Effect of the Different Preparation Methods on the Characterization and the Catalytic activity of the Nano-Structured Cryptomelane Materials

Abstract

Manganese-based octahedral molecular sieves of the type K-OMS-2 (cryptomelane structure) were prepared either by synproportionation of KMnO₄ and Mn²⁺sulfate salts, in acidic aqueous suspension (reflux method) and or by the reduction  of Mn⁺⁷ using benzyl alcohol and CTAB (as template) in neutral medium. The prepared K-OMS-2 materials were characterized using several techniques: X-ray diffraction, Fourier transformer-Infrared & Raman spectroscopy, Transmission electron microscopy, differential & gravimetric thermal analysis, H₂-temperature programmed reduction and N₂ adsorption-desorption measurements. The results revealed that, the two prepared samples are mainly nano-structured cryptomelane materials (crystallite diameter 9- 32.5 nm.). The obtained K-OMS-2 material on using the sulfate anion (reflux method) has more available lattice oxygen as compared to that prepared by the other method. On using CTAB the obtained cryptomelane material exhibits a high thermal stability. The catalytic activity of the prepared K-OMS-2 cryptomelane samples were studied towards the oxidation reaction of cyclohexane at reaction temperature: 300-400^oC. The catalytic activity for K-OMS-2 “neutral” is higher (the conversion increases with the increase in reaction temperature from 27 to 95% successively) with respect to K-OMS-2 ”reflux” (the conversion increases from 30 to 79% successively).

Keywords: Cryptomelane, acidic, neutral, lattice oxygen, thermal stability.