Tetrahedral Connection of ε-Keggin-type Polyoxometalates To Form an All-Inorganic Octahedral Molecular Sieve with an Intrinsic 3D Pore System

Microporosity and Catalytic Activity for Hydrodesulfurization of Pharmacosiderite Mo4P3O16 Synthesized at a Moderate Temperature

Pharmacosiderite Mo4P3O16 (Pharma-MoPO) consists of [Mo4O4] cubane unit and [PO4] tetrahedral to form an open framework with a microporous structure similar to that of LTA-type zeolite. Although attractive applications are expected due to its microporous structure and redox-active components, its physicochemical properties have been poorly investigated due to the specificity of its synthesis, which requires a high hydrothermal synthesis temperature of 360 °C. In this study, we succeeded in synthesizing Pharma-MoPO by hydrothermal synthesis at 230 °C, which can be applied using a commercially available autoclave by changing the metal source. Through the study of the solids and liquids obtained after hydrothermal syntheses, the formation process of Pharma-MoPO under our studied synthesis conditions was proposed. Advanced characterizations provided detailed structural information on Pharma-MoPO, including the location site of a countercation NH4+. Pharma-MoPO could adsorb CO2 with the amount close to the number of cages without removing NH4+. Pharma-MoPO exhibited stable catalytic activity for the hydrodesulfurization of thiophene while maintaining its crystal structure, except for the introduction of sulfide by replacing lattice oxygens. Pharmacosiderite Mo4P3O16 was successfully obtained by hydrothermal synthesis at a moderate temperature, and its microporosity for CO2 adsorption and catalytic properties for hydrodesulfurization were discovered.

Assembly of ϵ-Keggin Polyoxometalate from Molecular Crystal to Zeolitic Octahedral Metal Oxide

Zeolitic octahedral metal oxides are inorganic crystalline microporous materials with adsorption and redox properties. New ϵ-Keggin nickel molybdate-based zeolitic octahedral metal oxides have been synthesized. ³¹ P NMR spectroscopy shows that reduction of Mo^VI -based molybdates forms an ϵ-Keggin polyoxometalate that immediately transfers to the solid phase. Investigation of the formation process indicates that a low Ni concentration, insoluble reducing agent, and long synthesis time are the critical factors for obtaining the zeolite octahedral metal oxides rather than the ϵ-Keggin polyoxometalate molecule. The synthesized zeolitic nickel molybdate with Na⁺ is used as the adsorbent, which effectively separates C₂ hydrocarbon mixtures.

Investigation of the Synthesis of Zeolitic Vanadotungstate and its Use in the Separation of Propylene/Propane at High Temperature and Humidity

Synthetic conditions for the zeolitic octahedral metal oxide based on vanadotungstate are studied. The temperature, time, acidity, W/V ratio, cation species, and concentration affect the resulting materials. The study shows that mixing tungstate and VO²⁺ in an aqueous solution generates cubane units ([W₄O₁₆]^8-) at room temperature. The cubane units assemble with VO²⁺ immediately to form a solid with an amorphous phase and nonporosity, which further crystallizes under a hydrothermal condition to form the crystalline microporous vanadotungstate. The zeolitic vanadotungstates act as effective adsorbents for the separation of propylene/propane. The active materials effectively separate propylene/propane even at high temperatures and high humidities.

Zeolitic Octahedral Metal Oxides with Ultra-Small Micropores for C2 Hydrocarbon Separation

Separation of C₂ hydrocarbons, C₂ H₆ , C₂ H₄ , and C₂ H₂ , remains significant challenges in chemical industry. However, there are only few adsorbents that can effectively isolate C₂ hydrocarbons from their mixtures particularly at a high temperature. Herein, we design a zeolitic octahedral metal oxide based on ϵ-Keggin polyoxometalates with metal ion linkers. Single gas adsorption of the material shows the different adsorption performances for the C₂ hydrocarbons and the strong interaction of the material with the C₂ hydrocarbons. Dynamic competitive adsorption experiments show that the material efficiently separates each of the binary C₂ hydrocarbon mixtures and even the ternary C₂ hydrocarbon mixtures with high selectivity. The material keeps high separation performance even the temperature was increased to 85 °C. The material is stable and is able to be reused without loss of the separation performance.

