Four distinct structural types (orthorhombic, trigonal, tetragonal and amorphous) of Mo 3 VO x catalyst were each synthesized by a hydrothermal method as a single phase, characterized structurally and tested for oxidative dehydrogenation of ethane. A common structural feature of the catalysts is that the materials are a layer-type structure and constructed with pentagonal {Mo 6 O 21 } units. The arrangement of the pentagonal units can form heptagonal channels to create different structural features. The orthorhombic Mo 3 VO x catalyst has microporosity due to the open heptagonal channels adsorbing nitrogen molecules and showed the highest activity for the reaction among four distinct catalysts. Furthermore, this phase appeared to be most active, currently, compared to other complex metal oxide catalysts reported. An observed positive relation between the microporosity and the oxidation activity suggests that the catalytic oxidation takes place at the heptagonal channels.

Redox treatments of an orthorhombic Mo 29 V 11 O 112 catalyst (MoVO) were conducted and its crystal structure, microporosity, and catalytic activity were investigated. TPR and TG revealed that MoVO evolved two kinds of lattice oxygen (α-oxygen and β-oxygen) from the structure by reduction treatment. In the early stage of reduction, αoxygen was evolved from the structure, causing expansion of the micropore channel. With further reduction, the atoms in the pentagonal [Mo 6 O 21 ] 6− unit moved toward the micropore channel, resulting in a decrease in micropore size. Expansion of the micropore drastically increased catalytic activity for selective oxidation of ethane, but the activity was decreased by a reduction in the micropore channel size. Strong relationships were found between crystal structure, microporosity, and catalytic activity for selective oxidation of ethane.