Dealumination acid leaching

Macedo et al. [195] studied hydrothermally dealuminated, acid leached and chemically dealuminated Y zeolites and correlated the heat of adsorption of ammonia to the activity in the conversion of cyclohexene. After milA selective acid leaching (with 1.5N HCl), Bronsted acid sites, which were blocked by extra-framework alumina species after steaming, became available and, therefore, the conversion of cyclohexene was enhanced. Severe acid leaching (with 4N HCl), on the other hand, also attacked the framework aluminum atoms, partially destroyed the zeohte structure, and hence had a negative effect on the conversion. For chemically dealuminated zeolites an increase in... 

Dwyer et al. (43) have also reported that dealumination of Y zeolites by a steam/acid leaching process produces a more uniform composition than dealumination by EDTA. The later method caused a depletion of Al in the outermost surface layer, producing a compositional gradient in the zeolite crystals. The conclusions reached by J. Dwyer in his studies of aluminum-deficient zeolites using the FABMS method are summarized in Table IV. 

TT-mordenite, which contained 11.2 wt % alumina, was dealuminated by A A heating for 4 hours at 700° C and then leaching under 6N HC1 reflux to 0.1 wt % alumina content (1) (aluminum-deficient H-mordenite). The intrinsic cracking activities of the parent H-mordenite and acid-leached H-mordenites are proportional to aluminum content (2) (i.e., the associated Bronsted acid centers). The deactivation rate of the aluminum-deficient H-mordenite is much lower than that of the parent H-mordenite. 

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. 

The total concentration of OH groups in dealuminated zeolite Y samples was determined using 1H-NMR techniques described (40) and is in reasonable agreement with the concentration of tetrahedrally coordinated extra-framework Al. The concentration of SiOH groups does not change in hydrothermal dealumination, but increases on acid leaching due to removal of framework Al. 

Commercial zeolite based hydroisomerization catalysts comprise alumina bound and platinum impregnated dealuminated mordenite. The activity and selectivity of the hydroisomerization of n-paraffins is strongly influenced by acid leaching. The influence of silica to alumina ratio has been studied for pentane isomerization over platinum mordenite many times since one of the first papers published (6). 

The acidity of high-silica zeolites produced either by direct synthesis or by chemical dealumination of parent zeolites (either by steaming and acid leaching or by SiCfi vapor treatment), which makes it possible in principle to extract the aluminum from the network without the structure collapsing, was extensively studied in the 1990s. 

Dealuminated by reaction with SiCl between 473 and 723 K followed by acid leaching with HQ. Steam dealuminated. 

Dealuminated by steaming between 973 and 1163 K followed by acid leaching with HCl. Values in parentheses are Si/Al ratios determined with NMR. 

Figure 5. J9Si-NMR spectra of mordenite dealuminated by acid leaching and subsequent steaming. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.)...

Table V. Variation of relative HRMAS Si-NMR line intensities of mordenite at various degrees of dealumination by acid leaching...

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