Sol–Gel Synthesis and Characterization of Silica Polyamine Composites: Applications to Metal Ion Capture

Abstract

A sol-gel method has been developed for the synthesis of composite materials analogous to the previously reported and commercialized silica polyamine composite (SPC) materials made from amorphous silica. Monolithic xerogels were formed using a two-step procedure with no templating agent using acid catalyzed followed by base catalyzed hydrolysis. This reaction was followed by (1)H NMR. Initial sol-gels were formed using a methyltrimethoxysilane (MTMOS) and 3-chloropropyltrimethoxysilane (CPTMOS) mixture. Elemental analyses and (13)C CPMAS NMR confirmed incorporation of both monomeric units into the surface structure. Some control over surface morphology was achieved by adjusting synthetic conditions. The resulting xerogels were reacted with poly(allylamine) (PAA) to give composite materials which showed much lower metal ion capacities than the commercially available amorphous silica analogs. The low degree of reaction of the chloropropyl groups indicated they were not surface-available to the polyamine. Addition of tetramethoxysilane (TMOS) produced a structural matrix and resulted in higher chloride utilization (reaction of surface chloropropyl groups with the polyamine). The ratio of the three siloxane monomeric components was varied until the resulting polyamine composite xerogels had metal capacities comparable with the commercialized SPC materials. These composites had narrower average pore size distributions and fewer small pores. Further modification of these composites with metal selective ligands showed material characteristics similar to those of commercially available SPC materials. Subjecting a composite made by the sol-gel route to one thousand load-strip cycles with Cu(2+) shows essentially no loss in metal capacity, and this robustness compares favorably with that observed for the SPC made from amorphous silica gels.