Evaluation of the role played by bismuth molybdates in Bi 2Sn 2O 7–MoO 3 catalysts used for partial oxidation of isobutene to methacrolein

Abstract

The catalytic behaviour of multiphasic catalysts based on α-bismuth pyrostannate, Bi 2Sn 2O 7, was investigated in the selective oxidation of isobutene into methacrolein. When α-Bi 2Sn 2O 7 is mixed with MoO 3, 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 α-bismuth molybdate, α-Bi 2Mo 3O 12, 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 Bi 2Mo 3O 12 in the synergetic effects observed in the Bi 2Sn 2O 7–MoO 3 system. The experimental results are discussed in terms of several hypotheses. First, the intrinsic activity of Bi 2Mo 3O 12 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 Bi 2Sn 2O 7–Bi 2Mo 3O 12 mechanical mixtures have evidenced a cooperation between these two ternary oxides, particularly when Bi 2Sn 2O 7 was the major component of the mixture. Consequently, it is likely that a synergy between Bi 2Sn 2O 7 and the in situ generated Bi 2Mo 3O 12 might play a role in the synergy observed in the Bi 2Sn 2O 7–MoO 3 association. Third, as bismuth pyrostannate was previously shown to behave as an oxygen donor phase with respect to WO 3, a remote control mechanism could therefore occur between Bi 2Sn 2O 7 and MoO 3, independently from the formation of α-Bi 2Mo 3O 12.