Keggin-type polyoxometalate 1 : 1 complexes of Pb(II) and Bi(III): experimental, theoretical and luminescence studies

Bi3+ and Pb2+ uptake by a monolacunary Keggin-type [PW11O39]7- anion leads to the formation of [PW11O39Bi]4- and [PW11O39Pb]5- complexes with a stereochemically active lone pair at the incorporated heterometal. The two complexes were isolated as (TBA)4[PW11O39Bi] (1) and (TBA)5[PW11O39Pb] (2) and characterized by 31P and 183W NMR spectroscopy, high-resolution electrospray mass-spectrometry (HR-ESI-MS) and cyclic voltammetry (CV). EXAFS and XANES data confirm the unchanged oxidation state and ψ-square pyramidal geometry of Bi3+ and Pb2+ in 1 and 2. DFT calculations were used in order to (i) confirm the absence of ligands attached to the heterometal sites in both complexes and localize the lone pair, and (ii) assign all signals in the 183W NMR spectra. Complexes 1 and 2 demonstrate photoluminescence (PL). A reversible change in the PL spectra of both complexes in the presence of water vapor has been detected. On the contrary, PL data for sandwich-type ((CH3)4N)4K3[H4(PW11O39)2Bi]·25H2O (3) do not show sensitivity to water vapor.

Zeolitic octahedral niobium oxide with microchannels of seven-membered rings for photocatalytic H2 evolution from saline water

Zeolitic octahedral metal oxides (ZOMOs) are fully inorganic crystalline materials, mostly containing transition metals, and possess a defined framework as well as regular hollow channels. Therefore, they have significant application potential in many fields, particularly catalysis. Herein, we report the synthesis and characterization of zeolitic octahedral niobium oxides (ZOMO-NbOx) and the framework was made of {NbOx} polyhedra ({NbO6} octahedron and {Nb6O27} pentagon units). Microchannels consisting of 7-membered rings with ∼0.4 nm diameter were realized, and rarely reported plural {Nb6O27} units including dimers, trimers, and tetramers were discovered. Owing to the continuous hollow microporous structure that provides a large surface area as well as a shortened transfer path for charge carriers from bulk to the surface, considerably enhanced activities beyond pristine Nb2O5 were achieved towards photocatalytic H2 evolution from (saline) water. The results show a practical case of solo-Nb-based zeolitic materials, which expand the ZOMO family and provide further insights into the design and synthesis of efficient zeolitic semiconductors for artificial photocatalysis.

Zeolitic octahedral metal oxide-based membranes for pervaporative desalination of concentrated brines

An all-inorganic zeolitic octahedral metal oxide based on cobalt tungstoselenate with porosity and hydrophilicity is successfully used to fabricate a membrane. The as-synthesized membrane and its ion-exchanged membranes exhibit extraordinary permeation flux with high salt rejection by pervaporative desalination for high-salinity brines up to 25 wt%.

Preparation of zeolitic bismuth vanadomolybdate using a ball-shaped giant polyoxometalate for olefin epoxidation

Bismuth vanadomolybdate-based zeolitic octahedral metal oxide was synthesized from {Mo72V30} and used as a heterogeneous catalyst for olefin epoxidation.

