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 pyr ostannate, Bi2Sn2O7, was investigated in the selective oxidation of isobute ne into methacrolein. When alpha-Bi2Sn2O7 is mixed with MoO3, strong cooper ation effects on the yield and selectivity in methacrolein occur. However, XRD analyses performed on samples after test revealed the formation of a lo w quantity of a-bismuth molybdate, alpha-Bi2Mo3O12, when the reaction tempe rature 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 Bi2Mo3O12 in the synergetic effects observed in the Bi2Sn2O7-MoO3 system. The experim ental results are discussed in terms of several hypotheses. First, the intr insic activity of Bi2Mo3O12 is probably the simplest explanation for the sy nergetic effects although experiments have shown that this phase present in a low quantity is only poorly active. Second, catalytic tests made on Bi2S n2O7-Bi2Mo3O12 mechanical mixtures have evidenced a cooperation between the se two ternary oxides, particularly when Bi2Sn2O7 was the major component o f the mixture. Consequently, it is likely that a synergy between Bi2Sn2O7 a nd the in situ generated Bi2Mo3O12 might play a role in the synergy observe d in the Bi2Sn2O7-MoO3 association. Third, as bismuth pyrostannate was prev iously shown to behave as an oxygen donor phase with respect to WO3, a remo te control mechanism could therefore occur between Bi2Sn2O7 and MoO3, indep endently from the formation of alpha-Bi2Mo3O12 (C) 1999 Elsevier Science B. V. All rights reserved.