Synthesis of microporous silica spheres

Spherical microporous silica powders with a narrow size distribution have b een prepared by a precipitation technique involving the hydrolysis reaction of a silicon alkoside in ethanol. The formation of the important microporo sity has been investigated following two templating methods: the co-hydroly sis and condensation of two alkoxides, one of which presents porogen functi on, and the adsorption of an organic compound (glycerol) as the porogen. In both processes, the organic porogen is removed by a simple calcination. In the first method, the addition of more than 20 mol% of the porogen alkoxid e, necessary for generating enough microporosity, disturbs completely the c ondensation process resulting in microporous, nonuniform silica particles o f large size distribution. The best result has been obtained with the glyce rol method where submicrometer-sized silica spheres with a very narrow size distribution and about 40 vol% porosity have been synthesized. The presenc e of glycerol during the synthesis considerably affects the precipitation m echanism, resulting in a larger mean particle size. The use of an aggregati ve growth model has successfully been employed to explain the effect of the porogen during particle formation. The precipitation mechanism of silica i nvolves the aggregation between particles of similar size until a critical size is reached, resulting in a uniform particle size distribution. In the presence of glycerol, it has been shown that a second aggregative growth be tween still-nucleating primary particles and large particles occurred with increasing reaction time. This second aggregative growth appears at an inte rmediate stage of the precipitation process and is due to both the precipit ation of smaller primary particles and the destabilization of the colloidal stability of the system. This explains why the final particle size reached in this system is larger compared to silica particles synthesized without glycerol and shows how glycerol is incorporated in the silica particles. Th e synthesis of silica microporous spheres of narrow size distribution, by v arying particle size and porosity, should yield a wide range of aqueous sil ica slurries for particular chemical mechanical polishing applications. (C) 2000 Academic Press.