SYNTHESIS OF VPI-8 .2. MECHANISM OF CRYSTALLIZATION

A model for the crystallization mechanism of VPI-8 from lithium zincos ilicate reaction mixtures of typical composition Si:Zn = 10 and Li:Zn = 2 is presented. When the reaction mixture is heated to temperatures above 135 degrees C, an amorphous gel is formed which contains a homog eneous distribution of silicon and zinc; Si:Zn=8. With heating time, z inc migrates to the superficial region of the gel particles creating a zinc-depleted core region where the nucleation of VPI-8 occurs. Simul taneous with the nucleation of VPI-8 in the core region, two other zin c-rich phases form in the superficial region. As the heating time is i ncreased, one of the zinc-rich phases is dissolved and the other remai ns while the yield of VPI-8 with a framework Si:Zn of around 27 increa ses. Ultimately, VPI-8 with a small amount of a contaminating phase wi th Si:Zn = 1 is obtained. The model presented for the crystallization mechanism of VPI-8 is consistent with all reported synthesis and physi cochemical data. (C) 1997 Elsevier Science B.V.