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

Crystalline trigonal Mo3VOx (Tri-MoVO) catalyst is constructed of {Mo6O21}6– pentagonal units and octahedral {MO6} units that form hexagonal and heptagonal channels in the crystal structure. Tri-MoVO is an extremely active catalyst for selective oxidation of acrolein (ACR) to form acrylic acid (AA), with the local structure around the heptagonal channel responsible for the catalysis. Here, promoter elements that are widely used in industrial Mo–V-based mixed oxide catalysts (W and Cu) were introduced without altering the crystal structure of Tri-MoVO. W was located at the pentagonal unit sites by replacing Mo, and Cu was located in the interstitial spaces of lattice oxygens in the corner of the heptagonal channel micropore, forming a sophisticated crystal structure involving the four constituent elements. Introduction of W improved the ACR conversion without altering AA selectivity even when the water pressure in the reaction gas was decreased, owing to the promotion of the dissociative adsorption of water. The introduction of Cu resulted in improvement of the AA selectivity from ca. 93–94% to ca. 97%. Based on the structural and elemental similarities between Tri-MoVWCuO and industrially utilized Mo–V-based mixed oxide catalysts synthesized according to the patented procedure, we concluded that Tri-MoVWCuO is the true catalytically active structure for selective oxidation of ACR.