EFFECTS OF INORGANIC CATION TEMPLATES ON OCTAHEDRAL MOLECULAR-SIEVES OF MANGANESE OXIDE

Five hydrated inorganic divalent cations, Mg2+, Co2+, Ni2+, CU2+ and Z n2+, have successfully been used as templates for the synthesis of man ganese oxide octahedral molecular sieves (OMS-1) having the todorokite structure. The OMS-1 samples have been well characterized by X-ray di ffraction, differential scanning calorimetry, thermogravimetric analys is, scanning electron microscopy/energy dispersive X-ray studies, indu ctively coupled plasma analysis, electron paramagnetic resonance, Four ier transform infrared spectroscopy, thiosulfate titration, and cycloh exane sorption. Catalytic CO oxidation and 2-propanol decomposition we re carried out. Results show that these OMS-1 samples with a tunnel si ze of about 6.9 Angstrom are crystalline and chemically pure. They hav e the following formulas: Mg3.17Mn5.05O12.4.52H(2)O, Co1.84Mn5.59O12.3 .45H(2)O, Ni1.64Mn5.75O12.4.19H(2)O, CU3.50Mn4.45O12.5.33H(2)O, and Zn 3.55Mn4.47O12.2.59H(2)O. Their thermal stability largely depends on th e nature of the cations: Mg-OMS-1 is thermally stable in air up to 600 degrees C; Co-OMS-1 and Ni-OMS-1 are stable to 500 degrees C, and Cu- OMS-1 and Zn-OMS-1 are stable to 300 degrees C. The crystal morphologi es of the OMS-1 samples can be plates, needles, or fibrous shapes, dep ending on the nature of the cations. The cations also have profound ef fects on acidity: Mg-OMS-1, Co-OMS-1, and Ni-OMS-1 have intermediate s trength Bronsted and Lewis acid sites, while Cu-OMS-1 and Zn-OMS-1 hav e no Bronsted acid sites and weak Lewis acid sites. All the OMS-1 samp les have six hyperfine electron paramagnetic resonance lines for Mn2located in an octahedral environment.