New insights in the understanding of the behaviour and performances of bismuth molybdate catalysts in the oxygen-assisted dehydration of 2-butanol

Pure Bi₂Mo₃O₁₂, Bi₂Mo₂O₉, Bi₂MoO₆, MoO₃ and -Sb₂O₄ and their mechanical mixtures were investigated in the oxygen-assisted dehydration of 2-butanol at atmospheric pressure and at low temperature (220 and 250°C). All catalysts were characterized before and after the catalytic reaction by BET surface area measurement, Raman spectroscopy, XRD and XPS. A strong parallelism is confirmed with the results obtained in the selective oxidation of olefins. In the frame of the remote control concept, , β and γ-bismuth molybdates are able to play a dual role: donor of spillover oxygen (Oso) with respect to MoO₃, and acceptor of Oso with respect to -Sb₂O₄. On one hand, this duality leads to mutual increase of activity when the bismuth molybdates are mixed together. In the presence of MoO₃, the phase seems to be a stronger Oso donor than β and γ, and β has a donor strength between and γ. On the other hand, when the Bi molybdates are reacted in the presence of a big quantity of spillover oxygen, like in a mixture with -Sb₂O₄, they undergo a dramatic decrease of activity. The phenomenon originates from the full oxidation of the reduced Mo species to Mo⁶⁺ induced by Oso. In parallel with other reactions involving oxygen, this confirms that the real active and selective state of molybdenum-containing oxides is that slightly reduced possessing Mo⁵⁺.