Crystallization Mechanism of Pure-Silica ZSM-22 in the Seed-Assistant System

The crystallization mechanism of pure-silica ZSM-22 was investigated in the presence of seeds. Seed crystals played a significant role in the nucleation and crystal growth. Experimental findings revealed that the seed crystals acted as a substrate for crystal crystallization, and nucleation only occurred via the interaction between the organic structure-directing agent (OSDA), 1,6-hexanediamine, and seed crystal surface. Oligomeric silicate species in the liquid phase as nutrients directly crystallized on the crystals. The crystal growth occurred via layer-by-layer construction on the external surface of the seed crystals. The major chemical process occurring in the crystallization process was (i) dissolution of the large seed crystals into many thin needle-like crystals, (ii) dissolution of silica particles to form oligomeric silicate species, (iii) interaction between template molecules and the crystal framework which promoted nucleation, and (iv) formation of fully crystalline pure-silica ZSM-22 via layer growth. The morphology of pure-silica ZSM-22 can be controlled by the interaction between OSDA and the crystal framework which affected crystal growth in the a- and b-orientation.