Highly Stable Mn-Doped Metal-Organic Framework Fenton-Like Catalyst for the Removal of Wastewater Organic Pollutants at All Light Levels.

A novel Mn-doped Fe-based metal-organic framework (MOF) Fenton-like catalyst was prepared for the removal of wastewater organic pollutants. The catalyst exhibited good degradation performance, stability, and recyclability for the removal of phenol from water with a maximum catalytic efficiency of 96%. Incorporating a long persistent phosphor in the MOF ensured optimum performance in the dark.

Introduction

The Fenton oxidation process has been recognized for some time as a promising and effective method for the treatment of toxic, refractory, and non-biodegradable wastewater organic pollutants.[1−3] The hydroxyl radical (•OH) is the main reactive species in the Fenton reaction, which, owing to its nonselective nature, can attack many organic pollutants. The powerful oxidizing ability of the •OH radical can be attributed to its high oxidation potential. For heterogeneous Fenton-like and photo-Fenton reactions, the catalytic decomposition of H2O2 to •OH includes two main steps: interaction between H2O2 and the active site, and reversible electron transfer between H2O2 and the active site. The overall rate of reaction largely depends on the number of exposed active sites and the H2O2 reduction of Fe(III) to Fe(II), which are believed to be the rate limiting steps in homogeneous and heterogeneous Fenton-like reactions.[4,5] However, the catalyst-based heterogeneous Fenton process shows a low •OH production activity. Several strategies have been employed to improve the activity of the catalyst, such as reducing the size of the catalyst and immobilizing Fe species on various supports. Nonetheless, there is still a demand for the design and preparation of catalysts with effective exposed active sites.[6−8]

An Fe-based metal–organic framework (MOF) is a hybrid solid with a periodic network structure composed of metal ions/clusters and organic ligands.[9,10] The unique textural characteristics of the MOF, such as a wide distribution of single Fe sites and their porous structure and large surface area, provide many exposed active sites, facilitating the contact with reactants. Recently, some Fe-based MOFs have been studied for the Fenton degradation of organic pollutants. MIL-88B-Fe is a three-dimensional (3D) porous MOF composed of 1,4-phthalic acid and trimeric Fe octahedral (Fe3-μ3-oxo) clusters. MIL-88B-Fe exists in trimeric (Fe3-μ3-oxo) clusters, which are accessible and coordinatively unsaturated. In addition, the Fe atom has an octahedral environment with open sites. These are bound by nonbridging ligands (e.g., water or halogen/hydroxide anions) that can be substituted by Lewis bases.[11−15] These characteristics show that the Fe-based MOF materials have abundant exposed active sites and good accessibility for reactants. Previous studies have shown that the catalytic activity of MOF is related to its metal composition, regardless of whether it is an isolated metal center or a metal cluster. In recent years, the doping of MOFs with different metals, nonmetals, or multiple metal centers, to prompt specific activities, has attracted considerable attention. However, the inability to generate electrons in the absence of sunlight remains a challenge for these materials. Since photosensitizers cannot produce electron–hole pair separation in the absence of solar excitation, catalytic activity in the dark–light conditions is impaired.[16−19] Long persistent phosphors (LPPs) are luminescent materials with unique energy storage abilities and long-lasting emission (up to several hours) post excitation. After incorporation into a catalytic material, LPPs can store light energy for utilization in low-light conditions.[20] Based on this concept, optically independent Mn-doped Fe-MOF customized LPPs could meet the specific requirements of applications in pollution control.

This study sought to characterize the physicochemical properties of Mn doping on MIL-88B-Fe. The influence of reaction parameters on the Fenton-like removal of a target pollutant (phenol) from water was investigated together with the degradation mechanism and the stability and reusability of the catalyst.

Experimental Section

Chemicals and Materials

All chemicals were of analytical grade and were used without further purification. H2O2 (30%), FeCl3·6H2O, N,N-dimethyl formamide (N,N-DMF), methanol, isopropyl alcohol, and NaOH were purchased from Damao Chemical Reagent Factory (Tianjin, China) and Fuyu Chemical Co., Ltd. (Tianjin, China). tert-Butyl alcohol (TBA) was obtained from Bodi Chemical Co., Ltd. (Tianjin, China). Ultrapure water (18.2 MΩ·cm) was used throughout the experiments.

