Direct synthesis of metal-chelating mesoporous silica: Effects of added organosilanes on silicate formation and adsorption properties

SBA-15 mesoporous silicates with surface functional groups were synthesized by cohydrolysis of tetraethoxyorthosilicate and ethyl-, carboxylate-, and ethylenediaminetriacetic acid-functionalized organosilanes (ETES, CTES, EDATAS, respectively) using the nonionic surfactant poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (PEO20PPO70PEO20, Pluronic P123). X-ray diffraction revealed that silicates synthesized with up to 5% (wt/wt total silica) of added functionalized silane yielded ordered mesoporous materials with P-6mm hexagonal symmetry. Further increasing the amount of added functionalized silanes to 20% resulted in a significant decrease in the structural ordering of the resulting silicate. Surface areas, pore volumes, and pore diameters were determined from nitrogen gas adsorption/desorption isotherms. Pore size contraction was observed as the wt % of added ETES was increased but not for the silicates formed with added CTES or EDATAS, with the exception of the silicate formed using 20% added EDATAS. Inclusion of 1,3,5-trimethylbenzene (TMB) during synthesis resulted in silicates with larger pore sizes but a loss of structural order. Appreciable adsorption of copper ions from solution was observed only for the EDATAS-functionalized silicates. X-ray photoelectron spectroscopy of Cu2+-bound EDATAS-functionalized silicates revealed an Cu/N ratio of 0.15, smaller than expected for 1:1 stoichiometry of copper ions and etylenediaminetriacetic acid groups. Adsorption isotherms for Cu2+ binding to EDATAS-functionalized silicates were fit to a double-component Langmuir equation. Binding constants, but not capacity, were dependent on the amount of added EDATAS used in the synthesis and were several orders of magnitude smaller than that reported for structurally similar nonimmobilized HEDTA.