Dealumination zeolites

Zeolite dealumination was first reported by Barrer and Makki (1), who progressively removed aluminum from clinoptilolite by treating the zeolite with hydrochloric acid of different strengths. Subsequent dealumination studies were carried out primarily on mordenite (2-5) and Y zeolites. 

Reactions with acids. Hydrochloric acid was used in the dealumination of clinoptilolite (1), erionite (14) and mor-denite (2,3,15,92). In the case of Y zeolite, dealumination with mineral acids was successful only after conversion of the zeolite into the ultrastable form (vide infra). Barrer and Makki (1) were the first to propose a mechanism for the removal of aluminum from mordenite by mineral acids. It involves the extraction of aluminum in a soluble form and its replacement by a nest of four hydroxyl groups as follows ... 

Reaction with chelating agents. Such reactions have been used primarily for partial dealumination of Y zeolites. In 1968, Kerr (8,21) reported the preparation of aluminum-deficient Y zeolites by extraction of aluminum from the framework with EDTA. Using this method, up to about 50 percent of the aluminum atoms was removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity. Later work (22) has shown that about 80 percent of framework aluminum can be removed with EDTA, while the zeolite maintains about 60 to 70 percent of its initial crystallinity. Beaumont and Barthomeuf (23-25) used acetylacetone and several amino-acid-derived chelating agents for the extraction of aluminum from Y zeolites. Dealumination of Y zeolites with tartaric acid has also been reported (26). A mechanism for the removal of framework aluminum by EDTA has been proposed by Kerr (8). It involves the hydrolysis of Si-O-Al bonds, similar to the scheme in Figure 1A, followed by formation of a soluble chelate between cationic, non-framework aluminum and EDTA. 

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. 

The HYF zeolite dealuminated with (NH ) F Si only shows lines at 0 ppm and 60 ppm, even after deep bed calcination the lines at vSO ppm and 54 ppm are not visible. 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

The zeolite dealumination mechanism is illustrated in Scheme 2.1.6.2. During treatment with silicon tetrachloride, a dealumination method first reported by Beyer et al. [50], the faujasite s framework aluminum was isomorphously replaced by silicon while maintaining the microporous structure. The reaction was self-... 

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. 

This method can be applied to zeolites dealuminated by various techniques. In steamed faujasites different extra-framework Al and Si species are observed. The dealuminated samples are soaked in a 38% acac-methanol solution at room temperature under a flow of dry air. After one hour the ethanol and excess acac are evaporated. 

Catalytic activity, assessed by cumene cracking on separated fractions and also by analysis of residual coke on catalyst fractions, shows a sharp decline with increasing density (age). This rapid loss of initial activity coincides with zeolite dealumination which is largely completed as a slow rate of zeolite destruction is established. Subsequent loss of crystallinity has little additional effect on activity. The associated loss of microporosity leads to an apparent increase in skeletal density with increasing age. 

In hydrocarbon conversion over zeolite catalysts, the formation and retention of heavy products (carbonaceous compounds often called coke ) is the main cause of catalyst deactivation. 5X 77 XI1 These carbonaceous compounds may poison or block the access of reactant molecules to the active sites. Moreover, their removal, carried out through oxidation treatment at high temperature, often causes a decrease in the number of accessible acid sites due to, e.g., zeolite dealumination or sintering of supported metals. 

In addition to the Bronsted acidity in zeolites, in these materials the Lewis acidity is present as well. According to Lewis, an acid is an electron pair acceptor, a definition which is broader than that given by Bronsted, since a proton is a particular case of an electron pair acceptor. Then, the definition of Lewis covers practically all acid-base processes, whereas the definition of Bronsted represents only a particular type of process [128], The Lewis acidity is related to the existence of an extra-framework A1 (EFAL) species formed during the zeolite dealumination process [128],... 

In zeolite-based catalysts, the Lewis acidity is related to the existence of extra-framework A1 (EFAL) species formed during the zeolite dealumination process [18], It occurs frequently in zeolite activation, for example, during the calcination process... 

