Synthesis of VPI-8. II. Mechanism of crystallization

A model for the crystallization mechanism of VPI-8 from lithium zincosilicate reaction mixtures of typical composition Si:Zn=10 and Li:Zn=2 is presented. When the reaction mixture is heated to temperatures above 135°C, an amorphous gel is formed which contains a homogeneous distribution of silicon and zinc; Si:Zn=8. With heating time, zinc migrates to the superficial region of the gel particles creating a zinc-depleted core region where the nucleation of VPI-8 occurs. Simultaneous with the nucleation of VPI-8 in the core region, two other zinc-rich phases form in the superficial region. As the heating time is increased, one of the zinc-rich phases is dissolved and the other remains while the yield of VPI-8 with a framework Si:Zn of around 27 increases. Ultimately, VPI-8 with a small amount of a contaminating phase with Si:Zn=1 is obtained. The model presented for the crystallization mechanism of VPI-8 is consistent with all reported synthesis and physicochemical data.