Synthesis and characterization of nanofibrous sodium manganese oxide with a 2 x 4 tunnel structure

Sodium manganese oxide octahedral molecular sieves with a 2 x 4 tunnel stru cture (Na-2 x 4) have been hydrothermally synthesized from Na-birnessite ma terials at low temperatures and pressures. The synthetic template materials , the pH value of the medium, and the autoclaved temperature are critical i n the synthesis. Sodium salts, such as NaCl, NaNO3, and Na2SO4, are good te mplates for Na-2 x 4. In strong basic solution or below 160 degreesC, Na-bi rnessite 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+M n0.674+O2. xH(2)O, and it is monoclinic with space group C2/m. The unit cel l 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 da ta for Na-2 x 4 are slightly different from the data for Rb-2 x 4 synthesiz ed under high pressure and high temperature, which were reported by Rziha e t al. (Eur. J. Miner. 1998, 8, 1155-1161). The surface area of Na-2 x 4 is about 57 m(2)/g. Na-2 x 4 materials are thermally stable up to 450 degreesC as indicated by TGA and TPD data.