Transformation from layered to tunnel structures: Synthesis, characterization, and applications of manganese oxide octahedral molecular sieves

Manganese oxide based octahedral molecular sieves (OMS) have been found to have a wide variety of applications as catalysts, absorbents, and battery materials due to their unique structures and physical and chemical properties. OMS materials are made up of manganese oxide octahedral building blocks sharing comers and edges to form tunnel structures. Manganese species in the framework of OMS materials are mixed valent with various ion-exchangeable cations residing in the tunnels playing important roles in charge balancing and special chemical activities. ^ With different synthetic parameters such as the template used, temperature, pressure, and the pH of the synthetic media, layered birnessite materials were hydrothermally transformed into distinct tunnel structures with different tunnel sizes, including Mg-3x3 (OMS-1), NH4-2x2 (NH4-OMS-2), Na-2x4 (OMS-5), and other manganese oxides. Characterization of the OMS materials with a wide variety of instruments has revealed that most of them are nano-fibrous hollow crystals ith large surface areas, high ion-exchange capabilities, and relatively high thermal stabilities. The Na-2x4 tunnel structure sodium MnOx has been synthesized for the first time and studied in detail, including synthetic strategies, structural analyses, and other physical and chemical property analyses. ^ As catalysts, the synthetic OMS materials show high catalytic activities and shape-selective properties. For example, the results of the competitive oxidation of cycloalkanes with tertiary butyl hydrogen peroxide (TBHP) over different tunnel sized ONIS materials have proven that the OMS materials with larger tunnels are more favorable for the oxidation of the biggest molecule, cyclooctane, than the smallest one, cyclohexane. ^ Besides the tunnel size effects, tunnel cations in the OMS materials also have influences on their catalytic activities. The study of carbon monoxide cleanup for fuel cell applications demonstrates that Ag-OMS-2 (a hollandite structured OMS catalyst with Ag cations residing in the tunnels) is the best oxidative catalyst among many other catalysts. The amount of Ag loading and the average oxidation state of Mn in a Ag-OMS-2 catalyst are the major influences on their catalytic performance. A suitable working temperature range for complete removal of CO from hydrogen-rich reformates using a Ag-OMS-2 catalyst may be adjusted by changing the Ag loading.