Sunlight-driven photocatalytic oxidation of 5-hydroxymethylfurfural over a cuprous oxide-anatase heterostructure in aqueous phase
Qizhao Zhang a , Hao Zhang a , Bang Gu a , Qinghu Tang b , Qiue Cao a , Wenhao Fang a,*
a School of Chemical Science and Technology, National Demonstration Center for Experimental Chemistry and Chemical Engineering Education, Yunnan University, 2 North Cuihu Road, 650091 Kunming, China
b School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, 453007 Xinxiang, China
Chemical conversion of renewable and abundant lignocellulosic biomass sources into sustainable fuels and chemicals has created a large consensus [1,2]. 5-Hydroxymethylfurfural (HMF) as a typical model compound for C6 carbohydrates is recently studied for conversion of cellulose to the upgraded production of various building-block chemicals [3,4]. Of all, oxidative transformation of HMF is an important valorization route [5,6]. 2,5-Diformylfuran (DFF), the symmetrical furanic dialdehyde obtained from the selective oxidation of HMF, can serve as an essential intermediate for synthesizing diamines and Schiff bases [7,8]. Besides, DFF is viewed as an important monomer for furyl polymers, pharmaceutical intermediates, antimicrobial, etc [9,10]. As is known, the product distribution during oxidation of HMF can be complicated. Hence, the high production of DFF from HMF strongly relies on a suitable catalyst and some mild reaction conditions.
Photocatalysis enables to drive the selective oxidation of HMF to DFF under mild reaction conditions [10–14]. In that case, only room or near-room temperature and atmospheric O2 are used. This is a promising alternative to thermal catalysis thanks to the photocatalysts with photogenerated holes and reactive oxygen species generated in the presence of O2. However, very few effective catalysts have been reported in literature. Although MnO2 nanorods , C3N4-supported Ti3C2 , and lead-halide perovskite  are described as photocatalysts to give ≥ 90% yield of DFF using atmospheric O2 upon ultraviolet or visible-light irradiation, the catalytic efficiency in terms of productivity to DFF is found to reach only a moderate value (≤30 mg gcatal. − 1 h− 1 ). Notably, the complex preparation of catalysts and the compulsory use of organic solvents (i.e., nitriles and benzotrifluoride) can badly hamper the green footprint of the photocatalysis. Thereby, oxide catalysts that are low-cost, easily-prepared and readily-tunable are highly demanded. Moreover, using water instead of organic solvent as reaction medium is greener but can be much challenging for oxide catalysts. Water as a byproduct tends to block the forward reaction and water may also aggravate metal leaching. Recently, a typical Bi2WO6 photocatalyst was shown to afford 19% yield of DFF at a conversion of 26% using air and water and driven by visible-light .
Titanium dioxide (TiO2) is regarded as one of the most common oxide photocatalysts [18–20]. TiO2 has been extensively explored for photocatalytic degradation of organic pollutants [20–22]. However, very few applications of TiO2 on the photocatalytic selective oxidation of biomass-derived compounds are reported, e.g., selective oxidation of HMF to DFF in water phase [23–25]. Earlier, S. Yurdakal et al. prepared TiO2 with different crystalline phases (i.e., anatase, rutile, and brookite) and investigated their performances in photocatalytic oxidation of HMF to DFF in aqueous phase . It was found that TiO2 showed a strong oxidation power to quickly drive the reaction under ultraviolet irradiation at 35 ◦C. However, the used crystalline allotropic phases seemed not to influence the global selectivity (about 21%) of the process. The conversion could be increased but with formation of total oxidized products (i.e., CO2 and H2O). Thereafter, to further explore the selective photo-oxidation property of TiO2, I. Krivtsov et al. prepared nitrogen-doped and oxygen-enriched TiO2. This N/TiO2 catalyst may show a higher selectivity of DFF (26%) at 40% conversion of HMF at 25 ◦C in water using visible-light irradiation . It was demonstrated that the doping of nitrogen formed Ti-O-N bonds and thus rendered the surface devoid of hydroxyl groups. This can suppress the generation of •OH radicals on TiO2 surface and interfered the interaction of surface sites with products, thus reducing the decomposition of DFF. Recently, A. Allegri et al. prepared Au-Cu alloy particles loaded on porous TiO2-SiO2 nanocomposites but this Au3Cu1/Ti15Si85 photocatalyst afforded an unsatisfactory performance . Only 21% conversion and 34% selectivity were obtained upon simulated sunlight irradiation at 30 ◦C. It was found that photogenerated holes and •OH radicals gave rise to the decomposition of HMF and its derived oxidation products.
