Metal–Organic Frameworks Based on Fe and Cu

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This entry is adapted from the peer-reviewed paper 10.3390/catal13010159

Advanced oxidation processes (AOPs) have been postulated as viable, innovative, and efficient technologies for the removal of pollutants from water. Among AOPs, photo-Fenton processes have been shown to be effective for the degradation of various types of organic compounds in industrial wastewater.

bimetallic catalysts copper heterogeneous catalysis iron photo-Fenton

1. Introduction

In recent years, the rapid development of the industrial sector has resulted in numerous effluents that contain toxic substances and affect the environment in a dangerous way ^[1]. Some of these pollutants, due to their chemical nature, are resistant to the conventional physical and biological processes commonly used in wastewater treatment plants ^[2]. For this reason, advanced oxidation processes (AOPs) have been postulated as viable, innovative, and efficient technologies for the removal of these compounds from water bodies ^[3]. Among the AOPs, the heterogeneous Fenton and photo–Fenton processes have been shown to be effective for the degradation of various types of organic compounds in industrial wastewater ^[4]^[5].

Metal–organic frameworks (MOFs) are inorganic–organic porous polymers composed of metal ions or metal clusters linked to each other by bridging organic ligands to form three–dimensional structures with a high specific surface area. In particular, Fe–based MOFs, made by polydentate organic ligands as linkers and inorganic Fe ions or Fe–oxo clusters as nodes, have recently attracted great attention in various applications such as gas adsorption, catalysis, and sensor development ^[6].

2. Metal–Organic Frameworks Based on Fe and Cu

Within the applications, Fe–based MOFs have been considered promising catalysts in photo–Fenton processes due to their interfacial electron transfer properties and the cycling of the Fe(III)/Fe(II) redox pair. Additionally, the band gaps of Fe–based MOFs are suitable for visible light photoactivation and thus, Fe–O clusters could be directly excited to generate electron hole pairs (e⁻ − h⁺), which subsequently degrade organic pollutants or lead to the generation of reactive oxygen species ^[7]. Likewise, it has been shown that the separation efficiency of photogenerated carriers in the Fe–O cluster remains low due to fast electron–hole recombination ^[7]^[8]. Taking this into account, different modifications in the structure of MOFs have been proposed to delay the recombination rate and improve pollutant degradation efficiencies ^[8]^[9]. In this sense, the combination of MOFs with metals or metal oxide nanoparticles may form a new interface or heterojunction that efficiently couples the catalytic process of photo–induced electrons and radicals’ generation ^[10]^[11]. Moreover, the addition of a second metal ion into the nodes of frameworks could significantly improve the catalytic properties of MOFs ^[12]. Recently, different reports have incorporated Cu into Fe–based MOFs structure yielding Fe–Cu bimetallic MOFs in order to improve the performance of these materials in photo–Fenton processes.

Do et al. ^[13] reported the preparation of Fe–doped Cu 1,4–benzenedicarboxylate MOFs (Fe–CuBDC) and its application as a heterogeneous photo–Fenton catalyst for the degradation of MB in aqueous solution under visible light irradiation. The authors compared the efficiency of the bimetallic Fe–CuBDC with each single metal MOF. The degradation efficiency of MB with Fe–CuBDC was much higher than those obtained with the others, which indicates that the partial substitution of Cu by Fe significantly improves the photocatalytic activity of the material. Complete removal of MB was achieved after 70 min under light irradiation with 1.0 g L⁻¹ of Fe–CuBDC (Table 1). The degradation performance of the Fe–CuBDC catalyst was relatively constant with a slight reduction in removal efficiency from 99.9% to 97.2% after five reuse cycles.

Table 1. Summary of recently used Fe–Cu bimetallic MOFs as photo–Fenton photocatalysts in wastewater treatment.

Material ^(a)	Conditions	Contaminant	Degradation Efficiency	Reference
Fe–CuBDC^(b)	Simulated sunlight, pH 6, [H₂O₂] = 50 mM, [Catalyst] = 1 g L⁻¹	Methyl Blue (50 mg L⁻¹)	100% removal of the dye in 70 min	^[13]
MCuFe MOF ^(b,c)	300 W xenon lamp equipped with a UV cut–off filter (λ > 400 nm), pH 4–9, [H₂O₂] = 5 mM, [Catalyst] = 0.05 g L⁻¹	Methyl Blue (50 mg L⁻¹)	100% removal of the dye in 40 min	^[14]
Cu₂O/MIL(Fe/Cu) ^(b)	500 W xenon lamp, pH 7.47, [H₂O₂] = 49 mM, [Catalyst] = 0.5 g L⁻¹	Thiacloprid (80 mg L⁻¹)	100% removal of TCL in 20 min; 82% TOC removal in 80 min	^[15]
L–MIL–53 (Fe, Cu) ^(b)	300 W xenon lamp equipped with a UV cut–off filter (λ > 420 nm), pH 7, [H₂O₂] = 5 mM, [Catalyst] = 0.1 g L⁻¹	Ciprofloxacin (20 mg L⁻¹)	60% removal of CIP in 30 min	^[16]

^(a) The synthesis method is shown in the footnotes. ^(b) solvothermal method. ^(c) Hydrothermal method.

