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 Å2 and a narrow mouth hydrophobic channel of dimension 2.33 × 1.926 Å2. 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′.