Characterization of Manganese Oxide Octahedral Molecular Sieve (M−OMS-2) Materials with Different Metal Cation Dopants

Abstract

Reflux methods were used to prepare different metal cation (M: Cu2+, Zn2+, Ni2+, Co2+, Al3+, or Mg2+) doped manganese oxide octahedral molecular sieve (M−OMS-2) materials with cryptomelane structure. Thorough characterization has been carried out for these M−OMS-2 materials. SEM micrographs of M−OMS-2 materials reveal their typical fibrous morphology. The results of ICP−AES elemental analyses for M−OMS-2 materials shed light on the distribution of doped metal cations in the cryptomelane structure. TGA and XRD studies indicate that the prepared M−OMS-2 materials are more thermally stable in O2 (up to 600 °C) than in inert (e.g., N2 or He) atmospheres (up to 500 °C). Phase transformations from cryptomelane to hausmannite or bixbyite were observed when M−OMS-2 materials were heated above these stable temperatures. Comprehensive N2 sorption analyses confirmed that M−OMS-2 materials are porous materials, which contain micropores and mesopores, with total BET surface areas in the range of 70−110 m2/g. Basicity studies of synthetic M−OMS-2 materials with TPD of adsorbed CO2 revealed that all pretreated M−OMS-2 materials have several different basic sites with varied strengths and amounts on their surfaces. TPD studies of NH3 adsorbed on the pretreated M−OMS-2 materials demonstrate that all M−OMS-2 materials also have similar acidities.