Dynamic Surface Structures and Reactivity of Vanadium-Containing Molybdophosphoric Acid (H3+xPMo12–xVxO40) Keggin Catalysts during Methanol Oxidation and Dehydration

Soft nanobrush-directed multifunctional MOF nanoarrays

Controlled growth of well-oriented metal-organic framework nanoarrays on requisite surfaces is of prominent significance for a broad range of applications such as catalysis, sensing, optics and electronics. Herein, we develop a highly flexible soft nanobrush-directed synthesis approach for precise in situ fabrication of MOF nanoarrays on diverse substrates. The soft nanobrushes are constructed via surface-initiated living crystallization-driven self-assembly and their active poly(2-vinylpyridine) corona captures abundant metal cations through coordination interactions. This allows the rapid heterogeneous growth of MOF nanoparticles and the subsequent formation of MIL-100 (Fe), HKUST-1 and CUT-8 (Cu) nanoarrays with tailored heights of 220~1100 nm on silicon wafer, Ni foam and ceramic tube. Auxiliary functional components including metal oxygen clusters and precious metal nanoparticles can be readily incorporated to finely fabricate hybrid structures with synergistic features. Remarkably, the MIL-100 (Fe) nanoarrays doped with Keggin H₃PMo₁₀V₂O₄₀ dramatically boost formaldehyde selectivity up to 92.8% in catalytic oxidation of methanol. Moreover, the HKUST-1 nanoarrays decorated with Pt nanoparticles show exceptional sensitivity to H₂S with a ppb-level detection limit.

Tailored growth of MOFs is of great interest for a broad range of applications. Here authors present the use of soft nanobrushes to direct the growth of MOF nanoarrays on requisite substrates, which simultaneously allows the loading of auxiliary functional species for advanced catalysis and sensing applications.

Incorporation of Keggin-based H3PW7Mo5O40 into bentonite: synthesis, characterization and catalytic applications

The Keggin-based molybdo-substituted tungstophosphoric acid, H3[PW7Mo5O40]·12H2O, were synthesized and incorporated with a bentonite clay by using a wetness impregnation method. The catalysts were characterized using several methods, such as inductively coupled plasma-atomic emission spectroscopy (ICP-AES), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), and thermogravimetric and differential thermal analysis (TG-DTA). This extremely active catalytic system provides a green strategy for the synthesis of 1,8-dioxo-octahydroxanthene and 1,8-dioxo-decahydroacridine derivatives under solvent free conditions at 80 °C with a good reaction mass efficiency, effective mass yield, and excellent atom economy. Both the surface acidity and catalytic activity sharply increased after H3[PW7Mo5O40]·12H2O was impregnated with bentonite clay. In addition, the PW7Mo5/bentonite catalyst can be conveniently recovered and reused numerous times without demonstrating a significant loss in activity.

Keggin-Type Polyoxometalate-Based ZIF-67 for Enhanced Photocatalytic Nitrogen Fixation

The photocatalytic reduction of N₂ to NH₃ is considered a promising strategy to alleviate human need for accessible nitrogen and environmental pollution, for which developing a photocatalyst is an effective method to complete the transformation of this process. We firstly design a series of highly efficient and stable polyoxometalates (POMs)-based zeolitic imidazolate framework-67 (ZIF-67) photocatalysts for N₂ reduction. ZIF-67 can effectively fix N₂ owing to its porosity. Integration of POMs cluster contributes enormous advantages in terms of broadening the absorption spectrum to improve sunlight utilization, enhance the stability of the materials, effectively inhibit the recombination of photo-generated electron-hole pairs, and reduce charge-transfer impedance. POMs can absorb light to convert into reduced POMs, which have stronger reducing ability to provide ample electrons to reduce N₂ . The reduced POMs can recover their oxidation state through contact with an oxidant, which forms a self-recoverable and recyclable photocatalytic fixing N₂ system. The photocatalytic activity enhances with the increasing number V substitutions in the POMs. Satisfactorily, ZIF-67@K₁₁ [PMo₄ V₈ O₄₀ ] (PMo₄ V₈ ) displays the most significant photocatalytic N₂ activity with a NH₃ yield of 149.0 μmol L^-1  h^-1 , which is improved by 83.5 % (ZIF-67) and 78.9 % (PMo₄ V₈ ). The introduction of POMs provides new insights for the design of high-performance photocatalyst nanomaterials to reduce N₂ .

Investigation of the acrolein oxidation on heteropolyacid catalysts by transient response methods

Isotopic assessment by oxygen, water and acrolein labeling delivers more detailed mechanistic insights.

