Multi-dimensional Crystal Structuring of Complex Metal Oxide Catalysts of Group V and VI Elements by Unit-Assembling

Recent Advances in Tetra- (Ti, Sn, Zr, Hf) and Pentavalent (Nb, V, Ta) Metal-Substituted Molecular Sieve Catalysis

Metal substitution of molecular sieve systems is a major driving force in developing novel catalytic processes to meet current demands of green chemistry concepts and to achieve sustainability in the chemical industry and in other aspects of our everyday life. The advantages of metal-substituted molecular sieves include high surface areas, molecular sieving effects, confinement effects, and active site and morphology variability and stability. The present review aims to comprehensively and critically assess recent advances in the area of tetra- (Ti, Sn, Zr, Hf) and pentavalent (V, Nb, Ta) metal-substituted molecular sieves, which are mainly characterized for their Lewis acidic active sites. Metal oxide molecular sieve materials with properties similar to those of zeolites and siliceous molecular sieve systems are also discussed, in addition to relevant studies on metal-organic frameworks (MOFs) and some composite MOF systems. In particular, this review focuses on (i) synthesis aspects determining active site accessibility and local environment; (ii) advances in active site characterization and, importantly, quantification; (iii) selective redox and isomerization reaction applications; and (iv) photoelectrocatalytic applications.

Design and synthesis of highly active MoVTeNb-oxides for ethane oxidative dehydrogenation

Ethane oxidative dehydrogenation (ODH) is an alternative route for ethene production. Crystalline M1 phase of Mo-V mixed metal oxide is an excellent catalyst for this reaction. Here we show a hydrothermal synthesis method that generates M1 phases with high surface areas starting from poorly soluble metal oxides. Use of organic additives allows control of the concentration of metals in aqueous suspension. Reactions leading to crystalline M1 take place at 190 °C, i.e., approximately 400 °C lower than under current synthesis conditions. The evolution of solvated polyoxometalate ions and crystalline phases in the solid is monitored by spectroscopies. Catalysts prepared by this route show higher ODH activity compared to conventionally prepared catalysts. The higher activity is due not only to the high specific surface area but also to the corrugated lateral termination of the M1 crystals, as seen by atomic resolution electron microscopy, exposing a high concentration of catalytically active sites.

Crystalline M1 phase of Mo-V-Te-Nb mixed oxide is an excellent catalyst for ethane oxidative dehydrogenation to ethene. Here, the authors show a method that synthesizes highly active materials by generating M1 crystals with corrugated terminations, thus exposing a large concentration of active sites.

Thermal Behavior, Crystal Structure, and Solid-State Transformation of Orthorhombic Mo-V Oxide under Nitrogen Flow or in Air

Orthorhombic Mo-V oxide is one of the most active solid-state catalysts for selective oxidation of alkane, and revealing its detailed structure is important for understanding reaction mechanisms and for the design of better catalysts. We report the single-crystal X-ray structure analysis of orthorhombic Mo-V oxide heated under a N₂ flow; V is present in 6-membered rings with partial occupancy, similar to the structure reported by Trunschke's group for orthorhombic Mo-V oxide heated under an Ar flow (Trunschke, ACS Catal.2017, 7, 3061). Our previous paper (Ishikawa, J. Phys. Chem. C, 2015, 119, 7195) reported that V is not present in the 6-membered rings when orthorhombic Mo-V oxide is calcined in the presence of oxygen. Furthermore, Trunschke's paper reported that V in the 6-membered rings moves to the surface of the crystals under oxidation reaction conditions in the presence of H₂O. Our present results provide additional evidence for V migration in the 6-membered rings during heat treatment. We also report the differences in the thermal behaviors, ultraviolet-visible absorptions, N₂ isotherms, and elemental analysis results of Mo-V oxide heated in air and under a N₂ flow. Furthermore, we report the solid-state transformation of orthorhombic Mo-V oxide to tetragonal Mo-V oxide by controlled heat treatment.