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

Abstract

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

Synthesis of manganese oxide octahedral molecular sieves containing cobalt, nickel, or magnesium, and the catalytic properties for hydration of acrylonitrile

“…The all diffraction peaks for Mg-todorokite sample was attributed to the todorokite phase of PDF card #38-475 [34] of natural todorokite and data reported for synthesized todorokite samples [11,12], except for a peak of manganite at 0.340 nm. In Figure 3, Mg-todorokite contains both needle and platelet shaped crystallites.…”

“…Synthesis of todorokite-type manganese oxides containing a "foreign cation" Co 2+ , Ni 2+ , or Mg 2+ , involved three major steps, as shown in Figure 1 [11,33].…”

“…Although todorokite in the natural form is a hydrated manganese oxide with impurities, the todorokite-type manganese oxide has been synthesized hydrothermally as a single-phase material [10][11][12][13]. Golden et al have reported a synthetic todorokite-type manganese oxide that was prepared by the hydrothermal treatment of Mg 2+ -exchanged birnessite-type manganese oxide [10].…”

“…It was interesting to note the activity and the selectivity of todorokite catalysts for the hydration. In the last two decades, there have been many reports of todorokite-type octahedral molecular sieves prepared by using magnesium salts and/or Mg (MnO 4 ) 2 [2,[10][11][12]. Some of divalent magnesium cations exist in the framework and play a role in maintaining the stable structure of these manganese oxide compounds [31].…”

“…Golden et al have reported a synthetic todorokite-type manganese oxide that was prepared by the hydrothermal treatment of Mg 2+ -exchanged birnessite-type manganese oxide [10]. Shen et al have developed a thermally stable todorokite-type manganese oxide containing magnesium, called OMS-1, and found that it can be used as a molecular sieve with a pore size of 0.69 nm estimated by adsorption of various kinds of organic compounds [11].…”