Chemical syntheses of manganese and tantalum oxide octahedral molecular sieves and their structural characterization by powder x-ray diffraction

Date of Completion

January 1999


Chemistry, Inorganic|Engineering, Materials Science




This research consists of soft chemical synthesis and structural investigations of manganese and tantalum oxide octahedral molecular sieves. These include the use of sol-gel, hydrothermal and reflux methods for syntheses and the employment of various techniques for characterization, especially the Rietveld analysis of powder X-ray diffraction data for structural refinements. ^ The manganese oxide cryptomelane (K-OMS-2) with a tunnel structure and birnessite layered materials (OL-1) were prepared by the sol gel method. The synthesis consists of reacting MnO4 solutions with fumaric acid or glucose to form a gel, and heating the xerogels at a temperature effective to produce the final manganese oxide materials. These sol-gel methods give many advantages, such as high thermal stabilities of products, over other preparation routes. The synthetic parameters have been optimized to prepare pure K-OMS-2 and OL-1. The crystal structure of K-OMS-2 has been refined by the Rietveld method from in-house powder X-ray diffraction data. ^ The potassium tantalate defect pyrochlores were prepared by hydrothermal methods at a temperature of 200°C. The materials were crystallized from tantalum pentoxide in a potassium hydroxide solution, with a uniform crystal size of about 1 μm. They were ion-exchanged with H+ at low temperatures in a nitric acid solution. A BET surface area of 15 m 2/g was obtained. The structure of this defect pyrochlore and its acid exchanged form were determined by Rietveld refinement from powder X-ray diffraction data. ^ The reflux method was also employed to search for new manganese oxide microporous materials. A ramsdellite material with a 1 x 2 tunnel structure and high surface area of 70 m2/g was prepared. Catalytic oxidations of hexane and cyclohexane with tert-butyl hydroperoxide have shown good activities with this catalyst. This investigation suggests that shape selectivity plays a role in the high catalytic activities of the oxidations of these saturated hydrocarbons. ^