Synthesis of MIL-53(Fe) Metal-Organic Framework Material and Its Application as a Catalyst for Fenton-Type Oxidation of Organic Pollutants

-e iron (III) benzene dicarboxylate metal-organic framework material (MIL-53(Fe)) was synthesized with either the solventthermal or hydrothermal method under different conditions. -e influence of the type of solvents, molar ratio of precursors and solvent, temperature, and reaction time on the structure of MIL-53(Fe) was investigated. -e material was characterized by using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption isotherm. -e MIL-53(Fe) structure formed in N′, N-dimethylformamide (DMF) and methanol (MeOH) but not in water. In DMF, the molar ratio of precursors and solvent, temperature, and reaction time had a significant effect on the crystal structure of MIL53(Fe). Under optimal conditions, MIL-53(Fe) has high crystallinity and a large specific surface area (SBET � 88.2m /g). -e obtained MIL-53(Fe) could serve as a potential heterogeneous catalyst to oxidize phenol (PhN), rhodamine B (RhB), and methylene blue (MtB) in the Fenton-like reaction system at the different solution pHs.

Hydrogen peroxide (H 2 O 2 ) decomposition in the presence of iron catalysts to hydroxyl radicals (HO • ) has been widely developed into advanced oxidation processes (AOPs) for application in the environmental treatment. e hydroxyl radical has a strong standard oxidation potential (E 0 (HO • /H 2 O � +2.8 V NHE )) and a high bimolecular reaction rate constant (10 8 −10 11 M −1 ·s −1 ).
is radical is capable of nonselective oxidation of numerous organic substances to nontoxic products, such as CO 2 , H 2 O, and inorganic salts. Various types of catalytic iron, including metal salts (in the form of Fe 2+ or Fe 3+ ), metal oxides (e.g., Fe 2 O 3 and Fe 3 O 4 ), and zero-valence metal (Fe 0 ), have been used in chemical (classical Fenton), photochemical (photo-Fenton), and electrochemical (electro-Fenton) degradation pathways. However, the prevention of iron precipitation remains the bottleneck for iron-based AOPs [31].
is motivates the development of heterogeneous Fenton-like processes. Various researchers have utilized different inorganic and organic materials as a supporting matrix for active iron ions in the heterogeneous Fenton-like process, such as SBA-15 [32], carbon [33], kaolin [34], and MCM-41 [35]. Recently, MIL-53(Fe) has attracted the attention of researchers due to its framework containing iron that acts as the structural nodes functioning as a new photocatalyst [5,24] and a catalyst in the Fenton-like reaction system [6][7][8]25].
MIL-53(Fe) has been successfully synthesized and applied to degrade methyl orange in an ultraviolet-assisted heterogeneous Fenton-like process [36]. In this paper, e detailed synthetic conditions will be presented. e influence of solvent type, solvent content, temperature and time of reaction, and the molar ratio of precursors has been investigated to produce MIL-53(Fe) with high crystallinity and a large specific surface area. Addition, the obtained material is used as a catalyst in the Fenton-like reaction system to oxidize phenol (PhN), rhodamine B (RhB), and methylene blue (MtB).

Synthesis of Iron (III) Benzene Dicarboxylate.
e synthesis of iron (III) benzene dicarboxylate was adapted, Du et al. [5]. In a typical process, a mixture of 1.2469 g FeCl 3 ·6H 2 O, 0.7663 g BDC, and a quantity of the solvent (or DMF/MeOH/water), such that the molar ratio of the precursors and solvent is 1 : 1 : 400, was placed in a Teflon-lined steel autoclave (volume 200 mL) in an oven at 150°C for 15 hours. A yellow powder was obtained by filtering and drying overnight at 150°C. en, the powder was cooled to ambient temperature and washed with distilled water, filtered, and dried to obtain iron (III) benzene dicarboxylate.
In addition to the effects of solvent type on the formation of MIL-53(Fe) crystalline phase, the influence of the DMF content, reaction temperature, reaction time, and molar ratio of Fe (III)/BDC was also investigated.

