DIRECT MEASUREMENTS OF THE CRYSTAL-GROWTH RATE AND NUCLEATION BEHAVIOR OF SILICALITE, A ZEOLITIC SILICA POLYMORPH

The method of Zhdanov and Samulevich has been used to analyse the crystal growth rate and nucleation behaviour of silicalite, a silica molecular sieve. Four reactions run under different conditions at 368 K gave values for the crystal linear growth rate (0.5 dl/dt) in the range (1.9-2.0)×10-2 μm h-1. However, the reactions do not show the same pattern of crystal mass increase with time, the variations reflecting differences in nucleation behaviour. All the nucleation rate curves were either bimodal or trimodal, suggesting that at least two separate nucleation mechanisms were operating. Early in the reaction, nucleation is probably heterogeneous and associated with the amorphous gel or colloidal material present in the mixture. Later on, when appreciable quantities of crystalline product have been formed, a further crop of crystals nucleates either by a secondary mechanism or by release of further hetero-nuclei from within the dissolving amorphous component. Nucleation was facilitated by early addition of the mineralising organic "template" (tetrapropylammonium cation), and (probably) by rapid stirring. Although all four reaction compositions were identical, the differences seen in nucleation behaviour were linked to observable differences in chemical parameters (e.g. the pH profile of the reactions) as expected for systems governed by the competitive kinetics of many overlapping reactions. The results confirm that simple growth curves based on, for example, XRD crystallinity are of limited use in helping to understand the complex processes occurring in zeolite synthesis, and that a more detailed analysis is justified. © 1990.