Stabilization after dealumination

Since the Si—O bond length (1.66 A) is shorter than that (1.75 A) of Al—O, the crystal lattice of zeolites is shrunken and the structure is stabilized after dealumination and silicon enrichment, as confirmed as follows. Of course, unavoidably there exist silicon-deficient hydroxyl nests after both dealumination and ultra-stabilization, and some mesopores will be generated in the USY framework. 

The aluminum defects (III) in the above scheme created by dealumination may be eliminated by silicon species from the zeolite structure of amorphous silica contained in the material. The dealumination can be achieved also by the reaction with silicon tetrachloride. In this case, no vacancies are formed since aluminum in the structure is directly substituted by silicon. Since the thermal stability of zeolites increases with increasing Si/Al ratio, zeolites become thermally more stable after dealumination. In the case of Y-zeolites, the stabilized zeolites are called ultra-stable zeolites. 

ZSM-5 was reported to release framework aliunimun completely upon treatment with 1 N hydrochloric acid [18]. In contrast, Kornatowski et al. [ 19] foimd that ZSM-5 can be only partly dealuminated by acid treatment Recently, Kooy-man et al. [20] reported that the bulk alumimun content of ZSM-5 zeolites could not be significantly decreased by extraction with 1 N hydrochloric acid even at temperatures as high as 160°C. From this it follows that the framework aluminum content was little affected since the amoimt of extra-framework aluminum detected by Al MAS NMR spectroscopy after acid leaching at 80 C, somewhat dependent on the zeolite preparation mode, was found to be rather small. HBr and H2SO4 proved to be even less effective than HCl. The high stability towards dealumination by acid leaching is attributed to the virtual absence of structural defects in the ZSM-5 samples studied. 

Dilute fluorine gas (0-20%) can be used to treat zeolites at near-ambient temperature and pressure. Most of the resulting materials retain very high crystallinity even after 600°C postcalcination for two hours. Both framework infrared spectra and X-ray powder diffraction patterns clearly show structural dealumination and stabilization. The hydrophobic nature of the fluorine-treated and 600sC-calcined material is shown by a low water adsorption capacity and selective adsorption of n-butanol from a 1 vol.% n-butanol-water solution. Fluorination also changes the catalytic activity of the zeolite as measured by an n-butane cracking method. 

The spectrum for the 600°C-calcined fluorine-treated erionite sample shows substantial shifts in band positions, but band sharpening is less obvious. The bands at 1082, 792, 578, 470 and 438 cm- are shifted to 1098, 814, 585, 477 and 444 cm , respectively, after fluorine treatment and 600°c calcination. The large shifts observed are evidence of dealumination. The splitting of the 1082 cm-1 band into a doublet located at 1098 and 1085 cm-1 and some degree of band sharpening imply structure stabilization for fluorine treated erionite. 

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... 

For a zeolite T (OFF stmcture, 0.68 nm XRD pore diameter), Tanaka et al. [131], observed that the separation factor of a water/acetic acid (50/50 wt%) measured at 75°C decreased monotonically after the immersion of the membrane into the acetic acid mixmre. Initially, the separation factor and water flux were 182 and 1.46 kg/m h, respectively, and after 32 h these values changed to 86 and 1.77 kg/m h, showing a deterioration of the membrane. Cui et al. [130] also smdied the stability of crystals and membranes of zeolite T in acid medium. The powders were immersed in a 50/50 wt% water/acetic acid mixture for 7 days at 75°C and also in HCl solutions 0.5 and 1 M for 1 h at 50°C. The analysis of the samples after the treatment by ICP and XRD indicated that the sample treated in the acetic acid solution maintained its original Si/Al ratio equal to 4 however, the hydrochloridric acid treatment with the 1 M solution destroyed the zeolite stmcmre and the 0.5 M solution dealuminated the zeolite to a Si/Al equal to 8.9 and the XRD analysis corresponded to zeolite T. The membrane performance, after being used for 1 week at different water/acetic acid concentrations, remains almost unchanged and the separation factor of the membrane treated in HCl dramatically decreased as was expected. 

Compared to the NaCaA sample the slightly higher stability of the NaY zeolite is caused by the lower content of framework aluminium. With further dealumination of this framework a further stabilization would be expected. However, this effect is not observed. Also the assumption of a strong distortion of the framework after the dealumination process as the sole reason for the reduced stability is incorrect. 

Under these conditions, the zeolite HY was very stable. During the first cycle (Table 14.7) we have observed a slight dealumination of the HY zeolite, probably by dissolution of extraframework alumina and generation of mesopores, because we did not observed any impact on the integrity of the catalyst. After the first cycle, the kinetics of the reaction increased and then stabilized. 