2.3.
Characterization. X-ray diffraction (XRD) patterns were recorded on a VNU-D8 Advance Instrument (Bruker, Germany) under Cu Kα radiation (λ �1.5406Å). Scanning electron microscopy (SEM) images were obtained on an SEM JMS-5300LV (Japan), and high-resolution transmission electron microscopy (HR-TEM) images were obtained by using a JEM-2100. Fourier-transform infrared spectra (FT-IR) were recorded with a Jasco FT/IR-4600 spectrometer (Japan) in the range of 4000−400 cm −1 . e X-ray photoelectron spectroscopy (XPS) was studied by using a Shimadzu Kratos AXIS Ultra DLD spectrometer (Japan). Peak fitting was performed on CasaXPS software. e N 2 adsorption/desorption isotherm measurement test was performed at 77 K on a Tristar 3000 analyzer. Before setting the dry mass, the samples were degassed at 250°C with N 2 for 5 h.

Determination of the Point of Zero Charge.
e point of zero charge (pH PZC ) of the MIL-53(Fe) material was determined following the process described by Jing et al. [23]. 50 mL of NaCl 0.01 M solution was added to a series of 100 mL Erlenmeyer flasks. e initial pH value of the solutions (pH i ) was adjusted to 2-12 by adding either the HCl 0.1 M or NaOH 0.1 M solution. en, 0.05 g of MIL-53(Fe) was added to each flask, and the mixture was shaken for 48 hours. e final pH (pH f ) of the solutions was measured. e plot representing the relationship between the difference of the final and initial pH value (ΔpH � pH f − pH i ) and pH i was drawn, and the point of intersection of the curve with the abscissa provides pH PZC .

Oxidizing Organic Compounds.
e oxidation of PhN, RhB, and MtB by H 2 O 2 in the aqueous solution was carried out at 30°C in a 500 mL round flask with a reflux condenser in the presence of MIL-53(Fe). In each experiment, 0.05 g of the catalyst was stirred for 30 minutes with 100 mL of a solution containing PhN (or RhB, or MtB) at a specific concentration and pH (the pH of the solution was adjusted with a solution of HCl 0.1 M or NaOH 0.1 M) to achieve adsorption/desorption equilibrium. en, H 2 O 2 was added to the solution (the concentration of H 2 O 2 in the solution is 0.096 M). After a certain interval, 5 mL of the solution was withdrawn and centrifuged to remove the catalyst, and the concentration of the remaining solution was determined.
e experiments were carried out in three replicates, and the data were analyzed using Microsoft Excel software. e concentration of PhN in the solution was determined with high-performance liquid chromatography (HPLC) on the UFLC Shimadzu (Japan) equipment. e HPLC apparatus was equipped with a C-18 column with an acetonitrile/methanol/water (1 : 1 : 8) mobile phase and an SPD-20A detector at a wavelength of 254 nm. e products of PhN oxidation were determined with the LC-MS/MS method on the Agilent 1900/6500 series Q-TOF (USA).
e RhB (or MtB) concentration in the supernatant was determined with the UV-Vis method on the Jasco V-770 (Japan) at λ max � 554 nm for RhB (λ max � 664 nm for MtB).

