Evaluation of the role played by bismuth molybdates in Bi_2Sn_2O_7-MoO_3 catalysts used for partial oxidation of isobutene to methacrolein

摘要

The catalytic behaviour of multiphasic catalysts based on #alpha#-bisnuth pyrostannate, Bi_2Sn_2O_7, was investigated in the selective oxidation of isobutene into methacrolein. When #alpha#-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 #gamma#-bismuth molybdate, #gamma#-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_2Mo_3O_(12) is probably is only poorly active. Second, catalytic tests made on Bi_2Sn_2O_7-Bi_2Mo_O_(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 pyrostammate 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 #alpha#-Bi_2Mo_3O_(12)