al ratio in HY zeolites.

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Fig. 2. The relation between the amount of Brønsted acid sites to SBET ratios (B/S) and a: furfural

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yield; b: HMF yield.

The formation of HMF through cyclic fructose dehydration was competitive with the production of furfural through acyclic fructose selective C-C bond cleavage

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reaction[14, 28]. If the Brønsted acid sites present densely on the external surface, the cyclic fructose quickly dehydrated on the external surface of HY zeolite, significantly

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reduce the amount of acyclic fructose which diffused into the pores, thereby reducing the yield of furfural. Oppositely, the Brønsted acid sites density of the external surface is sufficiently low, cyclic fructose will have time to diffuse into micropores, and then they were converted into furfural. In conclusion, the HMF formation was enhanced

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when the Brønsted acid sites density of the external surface of HY was dense. A low Brønsted acid sites density of the external surface of HY could inhibit the generation of HMF, and the cyclic fructose had more opportunities to diffuse into the channels of HY to be transformed as acyclic fructose, and a high furfural yield was achieved with the help of Brønsted acid sites in the channels of HY.

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3.3. Effect of reaction conditions on furfural and HMF yields from fructose The effect of the reaction time on the conversion of fructose over HY-1, HY-2 and HY-3 were investigated (Fig. 3). The fructose conversion increased with the increase of reaction time, and reaching maximum value at 70 minutes in presence of HY-1, HY-2 and HY-3. The furfural and HMF yields increased with the increase of reaction time before achieving maximum value, and then decreased over HY-1, HY-2

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and HY-3. The HMF yield decreased more sharply compared with furfural yields over all three HY zeolites. GBL can inhibit furfural degradation but had no effect on the

HMF degradation[14]. HY-1 which had the highest Lewis acid sites amount,

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exhibited a decrease of HMF yield from 66.3% to 55.4% when the reaction time

changed from 60 minutes to 80 minutes, much higher than that over HY-2 and HY-3. The Lewis acid sites in HY which promoted the formation of humins through several

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side reactions, should be also responsible for those results[29]. The high density of

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Brønsted acid sites of the external surface of HY-1 favored the producing of HMF, resulting in a much higher HMF yield than furfural yield. The similar result was

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observed in presence of HY-2, and the highest HMF yield of 37.1% and the highest

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furfural yield of 23.9% were obtained. HY-3 showed the highest HMF yield of 22.5% and the highest furfural yield of 29.1%, which might be caused by the low Brønsted

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acid sites density of the external surface. The HY-3 showed the highest furfural yield compared with HY-1 and HY-3. Therefore the effect of the HY-3 amount on converting fructose was studied (Fig. 4a). A higher amount of HY-3 promoted the

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conversion of fructose, and the highest furfural yield achieved 36.5% with 0.1 g HY-3 while the highest HMF yield reached 16.9% with 50 mg HY-3. The further increase

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of the HY-3 amount facilitated the side reactions, resulting in the decrease of furfural and HMF yields[30]. The effect of reaction temperature was studied when the reaction time were at the range of 0~50 minutes (Fig. 4b). With the increase of

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reaction temperature, the fructose conversion increased when reaction time was 0 minute and 50 minutes. When the reaction time was 0 minute, the highest furfural yield achieved 35.2% at 150°C while the highest HMF yield (13.2%) was obtained at 140°C. The further increase of the temperature had a negative influence for the formation of furfural and HMF, which might cause by the formation of humins from

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Fig. 3. The effect of reaction time on the conversion of fructose. Conditions: 50 mg HY zeolites. 1.125 g H2O, 13.125 g GBL-water, 0.75g fructose, 2 MPa N2

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Fig. 4. Effect of (a) catalyst amount, (b) reaction temperature and (c) water content on fructose conversion. Conditions: 15 g fructose/GBL-water solution for each run, 0.75 g fructose, 2 MPa N2. a: 1.125 g water, 150°C, 60 minutes; b: 1.125 g water, 50 minutes or 0 minute (30 minutes from 25°C to desired temperature), 0.1 g HY-3; c: 150°C, 50 minutes, 0.1 g HY-3.

