Role of the mutual contamination in the synergetic effects between MoO3 and SnO2

This contribution is an answer to the criticism that the synergetic effects observed between SnO2 and MoO3 in the catalytic dehydration-dehydrogenation of 2-butanol to butene (BUT) and methyl-ethyl-ketone (MEK), in the presence of oxygen and at low temperature (463 K), are not due to a remote control mechanism via the migration of spillover oxygen (Oso) but to the formation of a more active and selective mutual contamination of the oxides during catalysis. Experiments especially designed to artificially induce different types of mutual contamination between SnO2 and MoO3 and then to evaluate their catalytic performances have been conducted. Three types of contaminated samples were synthesized: Mo ions deposited at the surface of SnO2, a solid solution of Mo in SnO2 lattice and a Sn-Mo-O mixed oxide precursor. The contaminated samples were tested alone and in the presence of SnO2 and MoO3. The conclusion is that, for reasons of unstability and/or of low performance, none of the mutual contamination compounds investigated can account for the synergetic effects between SnO2 and MoO3. Other experiments conducted with SnO2 and MoO3 in the selective oxidation of isobutene at high temperature (693 K) confirm the existence of a synergism between the oxides. In this case, SnO2 is very active, which induces its continuous reduction during the reaction. To maintain its initial high oxidation level, SnO2 pumps lattice oxygen from MoO3. The consequence is that MoO3 turns progressively reduced. It is shown that an additional supply of Oso succeeds to prevent this phenomenon. Finally, more specific experiments lead to a better understanding of the mechanism by which active sites are formed under the action of Oso. Oso is formed at the surface SnO2 (which is an Oso donor). It then migrates onto the surface Of MoO3 (which is an Oso acceptor) where it increases the acidity, and thus the dehydration activity, Of MoO3. This accounts for the synergetic increase of the production of BUT observed for mechanical mixtures of SnO2 and MoO3. (C) 2002 Elsevier Science B.V. All rights reserved