The control of phase and orientation in zeolite membranes by the secondary growth method

Mordenite and ZSM-5 seeds could grow on the surface of an asymmetric α-Al₂O₃ support using an identical organic-free aluminosilicate solution under the same hydrothermal conditions. It suggests that the phase of zeolite can be controlled by varying the type of seed. Continuous mordenite and ZSM-5 membranes were prepared by the secondary growth method using corresponding seeds under appropriate conditions. The water content in the secondary growth mixture strongly influenced the growth of zeolite crystals. ZSM-5 seeds grew at a smaller rate and showed a habit of thin hexagonal plate in a concentrated mixture, while the crystals with a habit of rectangular parallelepiped were formed from a diluted secondary growth mixture. ZSM-5 crystals had poor intergrowth in the former case, whereas the crystals were highly intergrown in the latter case. ZSM-5 crystals in either membrane have a random orientation. In contrast to ZSM-5 zeolite, the crystal orientation in the secondary grown mordenite membranes strongly depended on the water content in the secondary growth mixture. The crystals in the mordenite membrane developed in a diluted secondary growth mixture were oriented with the (1 5 0) face parallel to the support surface, whereas the crystals in the membrane grown in a concentrated mixture were oriented with the (0 0 2) face parallel to the support surface. These results suggest that the crystal orientation in mordenite membranes can be manipulated only by changing the water content in a secondary growth mixture. The formation of the preferred crystal orientation in the secondary grown mordenite membrane is discussed in terms of an "evolutionary selection" process.