Research Article pubs.acs.org/journal/ascecg
Photocatalytic Selective Oxidation of 5‑Hydroxymethylfurfural to 2,5-Diformylfuran over Nb2O5 under Visible Light Huili Zhang, Qi Wu, Cong Guo, Ying Wu,* and Tinghua Wu* Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China ABSTRACT: The photocatalytic selective oxidation of biomass derivate compound 5-hydroxymethylfurfural (HMF) to 2,5diformylfuran is an environmentally friendly and energy-saving process. Nb2O5 was prepared and applied in the oxidation of HMF under visible light, although its bandgap energy is higher than 3.2 eV. The reaction process follows a special mechanism over a Nb2O5 catalyst different from the normal oxide semiconductor. The effect of calcination temperature, incident wavelength, and reaction time on photocatalytic performance was investigated. Nb2O5-800 exhibits the best performance in the presence of O2 with benzotrifluoride as a solvent under visible light irradiation. KEYWORDS: 5-Hydroxymethylfurfural, 2,5-Diformylfuran, Photocatalytic oxidation, Visible light irradiation, Nb2O5
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INTRODUCTION The biomass derivate chemical 5-hydroxymethylfurfural (HMF) is considered to be a versatile platform compound and a crucial intermediate for connecting biomass resources and the fossil fuel industry. Many high-efficiency fuels, including C9−C15 alkanes and 2,5-dimethylfuran (DMF), or high-value chemicals such as 2,5-diformylfuran (DFF)1−5 can be synthesized from HMF. Partial oxidation of the hydroxyl of HMF into a corresponding aldehyde group has attracted a great deal of attention because the target product DFF can be used in a widespread manner as a monomer of furan-based biopolymers and intermediates of pharmaceuticals, antifungal agents, nematocides, and ligands, as well as in photography, analytical chemistry, metal electroplating, and electrooptical devices.6−10 The catalytic process from HMF to DFF is usually performed in organic solvents with hydrogen peroxide or air as an oxidant at high pressures and temperatures. A high selectivity for DFF in the range of 60−99% can be achieved by these reaction processes, but they are normally environmentally unfriendly and do not qualify as green chemistry. Photocatalysis is viewed as an economic and clean technique because only sunlight is needed to realize reaction.11,12 Its application appears to be more appealing than the conventional chemical catalytic oxidation methods.13 Although photocatalytic processes usually have been deemed to be highly unselective, recent progress indicates that semiconductor photocatalysis can also serve as a feasible alternative method for conventional thermocatalytic routes in the synthesis of fine chemical or biomass-derived carbohydrates via the choice of appropriate photocatalysts and controlling reaction conditions.14 In recent years, heterogeneous photocatalysis has been developed as part of oxidation of an alcohol to its corresponding aldehyde at the expense of solar energy.15−19 However, few studies of the photocatalytic partial oxidation of © 2017 American Chemical Society
HMF to DFF have been reported. Only TiO2 has been studied and used in this reaction. Yurdakal et al.20 compared TiO2 samples with different crystalline phases and crystallinities under light irradiation at 365 nm and found that a DFF selectivity of 22% could be obtained even on a poorly crystalline TiO2 catalyst. However, the selectivity of DFF is very low, which might be ascribed to its wide energy gap and strong oxidation ability under ultraviolet (UV) light. Nb2O5 is deemed to be a potential photocatalyst because of its abundant surface properties, good stability, and superior optical properties.21,22 Considerable progress has been made in its preparation and application. Until now, only a few studies that focused on the photocatalytic oxidation of alcohols by using Nb2O5 as a catalyst have been reported. Furukawa et al.23 used Nb2O5 to catalyze photooxidation of various aliphatic and aromatic alcohols with the help of O2, and a high selectivity of >90% can be achieved. However, to the best of our knowledge, no research of the application of Nb2O5 for photocatalytic conversion of HMF to DFF has been reported. Herein, Nb2O5 was prepared by a simple calcination process and applied in the photocatalytic selective oxidation of HMF to DFF. The effects of calcination temperature during photocatalyst preparation, reaction time, and irradiation light wavelength on photocatalytic performance are investigated.
