Vapor phase versus liquid phase grafting of meso-porous alumina

Functionalization of meso-porous c-alumina has been performed by grafting of 3-Aminopropyltrimethoxysilane (3APTMS) simultaneously from either the liquid phase or from the vapor phase. In both cases, after grafting nitrogen physisorption indicates that the materials remain meso-porous with significant differences in pore morphology, and infrared spectroscopy analysis reveals the presence of adsorbed APTMS in different quantities. Grafting from the liquid phase is affected by pronounced oligomerization of the precursor molecules, resulting in relatively non-homogeneous grafting and ultimately in pore blocking. In contrast, grafting via the vapor phase results in a homogeneous layer of grafted molecules on the surface of the pores, with a thickness similar to the length of the 3APTMS molecule. Grafting via the vapor phase is relatively insensitive to the reaction temperature (50 or 80 C) and the presence of water (50 ppm). This is explained by the low concentration of oligomers in the vapor phase, due to their low vapor pressure. This study clearly shows that for obtaining a thin layer on the pore surface of c-alumina vapor phase silanization is highly preferable.