Alkene oxidation over the (Pb,Bi)MoO4/Au | yttria-stabilized zirconia | Ag system
Oxidations of propene and but-1-ene have been carried out at 723–748 K over an electrochemical reactor composed of (Pb,Bi)MoO4 catalyst coupled with an oxygen pump, Au | YSZ | Ag (YSZ: 8 mol% yttria-stabilized zirconia). Two scheelite-type molybdate, Pb1–3xBi2xϕxMoO4(ϕ= cation vacancy, x= 0 and 0.04), catalyst films were successfully prepared on the Au anode by vacuum evaporation and used in the oxidation under oxygen pumping, which enabled us to control the mobility of the lattice oxide ions. The oxidation over the scheelite-type molybdates revealed that oxygen species transported through the bulk of the catalyst insert into the allylic intermediate to form the oxygenated products, e.g. acrylaldehyde from propene and methyl vinyl ketone from but-1-ene, while oxygen species chemisorbed on the catalyst surface accelerates dehydrogenation of the allylic intermediate of but-1-ene to buta-1,3-diene or complete oxidation of alkene to carbon oxides. It was also revealed that the bulk diffusion of lattice oxide ions in the catalyst had an important role in both activity and selectivity, and was enhanced by the addition of bismuth metal ions into PbMoO4, which was originally inactive and had no oxide-ion mobility.