Synthesis of amino-functionalized MCM-41 via direct co-condensation and post-synthesis grafting methods using mono-, di- and tri-amino-organoalkoxysilanes

Amino-functionalized mesoporous MCM-41 materials were synthesized directly by co-condensation of 3-aminopropyltrimethoxysilane (N silane), [1-(2-aminoethyl)-3-aminopropyl] trimethoxysilane (NN silane) or 1-[3-(trimethoxysilyl) propyl] diethylenetriamine (NNN silane) with tetraethyl orthosilicate. The maximum proportions of N, NN and NNN silanes in the Si sources for obtaining the hexagonally structured functionalized silica were 0.5, 0.4 and 0.4, respectively. The Si-29 MAS NMR measurements of the materials thus obtained indicate that the Si atoms containing Si-C bonds accounted for 26.0, 17.0 and 7.8 Si mol% of the total silicons, respectively. By comparing with the results from the elemental analyses and the argentometric titrations of the surfactant-extracted samples, it was revealed that all the amino moieties incorporated were not present on the surface, but some of them were in the wall of the hexagonal channels. Amino-functionalized MCM-41 samples were also synthesized via a post-synthesis grafting on dehydrated MCM-41. At the maximum level of silylation with N, NN and NNN silanes, Si atoms containing Si-C bonds of about 20.6, 19.2 and 17.9 Si mol%, respectively, were obtained, indicating that the surface coverage of amino-organoalkoxysilanes anchored to silanol groups on the surface of MCM-41 were almost the same, irrespective of the size of the amino-organoalkoxysilanes. The adsorption experiments of Co2+ and Fe3+ were conducted in order to comparatively investigate the activity and the location of the amino-organic moieties introduced via the two methods, and some differences in the activity of amino groups derived from N, NN and NNN silanes were found. The cations adsorbed on the amino-functionalized samples via the co-condensation method were increased with an increase in the surface density of amino groups regardless of the amino-organoalkoxysilanes. When N and NN silanes were grafted onto the surface of MCM-41, the adsorption capacity was increased with an increase in the surface density of amino groups. However, when NNN silane was anchored to the surface, the adsorption capacity was decreased with an increase in the surface density of the amino groups. These results imply that the locations of the amino groups introduced to the silica via the direct co-condensation and the post-synthesis grafting methods are clearly different.