Evaluation of the role played by bismuth molybdates in Bi2Sn2O7–MoO3 catalysts used for partial oxidation of isobutene to methacrolein

The catalytic behaviour of multiphasic catalysts based on -bismuth pyrostannate, Bi₂Sn₂O₇, was investigated in the selective oxidation of isobutene into methacrolein. When -Bi₂Sn₂O₇ is mixed with MoO₃, strong cooperation effects on the yield and selectivity in methacrolein occur. However, XRD analyses performed on samples after test revealed the formation of a low quantity of -bismuth molybdate, -Bi₂Mo₃O₁₂, when the reaction temperature exceeded 673 K. Additional experiments were therefore carried out on the “Bi–Sn–Mo–O” catalysts in order to shed light on the role of Bi₂Mo₃O₁₂ in the synergetic effects observed in the Bi₂Sn₂O₇–MoO₃ system. The experimental results are discussed in terms of several hypotheses. First, the intrinsic activity of Bi₂Mo₃ O₁₂ is probably the simplest explanation for the synergetic effects, although experiments have shown that this phase present in a low quantity is only poorly active. Second, catalytic tests made on Bi₂Sn₂O₇–Bi₂Mo₃O₁₂ mechanical mixtures have evidenced a cooperation between these two ternary oxides, particularly when Bi₂Sn₂O₇ was the major component of the mixture. Consequently, it is likely that a synergy between Bi₂Sn₂O₇ and the in situ generated Bi₂Mo₃O₁₂ might play a role in the synergy observed in the Bi₂Sn₂O₇–MoO₃ association. Third, as bismuth pyrostannate was previously shown to behave as an oxygen donor phase with respect to WO₃, a remote control mechanism could therefore occur between Bi₂Sn₂O₇ and MoO₃, independently from the formation of -Bi₂Mo₃O₁₂.