Some observations on the preparation and properties of colloidal silicalites. Part I: synthesis of colloidal silicalite-1 and titanosilicalite-1 (TS-1)

Abstract

Possible methods for the preparation of colloidal zeolite sols are discussed: direct synthesis is at present the only practicable procedure. The linear crystal growth rate for colloidal TS-1 (≈2.5 at.% Ti) at 175 °C was estimated to be ⩽80 nm h −1, which is lower than that for colloidal silicalite by at least a factor of 3. The introduction of titanium also caused a reduction in the number of crystals nucleated. These effects are ascribed to the difficulty of incorporating the heteroelement into the lattice. No evidence for Ostwald ripening in colloidal silicalite synthesis was found, even at long reaction times. In seeded growth experiments at 100 °C, (i) TPA-silicalite-1 product crystal size could be closely controlled by the quantity of 43 nm seed added, and (ii) crystal linear growth rates in thermally and microwave heated reactions were the same within experimental error. Ageing the precursor sols at 22 °C before crystallisation at 90–175 °C (microwave or thermal heating) demonstrated the generation of proto-nuclei during the room-temperature ageing process. On subsequent heating, these mature into viable nuclei and grow into crystals. An interpretation of this behaviour establishes the relationship of the present results to those of other workers. This identifies the generation of nuclei with a series of chemical steps which lead from the initial equilibration reactions to the formation of the primary replicating unit.