PORE STRUCTURE EVOLUTION IN SILICA-GEL DURING AGING/DRYING .4. VARYING PORE FLUID PH

In an effort to understand the various phenomena which occur during aging of silica gels at different pH, silica particles prepared by three routes distinguished principally by their silicon coordination (Q(n) distributions) (Stober silica spheres, Ludox and Cab-O-Sil) were employed. In situ techniques were used including proton spin-lattice relaxation, SAXS and zeta potential for the wet state and Si-29 MAS-NMR, SEM, TEM and N-2 sorption for dry samples. In solution, the surface area increased in the following order after aging at pH 10 as compared with that at pH 7 and the initial dry surface area: Cab-O-Sil < Ludox << Stober which is inversely proportional to the degree of silicon coordination. This same trend was observed for the surface charge measurements. Surface areas calculated from Si-29 MAS-NMR for four different Stober sphere sizes were of order 1000 m(2)/g as compared with nitrogen/BET surface areas which varied from 9 to 370 m(2)/g, indicating significant internal surface which is inaccessible to nitrogen. The Si-29 NMR results for the Stober spheres were consistent with a close packed aggregate of similar to 3 nm primary particles. In addition to particulate silica, aging at varying pH of a two-step acid/base-catalyzed silica gel was studied. At all pH values, the surface area was higher in the wet gel as compared with the xerogel. Aging at higher pH was found to yield lower surface area, larger pore volume and a narrower pore size distribution depending upon the pH, aging time and surface tension of the final pore fluid.