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.