MANGANESE OXIDE OCTAHEDRAL MOLECULAR-SIEVES - PREPARATION, CHARACTERIZATION, AND APPLICATIONS

A thermally stable 3 x 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 MnO4- and Mn2+ under markedly alkaline conditions. The nature and thermal stability of products depend strongly on preparation parameters, such as the MnO4-/Mn2+ 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-degrees to 180-degrees-C for more than 2 days yield OMS-1, which is as thermally stable (500-degrees-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-degrees-C. At 600-degrees-C, the tunnel structure collapses. Both Lewis and Bronsted acid sites have been observed in OMS-1. Particular applications of these materials include adsorption, electrochemical sensors, and oxidation catalysis.