The third kind of spectrum, pictured on Figure 7c, is found on dealuminated zeolites Y15 and Y 5 at medium and high nickel loadings. This spectrum reveals that only a small part of nickel is stabilized in the cationic sites. But the bands at 23460 cm-1 and 5500 cm-1 are inconsistent with an octahedral coordination. Such bands may be attributed to coordinatively unsaturated nickel ions. A similar type of nickel species was recently found on an alumina surface after moderate heating (16) with characteristic bands at 5830 and 23500 cm 1. 

These zeolites after hydrothermal treatment and dealumination were examined by XRD for crystallinity and structural stability. The nature and extent of extraffamework aluminum (AEFAL) as well as framework aluminum (Al F), acid soluble extraframework aluminum (AIEFAL) assessed by chemical analysis and XRF for total aluminum (ALT), Si NMR for framework aluminum, acid extraction for acid soluble extrafarmework aluminum. From the above mentioned measurements acid insoluble extra framework aluminum (AIEFAL) was derived. The amounts of various alumina as a function of severity of hydrothermal treatment are presented in (Table-1). It can be noted that acid insoluble extraframework aluminum (AIEFL) per unit cell passes through a maximum at a steaming temperature of400 C. 

The effect of cationic EFAL species on catalytic activity, has been shown by comparing steam stabilized and zeolite Y dealuminated by (KH4)2SiFj treatment. While Creighton et al. (100) found the same selectivity, Beyerlain et al. (101) show that clean framework "fresh" zeolite Y dealuminated by (NH4)2SiFs, gave a lower i-butane cracking activity that the same sample after mildly steamed. 

To charge-balance the anionic framework, there are several structurally distinct cation locations, originally filled by sodium ions after synthesis, and then ion-exchanged to introduce acidic species into the product. Hydrothermal treatment after ion exchange results in at least partial framework dealumination, leading to hydroxo-aluminum cations as well as the functionally similar rare earth cations, which provide acid sites throughout the structure as well as increased thermal stability. 

Zhang et al. (2001) studied the HTT effects on the structure of zeolite HZSM-5 (with micro-and nanosized particles) using (Figure 2.96), Al, and Si solid-state MAS NMR. The thermal stability of nanosized HZSM-5 ( 70 nm) is lower than that of microsized particles ( 1 J.m) due to dealumination and desilicification. After HTT at 700°C, the Brpnsted acid sites (6h = 3.9 ppm) disappear (as well as nonframework AlOH at 8h = 2.7 ppm) in nanosized HZSM-5 in contrast to microsized HZSM-5 (Figure 2.96). However, the silanol peak at 1.7 ppm increases for nanosized particles. For microsized particles, the dealumination process is dominant. Upon HTT, the amorphous silica can heal dealuminated fragments. Therefore, the hydrothermal stability of nanosized HZSM-5 particles is similar to that of the microsized HZSM-5 (Zhang et al. 2001). 

Faujasite stability is frequently interpreted as the retention of zeolite surface area after steaming where the surface area is estimated using the t-plot method (17, 18). However, the zeolite framework will initially contain 15 % to 18 % alumina. During deactivation and dealumination in the regenerator most of this alumina will be removed from the framework. Microprobe studies have shown that the alumina migrates to the outside surface of the zeolite particle during the... 

In terms of chemical modifications, treatment of ZSM-5 with phosphorus compounds [42,43] seems to be an interesting route to enhance the selectivity to light olefins in cracking reactions. It has been demonstrated that after phosphorus treatment, the strong acid sites of the original zeolite are replaced by an increased number of weaker acid sites, whose concentration increases after steam treatment. Finally, the combined treatment phosphorus/ REs [44] results in an improvement in both stability and activity. Apparently, REs reduce aromatics formation on the external surface area of the zeolite, whereas phosphorus reduces the loss in activity caused by dealumination. 

It is highly probable that lattice vacancies created by removal of silicon may be filled up in the same way and under similar conditions as those remaining after release of aluminum. Thus, recrystallization of the desilicated products to particles with well-ordered crystal structure but traversed by nanopores is obviously effected by water steam present as reaction product of the dehydroxy-lation of hydroxyl nests . After the desihcation process, the zeolite is in the sodium (or potassium) form and this is known to be highly resistant towards hydrothermal effects. Therefore, it is to be expected that steaming represents the most effective method for the stabilization of desilicated zeolites avoiding the risk of concurrent dealumination. 

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