Role of the heptagonal channel of crystalline Mo3VOx catalyst for the selective oxidation of acrolein and methacrolein

Abstract

Crystalline orthorhombic Mo₃VO_x (MoVO) is a promising catalyst for the selective oxidation of acrolein (ACR) and methacrolein (MCR), and the heptagonal channel texture acts as a catalysis field for these reactions. However, the catalytic performance for the former reaction is far superior to that for the latter, even though their molecular structures are similar except for the presence of a methyl group. This activity difference has long been a controversial issue in the oxidation chemistry of Mo–V based mixed metal oxides, although no clear conclusion has yet been provided. Here, the catalytic properties of MoVO for these reactions were investigated in detail. Based on the structure–activity relationship, it was found that the ACR oxidation takes place over the cross-section of the rod-shaped crystal of MoVO ((001) plane), while the MCR oxidation occurs over the lateral-section. This difference was derived from the interaction between the substrates and the crystal structure of MoVO; the heptagonal channel texture in the (001) plane captured ACR inside the channel, whereas MCR was hardly accessible to the same site due to its large molecular size. Based on the fact that MoVO generates electrophilic oxygen species effective for aldehyde oxidation at the heptagonal channel, the exceptionally high catalytic performance of MoVO for the selective oxidation of ACR could be associated with the two striking properties of the heptagonal channel; capturing of ACR and generation of the electrophilic oxygen species. The former was lacking in the selective oxidation of MCR, which accounted for the significantly lower catalytic activity than that for the ACR oxidation. However, its catalytic performance was even superior to that of other reported MCR oxidation catalysts because MoVO could form the electrophilic oxygen species at the heptagonal channel exposed on the lateral-section of the rod.