A novel porous molybdophosphate-based FeII,III-MOF showing selective dye degradation as a recyclable photocatalyst

A review of metal-organic framework (MOF) materials as an effective photocatalyst for degradation of organic pollutants

Water plays a vital role in all aspects of life. Recently, water pollution has increased exponentially due to various organic and inorganic pollutants. Organic pollutants are hard to degrade; therefore, cost-effective and sustainable approaches are needed to degrade these pollutants. Organic dyes are the major source of organic pollutants from coloring industries. The photoactive metal-organic frameworks (MOFs) offer an ultimate strategy for constructing photocatalysts to degrade pollutants present in wastewater. Therefore, tuning the metal ions/clusters and organic ligands for the better photocatalytic activity of MOFs is a tremendous approach for wastewater treatment. This review comprehensively reports various MOFs and their composites, especially POM-based MOF composites, for the enhanced photocatalytic degradation of organic pollutants in the aqueous phase. A brief discussion on various theoretical aspects such as density functional theory (DFT) and machine learning (ML) related to MOF and MOF composite-based photocatalysts has been presented. Thus, this article may eventually pave the way for applying different structural features to modulate novel porous materials for enhanced photodegradation properties toward organic pollutants.

A New 2D Metal–Organic Framework for Photocatalytic Degradation of Organic Dyes in Water

Two–dimensional (2D) metal–organic frameworks (MOFs) are fascinating photocatalytic materials because of their unique physical and catalytic properties. Herein, we report a new (E)–4–(3–carboxyacrylamido) benzoic acid [ABA–MA] ligand synthesized under facile conditions. This ABA–MA ligand is further utilized to synthesize a copper-based 2D MOF via the solvothermal process. The resulting 2D MOF is characterized for morphology and electronic structural analysis using advanced techniques, such as proton nuclear magnetic resonance, Fourier-transform infrared spectroscopy, ultraviolet–visible spectroscopy, and scanning electron microscopy. Furthermore, 2D MOF is employed as a photocatalyst for degrading organic dyes, demonstrating the degradation/reduction of methylene blue (MeBl) dye with excellent catalytic/photodegradation activity in the absence of any photosensitizer or cocatalyst. The apparent rate constant (kap) values for the catalytic degradation/reduction of MeBl on the Cu(II)–[ABA-MA] MOF are reported to be 0.0093 min−1, 0.0187 min−1, and 0.2539 min−1 under different conditions of sunlight and NaBH4. The kinetics and stability evaluations reveal the noteworthy photocatalytic potential of the Cu(II)–[ABA–MA] MOF for wastewater treatment. This work offers new insights into the fabrication of new MOFs for highly versatile photocatalytic applications.

Organic Moiety-Regulated Photocatalytic Performance of Phosphomolybdate Hybrids for Hexavalent Chromium Reduction

Visible-light-driven photocatalytic Cr(VI) reduction is a promising pathway to moderate environmental pollution, in which the development of photocatalysts is pivotal. Herein, three hourglass-type phosphomolybdate-based hybrids with the formula of: (H₂ bpe)₃ [Zn(H₂ PO₄ )][Zn(bpe)(H₂ O)₂ ]H{Zn[P₄ Mo₆ O₃₁ H₆ ]₂ } ⋅ 6H₂ O (1) Na₆ [H₂ bz]₂ [ZnNa₄ (H₂ O)₅ ]{Zn [P₄ Mo₆ O₃₁ H₃ ]₂ } ⋅ 2H₂ O (2) and (H₂ mbpy) {[Zn(mbpy)(H₂ O)]₂ [Zn(H₂ O)]₂ }{Zn[P₄ Mo₆ O₃₁ H₆ ]₂ } ⋅ 10H₂ O (3) (bpe=trans-1,2-bi(4-pyridyl)-ethylene; bz=4,4'-diaminobiphenyl; mbpy=4,4'-dimethyl-2,2'bipyridine) were synthesized under the guidance of the functional organic moiety modification strategy. Structural analysis showed that hybrids 1-3 have similar 2D layer-like spatial arrangements constructed by {Zn[P₄ Mo₆ ]₂ } clusters and organic components with different conjugated degree. With excellent redox properties and wide visible-light absorption capacities, hybrids 1-3 display favourable photocatalytic activity for Cr(VI) reduction with 79%, 70% and 64% reduction rates, which are superior to that of only inorganic {Zn[P₄ Mo₆ ]₂ } itself (21%). The investigation of organic components on photocatalytic performance of hybrids 1-3 suggested that the organic counter cations (bpe, bz and mbpy) can effectively affect the visible-light absorption, as well as the recombination of photogenerated carriers stemmed from {Zn[P₄ Mo₆ ]₂ } clusters, further promoting their photocatalytic performances towards Cr(VI) reduction. This work provides an experimental basis for the design of functionalized photocatalysts via the modification of organic species.