Green synthesis of Ag/TiO2 composite coated porous vanadophosphates with enhanced visible-light photo-degradation and catalytic reduction performance for removing organic dyes

As environmental pollution and energy shortages have become global concerns, the construction of highly efficient catalysts using facile and green methods remains a long-term goal. In the present study, we proposed a facile catalyst preparation method in which Ag/TiO₂ composites were coated on the surface of the porous pure inorganic crystalline vanadium phosphates (VPO) by a one-step strategy. More importantly, the in situ reduction of Ag nanoparticles was achieved at room temperature without severe conditions or hydrogen atmosphere in which the porous VPO was employed as the reductant. The prepared Ag/VPO@TiO₂ composites act as a class of efficient bifunctional catalysts for visible light photodegradation of MB molecules and catalytic reduction of p-nitrophenol (4-NP). Among these samples, the 6.82%Ag/VPO@TiO₂ composite exhibited a superior photocatalytic activity in the degradation of MB and an ultrafast reduction rate for 4-NP of about 0.1 mM/40 s. The photocatalytic mechanistic studies revealed that the encapsulated VPO with a narrow band gap not only efficiently enhances the photosensitivity of the TiO₂ but also largely facilitates the photogenerated charge separation. The subsequent deposition of Ag NPs is able to further promote electron transfer ability, which leads to the higher photocatalytic activity. Moreover, the contact of Ag NPs with the surface of semiconductor TiO₂ can result in an electron-enhanced area in their interface that could effectively facilitate the uptake of electrons by the 4-NP molecules and then improve the reduction activity.

Zeolitic Vanadomolybdates as High-Performance Cathode-Active Materials for Sodium-Ion Batteries

Developing new cathode-active materials for rechargeable batteries is important to fulfill the growing demands of energy transformation, storage, and utilization. Zeolitic transition-metal oxides based on vanadomolybdate, constructed by pentagon metal-oxygen clusters as building blocks and metal ions as linkers in a trigonal symmetry, are good candidates for cathodes of Na rechargeable batteries. The material is activated via amorphization of the crystal structure in the ab plane during discharging process, keeping the molecular structure of the building blocks stable, which causes high specific capacity and good cycle performance.

Ion-Exchangeable Microporous Polyoxometalate Compounds with Off-Center Dopants Exhibiting Unconventional Luminescence

The synthesis of luminescent polyoxometalates (POMs) typically relies on the assembly of POM ligands with rare earth or transition metals, placing significant constraints on the composition, structure, and hence the luminescence properties of the resultant systems. Herein, we show that the ion-exchange strategy can be used for the synthesis of novel POM-based luminescent materials. We demonstrate that introducing bismuth ions into an ion-exchangeable, microporous POM compound yields an unconventional system luminescing in the near-infrared region. Experimental characterization, coupled with quantum chemical calculations, confirms that bismuth ions site-specifically occupy an off-center site in the lattice, and have an asymmetric coordination geometry unattainable by other means, thus giving rise to peculiar emission. Our findings offer an effective strategy for the synthesis of POM-based luminescent materials, and the design concept may potentially be adapted to the creation of POM-based systems with other functionalities.

A Novel Ruthenium-Decorating Polyoxomolybdate Cs₃Na₆H[MoVI14RuIV₂O50(OH)₂]·24H₂O: An Active Heterogeneous Oxidation Catalyst for Alcohols

The first example of wholly inorganic ruthenium-containing polyoxomolybdate Cs₃Na₆H[MoVI14RuIV₂O50(OH)₂]·24H₂O (1) was isolated and systematically characterized by element analysis, infrared spectroscopy (IR), thermogravimetric analyses (TGA), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX) and single-crystal X-ray diffraction. Compound 1 is composed of an unprecedented {Mo14}-type isopolymolybdate with a di-ruthenium core precisely encapsulated in its center, exhibiting a three-tiered ladder-like structure. The title compound can act as an efficient heterogeneous catalyst in the transformation of 1-phenylethanol to acetophenone. This catalyst is also capable of being recycled and reused for at least ten cycles with its activity being retained under the optimal conditions.

High-Performance Cathode Based on Microporous Mo-V-Bi Oxide for Li Battery and Investigation by Operando X-ray Absorption Fine Structure

The development of cathode-active material of Li battery is important for the current emerging energy transferring and saving problems. A stable crystalline microporous complex metal oxide based on Mo, V, and Bi is an active and suitable material for Li battery. High capacity (380 Ah/kg) and stable cycle performance are achieved. X-ray absorption near-edge structure analyses demonstrate that the original Mo⁶⁺ and V⁴⁺ ions are reduced to Mo⁴⁺ and V³⁺ in the discharging process, respectively, which results in a 70-electron reduction per formula. The reduced metal ions can be reoxidized reversibly in the next charging process. Furthermore, extended X-ray absorption fine structure analyses reveal that the Mo-O bonds in the material are lengthened in the discharging process probably due to interaction with Li⁺ without change of the basic structure.