Structural Properties of Iron (III) Benzene Dicarboxylate
Synthesized under Different Conditions 3.1.1. Influence of Solvent Type. e influence of DMF, MeOH, and water on the formation of the crystal structure of iron (III) benzene dicarboxylate is evaluated from XRD data. e XRD patterns of the samples synthesized in DMF and MeOH exhibit diffraction peaks at 7.24, 8.92, 9.72, 10.15, and 12.50° (Figure 1(a)). ese peaks indicate that the metalorganic framework structure is formed in the reaction. However, the sample synthesized in MeOH has diffraction peaks with higher 2θ at 17, 25, and 27.6° (Figure 1(b)), and they are the typical diffraction peaks for BDC. is evidence indicates that a large amount of BDC does not react or only binds with iron (III) in the form of individual structures (not forming a framework). e sample synthesized in water also exhibits only the characteristic diffraction peaks of BDC without diffraction peaks at smaller angles ( Figure 1 is finding indicates that water is not a suitable solvent for the formation of the MIL-53(Fe) structure. e MIL-53(Fe) structure is formed via the bonds between iron (III)/iron oxide and the BDC anion, and therefore, the number of these bonds has a significant influence on the synthesis efficiency. When the solvent is MeOH and water, iron (III) easily hydrolyzes at high temperatures. In contrast, DMF is suitable for the deprotonation of BDC to form BDC anions because the dipole moment of DMF is much greater than that of MeOH and water (3.86 as opposed to 1.69 and 1.85 D). Among the three solvents, DMF is the most suitable for the synthesis of MIL-53(Fe). Numerous authors report their synthesis of MIL-53(Fe) in DMF [1-3, 5, 6, 8-10, 19, 24, 25, 28]. e FT-IR spectra of BDC and the iron (III) benzene dicarboxylate samples synthesized in the solvents are shown in Figure 2. For BDC, the absorption peak at 1681 cm −1 characterizes the stretching vibration of the C � O group, and the absorption peaks at 1423 and 937 cm −1 are attributed to the bending vibration of the O−H group, while the absorption peak at 784 cm −1 represents the stretching vibration of the C−H bond in the aromatic ring [24]. For the sample synthesized in water, the absorption peaks are almost identical to those of BDC, indicating that the MIL-53(Fe) structure is hardly formed. e absorption peak of ] Fe−O at 565 cm −1 is probably due to the iron hydroxide formed by hydrolysis. For the sample synthesized in MeOH, the appearance of the absorption peak at 1508 cm −1 indicates the existence of a coordinated bond between the central metal atom and the carboxyl groups, and the absorption peak at 559 cm −1 corresponds to the stretching vibration of Fe−O.
ese absorption bands indicate the formation of the metaloxo bonds between the carboxylic group of BDC acids and Fe (III) [1,6,24]. ese bonds indicate the presence of the MIL-53(Fe) metal-organic framework. However, the absorption peak characteristic for the stretching vibration of the C � O group (1704 cm −1 ) remains intense, manifesting that a large amount of BDC is not involved in the formation of the framework. For the sample synthesized in DMF, the absorption peaks at 1529, 1383, 748, and 537 cm −1 characterize the structure of the MIL-53(Fe) material [1,6,24]. Furthermore, the absorption band at 1681 cm −1 (] C � O ) almost disappears, indicating that the obtained MIL-53(Fe) sample no longer has free carboxylic groups, and they form bonds with Fe (III). ese results are completely consistent with the XRD results presented in Figure 1.
e SEM images indicate that the material synthesized in DMF consists of rods of varying sizes with a smooth surface, sharp edge, and multiple cracks/fractures (Figures 3(a) and  3(b)). e cracks/fractures in the material structure are probably responsible for the MIL-53(Fe) characteristic diffraction with low intensity, as seen in Figure 1(a). In MeOH, the obtained sample also has a rod form (Figure 3(c)) similar to the sample synthesized in DMF, but the rod shape is irregular, and the surface has small particles attached to it. For the sample synthesized in water (Figure 3(d)), the obtained material has a cube shape with a size of 1−2 μm and a relatively regular nodule surface.
us, DMF and MeOH enable the formation of the MIL-53(Fe) structure with similar morphology. e metal-organic framework structure is not formed in water. erefore, the cubes are probably encapsulated compounds with iron oxide clusters in the core, and the surface consists of various forms of benzene dicarboxylate bridges.

