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

Effect of partially neutralized aluminum solutions on the texture and pore structure of silica

N-2 adsorption measurements at 77 K were performed on two types of silica of different porosities, and on their products obtained by soaking in partially neutralized aluminum solutions of varying R = [OH]/[Al] = 0.5-2.0 (PH 3-4). TG analysis showed that the Al taken by the mesoporous silica was accompanied by its hydroxyls, but with the predominantly microporous silica it was stripped of some of them, where a strong potential field in the narrow pores compensated for any charge differences. IR spectral analysis showed that for microporous silica the peak (due to Si-O stretching of surface OH) is located at 950 cm(-1) instead of approximately 970 cm(-1) in the case of mesoporous silica, which seems to result from the strong perturbation between hydroxyls at close range in the narrow pores. A reduction in this peak lakes place upon Al treatment that increases with R, with the probable formation of a surface complex of the type =Si-O-Al. Uptake of Al ions produced a narrowing of the pores, that increased with the increase in Al content. From pore structure analysis, the micropore fraction could be sorted into ultramicropores (less than or equal to 10 Angstrom) and supermicropores (10-20 Angstrom). The volume of the former is almost unaffected by any change in Al content. The sizes of the supermicropores and mesopores (greater than or equal to 20 Angstrom) are crucial to the Al uptake. Changes in pore sizes at R greater than or equal to 1.0 facilitate the initial attack of the large Al polymeric species