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Abstract

We investigated the possible role of binary oxides associating Sb and Mo in the synergetic effects between physically mixed MoO₃ and α-Sb₂O₄ in the selective oxidation of isobutene to methacrolein. The catalytic behavior of three Sb−Mo−O phases - Sb₂MoO₆ on one hand, a mixture of Sb₂Mo₁₀O₃₁ and Sb₄Mo₁₀O₃₁ on the other hand - was investigated. The samples were characterized using X-ray diffraction (XRD), confocal laser Raman microscopy, specific area measurements, and scanning electron microscopy. Additional in situ high-temperature XRD was also performed. At similar conversions of isobutene, the selectivities to methacrolein for the Sb−Mo−O phases were always lower than those of mixtures of MoO₃ with α-Sb₂O₄ with the same atomic compositions. A second result was that the three Sb−Mo−O phases were unstable in the conditions of reaction and tended to decompose to simple molybdenum and antimony oxides. An improvement of the catalytic performances of the Sb−Mo−O catalysts occurred in parallel to their decomposition. Some apparent cooperation effects were observed for mechanical mixtures of the Sb−Mo−O phases with MoO₃ or α-Sb₂O₄. These effects are actually related to the decomposition of the Sb−Mo−O phases and are due to the cooperation between MoO₃ and α-Sb₂O₄ formed at their surface. The spontaneous decomposition of the Sb−Mo−O phases shows that these phases cannot form in operandi (during the catalytic reaction) starting from the simple oxides. Consequently, this shows that they cannot be the real most performant phases in the mixtures of MoO₃ with α-Sb₂O₄ and hence that they cannot be at the origin of the synergism between the simple oxides.