In this work, quinone-modified metal-organic framework MIL-101(Fe)(Q-MIL-101(Fe)), as a novel heterogeneous Fenton-like catalyst, was synthesized for the activation of persulfate (PS) to remove bisphenol A (BPA). The synthetic Q-MIL-101(Fe) was characterized via X-ray diffraction, scanning electron microscope, Fourier transform infrared, electrochemical impedance spectroscopy, cyclic voltammetry and X-ray photoelectron spectroscopy. As compared to the pure MIL-101(Fe), Q-MIL-101(Fe) displayed better catalytic activity and reusability. The results manifested that the Q-MIL-101(Fe) kept quinone units, which successfully promoted the redox cycling of Fe3+/Fe2+ and enhanced the removal efficiency. In addition, the reaction factors of Q-MIL-101(Fe) were studied (e.g. pH, catalyst dosage, PS concentration and temperature), showing that the optimum conditions were [catalyst] = 0.2 g/L, [BPA] = 60 mg/L, [PS] = 4 mmol/L, pH = 6.79, temperature = 25 °C. On the basis of these findings, the probable mechanism on the heterogeneous activation of PS by Q-MIL-101(Fe) was proposed.
Advanced oxidation processes (AOPs) are usually recognized as innovative, eco-friendly and high-efficiency water treatment technologies for removal of various organic contaminants due to the production of highly reactive radicals, for instance, sulfate radical (SO4−•), hydroxyl radical (HO•), among others (Zeng et al. 2015; Tang & Wang 2018). In recent years, SO4−• based technology, as one of the AOPs, has attracted extensive attention from researchers. SO4−• possesses a higher redox potential (2.5–3.1 V) than HO• (1.9–2.7 V) and is more efficient than HO• to degrade some stubborn organic pollutants owing to the advantages of high selectivity oxidizing power (Zeng et al. 2015). In general, persulfate (PS) or peroxymonosulfate (PMS) can be activated effectively to produce highly reactive SO4−• via external energy (such as ultrasound, radiolysis and ultraviolet) or catalyst (Mehdi et al. 2015). Compared with those methods employing different external energy, a catalyst can activate PS or PMS effectively with much less energy to produce SO4−•. Up to now, various Fe-based catalysts, for instance, Fe-immobilized materials (Shi et al. 2015), Fe-oxygen compounds (Leng et al. 2013), and zero-valent iron (Yan et al. 2015), have been demonstrated to efficiently catalyze the activation of PS or PMS. However, because many of these heterogeneous Fenton catalysts generally show good catalytic activity under neutral or acidic conditions, it is of extreme significance to develop a stable and highly efficient heterogeneous Fenton catalysts with applicability in broad pH range.
Fe-based metal organic frameworks (MOFs), as a newly-developing category of multifunctional porous materials, have caused increasing attention in the fields of drug delivery, gas adsorption/separation and catalysis. These materials usually consist of polydentate bridging ligands, metal clusters or metal nodes, resulting in forming well-defined network structures (Du et al. 2011; Gao et al. 2017; Hu et al. 2019). In addition, compared with traditional Fenton-like materials, MOFs catalysts possess the characteristics of large specific surface area, high porosity, adjustable porosity, good thermal stability and abundant active sites (Gao et al. 2017), so as to provide desired adsorption selectivity. Recently, some Fe-based MOFs have been researched in heterogeneous Fenton reaction for removal of organic pollutants. Du et al. (2011) found that MIL-53(Fe) could be able to activate H2O2 for realizing methylene blue (MB) degradation under visible light. Hu et al. (2019) proposed MIL-101(Fe) acting as Fenton-like catalysts to degrade tris(2-chloroethyl)phosphate. However, these reports exhibited that the pure Fe-based MOFs materials contained low iron concentration and possessed few active sites (Fe3+) with weak Fenton activity, leading to restricting Fenton oxidation catalytic activity. Therefore, a novel Fe-based metal-organic framework FeII@MIL-100(Fe) was proposed acting as heterogeneous Fenton-like catalyst to degrade high concentration MB (Lv et al. 2015). The catalytic activity was markedly heightened via synergistic effect of Fe2+ and Fe3+.
