Classic Model for Zeolite Growth

Zeolite synthesis can also be approached from classical nucleation-crystalli-zation theory [105]. A viable nucleus is estimated to have a size between one and eight unit cells, depending on the structure type and the experimental synthesis conditions [106]. However, because of crystallization from high surface area systems, the energetics of zeolite nucleation can be significantly different from that of more dense phases [107]. Advanced techniques such as HRTEM and cryo-TEM are the basis of the experimental information about zeolite nucleation [108]. 

Nucleation involves activation of dormant nuclei in the amorphous phase upon release in the solution phase via gel dissolution [109]. The dormant nuclei are located near the surface of the gel particles, thus becoming activated in the early phase of the gel dissolution/growth process [110], For ZSM-5 synthesis, gel dissolution and nucleation were experimentally proven and successfully modeled to be interfacial phenomena at the gel/solution boundary [111]. 

Experimentally observed nucleation rates typically go through a maximum with increasing degree of supersaturation of the synthesis solution, and thus are related to the increasing viscosity of the medium, inhihiting migration of intermediate species [98]. If nucleation and growth consume the same species, this phenomenon, viz. the maximum observed in the nucleation rate, is rationalized. 

Chemically, the increase in ordering degree of the primary gel has been visualized as follows [117]  

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