Hexagonal and cubic phases of manganese oxide mesoporous structures (MOMS) have been prepared by means of the oxidation of Mn(OH) 2 . The hexagonal MOMS materials form a hexagonal array of pores with an open porous structure, thick walls (1.7 nanometers), and exceptional thermal stability (1000°C). The walls of the mesopores are composed of microcrystallites of dense phases of Mn 2 O 3 and Mn 3 O 4 , with MnO 6 octahedra as the primary building blocks. The calcined hexagonal MOMS have an electrical conductivity of 8.13 × 10 −6 per ohm·centimeter, an average manganese oxidation state of 3.55, and a band gap of 2.46 electron volts. Catalytic oxidations of cyclohexane and n -hexane in aqueous solutions in a batch reactor show conversions of ∼10 and ∼8 percent, respectively. Characterization and catalytic data suggest that MOMS systems show significant enhancement in thermal stability with respect to octahedral molecular sieve materials.

This review concerns the synthesis, characterization, and applications of porous manganese oxides during the last two years. The synthesis of porous tunnel structures, layered structures, and related materials is discussed. Both microporous and mesoporous systems materials are covered here. Characterization discussed here focuses around structural studies. The focus of the application sections include electrochemical and catalytic studies.

The sol-gel reaction between KMnO 4 and fumaric acid in a 3:1 mole ratio generates a flocculant gel that serves as a precursor to the tunneled manganese oxide, cryptomelane. The elemental composition of sol-gel cryptomelane has been determined to be K 0.12 MnO 2.0 -(H 2 O) 0.09 . Further characterization has been performed using powder X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy. The sol-gel process is heavily dependent on reactant concentration. Solutions that are too concentrated produce the layered manganese oxide birnessite, whereas overly dilute reactions yield mixtures of cryptomelane and Mn 2 O 3 . The preference for cryptomelane over birnessite correlates with low potassium content in the gel. The sol-gel procedure for synthesizing cryptomelane is not easily transferred to the preparation of analogous manganese oxides with different tunnel cations. Reactions that employ permanganates other than KMnO 4 generally yield Mn 2 O 3 , with cryptomelane being a minor product at best. Thermal analyses of cryptomelane gels indicate that calcination proceeds through a series of stages that involve loss of water, loss of residual organics, conversion to cryptomelane, and finally degradation to Mn 3 O 4 . The extraction of potassium ions from sol-gel cryptomelane by various foreign cations is minimal, with the loss of K + being on the order of 10%.

Abstract:Sodium manganese oxide octahedral molecular sieves with a 2 x 4 tunnel structure (Na-2 x 4) have been hydrothermally synthesized from Na-birnessite materials at low temperatures and pressures. The synthetic template materials, the pH value of the medium, and the autoclaved temperature are critical in the synthesis. Sodium salts, such as NaCl, NaNO 3 , and Na 2 SO 4 , are good templates for Na-2 x 4. In strong basic solution or below 160 degreesC, Na-birnessite does not transform to the Na-2 x 4 structure. TEM images show the synthesized Na-2 x 4 is made up of thin slablike single crystals elongated along the b axis. The formula of Na-2 x 4 can be written as Na0.33Mn0.333+Mn0.674+O2. xH( 2 )O, and it is monoclinic with space group C2/m. The unit cell parameters (a, b, c, and beta) for Na-2 x 4 are 14.434(5) Angstrom, 2.849(7) Angstrom, 23.976(6) Angstrom, and 98.18 degrees, respectively. These data for Na-2 x 4 are slightly different from the data for Rb-2 x 4 synthesized under high pressure and high temperature, which were reported by Rziha et al. (Eur.

No abstract