Recently, many studies showed that quinone compounds (QCs), as redox mediators (RMs), have been proved to markedly accelerate the removal of organic contaminants in the Fenton system (Ma et al. 2006; Li et al. 2016a, 2016b). RMs acting as electron-transfer mediators can promote electron transfer from a primary electron donor to a final electron acceptor. Ma et al. (2006) proposed that QCs acting as RMs could be cycled back and forth from the reduced semiquinones (SQ) or hydroquinone (HQ) state to the oxidized quinone, resulting in promotion of electron transfer processes. Simultaneously, this effect promoted the redox cycling of Fe by rapidly reducing Fe3+ to Fe2+. Hence, the Q/HQ and Fe3+/Fe2+ cycles synergistically accelerate the removal of pollutants in the Fenton system. However, the direct application of RMs could cause secondary pollution for the environment and increase the expenses related to procurement of the chemical. Hence, the immobilization of RMs has attracted the attention from many researchers. Ma et al. (2011) have successfully prepared a simple, efficient, and recyclable anthraquinone-resin hybrid co-catalyst for heterogeneous Fenton reactions. The Fe cycle was accelerated by utilizing a quinone cycle under visible irradiation, which could greatly enhance Fenton-like reactions.
In this work, quinone-modified metal-organic framework MIL-101(Fe) (Q-MIL-101(Fe)), as a novel heterogeneous Fenton-like catalyst, was synthesized via a green and facile chemical method and used for the degradation of bisphenol A (BPA). The Q-MIL-101(Fe) sample was characterized via scanning electron microscope (SEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). Therefore, the present work focuses on: (1) studying the heterogeneous activation of PS by the prepared catalyst for the removal of BPA; (2) investigating the role of Q in PS oxidation; and (3) proposing the probable mechanism on the heterogeneous activation of PS by Q-MIL-101(Fe).
Materials and instruments
All chemicals were used directly as obtained with no further purification in this study. Resorcinol, BPA (purity 99.5%) and 2-aminoterephthalic acid (NH2-BDC) were obtained from Sigma Aldrich. H2O2 (30%), N,N-dimethylformamide (DMF), ferric chloride hexahydrate (FeCl3·6H2O) and potassium PS (K2S2O8) were purchased from China National Medicines Corporation Ltd (Beijing, China).
The XRD patterns of the as-prepared products were obtained using a RigakuDmax/Ultima IV diffractometer in the range of 2θ = 2°–50° with Cu Kα radiation. The morphologies of the as-prepared products were observed by using a XL30ESEM-TMP SEM. FTIR spectra were performed by KBr pellets on Nicolet 6700 spectrometer in the range of 4,000–500 cm−1 at indoor temperature. The chemical states of samples were studied by XPS, and the XPS spectra were recorded with a Surface Science Instrument SSX-100. CV and EIS patterns of products were measured by using a CHI 660E electrochemical station (Shanghai Chenhua, China) with a three-electrode system.
Synthesis of Q-MIL-101(Fe)
MIL-101(Fe) sample was synthesized via a solvothermal method, which was prepared according to Xu et al. (2019).
Q-MIL-101(Fe) sample was prepared chemically, using 0.05 g MIL-101(Fe) and 0.005 g resorcinol added to 25 mL of aqueous solution. The mixture was magnetically stirred. Meanwhile, H2O2 (0.1 mL, 5.55 g/L) was added to the mixed solution every 20 min within 2 h. After separation from the mixture via centrifugation, the final products were washed with ultrapure ethanol and water, then re-collected and dried at 60 °C in an oven.
The catalytic performances of Q-MIL-101(Fe) were evaluated by the removal of BPA in aqueous solution. The adsorption and degradation tests were conducted in a typical batch mode by using three necked flasks (250 mL) at ambient temperature. Q-MIL-101(Fe) (5 mg) was dispersed into BPA solution (25 mL, 60 mg/L), then the mixture was mechanically stirred for 60 min to set up the balance of adsorption/desorption, followed by the addition of PS (0.2 mL, 0.5 mol/L) to the mix solution. At different time intervals, a small quantity of the mixture was sampled and shortly centrifuged at a speed of 9,000 rpm for 2 min for removal of the solid catalyst. The residual concentration of BPA was measured by high performance liquid chromatography (HPLC). HPLC (UltiMate3000) equipped with a C18 reversed-phase column (4.6 × 250 mm) and a diode array detector. The mobile phase consisted of a mixed solution of water and acetonitrile (50:50, V/V) at a flow velocity of 1.0 mL/min at room temperature as well as at an analytical wavelength of 276 nm.
