Introduction

Fossil-based resources such as petroleum, coal and natural gas are deemed as the dominant raw materials to be used for energy and synthesis of organic chemicals1,2. Nevertheless, the mismatch between the increasing demand for and sharply diminishing supply of fossil-based resources implies that the search for alternative raw material sources is critically important. Renewable biomass is the most suitable candidate for alternative raw material sources since they are abundant, easy to obtain and rich of carbohydrates which can be converted to valuable chemicals3,4. Consequently, the conversion of renewable biomass to fuels and chemicals has received wide attention.

Cellulose as an important branch of biomass is composed of the basic glucose unit building blocks that can be transformed to the useful platform molecule 5-HMF. 5-HMF can acts as the raw material to be used to synthesize chemicals, liquid fuels and so on ref. 5 and 6. Hence, developing an approach to efficiently synthesize 5-HMF from rich and cheap glucose resources under mild conditions is extremely desirable.

5-HMF synthesis from glucose is difficult due to the high stability of the glucose ring, making the dehydration process more difficult7. In order to overcome this disadvantage, the catalysts were paid more attention to reduce the activation energy of this reaction. Catalysts used for glucose dehydration to 5-HMF are classified into heterogeneous and homogeneous catalysts which play the different performance in different reaction solvents such as aqueous, organic solvents and ionic liquids8,9. Homogeneous catalysts like ionic liquids and metal salts employed for 5-HMF conversion from glucose are limited due to several drawbacks including the high cost, toxicity, difficult separation and recovery10,11. In contrast, heterogeneous catalysts avoided aforementioned disadvantage have been widely utilized for biomass conversion into 5-HMF, e.g. oxides, phosphates, ion exchange resins and heteropolyacids12,13.

As for the heterogeneous catalysts, besides the main catalytic active sites like metal oxides such as WO3, TiO2, and ZrO2, the supports also play a very important role on catalytic process14,15. Generally, the conventional porous materials were used as supports because of their large surface areas. But the porosity and surface area will be reduced during the loading of active sites. Recently, various one-dimensional (1D) oxide nanofibers have been reported as the efficient heterogeneous catalyst supports, which can realize the loading of active sites without declining surface area16,17.

Herein, the active sites (acidic Nb2O5) were loaded on the surface of 1D γ-Al2O3 nanofibers by facile incipient-wetness impregnation method. Nb2O5-γ-Al2O3 nanofibers displayed the high catalytic activity in glucose conversion to 5-HMF with dimethyl sulfoxide as solvent, and it was found that Lewis acid site Nbδ+ promoted the isomerization of glucose to fructose, while Brønsted acid sites catalyzed the dehydration of generated fructose to 5-HMF.

Results and Discussion

Nb2O5/γ-Al2O3 nanofiber characterization

1D γ-Al2O3 nanofiber with different Nb2O5 loading of 0, 1, 3, 3.4, 4.7, 9.4, 26.6 and 33.9 wt% was prepared by facile incipient-wetness impregnation method. Table 1 presents the contents of Nb2O5 measured by ICP-AES technique (after the samples were dissolved by strong phosphoric acid). The characterized results reveal that the actual Nb2O5 contents are same or smaller than the controlled Nb2O5 content.

Table 1 The Nb2O5 contents of Nb2O5-γ-Al2O3 by ICP-AES.
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XRD patterns were collected from 5 to 80° (Fig. 1). It is found that the distinct diffraction peaks of these samples appear at 37.6°, 39.5°, 45.8°, 60.9° and 67°, which are assigned to the diffraction of (3 1 1), (2 2 2), (4 0 0), (5 1 1) and (4 4 0) of γ-Al2O3, respectively. With the increase of Nb2O5 loading, the intensive diffraction peaks at 2θ = 22.6° and 28.5° appeared, which are assigned to the diffraction of bulk Nb2O518.

Figure 1
figure 1

XRD of γ-Al2O3 with different Nb2O5 loadings.

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Textural properties of catalysts obtained from the nitrogen sorption at 77 K are listed in Table 2 and Fig. 2. These isotherms are similar with each other which belong to the type IV Van Der Waals isotherm19. Both pore volume and average pore diameters decrease with the increase of Nb2O5 loading, but the BET surface areas of Nb2O5-γ-Al2O3 nanofibers gradually increase with Nb2O5 loading augment. However, further increasing Nb2O5 loading will lead to BET surface area decrease, which is similar with the report of literature20.

