Dealumination high-temperature

The reaction mechanism during the thermal treatment step is similar to the one already described for thermal dealumination. High temperatures and steam will enhance the expulsion... 

An uitrastabie or a dealuminated zeolite (USY) is produced by replacing some of the aluminum ions in the framework with silicon. The conventional technique (Figure 3-9) includes the use of a high temperature (1,300-1,500°F [704-816°C]) steam calcination of... 

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. 

Thermal dealumination. The method involves calcination of the ammonium (or hydrogen) form of the zeolite at relatively high temperatures (usually over 500°C) in the presence of steam. This results in the expulsion of tetrahedral aluminum from the framework into non-framework positions, but does not remove the aluminum from the zeolite. The process consists essentially in a high-temperature hydrolysis of Si-O-Al bonds and leads to the formation of neutral and cationic aluminum species (Figure 1A). 

Kerr (7-9) has shown the critical role of the calcination environment and bed geometry in the formation of USY zeolites ("deep bed" vs."shallow bed"calcination). Ward (10) prepared USY zeolites by calcining ammonium Y zeolites in flowing steam. The work done by Kerr and Maher et al. (11) has clearly demonstrated that USY zeolites are formed as a result of aluminum expulsion from the framework at high temperatures in the presence of steam. The nature of the non-framework aluminum species has not been completely clarified. Obviously, their composition will be strongly affected by the preparation procedure of the USY zeolite. Table II shows different oxi-aluminum species assumed to be formed during thermal dealumination of the zeolite framework. 

More recently, dealumination was achieved by fluorination of zeolites at ambient temperature with a dilute fluorine-in-air stream, followed by high-temperature calcination (102). The suggested reaction mechanism involves the formation of different aluminum-fluorine compounds along with zeolites containing hydroxyl and fluorine nests. During the high-temperature calcination, it is assumed that silica insertion occurs, similar to the scheme in Figure IB. 

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). 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

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]. 

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. 

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. 

Bertea et /.173-751 reported the vapour phase nitration of benzene with aqueous nitric acid (65%) at 170 °C over post-synthetic dealuminated Y, ZSM-5 and Mordenite zeolites by high temperature and acid treatment. This treatment reduces the framework as well as the nonframework aluminium content, which results in... 

In both cases, framework Al may be exposed to water vapour at rather high temperature (525-875 K), which can lead to dealumination of the zeolite structure (production of nonframework Al species and decrease in the concentration of acid sites, modification of sorptive properties and catalytic behaviour). In order... 

Dealuminated zeolite samples were calcined in air at 540°C for three hours prior to catalytic testing. A portion of each sample was further modified using high-temperature steam. Zeolite samples were placed in a fixed-bed quartz tube 95% steam was passed through the bed at 750°C and atmospheric pressure for 4 hours. 

Dealumination of zeolites can be achieved using several other methods such as extraction with EDTA (e.g., 35, 76), high temperature steaming (e.g.,... 

Progressive steam dealumination of HY zeolite at high temperatures causes the same effect observed above for high temperature calcination (149,... 

X-Ray photoelectron spectroscopy data showed an increase of about 20% in the Al/Si ratio as the outgassing temperature was increased from 773 to 1173 K, indicating that the surface concentration of A1 increased (169). Treat-ment with HCl to dissolve nonstructural aluminum produced a 15% decrease in the aluminum content of the sample calcined at 1173 K with respect to the 773 K sample. Subsequently, a small extent of dealumination occurred during the dehydroxylation at high temperature. It was suggested that aluminum from the lattice was extracted on calcination and resulted in an aluminalike species that stayed within the cavities of the zeolite. 

A highly crystalline ZSM-12 (24) was hydrothermally synthesized by Fyfe et al. (25) and dealuminated by steaming at a high temperature (18). There were extra lines in... 

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. 

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. 

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