The induction Period and Nucleation

One of the most thoroughly studied zeolite crystallisations is that of the synthesis of the pure silica form of ZSM-5 from a clear solution using TP A as the structure-directing agent. A series of low-angle X-ray scattering experiments by de Moor et indicate the presence of populations of scattering 

Chang and Bell showed that major changes occur in the TPA-silica gel, with units being formed that contained the TPA cation, and that these units rearranged with time. Spectroscopic H- Si CP MAS NMR evidence from Burkett and Davis on the same system indicates that the TPA cations become included in organised inorganic-organic composites before crystallinity is apparent, and that their conformation is similar to that which they adopt in the 

The picture that emerges is therefore one in which amorphous precursors assemble from solution and then reorganise to give a colloidal suspension of zeolite crystals similar to that observed by Mintova et al at room temperature for zeolite The mechanism of structure direction in the TPA-ZSM-5 system, as proposed by Burkett and Davis on the basis of the experimental 

Step 1. A coordinating shell of water molecules arranges around the TPA cation. This process of cation hydration is known to occur from other studies of quaternary ammonium cations. 

Step 2. Hydrated silicate species interact with hydrated organic cations, resulting in replacement of water molecules in the coordination sphere of the TPA by silicate anions. The hydrophobicity/hydrophilicity of the organic is known to be critical in this stage. (Replacement of TPA with ethanoltripropylammo-nium ions results in much slower crystallisation of the pure silica ZSM-5, whereas diethanoldipropylammonium does not nucleate the structure.) 

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