Nb2O5-γ-Al2O3 nanofibers as heterogeneous catalysts for efficient conversion of glucose to 5-hydroxymethylfurfural

One-dimensional γ-Al2O3 nanofibers were modified with Nb2O5 to be used as an efficient heterogeneous catalyst to catalyze biomass into 5-hydroxymethylfurfural (5-HMF). At low Nb2O5 loading, the niobia species were well dispersed on γ-Al2O3 nanofiber through Nb–O–Al bridge bonds. The interaction between Nb2O5 precursor and γ-Al2O3 nanofiber results in the niobia species with strong Lewis acid sites and intensive Brønsted acid sites, which made 5-HMF yield from glucose to reach the maximum 55.9~59.0% over Nb2O5-γ-Al2O3 nanofiber with a loading of 0.5~1 wt% Nb2O5 at 150 °C for 4 h in dimethyl sulfoxide. However, increasing Nb2O5 loading could lead to the formation of two-dimensional polymerized niobia species, three-dimensional polymerized niobia species and crystallization, which significantly influenced the distribution and quantity of the Lewis acid sites and Brönst acid sites over Nb2O5-γ-Al2O3 nanofiber. Lewis acid site Nbδ+ played a key role on the isomerization of glucose to fructose, while Brønsted acid sites are more active for the dehydration of generated fructose to 5-HMF. In addition, the heterogeneous Nb2O5-γ-Al2O3 nanofiber catalyst with suitable ratio of Lewis acid to Brönsted sites should display an more excellent catalytic performance in the conversion of glucose to 5-HMF.

As for the heterogeneous catalysts, besides the main catalytic active sites like metal oxides such as WO 3 , TiO 2 , and ZrO 2 , the supports also play a very important role on catalytic process 14,15 . 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 area 16,17 .
Herein, the active sites (acidic Nb 2 O 5 ) were loaded on the surface of 1D γ -Al 2 O 3 nanofibers by facile incipient-wetness impregnation method. Nb 2 O 5 -γ -Al 2 O 3 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 Nb δ+ promoted the isomerization of glucose to fructose, while Brønsted acid sites catalyzed the dehydration of generated fructose to 5-HMF.

Results and Discussion
Nb 2 O 5 /γ-Al 2 O 3 nanofiber characterization. 1D γ -Al 2 O 3 nanofiber with different Nb 2 O 5 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 Nb 2 O 5 measured by ICP-AES technique (after the samples were dissolved by strong phosphoric acid). The characterized results reveal that the actual Nb 2 O 5 contents are same or smaller than the controlled Nb 2 O 5 content.
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 isotherm 19 . Both pore volume and average pore diameters decrease with the increase of Nb 2 O 5 loading, but the BET surface areas of Nb 2 O 5 -γ -Al 2 O 3 nanofibers gradually increase with Nb 2 O 5 loading augment. However, further increasing Nb 2 O 5 loading will lead to BET surface area decrease, which is similar with the report of literature 20 .
The scan electron microscopy (SEM) images of γ -Al 2 O 3 nanofiber with different Nb 2 O 5 loading from 0 to 33.9 wt% are presented in Fig. 3a 21,22 . Moreover, the characterization of γ -Al 2 O 3 and 1 wt% Nb 2 O 5 -γ -Al 2 O 3 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 -Al 2 O 3 . As shown in Fig. 5, γ -Al 2 O 3 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 bonds 23 . As for Nb 2 O 5 -γ -Al 2 O 3 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 species 24 .

