Mazzite omega

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 ). 

Fig. 24. 29Si MAS NMR spectra at 79.80 MHz (ahove) of zeolite Y, zeolite omega (synthetic mazzite), offretite, and mordenite, and their dealuminated forms (below).

The intensities of the two signals in the spectrum of the siliceous zeolite omega are in the 2 1 ratio. While zeolite omega is more appropriately called synthetic mazzite (94), both the previously suggested structure (95) and the structure of mazzite (96) call for two nonequivalent tetrahedral sites in a 2 1 population ratio. Accordingly, Thomas et al. (91) were able to assign the... 

Fig. SO. Projection drawing, viewed along [001] of the structure of zeolite omega (synthetic mazzite). There are two distinct tetrahedral sites, one more (A) and one less (B) accessible via large channels. The unit cell is enclosed within the dashed lines.

Crystal habit modification. Several crystal habits have been reported in the open and patent literature for zeolite omega. Elongated hexagonal rods (15,16) or fibres (17) have been reported when the zeolite resulted from the recrystallization of another zeolite, Y (15) or S (17), or of clays (16). Moreover the natural counterpart of zeolite omega, mazzite, appears as bundles of needle-shaped particles (18). All these solids have been grown at low supersaturation levels, hence under conditions in which our results show that the growth in the direction <001> prevails on the growth normal to the c-axis (Fig.8). 

FIGURE 2 Thermogravimetric analyses for ECR-1, Linde Omega (mazzite) and ECR-4 (faujasite), run at 10°C/min. in air, using a DuPont 951/1090 TGA. The distinctive high temperature weight losses represent the decomposition and desorption of the organic "templates". 

FIGURE 3 IR spectra for ECR-1, mordenite and Linde Omega (mazzite), showing common framework vibrations, and a distinctive 5-ring doublet at 1210 and 1240cm.-1 for ECR-1. 

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... 

Before going on to list these families, one further point needs to be made. This arises because there are several instances where a unique zeolite framework has been first characterized from a laboratory preparation and later identified as a natural species. The first of these was mazzite, prepared first as synthetic zeolite ZSM-4 (or omega) but known in naMe prior to the allocation of the lUPAC code hence its inclusion in Table 7. The other species have been discovered in naMe after an lUPAC code had been allocated to their synthetic analogs. The details for these materials are given in Table 8. 

Mazzite MAZ 7.4 1 LZ-217 TMA-0 Omega zsmI LZ-202 Gallosilicate mazzite Gallosilicate L... 

If templating is important, key ratios will define the cation distribution, Na Si02, K Si02, R4N Si02, and so forth, ratios which can range (individually) from zero to more than two. In Table III, examples are selected from the literature to illustrate cation effects in crystallization experiments. The Na-series examples show a shift in product from Y to mazzite-related structures (ZSM-4, Omega) on addition of only small amounts of TMA ion. When potassium replaces sodium, the L structure results but limited addition of TMA produces yet another structural type, the offretite-type zeolites. 

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