Insight on Structural Change in Sol–Gel-Derived Silica Gel with Aging under Basic Conditions for Mesopore Control
Structural rearrangement of sol–gel-derived silica gel by aging under basic conditions was investigated using small angle X-ray scattering (SAXS) and 29Si nuclear magnetic resonance (NMR). A wet silica gel prepared under acidic conditions had a fractal nature, and many unreacted silanols remained on the surface. During aging of the gel in ammonia solution, additional Si—O—Si bonds rapidly formed, whereas the change in mesoscale structure gradually proceeded. This result was compared with that of simulation modeling the Ostwald ripening, i.e. dissolution from positive curvature and reprecipitation on negative curvature. In the simulation, structural change in a cluster from fractal nature to particle aggregates was well visualized in 2-dimmensional square lattice. Both scattering profiles calculated from the model clusters and the change in average coordination number of monomers in the cluster well agreed with the experimental SAXS and NMR results, respectively. This agreement strongly ensures us that structural change by aging under basic conditions proceeds through the Ostwald ripening. The mesopore size as well as mesopore volume in calcined silica gel is determined by the shrinkage degree during drying and calcination. The sample aged in basic solution restrains the shrinkage because of the growth of particulate structure, and retains large size and volume of pores.
Keywordssilica gel Ostwald ripening small angle X-ray scattering mesopore structure cluster aggregation model
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