Omega dealumination

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

In contrast, there are relatively few publications on the conversion of polynuclear aromatics in zeolite catalysts. Lee et al. [16] found unusually high selectivities for 4,4 -diisopropylbiphenyl when dealuminated mordenite was used as catalyst for the alkylation of biphenyl with propene. The reactions of 1- and 2-methylnaphthalene on acid forms of zeolite X, Y, Omega, mordenite and ZSM-5 were studied byDimitrov etal. [17], S o 1 i n a s et... 

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).

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... 

Laboratory studies have shown that omega (MAZ structure type) based paraffin hydroisomerization catalyst shows higher activity than mordenite based catalyst and better selectivity, i.e. higher octane due to higher yield of di-branched paraffins compared to mordenite performance (17). The isomerization of a C5/C6 cut at 15 bar results in a final calculated RON of 80.4 for the alumina bound dealuminated PtH-MOR catalyst supplied by IFP with undisclosed (most likely similar) Si/Al ratio, measured at 265 °C compared to a RON value of 80.9 for an alumina bound dealuminated PtH-MAZ catalyst with bulk Si/Al = 16, measured at 250 °C. Both measurements were performed in a bench-scale tubular reactor with a volume of 50 cm3 of 2 mm diameter extrudates with WHSV of 1.5 h and H2/HC of 4. This... 

Cumene synthesis Dealuminated mordenite MCM-22 beta Y omega Lower inputities Transalkylation function Lower benzene-to-propylene ratio allows higher capacity, great unit efficiency High selectivity Regenerable, non-hazardous, non-corrosive... 

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.)...

The FTIR/KBr method was, inter aha, applied by Maache et al. [306] for the characterization of various samples of H-Beta dealuminated by acid leaching. In the case of framework vibrations of zeolite Beta, a good Hnear correlation was found between the wavenumber of the asymmetric O-T-O vibration, Vas(O T-o)> and the aluminum mole fraction, nAi/(nsj+nAi) [263]. Ponthieu et al. [307] attempted to find a correlation between the wavenumbers of framework vibrations of ojfretite and zeolite Omega and the degree of dealumination (aluminum mole fraction) similar to those observed with faujasites and mordenites. However, Hnear correlations were hardly obtained. 

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

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. 

---