Manganese Oxide Octahedral Molecular Sieves: Preparation, Characterization, and Applications

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Abstract
A thermally stable 3 × 3 octahedral molecular sieve corresponding to natural todorokite (OMS-1) has been synthesized by autoclaving layer-structure manganese oxides, which are prepared by reactions of MnO 4 - and Mn 2+ under markedly alkaline conditions. The nature and thermal stability of products depend strongly on preparation parameters, such as the MnO 4 - /Mn 2+ ratio, pH, aging, and autoclave conditions. The purest and the most thermally stable todorokite is obtained at a ratio of 0.30 to 0.40. Autoclave treatments at about 150° to 180°C for more than 2 days yield OMS-1, which is as thermally stable (500°C) as natural todorokite minerals. Adsorption data give a tunnel size of 6.9 angstroms and an increase of cyclohexane or carbon tetrachloride uptake with dehydration temperature up to 500°C. At 600°C, the tunnel structure collapses. Both Lewis and Brönsted acid sites have been observed in OMS-1. Particular applications of these materials include adsorption, electrochemical sensors, and oxidation catalysis.