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

Abstract

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 SrMnO 3 with high surface area was successfully synthesized by simple calcination of the amorphous precursor prepared using aspartic acid and metal acetates instead of metal nitrates, without pH adjustment. The specific surface area reached up to ca. 50 m 2 g −1 , which is much larger than that for SrMnO 3 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. ■ INTRODUCTIONPerovskite-type oxides with the general formula ABO 3 are a class of mixed oxides that exhibit compositional and structural varieties. The versatility and accessibility of perovskite-type oxides have attracted significant interest in broad fields of piezoelectric, ferroelectric, (anti)ferromagnetic, catalytic, and semiconducting materials. 1 A number of methods for the synthesis of perovskites, such as solid-state, coprecipitation, sol−gel, hydrothermal, freeze/spray drying, and microwave methods, have been developed. 2 In particular, an increase of surface area is important for catalytic applications, and many efforts have been made to synthesize nanoperovskites with high surface areas. The sol−gel methods represented by the Pechini method and polymerized complex (PC) method are among the most studied and frequently used techniques for the preparation of nanoperovskites because these methods can accurately control the final composition and yield pure and homogeneous perovskites. 3 However, these methods have some disadvantages in that they are (i) complicated procedures that include complex and polymer gel formation, pyrolysis to an amorphous precursor, and calcination and they require (ii) the use of toxic ethylene glycol and significant amounts of organic reagents and (iii) high-temperature calcination to remove carbonates formed from the carbonaceous precursors, which results in low specific surface area. In addition, the combustion of carbon species elevates the temperature of the material itself and further sinters the material, which decreases the surface area. Therefore, the development of simple and efficient synthesis methods to obtain highly homogeneous and dispersed perovskite nanomaterials with high surface area is still strongly required and a challenging research subject.We have recently reported that hexagonal SrMnO 3 (SMO-PC) synthesized by the PC method with a surface area of 25 m 2 g −1 can act as an efficient reusable heterogeneous catalyst for the selective liquid-phase oxidation of various organic substrates with O 2 . 4 The synthesis of high-surface-area SMO can improve its catalytic activity. Teraoka and co-workers reported La 0.8 Sr 0.2 MO 3 (M = Mn and Co) with high surface area (37 and 20 m 2 g −1 , respectively)...

Aerobic Oxygenation of Alkylarenes over Ultrafine Transition‐Metal‐Containing Manganese‐Based Oxides

“…Among the catalysts reported previously for the oxygenation of 1 a , nano‐sized hexagonal SrMnO 3 perovskite exhibited an excellent catalytic activity under relatively mild conditions (60–120 °C, 1 atm O 2 , no additives) . The oxygenation of 1 a using Ni‐MnO x was performed under the same reaction conditions as those reported for the SrMnO 3 ‐catalyzed oxygenation; the oxygenation of 1 a using Ni‐MnO x was completed more rapidly (within 1 h) than that using SrMnO 3 (within 8 h), which shows the effectiveness of Ni‐MnO x . Hereafter, we mainly utilized the most active Ni‐MnO x catalyst for further detailed investigations.…”

“…The Mn center is thought to accept an electron effectively by the SET mechanism from the n orbital of heteroatoms or the π orbital of aromatic rings . Alternatively, active oxygen species generated on the surface of the catalyst or oxygen atoms adjacent to the metal center are thought to abstract a hydrogen atom from weak C−H bonds directly by the HAT mechanism . To distinguish the predominant mechanism in the Ni‐MnO x ‐catalyzed oxygenation, the reactions of alkylarenes 4‐ethylanisole and 4‐isopropylanisole were conducted in the presence of Ni‐MnO x .…”

“…In this study, we prepared heterogeneous catalysts based on manganese oxide for alkylarene oxygenation and we considered the following factors. (1) Miniaturization of the particle size is important because manganese oxide with a high specific surface area typically exhibits a high catalytic performance . (2) Incorporation of additional metals into the manganese oxide structure is important to increase the redox stability.…”

“…[16][17][18][19][20][21][22][23][24] In particular,t heir oxidation catalysis is of great interest, and thus numerousa erobic oxidation systemst hat use catalysts based on manganese oxide have been developed. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] As fora lkylarene oxygenation, several catalysts based on manganese oxide have also been reported to be highly effective (Table S2); [40][41][42][43][44] for example, Hara and colleagues reported recently that nanosized hexagonal SrMnO 3 perovskite exhibits ah igh catalytic performance for alkylarene oxygenation under relatively mild conditions (60-120 8C, 1atm O 2 ,n oa dditives). [43,44] In this study,w ep repared heterogeneous catalysts based on manganese oxide for alkylarene oxygenation and we considered the following factors.…”

“…[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] As fora lkylarene oxygenation, several catalysts based on manganese oxide have also been reported to be highly effective (Table S2); [40][41][42][43][44] for example, Hara and colleagues reported recently that nanosized hexagonal SrMnO 3 perovskite exhibits ah igh catalytic performance for alkylarene oxygenation under relatively mild conditions (60-120 8C, 1atm O 2 ,n oa dditives). [43,44] In this study,w ep repared heterogeneous catalysts based on manganese oxide for alkylarene oxygenation and we considered the following factors. (1) Miniaturization of the particles ize is important because manganese oxide with ah igh specific surface area typicallye xhibits ah igh catalytic performance.…”