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 alpha-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. (C) 2003 Elsevier Inc. All rights reserved.