Stepwise Assembly of Surface Imprint Sites on MCM-41 for Selective Metal Ion Separations

A new stepwise surface functionalization methodology has been developed to synthesize mesoporous sol-gel sorbents containing the ethylenediamine functionality. The N-[3-(trimethoxysilyl)-propyl]ethylenediamine (TMSen) ligand is first coated on MCM-41, followed by sorption of Cu(II). A second exposure to TMSen results in the formation of a 2:1 TMSen:Cu(II) complex on the surface of the mesopores. Acid washing protonates the amino groups which release the copper ions. This results in the formation of binding sites that are uniquely designed with the coordination environment Cu(II) prefers. The mesoporous MCM-41 used for coating was synthesized under basic conditions using cetyltrimethylammonium bromide (CTAB) as the structure directing agent. Mesopores are formed by the calcination of the surfactant micelles giving this material relatively large surface areas and good mass transfer. A copper(II) imprinted gel has shown significant enhancement of copper(II) uptake capacities at various concentrations relative to a non-imprinted gel made without the metal ion template. The imprinted material also exhibits improved selectivity for removal of copper(II) ions from Cu(II)/Zn(II) aqueous solutions. This material has copper distribution coefficients (K_d) > 100,000 at concentrations less than 20 ppm. Separation factors, k > 100 w.r.t. copper(II), were achieved with an aqueous Cu(II)/Zn(II) system at pH 5.0.