Hierarchically order porous lotus shaped nano-structured MnO 2 through MnCO 3 : chelate mediated growth and shape dependent improved catalytic activity† Provas Pal, ab Sandip K. Pahari, ab Arnab Kanti Giri, ab Sagar Pal, c Hari C. Bajaj ab and Asit Baran Panda * abd 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 MnO 2 is synthesized using a simple aqueous solution based chelating agent (citric acid) mediated growth of MnCO 3 followed by calcination at 350  C. MnCO 3 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 MnO 2 shapes exhibit excellent shape dependent catalytic oxidation of a-pinene to verbenone using molecular oxygen as the oxidant. The lotus shaped porous MnO 2 shows superior activity, with 94% conversion of a-pinene and 87% selectivity of verbenone, to that of other MnO 2 shapes. The activity is reasonably high compared to heterogeneous as well as homogeneous catalysts reported in the literature and bulk MnO 2 with respect to both their conversion and selectivity. The synthesized lotus shaped MnO 2 also showed good catalytic activity towards oxidation of allylic compounds to corresponding ene–ones using molecular oxygen as oxidant and is reusable. Introduction During crystal growth, organic molecules and polymers perform multiple roles in the nucleation and growth process. 1–3 These molecules acts as templates, controlling both the nucleation and growth processes through coordination with metal ions as well as newly formed nucleates in specic geometry; and also controlling the mass transport. In favorable conditions, the organic molecules resulted in formation of hierarchical micro- structures with varying morphology from nanoscopic to macroscopic scale through controlled association of these effects. 2,3 It is possible to design the morphology of a crystal by a thorough understanding and tailoring the interaction of organic molecules with metal ions and specic crystal facets. In this context, controlled synthesis of hierarchical nanostructures has become one of the most leading areas, not only in terms of scientic interest but also in the area of their tremendous real technological applications originating from their size and shape dependent superior physico-chemical properties. There have been several reports on random 3D architectures. However, controlled chelating ligands, such as amino acid, citric acid (CA), tartaric acid (TA), oxalic acid (OA), and ethyl- enediaminetetraacetic acid (EDTA), mediating the growth of 3D hierarchical structures and a thorough understanding of them is rare. 2–4 It is a challenge to formulate a simple, favorable route for controlled growth of 3D hierarchical structures using the chelating molecules and gain a systematic understanding of the role of these chelating molecules in the structure formation. Manganese di-oxide (MnO 2 ), a well known functional transi- tion metal oxide with structural exibility, has attracted extensive attention due to its distinctive physico-chemical properties and wide application in catalysis, ion exchange, molecular adsorp- tion, biosensors, electrochemical super capacitors, electrode materials in Li-batteries and energy storage. 5–7 The properties of MnO 2 are strongly dependent on its size, shape, morphology and crystalline structure. 8 To date many efforts have been made to a Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR), G.B. Marg, Bhavnagar-364002, Gujarat, India. E-mail: abpanda@csmcri.org; Fax: +(91)278-2567562; Tel: +(91)278-2567760 ext. 704 b Academy of Scientic and Innovative Research, Central Salt and Marine Chemicals Research Institute, G.B. Marg, Bhavnagar-364002, Gujarat, India c Department of Applied Chemistry, Indian School of Mines, Dhanbad 826 004, Jharkhand, India d Central Glass & Ceramic Research Institute (CSIR), Raja SC Mullick Road, Kolkata- 32, West Bengal, India † Electronic supplementary information (ESI) available: Detailed experimental procedure, additional SEM images, N 2 sorption isotherm, H 2 -TPR results and catalytic oxidation results. See DOI: 10.1039/c3ta11440f Cite this: J. Mater. Chem. A, 2013, 1, 10251 Received 10th April 2013 Accepted 10th June 2013 DOI: 10.1039/c3ta11440f www.rsc.org/MaterialsA This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1, 10251–10258 | 10251 Journal of Materials Chemistry A PAPER Published on 10 June 2013. Downloaded by University of York on 24/08/2013 13:26:19. View Article Online View Journal | View Issue