Nano-porous bimetallic CuCo-MOF-74 with coordinatively unsaturated metal sites for peroxymonosulfate activation to eliminate organic pollutants: Performance and mechanism
Graphical abstract
Introduction
The worldwide concern about water pollution has caused serious attention due to the production of large amounts of wastewater with refractory organic pollutants during the anthropogenic activities, such as petrochemical, pharmaceutical, paper, oil, and other industries (Nidheesh and Gandhimathi, 2012; Gu et al., 2018; Li et al., 2018, 2019a). Organic dyes, which are among the main organic pollutants affecting water quality, are difficult to remove from the natural environment owing to their stable and complex structure (Li et al., 2019b; Ranjith et al., 2019; Oon et al., 2018). The discharge of dye wastewater without sufficient treatment would pose a great threat to our living environment and human health because of their reported abundance, recalcitrance, and toxicity (Liang et al., 2018; Li et al., 2019c).
Advanced Oxidation Processes (AOPs) based on highly Reactive Oxygen Species (ROSs) (such as SO4•−, ·OH, O2•−, etc.) are increasingly adopted for wastewater treatment, especially for the wastewater containing recalcitrant organic pollutants (Li et al., 2016a, 2018, 2019a). Among AOPs, oxidation using persulfate (PS) including peroxydisulfate (S2O82−, PDS) and peroxymonosulfate (HSO5−, PMS) is an attractive and effective technology for removal of a variety of hazardous and persistent organic pollutants due to the non-selective reaction between organic compounds and ROS (Zhang et al., 2015). However, PDS and PMS oxidants are relatively stable and unable to degrade organic pollutants at room temperature, extra energy (heat, UV) and/or catalysts are needed to activate them for the generation of ROS (Li et al., 2018; Waldemer et al., 2007; Ai et al., 2014). Compared to the symmetric structure of PDS (−3OS-O-O-SO3−, lO-O = 1.322 Å), the asymmetric structure (HO–O–SO3−) of PMS have a longer O–O bond (lO-O = 1.326 Å), which allows it to need lower activation energy (Guo et al., 2020). Cobalt-based catalysts (e.g. CoFe2O4, TiO2/Co3O4 and CuO–Co3O4@CeO2) (Tan et al., 2017; Ranjith et al., 2019; Li et al., 2020c) have been explored to activate PMS due to its outstanding catalytic efficiency for the decomposition of PMS to generate sulfate radicals (SO4•−) (Anipsitakis and Dionysiou, 2004). Nevertheless, the secondary pollution caused by the leaching Co2+ have been raising intensive concern due to its toxicity to human health. Thus, alternative catalysts with high efficiency are urgently needed.
Metal-organic frameworks (MOFs) have attracted significant attention in AOPs due to their highly ordered structure, large specific surface areas, and tunable structural features (Guesh et al., 2017; Kapelewski et al., 2014). MOFs consist of bridging organic linkers and inorganic secondary building units (SBUs) of metal ions (or oxo-clusters) (Kapelewski et al., 2014; Hu et al., 2014). The high density of undercoordinated metal cations is expected to be a key component of heterogeneous catalysts of PDS and PMS. Li et al. (2016b) tested the catalytic performance of Fe(III) based metal-organic frameworks (including MIL-101(Fe), MIL-100(Fe), MIL-53(Fe), and MIL-88B(Fe)) for activation of PDS to remove acid orange 7 (AO7), which showed low degradation efficiency. For Fe(II)-MOFs, 86.73% degradation efficiency of dibutyl phthalate (DBP) was achieved by PDS oxidation at a wide pH range (Chi et al., 2019). The excellent performance of Co-BTC (using Co2+ as nodes and 1,3,5-benzene tricarboxylate as a linker) as catalysts of PMS for the oxidation of DBP have been documented in our previous study (Li et al., 2016a), but the stability was not satisfied. To further improve the catalytic activity and stability, incorporating of new secondary metal nodes into the frameworks is proposed, that means to prepare bimetallic MOFs (Yang and Xu, 2017; Sun et al., 2017). Bimetallic MOFs have already been proven to exhibit higher catalytic activities than their monometallic counterparts (Li et al., 2019b; Sun et al., 2017; Wang et al., 2016). Sun et al. prepared three series of bimetallic Fe/Mn-MOFs, Fe/Co-MOFs, and Fe/Ni-MOFs to activate H2O2 for phenol degradation, which indicated that the introduction of another metal ion could significantly enhance the phenol degradation (Sun et al., 2017). Bimetallic MOF of FeCo-BDC (BDC- terephthalic acid) also exhibited higher catalytic activities toward PMS and PDS than their monometallic counterparts for the removal of methylene blue and phenanthrene (Li et al., 2019b, 2020a). As one of the important components of MOFs, the organic ligands showed a great influence on their intrinsic properties. For example, the use of bridging ligands containing OH rather than the halides is favorable for the formation of metal-oxo active sites (Rosen et al., 2020). Additionally, the redox activity of the framework contains a [MO]2+ site is greatly influenced by the donor strength of the organic linker (Rosen et al., 2020). As organic ligands, two more oxygens of 2,5-dihydroxy-1,4-benzene dicarboxylate (DHTA) from –OH groups would connect with metal ions compared with terephthalic acid (H2BDC). Then, it is hypothesized that the MOF used DHTA as organic ligand will contain much more redox-active metal centers than the isostructural framework containing H2BDC as the organic ligand.