A spherical deca-vanadophosphate covalent assembled all-inorganic open framework

Through covalent connection of phosphate groups, an all-inorganic open framework, [NH₄]₉[P₄V₁₀O₃₄]_1.5·3H₂O (1), has been prepared by a hydrothermal reduction-condensation method. It is built from a vanadophosphate (VPO) unit of [P₄V₁₀O₃₄]^6-, which represents a rare example of a spherical {V₁₀-type} structure among the VPO family. Further studies also demonstrate that compound 1 is an n-type semiconductor.

Synthesis of crystalline molybdenum oxides based on a 1D molecular structure and their ion-exchange properties

Crystalline molybdotellurate based on a 1D molecular structure exhibits interesting ion-exchange properties with both inorganic and organic cations.

A Robust Open Framework Formed by Decavanadate Clusters and Copper(II) Complexes of Macrocyclic Polyamines: Permanent Microporosity and Catalytic Oxidation of Cycloalkanes

The first decavanadate-based microporous hybrid, namely, [Cu(cyclam)][{Cu(cyclam)}2(V10O28)]·10H2O (1, cyclam = 1,4,8,11-tetraazacyclotetradecane) was prepared by reaction of (VO3)(-) anions and {Cu(cyclam)}(2+) complexes in NaCl (aq) at pH 4.6-4.7 and characterized by elemental analyses, thermogravimetry, and X-ray diffraction (powder, single-crystal) techniques. Compound 1 exhibits a POMOF-like supramolecular open-framework built of covalent decavanadate/metalorganic layers with square-like voids, the stacking of which is aided by interlamellar cementing complexes and generates water-filled channels with approximate cross sections of 10.4 × 8.8 Å(2). The framework is robust enough to remain virtually unaltered upon thermal evacuation of all water molecules of hydration, as demonstrated through single-crystal X-ray diffraction studies on the anhydrous phase 1a. This permanent microporosity renders interesting functionality to 1, such as selective adsorption of CO2 over N2 and remarkable activity as heterogeneous catalyst toward the H2O2-based oxidation of the highly-stable, tricyclic alkane adamantane.

New crystalline complex metal oxides created by unit-synthesis and their catalysis based on porous and redox properties

The development of new complex metal oxides having structural complexity suitable for solid-state catalysis is of great importance in fundamental catalysis research and practical applications. However, examples of these materials are rare. Herein, we report two types of crystalline complex metal oxides with new structures and their catalytic properties. The first one is an all-inorganic ε-Keggin polyoxometalate-based material with intrinsic microporosity. The framework of the material is formed by the assembly of ε-Keggin polyoxomolybdate units with metal ion linkers in a diamondoid topology. The micropores of the material can be opened without change of the structures, and the material adsorbs small molecules. This material has both redox properties and acidity and can be applied to O₂ adsorption, selective oxidation of methacrolein, and hydrolysis of cellobiose. The other material is a crystalline metal oxide based on molecular nanowires. The hexagonal POM units stack along the c axis to form prismatic clusters as molecular wires. The molecular wires further assemble in a hexagonal fashion to form the crystals, and NH₄⁺ and water are present in between the molecular wires. The material is active as an acid catalyst for cellobiose conversion.

The first tritopic bridging ligand 1,3,5-tris(4-carboxyphenyl)-benzene (H3BTB) functionalized porous polyoxometalate-based metal–organic framework (POMOF): from design, synthesis to electrocatalytic properties

A POMOF with substantial catalytic activity towards bromate reduction was isolated through the extension of transition-metal-grafted ε-Keggin by a tripodal ligand H₃BTB.