Amino Acid-Aided Synthesis of a Hexagonal SrMnO₃ Nanoperovskite Catalyst for Aerobic Oxidation

A simple and efficient synthetic method for preparing high-surface-area perovskites was investigated by focusing on the importance of the formation of an amorphous precursor. Hexagonal SrMnO₃ with high surface area was successfully synthesized by simple calcination of the amorphous precursor prepared using <i>aspartic acid</i> and metal <i>acetates</i> instead of metal <i>nitrates</i>, without pH adjustment. The specific surface area reached up to ca. 50 m² g^–1, which is much larger than that for SrMnO₃ synthesized by previously reported methods. The catalytic activity for heterogeneous liquid-phase aerobic oxidation was significantly improved in comparison with the polymerized complex method, and the present catalytic system was applicable to the oxidation of various substrates

Reversible Deprotonation and Protonation Behaviors of a Tetra-Protonated γ-Keggin Silicodecatungstate

The potentiometric titration of a γ-Keggin tetra-protonated silicodecatungstate, [γ-SiW₁₀O₃₄(H₂O)₂]^4– (H₄·<b>I</b>), with TBAOH (TBA = [(<i>n</i>-C₄H₉)₄N]⁺) showed inflection points at 2 and 3 equiv of TBAOH. The ¹H, ²⁹Si, and ¹⁸³W NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [γ-SiW₁₀O₃₄(OH)­(OH₂)]^5– (H₃·<b>I</b>), [γ-SiW₁₀O₃₄(OH)₂]^6– (H₂·<b>I</b>), and [γ-SiW₁₀O₃₅(OH)]^7– (H·<b>I</b>), with <i>C</i>₁, <i>C</i>_2<i>v</i>, and <i>C</i>₂ symmetries, respectively. Single crystals of TBA₆·H₂·<b>I</b> suitable for the X-ray structure analysis were successfully obtained and the anion part was a monomeric γ-Keggin divacant silicodecatungstate with two protonated bridging oxygen atoms. Compounds H₃·<b>I</b>, H₂·<b>I</b>, and H·<b>I</b> were reversibly monoprotonated to form H₄·<b>I</b>, H₃·<b>I</b>, and H₂·<b>I</b>, respectively

Reversible Deprotonation and Protonation Behaviors of a Tetra-Protonated γ-Keggin Silicodecatungstate

The potentiometric titration of a γ-Keggin tetra-protonated silicodecatungstate, [γ-SiW₁₀O₃₄(H₂O)₂]^4– (H₄·<b>I</b>), with TBAOH (TBA = [(<i>n</i>-C₄H₉)₄N]⁺) showed inflection points at 2 and 3 equiv of TBAOH. The ¹H, ²⁹Si, and ¹⁸³W NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [γ-SiW₁₀O₃₄(OH)­(OH₂)]^5– (H₃·<b>I</b>), [γ-SiW₁₀O₃₄(OH)₂]^6– (H₂·<b>I</b>), and [γ-SiW₁₀O₃₅(OH)]^7– (H·<b>I</b>), with <i>C</i>₁, <i>C</i>_2<i>v</i>, and <i>C</i>₂ symmetries, respectively. Single crystals of TBA₆·H₂·<b>I</b> suitable for the X-ray structure analysis were successfully obtained and the anion part was a monomeric γ-Keggin divacant silicodecatungstate with two protonated bridging oxygen atoms. Compounds H₃·<b>I</b>, H₂·<b>I</b>, and H·<b>I</b> were reversibly monoprotonated to form H₄·<b>I</b>, H₃·<b>I</b>, and H₂·<b>I</b>, respectively

Heterogeneously Catalyzed Aerobic Oxidation of Sulfides with a BaRuO₃ Nanoperovskite

A rhombohedral BaRuO₃ nanoperovskite, which was synthesized by the sol–gel method using malic acid, could act as an efficient heterogeneous catalyst for the selective oxidation of various aromatic and aliphatic sulfides with molecular oxygen as the sole oxidant. BaRuO₃ showed much higher catalytic activities than other catalysts, including ruthenium-based perovskite oxides, under mild reaction conditions. The catalyst could be recovered by simple filtration and reused several times without obvious loss of its high catalytic performance. The catalyst effect, ¹⁸O-labeling experiments, and kinetic and mechanistic studies showed that substrate oxidation proceeds with oxygen species caused by the solid. The crystal structure of ruthenium-based oxides is crucial to control the nature of the oxygen atoms and significantly affects their oxygen transfer reactivity. Density functional theory calculations revealed that the face-sharing octahedra in BaRuO₃ likely are possible active sites in the present oxidation in sharp contrast to the corner-sharing octahedra in SrRuO₃, CaRuO₃, and RuO₂. The superior oxygen transfer ability of BaRuO₃ is also applicable to the quantitative conversion of dibenzothiophene into the corresponding sulfone and gram-scale oxidation of 4-methoxy thioanisole, in which 1.20 g (71% yield) of the analytically pure sulfoxide could be isolated