Thermal dealumination

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). 

An example of such thermal dealumination is the formation of ultra-stable Y zeolites (USY zeolites). McDaniel and Maher (6) reported the preparation of two types of ultrastable Y zeolites (a) one type prepared by the hydrothermal... 

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. 

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... 

Using the n-buthylamine titration method, Scherzer and Humphries (18) have shown that USY-B zeolites have considerably less acidity than USY-A zeolites. This is due to the more advanced thermal dealumination of USY-B, which reduces both Bronsted and Lewis type acidity. 

Kuhl (84) reported a decrease in the number of acid sites in calcined or steamed H-mordenite, due to thermal dealumination of the framework. Barthomeuf et al. (85), using data obtained from ammonia and pyridine sorption, concluded that the total acidity and the number and strength of Lewis sites decreases upon acid-dealumination of H-mordenite to a SiO /Al O ratio of 18, while the Bronsted acidity remained unchanged. 

Preparation method. Mild acid-dealumination will generally result in a more active material than the parent zeolite due to (a) removal of amorphous materials from the zeolite channels, thus lowering the diffusion resistance for the reacting molecules and (b) generation of stronger acid sites during the dealumination process, which enhances the catalytic activity of the zeolite for acid-catalyzed reactions. However, thermal dealumination will generally result in less... 

For such treated samples it is not easy to discriminate between two possible effects of dealumination, namely, the removal of some acid sites and the decrease in microporosity due to the deposition of aluminum-containing debris in the pores. Thus, hydrothermally dealuminated FER, hydro thermally dealuminated acid-washed FER. acid-washed FER, and CsFER were compared under the same experimental conditions (62). The results indicate the following order of isobutylene selectivities untreated FER < acid-treated FER < hydrothermally treated FER < hydrothermally acid-treated FER < CsFER 61). These results, obtained with noncoked catalysts, reinforce the interpretation in terms of shape selectivity. The hydrothermally acid-treated sample has acid sites located only in the micropores, and the aluminium debris in the micropores creates an additional constraint playing a role identical to that of Cs" in FER. 

Reports on the thermal stabilities of faujasites and mordenites are largely confined to their resistance to collapse at elevated temperatures. There is, however, a need to extend these works to the investigations of reactions which occur during the thermal treatment of hydrogen zeolites. These include aluminum migration, dehydroxylation and formation of new active sites. The present study is concerned with the effect of calcination temperature on the crystallinity, the extent of thermal dealumination, concentration of hydroxyl groups and catalytic activity of hydrogen faujasites and mordenites with different Si/Al framework ratios. 

Support for this suggestion can be found in the five-fold increase in the activity of NH4M6.5 mordenite calcined at 650°C (Fig. 4). According to microcalori-metric and XRD measurements, this treatment results in thermal dealumination and formation of a small number of very strong add sites, which otherwise can be detected only in high-silica samples (Table 2). 

Figure 5.13. NMR spectra of thermally-dealuminated ammonium-exchanged Y-zeolite. A. 14.1T MAS NMR spectrum, B. 11.7T DOR spectrum, from Ray and Samoson (1993), by...

Thermal dealumination also tends to decrease the effective pore diameter either by shrinkage of the unit cell and/or by generating amorphous materials in the zeolite channels. This results in an increase of the resistance to diffusivity and an increased shape selectivity. This has opened several new potential applications for zeolite matrices with controlled selectivity. 

Fig. 9.15 Variation in the number of strong acid sites with (2nh3 = 122-136kJ/mol (a) and activity in cracking of isooctane as a function of Vai for Y zeolites (b) prepared by treating the stabilized Y zeolites with HQ (1), prepared by thermal dealumination at 600-700°C (2) and produced by treatment of sodium faujasites with SiCLj (3)...

Partial re-insertion of aluminum into the framework of H-ZSM-5 dealuminated by calcination at 800°C was observed upon treatment with alkaline solutions [ 225 ]. Similarly, non-framework aluminum species created by deep hydro-thermal dealumination of H-ZSM-5 could be partially re-introduced into the framework upon treatment (2 h, 77°C) with 0.1 M NaOH solution [226]. However, no re-alumination was observed after dealumination under mild hydro-thermal conditions when the Si/Al ratio of the dealuminated material was <25. 

Thermal treatments can be applied to modify the properties of a material, for example, dealumination and optimization of crystalHne phases. These techniques do not require oxidants. Oxidative thermal treatments are generally employed to activate molecular sieves, by removing the organic templates employed during synthesis. This is one of the key steps when preparing porous catalysts or adsorbents. In air-atmosphere calcination, the templates are typically combusted between 400... 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

This could be a result of dealumination of the catalysts in the presence of water formed in the thermal pyrolysis of woody biomass. 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

Some investigations have focused on the influence of the Si/Al ratio in zeolite BEA. Corma et al. (140) used various BEA samples synthesized with different Si/Al ratios and found a higher thermal stability towards dealumination with increasing Si/Al ratio. The most stable catalyst was also the most active one. Weitkamp et al. (141) compared the selectivities of four H-BEA samples with Si/Al ratios ranging from 12 to 90. The octane number selectivities ran through a maximum at a Si/Al ratio of 19, whereas the TMP/DMH ratio decreased... 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

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). 

Combination of thermal and chemical dealumination. This is a two-step method which was applied in the preparation of aluminum-deficient mordenite (4,5) and Y zeolites (28,29). In some instances the two-step treatment was repeated on the same material, in order to obtain a higher degree of dealumination (5,28). 

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). 

Aluminum-deficient Y zeolites prepared by partial removal of aluminum with a chelating agent (e.g. EDTA) also show improved thermal and hydrothermal stability compared to the parent zeolite. The optimum stability was found in the range of 25 to 50 percent of framework A1 extraction (8). However, the maximum degree of dealumination is also affected by the SiO /Al O ratio in the parent zeolite a higher ratio appears to allow more advanced dealumination without loss of crystallinity (8,25,45). Above 50 or 60 percent dealumination, significant loss of crystallinity was observed. Calcination of the aluminum-deficient zeolite resulted in a material with a smaller unit cell size and lower ion-exchange capacity compared to the parent zeolite. 

Stability. Being a fairly high-silicious zeolite, mordenite generally has high thermal stability. It was reported (77) that progressive acid dealumination results in an increase in thermal stability, followed by a decrease. Maximum stability was reached for a Si09/Al90 ratio of about 19. 1 1 J... 

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... 

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