Differences between silica and silica-alumina gels I. Factors affecting the porous structure of these gels

Abstract

By measurement of the particle densities and surface areas of a number of series of silica and silica-alumina gels the effects of several processing variables on their porous structure have been determined. From these data the average particle sizes and pore sizes of the dried gels were calculated. The calculations are based on an assumption that the ultimate particles making up the gels are essentially spherical in shape. The following preparational variables were studied: (a) the pH of the hydrosol, (b) the solids content of the hydrosol, (c) the length of the aging time prior to base exchange, and (d) the nature of the base exchange solution. It was found that these factors had markedly different effects on the two types of gel. The following general conclusions were drawn: (1) Low pH favors higher density and higher surface area for silica gel. This is true both in regard to the pH of formation of the hydrosol and with respect to the pH of the base exchange medium. (2) In the case of silicaalumina gel, lower pH favors higher density and lower surface area. (3) Increased solids content of the hydrosol gave decreased density with both types of gel. However, at the same time the surface area decreased for silica gel, but increased for silica-alumina gel. (4) The effect of increased aging time was to moderate any change which could be produced by subsequent base exchange treatment.