Manganese-based octahedral molecular sieves of the type K-OMS-2 (cryptomelane structure) with different morphologies and specific surface areas in the range of 20-135 m(2) g(-1) were prepared via synproportionation of KMnO4 and Mn2+ salts, either in acidic aqueous suspension (reflux method) or by a solid-state reaction, and via oxidation of MnSO4 either by K2Cr2O7 or by molecular oxygen in aqueous solution. For the reflux method, the influence of K+ cations for the formation of the OMS-2 structure was proven. When KMnO4 is replaced by Ba(MnO4)(2), the anion in the Mn2+ salt exerts a strong influence on the synthesis product. All materials were characterized by elemental analysis (ICP-OES), XRD, nitrogen sorption. scanning electron microscopy and TGA. For the oxidation of benzyl alcohol with molecular oxygen in liquid toluene at 110 degrees C, the catalytic activity of the K-OMS-2 materials is directly correlated to their specific surface area A(BET). Since A(BET) increases with decreasing average crystallite diameter, the catalytic conversion presumably occurs at the outer crystallite surface. These relations are independent of morphology orsynthesis procedure. The K-OMS-2 materials are more active than crystalline manganese oxides (MnO, Mn2O3, Mn3O4, beta-MnO2), but less active than amorphous MnO2. Regeneration of deactivated K-OMS-2 catalysts can be achieved by calcining at 300 degrees C in air, partly due to the reversible desorption of water. (C) 2008 Elsevier B.V. All rights reserved.