Influence of Solvent Content and Reaction Temperature.
In this section, DMF is used as a solvent to synthesize iron (III) benzene dicarboxylate. At a low solvent content ratio (the molar ratio of precursors and DMF is 1 : 1 : 200), low intensity of diffraction peaks indicates low crystallinity of the material (Figure 4(a)). At the 1 : 1 : 300 ratio, the obtained material exhibits a high-intensity diffraction peak at 12.5°, indicating the metal-organic framework formation, but the remaining diffraction peaks have low intensity, indicating a low-ordered material. At higher solvent content ratios (1 : 1 : 400 and 1 : 1 : 450), the XRD patterns have diffraction peaks at 9.29, 12.23, 17.68, 24.82, and 26.92°with higher intensity and clarity, manifesting that the metal-organic framework structure is formed with high crystallinity.
When synthesized at 120°C, MIL-53(Fe) exhibits a low crystal structure with low intensity on the XRD pattern (Figure 4(b)). e peak intensity increases when the synthesis is performed at 150°C, and the material is crystalline.
is crystalline structure practically disappears at higher synthetic temperatures (180 and 200°C). us, at the molar ratio of the precursors and DMF of 1 : 1 : 450 and 150°C, MIL-53(Fe) possesses a highly crystalline structure.
e SEM image and FT-IR spectrum of the iron (III) benzene dicarboxylate sample synthesized at 150°C and a precursors/DMF molar ratio of 1 : 1 : 450 are shown in Figure 5. e obtained sample has a full band of absorption Advances in Materials Science and Engineering characteristics for MIL-53(Fe) with strong intensities at 1526, 1380, 751, and 535 cm −1 [1,6,24]. No vibration for ] C � O is observed, indicating that all BDC is consumed in the reaction. e morphology of the obtained material is similar to that of the sample synthesized at the 1 : 1 : 400 ratio (Figure 3(a)) with a more regular shape and smoother surface. is increasing intensity proves that when the reaction time is extended, the MIL-53(Fe) structure is broken, and under these conditions, mainly iron oxide clusters are formed.

