Structural evolution upon heating of sol-gel prepared birnessites

Birnessites containing K⁺ and Na⁺ in the interlayer have been prepared by a sol-gel method using glucose and polyvinyl alcohol as reducing agents. The samples have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analysis (differential and thermogravimetric), FT-IR spectroscopy and specific surface area and texture assessment by nitrogen adsorption at −196 °C. The solids obtained are more crystalline than those prepared by oxidation with H₂O₂ in a basic medium. The samples are thermally stable and evolution upon calcination at high temperature depends on the precise nature of the interlayer cation. So, calcination at 600 °C of a K-birnessite leads to formation of cryptomelane (2×2 tunnel structure), while the Na analogue leads to formation of mixed Mn-Na oxides with different tunnel structures. Ion exchange of interlayer cations takes place topotactically at room temperature; Li⁺ enters easily in the interlayer, while the entrance of Mg²⁺ is more difficult, probably because of its high stability in the hydrated form; increasing of the temperature and contacting time during ion exchange does not improve very largely the process.