Shi et al. ^[14] prepared a binary bimetallic heterojunction, which consisted of a core–shell magnetic CuFe₂O₄@MIL–100(Fe, Cu) metal–organic framework, via an in–situ derivation strategy. The synthesis methods, which involved the in situ surface pyrolysis of CuFe₂O₄ nanoparticles and complexation with trimesic acid to derive MIL–100(Fe, Cu), allowed the formation of a bimetallic core–shell CuFe₂O₄@MIL–100(Fe, Cu) heterojunction (MCuFe MOF). These materials showed notable catalytic performance in a photo–Fenton process towards the degradation of various organic pollutants by increasing H₂O₂ activation efficiency and decreasing the required dosage of MCuFe MOF (0.05 g L⁻¹) over a wide pH range (4–9) (Table 1). Furthermore, the MCuFe MOF showed a high stability for the degradation of organic contaminants, with almost no decrease in activity and negligible metal leaching after five successive cycles. According to the proposed mechanism, the combination of the photothermal conversion effect with the formed heterojunction cannot only accelerate the generation and separation of photogenerated e⁻ and h⁺ but also improve the continuous and efficient transformation of ≡Fe(III)/Fe(II) and ≡Cu(II)/Cu(I) redox couples, leading to enhanced photo–Fenton efficiency.

Zhong et al. ^[15] performed Cu₂O growth on the surface of Cu–doped MIL–100 (Fe) to obtain a Cu₂O/MIL(Fe/Cu) composite, which showed an enhanced interfacial synergistic effect and was successfully used as photo–Fenton catalysts for thiacloprid (TCL) degradation. The authors reported that Cu doping into MIL(Fe) led to the reduction in the band gap, and a boost of the redox cycle Fe²⁺/Fe³⁺. Likewise, the growth of Cu₂O extended the light absorption range of MIL(Fe/Cu) from UV to the visible region. Cu₂O/MIL(Fe/Cu) composite exhibited a TCL degradation rate nearly 10 times faster than those of Cu₂O and MIL–100(Fe), separately, achieving a complete TCL degradation within 20 min of reaction. Moreover, TOC removal reached 82.3% within 80 min under neutral conditions, which highlights the good performance of the Cu₂O/MIL(Fe/Cu) composite. Catalyst reuse tests showed no significant efficiency loss in reaction rates even after the tenth cycle, reaching complete TCL degradation within 20 min in each cycle.

Low–crystalline MOFs, with long–range disorder but local crystallinity, allow the availability of more active sites and defects than highly crystalline materials. These materials could improve the activation capacity of Fe–based MOFs towards H₂O₂ by increasing the number of metallic coordinately unsaturated active sites (CUS) within the frameworks. Wu et al. ^[16] prepared low–crystalline bimetallic MOFs of MIL–53(Fe, M) (M: Mn or Cu), via a one–pot solvothermal method, as photo–Fenton catalysts for the degradation of CIP. The results showed a significant improvement in photo–Fenton performance for the low–crystallinity catalyst compared to the crystalline counterparts, which was mainly attributed to the enhancement of the synergism between the hetero–metal nodes. In particular, low crystallinity MIL–53(Fe, Cu) exhibited a much higher removal efficiency and a faster reaction rate than that of crystalline MIL–53(Fe, Cu) within 30 min (Table 1). Besides the increased metal CUSs in the low–crystalline state, both Cu and Mn could increase the specific surface area and promote the visible–light absorption and separation/transportation of carriers in the low–crystalline state, thus leading to the acceleration of Fe(II)/Fe(III) and M(red)/M(ox) cycles.

All the results described above are summarized in Table 1.

References

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Subjects: Nanoscience & Nanotechnology

Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Gabriela N. Bosio

Fernando S. García Einschlag

Luciano Carlos

Daniel O. Mártire

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Update Date: 21 Feb 2023

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1. Introduction

2. Metal–Organic Frameworks Based on Fe and Cu

References