Framework dealumination

A) Thermal or hydrothermal treatment of zeolites. This results in partial framework dealumination, but the aluminum remains in the zeolite cages or channels. 

It was also shown that thermal treatment of an ammonium zeolite under steam causes not only framework dealumination, but also a structural rearrangement in the zeolite framework. The defect sites left by dealumination are filled to a large extent by silica, which leads to a very stable, highly silicious framework (11,12) (Figure IB) Defect sites not filled by silica are occupied by "hydroxyl nests" (13). 

When the two-step process is repeated on the same material, the thermal treatment following the chemical dealumina t ion results in further expulsion of aluminum from the framework into zeolite cages or channels. The solubilization of non-framework aluminum during the first chemical treatment appears to facilitate further framework dealumination during the subsequent thermal treatment due to the altered steric and electrostatic parameters in the zeolite channels. The newly formed non-framework aluminum species can be readily solubilized by acid treatment. This cyclic method has allowed the almost total removal of aluminum from mordenite (5). 

Using Si-NMR spectra to calculate the SiO /Al O ratio in the framework and chemical analysis to determine the overall SiO /Al O ratio, Lippmaa et al. (38) concluded that in their sample of USY-A zeolite, 33 Al/u.c. are in the framework and 24 Al/u.c. are in non-framework positions (42 percent framework dealumination). In the USY-B zeolite prepared from the same parent NaY, Lippmaa et al. found 4 Al/u.c. in the framework and 53 Al/u.c. in non-framework positions (93% framework dealumination). 

The unusually high stability of DAY zeolites prepared from USY-B and having SiO /Al O ratios over 100 indicates that the non-framework aluminum species present in USY-B play no role in enhancing the stability of this zeolite. It is the highly silicious framework, in which most of the aluminum has been replaced by silicon atoms, that is responsible for the high stability of USY-B zeolites and of corresponding DAY zeolites. In zeolites with a lesser degree of framework dealumination (i.e. in USY-A), the non-framework aluminum species appear to play a role in the stabilization of the zeolites, since their removal results in materials of lesser stability (28). 

The differences in the shape of isotherms have been attributed to the formation of secondary pores during framework dealumination. The hysteresis loops observed were attributed to capillary condensation in the secondary pores. 

The spectrum of the aluminum-deficient mordenite shows no signal for Si(2 Al) and a considerable decrease in intensity of the Si(l Al) signal, due to framework dealumination. 

Degree of dealumination. Moderate dealumination generally increases the catalytic activity or leaves it unchanged, while advanced dealumination leads to a decrease in activity. Such a decrease is due to a loss of active sites with advanced framework dealumination. 

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

It is also of interest to use MAS NMR for the study of the thermal treatment of zeolites which are not in the ammonium-exchanged form. In an X-ray study, Pluth and Smith (179) found electron density at the center of the sodalite cages in dehydrated zeolites Ca-A and Sr-A and attributed this to a partial occupancy of these sites by a four-coordinated aluminous species. No such effect was found in zeolite A exchanged with monovalent cations. Corbin et al. (180) used 27A1 MAS NMR to examine commercial samples of K-A, Na-A and (Ca,Na)-A, as received (see Fig. 41). For K-A and Na-A, only framework tetrahedral Al species were observed, with chemical shifts of 57 and 52 ppm respectively. However, in (Ca,Na)-A an additional intense resonance at 78 ppm, typical of AlfOH) but definitely not due to framework aluminum, was also found (see Fig. 41). A much weaker signal, also at 78 ppm, was detected in zeolite Sr-A its intensity increased greatly on heating the sample to 550°C. Freude et al. (183) came to very similar conclusions in their NMR study of heat-treated zeolite Ca-A. They found that maximum framework dealumination occurs at 500°C and corresponds to ac. 17% of total Al. 

The combination of high temperature and steam in the regenerator causes framework dealumination (rapid) and crystalline zeolite destruction (slower). Dealumination both lowers activity and produces important changes in selectivity, while zeolite destruction leads primarily to a loss in activity. 

The results for residual carbon on equilibrium catalyst fractions (Figure 2) and for cumene cracking on regenerated equilibrium fractions (Table V) also indicate that cracking activity shows little dependence on zeolite content following completion of framework dealumination (minimum unit cell size). 

