An Infrared Study of the Amine-Catalyzed Reaction of Methoxymethylsilanes with Silica

Abstract

A thin film infrared technique was used to investigate the reaction of methoxysilanes and amines with the silica surface. The low-frequency region contains bands due to Si–O–Si modes that are used to distinguish between hydrogen-bonded and chemisorbed species. It is shown that the competitive adsorption of amines and CH3OSi(CH3)3 differs from the results obtained using (CH3O)2Si(CH3)2 or (CH3O)3SiCH3. The monomethoxysilane does not displace preadsorbed triethylamine whereas the triethylamine is displaced from the surface by both (CH3O)2Si(CH3)2 and (CH3O)3SiCH3. In the reverse sequence, the triethylamine displaces all three methoxysilanes on the surface. When 1:1 mixtures of methoxysilanes and triethylamine (or propylamine) are co-added to silica, the amine preferentially adsorbs and is only displaced by subsequent chemisorption of the silane. The implication of these results for using a two-step amine-catalyzed reaction of methoxysilanes on silica is discussed.