It is clear to see for TiO2-based photocatalysts that simultaneously tuning conversion of HMF and selectivity of DFF is critical to achieve a high yield of DFF. However, the stabilization of DFF at a high conversion of HMF remains greatly challenging. As expected, dual-functional photocatalytic systems are believed to show synergistic oxidation performance for selective formation of DFF from HMF . Particularly, incorporating the extra active sites responsible for the selective conversion of an alcohol to an aldehyde to TiO2 can be probably an effective strategy. For instance, recombining two semiconductors with different properties may often bring about unexpected results, such as formation of heterojunction or Schottky junction, which would provoke unique visible-light response or photothermal effect to a catalyst. Typically, S. Xie et al. investigated a series of traditional semiconductor catalysts for photocatalytic oxidative coupling of methanol to ethylene glycol. Interestingly, cuprous oxide (Cu2O) was shown to be highly selective to oxidize methanol to formaldehyde under visible-light irradiation . Recently, D. A. Giannakoudakis attempted a CuOx nanoclusters (<4 nm) decorated TiO2 photocatalyst which afforded 30% selectivity of DFF at 98% conversion of HMF but in acetonitrile under ultraviolet irradiation at 30 ◦C . To the best of our knowledge, no efforts have been made to utilize the catalytic property of TiO2 and Cu2O for synergistic oxidation catalysis in aqueous phase.
Herein, this work reports a (Cu2O)x‖TiO2 heterojunction photocatalyst that is prepared by chemical reductive precipitation method. The strong interaction between Cu2O and TiO2 semiconductors enables synergistic control of the conversion of HMF and the selectivity of DFF. Hence the (Cu2O)x‖TiO2 catalyst attains the optimum yield and productivity of DFF in aqueous phase under simulated sunlight irradiation, in comparison with a series of typical catalysts. The prime objective is to unravel the unique photocatalytic property of the Cu2O-TiO2 heterostructure and then to discuss the reaction mechanism in light of the structure-activity relationship and the energy-band structure of the catalyst.
2.1. Chemicals and reagents
Anatase (99.9%), rutile (99.5%), copper(II) acetate monohydrate (≥98%), glucose (99%) and Na2CO3 (99.8%) from Alfa Aesar, and P25 from Evonik Degussa were used for catalyst preparation. HMF (98%), DFF (98%), HMFCA (98%), FFCA (98%) and FDCA (98%) from Ark Pharm were used for catalytic reactions and quantitative analysis.
2.2. Catalyst preparation
(Cu2O)x‖TiO2 catalysts were prepared by the chemical reductive precipitation method. Different Cu2O/TiO2 molar ratios (x = 0.08, 0.16, 0.5 and 1.5) and TiO2 crystalline phases (anatase, rutile, as well as P25) were investigated. Unless it was specified, anatase was used as default. As an example for (Cu2O)0.16‖TiO2, copper acetate (1.4 mmol) and anatase (4.2 mmol) were mixed by 20 mL of deionized water and the mixture was ultrasonicated for 5 min. Subsequently, 20 mL of glucose solution (0.2 mol L− 1 ) was dropwise added under a magnetic stirring at 800 rpm and the mixture was stirred for 30 min. Following that, 0.2 mol L− 1 of Na2CO3 solution was added dropwise to adjust the pH to 9. Afterwards, the mixture was heated to 60 ◦C and aged for 2 h under stirring. The obtained solids were filtered and washed with deionized water and ethanol till the pH of filtrate reached neutral. Finally, the (Cu2O)0.16‖TiO2 catalyst was dried under vacuum at 60 ◦C for 12 h. Cu2O was prepared by the same method.
2.3. Photocatalytic reactions
Typically, HMF (0.1 mmol), catalyst (30 mg) and deionized water (100 mL) were charged into a 150 mL quartz reactor, as illustrated in Scheme S1. The reaction mixture was first ultrasonicated (40 kHz) for 15 min and then stirred at dark under 500 rpm for another 15 min. This allowed a desorption equilibrium between catalyst and reactant. A Xe lamp (λ: 350–780 nm, light intensity: 0.75 W cm− 2 ) CEL-HXF300-T3 from Beijing China Education Au-light Technology Co., Ltd. was used. The lamp was fixed vertically about 6 cm above the liquid surface, and the reactor was connected to a circulating water of 35 ◦C and an O2 flow of 10 mL min− 1 . Continuous sampling (0.5 mL) was done every 30 min. The quantitative analysis was conducted on an Agilent 1260 Infinity HPLC armed with a photodiode array detector and a Shodex SH-1011 sugar column (8 mm, 300 mm, 6 µm) using a dilute H2SO4 solution (5 mM) as mobile phase. Specific wavelength was set for the detector, i. e., 285 nm for analyzing HMF, 290 nm for DFF and FFCA, and 260 nm for HMFCA and FDCA, respectively. The quantification was based on an external standard method using the standard solutions of reactant and products at different concentrations. Each reaction was performed at least twice to guarantee reliable data. Conversion of HMF, selectivity of products, yield and productivity of DFF were calculated on the basis of carbon balance.
