Journal of the American Chemical Society, Vol.116, No.24, 11020-11029, 1994
Effects of Inorganic Cation Templates on Octahedral Molecular-Sieves of Manganese Oxide
Five hydrated inorganic divalent cations, Mg2+, Co2+, Ni2+, CU2+ and Zn2+, have successfully been used as templates for the synthesis of manganese oxide octahedral molecular sieves (OMS-1) having the todorokite structure. The OMS-1 samples have been well characterized by X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy/energy dispersive X-ray studies, inductively coupled plasma analysis, electron paramagnetic resonance, Fourier transform infrared spectroscopy, thiosulfate titration, and cyclohexane sorption. Catalytic CO oxidation and 2-propanol decomposition were carried out. Results show that these OMS-1 samples with a tunnel size of about 6.9 Angstrom are crystalline and chemically pure. They have 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 Zn3.55Mn4.47O12.2.59H(2)O. Their thermal stability largely depends on the 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 morphologies of the OMS-1 samples can be plates, needles, or fibrous shapes, depending on the nature of the cations. The cations also have profound effects on acidity : Mg-OMS-1, Co-OMS-1, and Ni-OMS-1 have intermediate strength Bronsted and Lewis acid sites, while Cu-OMS-1 and Zn-OMS-1 have no Bronsted acid sites and weak Lewis acid sites. All the OMS-1 samples have six hyperfine electron paramagnetic resonance lines for Mn2+ located in an octahedral environment.