Cooperation effects towards partial oxidation of isobutene in multiphasic catalysts based on bismuth pyrostannate

The catalytic behaviour of alpha-Bi2Sn2O7 was investigated within the framework of the remote control theory. Mixtures of bismuth pyrostannate with typical oxygen donors (Bi2O3, Sb2O4) or accepters (MoO3, WO3) were therefore prepared and tested in the isobutene-methacrolein conversion at 648 and 673 K. The catalysts were characterized before and after their use by X-ray diffractometry, X-ray photoelectron spectroscopy; scanning electron microscopy and BET specific surface area measurements. The experiments indicate that cooperation for partial oxidation occurs when Bi2Sn2O7 is mixed with acceptor phases. Several mechanisms allowing to explain the synergetic effects observed in the Bi2Sn2O7-WO3 and the Bi2Sn2O7-MoO3 mixtures are discussed. Ln the Bi2Sn2O7-WO3 catalysts, arguments are presented showing that surface enrichment by one oxide or the in situ formation of a bismuth tungstate phase cannot be reasonably considered as responsible for the cooperation effects. Bismuth stannate is consequently concluded to behave as an oxygen donor with respect to WO3. Bi2Sn2O7 could "a priori" display the same behaviour in the Bi2Sn2O7-MoO3 catalysts and cooperate with MoO3 via a remote control mechanism. But in the latter case, as alpha-Bi2Mo3O12 is generated in situ, the intrinsic catalytic activity of this bismuth molybdate must probably be taken into account as an additional explanation for the synergy observed in these mixtures. On the other hand, the hypothesis of coverage of one oxide by the other one seems to be ruled out in these Bi2Sn2O7-MoO3 catalysts. (C) 1998 Elsevier Science B.V. All rights reserved