Influence of Reaction
However, for the samples synthesized at the molar ratios of Fe (III)/BDC at 1 : 2 or 1 : 3, the XRD patterns have diffraction peaks at 9.17, 12.59, 17.51, 18.17, 18.47, and 25.37° (  Figure 6(b)), which are consistent with those reported in various publications on the MIL-53(Fe) material [1,5,6,8,19,24,25]. And the diffraction peaks of iron oxide do not appear. erefore, the MIL-53(Fe) metal-organic framework material is formed with high purity at the molar ratios of Fe (III)/BDC at 1 : 2 or 1 : 3. In particular, the intensity of the peaks characteristic for the MIL-53(Fe) material is high and sharp when synthesized at the molar ratio of Fe (III)/BDC at 1 : 2, demonstrating that the MIL-53(Fe) structure is formed with high order and crystallinity. e infrared spectra of the iron (III) benzene dicarboxylate samples synthesized at 150°C with different reaction times and different molar ratios of Fe (III)/BDC are depicted in Figure 7. e samples are synthesized at the ratio of 1 : 1 and 1 : 2 and 24 h have absorption bands at 1528, 1383, 750, and 542 cm −1 . ese bands are specific to MIL-53(Fe) [1,6,24], indicating the formation of a metal-organic framework structure. However, the sample synthesized at the ratio 1 : 1 still has free BDC, evidenced by the vibration band of ] (C � O) at 1693 cm −1 . For the sample synthesized at the ratio 1 : 1 and 96 h, the intensity of the specific absorption bands of MIL-53(Fe) significantly reduces. erefore, longer reaction time does not support the formation of the metal-organic framework, and under this condition, mainly iron oxide is formed.  e morphology of the iron (III) benzene dicarboxylate samples synthesized at 150°C with different reaction times and different molar ratios of Fe (III)/BDC indicates that at the ratio of 1 : 2 and 24 h (Figure 8(a)), the material has a shape of rods with a smooth surface and sharp edges (similar to those observed in Figures 3(a) and 5(b)). At high resolution, little cracking/fracture is observed inside the rods (Figure 8(b)). At the ratio of 1 : 1 and 24 h (Figure 8(c)), the material has a spherical shape with a particle diameter of about 0.3-0.4 μm, with interspersed larger plate particles. When the reaction time is extended to 96 h, the particles are mainly spherical (Figure 8(d)).  Figures 9(a) and 9(b)). is finding is consistent with the XRD result presented in Figure 6(a), indicating the formation of iron oxide clusters in the material. In Figure 9(b), it is also possible to see worm-hole-like structures illustrating the appearance of the MIL-53(Fe) metal-organic framework. At the ratio of Fe (III)/BDC 1 : 2 and 24 h, the material has honeycomb-like structures with a diameter of    (Figure 9(c)). At higher magnification, the HR-TEM image shows worm-hole-like structures (Figure 9(d)), indicating that the metal-organic framework structure is very uniform with high purity. e XPS survey spectra of the samples (Figures 10(a) and 10(c)) confirm the presence of Fe, C, and O elements on the surface of the samples. Figures 10(b) and 10(d) show the high-resolution XPS spectra of Fe 2p with two peaks corresponding to the binding energy of 709.3-709.4 eV and 722.1-722.5 eV, which are assigned to Fe 2p 3/2 and Fe 2p 1/2 of Fe (III), respectively [1,5,24]. e peak separation, namely, ΔFe2p � 2p 1/2 − 2p 3/2 ≈ 13 eV, is very similar to the theoretical value of 13.6 eV for Fe 2 O 3 [38]. erefore, these peaks belong to Fe 3+ of the iron (III) benzene dicarboxylate samples. e nitrogen adsorption-desorption isotherms reveal that the iron (III) benzene dicarboxylate samples synthesized under different conditions exhibit between type I and type IV, with microporous and mesoporous structure (Figure 11). e samples undergo condensation at relatively high pressure (P/P 0 ∼ 1) with a H3-type hysteresis loop, indicating the existence of the pores formed between particles of uneven sizes. For the sample synthesized at the molar ratio of Fe (III)/BDC 1 : 2, strong condensation at low relative pressures indicates the existence of multiple micropores. e textural properties of the samples are presented in Table 1.
e specific surface area increases gradually with the crystallinity of the synthesized MIL-53(Fe) material and reaches the highest value (88.2 m 2 /g) for the sample synthesized with the molar ratio 1 : 2. is value is consistent with that reported by Pu et al. [25] (77.6-89.7 m 2 /g) for MIL-53(Fe) and much higher than the value reported in other publications [1][2][3]6, 30] (from 5.5 to 52 m 2 /g).
In summary, the optimal conditions for forming the MIL-53(Fe) material in this study are as follows: solvent DMF, molar ratio of Fe(III)/BDC/DMF 1 : 2 : 450,   Advances in Materials Science and Engineering temperature 150°C, and reaction time 24 h. is MIL-53(Fe) sample is used as the catalyst for the Fenton-like process to oxidize organic substances in aqueous solutions.

