Synthesis of a Polyoxometalate-Encapsulated Metal–Organic Framework via In Situ Ligand Transformation Showing Highly Catalytic Activity in Both Hydrogen Evolution and Dye Degradation

In Situ Ligand-Transformation-Assisted Assembly of a Polyoxometalate and Silver-Phosphine Oxide Cluster for Colorimetric Detection of Phenol Contaminants

In situ ligand transformation strategies represent an efficient pathway for constructing function-oriented polyoxometalate (POM)-based crystalline materials. Herein, three POM-based hybrid networks were synthesized through in situ transformation of the phosphine ligand, formulated as [Ag(dppeo)6][H2PMo12O40]·5H2O (1), [Ag(dedpo)]4[SiW12O40]·6H2O (2), and [Ag(dppeo)]3[PW12O40]·3H2O (3) (dedpo = (2-(diphenylphosphaneyl)ethyl)diphenylphosphine oxide; dppeo = ethane-1,2-diylbis(diphenylphosphine oxide)). During the synthesis of these compounds, the 1,2-diphenylphosphine ethane molecule underwent in situ oxidation, transforming into dppeo and dedpo ligands, respectively. Compound 1 features a supramolecular architecture assembled from [Ag(dppeo)3]+/[Ag2(dppeo)6]2+ cationic clusters with disordered Ag centers and protonated [H2PMo12O40]- anions. Compound 2 presents a 3-D POM-supported metal-organic framework consisting of binuclear [Ag(dedpo)]22+ units, {-dedpo-Ag-dedpo-} chains, and [SiW12O40]4- polyoxoanions. Compound 3 displays a 2-D layered structure formed by {-dppeo-Ag3-dppeo-} chains and [PW12O40]3- clusters. Pronounced argentophilic interactions are observed in compounds 1 and 3. The three compounds demonstrate satisfactory heterogeneous catalytic activity in the colorimetric detection reactions toward phenol pollutants with detection limits of 1.73, 1.92, and 4.6 μM, respectively. Additionally, compounds 1-3 show high anti-interference capabilities and high sensitivity in differentiating phenol from its halogenated derivatives. This work presents some guidance for designing specific function-oriented POM-based materials via an in situ ligand transformation strategy.

Encapsulation of H4SiW12O40 into an Amide-Functionalized MOF: A Highly Efficient Nanocomposite Catalyst for Oxidative Desulfurization of Diesel Fuel

Production of hydrocarbon fuels containing sulfur in ultralow levels is in high demand and requires the development of novel catalytic systems for oxidative desulfurization (ODS). Herein, a new nanocomposite SiW12@ZSTU-10 catalyst containing H4SiW12O40 (SiW12) encapsulated into a zinc(II) 3D metal-organic framework (MOF) (ZSTU-10) was assembled and characterized. The intricate structure and porosity of ZSTU-10 permit efficient encapsulation of the catalytically active SiW12 cages. The impact of different experimental parameters on the ODS of model oil containing dibenzothiophene as a typical S-based contaminant was evaluated. The SiW12@ZSTU-10 catalyst exhibits remarkable activity with up to 99.8% sulfur removal in 30 min. Kinetic features, trapping tests, and mechanistic studies were also performed. Furthermore, the catalyst offered an outstanding thermal and chemical stability, without apparent leaching and decline in the activity after six cycles. Such an improved catalytic efficiency of SiW12@ZSTU-10 can be assigned to (i) size-matched occupation of the ZSTU-10 pores by SiW12-active species, (ii) prevention of polyoxometalate (POM) leaching from the MOF matrix, (iii) facilitation of the access of S-based substrates to the active sites of SiW12, and (iv) excellent stability and recyclability of the obtained nanocomposite. The preset work widens a family of promising nanocomposite catalysts for improving the desulfurization performance of hybrid POM-MOF catalytic systems.