Reduced State of the Graphene Oxide@Polyoxometalate Nanocatalyst Achieving High-Efficiency Nitrogen Fixation under Light Driving Conditions

The nitrogen (N₂) reduction to generate ammonia (NH₃) is a prerequisite for inputting fixed nitrogen (N) into a global biogeochemical cycle. Developing highly efficient photocatalysts for N₂ fixation under mild conditions is still a challenge. Herein, we first report three kinds of reduction states of graphene oxide (GO)@polyoxometalate (POM) composite nanomaterials, which have outstanding photocatalytic N₂ fixation activities in pure water without any other electronic sacrificial agents and cocatalysts at atmospheric pressure and room temperature. A lot of experiments show that the remarkable photocatalytic N₂ fixation performance of these three nanocatalysts is due to three factors that doping the reduced POMs (also called heteropoly blues) into the reduce GO (rGO) reduces the aggregation state of rGO (from 5 to 2 nm), resulting in rGO exposing many active sites to enhance the N₂ adsorption amount, these three nanocatalysts possess a wide absorption spectrum and strong reducibility, which facilitate absorb light energy exciting abundant photoelectrons to activate N₂, and rGO can effectively suppress the electrons recombination and rapidly transfer electrons to the absorbed N₂ to accelerate NH₃ production. Among them, r-GO@H₅[PMo₁₀V₂O₄₀] (PMo₁₀V₂) exhibits the highest NH₃ generation efficiency of 130.3 μmol L^-1 h^-1, which is improved by 65.9 and 97.3% compared to the reduced PMo₁₀V₂ (rPMo₁₀V₂) and PMo₁₀V₂. Introduction of POMs provides a new perspective in the design of high-performance photocatalytic N₂ fixation nanomaterials.

Zirconia-supported 11-molybdovanadophosphoric acid catalysts: effect of the preparation method on their catalytic activity and selectivity

The oxidation of adamantane with hydrogen peroxide catalyzed by zirconia-supported 11-molybdovanadophosphoric acid is shown to be a suitable green route for the synthesis of adamantanol and adamantanone. This work evaluates how the catalyst activity and selectivity are affected by some of its preparative parameters, such as the method for supporting the catalytically active heteropoly acid over the zirconia matrix or the pretreatments applied to the resulting materials before being used as heterogeneous catalysts. Our results indicate that the most effective catalysts able to maintain their activity after several reaction runs are those prepared by following the sol-gel route, whereas the most selective catalysts are those obtained by impregnation methods. Moreover, the calcination temperature has also been identified as a relevant parameter influencing the performance of catalysts based on supported heteropoly acids. The increasing catalytic activity observed over several consecutive reaction runs has been attributed to the formation of peroxo derivatives of polyoxometalate clusters at the surface of the catalyst and their accumulation after each reaction cycle.

Dispersed Vanadium in Three-Dimensional Dendritic Mesoporous Silica Nanospheres: Active and Stable Catalysts for the Oxidative Dehydrogenation of Propane in the Presence of CO2

The uniform monodispersed vanadium-doped three-dimensional dendritic mesoporous silica nanospheres (nV-MSNSs) were successfully synthesized in a heterogeneous oil-water biphase stratification reaction system and characterized by several state-of-the-art methods. The synthesized nV-MSNSs were applied to the oxidative dehydrogenation of the propane (ODHP) reaction with the presence of CO₂ and exhibited excellent catalytic performances. The results show that the vanadium loading (1.3-8.0 wt %) evidently influences the textural properties, oxidation state, and polymerization degree of vanadium species of nV-MSNSs. The specific surface area (S_BET), pore diameter (D_p), and pore volume (V_p) of nV-MSNSs decrease with the loading of vanadium species. The excessively high vanadium loading leads to the slight connection of nanospheres, but does not affect the assembly and growth of the three-dimensional (3D) dendritic channels. The percentage of highly dispersed vanadium V^V species gradually increases and attains the maximum value for 5.2V-MSNSs with the loading of vanadium and then decreases with further vanadium addition. The higher-polymerized VO_x species gradually generates above 5.2 wt % vanadium content for nV-MSNSs. The lower-polymerized VO_x species appear to be more active than the higher-polymerized VO_x species. Markedly, the 5.2V-MSNSs exhibit the highest catalytic activity with the initial propane conversion of 58% for the ODHP. The excellent catalytic performance can be maintained after eight reaction-regeneration cycles. The silica mesoporous frameworks can be well preserved in the reaction-regeneration cycles; meanwhile, the highly dispersed vanadium oxide also can be fully recovered after in situ regeneration. Hence, nV-MSNS catalysts exhibit outstanding activity and stability, and it would have a promising application in the DH of alkanes.

Self-assembly of [PNbxW12−xO40]n− Keggin anions – a simple way to mixed Nb–W polyoxometalates

This paper summarizes preparations of mixed phosphoniobotungstates [PNb_xW_12−xO₄₀]^(3+x)− (x = 1–3) based on two different ways: incorporation of Nb into pre-existing lacunary-type phosphotungstates and self-assembly reactions.