Table 2 The textural properties of γ-Al2O3 with different Nb2O5 loadings.
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Figure 2
figure 2

The N2 sorption isotherms at 77 K for γ-Al2O3 (solid circles: adsorption; open circles: desorption).

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The scan electron microscopy (SEM) images of γ-Al2O3 nanofiber with different Nb2O5 loading from 0 to 33.9 wt% are presented in Fig. 3a–h. All samples have the homogeneous length of 200–300 nm and the similar diameter of 30–50 nm, which indicates Nb2O5 loading do not influence the morphology and structure of γ-Al2O3 nanofibers. However, the particles are observed to aggregate together when Nb2O5 loading reaches 4.7 wt%, due to the surface tension21,22. Moreover, the characterization of γ-Al2O3 and 1 wt% Nb2O5-γ-Al2O3 by transmission electron microscopy (TEM) was also performed. As shown in Fig. 4a,b, the average size and the morphology of these particles are similar. Elemental mapping by EDS was used to study the distribution of the Al, O, and Nb elements in nanofiber based samples (Fig. 4d). The abundant Al and O elements distribute homogeneously in a single nanofiber, however the content of Nb element is comparably smaller and Nb element mainly distributes on the surface of γ-Al2O3.

Figure 3
figure 3

SEM images of γ-Al2O3 with different Nb2O5 loadings: (a) 0 wt%; (b) 1 wt%; (c) 3 wt%; (d) 3.4 wt%; (e) 4.7 wt%; (f) 9.4 wt%; (g) 26.6 wt%; (h) 33.9 wt%.

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Figure 4
figure 4

TEM images of γ-Al2O3 with different Nb2O5 loadings: (a) 0 wt%; (b) 1 wt%. (c) EDX patterns of the selected area of the 1 wt% Nb2O5-γ-Al2O3. (d) The TEM image and elemental mapping of the 1 wt% Nb2O5-γ-Al2O3.

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As shown in Fig. 5, γ-Al2O3 nanofiber exhibits the very weak Raman bands in the region of 200~2000 cm−1 due to the low polarizability of light atoms and the ionic character of Al–O bonds23. As for Nb2O5-γ-Al2O3 nanofiber with 1~9.4 wt % load, the intensive Raman bands in the region of 1000~2000 cm−1 appeared due to the polarizability of Nb–O–Al species24.

Figure 5
figure 5

Raman spectra of γ-Al2O3 with different Nb2O5 loadings.

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Effects of different Nb2O5 loading on biomass selectivity conversion

Under the condition of 150 °C for 4 hours, 5-HMF yield from glucose is in the range of 32.5% to 59.0% with the different Nb2O5 loading (Fig. 6A), and the best 5-HMF yield is about 59.0% over the catalyst with 0.5 wt% Nb2O5 loading, while the yield is only 33.5% with 33.9 wt% Nb2O5 loading. 59.0% 5-HMF yield over 0.5 wt% Nb2O5-γ-Al2O3 nanofiber is significant which is higher than those reported in literatures20,25,26. Nb2O5-γ-Al2O3 nanofiber can well catalyze the conversion of fructose and xylose to 5-HMF and furfural, which is not as significant as glucose conversion (Fig. 6B,C). As Nb2O5 loading increased from 3 wt% to 33.9 wt%, 5-HMF yields from fructose conversion raised from 67.4% to 76.8%, but 3 wt% Nb2O5 loading resulted in the minimum 5-HMF yield. When xylose was dehydrated, the maximum 56.1% furfural yield was obtained over 1 wt% Nb2O5-γ-Al2O3 nanofiber, but furfural yield declined to 36.9% when Nb2O5 loading reached 33.9 wt%. Those results indicated that the niobia species existed on the γ-Al2O3 nanofiber support played the key role on biomass conversion or dehydration.

Figure 6
figure 6

Effect of different Nb2O5 loadings on γ-Al2O3 on the yield of: 5-HMF from the dehydration of glucose at for 4 hours (A) and fructose for 5 hours (B), furfural from the dehydration of xylose for 6 hours (C) at 150 °C.