14

furfural and HMF. In the case where the reaction time was 50 minutes, the furfural yield was increased rapidly from 25.0% to 32.4% when the temperature increased from 110°C to 120°C. The further increase of the temperature (from 120°C to 150°C) had a slight effect on furfural yield but increased the HMF yield from 11.4% to 21.4%. The increase of the temperature from 150°C to 160°C resulted in a slight decrease of furfural yield and a rapid decrease of HMF yield. GBL is a polar aprotic solvent which can dissolve furans as well as restrain the

IP T

formation of humins from furfural and sugars[14]. The water could make HY solvated and decrease it acidity, resulting in a low furfural yield. Moreover, the degradation of

SC R

furfural was more serious in water than GBL-water (Table S1, in ESM). Although water was adverse for furfural preparation from fructose, it still be indispensable as it could coordinate with GBL to dissolve fructose. Therefore, the effect of water content in GBL-water system on fructose conversion was investigated (Fig. 4c). The water

U

had no obvious effect on fructose conversion when it content was less than 15 wt%.

N

The fructose conversion was decreased rapidly (from 98.3% to 77.2%) when the

A

water content was changed from 15 wt% to 30 wt%. The highest yield of furfural was obtained with a water content of 7.5 wt%. The formation of humins was accelerated

M

with a water content of 0-5 wt %, while the degradation of furfural and HMF were enhanced with a water content over 7.5 wt %. Undesirable humins were formed in the

ED

conversion of sugars and thus led to the decrease of HMF and furfural yields[31].

PT

3.4. The solvent effect in fructose conversion GBL is a typical polar aprotic solvent. Polar aprotic solvents such as GVL,

CC E

1,4-dioxane, and tetramethylene sulfone were used as solvents in fructose conversion over HY-3 (Table 4). Generally, HMF was the main product in hexoses conversion over zeolites, and a trace of furfural was obtained in this process. However, the furfural yield reached a high level in fructose conversion over HY-3 in GVL,

A

1,4-dioxane, or tetramethylene sulfone solvents (Table 4). Meanwhile, a low yield of furfural was obtained in water. The fructose conversions in GVL, 1,4-dioxane, and tetramethylene sulfone were much higher than that over water. The solvent can not only dissolve but interact with fructose, regulating its conversion. Therefore, fructose adsorption test in water, GBL/water (mass ratio: 1:1), and GBL/water (mass ratio: 4:1) over HY-3 was investigated. The amount of the adsorbed fructose in HY-3 increased 15

with the increase of the GBL amount in solvent (Table S2 in ESM), indicating that the introducing of GBL in the solvent promoted the adsorption of fructose on HY-3. Cyclic fructose (8.6 Å) can not diffuse into the channels of HY zeolite (7.4 Å) unless Table 4 The conversion of fructose in various solvent a Yield mol%

Entry

Solvent

Conv. %

1

GVL

98.5

40.4

12.3

5.2

0.3

0.5

0.3

2

1,4-dioxane

97.0

18.6

48.1

6.4

0.4

0.6

0

3

Tetramethylene sulfone

98.8

33.2

32.0

1.7

0.4

0.4

0.3

4

H 2O

36.3

2.0

14.3

0

0.4

0.7

0

IP T

SC R

a 0.1

Furfural HMF LA+FA Glucose Xylose Arabinose

g HY-3 zeolites, 13.125 g solvent, 1.125 g water, 0.75 g fructose, 150°C, 50 minutes, 2 MPa

PT

ED

M

A

N

U

N2.b 14.25 g H2O, Carbon balance: 47.0%. LA: Levulinic acid, FA: formic acid.

