A Cationic Metal-Organic Solid Solution Based on Co(II) and Zn(II) for Chromate Trapping

We report the synthesis and characterization of a solid solution series of cationic metal-organic materials with full compositional range from pure Co(II) to Zn(II) end-members. The materials consist of [ZnxCo1-x(H2O)(4)(4,4'-bipy)(2)](2+) metal-organic clusters that pi-pi stack into 2-D positively charged layers, with the metal ratio tunable by molar ratio under hydrothermal conditions. The interlamellar alpha,omega-alkanedisulfonate serves as an anionic template and non-covalently interacts with the cationic layers. The weak interaction allows anion exchange for toxic oxometal anions, such as chromate, CrO42-. The highest chromate adsorption capacity was 68.5 mg/g (0.43 mol/mol) for the as-synthesized 50 mol % Co(II)-incorporated material. Our cationic material can also selectively trap these toxic oxo-anions when nontoxic anions (e.g., nitrate, sulfate) were present in an over 50-fold excess concentration.