The framework dealumination of NaY type zeolites with SiCl4 has also been studied by A1 n.m.r.60... 

FTIR spectra (lattice vibrations) of a stabilised (S3) and an aged (S6) catalyst are compared in Fig. 5. The shift to higher frequency of the O-T-O band at about 1100 cm-1 indicates framework dealumination of S6 relative to S3 [16J. In addition, the decrease of the intensity ratio I550/I450 for the bands at 550 cm l... 

The hydrothermally dealuminated zeolites (USY-1 and USY-2) used in this study were plant-grade materials and were used without further exchange. The chemically dealuminated zeolites (AFS-1 and AFS-2) were prepared in our laboratory following the method of Breck and Skeels(12). The AFS samples were washed thoroughly until residual fluoride was no longer detected in the filtrate AFS-1 received additional warm-water washes to reduce further fluoride levels in the solid. AFS samples were prepared so as to have the same extent of framework dealumination as the USY samples. 

However according to Kiihl [36] this process may be followed by framework dealumination which involves the formation of extra-lattice Lewis acid AlO species ... 

Recently, bifunctional Pt/beta catalysts have shown a good performance for LSR isomerization (9). Its catalytic activity was seen to depend on the synthesis condition of the zeolite (10). In this work we have studied the influence of post-synthesis treatments, i.e. framework dealumination and EFAL extraction, on the activity and selectivity of Pt/beta catalysts for LSR isomerization. 

Acid form of zeolite beta (Hp) was obtained from a commercial TEA-beta (Valfor CP806B-25) by calcination at 773 K for three hours to remove the template, followed by ion exchange with a 2M aqueous solution of NH4CI at 353 K for two hours, and a final calcination at 773 K for three hours. Then, the HP was dealuminated using three different procedures, i.e. steam calcination at 873 K during three hours (sample HPs,), acid treatment with HCl O.IM at reflux for two hours (sample HP, ), and ammonium hexafluorosilicate (HFS) treatment (sample HP ps2)- The latter two procedures produced almost EFAL-free beta samples. Moreover, the EFAL formed in HP during the calcination steps was also extracted with the required amount of ammonium hexafluorosilicate in order to avoid framework dealumination (sample HP fsi)- The HFS treatments were carried out in an ammonium acetate buffer at 348 K with slow addition of a 3M hexafluorosilicate solution (12 cm /h). Afterwards,... 

McDaniel, Maher, Kerr and Shipman have studied[21,22] framework dealumination under high-temperature conditions for (A) in detail, and they concluded that the dealumination product is complicated and in the extraframework channels, cages, and on the surface there are various extraframework aluminum species (generally denoted EFAL) (Figure 6.11). 

In solution, zeolite framework dealumination can be realized through interactions of zeolites with acids, salts, and chelates. 

Usually, inorganic and organic acids can be used for framework dealumination of zeolites, and the acids include hydrochloric acid, nitric acid, formic acid, acetic acid, and so on. According to its acid-resistance ability, hydrochloric acid can be used for high-silica zeolites such as mordenite, clinoptilolite, erionite, etc. We will take mordenite as an example to describe this dealumination method (Table 6.4). The first step in the treatment of mordenite using hydrochloric acid is to convert the zeolite into H-type, and further acid treatment can then enlarge the pore diameter through dealumination. After partial dealumination, the Si/Al ratio of the zeolite is increased and the heat-resistance, water-resistance, and acid-resistance abilities are enhanced. 

The activity of the catalyst samples for both hydration reactions depends strongly on Nai. For high framework aluminium contents (from 50, corresponding to the parent H-Y, to about 35) the catalyst samples are inactive. This is probably due to the hydrophilic properties of the zeolite s active surface. The catalytic activity increases with the framework dealumination to reach a maximum value at Nm = 22. This increase is likely to be due not only to the increase in the acid strength of the Bronsted sites, but also to the changing in the hydrophilic/hydrophobic balance of the zeolite s active surface. 

The effect-fif steaming severity on dealumination is shown in Figure 6. The ySi NMR spectra show that, for a specific zeolite composition, framework dealumination increases with increasing steaming severity. 

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