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EXPERIMENTAL SECTION
Materials. Niobic acid was purchased from Sahn Chemical Technology Co., Ltd. 5-Hydroxymethylfurfural, 2,5-diformylfuran, benzotrifluoride, and aenzotrifluoride were purchased from Sinopharm Chemical Reagent Co., Ltd., Tokyo Chemical Industry Co., Ltd., Received: January 21, 2017 Revised: February 22, 2017 Published: March 3, 2017 3517
DOI: 10.1021/acssuschemeng.7b00231 ACS Sustainable Chem. Eng. 2017, 5, 3517−3523
Research Article
ACS Sustainable Chemistry & Engineering Kermel Chemical Reagent Co., Ltd., and Aladdin Industrial Corp., respectively. All these agents were utilized without further purification. Catalyst Preparation. Nb2O5 was prepared by a simple calcination method. Niobic acid was ground and then calcinated at 300, 500, and 800 °C in the furnace for 6 h at a rate of 2 °C/min. The obtained white powders were denoted Nb2O5-300, Nb2O5-500, and Nb2O5-800, respectively. Catalyst Characterization. The structures of Nb2O5 samples were determined by powder X-ray diffraction (XRD) analysis performed on a Philips PW3040/60 X-ray diffractometer equipped with Cu Kα radiation (40 kV and 40 mA). The BET specific surface area was calculated by N2 physical absorption measurements recorded at 77 K by using with an Autosorb-1 apparatus (Quantachrome Instruments). Scanning electron microscopy (SEM) images were recorded using a field emission scanning electron microscope (Hitachi S-4800). The Fourier transform infrared (FT-IR) spectra of Nb2O5 samples were recorded using a Nicolet NEXUS670 FT-IR spectrometer at intervals of 4 cm−1. Ultraviolet−visible (UV−vis) diffuse reflectance spectra (DRS) were recorded by using a UV−vis spectrometer (Nicolet Evolution 500, Thermo) over a range of 200− 800 nm, where BaSO4 was used as a reference standard. The X-ray photoelectron spectroscopy (XPS) measurements were taken on an ESCLALAB 250Xi system with Al Kα radiation to analyze the chemical states of the catalysts. The electrochemical impedance spectroscopy (EIS) and photocurrent (PC) response measurements were taken on a CHI660B electrochemical workstation equipped with a standard three-electrode cell. The measurement processes were performed as reported previously.24 Catalyst Evaluation. The catalytic reactions were performed in a double-neck flask (20 mL) that was used as a photoreactor in at an O2 flow rate of 10 mL/min using a 300 W xenon lamp (Beijing ChangTuo Ltd.) as a light source. The distance between the light source and the center of the reaction solution is approximately 10 cm. The real light power density at the position of the reactor is determined to be 63.4 mW/cm2. The reaction solution temperature was kept at around 30 °C by cooling using a fan. The initial HMF concentration and volume of suspension were 0.1 mM and 5 mL, respectively. Before we switched on the lamp, the suspension containing 50 mg of catalyst was stirred for 30 min in the dark to attain the thermodynamic equilibrium. The quantitative analysis of reactants and products was performed on a high-performance liquid chromatography (HPLC) instrument (Waters 2487 dual-absorbance detector, Waters Binary HPLC pump) equipped with a C18 AQ column using an eluent consisting of 40% acetonitrile and 60% ultrapure water at a flow rate of 1 mL/min. The HMF conversion and DFF yield were calculated as follows:
conversionHMF =
yieldDFF =
MDFF 0 MHMF
0 MHMF − MHMF × 100% 0 MHMF
× 100%
Figure 1. SEM micrographs of (a) Nb2O5-300, (b) Nb2O5-500, and (c) Nb2O5-800.
crystallization of samples. The obtained Nb2O5-500 sample displays the structural characteristics of hexagonal Nb2O5 (TT phase, space group P, JCPDS Card No. 28-0317). The TT phase shows a low degree of crystallinity, which may be deemed to be a modification of the orthorhombic T phase. For the sample calcined at 800 °C, the peaks of Nb2O5 at 28.4° and 36.7° are split into two peaks, suggesting a transformation of the pseudohexagonal TT phase to the T phase.25 Clearly, the Nb2O5-800 catalyst is highly crystallized with an orthorhombic phase structure (space group Pbam, JCPDS Card No. 271003). Raman spectra of Nb2O5 calcined at different temperatures are shown in Figure 2B. Nb2O5-300 exhibits a broad and weak band at 661 cm−1, which is characteristic of the amorphous phase of Nb2O5.26 The spectra of the Nb2O5 samples calcined at 500 and 800 °C are very