Framework doping of iron in tunnel structure cryptomelane

A framework doping method was proposed to dope transition-metal iron ions into the framework instead of the tunnels of 2 x 2 tunnel structure manganese octahedral molecular sieve cryptomelane. Iron was first doped into the MnO6 octahedra layers of layered structure birnessite, which served as a synthetic precursor for tunnel structure cryptomelane. Iron-doped cryptomelane was obtained by the thermal transformation of iron-doped birnessite. The effects of temperature and the amount of iron doping on the thermal transformation from birnessite to cryptomelane were also studied. Iron was evenly doped into the framework without producing other phases if iron doping was less than 10% of the total manganese. Hematite phases appeared if iron doping reached 16% of the total manganese. A doping limit of iron was observed for tunnel structure cryptomelane. X-ray photoelectron spectroscopy and electron paramagnetic resonance were used to analyze the local chemical environment of manganese and iron in cryptomelane, suggesting iron was doped into the framework instead of the tunnels. Iron doping had a great impact on the microstructure and properties of cryptomelane. The amount of K+ in tunnels, the average oxidation state of manganese, and the vibrational frequency of the Mn-O band increased with the amount of iron doping. The thermal stability of cryptomelane was improved by iron doping. The prepared cryptomelane had a large number of basic sites, and the strength of these sites increased with increasing amounts of iron doping.