La_1–xSr_xFeO_3−δ Perovskite Oxide Nanoparticles for Low-Temperature Aerobic Oxidation of Isobutane to tert-Butyl Alcohol

Abstract

The development of reusable solid catalysts based on naturally abundant metal elements for the liquid-phase selective oxidation of light alkanes under mild conditions to obtain desired oxygenated products, such as alcohols and carbonyl compounds, remains a challenge. In this study, various perovskite oxide nanoparticles were synthesized by a sol−gel method using aspartic acid, and the effects of A-and B-site metal cations on the liquid-phase oxidation of isobutane to tert-butyl alcohol with molecular oxygen as the sole oxidant were investigated. Iron-based perovskite oxides containing Fe 4+ such as BaFeO 3−δ , SrFeO 3−δ , and La 1−x Sr x FeO 3−δ exhibited catalytic performance superior to those of other Fe 3+ -and Fe 2+based iron oxides and Mn-, Ni-, and Co-based perovskite oxides. The partial substitution of Sr for La in LaFeO 3 significantly enhanced the catalytic performance and durability. In particular, the La 0.8 Sr 0.2 FeO 3−δ catalyst could be recovered by simple filtration and reused several times without an obvious loss of its high catalytic performance, whereas the recovered BaFeO 3−δ and SrFeO 3−δ catalysts were almost inactive. La 0.8 Sr 0.2 FeO 3−δ promoted the selective oxidation of isobutane even under mild conditions (60 °C), and the catalytic activity was comparable to that of homogeneous systems, including halogenated metalloporphyrin complexes. On the basis of mechanistic studies, including the effect of Sr substitution in La 1−x Sr x FeO 3−δ on surface redox reactions, the present oxidation proceeds via a radical-mediated oxidation mechanism, and the surface-mixed Fe 3+ /Fe 4+ valence states of La 1−x Sr x FeO 3−δ nanoparticles likely play an important role in promoting C−H activation of isobutane as well as decomposition of tert-butyl hydroperoxide.