Encapsulation of the vanadium substituted Keggin polyoxometalates [α-PVW11O40]4- and [α-PV2W10O40]5- in HKUST-1

Two POM@MOF hybrid materials composed of a copper-based metal-organic framework (MOF) [Cu3(C9H3O6)2(H2O)3]n (HKUST-1) encapsulating vanadium-substituted Keggin polyoxometalates (POM), [α-PVW11O40]4- (PVW11) and [α-PV2W10O40]5- (PV2W10), were prepared and characterized. PVW11@HKUST-1 and PV2W10@HKUST-1 were synthesized hydrothermally by self-assembly of HKUST-1 in the presence of the preformed POMs, [α-PVW11O40]4- and [α-PV2W10O40]5-, respectively. The two POM@MOF composites were characterized by X-ray diffraction, TGA, BET surface area analysis and FT-IR and Raman spectroscopy. The electronic structure of the POM@MOF materials and their respective constituents is surveyed using solid state UV-vis reflectance spectroscopy. The UV-vis spectra order the oxidizing strength of the POM constituents ([α-PV2W10O40]5- > [α-PVW11O40]4-) and reveal the distinct electronic structure of the POM@MOF materials obtained by synthetic encapsulation of mono- and di-vanadium substituted Keggin polyoxotungstates in HKUST-1.

A ratiometric sensor for selective detection of Hg2+ ions by combining second-order scattering and fluorescence signals of MIL-68(In)-NH2

Ratio fluorescence has attracted much attention because of its self-calibration properties. However, it is difficult to obtain suitable fluorescent materials with well-resolved signals simultaneously under one excitation. In this work, we report a different strategy, using MIL-68(In)-NH₂ as both the fluorescence element and the scattered light unit, and coupling the fluorescence and the scattered light to construct the fluorescence and scattered light ratio system. Based on the optical properties and the second-order scattering (SOS) of the material nanoparticles, the synthesized MIL-68(In)-NH₂ can be used to realize the ratio detection of Hg²⁺. Because the scattering intensity of small particle MIL-68(In)-NH₂ is weak, SOS is not obvious. When Hg²⁺ is introduced the coordination reaction between the amino nitrogen atoms of MIL-68(In)-NH₂ and Hg²⁺ make the particles larger, resulting in the decrease of fluorescence and the enhancement of SOS. As a result, a novel Hg²⁺ ratiometric detection method is developed by using the dual signal responses of the fluorescence and scattering. Under the optimal conditions (pH = 6, reaction time 5 min, room temperature, and the maximum excitation wavelength 365 nm), the linear range of the method is 0-100 μM, and the detection limit is 5.8 nM (Ksv = 9.89 × 10⁹ M^-1). In addition, the probe is successfully used to evaluate Hg²⁺ in actual water samples. Compared with the traditional method of recording only the fluorescence signal, the proposed fluorescence-scattering method provides a new strategy for the design of ratiometric sensors.

Microwave-Assisted Synthesis of Tris-Anderson Polyoxometalates for Facile CO2 Cycloaddition

Four new tris-Anderson polyoxometalates (POMs), (NH₄)₄[ZnMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (1), (NH₄)₄[CuMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (2), (TBA)₃(NH₄)[ZnMo₆O₁₇(C₅H₉O₃)₂(OH)]·10H₂O (3) (TBA = n-C₁₆H₃₆N), and (NH₄)₄[CuMo₆O₁₈(C₅H₉O₃)₂]·16H₂O (4), were synthesized by a microwave-assisted method. Single-crystal X-ray diffraction revealed that 1 and 2 contained a tris (trihydroxyl organic compounds) ligand grafted on one side, while two tris ligands were grafted on two sides to form χ/δ and δ/δ isomers in 3 and 4, respectively. ¹H and ¹³C NMR spectra of the χ/δ isomer 3 were obtained for the first time, with six methylenes showing six peaks in the ¹H NMR spectrum and only four peaks in the ¹³C NMR spectrum. Mass spectrometry monitoring revealed that during the microwave-assistant process the tris ligand can graft onto POMs to form 1, while tris directly coordinates with metallic heteroatoms to form isopolymolybdates during the conventional reflux synthesis process. In addition, 1-4 can catalyze CO₂ with epoxides into cyclic carbonates with high selectivity and yields at an atmospheric pressure of CO₂, which is lower than the pressure of CO₂ in other catalysis using POMs as catalysts. Furthermore, 1-4 showed good catalytic stability and cycling properties. Mechanism studies substantiated POMs cocatalyzed with Br^- to improve the catalytic yields.

Heterogenisation of polyoxometalates and other metal-based complexes in metal–organic frameworks: from synthesis to characterisation and applications in catalysis

This review provides a thorough overview of composites with molecular catalysts (polyoxometalates, or organometallic or coordination complexes) immobilised into MOFs via non-covalent interactions.

Polyoxometalate-Based Metal-Organic Frameworks with Unique High-Nuclearity Water Clusters

The maximum exposure of polyoxometalates (POMs) is of great significance to enhance the catalytic performance of HKUST-1 with incorporated Keggin-type POMs. Herein, two phosphovanadomolybdates were encapsulated into the HKUST-1 via a hydrothermal method to obtain two polyoxometalate-based metal-organic frameworks, formulated as [Cu₁₂(BTC)₈(H₂O)₁₂][H₄PMo₁₁VO₄₀]@(H₂O)₃₀ (1) and [Cu₁₂(BTC)₈(H₂O)₁₂][H₅PMo₁₀V₂O₄₀]@(H₂O)₄₉ (2). Single-crystal X-ray diffraction analysis indicates that two compounds contain unique high-nuclearity water clusters without organic counter cations. The octahedral-shaped water cluster (H₂O)₃₀ was constructed from square-pyramid-shaped (H₂O)₅ for compound 1, while the huge cage-shaped water cluster (H₂O)₄₉ of compound 2 consisted of crown-like (H₂O)₈ and one water molecule, which substitute the organic counter cations involved in the structural construction. More importantly, after removing the water clusters via simple heat treatment, the active sites of the two compounds were fully exposed, leading to good catalytic activities for both benzene hydroxylation reaction and oxidative desulfurization. Furthermore, the catalytic test confirmed that compound 2 may be a bifunctional heterogeneous catalyst with great promise for both benzene hydroxylation and oxidative desulfurization.

Imidazole-induced self-assembly of polyoxovanadate cluster organic framework for efficient Knoevenagel condensation under mild conditions

Three new polyoxovanadates have been made, one of them displays highly efficient heterogeneous solvent-free catalytic activity and excellent recyclability in the Knoevenagel condensation at room temperature.

An in situ approach to functionalize metal–organic frameworks with tertiary aliphatic amino groups

Tertiary aliphatic amino modified UiO-67/66(Zr), IRMOF-n(Zn) and MIL-101(Fe) were synthesized by a facile and efficient one-pot strategy under the corresponding metal catalysis.