Selective carbon dioxide adsorption of ε-Keggin-type zincomolybdate-based purely inorganic 3D frameworks

Abstract

Polyoxometalate-based 3D frameworks, Na_1.5H_11.4[ZnMo₁₂O₄₀{Zn₂}]·5.5H₂O and (NH₄)_1.5H_8.5[ZnMo₁₂O₄₀{Zn₂}]·6H₂O, are synthesized in moderate yields. Rotation of the reactor under hydrothermal conditions is essential to improve the yield. The materials show zeolite-like selective molecule adsorption properties. Depending on the micropore aperture size of the materials, small molecules can be adsorbed in the materials, while large molecules cannot. The enthalpy of adsorption and DFT calculation indicate that the materials strongly interact with CO₂, but weakly interact with CH₄, due to electrostatic interactions between the materials and molecules. CO₂/CH₄ co-sorption experiments show that the materials can selectively adsorb CO₂, and CO₂ adsorption selectivity of the material with sodium cations is higher than that of the material with ammonium cations. The material with sodium ions can be utilized for gas chromatographic separation of CH₄ and CO₂.