Controlled dealumination

Different procedures can be used in practice to activate the zeolite, and the choice of a particular method will depend on the catalytic characteristics desired. If the main objective is to prepare a very active cracking catalyst, then a considerable percentage of the sodium is exchanged by rare earth cations. On the other hand, if the main purpose is to obtain gasoline with a high RON, ultrastable Y zeolites (USY) with very low Na content are prepared. Then a small amount of rare earth cations is exchanged, but a controlled steam deactivation step has to be introduced in the activation procedure to obtain a controlled dealumination of the zeolite. This procedure achieves a high thermal and hydrothermal stability of the zeolite, provided that silicon is inserted in the vacancies left by extraction of A1 from the framework (1). The commercial catalysts so obtained have framework Si/Al ratios in the... 

USY Catalysts. USY catalysts are advertised by catalyst vendors, as low coke/high octane catalysts. This behavior results from the smaller zeolite unit cell size due to dealumination (21.221. The controlled dealumination leads to fewer but stronger acid sites resulting in increased cracking relative to H-transfer. The decrease in the extent of the exothermic H-transfer reactions also results in net increase in the endothermic heat of cracking for USY catalysts (211. 

This procedure can be succeeded by the ultrastabilization process, which is one of the basic operations in the industrial production of acid catalysts, consisting of a controlled dealumination process produced by thermal treatment in a water vapor atmosphere, which increases the thermal stability of the zeolite. 

Controlled dealumination is reported as an instrument to enhance the activity of zeolites BEA by tailoring their acidity as we have previously shown for zeolites Dealuminated BEA shows, in the anisole... 

Ga-MOR was hydrothermally synthesized by using tetraethylammonium bromide as a template. Ga content was controlled by removing Ga through HCl treatment in the same way as the dealumination of zeolite [1], The numeral at the end of catalyst name stands for Si02/Ga203 ratio in Ga-MOR thus prepared. Ga203/Ga-M0R catalyst was prepared by... 

We have characterized HY zeolites dealuminated by different procedures and at different levels and have shown that the proportion of the different types of EFAL can be controlled. Furthermore, all the zeolite modifications clearly influence the product distribution during gas-oil cracking. 

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. 

Hydrophobic zeolites, as well as the all-silica zeolites or zeolites with a very small aluminum content, possess high capacity for adsorbing organic compounds dissolved in water. Some recent studies demonstrated that hydrophobic, dealuminated zeolites adsorbed organic compounds from water as effectively as activated carbon [2,37,88,89,214], The hydrophobicity of zeolites is controlled basically by changing the Si/Al ratio in the framework by synthesis conditions and postsynthesis modification treatments [215],... 

The optinum acid treatment conditions should he properly controlled to make only some few of framework A1 atoms to be removed while the zeolite framework could not be collapsed. The increased Si/Al ratio by acid dealumination may effectively suppress the carbon deposition on the surface of catalyst during the hydrocarbon cracking reactions. 

The samples were analyzed for Na, Si, and A1 content by x-ray fluorescence spectroscopy. The extent of dealumination was found to vary for different lots of Zelon 900H. For a given lot, the extent of dealumination increased with acid extraction time. The extent of dealumination was controlled by adjusting the extraction time. Boron oxide (1.50 g B2O3) was dissolved in 140 mL of 0.25 M KOH. Dealuminated mordenite (25 g Si/Al = 64) was added to the solution, and the pH was adjusted to 13 by addition of KOH. The resulting suspension was stirred at 80°C for 24 h in a Teflon lined Parr pressure vessel. The product was separated by filtration, thoroughly washed with deionized water, and dried at 120°C. The composition of the product was determined by XRF and atomic absorption spectroscopy. 

Aluminium can be isomorphously substituted for silicon in the framework of zeolite Y by hydrothermal treatment of the dealuminated (ultrastabilised) zeolite with aqueous solutions of strong bases at elevated temperatures. The extent and efficiency of the reaction depend on the temperature, duration of treatment and especially on the kind and concentration of the basic solution. The degree of crystallinity and the thermal stability of the products are primarily controlled by the composition of the parent material. 29Si and 27A1 magic-angle-spinning NMR (MAS NMR) indicates that the extent of realumination is determined by the number of available tetrahedral Si(OAl) sites. 

Dealumination with ammonium hexafluorosilicate, developed by Skeels and Breck47, was mainly applied to HFAU zeolites. NH4 zeolites are treated with solutions of (NH4)2SiF6 under controlled pH conditions at 80°C. Aluminium is removed from the... 

A combined XPS-SIMS study of dealuminated HFAU samples shows a significant gradient in the Si/Al ratio in the crystallites50. This gradient was ascribed both to a gradient of dealumination (i.e. dealumination is diffusion controlled) and to a selective deposit of silica on the outer surface of the crystallites. Silica was proposed to be formed from hydrolysis of SiF4 that results from the decomposition of hexafluorosilicate. 

To illustrate the dealumination and structure stabilization in treated zeolites, severely treated and postcalcined ones are chosen as examples. The degree of dealumination, nevertheless, can be controlled by varying the treatment conditions. 

In the present study, a commercial H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [3,4] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The fluorosilicate method is attractive because it allows to produce silicon-enriched zeolites which are in principle perfectly microporous and exempt from framework defects and non-ffamework A1 species. Typically, the AHFS treatment differs from many of the dealumination methods in that it is carried out in aqueous media under relatively mild conditions [5]. 

With increasing temperatures the NO conversion over most catalysts of this group passes throu a maximum (33). Cu/ZSM-5 has the highest maximum at the lowest temperature, namely, 500°C. The activity of Cu in other zeolites, e.g., mordenite, ferrierite, faujasite, /3 and L, is distinctly lower (33,35). A rough correlation seems to exist between activity and Si/Al ratio of these zeolites (33) Li and Hall state, however, that the Si/Al cannot be the sole controlling factor, because Cu/Y and dealuminated Cu/Y have very similar activities (i5). 

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