Synthesis, Characterization, and Catalytic Applications of Manganese Oxide Octahedral Molecular Sieve (OMS) Nanowires with a 2 × 3 Tunnel Structure

Abstract

Romanechite is a natural manganese oxide with a 2 × 3 tunnel structure containing a majority of Ba2+ and trace amounts of Na+, K+, and Sr2+ as tunnel cations. Many attempts have been made to synthesize the 2 × 3 tunnel structured manganese oxide in laboratories using Ba2+ as the template. However, no successful work has been reported due to (1) the presence of intergrown hollandite impurities (barium-2 × 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 d ≈ 9.7 and 7.0 Å, respectively. Hydrated Na+ ions have been utilized as structure directors to successfully synthesize the Na-2 × 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 N2 environments, the 2 × 3 tunnel structure is stable below 550 °C and transforms into hausmannite (Mn3O4) at temperatures of 550 °C or higher; however, under O2 environments, the 2 × 3 tunnel structure is preserved at 550 °C as measured by in situ XRD. The density functional theory (DFT) method indicated that the 2 × 3 tunnel structure has a major micropore size of 7.5 Å. NH3 and CO2 chemisorption results indicated that the amount of strong acidic and basic sites on the Na-2 × 3 material is about 0.22 and 0.002 mmol/g sample, respectively. Catalytic oxidation of indene by the Na-2 × 3 manganese oxide showed a 96% conversion for indene and a 73% selectivity toward phthalic anhydride for a 40 h reaction at 80 °C.