CC E

Fig. 5. The FTIR spectra of (a) fructose; (b) HY-3; (c) HY-3/fructose; (d) HY-3/fructose/water; (e) HY-3/fructose/GBL-water (mass ratio 1:1); (f) HY-3/fructose/GBL-water (mass ratio 4:1).

it transform to acyclic fructose. It should be noticed that the cyclic fructose was

A

formed when acycilc fructose diffused into the supercages which had higher size than cyclic fructose. The FTIR was applied to determine the structure of fructose in the series of systems, as shown in Fig. 5. The peak at 924 cm-1 was assigned to pure cyclic fructose. The peak at 933 cm-1 which assigned to cyclic fructose, were apperared in the HY/fructose/GBL-water systems, but not be observed in HY/fructose/water system. The small shift of the cyclic fructose peak in 16

HY/fructose/GBL-water systems indicated that the cyclic fructose was located in the channels of HY. Qi et al. also found that the GVL can accelerate the transfer of glucose from solution into NaX pores[32]. Therefore, it is likely that GBL can

U

SC R

IP T

promote the transfer of fructose from solution to HY pores.

A

N

Fig. 6. 13C-NMR spectra for D-[1-13C]fructose dissolved in GBL-water (4:1) solvent and water.

M

As reported by Kimura，the solvent had significant effect on the tautomeric composition of fructose which played a vital role in the products distribution in the conversion of fructose[19]. Some researchers had found that D-fructopyranose and

ED

D-fructofuranose were primary fructose conformers, and the two conformers is over 99%[19]. To investigate the effect of solvent on the conformers of fructose, the 13

PT

C-NMR of fructose was performed. As shown in Fig. 6, no peaks were found near

the chemical shifts for the acyclic fructose in the NMR spectra, and the tautomeric

CC E

compositions of fructose were found to be altered from GBL-water to water. Fructose existed in water as α-D-fructopyranose (α-P, 66.1 ppm), β-D-fructopyranose (β-P, 64.0 ppm), α-D-fructofuranose (α-F, 63.0 ppm) and β-D-fructofuranose (β-F, 62.8 ppm), which accounted for 0.7%, 69.9%, 5.6% and 23.8%, respectively. However, the

A

conformers composition of fructose was altered when the solvent changed from water to GBL-water, and the content of β-P decreased to 55.9% while the content of α-P, α-F and β-F increased to 1.1%, 13.4% and 29.6%, respectively. Furanose, which had a low content in the equilibrium tautomeric compositions of fructose in water, showed higher free energy compared with pyranose. Angyal pointed out that the conformational free energy of the stable conformer was lower 17

than the active ones[33]. The fructopyranose was more stable than fructofuranose. GBL promoted the formation of fructofuranose (active forms) compared with H2O solvent. The reactions activation energy of liner tautomer or HMF from fructofuranose is lower than that from fructopyranose. It can be found that the furfural yield increased from 2.0% to 37.8% and HMF yield increased from 14.3% to 21.4% when the solvent changed from water to GBL-H2O (Table 4, entry 4 and Table 3, entry 7). The synergy between the Brønsted acid sites in HY zeolites and GBL

IP T

promoted the formation of reactive forms for producing furfural and HMF. More remarkable, the fructofuranose seems more prefer to produce HMF rather than the

SC R

acyclic fructose which was essential for furfural. However, the furfural yield was

higher than HMF yield in fructose conversion in GBL-water solvent over HY-3 (Table 3, entry 7). Furfural and HMF have a tendency to polymerize in water solvent[2]. The furfural degradation was reduced (mass loss ratio from 20.1% to 5.9%)

U

while HMF degradation was maintained (mass loss ratio from 39.3% to 39.3%) when

N

the solvent transfer from water to GBL-water (Table S1 in ESM).

M

A

3.5. The soluble oligomers formed in fructose conversion in GBL-water solvent Insoluble humins and soluble oligomers (OG) which formed over acidic catalysts

ED

are likely account for the low HMF yield and low carbon balance in fructose conversion[34]. The spent HY-3 zeolite showed a certain mass loss in the TG result (Fig. S5, in ESM), indicating that a certain amount of insoluble humins were formed.

