Effects of Alkali Metal and Ammonium Cation Templates on Nanofibrous Cryptomelane-type Manganese Oxide Octahedral Molecular Sieves (OMS-2)

Abstract

Four alkali cations (Li+, Na+, K+, and Rb+) and ammonium (NH4+), have been successfully used as templates for the synthesis of cryptomelane-type manganese oxide octahedral molecular sieves (A−OMS-2, A = the template cation). H−OMS-2 material has been prepared by the calcination of NH4−OMS-2. These A−OMS-2 materials have been characterized by X-ray diffraction, transmission electron microscopy, high-resolution scanning electron microscopy, thermogravimetric analysis, temperature-programmed desorption, inductively coupled plasma analysis, and BET surface area pore size distribution measurements. Catalytic evaluation of these A−OMS-2 materials has been done with the aerobic oxidation of cyclohexanol to cyclohexanone. All of these A−OMS-2 materials have cryptomelane tunnel structures with different degrees of crystallinity and nanofibrous morphology. The nature of the cation templates affects the formation and microstructures of the A−OMS-2 materials. The physical and chemical properties, such as the thermal stability, chemical composition, and average oxidation state of manganese, greatly depend on the nature of cations and amount of tunnel water. Catalytic activities of these A−OMS-2 materials are related to their surface areas.