Catalytic performances and stability of three Sb-Mo-O phases in the selective oxidation of isobutene to methacrolein

We investigated the possible role of binary oxides associating Sb and Mo in the synergetic effects between physically mixed MoO and alpha-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 alpha-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 alpha-Sb2O4. These effects are actually related to the decomposition of the Sb-Mo-O phases and are due to the cooperation between MoO3 and alpha-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 alpha-Sb2O4 and hence that they cannot be at the origin of the synergism between the simple oxides.