Imaging the structures of zeolite L and synthetic mazzite

Abstract

Zeolite L (idealized formula K$_6$Na$_3$Al$_9$Si$_{27}$O$_{72}\cdot$ 21H$_2$O) and synthetic mazzite (idealized formula Mg$_2$(Ca,K$_2$)$_3$Al$_{10}$Si$_{26}$O$_{72}\cdot$ 28H$_2$O) otherwise known as zeolite omega and ZSM-4, are interesting microporous crystalline aluminosilicates of potential value as heterogeneous catalysts and catalyst supports. Their performance and future development for such purposes depend, however, upon their microstructural characteristics. We describe how these characteristics can be extracted from high resolution electron micrographs coupled with optical diffractometry and computer simulation. To improve the beam-stability of the zeolites they are first partly de-aluminated by using a process that is comparable with that employed industrially to enhance the thermal stability of zeolites in catalytic reactions. The correspondence between observed and calculated high-resolution images, both as a function of specimen thickness and extent of defocus of the objective lens, is good with synthetic mazzite, lending credence to the general correctness of the X-ray based structure, at least down to a resolution of 2.4 A (1 A = 10$^{-10}$ m = 0.1 nm). Several new types of structural faults have been discovered in the mazzite framework, and plausible models can be given for some, but not all, of these. In zeolite L, an unusual coincidence boundary, based on a mesh of $\surd$13 $\times \surd$13 times the hexagonal $a$ dimension, has been discovered. The presence of such boundaries on (00.1) planes in a sample of catalyst would lead to severe diminution in diffusivity of reactant molecules to, or products from, active sites that are predominantly located, as for most zeolitic catalysts, on the interior surfaces of the crystal.