Synthesis of the Catalyst

MIL-88B-Fe was prepared by a solvothermal method based on a previous study.[21] Briefly, a solution of FeCl3·6H2O (270 mg) and 1,4-benzenedicarboxylic acid (116 mg) was prepared in N,N-DMF (5 mL) and NaOH (2 M, 0.4 mL). After solvothermal treatment at 100 °C for 12 h, the as-synthesized MIL-88B-Fe was collected by filtration and washed with DMF, methanol, and water at 50 °C until the supernatant became colorless. Finally, it was activated overnight at 110 °C and the obtained catalysts were stored in a glass desiccator. MIL-88B-Fe (0.5 g) and Mn(NO3)2 (0.1 g) were then dispersed in a small amount of deionized water, stirred for 24 h, dried at 60 °C, and then uniformly ball-milled to obtain the Mn-doped catalyst.

Characterization of the Catalyst

Powder X-ray diffraction (XRD) was performed on an Empyrean diffractometer (Malvern Panalytical, Shanghai, China) with Cu Kα radiation (k = 1.54056 Å); scanning electron microscopy (SEM) images were obtained on an S-4800 SEM (Hitachi, Japan); and transmission electron microscopy (TEM) images were obtained on a Tecnai G2 F30 S-Twin TEM (FEI Company). Elemental analysis of catalysts and the concentrations of leached metal ions during the Fenton reaction were determined by inductively coupled plasma (ICP)-optical emission spectroscopy (OES) on an Optima 2000 ICP-OES (PerkinElmer Instruments).

Catalytic Activity Test

Phenol was selected as the target pollutant to evaluate the catalytic activity as it is an important intermediate in industrial processes and a ubiquitous pollutant. Degradation experiments were carried out using three-neck flasks (250 mL) at room temperature. In a typical experiment, a given amount of the catalyst was dispersed in phenol solution under magnetic stirring (50 mg/L, 150 mL) for 2 min and the degradation reaction was initiated by adding H2O2 and Xe lamp irradiation. Aliquots were withdrawn at predetermined time intervals and filtered through 0.22 μm membrane filters to remove the suspended catalyst material. Isopropyl alcohol (10 mL) was immediately added to quench the reaction, and the concentration of phenol was measured. After reaction, MIL-88B-Fe was separated by filtration, washed with water and methanol, and dried at 110 °C prior to reuse as required.

Results and Discussion

Catalyst Characterization

XRD was used to characterize the crystal structure of the synthesized MIL-88B-Fe catalyst doped with 8% Mn. The XRD pattern of the fresh MIL-88B-Fe catalyst (Figure a) was consistent with that reported previously, indicating a successful preparation of Fe-MOF.[22] The Mn-doped MOF also exhibited the same characteristic XRD pattern as MIL-88B-Fe and with almost no decrease in intensity, indicating that the crystal structure did not change after manganese doping. As shown in Figure S1, the Fourier transform infrared spectrum of the Mn-doped MOF showed peaks consistent with the expected structure.

Figure 1

Characterization of catalysts: (a) XRD patterns for MIL-88B-Fe and the Mn-doped MOF; (b) SEM images of the Mn-doped MOF; and (c) SEM mapping of C, O, Fe, and Mn on the surface of Mn-doped MOF.

The SEM images of the Mn-doped MOF sample and MIL-88B-Fe are shown in Figures b and S2. The surface of the newly synthesized Mn-doped MOF showed uniform needle-like structures of 0.6–0.8 μm in length and 800 nm in diameter, which also resembled MIL-88B-Fe.[23] This indicates that the incorporation of Mn did not cause significant changes in the morphology of the catalyst. Figure c shows the SEM image of Mn-doped MOF and the corresponding elemental maps. The SEM image showed good agreement with the Fe and Mn elemental maps confirming that the Mn atoms were uniformly dispersed in the Mn-doped MOF crystal. XPS tests were carried out to identify the chemical state and binding energy of Fe and Mn on the surface of the Mn-doped MOF material. The XPS spectra of Fe 2p and Mn 2p for the Mn-doped MOF are shown in Figure S3.