PT

The ESI-MS was applied to identify the soluble OG which formed in fructose conversion in GBL-water solvent over HY-3 (Fig. 7A). The MS/MS spectrum of peak

CC E

at m/z of 213.0742 with two charges revealed fragment peaks attributed to [HMF + H]+ (m/z = 127), [Furfural + H2O + Na]+ (m/z = 137), [Furfural + HMF + K + Na]2+ (m/z = 141), [HMF + K]+ (m/z = 165) and [Hexose + H]+ (m/z = 181) (Fig. 7a), indicating the formation of OG-1 with a structure of (HMF + Furfural + Hexose). The

A

MS/MS spectrum of peak at m/z of 235.0591 revealed fragment peaks attributed to [HMF + Na]+ (m/z = 149), [HMF2 + H2O + 2Na]2+ (m/z = 158) and [HMF + H2O + Na]+ (m/z = 167) (Fig. 7b), indicating the formation of OG-2 with a structure of (HMF2). The MS/MS spectrum of peak at m/z of 257.0420 revealed fragment peaks attributed to [HMF2 + CH3OH + 2H]2+ (m/z = 143) and [HMF2 + CH3OH + H2O + NH4 + H]2+ (m/z = 159) (Fig. 7c), indicating the formation of OG-3 with a structure 18

of (HMF2). The MS/MS spectrum of peak at m/z of 321.0968 revealed fragment peaks assigned to [HMF + Na]+ (m/z = 149), [HMF + Furfural + LA - H2O + 2 H]2+ (m/z = 161) and [HMF - H2O + H]+ (m/z = 235) (Fig. 7d), indicating the formation of OG-4 with a structure of (HMF + Furfural + LA - H2O). OG-5 revealed peak at m/z of 343.0788 which assigned to [HMF + Furfural + LA - H2O + Na]+, which had a same structure with OG-4. The peak at m/z of 407.1335 was assigned to [HMF + Furfural + LA - H2O + Na]+, and OG-6 might had a structure of (HMF + Furfural +

A

CC E

PT

ED

M

A

N

U

SC R

IP T

LA - H2O).

Fig. 7. ESI-MS spectra of (A) the products of fructose conversion over HY-3 in GBL-water; (a) , (b), (c) and (d) are the MS/MS spectra of the peaks at m/z of 213.0742 (z = 2), 235.0591 (z = 1), 257.0435 (z = 1) and 321.0989 (z = 1), respectively.

Pathways for the formation of oligomers in fructose conversion over HY-3 in GBL-water were shown in Scheme 1. The etherification of two HMF molecules 19

through hydroxyl groups produced OG-2 and OG-3. LA occurred esterification with the product obtained in the etherification of HMF and the furfural hydrolysis product, and OG-4 and OG-5 were formed. The two furfural hydrolysis products occurred etherification through hydroxyl groups, the obtained product could react with two LA molecules through esterification to yield OG-6. The hydrolysis product of furfural and HMF occurred etherification, the product obtained in this process reacted with hexose through etherification, and OG-1 was formed. Obviously, the HMF

IP T

participated in the formation of the main soluble oligomers including OG-1, OG-2,

OG-3, OG-4 and OG-5 through etherification. Therefore, the etherification of HMF

SC R

was the main factor for the low HMF yield and low carbon balance in fructose

PT

ED

M

A

N

U

conversion in GBL-water solvent.

Scheme 1. Reaction pathways for the formation of oligomers in fructose conversion over HY-3 in

CC E

GBL-water.

