Preparation and characterization of a newly constructed multifunctional Co(ii)–organic framework: proton conduction and adsorption of Congo red in aqueous medium

Abstract

A novel multi-functional Co(II)–organic framework {[Co₂(SBTB)(BDBM)₂(H₂O)]·3H₂O}_n (Co-MOF 1) was successfully synthesized through the assembly between Co²⁺ and 5-sulfo-1,2,4-benzoic acid (H₄SBTB) in the presence of an auxiliary flexible ligand, 1,1′-(1,4-butadiyl)dibenzimidazole (BDBM). 1 shows a 2D wave-like layered structure in which the discrete hydrogen-bonded water tetramer exists. The solid-state proton-conductive properties of 1 as well as its composite Co-MOF–Nafion membrane were studied by alternating current (AC) impedance spectroscopy. The composite Co-MOF–Nafion membrane shows tunable proton conduction under various pH and temperature conditions. Interestingly, the composite Co-MOF–Nafion membrane demonstrates a pH-dependent proton-conducting mechanism. The proton transfer in Co-MOF 1 follows a typical Grotthuss mechanism at pH 3.0 and a typical vehicle mechanism at pH 5.0 and 7.0. In addition, adsorption of highly toxic Congo red (CR) over Co-MOF 1 was studied in view of adsorption kinetics and isotherms. The effects of initial pH, concentration and adsorption time on the adsorption of CR were investigated. It was found that the pseudo-second-order kinetic model and Langmuir adsorption isotherm matched quite well for the adsorption of CR onto Co-MOF 1, indicating that CR is adsorbed on the Co-MOF surface in a single layer. The observed maximum adsorption capacity for CR removal is 1100 mg g⁻¹. Moreover, 1 can adsorb CR selectively in the presence of methylene blue (MB) or methyl orange (MO).