PANI/PbS QD nanocomposite structure for visible light driven photocatalytic degradation of rhodamine 6G

Among conducting polymers, polyaniline (PANI) is one of the most widely used materials due to its unique properties (e.g., high electrical conductivity, outstanding electrochemical properties, easy polymerization, high stability, and low-cost synthesis). In this study, we report the synthesis of a composite of polyaniline with lead sulfide quantum dots (PbS QDs), which was subsequently employed for photocatalysis of a dye, rhodamine 6G (Rh-6G). This PANI/PbS composite was prepared by employing the chemical oxidative polymerization of aniline monomer in the presence of PbS QDs. The composite has been characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and ultraviolet-visible spectroscopy. The composite formation turned out to be beneficial not only for the dispersion of PbS QDs but also for increasing the conductivity of the whole catalyst. They exhibited ~87% degradation of the dye content for 50 min. The kinetic rate for its destruction is 5.03 mmol g^-1 h^-1 with the quantum efficiency (QE) of 7.98E-06 molec/photon. Due to enhanced charge transfer characteristics, the PANI/PbS photocatalyst was capable of efficiently degrading the dye molecules across varying concentrations. The electron-hole pair generated after the visible light irradiation on the PANI/PbS composite led to an efficient oxidative degradation of Rh 6G.

A novel 3D heteropoly blue type photo-Fenton-like catalyst and its ability to remove dye pollution

A environment-friendly 3D inorganic heteropoly blue (HPB) Ba₂Na₂ [HPW^V₄W^VI₈O₄₀]·26H₂O was directly synthesized by hydrothermal method and characterized by means of ICP, IR, XPS, X-ray single crystal and X-ray powder diffraction. It was an efficient heterogeneous photo-Fenton-like catalyst to degrade anionic dye methyl orange under visible light irradiation. It removed cationic dyes methylene blue in neutral environment and rhodamine B in acidic condition via flocculation. The removal efficiency of methylene blue and rhodamine B by flocculation was more than 95%. Moreover, it could degrade methyl orange and flocculate rhodamine B at the same time. For MO and MO-RhB solutions, the degradation rates of MO in 60 min were 85.5% and 49.1%, respectively. Furthermore, the possible pathways for the production of active species in the MO degradation reaction were discussed. This is the first HPB constructed with 4e-reduced phosphotungstate, Ba and Na ions, having the properties of photo-Fenton-like catalyst and flocculant.