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As shown in Fig. 7, the states of niobia species dispersed on the γ-Al2O3 nanofibers can be expressed in such three kinds of structure as a single NbO6 unit, two-dimensional aggregation and three-dimensional aggregation27,28,29,30. If the niobia species exist in the form of a highly dispersed monomer NbO6 unit, Lewis acid sites are originated from Nbδ+ ion. At low Nb2O5 loading, the niobia species dispersed on the γ-Al2O3 nanofiber support through Nb–O–Al bridge bonds. γ-Al2O3 has Lewis acid sites with different acid strengths and weak Brønsted acid sites, and the reaction between Nb2O5 precursor and hydroxyl groups on the surface of γ-Al2O3 nanofiber results in strong metal-support interaction, generating Nb2O5-γ-Al2O3 nanofiber with both strong Lewis acid sites and relatively intensive Brønsted acid sites14. With the increase of Nb2O5 loading, the interaction between the isolated niobia species and their nearest neighbors (either isolated or polymerized species) resulted in the formation of Nb–O–Nb bridge bonds. The Brönst acid sites originated from the Nb–OH–Nb bridge bonds27,28,29, and the abundance and intensity of Brønsted acid sites could be raised obviously because Nb2O5 loading increase could lead to the formation of three-dimensional polymerized niobia species. Nb2O5 crystallization caused a rapid decline of the L and B acid sites of Nb2O5-γ-Al2O3 nanofiber, which indicated that the crystalline phase Nb2O5 has few L and B acid sites30.

Figure 7
figure 7

The states of niobia species dispersed on the γ-Al2O3 nanofibers.

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It is known that the conversion of glucose to 5-HMF is a two-step reaction. The first step is the isomerization of glucose to fructose catalyzed by Lewis acid and the second step is the dehydration of generated fructose from glucose to 5-HMF under Brønsted acid conditions31. Herein, Nb2O5 loading increase could lead to the formation of two-dimensional polymerized niobia species, three-dimensional polymerized niobia species and crystallization, which influenced the distribution and quantity of the Lewis acid sites and Brønsted acid sites. On one hand, the Lewis acid site Nbδ+ play a key role on the isomerization of glucose to fructose, and Brønsted acid sites are more active in the dehydration of generated fructose to 5-HMF14,32. The heterogeneous catalyst with the suitable ratio of Lewis acid sites to Brønsted sites should display an more excellent catalytic performance in the conversion of glucose to 5-HMF in organic solvents33. Herein, the γ-Al2O3 nanofibers loaded with 0.5~1 wt% Nb2O5 offers the optimum ratio of Lewis acid sites to Brønsted acid sites, thus they exhibits the best performance in 5-HMF (or furfural) yield from glucose (or xylose) (see Fig. 8). On the other hand, the 1D γ-Al2O3 nanofiber support may play an important role on improving 5-HMF yield. For instance, the active Nb2O5 catalytic centers are decorated on the external surface of γ-Al2O3 fibers, improving the direct interaction between the active sites and glucose. The randomly oriented nanofibers form a large interconnected void (10~20 nm), which made glucose to well contact with the active sites34.

Figure 8
figure 8

Glucose conversion into 5-HMF over the Nb2O5-γ-Al2O3.

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The catalyst re-usability was studied using 1 wt% Nb2O5-γ-Al2O3 nanofibers. After reaction, the catalyst was separated from DMSO by centrifugation, and then washed with deionized water and ethanol, dried at 80 °C under vacuum before the next run. From Figs 9 and 10, it is found that the XRD pattern and morphology of catalyst well maintain after one recycle. However, the color of catalyst changed from white to brown, which maybe result from an accumulation of humans on the surface of catalyst12, which caused some decrease of catalytic performance.

Figure 9
figure 9

XRD of 1 wt% Nb2O5-γ-Al2O3 before and after the catalytic reaction.

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Figure 10
figure 10

SEM of 1 wt% Nb2O5-γ-Al2O3 before (A) and after the catalytic reaction (B).

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Conclusions

Nb2O5-γ-Al2O3 nanofibers have been prepared by facile incipient-wetness impregnation method to catalyze the conversion of glucose (fructose and xylose as well) into 5-HMF. It is found that Nb2O5/γ-Al2O3 nanofibers can efficiently promote the dehydration of glucose, fructose and xylose. The sample with 0.5~1 wt% Nb2O5 load exhibits the best performance in glucose conversion into 5-HMF, and 5-HMF yield come up to 55.9~55.9%. This excellent performance of 0.5~1 wt% Nb2O5/γ-Al2O3 nanofibers in glucose conversion into 5-HMF is ascribed to the synergistic effect of suitable ratio of Lewis acid sites to Brønsted acid sites on Nb2O5-γ-Al2O3 nanofibers.

