Structural and chemical disorder of cryptomelane promoted by alkali doping: Influence on catalytic properties

Cryptomelane-type manganese oxide with cesium and lithium incorporated in its tunnel structure is a highly active catalyst for the oxidation of ethyl acetate. [Display omitted] ► Li and Cs were successfully incorporated in the cryptomelane structure. ► Alkali metals doping enhances both the reducibility and the basicity of cryptomelane. ► These parameters have a significant effect on the catalytic properties. ► Li and especially Cs-doped catalysts are highly active for ethyl acetate oxidation. ► The amounts of Cs or Li incorporated do not influence the performance (pore mouth activity). Different amounts of cesium (0.8–1.3at.%) and lithium (up to 0.5at.%) were introduced in the tunnel structure of cryptomelane in order to tailor its physical and chemical properties. The mechanism by which cryptomelane accommodates these alkali cations is different; Li occupies mainly empty sites by a redox-type reaction, while Cs is ion-exchanged by H3O+ cations. The adsorption of these alkali metals into cryptomelane was highly selective and is dictated by the solvation energy and ionic radius. Extrinsic defects (e.g. positive holes and oxygen vacancies) are created upon doping, enhancing the redox-properties of cryptomelane. The basicity of the framework also increases when Cs and Li are incorporated. The catalytic properties of this material were tested in the oxidation of ethyl acetate, and greatly improved after doping. The observed activity increase is explained by the redox and basic properties of the modified materials. The amount of cesium loaded has no effect on the activity, as a result of pore mouth catalysis.