Catalytic Degradation of Methylene Blue by Fenton-like Oxidation of Ce-doped MOF
Graphical abstract
Ce-doped UiO-67 was synthesized by adding cerium instead of a part of zirconium into the UiO-67 skeleton by one-step hydrothermal synthesis method, and then the synthesized material was calcined at high temperature. The prepared nanocomposites were carefully characterized by a series of instruments and a surface analyzer. The synthesized materials were used as catalyst in the Fenton-like oxidation of methylene blue model reaction with H2O2 as oxidant. The catalyst has a high catalytic capacity for 500 mg L−1 methylene blue.
Introduction
With attracting growing attention to clean environment and human health, it is urgent to need high-efficiency and low-cost technology to reduce the pollutant in wastewater [[1], [2], [3]]. It has been recognized that the dye wastewater has seriously polluted the environment and caused serious consequences, such as aesthetic pollution, toxicity and interference to aquatic organisms [[3], [4], [5]]. Moreover, most textile dyes have persistent color and nonbiodegradable properties [6]. It is a difficult task to remove most dyes from wastewater. Only a few water treatment technologies have been successfully used to remove dyes from wastewater, including physical methods, biotechnology and chemical technologies. The removal of organic pollutants by activated carbon adsorption is one of the most effective methods for dye removal, but the treatment may be costly. Ozonation and hypochlorite oxidation are effective decolorization methods, but they are not ideal methods for dye removal due to high equipment cost, high operation cost and secondary pollution [6].
Recently, the chemical treatment method based on the production of hydroxyl radicals is the so-called advanced oxidation process. These technologies can produce a hydroxyl radical (·OH), which can degrade most organic pollutants rapidly and non-selectivity [6,7]. Fenton reaction is a well-studied advanced oxidation process, which reacts in the coexistence of catalyst and oxidant [8,9]. The final product is produced by the nonselective oxidation of organic compounds by hydroxyl radicals. The classical Fenton reagent consists of homogeneous solution of iron ion and hydrogen peroxide. The process is extremely difficult to recycle effectively. In the alkali precipitation process after the reaction, a large number of iron sludge will be produced as hazardous waste [10]. On the other hand, iron catalyzed Fenton or Fenton like reaction of H2O2 decomposition to ·OH has been widely studied [11]. Fenton-like systems using non-iron Fenton catalysts have also been developed. Cerium, cobalt, copper, manganese and other redox metal oxides were used as catalysts to study their reactivity to H2O2 decomposition into ·OH [10,12,13]. Chengjie Zang et al. [14] prepared ceria nanorods by calcining ceria nanocrystals at high temperature, which are easy to decompose H2O2 into ·OH, then applied in the Fenton-like degradation of Orange II (AO7). Yiwen Jiang et al. [15] also synthesized CeO2-supported Mn catalyst MnOx-CeO2-s as Fenton type composite catalyst, which catalyze ethyl acetate.
MOFs is considered to be a promising and unique porous material because of its crystallinity, rich structural diversity and customizability [16]. The periodic structure of MOFs leads to high dispersion and density of active metals [17]. Despite the above advantages, the content of active metal species in MOFs is low, which limits the catalytic reaction. Therefore, it is an urgent and interesting task to introduce a variety of active metal species into multi-functional surfactants. The multi-functional composite materials have synergistic effect, and give full range of performance to their role in catalytic reactions. Therefore, it is a meaningful approach to rationally design composite materials having capability in forming a self-assembled structure [[18], [19], [20]]. For example, Rui Geng et al. use a simple in situ construction method to design a heterojunction model based on molecular self-assembly [21]. The self-assembled porous g-C3N4 nanotube were able to hamper carrier aggregation and it provides numerous catalytic active sites, mainly via the coordination of Ag+ ions.
Therefore, in this work, a self-assembled structure was constructed through introducing Ce3+ into the MOF unit to form a non-iron Fenton catalyst (named as Ce-doped UiO-67-400 here after) with the excellent structural characteristics achieved by calcination at higher temperature. The one-step hydrothermal synthetic approach was found straightforward, where the synthesized product has both adsorption and catalytic dual functions without secondary pollution. The SEM, XRD, FTIR, XPS and N2 adsorption-desorption techniques were used to characterize the so-obtained bimetallic organic framework material. The performance of the material to promote MB degradation in Ce-doped UiO-67-400/H2O2 system was tested under different H2O2 concentration, pH, catalyst dosage and calcination temperature. The stability and reusability of the material were also investigated.
Section snippets
Sample preparation
Materials and chemicals: ZrCl4 (98 %) and Biphenyl-4,4ˊ-dicarboxylic acid (H2BPDC, 98 %) were purchased from Shanghai Macklin Biochemical Co, Ltd. Ce(NO3)3·6H2O (98 %) and Methylene blue (MB) was purchased from Sinopharm Chemical Reagent Co, Ltd. Acetic acid (HAc, 99.5 %) and N,N-dimethylformamide (DMF, 99.5 %) were obtained from Beijing Chemical Works. All reagents and solvents were analytical grade and used directly without any further purification.
Synthesis of sample
Ce-doped UiO-67: at room temperature, ZrCl4
Synthesis and characterization
The morphology of Ce-doped UiO-67 and Ce-doped UiO-67-400 catalysts was studied by SEM and TEM. Fig. 1(a) and (c) show SEM and TEM images of Ce-doped UiO-67, in which octahedral structure with smooth surface can be observed. The side length of octahedron is between 200−400 nm. After calcination at 400 °C for 4 h, Ce-doped UiO-67-400 was obtained. Fig. 1(b) and (d) are SEM and TEM images of Ce-doped UiO-67−400. Except for rough surface, the shape and size of calcined products are almost
Conclusions
Ce-doped UiO-67-400 was successfully synthesized by adding Ce3+ instead of Zr4+ into the framework of UiO-67 by one-step hydrothermal synthesis followed by calcination at high temperature. The results showed that the Ce-doped UiO-67-400 catalyst calcined at 400 °C has a large pore size, which is conducive to the decomposition of H2O2 to generate ·OH to oxidize MB. Under optimized conditions and initial concentration of MB is 500 mg L−1, Ce-doped UiO-67-400 as catalyst has exhibited a good
Declaration of Competing Interest
The authors reported no declarations of interest.
Acknowledgement
This work was financially supported the National Natural Science Foundation of China (Grants 21201160).
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