The effect of stirring on the synthesis of intergrowths of zeolite Y polymorphs

Abstract

This paper is concerned with the role of organic structure-directing agents during competitive synthesis of zeolite intergrowths. The system chosen for study is that of the intergrowth between the cubic (FAU) and hexagonal (EMT) polymorphs of zeolite Y. These two polymorphs can be synthesised using crown ether templates and it has been previously shown that mixtures of crown ethers produce intimate intergrowths of FAU and EMT. It was also previously suggested that concentration gradients of crown ethers at crystal growing surfaces are responsible for an oscillation of the structure between the two phases rather than a completely random intergrowth. In this current work we have made a detailed study of comparative syntheses, both stirred and static, in order to assess the role of diffusion of structure-directing agents to a growing crystal surface. Using a combination of high-resolution electron microscopy, X-ray diffraction and modeling we have established that diffusion-limited transport of structure-directing agents to growing crystal surfaces is an important factor controlling the final product. We also establish the “true” threshold concentration of crown ethers to synthesise each individual phase. This work has consequences for our general understanding of structure-directed growth of microporous crystals.