Offretite dealumination

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

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).

Monodirectional 12 membered ring zeolites (offretite, L, mordenite and 0) are very inefficient as catalysts for formaldehyde benzene condensation to give diphenylmethane, esterification of phenylacetio acid with equimolar amounts of ethanol, Friedel-Crafts acylation of 3-phenylpropanoyl chloride with anisole and Claisen-Schmidt condensation of acetophenone with benzaldehyde. This fact has been attributed to diffusional constraints of organic compounds inside the channels. By contrast, the behaviour of the tridireotional f zeolite is very similar to that of dealuminated HY zeolites, inoreasing the turnover of the acid sites with the framework Si-to-Al ratio. 

Mordenite samples (MOR) were obtained from different sources (ref.5). Zeolites beta (BETA), mazzites (MAZ), offretites (OFF) and ZSM5 were synthesized according to procedures described elsewhere (refs.10-12). Some parent offretites, mordenites and mazzites suffered hydrothermal treatments and acid leachings to obtain dealuminated materials (refs.13,14). A wide variety of samples were thus prepared with Si/Al ratios in the range BETA 6.3 < Si/Al < 31.5, ZSM5 13.2 < Si/Al < 44, OFF 3.4 < Si/Al < 26, MAZ 2.5 ( Si/Al < 5, MOR 4.4 < Si/Al < 39.5. The protonic forms of the zeolites, e.g. HZSM5, were obtained by calcination of the ammonium forms at different temperatures. 

Finally the spectra are simulated by varying R, x, and w, where w is the linewidth at half maximum. The experimental and theoretical spectra for steam dealuminated omega and offretite samples are illustrated in Figure 3. The correspondence between the experimental and theoretical spectra are excellent. One has also to emphasize, that the computation yields R = Si/Al, where the A1 atoms are only those in tetrahedral sites of the structure noted as (Si/Al)jy in Figure 3. The Si/Al ratio determined by chemical analysis corresponds, on the other hand, to global composition including both tetrehedral framework and tetra-and/or octahedral non-framework aluminium atoms. The presence of extraframework A1 can be noted in both cases because the Si/Al values are systematically lower than the (Si/Al)jy values. 

Figure 3. Experimental and theoretical Si-NMR spectra of steam dealuminated offretite and omega. (Reproduced with permission from ref. 23. Copyright 1988 Elsevier.)...

Fig. 7. MAS NMR spectra of zeolites Y, omega, offretite, mordenite and ZSM-5 (top) and their dealuminated forms (bottom) [71]...

Fig. 3.70 (a)Change in wavenumber of the hydroxyl vibrating in the large cavities as a iunc-tion of Si/Al ratio. From left to right A, GeX, X, Y, chabazite, L, Q, dealuminated Y, dealuminated Y, offretite, mordenite, clinoptilolite, dealuminated Y, dealuminated mordenite, ZSM—5. 

Fernandez et al. [133] adapted the combined technique consisting of hydro-thermal treatment and successive extraction with acids, already successfully used for the dealumination of Y zeoHte [86, 87], for the dealumination of K,TMA-offretite. With an increasing number of treatment cycles (up to 4), the aluminum content could be progressively decreased from 3.41 to 1.68 Al per u. c. The process was found to be associated with a significant increase in the thermal stability of the lattice, but was also accompanied by the formation of defects and holes in the crystal (mesopore system). 

The framework Si/Al ratio of zeolite Q (synthetic mazzite) was increased by treatment with SiCl4 at 500 °C from 4.24 to 6.00 without significant loss in crystalHnity [177]. The dealumination reaction was accompanied by a sfight increase in the hexagonal lattice parameter a while c remained unaffected. This unusual phenomenon, i.e., cell expansion upon isomorphous substitution of silicon for aluminum, as well as AL MAS NMR spectroscopic results, pointed to a redistribution of aluminum on at least two crystallographically different framework T-sites. Si MAS NMR spectra of offretite, erionite and zeofite Q all dealuminated with SiCl4 were presented in [178]. 

The dealumination of faujasite,mazzite and offretite with ammonia hexaflu-orosilicate and the characterization of the products with various techniques have been reported [192]. The maximum level of dealiunination, which could be achieved without loss of X-ray crystallinity, corresponded to 50% for faujasite and 30% for mazzite and offretite. The dealiunination capability was foimd to depend on the texture of the crystals, which may have indicated that the process was diffusion-controlled. 

Dealumination of zeolites with low initial Si/Al ratios leads to materials with new textural properties. The creation of extensive secondary porous network has been observed in dealuminated zeolites with low iiutial Si/Al ratios, such as Y (refs.1-4), omega (ref. 5), offretite (ref. 6), mordenite (ref. 7) and CS25-1 (ref. 8) structures. 

---