Microporous mixed-metal mixed-ligand metal organic framework for selective CO2 capture

Abstract

A microporous mixed-ligand-based metal–organic framework has been synthesized using two different dicarboxylic acid-based ligands (4,4′-biphenyldicarboxylate (BPDC) and imino diacetate (IMDA)) and two different metal ions (Ce³⁺ and Na⁺), namely Ce₃Na₃(BPDC)₃(IMDA)₃·(DMF)₂(H₂O)₉. The framework built from Ce–Na–carboxylate layers and BPDC pillars consists of 2D slit-shaped pores occupied by extra-framework Na⁺ ions. The desolvated framework is permanently porous with a BET surface area of ∼771 m² g⁻¹ and displays moderate CO₂ uptake of 2.0 mmol g⁻¹ with a CO₂/N₂ selectivity (S) of 68 at room temperature and 1 bar. A modest heat of adsorption (23 kJ mol⁻¹) and smooth diffusion kinetics are observed, as reflected in the facile CO₂ cycling. Using GCMC methods, the CO₂ adsorption isotherm at 298 K was simulated, which matches the experimental isotherm well. The CO₂ positions observed from the simulations showed that Na⁺ ions in the channels serve as favorable adsorption sites for the oxygen atoms in CO₂ pointing toward the Na⁺ ions (OCO⋯Na⁺ = 3.34–5.87 Å), while some CO₂ molecules sit flat on the phenyl rings of the BPDC at a CO₂⋯centroid distance of 3.6–3.7 Å.