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.