Particle Size Control of Cryptomelane Nanomaterials by Use of H₂O₂ in Acidic Conditions

Abstract

A new soft-step chemistry method has been developed to prepare pure cryptomelane-type manganese oxide materials (OMS-2) with the smallest particle sizes ever reported. The synthetic procedure is based on the reduction of KMnO4 by H2O2 under acidic conditions followed by reflux. An acetate-containing buffer solution and HNO3 are used to control the pH of the reaction mixture. The formation process, particle size, crystallite size, crystal structure, and properties of these nanomaterials have been investigated by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), potentiometric titration, thermogravimetric analysis, and N2 sorption analyses. Both the concentration of H2O2 and the nature of the acid used affect the crystalline phase formation, microstructure, thermal stability, and the composition of the final product. HRTEM images reveal that the OMS-2 nanofibers are not oriented preferentially and present significant twinning, along with discontinuity in the growth of the tunnel. Catalytic studies of these OMS-2 nanomaterials for oxidation of benzyl alcohol and fluorene have been performed. These nanomaterials show a low performance for the oxidation of benzyl alcohol and a unique catalytic activity for the oxidation of fluorene compared to OMS-2 materials prepared by conventional methods.

Manganese Oxide Octahedral Molecular Sieves (OMS-2) Multiple Framework Substitutions: A New Route to OMS-2 Particle Size and Morphology Control

“…Consistent with the work of Villegas et al [ 16 ] the characteristic tunnel structure of cryptomelane is manifested in our material, see HRTEM image in Figure 6 e. However, unlike other observations of these reported tunnels, 1 × 1 and 2 × 2 tunnels, which for a very long time have only been observed in simulated structures of OMS-2, have now been confi rmed experimentally for the fi rst time, see top-right inset of Figure 6 e. The tunnels observed in our synthesized multidoped OMS-2 material are akin to the ones in the simulated OMS-2 structure (top-left inset of Figure 6 e) drawn with CrystalMaker. [ 29 ] …”

Synthesis and Characterization of Manganese Oxide Catalysts for the Total Oxidation of Ethyl Acetate

“…Extensive research efforts have been devoted to the synthesis of this kind of material, the so called octahedral molecular sieves (OMS) with layer and tunnel structures [4][5][6][7][8][9][10]. The open frameworks of these materials are composed of edge and corner shared MnO 6 octahedra, the internal pores being occupied by cations and water molecules.…”

“…The open frameworks of these materials are composed of edge and corner shared MnO 6 octahedra, the internal pores being occupied by cations and water molecules. Cryptomelane-type manganese oxide (K-OMS-2) is one group of the OMS family called hollandite, and has tunnel structures that consist of 2 9 2 arrays of MnO 6 octahedra [7,11,12]. Inside the tunnels, potassium cations and small amounts of water stabilize the structure, and can be partially ion-exchanged with other cations with appropriate sizes.…”

“…The open tunnel structure and mixed valences of OMS-2 make this material very attractive for oxidation catalysis and, therefore, the synthesis of OMS-2 material with high purities, small particle size, high surface areas, and high yield has been a challenge in recent years. Various synthetic routes have been explored [13], including thermal and hydrothermal treatment of layered birnessite [14], redox precipitation under reflux [7,15], microwave heating [16], sol-gel methods [17,18] and a solvent free procedure [19]. The reflux method, involving the oxidation of Mn 2?…”

Kinetics and Mechanism of 9H‐Fluorene Oxidation Catalyzed by Manganese Oxide Octahedral Molecular Sieves

“…Temperature-programmed desorption mass spectrometry and thermogravimetric analyses of this catalyst show that at 170 8C the only loss of weight from the catalyst is by evolution of water as no other species are detected with the mass spectrometer, but above 300 8C oxygen species are detected. [19] This observation implies that surface oxygen species are still present at a lower temperature. There was a drastic drop in the rates of generation of 9-fluorenone when carbon tetrachloride (CCl 4 ) was added into the reaction vessel.…”