Alkene oxidation over the Mo–Bi mixed oxides/Au | yttria-stabilized zirconia | Ag system

Abstract

Oxidation of propene and but-1-ene has been carried out in an MoO₃–Bi₂O₃/Au | yttria-stabilized zirconia (YSZ)| Ag electrochemical reactor at 723 K and compared with the result obtained by means of a mixed-gas flow reaction over the MoO₃–Bi₂O₃ catalyst. The MoO₃–Bi₂O₃ catalyst was prepared from MoO₃, three phases of bismuth molybdate [Bi₂Mo₃O₁₂(α), Bi₂Mo₂O₉(β), Bi₂MoO₆(γ)] and Bi₂O₃ and each catalyst was successfully deposited by vacuum evaporation as a compact thin film on the Au anode. For propene oxidation, MoO₃ showed the highest activity among the catalysts used on the Au anode and its activity was up to 600 times higher than that obtained with the same catalyst using a mixed-gas flow reaction. These results suggest that oxygen species bound to molybdenum metal possess a definite role for both α-H abstraction from propene and oxygen insertion into the allylic intermediate under a sufficient oxygen supply. On the other hand, oxygen can permeate through a Bi₂O₃ film, suggesting that the bismuth species is effective for O₂ incorporation into the catalyst and also for transportation of oxide ions through the catalyst bulk. Each phenomenon, i.e. the insertion of lattice oxide ions into the allylic intermediate and O₂ incorporation into the lattice was separately observed. The electrochemical-reactor system showed some advantages for propene oxidation, e.g. high selectivity to arylaldehyde under high reaction temperatures and higher activity, compared with the mixed-gas flow reaction. In but-1-ene oxidation the main product was buta-1,3-diene by oxidative dehydrogenation and Bi₂Mo₂O₉(β) showed the highest activity. The critical effect of molybdenum species was not observed in this reaction but high activity was still obtained over the molybdenum-rich catalysts.