Steam-dealuminated HY zeolites

The A1 NMR spectra of three steam-dealuminated HY zeolites are shown in Figure 1. They Sjt w one intense line centered at 60 ppm (tetrahedral aluminum, A1 ) d two more lines at ppm and 0... 

In heavily steam-dealuminated zeolites, most of the activity should come from the EFAL that is concentrated on the external siirface (3,4). We believe that Levyis acidity, which can stabilize radicals, can play an important role in the radical cracking observed with strongly steam-dealuminated HY zeolites. Furthermore,... 

Finally, the variation of the yields of gasoline and diesel versus A1 per unit cell allow us to conclude that highly steam-dealuminated HY zeolites behave, from the point of view of gas-oil cracking, like a well-dispersed active alumina. 

The cracking activity and selectivity for n-heptane and gas-oil, of a series of dealuminated HY zeolites (SiCl4 or steam dealuminated) have been measured. The results have been related with the different types of aluminium and acid groups present and to the pore volume distribution. It is clearly shown that the unit cell parameter (framework Si/Al ratio) can not explain, by itself, the activity and selectivity of dealuminated zeolites. 

The starting NaY zeolite was an SK-40 from Union Carbide with a framework Si/Al ratio of 2.4. Ultrastable HY zeolites (HYUS) were prepared by steam-calcination of partially ammonium exchanged zeolites at atmospheric pressure and 550-750 °C during 3-20 hours. After dealumination they were exchanged twice with an NH solution at 80 C for one hour and then calcined at 550 °C for 3 hours. In this way dealuminated samples containing less than 2% of the original Na were obtained. One of these (HYUS-8) was subjected to different treatments (1) washed with a solution of citric acid or HCl (pH=3) at 25 °C for one hour (samples HYUSAC and HYUSl, respectively) (2) washed with a solution 0.1 M of NaOH at 40 °C for one hour (HYUSN), and (3) washed with a 38% v/v solution of acetylacetone in ethanol at 20 °C for 2 hours (HYUSA). 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

As observed previously (11,12) dealumination of HY zeolites during steaming leads to the formation of extraframework aluminium species (AlNF) a part of which remains in the microporous structure. These AlNF species (AlOx groups) act as... 

Progressive steam dealumination of HY zeolite at high temperatures causes the same effect observed above for high temperature calcination (149,... 

The Bronsted acid sites of HY zeolites dealuminated either by conventional treatment (steaming + acid leaching) or isomorphous substitution (fluorosilicate) have been characterized at each step of the preparation procedures through IR spectroscopy of probe molecules with various basic strengths (pyridine, C2H4,... 

Xenon ( Xe)(I = lj2). The electric-field gradients (EFG) in the supercage of HY, steam-dealuminated Y (STY) and lanthanum-exchanged Y zeolites (LaY) were quantified using the difference in chemical shift 5CS( Xe)-8CS( Xe) of the two NMR-observable isotopes Xe and The... 

Early development work by Mobil had shown that the exchange of sodium with higher-valency ions, particularly those derived from rare earth metals, increased stability. Rare earth-exchanged NaY-zeolite (REY-zeolite) was not as easily dealuminated by steam and high temperatures in the regenerator as HY-zeolite. Consequently, catalysts manufactured from REY-zeolite were soon being used to maximize gasoline production in most FCC units. 

Adsorption and desorption isotherms of water vapor on HY zeolites change in shape with increasing dealumination. Thus, for only steamed samples the isotherm is close to the type I whereas for zeolites submitted to an additional acid leaching, it is close to the type IV with a progressive lowering of the point B towards the pressure axis, what emphasizes a more and more hydrophobic character of adsorption. The micropore volume accessible to water and the unit cell parameter are simply correlated with the framework Si/Al ratio. The water molecule does not allow to determine quantitatively structural and secondary pore volumes, but appears to be a selective molecular probe of the structural aluminium ions. For HY zeolites and without any restraint, 8H2O are associated with such a framework aluminic site. 

Volumes of mesopores with diameters from 1.7 to 50 nm, as derived from the desorption curves, are given in Table 3. The mesopore volume of the dealuminated HY, LiY and NaY zeolites with a high crystallinity, except NaY with Si /Al of 15.6, doesnot exceed that of the parent NaY zeolite, for which a value of 35 mm g"l was determined (Table 3). For comparison, the mesopore volume of NH4Y zeolite, dealuminated by self-steaming at 973 K and with SiF/Al ratio of 5.5 is 100 mm g l. 

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