The effect of chelating anions on the retention of Co(II) by γ-alumina from aqueous solutions under the unadjusted pH condition of supported catalyst preparation

This study analyzes the effect of the addition of acetate, citrate, and nitrilotriacetate anions on the retention of Co(II) cations by the gamma-alumina surface in view of the preparation of alumina supported cobalt catalysts. The emphasis was placed on the way the Co(11) species attach to the solid surface when adsorbed from aqueous solutions under the unadjusted pH condition. The individual adsorption isotherms onto gamma-Al2O3 support for cobalt and a given ligand were determined by following the solution depletion method in single-solute and bi-solute systems. These adsorption data were supplemented by the results of potentiometric titrations. In the case of bi-solute systems, the adsorption procedures allowed either co-impregnation of gamma-alumina with equimolar solutions of cobalt and ligand salts or pre-impregnation of gamma-alumina with the ligand anions and the subsequent adsorption of cobalt. Changes in the pH of the equilibrium solid-liquid suspension were also monitored along the adsorption isotherms. The adsorption of Co(II) onto gamma-Al2O3 in the presence of acetate and nitrilotriacetate led to the formation of the type A (i.e., solid-metal-ligand) ternary complexes. The use of citrate anions together with Co(II) cations was shown to improve the impregnation process through the formation of ternary complexes of type B (i.e., solid-ligand-metal). The comparison with a system containing tricarballylate anions allowed concluding that the presence of the hydroxyl group in the citrate anion enhanced its affinity for the alumina surface by contributing to the inner-sphere character of its surface-bound complexes. (C) 2018 Elsevier Inc. All rights reserved.