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 alpha-bismuth pyrostannate, Bi(2)Sn(2)O(7), was investigated in the selective oxidation of isobutene into methacrolein. When alpha-Bi(2)Sn(2)O(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 a-bismuth molybdate, alpha-Bi(2)Mo(3)O(12), when the reaction temperature exceeded 673 K. Additional experiments were therefore carried out on the "Bi-Sn-M-O" catalysts in order to shed light on the role of Bi(2)Mo(3)O(12) in the synergetic effects observed in the Bi(2)Sn(2)O(7)-MoO(3) system. The experimental results are discussed in terms of several hypotheses. First, the intrinsic activity of Bi(2)Mo(3)O(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(2)Sn(2)O(7)-Bi(2)Mo(3)O(12) mechanical mixtures have evidenced a cooperation between these two ternary oxides, particularly when Bi(2)Sn(2)O(7) was the major component of the mixture. Consequently, it is likely that a synergy between Bi(2)Sn(2)O(7) and the in situ generated Bi(2)Mo(3)O(12) might play a role in the synergy observed in the Bi(2)Sn(2)O(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(2)Sn(2)O(7) and MoO(3), independently from the formation of alpha-Bi(2)Mo(3)O(12) (C) 1999 Elsevier Science B.V. All rights reserved.