Determination of 5-hydroxymethylfurfural in honey, using headspace-solid-phase microextraction coupled with a polyoxometalate-coated piezoelectric quartz crystal

High concentrations of 5-hydroxymethylfurfural (HMF) in honey provide an indication of overheating under inappropriate storage conditions or aging. Conventional methods for determining HMF are cumbersome and require expensive equipment or hazardous reagents. Hence the aim of this study was to propose a new analytical tool for HMF determination in honey, using a low cost acoustic wave sensor. Volatile organic compounds (VOCs) of honey samples were extracted, using the solid phase microextraction (SPME) technique, and HMF was quantified, using a piezoelectric quartz crystal with gold electrodes coated with a layer of decamolybdodivanado phosphoric acid, sensitive to HMF. The reliability of the proposed method was confirmed after comparing the results of HMF quantification with those obtained by the conventional spectrophotometric White method, and no statistical differences were found (α=0.05).

Self-patterning porous films of giant vesicles of {Mo72Fe30}(DODMA)3 complexes as frameworks

This work describes the preparation and properties of self-patterning porous films consisting of giant vesicles formed by a 2.5-nm-diameter, polyoxometalate (POM) cluster {Mo72Fe30} macroanion, and a double-tailed cationic surfactant dimethyldistearylammonium bromide (DODMABr) in CHCl3-CH3OH mixture solvent (VCHCl3: VCH3OH=3:1). These inverse vesicles with the diameter in the range of 0.45~1.3μm in organic solution and the porous films consisting of the giant vesicles of the {Mo72Fe30}(DODMA)3 complexes were characterized by SEM, TEM, XPS and AFM observations. Self-patterning of these giant vesicles into porous films that are highly ordered honeycomb films on solid surfaces can survive drying as the frameworks are firstly studied in detail. Water contact angle measurements proved that the porous films of {Mo72Fe30}(DODMA)3 vesicles were endowed hydrophobic property from the hydrophilic surface. This porous film materials consisting of giant vesicles may be promising new options in many fields like photoelectrochemistry, sterilization, template, catalysis, in-situ synthesis.

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.

A molybdovanadophosphate-based surfactant encapsulated heteropolyanion with multi-lamellar nano-structure for catalytic wet air oxidation of organic pollutants under ambient conditions

Molybdovanadophosphate-based surfactant encapsulated heteropolyanion with multi-lamellar nano-structure for ambient catalytic wet air oxidation of BPA.

Structure and special chemical reactivity of interface-stabilized cerium oxide nanolayers on TiO2(110)

Novel interface-stabilized ceria nanophases have been grown on TiO2(110) by physical vapor deposition. At low coverage, dumbbell nanostructures constituted by reconstructed titania and ceria clusters are formed, while long range ordered nanoxides can be obtained by increasing the ceria dose. Scanning tunneling microscopy and photoemission spectroscopy were used to characterize the electronic properties of the films, showing that the TiO2 substrate can effectively stabilize ceria in reduced form over a wide range of experimental conditions. Epitaxial coupling is a very useful tool for tuning the chemical properties of mixed oxide systems. The special electronic properties of the films have a direct counterpart in the chemical activity, which has been investigated by temperature programmed desorption using methanol as a probe molecule. The experimental results indicate an exceptional activity of the ceria-titania interface in the selective dehydration of methanol to formaldehyde at an unprecedented low temperature (330 K).

Computational insight into the initial steps of the Mars-van Krevelen mechanism: electron transfer and surface defects in the reduction of polyoxometalates

Metal oxides as a rule oxidize and oxygenate substrates via the Mars-van Krevelen mechanism. A well-defined α-Keggin polyoxometalate, H(5)PV(2)Mo(10)O(40), can be viewed as an analogue of discrete structure that reacts via the Mars-van Krevelen mechanism both in solution and in the gas phase. Guided by previous experimental observations, we have studied the key intermediates on the reaction pathways of its reduction by various compounds using high-level DFT calculations. These redox reactions of polyoxometalates require protons, and thus such complexes were explicitly considered. First, the energetics of outer-sphere proton and electron transfer as well as coupled proton and electron transfer were calculated for seven substrates. This was followed by identification of possible key intermediates on the subsequent reaction pathways that feature displacement of the metal atom from the Keggin structure and coordinatively unsaturated sites on the H(5)PV(2)Mo(10)O(40) surface. Such metal defects are favored at vanadium sites. For strong reducing agents the initial outer-sphere electron transfer, alone or possibly coupled with proton transfer, facilitates formation of metal defects. Subsequent coordination allows for formation of reactive ensembles on the catalyst surface, for which the selective oxygen-transfer step becomes feasible. Weak reducing agents do not facilitate defect formation by outer-sphere electron and/or proton transfers, and thus formation of metal defect structures prior to the substrate activation is suggested as an initial step. Calculated geometries and energies of metal defect structures support experimentally observed intermediates and demonstrate the complex nature of the Mars-van Krevelen mechanism.