Ordered mesoporous silica (SBA-15) derivatized with imidazole-containing functionalities as a selective adsorbent of precious metal ions

Organic ligand containing the imidazole [N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole], precious metal-chelating functionality, was immobilized on the surface of ordered mesoporous silica, SBA-15. The resulting functionalized ordered mesoporous silica (Imi-SBA-15) was characterized and tested as an effective adsorbent for Pt2+ and Pd2+. N-2 adsorption/desorption measurements and small angle X-ray scattering spectroscopy (SAXS) demonstrate both the maintenance of pore symmetry (space group p6mm) of SBA-15 and the introduction of the imidazole functional group inside the mesopore channel. The functionalized material shows an exceptional selectivity for Pt2+ and Pd2+ ions over other metal ions such as Ni2+, Cu2+, and Cd2+, even in the presence of an excess of the latter. Adsorption capacities of the imidazole functionalized mesoporous silica for Pt2+ and Pd2+ are comparable to those of polymeric adsorbents. Moreover, as-synthesized adsorbent shows a faster adsorption rate than the polymeric one. Adsorbed precious metal ions can be readily recovered by simple acid (HNO3) treatment. The results suggest that the hydrophilic ligand-immobilized mesostructured adsorbent prepared here could be a good alternative to ligand-immobilized organic polymers that are presently used for the adsorption of precious metals.