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

Abstract

The iron (III) benzene dicarboxylate metal-organic framework material (MIL-53(Fe)) was synthesized with either the solvent-thermal or hydrothermal method under different conditions. The 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. The 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. The 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 MIL-53(Fe). Under optimal conditions, MIL-53(Fe) has high crystallinity and a large specific surface area ($S_{\mathrm{BET}}$ = 88.2 m²/g). The 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.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Metal-organic frameworks (MOFs) are porous nanomaterials with large specific surface area and considerable pore volume [1–11]. These materials have attracted a lot of attention from scientists due to their perfect structure and applicability in such fields as gas separation and storage [12–17], molecular sensors [18, 19], adsorption [1–4, 20–22], and catalysis [5–8, 23–27]. MOFs are usually synthesized with the hydrothermal method [5, 22, 28] or the solvent-thermal method [1–3, 5, 6, 8–10, 17, 19, 24, 25, 28]. So far, research on MOFs has mainly focused on discovering new types of MOFs and their applicability in different fields. In the literature, little attention is paid to the detailed examination of the role of synthetic conditions.

Among MOFs, MIL-53 (M^III) (MIL: Materials of Institute Lavoisier; M^III = Fe, Al, Cr, Sc, Ga, In, …) with the formula M^III(OH)·(O₂C−C₆H₄−CO₂)·H₂O has great chemical flexibility and high chemical stability [3, 28, 29]. Iron (III) benzene dicarboxylate, denoted MIL-53(Fe), is a porous solid. It possesses a three-dimensional structure built from infinite one-dimensional bonds −Fe−O−O−Fe−O−Fe− with 1, 4-benzene dicarboxylate bridges [5, 24]. MIL-53(Fe) opens its pores only in the presence of guest molecules. Unlike other MOF materials, MIL-53(Fe) does not possess a high specific surface area [9]. While Pu et al. [25] claim a large specific surface area of 89.7 m²/g for MIL-53(Fe), others prove that MIL-53(Fe) has a really small specific surface area [1–3, 6, 30]. The cause of this controversy is probably because these authors have not found the required appropriate synthetic conditions to prepare MIL-53(Fe).

Hydrogen peroxide (H₂O₂) decomposition in the presence of iron catalysts to hydroxyl radicals (${\text{HO}}^{•}$) has been widely developed into advanced oxidation processes (AOPs) for application in the environmental treatment. The hydroxyl radical has a strong standard oxidation potential (E⁰(${\text{HO}}^{•}$/H₂O = +2.8 V_NHE)) and a high bimolecular reaction rate constant (10⁸−10¹¹ M⁻¹·s⁻¹). This radical is capable of nonselective oxidation of numerous organic substances to nontoxic products, such as CO₂, H₂O, and inorganic salts. Various types of catalytic iron, including metal salts (in the form of Fe²⁺ or Fe³⁺), metal oxides (e.g., Fe₂O₃ and Fe₃O₄), and zero-valence metal (Fe⁰), 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]. This 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 [6–8, 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, The detailed synthetic conditions will be presented. The 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).

2. Experimental

2.1. Reagents and Chemicals

Iron chloride hexahydrate (FeCl₃·6H₂O, 99.1%, Merck, EMD Millipore Corporation, Germany), benzene-1, 4-dicarboxylic acid (C₈H₆O₄, 99%, Acros, Janssen Pharmaceuticalaan, Belgium) (denoted BDC), N′, N-dimethylformamide (C₃H₇NO, ≥99.5%, Fisher, Leicestershire, England) (denoted DMF), methanol (CH₃OH, 99.9%, Fisher, Seoul, Korea) (denoted MeOH), phenol (C₆H₅OH, Merck, Germany) (denoted PhN), rhodamine B (C₂₈H₃₁ClN₂O₃, HiMedia, India) (denoted RhB), and methylene blue (C₁₆H₁₈ClN₃S, Merck, Germany) (denoted MtB) were used in this study.

2.2. Synthesis of Iron (III) Benzene Dicarboxylate

The synthesis of iron (III) benzene dicarboxylate was adapted, Du et al. [5]. In a typical process, a mixture of 1.2469 g FeCl₃·6H₂O, 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. Then, 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 $K\alpha$ radiation ($\lambda$ = 1.5406 Å). 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. The X-ray photoelectron spectroscopy (XPS) was studied by using a Shimadzu Kratos AXIS Ultra DLD spectrometer (Japan). Peak fitting was performed on CasaXPS software. The N₂ 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 N₂ for 5 h.