A

3.6. The effect of solvent on producing furfural from hexose and pentose The conversion of hexose and pentose to furfural in GBL-water or water were

investigated (Table 5). The furfural yields were 2.5% and 2.2% with xylose and arabinose used as substrates in water, respectively. The conversion of xylose and arabinose increased rapidly when the solvent transformed from water to GBL-water. The furfural yields reached 57.7% and 29.5% with xylose and arabinose used as feedstocks in GBL-water solvent, respectively. Wang et al. found that the GVL could 20

increase the reactivity of pentose and restrict the degradation of furfural as compared with the pure-water solvent[23]. The nature of GVL is similar to GBL, and the GBL could increase the reactive forms of fructose, indicating that the GBL may also promote the formation of reactive forms of xylose and arabinose. The glucose conversion increased rapidly when the solvent transformed from water to GBL-water. The furfural and HMF yields which received from glucose were much lower than those from fructose under the same reaction condition. The isomerization of glucose

IP T

to fructose was an essential process for furfural and HMF. Both alkali and Lewis acid

are active for the isomerization of glucose[35]. HY-3, with a low content of Lewis

SC R

acid sites and supercages of 1.3 nm[36], had poor activity in the glucose isomerization reaction, resulting in a low yield of furfural and HMF in water or GBL-water solvent. Table 5

Yield mol%

Furfural HMF LA+FA Fructose Xylose Arabinose

water

8.8

2.5

2

arabinose

water

8.6

2.2

3

glucose

water

8.3

4

xylose

GBL-water

arabinose GBL-water glucose

GBL-water

0

--

--

0.5

--

0

--

0.2

--

0.7

2.5

0

0.3

0

0.4

91.3

57.7

--

0

--

--

0.2

80.4

29.5

--

0

--

1.9

--

73.8

6.3

4.6

9.3

0

6.2

0

ED

6

--

M

xylose

N

Conv. %

1

5

a

Solvent

A

Entry Substrate

U

The conversion of carbohydrates to furfural in different solvents a

0.1 g HY-3 zeolites, 15 g carbohydrates/GBL-water (or water) solution for each run, water

N2.

PT

concentration 7.5 wt%, glucose: 0.75 g, xylose (or arabinose): 0.625 g; 150°C, 50 minutes, 2 MPa

CC E

3.7. The reaction pathways of fructose conversion over HY zeolites The reaction pathways for furfural obtained from C6 sugars have been reported

A

by several workers[37, 38]. One possible pathway is fructose dehydrating to produce HMF, following by the loss of -CH2O group to form furfural[39]. Another potential route is fructose selective cleaving the C-C bond to yield pentose, following by the dehydration to produce furfural[11]. The first reaction pathway was eliminated by 5-HMF converting test since furfural was not detected in the products (Table S1, in ESM). Only recently, Cui et al. suggested that arabinose was formed through the selective C-C bond 66cleavage of fructose with the cooperative effects between 21

H-Beta and GBL solvent[14]. In this work, a certain amount of xylose was appeared in the products of fructose conversion over HY-3 zeolite (Table 3, entry 7). Meanwhile, the furfural yield (29.5%) obtained from arabinose directly was lower than that (37.8%) from fructose indirectly. Those results indicated that xylose was the intermediate for furfural from fructose. The process for fructose dehydration to HMF was competitive with the process of the selective C-C bond cleavage of acyclic fructose. The dehydration of fructose to

IP T

HMF was occurred easily over HY-1 which had dense Brønsted acid sites density of

the external surface, and sequentially inhibited the formation of furfural. A sparse

SC R

Brønsted acid sites density of the external surface of HY-3 restricted the formation of

HMF by the dehydration of acycilc fructose. The GBL facilitated the diffusion of cyclic fructose from solution to the channels of HY-3. The pore sizes of HY was lower than the kinetic diameter of cyclic fructose, and acyclic fructose was formed in

U

this process. Meanwhile, the GBL reduce the activation energy of the process for the

N

conversion of cyclic fructose to acyclic fructose. The Brønsted acid sites in the

A

channels of HY-3 accelerated the selective C-C bond cleavage of acyclic fructose to xylose, and furfural was formed by xylose dehydration. Additionally, if the selective

M

cleavage of C-C bond in acyclic fructose was not occurred, the supercages of 1.3 nm in HY might favor the formation of cyclic fructose from acyclic fructose as the cyclic

ED

fructose was more stable than acyclic fructose, and HMF was formed [27]. Based on

A

CC E

PT

these understandings, possible reaction pathways were proposed (Scheme 2).