Effects of different Nb 2 O 5 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 Nb 2 O 5 loading (Fig. 6A), and the best 5-HMF yield is about 59.0% over the catalyst with 0.5 wt% Nb 2 O 5 loading, while the yield is only 33.5% with 33.9 wt% Nb 2 O 5 loading. 59.0% 5-HMF yield over 0.5 wt% Nb 2 O 5 -γ -Al 2 O 3 nanofiber is significant which is higher than those reported in literatures 20,25,26 . Nb 2 O 5 -γ -Al 2 O 3 nanofiber can well catalyze Sample (wt%) BET surface (m 2 g −1 ) Pore volume (cm 3 g −1 ) Average pore diameter (nm)  the conversion of fructose and xylose to 5-HMF and furfural, which is not as significant as glucose conversion (Fig. 6B,C). As Nb 2 O 5 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% Nb 2 O 5 loading resulted in the minimum 5-HMF yield. When xylose was dehydrated, the maximum 56.1% furfural yield was obtained over 1 wt% Nb 2 O 5 -γ -Al 2 O 3 nanofiber, but furfural yield declined to 36.9% when Nb 2 O 5 loading reached 33.9 wt%. Those results indicated that the niobia species existed on the γ -Al 2 O 3 nanofiber support played the key role on biomass conversion or dehydration. As shown in Fig. 7, the states of niobia species dispersed on the γ -Al 2 O 3 nanofibers can be expressed in such three kinds of structure as a single NbO 6 unit, two-dimensional aggregation and three-dimensional aggregation [27][28][29][30] . If the niobia species exist in the form of a highly dispersed monomer NbO 6 unit, Lewis acid sites are originated from Nb δ+ ion. At low Nb 2 O 5 loading, the niobia species dispersed on the γ -Al 2 O 3 nanofiber support through Nb-O-Al bridge bonds. γ -Al 2 O 3 has Lewis acid sites with different acid strengths and weak Brønsted acid sites, and the reaction between Nb 2 O 5 precursor and hydroxyl groups on the surface of γ -Al 2 O 3 nanofiber results in strong metal-support interaction, generating Nb 2 O 5 -γ -Al 2 O 3 nanofiber with both strong Lewis acid sites and relatively intensive Brønsted acid sites 14 . With the increase of Nb 2 O 5 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 bonds [27][28][29] , and the abundance and intensity of Brønsted acid sites could be raised obviously because Nb 2 O 5 loading increase could lead to the formation of three-dimensional polymerized niobia species. Nb 2 O 5 crystallization caused a rapid decline of the L and B acid sites of Nb 2 O 5 -γ -Al 2 O 3 nanofiber, which indicated that the crystalline phase Nb 2 O 5 has few L and B acid sites 30 .
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 conditions 31 . Herein, Nb 2 O 5 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 Nb δ+ 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-HMF 14,32 . 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 solvents 33 . Herein, the γ -Al 2 O 3 nanofibers loaded with 0.5~1 wt% Nb 2 O 5 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 -Al 2 O 3 nanofiber support may play an important role on improving 5-HMF yield. For instance, the active Nb 2 O 5 catalytic centers are decorated on the external surface of γ -Al 2 O 3 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 sites 34 .
The catalyst re-usability was studied using 1 wt% Nb 2 O 5 -γ -Al 2 O 3 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 catalyst 12 , which caused some decrease of catalytic performance.   can efficiently promote the dehydration of glucose, fructose and xylose. The sample with 0.5~1 wt% Nb 2 O 5 load exhibits the best performance in glucose conversion into 5-HMF, and 5-HMF yield come up to 55.9~55.9%. This

Methods
Synthesis of supports. All commercially available chemicals and solvents are of reagent grade and were used as received without further purification. The γ -Al 2 O 3 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(NO 3 ) 3 ·9H 2 O was dissolved in 50 mL deionized water. The buffer solution was loaded into the solution of Al(NO 3 ) 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 -Al 2 O 3 nanofibers.   Preparation of catalysts. Nb 2 O 5 -γ -Al 2 O 3 nanofibers were prepared by the incipient-wetness impregnation method where NbCl 5 was selected as the niobium precursor and incorporated into γ -Al 2 O 3 nanofibers. Firstly, the appropriate amount of NbCl 5 was mixed together with the prepared γ -Al 2 O 3 nanofibers (0.5 g) in order to obtain catalysts with the controlled Nb 2 O 5 loading [wt% = Nb 2 O 5 /(Nb 2 O 5 + Al 2 O 3 )] 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 NbCl 5 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 Nb 2 O 5 -γ -Al 2 O 3 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 N 2 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 Nb 2 O 5 contents of Nb 2 O 5 -γ -Al 2 O 3 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 .