Comparative study for low-temperature catalytic oxidation of o-xylene over doped OMS-2 catalysts: Role of Ag and Cu

Abstract

Ag or Cu doped manganese oxide octahedral molecular sieve (OMS-2) was prepared through the one-step reduction of KMnO4, Tollens’ reagent, or Fehling’ reagent with propionaldehyde. The influences of the dopants on the structure, redox ability and catalytic performance of OMS-2 were investigated. The results indicate that Ag-OMS-2 and Cu-OMS-2 exhibited relatively higher catalytic activity for o-xylene oxidation at higher (>185°C) and lower temperatures (≤185°C), respectively. This can be illustrated as follows. The doped Ag increased the content of surface absorbed oxygen and inhibited the adsorption of CO2. The doped Cu increased the content of lattice oxygen and the surface area of OMS-2, but increased its adsorption capacity toward CO2. In situ DRIFT study demonstrates that the oxidation of low molecular weight carboxylates (the stable intermediates in the oxidation of o-xylene) is the rate-determining step, which is closely related with the active oxygen species. The doped Ag increased the content of electrophilic oxygen (O2−, O− or O22−) and promoted the transformation of O2 into nucleophilic oxygen (O2−), which facilitated the oxidation of benzene ring and small molecule carboxylate, respectively.