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.

Crystalline trigonal Mo3VO x (Tri-MoVO) catalyst is constructed of {Mo6O21}6– pentagonal units and octahedral {MO6} units that form hexagonal and heptagonal channels in the crystal structure. Tri-MoVO is an extremely active catalyst for selective oxidation of acrolein (ACR) to form acrylic acid (AA), with the local structure around the heptagonal channel responsible for the catalysis. Here, promoter elements that are widely used in industrial Mo–V-based mixed oxide catalysts (W and Cu) were introduced without altering the crystal structure of Tri-MoVO. W was located at the pentagonal unit sites by replacing Mo, and Cu was located in the interstitial spaces of lattice oxygens in the corner of the heptagonal channel micropore, forming a sophisticated crystal structure involving the four constituent elements. Introduction of W improved the ACR conversion without altering AA selectivity even when the water pressure in the reaction gas was decreased, owing to the promotion of the dissociative adsorption of water. The introduction of Cu resulted in improvement of the AA selectivity from ca. 93–94% to ca. 97%. Based on the structural and elemental similarities between Tri-MoVWCuO and industrially utilized Mo–V-based mixed oxide catalysts synthesized according to the patented procedure, we concluded that Tri-MoVWCuO is the true catalytically active structure for selective oxidation of ACR.

High-dimensionally structured Mo–Fe oxide (HDS-MoFeO) was synthesized through an assembly of structural units supplied from Keplerate-type polyoxometalate, {Mo72Fe30}, under an appropriate hydrothermal condition. HDS-MoFeO showed excellent catalytic activity for the selective oxidation of methanol with slightly lower selectivity for formaldehyde than that of a conventional Mo–Fe oxide catalyst.

High-dimensionally structured (HDS) mixed oxides of vanadium with metals (M) (e. g., Nb, Mo, and W; denoted as HDS-MVO) were constructed by {M 6 O 21 } 12À pentagonal units and {MO 6 } (M=Nb, Mo, W, or V) octahedra as linkers. The materials were synthesized using a hydrothermal method and rod-shaped solids. The random assembly of the pentagonal units and octahedra in the cross-sectional plane of the rods facilitated the formation of micropore channels along the long axis of the rods. Micropore formation was directly observed in the crosssection by HAADF-STEM. These structural features are common to HDS-NbVO, HDS-MoVO, and HDS-WVO. The catalytic activity of these three HDS-MVOs with V/Mo ratios in the range 0.35-0.39 was tested for the oxidative dehydrogenation of ethane and propane. The reaction rates per surface area for ethane oxidation and propane oxidation over the HDS-MoVO and HDS-WVO catalysts were comparable, whereas the HDS-NbVO catalyst showed an appreciable difference between the two reaction rates. Both HDS-MoVO and HDS-WVO exhibited higher selectivity for olefin formation during ethane oxidation than propane oxidation. Interestingly, the olefin selectivity over the HDS-NbVO catalyst was found to be almost independent of the alkane substrate. These catalytic features were discussed on the basis of VÀ OÀ V or VÀ OÀ Mo redox coupling and pore structure effects in HDS-MoVO and HDS-WVO and also of isolated and valence stable surface V in HDS-NbVO.

Microporous vanadotungstate (W 4 V 3 O 19 , VT-1) was constructed via the connection of [W 4 O 16 ] 8− cubane units with VO 2+ linkers to form an LTA-zeolite-type crystal structure. We found that the VO 2+ sites connecting the cubane units of VT-1 could be partially removed by an appropriate acid treatment without destroying its crystal structure. This process involved a redox reaction between the [W 4 O 16 ] 8− cubane units and VO 2+ linkers, and V was removed as V 5+ during the acid treatment. TiO 2+ or ZrO 2+ could be introduced into the VO 2+ vacancies. The introduction of Ti or Zr substantially improved the thermal stability of VT-1 under oxidizing conditions. The catalytic activity toward the selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) was improved by the introduction of Ti or Zr.