Bi-porous metal–organic framework with hydrophilic and hydrophobic channels: selective gas sorption and reversible iodine uptake studies

Abstract

A new three-dimensional (3D) bi-porous metal–organic framework (MOF), {[(Me₂NH₂)₂]·[Cd₃(5-tbip)₄]·2DMF}_n (1) (5-tbipH₂: 5-tert-butylisophthalic acid) has been synthesized. The 5-tbip ligand containing a hydrophobic t-butyl group and hydrophilic carboxylate groups is used to synthesize the bi-porous framework. This 3D MOF contains two types of channels, a wide mouth hydrophilic channel of dimension 7.448 × 7.676 Ų and a narrow mouth hydrophobic channel of dimension 2.33 × 1.926 Ų. Hydrophilic channels are lined with the orderly arranged dimethyl ammonium (DMA) cations which neutralize the anionic 3D framework. The guest-free form of the MOF (1′) showed interesting CO₂ selectivity over other gases such as N₂, CH₄, and H₂. Since the effective pore size of the desolvated compound 1′ (∼0.6 nm: from the pore size distribution curve of CO₂ adsorption measurement) is much more than the kinetic diameter of all measured gases (CO₂ = 3.3 Å, CH₄ = 3.76 Å, N₂ = 3.64 Å and H₂ = 2.8 Å), selective capture of CO₂ by 1′ could be ascribed to the strong electrostatic interaction of CO₂ with the framework. Compound 1′ also shows reversible iodine uptake (1′ ⊂ 4I₂: based on the sample weight measurements and TGA data) with visible color change of the compound. Interestingly, the iodine-loaded MOF showed ∼76 times increase in electrical conductivity compared to 1′.