ASSOCIATION OF SOLUBLE ALUMINUM IONIC SPECIES WITH A SILICA-GEL SURFACE - A SOLID-STATE NMR-STUDY

A series of high-area silica gels treated with well-characterized aluminum solutions, hydrolyzed in the range OH/Al = 0-2, were analyzed by Al-27 and Si-29 solid-state NMR (static, MAS, and CP-MAS). The Al-27 NMR data show that even in the case of nonhydrolyzed solutions, aluminum is incorporated into the silica tetrahedral framework. For samples treated with hydrolyzed solutions, this incorporation becomes substantial (up to Si/Al = 20) and depends very little on the time of contact between solid and aluminum solution. Octahedral surface aluminum is also present; its amount increases with both degree of hydrolysis and solution contact time. In the case of nonhydrolyzed solutions, the octahedral aluminum detected on the solid is fully hydrated and mobile, whereas for hydrolyzed solutions an association of the type inner-sphere complex is formed. The Si-29 NMR results are complementary to these observations as they show, when silica is treated with hydrolyzed solutions, a redistribution of the Si, Al, and proton populations together with a loss in silanol groups. The implication of these findings are discussed in terms of both chemical modifications of the surface and lattice structural variations.