Conv.(%) = nHMF,initial − nHMF,final nHMF,initial (1)
Select.(%) = nproduct nHMF,initial − nHMF,final (2)
Yield(%) = nDFF nHMF,initial − nHMF,final (3)
Prod. ( mgDFFg− 1 catal. h− 1) = WDFF Wcatal. × reaction time (4)
2.4. Characterization methods
X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance diffractometer with Cu Kα radiation and a beam voltage of 40 kV. The patterns were registered in the 2θ domain of 10–90◦ at a screening rate of 0.1◦ s − 1 . Transmission electron microscopy (TEM) was carried out on a field-emission JEOL-2100 F system with an acceleration voltage of 200 kV. Energy dispersive spectroscopy (EDS) elemental mapping was conducted on an X-MaxN 80 T IE250 at 200 KV. Photoluminescence spectroscopy (PL) was recorded on an Edinburgh Instruments FSL980 with 200–1700 nm steady-state spectrum. The excited wavelength was fixed at 480 nm. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were carried out on a Thermo Scientific Kα+ system equipped with Al Kα radiation under ultrahigh vacuum. The binding energy shift due to the surface charging was adjusted with a reference of the C 1 s line at 284.8 eV. Ultravioletvisible diffuse reflectance spectrum (UV–vis DRS) was recorded on a Shimadzu UV3600 apparatus. Low-temperature electron paramagnetic resonance (EPR) spectroscopy was carried out at 100 K on a Bruker A300 EPR spectrometer with X-band frequency of 9.4 GHz. Quantitative calculation was based on the DPPH standard. Photoelectrochemical (PEC) performance of the catalyst was analyzed on an AutoLab electrochemical workstation model no. PGSTAT302N using a standard three-electrode cell. Indium tin oxide (ITO) glass deposited by the catalyst was used as the working electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and a Pt wire was used as the counter electrode. To prepare the working electrode, 5 mg of catalyst was dispersed into 0.5 mL of ethanol, and then 10 μL of Nafion® solution was added and the mixture was sonicated for 2 h to make it homogeneous. Afterwards 10 μL of the mixed droplets were coated on the ITO glass with a controlled area of 0.25 cm2 , followed by drying at room temperature for 30 min to form a film electrode. The photoelectrochemical test was conducted under a 300 W xenon lamp, and a Na2SO4 solution (0.5 mol L− 1 , 100 mL, pH = 6.8, 25 ◦C) was used as the electrolyte.
3. Results and discussion
3.1. Formation of Cu2O‖TiO2 p-n heterojunction and synergistic photocatalytic behavior
The actual molar ratios of Cu to Ti were precisely measured by ICPMS. The data show almost the same ratios to the theoretical values (Table S1), which reflects the effective preparation of the (Cu2O)x‖TiO2 catalysts by the chemical reductive precipitation method. The crystalline structure of catalyst was analyzed by XRD. As shown in Fig. 1A, well crystallized Cu2O can be prepared from copper(II) acetate by the chemical reductive precipitation method. The characteristic diffraction peaks due to the (110), (111), (200), (220) and (311) planes of facecentered cubic Cu2O (JCPDS #05–0667) are detected. In addition, from AES spectrum reported in Fig. S1, Cu1+ ions with a typical kinetic energy at 916.5 eV are found to be the dominant species on surface . For the (Cu2O)x‖TiO2 catalyst, Cu2O and TiO2 (anatase, JCPDS #21–1272) phases are clearly observed to coexist. Moreover, the intensity of Cu2O peak apparently grows higher with increasing the Cu/Ti ratio, but the position of TiO2 peak preserves unchanged. This indicates that Cu2O and TiO2 can be combined to form a mixed oxide . The microstructure of the representative (Cu2O)0.16‖TiO2 catalyst was further inspected by TEM and EDS. As displayed in Fig. 1B, this catalyst shows relatively uniform nanoparticulated morphology (ca. 20 nm). EDS elemental maps present well distributed and evenly overlapped signals of Cu, Ti and O. In addition, as shown in the high-resolution TEM image (Fig. 1C), the lattice fringe with d spacing of 0.234 nm and 0.350 nm can be attributed to the (111) plane of Cu2O  and the (101) plane of TiO2 , respectively. Notably, these crystal planes are closely contacted, which implies the formation of p-n heterojunction between Cu2O and TiO2 through the combination of a p-type and an n-type semiconductors.
