OMEGA, ZEOLITE

Unlike the usual amorphous, microporous adsorbents, it is possible to calculate the theoretical micropore volume of a dehydrated zeolite from the known crystal structure. We have performed these calculations here for several of the better known zeolites including zeolite A, zeolite X, zeolite L, mordenite (Zeolon), (8) zeolite omega, (4) and the zeolite 0 (offretite... 

Interpretation of infrared spectra of zeolite omega is inconsistent with the proposed structure (16) and is more consistent with a structure based on sodalite-type units (17). The adsorption saturation values can only be accounted for by the proposed structure if it is assumed that H20 and N2 can occupy the gmelinite-type cages. However, this proposed structure does not provide for access to the gmelinite-type units from the main channel which are large enough to pass N2. Access to the two dimensional... 

In the reaction mechanisms described above the acidity of the catalyst plays an important role. Zeolites can be converted into the H+ form and as such are powerful catalysts for acid-catalyzed reactions. We discuss below some aspects of isomerization catalyst preparation to demonstrate factors which influence the activity of catalysts based on zeolites. In this discussion we are concerned with zeolite Y and mordenite. Data on paraffin isomerization over dual function catalysts besed on other zeolites are scarce, and no data have been published showing that materials like zeolite X, zeolite L, offretite, zeolite omega, or gmelinite can be converted into catalyst bases having an isomerization activity comparable with that of H-zeolite Y or H-mordenite. 

A study is presented of the synthesis and properties of the novel synthetic zeolite omega. The synthesis variables and kinetics of formation are discussed, as well as the ion exchange, sorption, and thermal properties. By decomposition of imbibed tetra-methylammonium ions and exhaustive treatments of the zeolite with ammonium ions, a pure hydrogen form can be obtained which is a suitable substrate for the preparation of hydrocarbon conversion catalysts. Several catalysts were prepared and utilized to isomerize n-hexane, and to hydrocrack a heavy gas oil. 

This speculation was not only completely vindicated in the preparation of silica-rich forms of a variety of known zeolites (2, 3) it was found that direct use of organic cations in syntheses promoted unusual structural designs which led to novel zeolites. Zeolite omega (S2) was one such material and was first synthesized by Flanigen and Kellberg (4) These... 

Figure 1. A schematic representation of the structure of zeolite omega.

Characterization of Zeolite Omega. Chemical Composition. Analytical data for several samples of zeolite to are collected in Table II. The... 

Figure 2. Synthesis of zeolite omega from silica-low-alumina cracking catalyst...

Table in. Exhaustive Ion Exchange of Na +/TMA +-Zeolite Omega with NH4NO3 Solutions at 100°C, without Intermediate Calcinations... 

Pyrolysis of Sodium-Tetramethylammonium Zeolite Omega. Preliminary calcinations of the Q zeolites showed that intracrystalline diffusion restrictions interfered greatly with transport both of oxygen and of calcination products. Under mild conditions, coking was observed, and even under favorable conditions (550° C, thin beds, good venting) the reaction was slow. Some samples of zeolite were pyrolyzed under vacuum, and the products were identified by low resolution mass spectrometry. 

Table IV. Products Identified by Mass Spectrometry during the Vacuum Pyrolysis of Na +/TM A +-Zeolite Omega at 350-650°C...

Figure 8. Differential ihermogravimetric analysis of zeolite omega in air and in...

Table V. Sorption of n-Hexane on Zeolite Omega Influence of Thermal and Ion-Exchange Treatments0...

Table VI. Preparation, Properties, and Activity for n-C6 Isomerization of Catalysts Based on Zeolite Omega...

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... 

Silicon tetrachloride treatment can produce faujasites with very high Si/Al ratios in a single step, but works less well with other zeolites. The reasons for this are not clear. However, 27A1 MAS NMR shows unambiguously that other zeolites, notably mordenite (208,209), zeolites omega (209) and ZSM-5 (57), are also dealuminated in this way. This is often not detectable by other methods since, after being removed from the framework, the aluminum may... 

Fig. 48. 27A1 MAS NMR spectrum at 130.32 MHz of zeolite omega dealuminated with silicon tetrachloride vapor (209).

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.

Kinetics and Mechanism of Crystal Growth of Zeolite Omega... 

The change in morphology of crystals of zeolite omega has been analyzed with respect to the evolution of the concentration of aluminium in the parent liquor. In agreement with the overlapping principle, crystal habit is determined by the faces with the lowest growing rate. 

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