Advances in Polyoxometalates as Electron Mediators for Photocatalytic Dye Degradation

The increasing concerns over the environment and the growing demand for sustainable water treatment technologies have sparked substantial interest in the field of photocatalytic dye removal. Polyoxometalates (POMs), known for their intricate metal-oxygen anion clusters, have received considerable attention due to their versatile structures, compositions, and efficient facilitation of photo-induced electron transfers. This paper provides an overview of the ongoing research progress in the realm of photocatalytic dye degradation utilizing POMs and their derivatives. The details encompass the compositions of catalysts, catalytic efficacy, and light absorption propensities, and the photocatalytic mechanisms inherent to POM-based materials for dye degradation are exhaustively expounded upon. This review not only contributes to a better understanding of the potential of POM-based materials in photocatalytic dye degradation, but also presents the advancements and future prospects in this domain of environmental remediation.

Two 3D Two-Fold Interpenetrated Dia-Like Polyoxometalate-Based Metal-Organic Frameworks: Synthesis and Sulfide Selective Oxidation Activity

Two new three-dimensional (3D) polyoxometalate-based metal-organic frameworks (POMOFs), [M2(btap)4(H2O)4(HPMo10VI Mo2VO40)] (M = Co (1) and Cd (2); btap = 3, 5-bis(1', 2', 4'-triazol-1'-yl)pyridine), have been synthesized under mild hydrothermal conditions and characterized in detail. Single-crystal X-ray diffraction (SXRD) analysis indicates that 1 and 2 are isostructural. In complexes 1 and 2, the metal ion is coordinated with the ligand to form two different left and right helical one-dimensional chains, which are alternately connected in a twisted form to build a two-fold interpenetrated three-dimensional structure, and the polyoxometalate is encapsulated into in the pores generated by the interpenetrating structure. It is noteworthy that 1 and 2, as recyclable catalysts, possess favorable heterogeneous catalytic activity and excellent sulfoxide selectivity in sulfide oxidation reactions, with H2O2 as an oxidant. By reason of the high dispersion of polyoxometalate with good intrinsic activity in the skeleton structure, the title complex has high activity. In addition, no obvious decrease of sulfoxide yield is observed after at least five cycles. These results indicate the excellent catalytic activity and sustainability of 1 and 2.

Postsynthetic Modification of NU-1000 for Designing a Polyoxometalate-Containing Nanocomposite with Enhanced Third-Order Nonlinear Optical Performance

For the advancement of laser technologies and optical engineering, various types of new inorganic and organic materials are emerging. Metal-organic frameworks (MOFs) reveal a promising use in nonlinear optics, given the presence of organic linkers, metal cluster nodes, and possible delocalization of π-electron systems. These properties can be further enhanced by the inclusion of solely inorganic materials such as polyoxometalates as prospective low-cost electron-acceptor species. In this study, a novel hybrid nanocomposite, namely, SiW₁₂@NU-1000 composed of SiW₁₂ (H₄SiW₁₂O₄₀) and Zr-based MOF (NU-1000), was assembled, completely characterized, and thoroughly investigated in terms of its nonlinear optical (NLO) performance. The third-order NLO behavior of the developed system was assessed by Z-scan measurements using a 532 nm laser. The effect of two-photon absorption and self-focusing was significant in both NU-1000 and SiW₁₂@NU-1000. Experimental studies suggested a much superior NLO performance of SiW₁₂@NU-1000 if compared to that of NU-1000, which can be assigned to the charge-energy transfer between SiW₁₂ and NU-1000. Negligible light scattering, good stability, and facile postsynthetic fabrication method can promote the applicability of the SiW₁₂@NU-1000 nanocomposite for various optoelectronic purposes. This research may thus open new horizons to improve and enhance the NLO performance of MOF-based materials through π-electron delocalization and compositing metal-organic networks with inorganic molecules as electron acceptors, paving the way for the generation of novel types of hybrid materials for prospective NLO applications.