In this study, bimetallic CuCo-MOF-74 that based on the organic linker 2,5-dihydroxy-1,4-benzenedicarboxylate (DHTA) and divalent cations (Cu and Co) were successfully synthesized by varying the concentration of the two metal ions and temperatures during the synthesized procedure. The morphology, chemical composition, and textural properties of the CuCo-MOF-74 were characterized by X-ray powder diffraction (XRD), Fourier-transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption isotherms. Degradation studies of methylene blue (MB) by PMS were carried out with these CuCo-MOF-74 materials observing the effect of main parameters, including metal cluster composition, synthesis temperature, PMS concentration, catalyst dosage, initial pH, and inorganic anions on the degradation behavior. Furthermore, the stability of the CuCo-MOF-74, intermediate products of MB, and possible MB degradation pathway were also explored. The findings of this study will highlight the great potential of developing MOFs as a heterogeneous catalyst for organic pollutant degradation.
Section snippets
Materials
Details of all chemicals used in this study were included in the Supplementary Material (Text S1).
Preparation of bimetallic metal-organic frameworks (CuCo-MOF-74)
The synthesis of bimetallic metal-organic frameworks (CuCo-MOF-74) was based on a previous procedure with slight modification (Li et al., 2020a). Briefly, 0.594 g of 2,5-dihydroxy-1,4-benzene dicarboxylate (DHTA, 3 mM), 0.873 g of cobalt-nitrate hexahydrate (Co(NO3)2·6H2O, 3 mM), and 0.725 g of cupric nitrate trihydrate (Cu(NO3)2·3H2O, 3 mM) were added into the mixed solvent of
Morphology characterization
SEM and TEM instruments were applied to investigate the morphological structures of the CuCo-MOF-74 prepared under various conditions. As shown in Fig. 1a and Fig. 1b, the prepared Cu1Co1-MOF-74 obtained from 110 °C to 130 °C with 24h appeared uniformly hexagonal rod-like three-dimensional (3D) structure, where the diameter of the hexagonal rod was 0.5–3.0 μm with a length of 1.0–7.0 μm. Meanwhile, visible cracks and crevices were found in the as-prepared Cu1Co1-MOF-74, especially in the Cu1Co1
Conclusion
In this study, nano-porous bimetallic CuCo-MOF-74 with coordinatively unsaturated metal sites with different stoichiometric Cu/Co ratios were successfully synthesized and characterized by XRD, FT-IR, SEM, TEM, BET, and XPS techniques. Compared to CuCo-MOF-74 with other Cu/Co ratios, FeCo-MOF-74 with Cu/Co ratio of 1:1 exhibited the highest activity towards PMS decomposition, nearly 100% of MB degradation was achieved within 30 min, along with a pseudo-first-order kinetic rate constant of
Credit author statement
Huanxuan Li: Conceptualization, DFT calculations, Funding acquisition. Zongxiang Yang: Investigation, Writing – original draft preparation. Shun Lu: Investigation, Data curation. Liya Su: Validation, Visualization. Chunhui Wang: Catalyst preparation and data analysis, Funding acquisition. Jingang Huang: Resources, Writing – review & editing. Jie Zhou: Developing mechanics modelling and analysis. Junhong Tang: Resources, Conceptualization, Supervision, Mingzhi Huang: Writing – review & editing,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by Natural Science Foundation of Zhejiang Province (No. LY20B070006), National Natural Science Foundation of China (No. 51808177, No. 21802029), and Guangdong Provincial Natural Science Foundation (No. 2016A030306033).
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