MOFs containing a linear bis-pyridyl-tris-amide and angular carboxylates: exploration of proton conductivity, water vapor and dye sorptions

Abstract

Exploration of a new type of water-stable proton conducting material is of great interest owing to its applications as an electrolytic material in proton exchange membrane fuel cells (PEMFC). Herein, we report four such new metal organic frameworks of Cd(II) and Co(II) containing a new linear bis-pyridyl-tris-amide ligand (L) and angular dicarboxylates 4,4′-oxybisbenzoic acid (OBA) and benzene-1,3-diacrylic acid (BDA) as linkers. The MOFs are found to exhibit a general formula of {[M(L)(dicarboxylate)]·xDMF·yH₂O}_n with varying amounts of solvent. Two MOFs containing BDA exhibited two-dimensional rectangular grid-like networks with grid dimensions of 25 × 23 Ų, while those with OBA exhibited 3D-networks consisting of the mab topology with rectangular channels of dimensions 31 × 21 Ų and 31 × 17 Ų. All four exhibited a solvent accessible volume of 36–49%. AC impedance analyses on MOFs 1–4 reveal that they exhibit appreciable proton conductivities (PC): σ = 2.2 × 10⁻³, 9.5 × 10⁻⁴, 1.2 × 10⁻³ and 6.6 × 10⁻⁴ S cm⁻¹, respectively, at 353 K and 98% relative humidity (RH). The high PC values could be attributed to the hydrogen bonding between water molecules and also with amides and carboxylates of the framework. Furthermore, a gas sorption study revealed that MOFs 1–4 show preferential adsorption of water vapor (195–330 cc g⁻¹ at p/p₀ ∼ 1) over nitrogen given the hydrophilic nature of the channels and the stability of MOFs under humid conditions. In addition, all these materials have shown capability in selectively adsorbing cationic dyes (methylene blue, MB and crystal violet, CV).