It was known that adding fluoride ion to the synthesis gel will increase the hydrolysis rate of silica precursors. But at the same time, its also proved that fluoride route is THE BEST to obtain the single crystals with low defects/grains especially siliceous frameworks.
More the hydrolysis rate of silica precursor, more is the availability of silica monomers to form/grow the nuclei (which is similar to alkaline conditions). Even though the saturation of monomers, SBUs is high, why the growth is slow, smooth and grain-less ?
It is a interesting question. The answer, I assume, actually silica in presence of hydrofluric acid becomes Hydroflurosilicylic acid. This condence with aluminium species to form zeolites. As silicylic acid is bulker to have effective collision with alumina species will take more time. So crystallization rate is decreased, so bulkier particles are formed with more time.
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