New short-channel SBA-15 mesoporous silicas functionalized with polyazamacrocyclic ligands for selective capturing of palladium ions in HNO3 media

Abstract

In this study, a new kind of short-channel SBA-15 mesoporous silica decorated with polyazamacrocyclic ligands was developed, showing selective binding ability to palladium ions based on host–guest interaction. The established synthesis protocol involved the co-condensation synthesis of an SBA-15 precursor with halogen atoms uniformly incorporated in the mesoporous silica matrix, followed by the anchoring of 1,4,7,10-teraazacyclododecane (Cyclen) ligands via post-grafting. Due to the short straight channels and large pore size facilitating the diffusion of the molecules and ions, the mesoporous silicas were found to possess a high density of the functional Cyclen ligands, as well as high adsorption capacity of Pd(II) in HNO₃ solutions. The structure and morphology of the Cyclen functionalized mesoporous silicas were fully characterized. And, the adsorption behavior toward Pd(II) was investigated combined with the theoretical interpretation of the experimental data based on typical kinetic equations, isotherm models and thermodynamic equations. Furthermore, the detailed coordination mechanism between the Cyclen ligands and Pd(II) was examined by high resolution X-ray photoelectron spectroscopy (XPS). A suggested mechanism involving the synergistic effect of four cyclic amines in the Cyclen ligands was proposed to describe the coordination to Pd(II) in HNO₃ solutions. Overall, this work provides a facile and effective pathway to build polyazamacrocycle ligand decorated mesoporous silicas with short-channels and large pores, which might be potentially used for molecule recognition and selective enrichment of precious metals.