Chalcogels: Porous metal-chalcogenide networks from main-group metal ions. Effect of surface polarizability on selectivity in gas separation

We report the synthesis of metal-chalcogenide gels and aerogels from anionic chalcogenide clusters and linking metal ions. Metal ions such as Sb ³⁺ and Sn²⁺, respectively chelated with tartrate and acetate ligands, react in solution with the chalcogenide clusters to form extended polymeric networks that exhibit gelation phenomena. Chalcogenide cluster anions with different charge densities, such as [Sn₂S ₆]^4- and [SnS₄]^4-, were employed. In situ rheological measurements during gelation showed that a higher charge density on the chalcogenide cluster favors formation of a rigid gel network. Aerogels obtained from the gels after supercritical drying have BET surface areas from 114 to 368 m²/g. Electron microscopy images coupled with nitrogen adsorption measurements showed the pores are micro (below 2 nm), meso (2-50 nm), and macro (above 50 nm) regions. These chalcogels possess band gaps in the range of 1.00-2.00 eV and selectively adsorb polarizable gases. A 2-fold increase in selectivity toward CO₂/C₂H₆ over H₂ was observed for the Pt/Sb/Ge₄Se₁₀- containing aerogel compared to aerogel containing Pt₂Ge ₄S₁₀. The experimental results suggest that high selectivity in gas adsorption is achievable with high-surface-area chalcogenide materials containing heavy polarizable elements.