Graphene oxide (GO) contains abundant oxygen-containing functional groups acting as hydrogen bond acceptors for proton conduction on its basal plane. However, the dilemma in realizing bulk in-plane conduction and the metastability at room temperature of GO films both obstruct its application. Polyoxometalate-modified sponge-like GO monolith (PEGO) with 3D cross-linking inner structure, which exhibits unique “shrink-expand” effect to polar solvent, are synthesized. Owing to the introduction of polyoxometalates and the replacement of unstable epoxy groups by ethylenediamine, PEGO exhibits hitherto the highest proton conductivity under low relative humidity (1.02 × 10⁻² S cm⁻¹ at 60% relative humidity) and excellent long-term stability (more than 1 month). The outstanding conductivity originates from 3D transporting pathways, high-density hopping sites, and eliminated grain boundary resistance. This study provides a practical way to design GO-based proton-conducting material dominated by in-plane diffusion.

Two new heteropolyniobate compounds, (C₂N₂H₁₀)₆[(GeOH)₂Ge₂Nb₁₆H₂O₅₄]·25H₂O (1) and [Cu(en)₂(H₂O)₂]₅K₁₀[K(GeOH)₂Ge₂Nb₁₆H₃O₅₄]₂·38H₂O (2, en = ethylenediamine), have been prepared under hydrothermal–conventional combination conditions and structurally characterized by elemental analysis, IR spectroscopy, powder and single-crystal XRD, diffuse reflection spectroscopy, and thermogravimetric (TG) analysis. Compounds 1 and 2 are based on [(GeOH)₂Ge₂Nb₁₆H₂O₅₄]^12– and [K(GeOH)₂Ge₂Nb₁₆H₃O₅₄]^10– building blocks, respectively. Compound 1 represents the initially isolated Nb/Ge lacunary derivative and compound 2 exhibits 1D ladderlike chains. The photoluminescence properties and electrochemical properties of compounds 1 and 2 have been investigated.

Reducing the level of sulfur content in fuel oils has long been desired for environmental reasons. Polyoxometalates (POMs) can act as catalysts to remove sulfur-containing heterocyclic compounds by the process of oxidative desulfurization under mild conditions. However, one key obstacle to the development of POM-based catalysts is the poor solubility of POMs in the overall nonpolar environment. We report a novel strategy for the introduction of catalytically active POMs into nonpolar reaction systems by encapsulating the inorganic catalyst within the pores of a metal–organic framework structure in which the organic ligands act as hydrophobic groups. The nanocrystalline catalysts, obtained rapidly and conveniently by both solution and mechanochemical synthesis, showed remarkable activities in catalytic oxidative desulfurization reactions in both a model diesel environment and in real diesel wherein dibenzothiophene was converted rapidly and quantitatively into dibenzothiophene sulfone.

Two isostructural polyoxotungstates, K₆Na₄H₈[Eu₆(H₂O)₃₈(P₂W₁₅Nb₃O₆₂)₄]·45H₂O (1) and K₆Na₄H₈[Ce₆(H₂O)₃₈(P₂W₁₅Nb₃O₆₂)₄]·56H₂O (2), have been synthesized by reactionof saturated Nb/W mixed-addendum polyoxometalate [P₂W₁₅Nb₃O₆₂]^9– and lanthanide ions in acidic solution. They are the first examples of lanthanide derivatives based on saturated Dawson-type Nb/W mixed-addendum polyoxometalates. All the compounds were fully characterized by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis, elemental analysis, and electrochemistry. XRD analysis reveals that 1 and 2 display one-dimensional chains constructed from sandwich-type fragment {Ln₃(P₂W₁₅Nb₃O₆₂)₂} linked by alternating Ln–O–W bridging bonds. In 1 and 2, all the lanthanide ions selectively bind to O_t(Nb) of {P₂W₁₅Nb₃O₆₂} fragments (O_t: terminal oxygen atoms), indicating the high nucleophilicity of O_t(Nb). The photoluminescence behavior of 1 in the solid state was investigated at room temperature; it exhibits the characteristic transition of Eu under the excitation wavelength (394 nm). Additionally, the cyclic voltammogram of 2 indicates good electrocatalytic activity towards the reduction of nitrite.

