HYDROLYSIS OF (GAMMA-AMINOPROPYL)TRIETHOXYSILANE-SILYLATED IMOGOLITE AND FORMATION OF A SILYLATED TUBULAR SILICATE-LAYERED SILICATE NANOCOMPOSITE

The external AlOH and internal SiOH surfaces of the tubular aluminosilicate imogolite have been silylated by reaction with hydrolyzed (gamma-aminopropyl)triethoxysilane (APS) in acidic aqueous solution. By monitoring the depletion of APS from the imogolite surfaces upon dialysis against distilled water at 25-degrees-C, we observed two pseudo-first-order hydrolysis processes. The faster depletion process with k(obs) = 6.7 +/- 0.9 x 10(-2) h-1 was attributed to the hydrolysis of APS from the external AlOH surfaces of the tubes. This assignment was consistent with the depletion behavior of silylated gamma-alumina at comparable APS loadings in the range 46-78 wt % APS. Also, proton-decoupled Si-29 MAS NMR spectroscopy indicated that APS-silylated imogolite and gamma-alumina possess similar siloxane polymer structures. The slower APS depletion process, for which k(obs) = 1.4 +/- 0.1 x 10(-2) h-1, was assigned to the hydrolysis of APS from the internal SiOH surfaces of the imogolite tubes. The reaction of the fully silylated tubes with Na+-mont-morillonite afforded a tubular silicate-layered silicate nanocomposite in which the imogolite tubes and clay plates were irregularly aggregated, as judged by X-ray diffraction. Selective hydrolysis of APS from the external imogolite surfaces occurs upon washing of the irregularly aggregated products. This resulted in the formation of a well-ordered tubular silicate-layered silicate intercalation compound in which only the internal surfaces of imogolite were silylated.