Steam-dealuminated zeolite

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,... 

Catalytic Behaviour of "leached Steam-Dealuminated Zeolites... 

High degrees of dealumination are difficult to achieve using AHFS compared with those obtained via hydrothermal treatment, because of loss of framework crystallinity. The characterisation techniques used here have shown that silicon enrichment occurs during the AHFS treatment, leading to higher bulk Si/Al ratios. 7a1 MAS NMR appears to show the presence of aluminium species other than those teU a- or octahedrally co-ordinated. These may be the fluorinated aluminium species mentioned in earlier works. The textural properties of AHFS treated zeolites are not changed relative to the parent material in contrast to the steam dealuminated zeolites, where the introduction of secondary mesopores occurs. 

Figure 2 shows for HYUS-5 that there are several types of differently coordinated aluminium. Indeed, there are bands at 60, 30 and 1.4 ppm, which are related to tetrahedral, pentahedral (5) or tetrahedrally distorted aluminium (6), and different types of octahedrally coordinated aluminium, respectively. The tetrahedr aluminium appearing at 60 ppm has always being attributed to FAL (7). However, the intensity of the 60 ppm lime (34%) becomes difficult to reconcile with the value of 140 for the zeolite framework Si/Al ratio obtained from the unit cell size (24.24 A). Indeed, from the X-ray unit cell contraction one should expect 4% of the total aluminium to be in framework tetrahedral position. Then we conclude that in steam dealuminated zeolites there is some aluminium which is not in framework positions but it is also tetrahedrally coordinated (EFAL ). The same is observe in the HYD samples. We have recently claimed (8) that the EFAL could be present as a special type of non-crystalline silica-alumina formed during dealuminatij. If this is so, there is not doubt that at least some of the EFAL could be associated with BronsJ d acid sites, which would not be taken into account if only the FAL is considered to be related to active Bronsted sites. 

SiCl dealuminated zeolites O Steam dealuminated zeolites... 

Figure 8. Selectivities to different products of cracking of gasoil (60 % level of conversion) as a function of FAL per unit cell for steam dealuminated zeolites.

These results show that, together with the typical carbocation cracking, a radical cracking mechanism is also taking place, especially on highly steam dealuminated zeolites. This radical mechanism, which will become proportionally more important when the framework aluminium content of the zeolite will decrease, could explain the strong increase in ethylene formation and decrease in branched/unbranched C4 ratio when decreasing the unit cell size of the zeolite. 

An uitrastabie or a dealuminated zeolite (USY) is produced by replacing some of the aluminum ions in the framework with silicon. The conventional technique (Figure 3-9) includes the use of a high temperature (1,300-1,500°F [704-816°C]) steam calcination of... 

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... 

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. 

Poisoning studies carried out by several groups have shown that the equivalents of poison needed to quench the catalytic activity of the dealuminated Y-type zeolites are much less than the number of Alf atoms. Beyerlein et al. (9) reported that residual sodium cations extensively decreased the isobutane cracking activity of steam-dealuminated Y-type zeolites. From their results it was concluded that only one-third of the Alf atoms were associated with strong acidity throughout the Si/Al > 5 composition domain. 

Infrared bands of dealuminated zeolites in the 0-H stretching region have been extensively studied (14-17) however, emphasis will be placed on the study by Fritz and Lunsford (10), which describes the effect of Na+ on these infrared bands. Perhaps the most informative example is the SiClA-treated zeolite (series A of Figure 3). The SiCl4-treated zeolites have less extraframework aluminum than do the steamed samples therefore, the infrared spectra in the 0-H stretching region are less complicated. 

Evidence that something more than a proper Alf distribution is needed to create strong acidity first came from the experiments of Beyerlein et al. (7). Using the acid-catalyzed conversion of isobutane as a measure of strong acidity, they found that a dealuminated zeolite prepared by treatment with ammonium hexafluorosilicate (AHF) exhibited much less carbonium ion activity than might be expected, based on the number of Alf atoms. This treatment leaves very little extraframework Al in the zeolite. When the sample was mildly steamed, the activity became considerably greater. The authors concluded that the enhanced acidity was a result of a synergism between the framework Bronsted sites and the Lewis sites associated with extraframework aluminum. 

In a previous paper (14) we have shown that by an adequate treatment of steam dealuminated Y zeolites with (NH4)2SiF6 it is possible to remove selectively the EFAL. The physicochemical characterization of these samples shows that the presence of EFAL is the responsible for the formation of superacid sites, as well as for the neutralization of a part of framework hydroxyls. 

Cruz et al. (36) also found that AFS preferentially dealumi-nates the surface of zeolite Y crystals to a depth of about 100 A and produces an extra-framework-Al-free zeolite. They found that the activity for n-heptane cracking is the same as for a steamed dealuminated sample, while the gas oil cracking was lower, being controlled by the outer shell. 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

Dealuminated zeolite samples were calcined in air at 540°C for three hours prior to catalytic testing. A portion of each sample was further modified using high-temperature steam. Zeolite samples were placed in a fixed-bed quartz tube 95% steam was passed through the bed at 750°C and atmospheric pressure for 4 hours. 

Zeolite catalysts are frequently applied after treatments that tend to increase their stability and also to further enhance surface acidity and shape selectivity effects. These treatments, such as steam dealumination, can cause a decrease in the framework A1 content and the release of aluminum-containing species from the framework. This can contribute to the stability of the framework, but extraframework species can also contain additional catalyticaUy active acid sites. These particles can also narrow the size of the zeolite charmels or of their mouths, so improving the shape selectivity effects. Extra-framework material (EF) can also... 

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

Temperature programmed activity profiles for parent, hydrothermally and AHFS treated zeolites showed that the hydrothermally U eated zeolites yield a much greater activity than do the parent or AHFS treated materials. Selectivity profiles of the catalyst also showed significant differences between the AHFS and steam dealuminated samples. Figures 5 a and b show the... 

The cracking activity of AHFS treated material is greater than that of the parent zeolite, but not as great as that of the steam dealuminated samples. Selectivity studies have shown that different reaction mechanisms occur within the zeolite depending on the method of dealumination used. 

---