similar, attributed to the similar atomic structures of orthorhombic and hexagonal Nb2O5. A main vibration mode at ∼690 cm−1 is related to the vibration of weakly distorted NbO6 octahedral. The peaks in the range of 420−780 cm−1 are assigned to the symmetrical stretching of the Nb2O5 polyhedron, and other peaks between 200 and 360 cm−1 are attributed to the angular deformation modes of the Nb− O−Nb bonds.27 Compared with that of Nb2O5-500, the peaks of Nb2O5-800 seemed to be narrower, indicating the increase in crystallinity during the calcination process. The Raman results are consistent with the XRD data, further confirming the structures of Nb2O5 treated at different temperatures. Nb2O5 catalysts calcined at different temperatures were evaluated for photocatalytic selective oxidation of HMF under UV light by using O2 as the oxidant. In the blank test, no target product DFF was obtained either in absence of a photocatalyst or in the dark, indicating the catalyst and light are indispensible for this reaction. As shown in Figure 3, Nb2O5-800 exhibits a photocatalytic activity higher than those of Nb2O5-500 and Nb2O5-300, which is attributed to the fact that its high crystallinity delays the recombination of photogenerated charge carriers. Of the three different solvents tested, the highest photoactivity can be achieved using benzotrifluoride as a solvent, which might be ascribed to its lower polarity and better dissolving capacity for oxygen. In contrast, polar solvents (water or acetonitrile) cause a drop in conversion because they may compete with the reactants for the catalytic sites. Therefore, benzotrifluoride is chosen as the reaction solvent in the later experiments. To explore the influence of incident wavelengths, the reaction was also evaluated on a Nb2O5-800 catalyst under irradiation with different wavelengths of light. The results are summarized in Figure 4A. Except for the target product DFF, the main byproduct is overoxdation product 2,5-furandicarboxylic acid (FDCA). The total yield of DFF and FDCA is higher than 99%, along with 400 nm) irradiation, P25 shows no photoactivity under our mild reaction conditions, although there is a report that benzene- or toluene-modified hydroxylated TiO2 can absorb visible light and degrade 4-CP.28 It is interesting and noteworthy that under visible light irradiation TiO2 presents as inactive for oxidation of HMF to DFF, while Nb2O5 shows photoactivity, although the bandgap of Nb2O5 is >3.2 eV. It suggests that the Nb2O5 catalyst follows a special mechanism different from that of TiO2. To explore the reaction mechanism of the Nb2O5-800 photocatalyst, a series of characterizations were performed. Figure 5A represents the IR spectra of the fresh Nb2O5-800 catalyst and the catalyst after adsorbing reactant in the reaction system for 30 min in the dark. Compared with curve b, in curve c some new bands appear at 1672 and 1522 cm−1, which correspond to ν(CO) and the stretching vibration of CC, respectively, as well as 1281 cm−1 (C−H bending vibration) and 1193 cm−1 (stretching mode of a C−O bond of HMF). This result suggests that HMF could be adsorbed on the surface of niobium oxide. The photoabsorption performance of the adsorbed catalyst was further analyzed by DRS. As Figure 5B shows, the fresh
Figure 3. Effect of solvent on the photocatalytic activity of Nb2O5 catalysts calcined at different temperatures (reaction condition, λ > 300 nm; reaction time, 6 h).
the reverse. The achieved DFF selectivity under visible light (>400 nm) is better than that obtained under UV light (>350 and >300 nm), which suggests visible light is beneficial for the formation of DFF. In contrast, the higher energy of UV light leads to the deep oxidation of HMF to FDCA. Therein, Nb2O5800 exhibits the best catalytic performance under visible light (>400 nm) for 6 h (Figure 4B), with 19.2% conversion of HMF
Figure 4. Effect of (A) incident wavelength and (B) reaction time on the catalytic performance of the Nb2O5-800 catalyst. 3519
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Figure 5. (A) IR spectra of (a) fresh Nb2O5-800 and (b) Nb2O5-800 after adsorption in the dark for 30 min and (c) HMF. (B) DRS spectra of (a) fresh Nb2O5-800 and (b) Nb2O5-800 after adsorption in the dark for 30 min.
Figure 6. (A) EIS Nyquist plots and (B) transient photocurrent responses of Nb2O5 photocatalysts calcined at different temperatures.
