Functionalization of silica surfaces with mixtures of 3-aminopropyl and methyl groups

The surface of mesoporous M41S silica material was functionalized with 3-aminopropyl and methyl groups and with mixtures of both. The ratio of the two groups on the silica surface could be adjusted by changing the molar ratio of the corresponding alkoxysilanes in the reaction mixture. The fraction of 3-aminopropyl on the surface was always larger than in the functionalization reaction mixture. Moreover, functionalization with the 3-aminopropyl group or with mixtures of the 3-aminopropyl and methyl groups led to higher molar coverages of the silica surface than functionalization with the methyl group alone. One reason for these effects is that the amino group base-catalyzes the hydrolysis of the alkoxysilanes and thereby enhances the reaction of the silanes with the silica surface. The 3-aminopropyl/methyl-functionalized materials showed a very high stability against hydrolysis in boiling water because the siloxane bonds of the silica framework were protected by the organic moieties.