Differences between silica and silica-alumina gels II. A proposed mechanism for the gelation and syneresis of these gels

Abstract

An ionic mechanism for the gelation of silica and silica-alumina gels is proposed as explanation of the very rapid gelation occurring within a certain pH range. The intermicellar bonds are hydrogen bonds and the condensation reaction is believed to be a slow reaction. This reaction is the essential feature of the syneresis, or aging, process in silica gel. Interruption of this process by introduction of an acidic medium causes rupture of the hydrogen bonds which, in turn, results in smaller particles, higher density and higher surface area. In the case of silica-alumina gel the properties of the alumina overshadow those of the silica groups. Thus, the most important reaction occurring during syneresis is the rupture of the hydrogen bonds joining alumina groups to silica groups. This reaction, furthermore, is accelerated by hydroxyl ions. As a result, increased syneresis time and/or exchange with slightly basic media give smaller particles and, therefore, greater surface area. Acidic exchange media promote the condensation reaction, thus giving increased particle size. In the case of silica-alumina gels decreased density invariably accompanies increased surface area ( i.e., decreased particle size). This is contrary to the effect found in silica gels. The explanation lies in the fact that the silica-alumina micelles are charged particles. An important consequence of the proposed theory is that the alumina particles must be “terminal” groups. Thus the base exchange properties of such gels do not arise by substitution of A1 for Si in tetrahedral groups.