Fluidized bed processing of sodium tungsten manganese catalysts for the oxidative coupling of methane

Na–W–Mn/SiO2 catalysts have been prepared via a new processing route using optimized fluidized bed processing. The coating process enables the production of large batches of catalysts closely related to industrial application. Thus, amorphous silica granules as core structures have been homogenously coated with finely dispersed Mn2O3 and Na2WO4 crystallite layers in a fluidized bed. Due to a calcination procedure at 800 °C for 8 h under air, a transformation of the amorphous silica support into macroporous crystalline α-cristobalite was observed. The catalytic performance in OCM was evaluated in a packed-bed tubular reactor. 76% of C2-selectivity and 7% CH4-conversion have been obtained in a co-feed of CH4, O2 and N2 in a ratio of 4:1:4 at 750 °C. During the stability test, the catalytic performance remained constant over 24 h on stream.Characterization of the post reaction catalyst indicates a beginning phase transformation from Mn2O3 to MnWO4 which implies the reduction from Mn(III) to the more stable Mn(II) oxidation state. Thus, the number of WO42− tetrahedrons which are thought to be essential for catalytic performance maintained constant over the whole testing period. However, the local coordination of the WO42− tetrahedron changed partially from a symmetrical tetrahedron with equal W–O distances for the Na2WO4 phase to distorted tetrahedron for the MnWO4 phase. The catalytically active coating was stable during OCM and did not flake off. However, it appeared to have melted partly. EDX mapping indicates the diffusion of sodium ions into the SiO2 substrate.