Thermally Induced Transformation of Sb-Containing Trigonal Mo₃VO_x to Orthorhombic Mo₃VO_x and Its Effect on the Catalytic Ammoxidation of Propane

Abstract

Crystalline orthorhombic Mo3VO x and trigonal Mo3VO x (Orth-MoVO and Tri-MoVO) are structurally analogous, both being constituted by pentagonal {Mo6O21}6– units and {MO6} (M = Mo, V) octahedra to form hexagonal and heptagonal channels in the crystal structure. In the present study, we found that Sb could be introduced into the Tri-MoVO structure by dispersing Tri-MoVO in 1.2 M HCl solution with dissolved Sb2O3. Sb was introduced into the Tri-MoVO structure through a redox process with constituent elements. Sb was located at the heptagonal channel of the Tri-MoVO structure in the form of infinite [O–Sb­(O)−]∞ chains along the channel. Interestingly, crystal structure transformation from the Tri-MoVO structure to the Orth-MoVO structure was observed by heat treatment for Tri-MoVSbO in a crystalline structure to crystalline structure manner. After the structure transformation, Sb was located at the hexagonal channel of the Orth-MoVO structure in the form of dimers of SbO3E trigonal pyramids along the channel. We concluded that the structure transformation proceeded by the movement of the pentagonal {Mo6O21}6– unit column so as to transform the heptagonal channel of the Tri-MoVO structure to the hexagonal channel of the Orth-MoVO structure. Orth-MoVSbO formed by the above process showed substantial catalytic activity for selective ammoxidation of propane.

True Catalytically Active Structure in Mo–V-Based Mixed Oxide Catalysts for Selective Oxidation of Acrolein

“…We previously reported that an oligomer unit isolated by micropore channels is responsible for redox reactions in the C-MoVO catalysts. 13,23,29,47 Therefore, we now propose that a trimer unit (light green in Figure 3f) in the crystal structure serves as an adsorption and activation site of ACR. Based on the crystal structure and the role of W, we conclude that W promotes the dissociative adsorption of water and generates B acid sites not only over the W pentagonal unit but also over the trimer unit as the adsorption and activation site of ACR (Figure 6d).…”

“…22 It is still unclear why the heptagonal channel in Tri-MoVO accommodates different types of countercations at the same time, although the locations of the different types of elements in the heptagonal channel have been reported. 23 External surface areas and micropore volumes for xW-(amm) are listed in Table S2. The external surface areas and the micropore volumes for xW were comparable when 0.7≤x and were also similar to those for Tri-MoVO, while the micropore volumes for x ≤ 0.3 were higher than those for 0.7 ≤ x. xW-amm showed considerably smaller external surface areas and micropore volumes when x ≤ 1.0, although these values were similar to those for xW (0.7 ≤ x) when x exceeded 1.4.…”

Nb−V Mixed Oxide with a Random Assembly of Pentagonal Units: A Catalyst for Oxidative Dehydrogenation of Ethane and Propane

“…Based on the above discussion, we propose that the catalytically active oxygen species in HDS-MoVO and HDS-WVO are M 6 + À OÀ V 4 + (M=Mo, W) in the oligomer units facing the heptagonal channels, because it has been reported that oligomer units are redox active during oxidation catalysis. [8,20,21,29,33] In the case of HDS-8Nb4V, we propose that trans V 5 + =O in {VO 6 } octahedra connecting the pentagonal units is the catalytically active oxygen species. This is based on the following observations: (1) B-7MR is statistically high in HDS-8Nb4V; (2) almost no UV absorption derived from IVCT was seen even after the oxidation reactions.…”

“…These textures are redox-active and thus produce active oxygen species inside the heptagonal channel. [8,20,21,29,33] Based on the similarity of the constituent structural units and elemental composition, it is reasonable to assume that the heptagonal channel texture acts as a highly catalytically active structure for ODHE and ODHP over HDS-MVO. Table 3 shows the number of heptagonal channels in the structure of HDS-MVO counted using HAADF-STEM images.…”

“…In these materials, oligomer units such as trimer or pentamer units are facing to the heptagonal channels. These textures are redox‐active and thus produce active oxygen species inside the heptagonal channel [8,20,21,29,33] . Based on the similarity of the constituent structural units and elemental composition, it is reasonable to assume that the heptagonal channel texture acts as a highly catalytically active structure for ODHE and ODHP over HDS‐MVO.…”