Mazzite sheets

Three well known zeolite structure types - Linde L(18), mazzite(19) and the hypothetical omega-structure(20) - have essentially identical hexagonal lattice constants with =18A and .=7.5A, but only mazzite has a 7.5A x 18A layer that contains an n-glide plane operator the thickness of this layer is -15.5A. If the hexagonal overgrowth is mazzite, the electron micrograph interpretation implies that ECR-1 comprises mazzite sheets interconnected with some other subunit to form the observed 26A repeat distance. The... 

The refinement performed by Gualtieri et al. [5] evidenced that ECR-1 is formed by a strict alternation of mazzite (MAZ) and mordenite (MOR) sheets, with 4-, 5-, 6-, 8- and 12-membered tetrahedral ring forming a three dimensional ring. In the Na- form, sodium cations are distributed over 5 different extra-framework sites. The thirty-five water molecules are distributed over eleven sites. It is worth noting that the position of the cations found in ECR1 does not correspond with the site found for mazzite and mordenite. In the NH4 -exchanged form, the NH4 ions occupy three distinct extraframework sites, whereas the water molecules are distributed over the same eleven sites found for the Na-form [5],... 

Figure 1. The proposed structural framework of ECR-1, showing alternate sheets of mazzite and mordenite, the 26.5A repeat unit and the single 12-ring channel.

ECR-1 is a true boundary phase between two compatible structures - mazzite and mordenite - which normally crystallize with greatly disparate Si/Al ratios (3.4 and >5 respectively). Yet ECR-1 has an Si/Al ratio identical to mazzite, even though half of its component sheets are structurally characteristic of mordenite (in the structure proposal). This compositional incompatibility is further reflected in the almost complete lack of mixtures of mazzite with mordenite in the experiments so far completed. The ongoing search for ECR-1 with mordenite overgrowths is therefore an important objective in the understanding of crystallization mechanisms in this system, with particular interest in the possible compositional variation between all three component zeolites. 

Results from diverse experimental methods were combined to arrive at structure proposals for ECR-1, a typical example of a microcrystalline zeolite. The zeolite features a 12-ring single channel, formed by a regular alternation of connected sheets of mordenite and mazzite. Crucial clues to the structure came from the HREM observation of a mazzite epitaxial overgrowth on an ECR-1 crystal. Electron and x-ray diffraction, infra-red spectroscopy and synthesis phase relationships were essential additional data sources. 

Even within the unit cell and symmetry constraints of this system, there are two ways to interconnect mazzite and mordenite sheets in three dimensions - one related to the other by a shift of a/2. Differentiation of the two models will be best resolved by full Rietveld refinement of the observed data. This situation of several related structures having the same two dimensional projections but different three dimensional connectivity is common in zeolite structural chemistry ( eg. mazzite - omega several members of the ABC-6 family of structures ). 

Vaughan.One example of this is the zeolite ECR-1 and its gallosilicate analogue TNU-7, which are made up of alternating mazzite (MAZ) and mordenite (MOR) sheets that have a close structural fit in two directions (Figure 2.11) and which crystallise at the boundary between the compositional fields responsible for the end-members. Synthetic Paulingite (ZSM-25) is an example where many different structural units are present in the structure, which crystallises at the boundary between phases containing smaller subsets of these SBUs. ... 

Figure 2.11 The structure of the gallosilicate TNU-7 (and the aluminosilicate ECR-1), shown below, projected down the large channels, can be thought of as being built up from strictly alternating sheets maz and mot) that are found in the mazzite and mordenite structures, shown above left and right, respectively.

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