Catalytic Activity of MIL-53(Fe).
e homogenous Fenton reaction is strongly restricted by the pH of the solution because Fe 3+ does not exist at pH ≥ 4 and forms ferric hydroxide sludge. e optimal working pH range for a homogenous Fenton reaction is 2.8-3.2 [31,39]. erefore, heterogeneous Fenton-like processes are developed, in which the catalysts containing iron species (mostly Fe 3+ ) are stabilized by confining in substrates, and they create HO • radicals under uncontrolled pH conditions. MIL-53(Fe) is a metal-organic framework material with Fe (III) (or iron clusters) structural nodes, and it can be used as a heterogeneous catalyst in heterogeneous Fenton-like reactions. erefore, in this section, the catalytic activity of MIL-53(Fe) is assessed via the oxidation of phenol (PhN), rhodamine B (RhB), and methylene blue (MtB) at different solution pHs in a Fenton-like reaction system. e decomposition efficiency of the organic compounds is depicted in Figure 12.  Figure 12(a) shows that the PhN decomposition efficiency is approximately 100% after only 20 minutes of reaction at pH 2-4. In the pH range 6-10, the PhN decomposition rate decreases gradually. After 20 minutes, the efficiency reaches 96% at pH � 6, 90% at pH � 8, and only 52% at pH � 10. It does not reach ∼100% until after 40-60 minutes of reaction. Meanwhile, at pH 12, the decomposition efficiency of PhN is very low, only achieving ∼23% after 120 minutes of reaction. It is well known that PhN is a very stable compound and difficult to decompose completely by a catalytic oxidation reaction. However, the HPLC data presented in Figure 12(b) show that PhN decomposes almost completely after 40 minutes of reaction in the pH range 2-10, and the remaining intermediate product is mainly formic acid in the pH range 2-8. is indicates that MIL-53(Fe) is applicable to catalyze the complete oxidation of PhN in an aqueous solution by H 2 O 2 . e influence of pH on the efficiency of decomposition of RhB by H 2 O 2 on MIL-53(Fe) catalyst is depicted in Figure 12(c). At pH � 2, RhB decomposes very quickly. After 30 minutes of reaction, the efficiency is 93%, and it decomposes completely after 60 minutes. In the pH range 4-10, the efficiency remains stable in the first period and reaches approximately 100% after 90 minutes of reaction. At pH � 12, the decomposition of RhB is slow, and the efficiency does not reach 50% until after 120 minutes of reaction.
As for MtB ( Figure 12(d)), at pH � 2, this dye decomposes very quickly with a 98% efficiency only after 15 minutes of reaction and 100% after 20 minutes. In the pH range 4-8, the decomposition rate is practically stable, and the dye decomposes completely after 120 minutes of reaction. At pH 10 or 12, MtB also decomposes completely after 120 reactions, but the rate of MtB decomposition at pH 12 is higher. Over half of methylene blue decomposes (56%) after 15 minutes of reaction, compared with 18% at pH 10.
In summary, the MIL-53(Fe) material has catalytic activity over a wide pH range (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), indicating that this is a heterogeneous Fenton catalyst, i.e., iron in the material does not dissolve in solution to form a homogenous Fenton system. With all three pollutants, the decomposition rate is high in the pH range of 2-10. At pH 12, the decomposition rate of PhN and RhB decreases, while MtB decomposes faster. It is obvious that the surface of the MIL-53(Fe) material becomes negative at high pH due to the adsorption of OH − ions from solution (pH PZC of MIL-53(Fe) is 3.3, Figure 13). At this pH, PhN exists as an anion (PhN − ), and      therefore, the electrostatic repulsion hinders its adsorption, leading to a significant reduction of the decomposition rate. RhB exists in the form of "closed" entirety in the alkaline environment (electrically neutral), so the rate of decomposition also decreases. In contrast, MtB exists as a cation (MtB + ), and it adsorbs onto the surface of MIL-53(Fe) (due to electrostatic interactions), causing the rate of MtB degradation to increase.
To determine whether iron in the catalyst dissolves in the reaction solution, a leaching experiment is also conducted by monitoring the solution after filtering out the MIL-53(Fe) catalyst by centrifugation after 30 minutes of reaction. It is clearly that the degradation of RhB is quenched although H 2 O 2 remains in the solution (Figure 14).
is finding confirms that MIL-53(Fe) acts as a heterogeneous catalyst in this oxidation process.

Conclusions
e present paper shows that the MIL-53(Fe) metal-organic framework material is successfully synthesized with the solvent-thermal method with DMF or MeOH as solvents, where DMF is more suitable. In addition, the molar ratio of precursors and solvents, temperature, and reaction time also significantly influence the formation of MIL-53(Fe). In DMF and the molar ratio Fe (III)/BDC of 1 : 2, the resulting MIL-53(Fe) has high crystallinity, a large specific surface area (S BET 88.2 m 2 ·g −1 ), and an even porous structure. MIL-53(Fe) is a potential heterogeneous Fenton catalyst for the    oxidation of organic pollutants in the aqueous environment by hydrogen peroxide over the pH range 2-12.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.