Performance and Characterization of Supported Rhenium Oxide Catalysts for Selective Oxidation of Methanol to Methylal

摘要

The unique performances of supported rhenium oxide catalysts for the selective oxidation of methanol to methylal (dimethoxymethane) [3CH_3OH + 1/2O_2 → CH_2(OCH_3)_2 + 2H_2O] wee examined in a fixed-bed flow reactor and in a pulse reactor. Rhenium oxides supported on α-Fe_2O_3, γ-Fe_2O_3, and V_2O_5 among many oxide supports showed high activities (15-49% conversion) and selectivities (90-94% selectivity) for catalytic methylal synthesis at 513 K. These Re catalysts are the first to show good performances for one-stage methylal synthesis from methanol. The catalysts were characterized by XRD, XPS, TPR, TPD, and FTIR spectroscopy to provide insight into the active Re species relevant to the selective oxidation of methanol to methylal. Rhenium oxides on α-Fe_2O_3 were dispersed as tetrahedral ReO_4 species up to one monolayer at 0.7 wt % Re loading. It was suggested that active Re-Fe-O mixed-oxide clusters were formed above 1 wt % Re. ReO_2 crystallites were also observed for the Re/α-Fe_2O_3 samples with Re loadings above 6 wt %. On γ-Fe_2O_3, which has a higher surface area, no ReO_2 species were observed. It is suggested that the redox capability of rhenium oxides, Re~(6-7+) → Re~(4+), is responsible for the catalytic performance, although appropriate Lewis acidity of the rhenium oxides is also necessary for the acetalization of formaldehyde with methanol to methylal. The oxide supports prevent rhenium oxides from being sublimated and reduced to ReO_2 particles.