RESULTS AND DISCUSSION
Characterization of Q-MIL-101(Fe)
As shown by Figure S1 (available with the online version of this paper), quinone-modified MIL-101(Fe) was prepared in one-pot through Michael addition reaction. Different amounts of H2O2 were injected into the suspension containing MIL-101(Fe) and resorcinol. First, resorcinol was oxidized to corresponding m-quinone by H2O2 under mild conditions. Quinone was captured by the free amino group of MIL-101(Fe) via a Michael-type addition reaction, and then rapidly auto-oxidized to Q-MIL-101(Fe) with an aminoquinone structure (Li et al. 2017).
The XRD patterns of MIL-101(Fe) and Q-MIL-101(Fe) are shown in Figure 1(a). The sharp and strong diffraction peaks of MIL-101(Fe) manifested a good crystallization, and the primary diffraction peaks at 2θ of 2.75, 5.03, 8.36 and 8.96 were in well agreement with the previous report (Taylor-Pashow et al. 2009). The XRD pattern of Q-MIL-101(Fe) showed alike characteristics to pure MIL-101(Fe). In addition, there were no diffraction peaks of other substances presenting for the Q-MIL-101(Fe) sample, manifesting that the crystal structure of MIL-101(Fe) had no change after modification with quinone.
The surface morphologies and microscopic structures of Q-MIL-101(Fe) were studied via SEM. It can be seen from Figure 1(b) that the Q-MIL-101(Fe) sample mainly displayed anomalous octahedron crystals and the diameters of Q-MIL-101(Fe) were in the range of 450–800 nm, which was similar to the results of the previous report (Li et al. 2016a, 2016b). Moreover, there was no remarkable change in the morphology of MIL-101(Fe) sample, which manifested that the structure of Q-MIL-101(Fe) was not markedly destroyed in the process of the quinone modification.
The FTIR spectra of MIL-101(Fe) and Q-MIL-101(Fe) are illustrated in Figure 1(c). Similar to MIL-101(Fe), the characteristic absorption peaks of carboxylate group vibrations at 770, 1,382, 1,578 and 1,657 cm−1 were also observed for the spectrum of Q-MIL-101(Fe) (Li et al. 2016a, 2016b). The broad band at around 1,657 cm−1 corresponded to –CONH (primarily stretching of the C = O) (Lu et al. 2014). The two bands appearing at 1,578 cm−1 and 1,382 cm−1 could be assigned to the asymmetric and symmetric vibrations of primary carboxyl groups, respectively. The broad band at around 770 cm−1 belonged to C–H bending vibrations of the benzene ring (Li et al. 2016a, 2016b). Besides, a characteristic absorption peak of C = O could be found at 1,676 cm−1 in the IR spectra of Q-MIL-101(Fe) (Yuan et al. 2012). All the above results suggested that quinone was successfully bonded on MIL-101(Fe).
As seen in Figure 1(d), the Q-MIL-101(Fe) sample consisted of Fe, O, N, and C elements. The XPS spectrum for the C 1s region around 285 eV is displayed in Figure S2(a) (Figure S2 is available online). The peak at 284.8 eV belonged to C = C, C–H and C–C bands (Du et al. 2015). The binding energy with peak at 287.5 eV corresponded to C = O bands and the peak of C–N bands was around at 286.3 eV. In the spectrum of O 1s (Figure S2(b)), the binding energy with peak at 530.8 eV was assigned to Fe-O. The peak located at 531.5 eV was associated with the surface hydroxyl groups. However, the peak centered at 533.6 eV was attributed to C = O bands (Du et al. 2015). As shown in Figure S2(c), the Fe 2p1/2 and Fe 2p3/2 binding energy peaks primarily located at 724.8 eV and 711.4 eV, respectively. The results were in good agreement with previous reports (Tang & Wang 2018). After the reaction, the content ratio of Fe2+/Fe3+ was increased owing to the presence of quinone as well as PS, promoting the degradation of organic contaminants in Fenton-like reaction. The above results implied the MIL-101(Fe) was modified with quinone successfully.
