C. L. O'Young

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� 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 Br�nsted acid sites have been observed in OMS-1. Particular applications of these materials include adsorption, electrochemical sensors, and oxidation catalysis.

Temperature Program Desorption and Reduction Studies of Octahedral Molecular Sieves

Publisher Summary The reactivity of oxygen species in single or mixed transition metal oxides plays an important role in determining the catalytic performance of an oxide as a consequence of the Mars–van Krevelen redox mechanism. Manganese dioxides, such as octahedral molecular sieves (OMSs), are a family of promising compounds for studying relationships between solid and catalytic properties. The incorporation of additional transition metal(s) into OMS materials lends great variability for adjusting oxidative and reductive stoichiometry and, consequently, the valency states and Mn–O binding strengths. Three or more oxygen species can be discriminated in the tunnel structure of both doped OMS 1 and OMS 2 in view of the emerging temperature of peaks during temperature-programmed desorption (TPD) in He. The doping cations do not markedly affect the evolution of TPD peaks. The predominance of oxygen species related to the high-temperature (HT) region peaks in TPD suggests that they are mainly under the influence of Mn ions in the framework because of their relative interatomic distances from Mn and from doping cations.