Mixed bifunctional surface-modified silicas using tethered aminofunctional silane catalysts

Abstract

Amines are known to catalyse silica surface reactions with alkoxy functional organosilanes. In this work, a substoichiometric amount of an aminofunctional silane is bound to the silica surface by reaction with available silanols. This surface bound amine is used to catalyse the reaction of nearby silanols in its vicinity. The result is a bifunctional modified silica surface with two different functional groups intimately mixed at the molecular level. Three types of silicas were modified with a substoichiometric amount of different aminofunctional silanes possessing various structures. These amino-modified silica surfaces were then exposed to a relatively unreactive alkylsilane that requires the presence of an amine for catalysis of the surface reaction. Select materials were characterized by FTIR, TGA, as well as 29Si and 13C solid state NMR. Elemental analysis data was obtained for all materials after aminosilane reactions and after alkylsilane reaction, with and without the presence of surface bound aminosilane present, for quantitative analysis. Each surface bound aminosilane catalyses the reaction of approximately one alkylsilane. An aminosilane that is bound with one bond to the surface catalyses a greater number of alklylsilanes per aminosilane. Surface modified aminosilanes on nanoparticulate fumed silica on average catalyse a greater number of alkylsilanes compared to silica gels. This is the first report demonstrating the feasibility of using a surface bound aminosilane as a catalyst to synthesize intimately mixed bifunctional silica surfaces.