The strategy for obtaining a crystalline catalyst based on a porous copper-based metal-organic framework and 12-tungstosilicic acid with different particle sizes is reported. Through the control of hydrothermal synthesis and some simple treatments, catalyst samples with average particle diameters of 23, 105, and 450 μm, respectively, were prepared. This crystal catalyst has both the Brønsted acidity of 12-tungstosilicic acid and the Lewis acidity of the copper-based metal-organic framework, and has high density of accessible acid sites. Its catalytic activity was fully assessed in the dehydration of methanol to dimethyl ether. The effect of particle size on the catalytic activity of catalyst was studied, in order to select the particle size appropriate for avoiding the diffusion limitation in heterogeneous gas-phase catalysis. In the selective dehydration of methanol to dimethyl ether, this catalyst exhibited higher catalytic activity than the copper-based metal-organic framework, γ-alumina, and γ-alumina-supported 12-tungstosilicic acid catalysts. It showed high catalytic performances, even at higher space velocity or in the presence of excess water. In addition, the catalyst was also preliminarily assessed in the formation of ethyl acetate from acetic acid and ethylene. It also exhibited a high activity which was comparable with that of silica-supported 12-tungstosilicic acid catalyst.

The dimeric, zirconium-sandwiching compound K₄H₆[Zr₄(OH)₆(CH₃COO)₂(α-PW₁₀O₃₇)₂]·23H₂O (1) was obtained by the reaction of lacunary [PW₁₁O₃₉]^7– and ZrCl₄ in a 1 M potassium acetate buffer. It was characterized by elemental and thermogravimetric analysis, X-ray crystallography, solution ¹⁸³W NMR, FTIR, and UV spectroscopy, and cyclic voltammetry (CV). All the studies indicate that compound 1 is stable both in the solid state and in solution. The cyclic voltammograms indicate good electrocatalytic activity in the reduction of nitrite.

Two polyoxometalate hybrid compounds based on porous metal-organic frameworks (PMOFs/POMs) have been prepared by lithium ion exchange and their crystal structures, stabilities and gas adsorption properties have been characterized. Both compounds consist of neutral Cu₃(BTC)₂ (BTC = 1,3,5-benzentricarboxylate) metal-organic frameworks (MOFs) as hosts and Keggin polyoxometalates (POMs) anions and lithium ions as guests with the MOFs maintaining their permanent porosity. H₂ adsorption studies demonstrated that the guests play a key role in increasing the H₂ adsorption capacity of the frameworks. With the introduction of POMs and lithium ions, the compounds not only display strong hydrogen adsorption behavior, but also exhibit some differences in H₂ binding energy.

The reactivity of polyoxoniobates has been studied in acidic solution by grafting niobium onto trivacant Keggin-type germanotungstates. Four niobium-containing compounds were obtained in the course of this study. Cs_6.5K_0.5[GeW₉(NbO₂)₃O₃₇]⋅6H₂O (Cs_6.5K_0.5-1) synthesized by the reaction of K₇H[Nb₆O₁₉] and A-α-Na₁₀[GeW₉0₃₄] in H₂O₂ solution is a tris(peroxoniobium)-substituted A-α-GeW₉ derivative. Cs_6.5K_0.5[GeW₉Nb₃O₄₀]⋅ 10H₂O (Cs_6.5K_0.5-2) is a peroxo-free compound obtained by eliminating the peroxo groups in 1. Monomers 1 and 2 as precursors can each afford two nanoscale POMs, dimer Cs₅[H₁₅Ge₂W₁₈Nb₈O₈₈]⋅18H₂O (Cs₅-3) and tetramer Cs₈K₃H₉[Ge₄W₃₆Nb₁₆O₁₆₆]⋅ 27H₂O (Cs₈K₃H₉-4), through the formation of NbONb bridges. Disassembly through the cleavage of NbONb bonds from 4 to 2 and 1 was achieved by controlling the pH and by adding H₂O₂, respectively. The transition from 1 to 2 can be achieved by simply adding H₂O₂ to a solution of 1. All four compounds were characterized in the solid state by elemental analysis, infrared spectroscopy, thermogravimetry, and single-crystal X-ray diffraction. ¹⁸³W NMR analysis proved that the solid-state structures of polyanions 1–4 were retained after dissolution. Disassembly from 4 to 1 and 2 in solution was observed by ¹⁸³W NMR spectroscopy. The UV/Vis spectra of 1 at different pH confirmed that it is stable in the pH range of 0.1–14.0 at room temperature.