Performance and Characterization of Novel Re−Sb−O Catalysts Active for the Selective Oxidation of Isobutylene to Methacrolein

Abstract

The catalytic behavior of three well-defined Re−Sb−O compounds, SbOReO4·2H2O, Sb4Re2O13, and SbRe2O6, was investigated in the selective oxidation of isobutylene to methacrolein at 673 K. We have directed our attention to the correlation between the catalytic performance of the compounds and their structural and superficial changes undergone under the reaction conditions. For the purpose, the effects of the pretreatment conditions for the catalysts and of the isobutylene/oxygen ratio in the reaction atmosphere on the performance of the catalysts were explored. Each Re−Sb−O compound was characterized before and after the pretreatments and the catalytic reactions by X-ray diffraction, X-ray photoelectron spectrocopy, scanning electron microscopy, and in situ laser Raman microscopy. Among the three Re−Sb−O compounds, a novel SbRe2O6 compound exhibited the highest catalytic performance for selective oxidation. The compound underwent a progressive decomposition to Sb4Re2O13 and Re2O7 when pretreated or reacted at 673 K in the presence of oxygen. The decomposition was favored under more oxidative conditions, and a close parallelism between the catalytic performance and the quantity of Re2O7 stabilized at the surface of the catalyst was observed. On the other hand, the activities of pure Re2O7 and a new Sb4Re2O13 compound were very low when each of them was used as a catalyst independently. The results indicate that the reactivity of SbRe2O6 is likely due to the presence of Re2O7 at the surface, benefitting from a cooperation with Sb4Re2O13. The model also accounts for the performance of SbOReO4·2H2O. SbOReO4·2H2O decomposed to Sb4Re2O13 and Re2O7 regardless of the reaction atmosphere, which explains a higher activity of SbOReO4·2H2O than Sb4Re2O13. However, the amount of Re2O7 stabilized at the surface was smaller than that for the case of SbRe2O6, which leads to lower performances of SbOReO4·2H2O than those of SbRe2O6.