Dealuminated Y-type zeolites

J.H., and O Donnell, D.J. (1985) Determination of framework aluminum content in dealuminated Y-type zeolites a comparison based on unit cell size and wavenumber of i.r. bands. Zeolites, 6, 225-227. 

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. 

Hydrothermally dealuminated Y type zeolites (HDY s) possess a number of characteristics which make them very useful catalyst components—very high activity for acid-catalyzed reactions and outstanding thermal stability Such zeolites were thus rapidly incorporated into catalysts for two major petroleum refining processes, catalytic cracking [lp2] and hydrocracking, [3,4] which operate at high temperatures, and for which resistance to process upsets and the ability to withstand oxidative regeneration are both important. 

The structural stability of a Variety of commercially available hydrothermally dealuminated Y type zeolites in high pH ammonium salt solutions was studied It was found that such zeolites can lose large amounts of crystallinity, the susceptibility to structure loss increasing with salt concentration and pH, and with increases in both the chemical and framework silica/alumina ratio of the zeolite,... 

Dallas amorphous material dealuminated Y-type zeolite diethylbenzene... 

It was observed that the exact wavenumber of bands indicating acid OH groups in zeolites was influenced by the nsj/nAi ratio of the framework. Generally, the bands were found to shift to higher frequencies when the silicon content increased. This was first studied systematically by Barthomeuf [395,396] (see also Sect. 5.4.1.2). Thus, the band observed at the highest frequency, i.e., at 3745 cm" (vide infra) is the only one in the case of completely dealuminated Y-type zeolite or other siliceous frameworks (such as silicaHte-1, cf Sect. 5.4.2) and coincides with the band well-known from pure, i.e., aluminum-free, non-acid, amorphous silica. In fact, it is indicative of a third type of OH groups of deammoniated... 

The microcalorimetric technique has also been appHed to investigate the adsorption of acetonitrile, dimethylether, water, pyrrole, and ammonia on ferrierite, a small pore zeolite [84]. This investigation showed that the results of the determination of the site strength distribution are dependent on the strong or weak basicity of the probe, on the acidity and porosity of the acid soUd, and on the adsorption temperature. A similar calorimetric investigation of the acidity of dealuminated Y-type zeolites was performed using the same various basic probes [85]. 

A. Auroux, Z.C. Chi, N. Echoufi, Y. Ben Taarit, Calorimetric investigation of the acidity of dealuminated Y-type zeolites using various basic probes, in Zeolites as Catalysts, Sorbents and Detergent Builders, vol. 46, ed. by H.G. Karge, J. Weitkamp (Elsevier, Amsterdam, 1989), pp. 377-387... 

H.G. Kmge, V. Dondur, J. Weitkamp, Investigation of the distribution of acidity strength in zeolites by temperature-programmed desorption of probe molecules. 2. Dealuminated Y-type zeolites. J. Phys. Chem. 95,283-288 (1991). doi 10.1021/jl00154a053... 

Acidic micro- and mesoporous materials, and in particular USY type zeolites, are widely used in petroleum refinery and petrochemical industry. Dealumination treatment of Y type zeolites referred to as ultrastabilisation is carried out to tune acidity, porosity and stability of these materials [1]. Dealumination by high temperature treatment in presence of steam creates a secondary mesoporous network inside individual zeolite crystals. In view of catalytic applications, it is essential to characterize those mesopores and to distinguish mesopores connected to the external surface of the zeolite crystal from mesopores present as cavities accessible via micropores only [2]. Externally accessible mesopores increase catalytic effectiveness by lifting diffusion limitation and facilitating desorption of reaction products [3], The aim of this paper is to characterize those mesopores by means of catalytic test reaction and liquid phase breakthrough experiments. 

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

Figure 5. Hexane cracking activity as a function of framework aluminum content O, Y-type zeolite dealuminated with SiClA ...

Y-type zeolite prepared by steaming a, Y-type zeolite dealuminated with ammonium hexafluorosilicate , after La3+ exchange to level of maximum activity , ZSM-20 g, after La3+ exchange to level of maximum activity v, as synthesized zeolite Y , after exchange to level of maximum activity , HLa-X. Reproduced with permission from Ref. 20. 

The solid-state interaction between Y-type zeolites and niobium oxide (Nb205) is strongly dependent on the form of the parent zeolite (sodium, hydrogen or dealuminated forms) and on the conditions of the high temperature treatment. 

Lanthanum is known to improve the hydrothermic stability of Y-type zeolites [20]. A second attempt to stabilize the catalyst was made by preparing La-exchanged EMT-type samples. The dealumination process was stopped but this catalyst did not present any catalytic activity in NO reduction. A La-Cu-EMT-3.8-70-10 sample was also prepared. A maximum extent of reduction of 15% was measured at 500°C. This low activity in NO reduction is probably due to the very low copper content. 

The effect of rare earth loading of Y-type zeolite on its dealumination during thermal treatment was monitored in a study by Roelofsen et al. [301],who found the band at 790 cm to be most appropriate for the determination of the nsi/n j ratio, since the other structure-sensitive bands were too much affected by the cation amount, crystallinity and water content. 

The mechanism of acid-site development in Y-zeolites was described in the previous section, but it is essenticdly same for other zeolites. In the case of more acid-resistant zeolites such as ZSM-5 or mordenite, proton can be introduced by direct ex-chEmge with dilute hydrochloric acid, though this often causes dealumination of the zeolite framework. The reason why NaY must be converted into NH4Y to obtain HY is the instability of Y-type zeolites in acidic solutions. 

Since the development of zeolite, chemistry transalkylation has been studied mainly using zeolite catalysts. Frilette used natural mordenite treated by acid. The activity was much hi er than amorophous Si02 —AI2O3, but the activity could not be maintained. Benesi reported that mordenite was about 8 times more active than Y-type zeolites and that the active centers were Brensted acid sites. Various efforts including dealumination and cation exchange have been made to improve the aging. 

Literature data on the deal umi nation with Si Cl 4 of X and Y type zeolites with Na, Li, H, Ca or La as charge compensating cations is reviewed. The properties of the final zeolite products, in particular the degree of dealumination, the crystallinity, mesoporosity, and the nature and content of extra-framework aluminium and silicon species are explained based on the chemistry involved in the different steps of the dealumination procedures. The reactivity towards SiCl4 of the framework aluminium atoms is explained based on the location of the associated charge compensating cations over accessible and hidden sites. 

The dealumination of X type zeolites with SiCl4 occurs via the same mechanism as explained for Y type zeolites. 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

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