Selective Surfaces: Quaternary Co(Ni)MoS-Based Chalcogels with Divalent (Pb²⁺, Cd²⁺, Pd²⁺) and Trivalent (Cr³⁺, Bi³⁺) Metals for Gas Separation

Abstract

Porous chalcogels with tunable compositions of Co_<i>x</i>M_1<i>–x</i>MoS₄ and Ni_<i>x</i>M_1–<i>x</i>MoS₄, where M = Pd²⁺, Pb²⁺, Cd²⁺, Bi³⁺, or Cr³⁺ and <i>x</i> = 0.3–0.7, were synthesized by metathesis reactions between the metal ions and MoS₄^2–. Solvent exchange, counterion removal and CO₂ supercritical drying led to the formation of aerogels. All chalcogels exhibited high surface areas (170–510 m²/g) and pore volumes in the 0.56–1.50 cm³/g range. Electron microscopy coupled with nitrogen adsorption measurements suggest the presence of both mesoporosity (2 nm < <i>d</i> < 50 nm) and macroporosity (<i>d</i> > 50 nm, where <i>d</i> is the average pore size). Pyridine adsorption corroborated for the acid character of the aerogels. We present X-ray photoelectron spectroscopic and X-ray scattering evidence that the [MoS₄]^2– unit does not stay intact when bound to the metals in the chalcogel structure. The Mo⁶⁺ species undergoes redox reactions during network assembly, giving rise to Mo^4+/5+-containing species where the Mo is bound to sulfide and polysulfide ligands. The chalcogels exhibit high adsorption selectivities for CO₂ and C₂H₆ over H₂, N₂, and CH₄ whereas specific compositions exhibited among the highest CO₂ enthalpy of adsorption reported so far for a porous material (up to 47 kJ/mol). The Co-Pb-MoS₄ and Co-Cr-MoS₄ chalcogels exhibited a 2-fold to 4-fold increase in CO₂/H₂ selectivity compared to ternary CoMoS₄ chalcogels