Two urea-functionalized pcu metal–organic frameworks based on a pillared-layer strategy for gas adsorption and separation

Abstract

Herein, by using pillaring ligands with different lengths, two copper-based metal–organic frameworks, compounds 1 (Cu₂(bada)₂(dabco)) and 2 (Cu₂(bada)₂(bipy)) (bada = 4,4′-carbonylbis(azanediyl)di-benzoic acid, dabco = 1,4-diazabicyclo[2.2.2]octane, bipy = 4,4′-bipyridine) with the urea-functionalized ligand bada, have been successfully synthesized using a pillared-layer strategy. Both 1 and 2 exhibit two-dimensional square lattice (sql) layers pillared by dabco and bipy, respectively, and feature pcu topology. Different to dabco in 1, the pillaring ligand 4,4′-bipyridine with longer length forms a doubly interpenetrated structure of 2. Both compounds display good thermal stabilities and permanent porosities. 1 exhibits good performance for CO₂ (32 cm³ g⁻¹), C₂H₆ (58 cm³ g⁻¹), and C₃H₈ (105 cm³ g⁻¹) capture among MOFs without open metal sites. In particular, 1 possesses high Q_st for CO₂ (32.8 kJ mol⁻¹), C₂H₆ (37.2 kJ mol⁻¹), and C₃H₈ (40.3 kJ mol⁻¹) adsorption and outstanding selectivities for CO₂ (5.7), C₂H₆ (28), and C₃H₈ (224) over CH₄ at 298 K. Density functional theory (DFT)-based calculations further prove the hydrogen-bond interactions and σ–π interactions between gas molecules and urea-based ligands. 2 exhibits a gate-opening behavior for N₂, Ar and CO₂. Compound 1 could be seen as a significant potential material for gas storage and separation.