Photocatalytic oxidation of HMF over the (Cu2O)x‖TiO2 catalyst was carried out under Xe lamp (λ = 350–780 nm) in aqueous phase. As shown in Fig. 2, as reference catalysts, anatase presents 100% conversion of HMF but no oxidation products are generated, whereas Cu2O affords 100% selectivity of DFF but it is barely active. Previously, S. Yurdakal et al. disclosed that TiO2 can effectively catalyze oxidation of HMF, but selectivity of DFF was low. They observed that CO2 production would accumulate during the oxidation process over the lab-made TiO2 catalysts, i.e., brookite and P25 . Later, S. Xie et al. found that Cu2O showed superior ability to selectively convert OH group to C– –O groups in photo-oxidation reactions, e.g., Cu2O was greatly selective for visible-light-driven oxidation of methanol to formaldehyde . Interestingly, on the (Cu2O)x‖TiO2 catalyst the conversion of HMF continuously declines from 61% to 11% while the selectivity of DFF steadily goes up to 85% from 29% with increasing the Cu2O/TiO2 ratio from 0.08 to 1.5, as plotted in Fig. 2. Meanwhile, FFCA from over-oxidation of DFF is obtained and stabilizes at about 15% (in select.). Other products mainly including CO2 decreases quickly and finally become zero. These results show that Cu2O and TiO2 can decide in synergy the performance of the (Cu2O)x‖TiO2 photocatalyst. As expected, a volcano-like relationship can be built between productivity of DFF and molar ratio of Cu2O/TiO2. The (Cu2O)0.16‖TiO2 catalyst achieves the optimal productivity of 64.5 mg g− 1 h− 1 .
To understand the unique photocatalytic results, further characterizations were carried out. Usually the intensity of PL spectrum can reflect the recombination efficiency of photogenerated electron-hole, and a low peak intensity indicates a low electron-hole recombination rate. As shown in Fig. 3A, TiO2 displays the lowest fluorescence intensity in PL spectroscopy, which can correspond to the most effective separation of photogenerated electrons and holes . In contrast, Cu2O presents the highest fluorescence signal. Obviously, the formation of Cu2O‖TiO2 heterostructure enables to modify the pristine photoelectric property of TiO2. These results can explain the huge difference in photocatalytic activity between TiO2 and Cu2O for HMF conversion. Notably, the (Cu2O)0.16‖TiO2 catalyst shows the optimum separation of photogenerated electrons and holes among all the dual metal catalysts. Furthermore, photocurrent response caused by the separation and diffusion of photogenerated electron-hole pairs under light irradiation can provide some deeper information. One can see from Fig. 3B that all the catalysts show photocurrent responses but only the (Cu2O)0.16‖TiO2 catalyst apparently presents the highest value, which can reach about 1.3 and 6 folds over those of TiO2 and Cu2O, respectively. This unique phenomenon confirms the best separation and mobility of photogenerated electron-hole pairs on (Cu2O)0.16‖TiO2 , which may probably explain the highest productivity of DFF on this catalyst.
To investigate the light utilization on a photocatalyst, UV–vis DRS was performed. As displayed in Fig. 3C, TiO2 is evidenced to show a strong absorption in UV-light region but no photoresponse in visiblelight region. Interestingly, the formation of Cu2O‖TiO2 heterojunction brings about an intense visible-light absorption. Moreover, UV-light response can be also enhanced in comparison with TiO2 and Cu2O. Following that, the band-gap energy (Eg) of the photocatalyst was estimated using the Tauc method by plotting (F(R) hν) 1/2 against hν , as displayed in Fig. 3D and Table S2. When the energy of photons is equal to or greater than the band gap of a photocatalyst, electrons in the ground state can be excited and move to the high-energy level. When the absorption rapidly increases with decreasing wavelength, the corresponding energy of photons is equal to the band gap of the photocatalyst, hence electrons can move from the top of valence band to the bottom of conduction band. As shown in Fig. 3C, D, on the pristine TiO2 and Cu2O semiconductors, the absorption is quickly elevated from 383 and 628 nm with the corresponding band gap of 3.25 and 1.98 eV, respectively. On the (Cu2O)x‖TiO2 photocatalyst, one can clearly see the absorption edges of both semiconductors, in which the absorption edge of TiO2 rises from 383 to 395 nm with increasing Cu2O content, so that the band gap becomes relatively narrower from 3.25 to 3.15 eV. On the contrary, Cu2O shows a decreasing absorption edge (628–610 nm) and a wider band gap (1.98–2.04 eV). It is well known that the wider the band gap is, the stronger its redox ability can be . Hence, the narrower the band gap is, the stronger ability to utilize light can be obtained. Therefore, the formation of Cu2O‖TiO2 p-n heterojunction by combining the two semiconductors is highly important for achieving a high yield of DFF via photocatalytic oxidation of HMF. That is because p-n heterojunction can enhance the light utilization ability of TiO2 and simultaneously improve the redox ability of Cu2O.