Scheme 2. Possible pathways of fructose conversion over HY zeolites; HY-1 and HY-3 had dense and sparse density of Brønsted acid sites of the external surface, respectively.

22

3.8. The catalyst recycling The process of reaction would be severely impacted by the stability of the catalyst[40]. Therefore, the HY-3 recycling test was studied (Fig. 8). The fructose conversion and yields of furfural and HMF decreased slightly after each run. The surface area and micropore volume of the spent HY-3 (HY-3-C) was reduced compared with the fresh HY-3 (Table S3). The spent HY-3 exhibited a certain mass

IP T

loss according to TG curve (Fig. S5, in ESM). The two distinguishing COx peaks were shown in the TPO curve (Fig. S6, in ESM). The low-temperature peak was

related to the humins on the external surface of HY-3, while the high-temperature

SC R

peak could be connected to the humins on the internal surfaces of HY-3[41]. These results indicated that the humins were located on both the external and internal surfaces of HY-3. The acid sites in HY-3 might be masked by the humins, resulting in

U

the decrease of fructose conversion and yields of furfural and HMF. The spent HY-3

N

was calcined in air at 500°C for 4 h and then used as catalyst in the fourth run. The fructose conversion reached 98.1% and the yields of furfural and HMF reached 37.6%

A

and 20.8% (Fig.8, run 4) respectively, indicating that the HY-3 was a recyclable

M

catalyst in the conversion of fructose. Minor change for the GBL amount was found

A

CC E

PT

ED

after the reaction, indicating the GBL is also stable in the reactions (Table S4).

Fig. 8. The recycling tests of catalyst for fructose conversion. Reaction conditions: 15 g fructose/GBL-water solution for each run, 1.125 g water, 0.75g fructose, 0.1 g catalyst, 2 MPa N 2, 150°C, 50 minutes; Run No 4: the HY-3 after 3 recycling runs was calcined in air at 500°C for 4 h. 23

4. Conclusions HY zeolites with different Brønsted acid sites density were used as catalysts in fructose conversion. The Brønsted acid sites density of HY zeolites and solvent played vital roles in the products distribution in fructose conversion. With the decrease of the density of Brønsted acid sites, the furfural yield increased while the HMF yield decreased. Maximum HMF yield of 69.2% was achieved over HY-1,while

IP T

Maximum furfural yield of 37.8% was obtained over HY-3. The amount of the adsorbed fructose in HY-3 was increased with the increase of GBL amount in the

solvent. The transfer of fructose from the solution to the channels of HY was

SC R

enhanced with the introducing of GBL in solvent system. The GBL solvent greatly facilitated the reaction process for HMF and furfural from fructose. It decreases the activation energy required for HMF and furfural formation from fructose when

U

solvent transfer from water to GBL-water. The synergy between the HY-3 and GBL

N

facilitated the formation of acyclic fructose which could be occurred selective cleavage of the C-C bond to yield xylose with the help of Brønsted acid sites in the

A

channels of HY-3. The acid sites of HY-3 zeolite might be covered by the carbon

M

deposition in the channels after the reactions, resulting in the decrease of the furfural and HMF yields. The furfural and HMF yields was recovered when the spent HY-3

ED

zeolite was regenerated in air at 500°C for 4 h. The etherification of HMF was the main factor for the low HMF yield and low carbon balance in fructose conversion in

PT

GBL-water solvent.

CC E

Acknowledgements

The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (No. 21703275), Science Foundation for Youth Scholars of State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy

A

of Sciences(No. 2016BWZ002). Electronic Annex E-Supplementary Material of this work can be found in online version of the paper. Reference 24

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ED

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