Effects of MoO3 crystalline structure of MoO3–SnO2 catalysts on selective oxidation of glycol dimethyl ether to 1,2-propandiol

Abstract

To improve the selectivity of 1,2-propandiol (PDO) by modifying the structure and morphology of the MoO₃/SnO₂ catalyst, orthorhombic (α), monoclinic (β) and hexagonal (h) MoO₃ crystalline phases were prepared to investigate the rational design requirements of the MoO₃–SnO₂ structure that are beneficial for the reaction of glycol dimethyl ether (DMET) to PDO. With an increase in the reaction temperature, the highest PDO selectivity of the oxidation reaction of glycol dimethyl ether to 1,2-propandiol was always obtained over the h-MoO₃–SnO₂ catalyst and the lowest PDO selectivity was always obtained over the β-MoO₃–SnO₂ catalyst. The MoO₃ bulk structure, the interaction between SnO₂ and MoO₃ and the surface properties of these three catalysts could account for this distinctive difference. Hexagonal MoO₃ is dispersed more homogeneously over the h-MoO₃–SnO₂ catalyst due to the hexagonal crystalline tunnel structure existing in the h-MoO₃–SnO₂ catalyst, and the weak interaction between MoO₃ and SnO₂; besides, the more hydrated surface of the h-MoO₃–SnO₂ catalyst can lead to more Brønsted acid sites being present on the catalyst surface and favor the dissociation of the C–O bond in DMET and association of the C–C bond to form PDO with the assistance of the redox and basic sites, which can explain why the highest PDO was obtained over the h-MoO₃–SnO₂ catalyst. The lattice strain and oxygen vacancies in the β-MoO₃–SnO₂ catalyst, induced by the substitution of Sn⁴⁺ ions with the smaller sized Mo⁶⁺ ions, enhance the oxidation ability of the β-MoO₃–SnO₂ catalyst, and consequently more CH₃O· can be formed and transformed to formaldehyde (FA) and methyl formate (MF), which can explain why the total selectivity of FA and MF was highest while the selectivity of PDO was lowest over the β-MoO₃–SnO₂ catalyst at the same time. These findings are pretty significant for further investigation of the rational design of the MoO₃–SnO₂ catalyst structure, applied to the conversion of DMET to PDO.