Catalytic Activity

Figure a shows a schematic of the photo-Fenton reaction. The Lewis base, H2O2, tends to coordinate directly with the unsaturated Fe active centers (Lewis acid sites) in the Fe-MOFs. As a result, the generated ≡Fe(II) became the active site and established a bond with the coordinated hydrogen peroxide on the surface. This triggers the heterogeneous Fenton reaction to generate HO• while ≡Fe(II) is oxidized to ≡Fe.[24] One strategy for improving the optical Fenton performance was to exploit the photocatalytic properties of Mn in Mn-doped Fe-MOF catalysts. Figure b shows the effects of Mn addition to MIL-88B-Fe on the removal of phenol from water. The reaction was activated with H2O2 using Xe lamp irradiation for 30 min. In the absence of a catalyst, the concentration of phenol showed little change. When 0.1 g/L MIL-88B-Fe was added, phenol was rapidly degraded during the first 10 min to reach a steady state of ∼30% removal efficiency after 30 min. Doping stoichiometric amounts of Mn (4–10 wt %) onto the surface of the MOF significantly improved the degradation performance of the catalyst, and a maximum degradation of 96% was attained with an addition of 8 wt % Mn. At 10 wt % Mn doping, adhesion of H2O2 to the mesoporous surface of the MOF weakened and the catalytic degradation of phenol decreased. Figure c shows the effects of irradiation (Xe lamp) time on the degradation of phenol using 8 wt % Mn-doped MOF catalyst. Optimum degradation was achieved at an irradiation time of 30 min. The corresponding chemical oxygen demand (COD) conversion for each catalyst is shown in Figure d. The overall catalytic performance for the degradation of phenol followed the order 0 wt % < 10 wt % < 4 wt % < 6 wt % < 8 wt % Mn-doped MOF.

Figure 2

Degradation of phenol using Mn-doped MOF catalysts (initial phenol concentration, 50 mg/L; cat, 0.1 g/mL; H2O2, 1 mL; Xe lamp irradiation): (a) schematic of the photo-Fenton degradation process; (b) effect of Mn wt % on degradation (30 min irradiation); (c) effect of irradiation exposure time on degradation using 8 wt % Mn-doped MOF; and (d) chemical oxygen demand (COD) conversion at each Mn wt % doped MOFs.

Stability and Reusability of Catalyst

Figure a shows the change in concentrations of Fe and Mn ions during the degradation of phenol using 8% Mn-doped MIL-88B-Fe. The concentration of leached Fe increased with increasing reaction time, while the concentration of Mn remained very low. The maximum concentration of leached Fe reached a maximum value of 1.82 mg/L, which was lower than the environmental standard (2 mg/L) set by the European Union. This stability was also important for optimum catalytic performance in practical applications. One of the features of MOFs is the stability of their open structures. To evaluate the reusability of Mn-doped MIL-88B-Fe, four successive cycles of degradation testing were performed. As shown in Figures b and S4, the removal efficiency of phenol was maintained in each cycle. Furthermore, there was no loss of degradation performance over 20 days, demonstrating good durability and stability. The high catalytic efficiency and stability suggested that Mn-doped MIL-88B-Fe had the potential to meet the requirements of applications, requiring the efficient removal of pollutants. Since dissolved Fe ions might catalyze the degradation of phenol, their contribution to the overall catalytic efficiency was determined. To measure this contribution, the catalyst was stirred for 30 min in the phenol reaction solution prior to removal by filtration. The degradation reaction was then initiated with the addition of H2O2, and the change in the concentration of phenol in the filtrate was measured (Figure c). The efficiency of dissolved Fe ions on the catalytic degradation of phenol by H2O2 from four separate experiments was ∼6% after 30 min of reaction, which was much lower than the degradation rate using MIL-88B-Fe. The experimental results showed that the dissolved Fe ions made little contribution to the performance of the homogeneous catalytic system; however, heterogeneous catalysis plays a major role. Hence, determination of the active species in the catalytic system was one of the important methods to study the reaction mechanism.[25−27] The use of free radical scavenging agents can facilitate the identification of key active species such as •OH in the reaction process. Figure d shows the effect of the radical scavenger TBA on the degradation of phenol. The degradation rate of phenol decreased from 96% (0% TBA) to 5% (300 mM/L TBA) with increasing addition of TBA, confirming that •OH was the main active species in the degradation of phenol using the MIL-88B-Fe catalyst.

Figure 3

Catalyst stability, reusability, and the mechanism for the degradation of phenol: (a) change in dissolved Fe and Mn concentrations during the photo-Fenton reaction; (b) degradation performance of the recycled catalyst; (c) degradation performance catalyzed by soluble Fe leached from catalysts; and (d) influence of the radical scavenger (TBA) concentration on catalytic degradation.

All Light Level Photocatalytic Activity

It is generally believed that HO· acts as an active oxidation intermediate in Fenton and Fenton-like processes. According to this mechanism, the reaction can be initiated by the reaction of H2O2 with Fe(II) or Fe(III) to generate HO• or HO2• (eqs and 2). Furthermore, Mn ions of different valence states can participate in the chain reaction of free radicals in the catalytic system.[28] For example, Mn can exhibit a redox pair Mn(II)/Mn(III), which can generate HO• via the decomposition of H2O2 (eq ).[29] Electron transfer between Mn and Fe species can also occur in the degradation process. Overall, Mn species can participate in the production of HO• and can effectively regenerate Mn(III) via electron transfer. Hence, compared with Fe-MOF, improved performance was obtained with Mn-doped MOF.[30] This synergistic mechanism is shown in Figure a.

