| Abstract
| - Romanechite is a natural manganese oxide with a 2 × 3 tunnel structure containing amajority of Ba2+ and trace amounts of Na+, K+, and Sr2+ as tunnel cations. Many attemptshave been made to synthesize the 2 × 3 tunnel structured manganese oxide in laboratoriesusing Ba2+ as the template. However, no successful work has been reported due to (1) thepresence of intergrown hollandite impurities (barium-2 × 2 tunnel structure manganeseoxide) in the synthesized products and (2) the absence of romanechite characteristic X-raydiffraction (XRD) peaks in the products, such as the (001) and (200) diffraction peaks whichcorrespond to d ≈ 9.7 and 7.0 Å, respectively. Hydrated Na+ ions have been utilized asstructure 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 goodagreement with the pattern of romanechite (JCPDS file 14-627) without impurity phases.High-resolution microscopy measurements showed a nanofibrous morphology of the materialswith an average fiber diameter of 40 nm. Under N2 environments, the 2 × 3 tunnel structureis stable below 550 °C and transforms into hausmannite (Mn3O4) at temperatures of 550 °Cor 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 thatthe 2 × 3 tunnel structure has a major micropore size of 7.5 Å. NH3 and CO2 chemisorptionresults indicated that the amount of strong acidic and basic sites on the Na-2 × 3 materialis about 0.22 and 0.002 mmol/g sample, respectively. Catalytic oxidation of indene by theNa-2 × 3 manganese oxide showed a 96% conversion for indene and a 73% selectivity towardphthalic anhydride for a 40 h reaction at 80 °C.
- Na-2 × 3 tunnel structure manganese oxides have been synthesized via hydrothermal treatment of Na-birnessite. The nanowire 2 × 3 tunnel structure has an average wire diameter of 40 nm and a major micropore size of 7.5 Å. OMS-6 shows a 96% conversion for catalysis of indene oxidation and 73% selectivity toward phthalic anhydride.
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