2.4. Determination of the Point of Zero Charge

The 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. The 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. Then, 0.05 g of MIL-53(Fe) was added to each flask, and the mixture was shaken for 48 hours. The final pH $\mathrm{p}{\mathrm{H}}_f$ of the solutions was measured. The plot representing the relationship between the difference of the final and initial pH value $\mathrm{\Delta }\mathrm{pH}=\mathrm{p}{\mathrm{H}}_f-\mathrm{p}{\mathrm{H}}_i$ and $\mathrm{p}{\mathrm{H}}_i$ was drawn, and the point of intersection of the curve with the abscissa provides $\mathrm{p}{\mathrm{H}}_{\mathrm{PZC}}$.

2.5. Oxidizing Organic Compounds

The oxidation of PhN, RhB, and MtB by H₂O₂ 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. Then, H₂O₂ was added to the solution (the concentration of H₂O₂ 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. The experiments were carried out in three replicates, and the data were analyzed using Microsoft Excel software.

The concentration of PhN in the solution was determined with high-performance liquid chromatography (HPLC) on the UFLC Shimadzu (Japan) equipment. The 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. The products of PhN oxidation were determined with the LC-MS/MS method on the Agilent 1900/6500 series Q-TOF (USA).

The RhB (or MtB) concentration in the supernatant was determined with the UV-Vis method on the Jasco V-770 (Japan) at ${\lambda }_{\max }$ = 554 nm for RhB (${\lambda }_{\max }$ = 664 nm for MtB).

3. Results and Discussion

3.1. Structural Properties of Iron (III) Benzene Dicarboxylate Synthesized under Different Conditions

3.1.1. Influence of Solvent Type

The influence of DMF, MeOH, and water on the formation of the crystal structure of iron (III) benzene dicarboxylate is evaluated from XRD data. The 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)). These peaks indicate that the metal-organic 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. This 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). The sample synthesized in water also exhibits only the characteristic diffraction peaks of BDC without diffraction peaks at smaller angles (Figure 1(b)). This finding indicates that water is not a suitable solvent for the formation of the MIL-53(Fe) structure.

[figures omitted; refer to PDF]

The 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].

The 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. The absorption peak of ν_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. These absorption bands indicate the formation of the metal-oxo bonds between the carboxylic group of BDC acids and Fe (III) [1, 6, 24]. These 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 (ν_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). These results are completely consistent with the XRD results presented in Figure 1.

[figure omitted; refer to PDF]

The 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)). The 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.

[figures omitted; refer to PDF]

Thus, DMF and MeOH enable the formation of the MIL-53(Fe) structure with similar morphology. The metal-organic framework structure is not formed in water. Therefore, the cubes are probably encapsulated compounds with iron oxide clusters in the core, and the surface consists of various forms of benzene dicarboxylate bridges.

3.1.2. 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.

[figures omitted; refer to PDF]

When synthesized at 120°C, MIL-53(Fe) exhibits a low crystal structure with low intensity on the XRD pattern (Figure 4(b)). The peak intensity increases when the synthesis is performed at 150°C, and the material is crystalline. This crystalline structure practically disappears at higher synthetic temperatures (180 and 200°C). Thus, at the molar ratio of the precursors and DMF of 1 : 1 : 450 and 150°C, MIL-53(Fe) possesses a highly crystalline structure.

The 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. The obtained sample has a full band of absorption characteristics for MIL-53(Fe) with strong intensities at 1526, 1380, 751, and 535 cm⁻¹ [1, 6, 24]. No vibration for ν_C = O is observed, indicating that all BDC is consumed in the reaction. The 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.

