Effect of framework and layer substitution in manganese dioxide related phases on the energetics

The energetics of a set of MnO₂-related phases having a framework or a layer structure were determined by high-temperature drop solution calorimetry at 977 K using sodium molybdate as a solvent. Enthalpies of formation were calculated from the measured enthalpies of drop solution. In these materials, different metal cations can be incorporated through framework, tunnel, and interlayer substitution. The stability of the structure depends mostly on the nature and the amount of the tunnel (or interlayer) cations. The enthalpy of formation of small-OL-1(Na_0.04) is more exothermic than that of large-OL-1(Na_0.26). However, framework substitution of Ni or Cu destabilizes the structure; [doped]-OL-1 or [doped]-OMS-1 materials are less stable in enthalpy than OL-1 or OMS-1. The energetics are more sensitive to the cations incorporated into the tunnels than to the cations substituting in the framework. A trend has been observed between the enthalpy of formation of the doped materials from the oxides and the ionic radius of the dopant. The average oxidation state of the manganese does not correlate directly with the energetics. These results confirm and extend previous work on manganese dioxide. The similarity of energetics of many different structures shows that the synthesis of these metastable microporous materials is not limited by energetic constraints.