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.

Molybdenum Oxide Constructed by {Mo6O21}6- Pentagonal Unit Assembly and Its Redox Properties.

Molybdenum oxides are widely used in various fields due to their electronic and structural characteristics. These materials can generate lattice oxygen defects by reduction treatments, which sometimes play central roles in various applications. However, little has been understood about their properties since it is difficult to increase the amount of lattice oxygen defects due to the crystal structure changes in most cases. Here, we report a new class of high-dimensionally structured Mo oxide (HDS-MoOx) constructed by the random assembly of {Mo6O21}6- pentagonal units (PUs). Since the PU is a stable structural unit, the structural network based on the PU hardly caused structural changes to make the lattice oxygen defects vanish. Consequently, HDS-MoOx could generate a substantial amount of lattice oxygen defects, and their amount was controllable, at least in the range of MoO2.64-MoO3.00. HDS-MoOx was more redox active than typical Mo oxide (α-MoO3) and demonstrated an oxidation ability for gas-phase isopropanol oxidation under the reaction conditions, whereas α-MoO3 affords no oxidation products.

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

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.

Synthesis of High Dimensionally Structured Mo-Fe Mixed Metal Oxide and Its Catalytic Activity for Selective Oxidation of Methanol.

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.