Submicron sized water-stable metal organic framework (bio-MOF-11) for catalytic degradation of pharmaceuticals and personal care products

Water-stable and active metal organic frameworks (MOFs) are important materials for mitigation of water contaminants via adsorption and catalytic reactions. In this study, a highly water-stable Co-based MOF, namely bio-MOF-11-Co, was synthesized by a simplified benign method. Moreover, it was used as a catalyst in successful activation of peroxymonsulfate for catalytic degradation of sulfachloropyradazine (SCP) and para-hydroxybenzoic acid (p-HBA) as representatives of pharmaceuticals and personal care products, respectively. The bio-MOF-11-Co showed rapid degradation of both p-HBA and SCP and could be reused multiple times without losing the activity by simply water washing. The effects of catalyst and PMS loadings as well as temperature were further studied, showing that high catalyst and PMS loadings as well as temperature produced faster kinetic degradation of p-HBA and SCP. The generation of highly reactive and HO radicals during the degradation was investigated by quenching tests and electron paramagnetic resonance. A plausible degradation mechanism was proposed based on the functionalities in the bio-MOF-11-Co. The availability of electron rich nucleobase adenine reinforced the reaction kinetics by electron donation along with cobalt atoms in the bio-MOF-11-Co structure.

Influence of Pb element on the catalytic properties of {P4Mo6}-polyoxometalate for redox reactions

To introduce Pb element into the crystal framework of a polyoxoanion, a series of hybrid compounds were synthesized and characterized: (H₂bpp)₅[PbMn(H₂O)₂]₂H₂{Mn[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}₂·8H₂O (1), (H₂bpp)₃[PbM(H₂O)₂]₂H₆{M[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}₂·6H₂O (M = Ni (2), Co (3)) and (H₂bpp)₃[PbCd(H₂O)₂]₂H₆{Cd[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}₂·4H₂O (4) (bpp = 1,3-bis(4-pyridyl)propane). Crystal structures were determined and characterized by IR, TG, XPS and single-crystal X-ray diffraction. Hybrids 1-4 have similar compositions; the inorganic moiety has a high-dimensional polyanionic arrangement formed from bi-metallic {PbM} subunits covalently bridging negative {M(P₄Mo)₂} clusters; the organic moiety is made up of protonated bpp cations that surround anionic clusters through many complex supramolecular interactions. Experimental results showed that the introduction of Pb can modulate the electrochemical properties of crystal materials, leading to the more reversible redox process of {M(P₄Mo₆)₂}-based hybrids. Hybrids 1-4 were employed as heterogeneous catalysts for redox reactions to evaluate their activity involving the transfer of electrons. It was found that these kinds of hybrids were catalytically active in the reduction reaction of [Fe(CN)₆]^3- (+S₂O₃^2-) → [Fe(CN)₆]^4-, but were inactive for Cr(vi) (+HCOOH) → Cr(iii) at 25-70 °C. The effect of temperature on the catalytic ability of hybrids was studied, and it was found that 50 °C was the optimal reaction condition for the four compounds to catalyze [Fe(CN)₆]^3- → [Fe(CN)₆]^4-. The reduction percentages of Fe(iii) in 270 min were about 95.7% (1), 76.95% (2), 89.39% (3) and 80.8% (4). The well-defined polyanionic structure with tunable structural features might be beneficial for exploring the mechanism of catalytic redox reactions at the molecular level.

Efficient photocatalytic degradation of rhodamine 6G with a quantum dot-metal organic framework nanocomposite

The hybrid structures of metal organic frameworks (MOFs) and nanoparticles may offer the realization of effective photocatalytic materials due to combined benefits of the porous and molecular sieving properties of MOF matrix and the functional characteristics of encapsulated nanoparticles. In this study, cadmium telluride (CdTe) quantum dots (QD) are conjugated with a europium-MOF for the synthesis of a novel nanocomposite material with photocatalytic properties. Successful synthesis of a QD/Eu-MOF nanocomposite was characterized with various spectroscopic and microscopic techniques. This QD/Eu-MOF is found to be an effective catalyst to complete the degradation of Rhodamine 6G dye within 50 min.

Facile synthesis of AC@TiO2-S with improved visible light photocatalytic activity and recyclability through a controllable sol–gel approach

AC@TiO₂-S prepared by controlled sol–gel approach exhibits improved photocatalytic activity over AC@TiO₂-G prepared by sol–gel approach for the degradation of methyl orange under visible light irradiation and has excellent recyclability.