Study on adsorption and catalytic properties of catalyst
For evaluating the effect of Q-MIL-101(Fe) dosage, a set of experiments were carried out. BPA removal efficiencies were enhanced under various dosages of Q-MIL-101(Fe), within the range of 0.1 to 0.2 g/L (Figure 3(b)). Compared to the PS reaction system, the addition of Q-MIL-101(Fe) markedly enhanced the removal efficiency of BPA, which could be ascribed to Q-MIL-101(Fe) acting as a good supplier of Fe2+ for the activation of PS (Zou et al. 2013). However, the removal efficiency of BPA was slightly increased by 3.42% with catalyst dosages above 0.3 g/L. Possibly responding to the agglomeration of solid catalysts that impeded the dispersion of reactants (Gao et al. 2017). Considering the practical application, 0.2 g/L Q-MIL-101(Fe) was adopted for further research.
Removal of organic contaminants was also affected by temperature. As displayed in Figure 3(d), the degradation rate of BPA enhanced when elevating temperature due to the acceleration in SO4−• formation. Optimal temperature was set at 25 °C for further testing.
To further investigate the catalytic reaction process, free radicals quenching experiments were conducted to obtain information on the effect of free radicals. The literature shows that methanol (MeOH) was used as an effective quencher of sulfate and hydroxyl radicals. Reaction rate constants of MeOH with SO4−• and •OH are 1.0 × 107 M−1 s−1 and 9.7 × 108 M−1s−1, respectively. Hydroxyl radicals were also quenched by tertiary butyl alcohol (TBA), the reaction rate constant of which is ((3.8–7.6) × 108 M−1s−1 (Li et al. 2016a, 2016b). It is apparent that the rate constant of TBA for •OH is several orders of magnitude greater than that of SO4−•. Therefore, MeOH and TBA were selected as alternative quencher to evaluate the contribution of SO4−• and ·OH for BPA degradation. In addition, the study on the effect of dissolved oxygen (DO) for BPA degradation was performed under anoxic conditions, using N2.
Figure S3 (available online) shows that the removal efficiency of BPA was 94.26% with no addition of TBA, after 135 min, while the catalytic performance of Q-MIL-101(Fe) was influenced by the presence of MeOH sharply decreasing the removal efficiency of BPA to 46.88%. It can be assumed that the majority of free radicals produced in the reaction were quenched. Meanwhile, the degradation of BPA was also influenced TBA. Removal efficiency decreased from 94.26 to 52.4% in the presence of TBA. It was apparent that the degradation of BPA was greater affected by methanol than TBA. Furthermore, for researching the effect of DO in the reaction system, the experiments were carried out under anoxic condition. The removal rate of BPA was also reduced under anoxic condition. On the above results, indicating that SO4−• radicals as the main oxidizing species were generated in the reaction system.
Typical CVs of MIL-101(Fe) and Q-MIL-101(Fe) are illustrated in Figure S4 (available online). Obviously, the Q-MIL-101(Fe) showed lower peak separations and higher currents than the MIL-101(Fe), indicating a better reactant diffusion and favorable electron transfer for oxygen reduction reaction. Moreover, the difference between potential between cathodic and the anodic peaks (ΔEp) was 0.301 V for Q-MIL-101(Fe), which was much lower than pure MIL-101(Fe) (0.655 V) (Li et al. 2013). These observations indicated that the presence of quinone could minimize charge transfer resistance. In addition, it was beneficial to improving electrocatalytic activity of reaction system.