Nb2O5-800 catalyst exhibits an absorption edge of ∼375 nm, indicating the band gap energy of 3.3 eV, which is calculated using the equation λg = 1240/Eg. After the Nb2O5-800 catalyst is adsorbed in the reaction system, a red shift of the absorption edge (∼410 nm) is observed, which may be attributed to the formation of visible light responsible alcoholate species between Nb2O5 and HMF. Consequently, the selective oxidation reaction can proceed under visible light irradiation. From the experimental results presented above, we can presume the alcoholic hydroxyl group of HMF can be adsorbed on Nb2O5 in the dark. The formation of alcoholate species reduces the bandgap energy of Nb2O5. In terms of theoretical calculation, Furukawa et al.29 found that the energy level of the HOMO of the dissociative adsorption of methanol onto the Tphase Nb2O5 surface [Nb12O42H25(OCH3)] is higher than that of the Nb2O5 model cluster (Nb12O43H26), while a lowerenergy transition takes place with this methanol−Nb2O5 model cluster than with Nb2O5. Because of the similar alcoholic hydroxyl group in the HMF molecule, we can speculate that in our case the formation of the Nb2O5−alkoxide surface complex is possible in energy level. By excitation of the surface complex (i.e., direct electron excitation from the O 2p orbital localized on the alkoxide oxygen to the conduction band of Nb2O5 consisting of the Nb 4d orbital), the photocatalytic oxidation of
HMF can proceed under the irradiation light with an energy lower than the band gap of Nb2O5. We also have performed EIS and PC response experiments, which are commonly regarded as an effective method for verifying the charge separation status in a photocatalyst. Figure 6A displays the EIS Nyquist plots of the Nb2O5-300, Nb2O5500, and Nb2O5-800 electrodes under visible light irradiation (λ > 420 nm). A small arc radius of the EIS Nyquist plot reflects the high reaction rate at the surface of an electrode and thus implies a high efficiency of charge transfer and separation. The arc radius on the EIS spectrum of Nb2O5-800 is smaller than those of Nb2O5-500 and Nb2O5-300, revealing the higher separation efficiency of photoinduced electron−hole pairs and a faster interfacial charge transfer on Nb2O5-800. PC analysis was performed to further investigate the separation efficiency of the charge carriers. Figure 6B shows the I−t curves of the Nb2O5 samples calcined at different temperatures. All three samples show good photocurrent reproducibility when the light is switched on or off. The photocurrent of Nb2O5-800 is much higher than those of Nb2O5-500 and Nb2O5-300, which proves the Nb2O5-800 catalyst is more able to generate and transfer the photogenerated charge carrier under light irradiation, which agrees well with the EIS analysis. It is interesting that the photocurrent increased when a few drops of HMF were dropped onto the 3520
DOI: 10.1021/acssuschemeng.7b00231 ACS Sustainable Chem. Eng. 2017, 5, 3517−3523
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Figure 7. XPS spectra of (a) fresh Nb2O5-800 and (b) used Nb2O5-800 after reaction for 1 h in the absence of O2.
electrode, suggesting that the HMF adsorbed on the Nb2O5 surface plays the role of electron donor under visible light, and were thereby oxidized. The XPS experiment was performed to obtain information about the chemical environment and elemental concentrations and prove the role of oxygen during reaction. The used Nb2O5800 catalyst after reaction for 1 h in the absence of O2 was compared with the fresh one. As Figure 7A shows, the Nb 3d XPS peaks of fresh Nb2O5-800 were split well into two peaks as Nb 3d3/2 at 209.7 eV and Nb5+3d5/2 at 207.0 eV, which is in accord with chemical interaction between Nb and O in a stable Nb2O5 lattice. According to the literature value, the BE value of Nb5+ is ∼207.0 ± 0.3 eV (Nb 3d5/2)30−32 and that of Nb4+ generally is 1.0 ± 0.2 eV lower.33−35 The Nb 3d5/2 binding energy in the used catalyst has a shift to lower values. The Nb 3d5/2 peak can be deconvoluted into two peaks at 207.0 and 206.2 eV, corresponding to Nb5+ and Nb4+, respectively. The results distinctly manifest Nb5+ can be partly reduced to a lower chemical valence during the reaction. The O 1s spectrum (Figure 7B) also presents a similar shift to a lower binding energy, which might be attributed to the higher electron density around O2−-bonded lower-valence Nb4+. Compared with higher-valence Nb5+, Nb4+ has more difficulty inducing O2−, so the electron density around O 2− increases, which subsequently results in a decrease in the O 1s binding energy, in the same way proving the possibility of Nb5+ being partly reduced to Nb4+ during the reaction in the absence of oxygen. Obviously, the XPS result demonstrates that oxygen plays key role in the reoxidation of reduced Nb4+ to Nb5+ so that the reaction can proceed continually. The reactive species trapping experiments were performed to ascertain the dominant active species in the photocatalytic oxidation of HMF on the Nb2O5-800 catalyst. Different scavengers were added to the reaction system using isopropanol (IPA) as a •OH scavenger, benzoquinone (BQ) as a •O2− scavenger, and ethylenediaminetetraacetic acid (EDTA) as a h+ scavenger. As shown in Figure 8, there is no distinct influence on the performance with the addition of IPA or BQ, indicating that •OH and •O2− is not the major active species for the reaction. In comparison, when EDTA was added, the photocatalytic performance apparently decreases, revealing that h+ is the dominant active species and plays key roles in the photocatalytic process.
Figure 8. Reactive species trapping experiment with the Nb2O5-800 catalyst under visible light.
On the basis of the aforementioned characterization results, a possible reaction mechanism is suggested as shown in Scheme 1. First, the alcoholic hydroxyl group of HMF is adsorbed on Nb2O5 in the dark. The formation of alcoholate species reduces the bandgap energy of Nb2O5 to the visible range (3.2 eV. We determined that the alcoholic hydroxyl group of HMF can be adsorbed on Nb2O5 to form alcoholate species, decreasing 3522
DOI: 10.1021/acssuschemeng.7b00231 ACS Sustainable Chem. Eng. 2017, 5, 3517−3523
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