Hierarchically order porous lotus shaped nano-structured MnO2 through MnCO3: chelate mediated growth and shape dependent improved catalytic activity

Abstract

Design of hierarchical nanostructures towards a specific morphology is an important research area due to their shape dependent properties. Here, 3D hierarchically assembled lotus shaped porous MnO₂ is synthesized using a simple aqueous solution based chelating agent (citric acid) mediated growth of MnCO₃ followed by calcination at 350 °C. MnCO₃ in other shapes, such as rods, spheres and nano-aggregates, is also synthesized just by varying the chelating agents. It is observed that the geometry and strength of the chelating ligands has a crucial role in the controlled shape selective synthesis and based on this a probable chelating agent driven formation mechanism is discussed. The synthesized porous MnO₂ shapes exhibit excellent shape dependent catalytic oxidation of α-pinene to verbenone using molecular oxygen as the oxidant. The lotus shaped porous MnO₂ shows superior activity, with 94% conversion of α-pinene and 87% selectivity of verbenone, to that of other MnO₂ shapes. The activity is reasonably high compared to heterogeneous as well as homogeneous catalysts reported in the literature and bulk MnO₂ with respect to both their conversion and selectivity. The synthesized lotus shaped MnO₂ also showed good catalytic activity towards oxidation of allylic compounds to corresponding ene–ones using molecular oxygen as oxidant and is reusable.