Figure 4

Light–dark photocatalytic activity: (a) synergistic mechanism in MOF species; (b) fluorescent image of LPP (image is free domain); (c) catalytic performance with and without LPP; and (d) performance of the LPP catalyst in the dark.

In the absence of light, photosensitizers cannot absorb photons to form electron–hole pairs, and subsequent degradation reactions are not possible. To realize experimental degradation in the dark, the MOFs were coated with LPPs (Figure b). Interestingly, the corresponding LPP-coated MOFs exhibited increased catalytic efficiency from 96 to 98% (Figure c). This enhancement could be due to the simultaneous irradiation of Fe(II) and LPP, which increased Fe–O clusters for the generation of electron–hole pairs (Figure S5). Photogenerated electrons could be transferred from ≡O(II) to Fe(III) leading to the formation of ≡Fe(II). Figure d shows that relative degradation with the LPP catalyst remained at 1.0 after removing the light source, indicating zero catalytic processes. This was surprising since it was known that LPPs can store light energy and emit light fluorescence for several hours. In this way, the fluorescence released from the LPP layer could be re-absorbed by Fe(II), thereby continuing to produce catalyst.

Conclusions

A Mn-doped MIL-88-Fe catalyst, coated with LPPs, was prepared for the degradation of organic pollutants in water. The catalyst exhibited good degradation performance, stability, and recyclability for the removal of phenol from water. A maximum degradation efficiency of 96% was obtained at 8 wt% Mn-doped MIL-88-Fe. There was no loss of performance over 20 days, and the material could maintain catalysis when light irradiation was removed. This work represents an important step toward establishing efficient MOF catalysts for practical application in all light levels.

Experimental Methods

H2O2 (30%), FeCl3·6H2O, N,N-dimethyl formamide, methanol, isopropyl alcohol and NaOH, were purchased from the Damao Chemical Reagent Factory (Tianjin, China) and the Fuyu Chemical Co., Ltd. (Tianjin, China). tert-Butyl alcohol (TBA) was bought from the Bodi Chemical Co., Ltd. (Tianjin, China). All chemicals were analytical grade and used without further purification. Ultrapure water (18.2 MX) was used throughout the experiments.

Synthesis of Catalyst

MIL-88B-Fe was prepared by a solvothermal method according to the procedure in the literature, briefly, hydrothermal treatment of FeCl3·6H2O (270 mg) and 1,4-benzenedicarboxylic acid (116 mg) in N,N-dimethyl formamide (5 mL) with NaOH (2 M, 0.4 mL) at 100 °C for 12 h. After solvothermal treatment, the as-synthesized MIL-88B-Fe was collected by filtration and washed with DMF, methanol, and water at 50 °C until the supernatant became colorless. Finally, it was activated overnight at 110 °C. The obtained catalysts were stored in a glass desiccator. Then 0.5 g of MIL-88B-Fe and 0.1 g of Mn(NO3)2 deionized water solution were weighed out, mixed evenly and stirred for 24 h, dried at 60 °C and then uniformly ball-milled to obtain Mn doping catalytic.

Characterization of Catalyst

Powder X-ray diffraction (XRD) of catalysts was performed on an EMYPREAN diffractometer with Cu Kα radiation (k = 1.54056 Å) over a 2 h range of 5–40 °C scanning electron microscopy (SEM) images were obtained on an S-4800 type SEM (Hitachi, Japan) and transmission electron microscopy (TEM) images were obtained on a Tecnai G2 F30 S-Twin type TEM (FEI Company). Elemental analysis of catalysts and the concentrations of leached irons during Fenton reaction were tested by inductively coupled plasma optical emission spectroscopy (ICP, PerkinElmer Optima 2000).

Catalytic Activity Test

Phenol was selected as a target pollutant to evaluate the catalytic activity of catalytic, as it is a ubiquitous pollutant and an important intermediate in industrial processes. The degradation experiments were carried out in a batch mode using a three-necked flask (250 mL) at room temperature. In a typical experiment, catalytic was dispersed in phenol solution (50 mg/L, 150 mL) for 2 min. Then the degradation reaction was initiated by adding H2O2 under magnetic stirring. At predetermined time intervals, samples were withdrawn and filtered through 0.22 μm membrane filters to remove suspended MOFs. Meanwhile, an aliquot of 1 M isopropyl alcohol was immediately added to quench the reaction, and then the concentration of phenol was analyzed. After reaction, MIL-88B-Fe was separated by filtration, washed with water and methanol, and then reused in a new reaction after being dried at 110 °C.