The electrode kinetics and interface reactions to the two catalysts can be further studied by EIS. It can be clearly seen from Figure S5 (available online), the semicircle diameter of Q-MIL-101(Fe) sample was observably decreased compared with the pure MIL-101(Fe). Hence, the charge separation of the Q-MIL-101(Fe) was much easier than the MIL-101(Fe) (Zhang et al. 2009), which corresponded to the experimental result of CV. This result revealed that the introduction to quinone could accelerate the processes of electron transfer.
XPS characterization was conducted to study the chemical state of the Fe species and atomic composition on the catalyst. Figure 4(a) illustrates XPS spectra of the before and after reactions of Q-NH-MIL-101(Fe). It can be noticed that the XPS spectrum of the used Q-MIL-101(Fe) was nearly the same as the fresh, showing that the catalyst was stable. As known to all, the generation of •OH and SO4−• radicals were induced by electron exchange between Fe2+/Fe3+ and PS in Fenton-like reactions. The chemical states of the Fe species in the fresh and the used Q-MIL-101(Fe) were also conducted. Figure 4(b) discloses XPS spectra of Fe2p of the before and after reactions to Q-MIL-101(Fe). For the fresh Q-MIL-101(Fe), the peaks of Fe2p1/2 and Fe2p3/2 were primarily located at around 724.8 and 711.4 eV, respectively (Tang & Wang 2018). The peaks at 711.3, 715.1, 724.3, and 726.1 eV were attributed to the Fe3+, which demonstrated that the Fe in Q-MIL-101(Fe) was mainly in the Fe3+ state. In contrast, a new peak located at 709.6 eV (Tang & Wang 2018), which was attributed to the Fe2+, presented the decomposed curves of Fe2p3/2 of Q-MIL-101(Fe). In addition, it could be seen from Table 1 that the relative content ratio of Fe2+/Fe3+ in Q-MIL-101(Fe) under the before and after reaction conditions were clearly different. Viewing the XPS Foundation Database released by the National Institute of Standards and Technology, the Fe2+/Fe3+ ratio of the used Q-MIL-101(Fe) was higher than the fresh, reaching 0.38. However, the Fe2+/Fe3+ ratio of the fresh Q-MIL-101(Fe) was only 0.12. The result could be attributed to that a part of Fe3+ converted to Fe2+ during the Fenton-like reaction.
|Q-MIL−101(Fe) .||Compound .||Peak area (%) .||Fe2+/Fe3+ .|
|Q-MIL−101(Fe) .||Compound .||Peak area (%) .||Fe2+/Fe3+ .|
The recycle of the catalyst plays a significant role in industrial application. The reusability of as-synthesized Q-MIL-101(Fe) was tested under the optimum conditions: [catalyst] = 0.2 g/L, [PS] = 4 mmol/L, [BPA] = 60 mg/L, pH = 6.79, temperature = 25 °C. At the end of the reaction, the as-synthesized Q-MIL-101(Fe) was recycled by filtration, washed by ethanol and deionized water, and dried at 60 °C overnight. Afterwards, the as-synthesized Q-MIL-101(Fe) was reused in the next test. As can be seen from Figure S6 (available online), the result displayed that the removal efficiency of BPA merely decreased from 94.26% to 89.43% after recycling three times. In the Q-MIL-101(Fe)/PS reaction system (pH = 6.79), 0.18 mg/L the leached iron was detected in the solution after a 135-min reaction. The XPS of the fresh catalyst and the recycled catalyst are illustrated with Figure 4(a), indicating that the structure of catalyst was still unaltered. It was signified that the as-synthesized Q-MIL-101(Fe) had a prominent stability and recyclability.
In this study, a novel heterogeneous Fenton-like catalyst Q-MIL-101(Fe) was synthesized by a simple chemical method. And the Q-MIL-101(Fe) catalyst can efficiently activate PS to remove BPA. As compared to the pure MIL-101(Fe), Q-MIL-101(Fe) displayed better catalytic activity and reusability. The results manifested that the existence of Q successfully enhanced the removal efficiency of BPA, due to the effect of Q promoting the redox cycling of Fe3+/Fe2+. This work proposed a method for providing novel insights into the development and rational design of high-efficiency heterogeneous Fenton-like catalysts.
This work was financially supported by the National Natural Science Foundation, China (Grant No. 51578264, 41877132).