Dissolution of silica and formation of a dispersed phase induced at low pH by the association of soluble aluminum ionic species with solid silica gel

It is shown that the overall behavior of silica in the presence of hydrolyzed Al solutions (pH range 3 - 4) is drastically modified: the total amount of dissolved silica and the initial rate at which silica passes into solution are increased compared to what is observed for a porous silica in water. By using a simple spectrophotometric approach, an estimate of the amount of monomeric and polymeric silica aqueous species can be obtained as a function of time. For all hydrolyzed solutions, the presence of a dispersed phase is also observed. Furthermore, when it is treated with these Al solutions the initial silica gel has its texture, surface charge, and global reactivity modified. This complex chain of events which occurs at acidic pH is in fact triggered by the presence in solution of highly charged Ails polymeric cations, which after adsorption onto the silica sample depolymerize in a process which involves the surface hydroxyls. This leads to the inclusion of tetrahedral Al in the silica famework and the presence of surface-coordinated octahedral Al. This globaly leads through a weakening of the Si-O bonds to an increased release of silica into solution. The Al cations left in solution interact with the aqueous Si species, leading with time to the formation of a metastable silica-aluminum dispersed phase which differs strongly from the treated silica beads. These interactions between Al hydrolyzed solutions and solid silica are therefore quite effective in spatially redistributing the initial silica beads. Many of these conclusions have been made possible by the use of solid-state NMR.