[figures omitted; refer to PDF]

3.1.3. Influence of Reaction Time and Molar Ratio Fe (III)/BDC

To investigate the influence of reaction time on the formation of the metal-organic framework structure, the reaction temperature (150°C) and the molar ratio of Fe (III)/BCD/DMF (1 : 1 : 450) are fixed. The X-ray diffraction patterns of the iron (III) benzene dicarboxylate samples show that the peaks at 2θ < 30° and characteristic for metal-organic framework materials have low intensity and decrease with increasing reaction time (Figure 6(a)). Meanwhile, the peaks at 24.09, 33.19, 35.71, 41.05, 49.59, and 54.04° corresponding to the (012), (104), (110), (113), (024), and (116) planes of iron oxide (α-Fe₂O₃) [37] have high intensity. This 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.

[figures omitted; refer to PDF]

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. Therefore, 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.

The 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. The 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. These 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 ν (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. Therefore, longer reaction time does not support the formation of the metal-organic framework, and under this condition, mainly iron oxide is formed.

[figure omitted; refer to PDF]

The 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 omitted; refer to PDF]

The HR-TEM images of the iron (III) benzene dicarboxylate samples synthesized at 150°C and 24 h show an interplanar spacing distance of about 0.25 nm, corresponding to the (110) plane of α-Fe₂O₃ [37] on the sample synthesized with the ratio of Fe (III)/BDC at 1 : 1 (Figures 9(a) and 9(b)). This 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 about 20–30 nm (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.

[figures omitted; refer to PDF]

The 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]. The peak separation, namely, $\mathrm{\Delta }\text{Fe}2\text{p}=2{\text{p}}_{1/2}-2{\text{p}}_{3/2}\approx 13\text{\hspace{0.17em}\hspace{0.17em}eV}$, is very similar to the theoretical value of 13.6 eV for Fe₂O₃ [38]. Therefore, these peaks belong to Fe³⁺ of the iron (III) benzene dicarboxylate samples.

[figures omitted; refer to PDF]

The 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). The samples undergo condensation at relatively high pressure $P/P_0\sim 1$ 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. The textural properties of the samples are presented in Table 1. The specific surface area increases gradually with the crystallinity of the synthesized MIL-53(Fe) material and reaches the highest value (88.2 m²/g) for the sample synthesized with the molar ratio 1 : 2. This value is consistent with that reported by Pu et al. [25] (77.6–89.7 m²/g) for MIL-53(Fe) and much higher than the value reported in other publications [1–3, 6, 30] (from 5.5 to 52 m²/g).

[figures omitted; refer to PDF]

Table 1

Textural properties of iron (III) benzene dicarboxylate samples synthesized under different conditions.

Synthesized conditions			Specific surface area, SBET (m2·g−1)	Micropore area (m2·g−1)	External surface area (m2·g−1)
Solvent	Molar ratio of Fe (III)/BDC/solvent	Temperature and time
MeOH	1 : 1 : 400	150°C for 15 h	7.3	3.6	3.7
DMF	1 : 1 : 400	150°C for 15 h	12.4	8.8	3.6
DMF	1 : 1 : 450	150°C for 15 h	24.6	9.6	15.0
DMF	1 : 2 : 450	150°C for 24 h	88.2	77.0	11.2

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, temperature 150°C, and reaction time 24 h. This MIL-53(Fe) sample is used as the catalyst for the Fenton-like process to oxidize organic substances in aqueous solutions.

3.2. Catalytic Activity of MIL-53(Fe)

The homogenous Fenton reaction is strongly restricted by the pH of the solution because Fe³⁺ does not exist at pH ≥ 4 and forms ferric hydroxide sludge. The optimal working pH range for a homogenous Fenton reaction is 2.8–3.2 [31, 39]. Therefore, heterogeneous Fenton-like processes are developed, in which the catalysts containing iron species (mostly Fe³⁺) are stabilized by confining in substrates, and they create ${\text{HO}}^{•}$ 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. Therefore, 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. The decomposition efficiency of the organic compounds is depicted in Figure 12.

[figures omitted; refer to PDF]

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. This indicates that MIL-53(Fe) is applicable to catalyze the complete oxidation of PhN in an aqueous solution by H₂O₂.

The influence of pH on the efficiency of decomposition of RhB by H₂O₂ 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 (2–12), 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.

[figure omitted; refer to PDF]

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 H₂O₂ remains in the solution (Figure 14). This finding confirms that MIL-53(Fe) acts as a heterogeneous catalyst in this oxidation process.

[figure omitted; refer to PDF]

4. Conclusions

The 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 m²·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.

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