Synthesis, characterization, and catalytic applications of manganese oxide octahedral molecular sieve (OMS) nanowires with a 2 x 3 tunnel structure

Romanechite is a natural manganese oxide with a 2 x 3 tunnel structure containing a majority of Ba(2+) and trace amounts of Na(+), K(+), and Sr(2+) as tunnel cations. Many attempts have been made to synthesize the 2 x 3 tunnel structured manganese oxide in laboratories using Ba(2+) as the template. However, no successful work has been reported due to (1) the presence of intergrown hollandite impurities (barium-2 x 2 tunnel structure manganese oxide) in the synthesized products and (2) the absence of romanechite characteristic X-ray diffraction (XRD) peaks in the products, such as the (001) and (200) diffraction peaks which correspond to approximate to 9.7 and 7.0 A, respectively. Hydrated Na(+) ions have been utilized as structure directors to successfully synthesize the Na-2 x 3 tunnel structure manganese oxide (OMS-6) from hydrothermal treatment of Na-birnessite. XRD gave a pattern in very good agreement with the pattern of romanechite (JCPDS file 14-627) without impurity phases. High-resolution microscopy measurements showed a nanofibrous morphology of the materials with an average fiber diameter of 40 nm. Under N(2) environments, the 2 x 3 tunnel structure is stable below 550 degreesC and transforms into hausmannite (Mn(3)O(4)) at temperatures of 550 degreesC or higher; however, under O(2) environments, the 2 x 3 tunnel structure is preserved at 550 degreesC as measured by in situ XRD. The density functional theory (DFT) method indicated that the 2 x 3 tunnel structure has a major micropore size of 7.5 A. NH(3) and CO(2) chemisorption results indicated that the amount of strong acidic and basic sites on the Na-2 x 3 material is about 0.22 and 0.002 mmol/g sample, respectively. Catalytic oxidation of indene by the Na-2 x 3 manganese oxide showed a 96% conversion for indene and a 73% selectivity toward phthalic anhydride for a 40 h reaction at 80 Cdegrees.