Methods

Synthesis of supports

All commercially available chemicals and solvents are of reagent grade and were used as received without further purification. The γ-Al2O3 nanofibers were prepared by the hydrothermal method. A buffer solution prepared by diluting ammonia (40 mL, 25%) with deionized water to 10%, was used as the precipitation agent. Besides, 30 g Al(NO3)3·9H2O was dissolved in 50 mL deionized water. The buffer solution was loaded into the solution of Al(NO3)3 by dropwise under vigorous stirring until the solution became milky and the initial pH of the mixture ranged from 2.0 to 5.0. The resulting uniform solution was then transferred into a PTFE-lined autoclave and heated in an oven at 200 °C for 48 h. Thereafter, the obtained precipitate was washed several times with deionized water and ethanol by centrifugation, and the obtained precipitate was dried overnight at 55 °C and subsequently calcined in air at 600 °C for 5 h to obtain γ-Al2O3 nanofibers.

Preparation of catalysts

Nb2O5-γ-Al2O3 nanofibers were prepared by the incipient-wetness impregnation method where NbCl5 was selected as the niobium precursor and incorporated into γ-Al2O3 nanofibers. Firstly, the appropriate amount of NbCl5 was mixed together with the prepared γ-Al2O3 nanofibers (0.5 g) in order to obtain catalysts with the controlled Nb2O5 loading [wt% = Nb2O5/(Nb2O5 + Al2O3)] equal to 1, 3, 5, 10, 15, 30 and 40, respectively. Secondly, the deionized water containing the oxalate with the mole about five times of the mole of NbCl5 was introduced and then the mixture was kept at room temperature for 48 h. Thirdly, the mixture was dried at 100 °C for 24 h to obtain the different Nb2O5-γ-Al2O3 catalysts.

Catalytic activity

The glucose, fructose and xylose dehydration reactions were performed in a 15 mL sealed tube (thick walled pressure bottle from Beijing synthware glass) under magnetic stirring. In a typical run, glucose (450 mg), catalyst (45 mg) and DMSO (2.5 ml) were loaded into sealed tube which was then immersed into the preheated oil bath and stirred for a required time. After reaction, the mixture cooled to room temperature naturally, and then the internal standard substances (1-chloronaphthalene) was added into reaction mixture which was further diluted by methanol. The filtered solution was analyzed by HPLC. The dehydration reaction procedures of fructose and xylose were similar to that of glucose, and the glucose (450 mg) was replaced by fructose (450 mg) or xylose (375 mg), respectively.

General Information

The surface morphology and composition of catalysts were characterized by field emission scanning electron microscopy (SEM, JSM-7001F, JEOL, Tokyo, Janpan). High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL JEM-2100F field emission electron microscope under an accelerating voltage of 200 kV equipped with an energy-dispersive X-ray spectroscopy (EDX) instrument (Quantax-STEM, Bruker). The phases structures of catalysts were characterized by powder X-ray diffraction (XRD) analysis using an X-ray diffractometer (DX-2700, China) with Ni-filtered Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA with a fixed slit, ranging from 10 to 80°. Surface areas were determined by low temperature N2 adsorption performed at 77 K, on a 3H-2000PS2 analysis instrument, after pretreatment performed for 8 h at 150 °C under vacuum. The BET (Brunauere-Emmete-Teller) method was used to derive surface areas from the resulting isotherms. Pore size distributions were obtained from analysis of the adsorption branch of the isotherms using Barrette Joynere Halenda (BJH) method. The Raman spectra of these catalysts were determined by Renishaw inVia plus from 200 to 2000 cm−1. The Nb2O5 contents of Nb2O5-γ-Al2O3 nanofibers were characterized by Optima 8000 (ICP-AES). The 5-HMF and furfural were determined by high performance liquid chromatography (HPLC) (L6, China) fitted with a Pgrandsil-TC-C18 column and the ultraviolet detectors for 5-HMF and furfural at 286 nm and 272 nm, respectively. The column oven temperature was set at 25 °C, and the mobile phase was methanol/water = 80:20 (V/V) at a flow rate of 1.0 mL min−1.

Additional Information

How to cite this article: Jiao, H. et al. Nb2O5-γ-Al2O3 nanofibers as heterogeneous catalysts for efficient conversion of glucose to 5-hydroxymethylfurfural. Sci. Rep. 6, 34068; doi: 10.1038/srep34068 (2016).