The objective of this work is to identify relationships between structure and catalytic performance in the specific case of hydrocarbon cracking over dealuminated Y zeolites. Dealuminated zeolites are prepared using chemical and hydrothermal methods and the effect of dealumination method on structure-performance... 

This evidence suggests that not all Na species are mobile. Some Na species must in fact have reacted irreversibly with components on the catalyst, leaving it unavailable to poison the acid sites. It is likely that these reactions occur during the early stages of hydrothermal deactivation. The exact mechanism is unclear, but may involve reactions with extraffamework alumina. As the zeolite dealuminates from 24.55 to 24.25A unit cell size, approximately 65% of the initial framework alumina (about 15 wt% of the zeolite) comes out of the zeolite structure. Sodium, which also must leave the exchange sites as the zeolite dealuminates may react with this very reactive form of alumina. The other possibility is that as kaolin undergoes its transition to metakaolin at 800K... 

The main objective of this work is to characterise the textural properties of a series of Y zeolites dealuminated by ammonium hexafluorosilicate treatment. It was observed that the fluorosilicate treatment produced a highly crystalline product with a contracted unit cell. Both textural and XRD analysis confirmed the samples to be at least 95% crystalline for dealumination degrees <50%. According to N2 adsorption only a minimal contribution of mesopores was observed. However, a notable loss of micropores accompanied by the formation of mesopores was noted for severe dealumination levels, along with a considerable structural degradation. 

In Section V it was shown that the Si/AI ratio has a strong influence o the acidic properties of zeolites. Dealumination, as discussed previously, is a widely used means of changing the acid character of zeolite catalysts. Such changes in the acid strength distribution are manifested as changes in catalytic behavior. For example, dealumination of HY zeolites increased the catalytic activity for cumene cracking at 573 K, reaching a maximum at a... 

Zeolite dealumination is one of the most important subjects in the field of zeolite secondary synthesis and modification. For decades, zeolite scientists have been investigating dealumination routes and techniques in order to optimize the properties and functions of zeolites. Generally, there are three dealumination routes presented as follows ... 

Cerium-, copper-cerium coexchanged ZSM-5, copper-MCM-22, copper- and cerium-EMT type zeolite, copper-FAU type zeolite and copper-Beta exhibit an activity of the same order as that of copper-ZSM-5 in NOx reduction under simulated Diesel exhaust conditions. Propene was used as the reducing agent. The catalysts were used in a powder form and their activities tested in a fixed-bed flow reactor at a space velocity of 50 000 H . Copper-SAPO-34 and cerium- and gallium-EMT type zeolite have a moderate activity under the same conditions. The presence of water vapor inhibits the activity of EMT-zeolites. When an ageing procedure is carried out on copper-EMT type zeolite, dealumination occurs. The increase of the Si/Al ratio of the zeolite does not limit the dealumination process. The exchange of the zeolite with lanthanum prevents the zeolite from dealumination but leads to a loss of the catalytic activity. 

The Bronsted acid sites of HY zeolites dealuminated either by conventional treatment (steaming + acid leaching) or isomorphous substitution (fluorosilicate) have been characterized at each step of the preparation procedures through IR spectroscopy of probe molecules with various basic strengths (pyridine, C2H4,... 

This was attributed to the increase in acidity due to the completely isolated framework Al. On the other hand, the hydrogen transfer reactions, which are believed to be responsible for olefin saturation and, consequently, for the parallel decrease in the RON observed, have been related to the density of acid sites. These reasonable assumptions can not fully explain, however, the product distribution observed during the cracking of gas-oil on a series of Y zeolites dealuminated at different levels and by different procedures. This is due to the presence, besides the framework-associated Bronsted sites, of Bronsted and Lewis sites which are associated with extraframework aluminium (EFAL) and which can catalyze carbonium ion as well as radical cracking reactions. 

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