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

Pure Bi(2)Mo(3)O(12), Bi(2)Mo(2)O(9). Bi(2)MoO(6). MoO(3) and alpha-Sb(2)O(4) and their mechanical mixtures were investigated in the oxygen-assisted dehydration of 2-butanol at atmospheric pressure and at low temperature (220 and 250 degrees 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, alpha, beta and gamma-bismuth molybdates are able to play a dual role: donor of spillover oxygen (Oso) with respect to MoO(3), and acceptor of Oso with respect to alpha-Sb(2)O(4). On one hand, this duality leads to mutual increase of activity when the bismuth molybdates are mixed together. In the presence of MoO(3), the alpha phase seems to be a stronger Oso donor than beta and gamma, and beta has a donor strength between alpha and gamma. On the other hand, when the Si molybdates are reacted in the presence of a big quantity of spillover oxygen, like in a mixture with alpha-Sb(2)O(4), they undergo a dramatic decrease of activity. The phenomenon originates from the full oxidation of the reduced Mo species to Mo(6+) 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 Mo(5+). (C) 2000 Elsevier Science B.V. All rights reserved.