Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects

Abstract

Keggin-type phosphomolybdic acid (PMo12O40), treated with pyridine (Py), forms a crystalline material (PyPMo-HT) following heat treatment under an inert gas flow at ∼420 °C. Although this material is known to have attractive catalytic properties for gas-phase oxidation, the origin of this catalytic activity requires clarification. In this study, we investigated the crystal structure of PyPMo-HT. PyPMo-HT comprises a one-dimensional array of Keggin units and pyridinium cations (HPy), with an HPy/Keggin unit ratio of ∼1.0. Two oxygen atoms were removed from the Keggin unit during crystal structure transformation, which resulted in an electron being localized on the Mo atom in close contact with the adjacent Keggin unit. Upon the introduction of molecular oxygen, electron transfer from this Mo atom resulted in the formation of an electrophilic oxygen species that bridged two Keggin units. The electrophilic oxygen species acted as a catalytically active oxygen species, as confirmed by the selective oxidation of propylene. PyPMo-HT showed excellent catalytic activity for the selective oxidation of methacrolein, with the methacrylic acid yield being superior to that obtained with PMo12O40 and comparable to that obtained with an industrial Keggin-type polyoxometalate (POM) catalyst. The oxidation catalysis observed over PyPMo-HT provides a deeper understanding of POM-based industrial catalytic processes.

Acid Catalysis over Crystalline Zr3SO9: Role of the Local Structure in Generating Acidity

“…The amount of H 2 O was estimated from the peak area at m/z = 18 after calibration with CaC 2 O 4 •H 2 O (Rigaku). As previously reported, CaC 2 O 4 •H 2 O decomposes as follows 29,30 (2)…”

“…The amount of H 2 O was estimated from the peak area at m / z = 18 after calibration with CaC 2 O 4 ·H 2 O (Rigaku). As previously reported, CaC 2 O 4 ·H 2 O decomposes as follows , CaC 2 normalO 4 · normalH 2 normalO → CaC 2 normalO 4 + normalH 2 normalO 0.25em ( ∼ 190 ° C ) CaC 2 normalO 4 → CaCO 3 + CO 0.25em ( ∼ 500 ° C ) CaCO 3 → CaO + CO 2 0.25em ( ∼ 780 ° C ) Thus, eq was used to calibrate the amount of H 2 O. Thermogravimetry-differential thermal analysis (TG-DTA) was performed on a Thermo Plus EVO2 instrument (Rigaku). The sample (10 mg) was placed in the instrument, and the measurements were performed by increasing the temperature from 25 to 700 °C at a rate of 10 °C min –1 under flowing N 2 (100 mL min –1 ).…”

Constructing Defective Co3V2O8 Hexagonal Prism for Solvent-free Selective Oxidation of Cyclohexane: Strategy of Ionic Liquid Mediation

“…As a key medium for electron transport between different metals, OVs promote the corresponding redox cycle, accelerate the reaction rate, and improve the catalytic performance. 15 The OVs in macroporous La 0.8 Sr 0.2 CoO 3 monolithic catalysts was constructed through a facile in situ solution assembly, which was considered as a powerful site for oxygen adsorption and dissociation based on the formation of monatomic oxygen ions. 16 Tang et al designed a porous CVO with OVs using hydrothermal and doping reconfiguration methods, in which the improvement of electron transfer capability and the reduction of the sodium energy barrier were achieved via OVs for high-energy aqueous sodium-ion capacitors.…”

“…Therefore, the construction of OVs coupled with the synergistic effect of MMOs provides a promising means to efficiently utilize the advantages of MMOs, which can generate more surface reactive oxygen species to facilitate catalytic performance. As a key medium for electron transport between different metals, OVs promote the corresponding redox cycle, accelerate the reaction rate, and improve the catalytic performance . The OVs in macroporous La 0.8 Sr 0.2 CoO 3 monolithic catalysts was constructed through a facile in situ solution assembly, which was considered as a powerful site for oxygen adsorption and dissociation based on the formation of monatomic oxygen ions .…”

Theoretical insight into the influence of different molecular design strategies on photovoltaic properties for a series of POM-based dyes applied in dye-sensitized solar cells

“…Polyoxometalates (POMs), discrete metal-oxo clusters, have been widely applied in catalysis, medicine science and analytical chemistry due to their fascinating structural and photophysical properties. [14][15][16][17][18][19][20] Recently, POMs have been considered to be important inorganic ligands that are able to attach organic molecules, constructing inorganic-organic hybrids. The maximum absorption bands of hybrids significantly redshift compared with individual POMs and organic groups, resulting from the synergistic effect, 21 which meets the requirement of long wavelength absorbed dyes.…”