Faujasites, thermal stability

Thermal measurements such as DSC and DTA can be used to determine the crystal collapse temperature. The presence of the exothermic peak is associated with the lattice collapse. As shown in Figure 4.44 for a steamed and unsteamed faujasite, the thermal stability improves with increasing silica/alumina framework. 

Figure 11 shows that maximum hydrothermal stability for this faujasite was obtained a rare earth oxide level of only 7 Wt.%. However, since maximum thermal stability was achieved at abou the 20% rare earth oxide level, the exchange should be carried out to the 20% level in order to maximize overall stability characteristics. 

The revolutionary zeolite cracking catalyst (synthetic Linde X and Y) was introduced commercially over 28 years ago, but considerable effort is still being expended on the improvement of its stability and catalytic properties. Decreasing the aluminum content of the zeolite framework and the replacing the rare-earth with the hydrogen form have greatly increased activity at the expense of stability. The thermal stability of the faujasites is fairly well understood, while the reasons for the increased catalytic activitity are still not fully known. 

Fe with the template ion. DTA studies indicate that Fe-faujasites have lower thermal stability than their Al—analogs.The (OH) vibration frequency shifts from 3540 and 3630 to 3570 and 3643 cm respectively on isomorphous substitution of Al by Fe. Relative changes in the intensity of the ESR peak at g = 4.3 at low temperatures also support the conclusion that iron can be inserted in the fauja-site lattice positions. 

The relative crystalline stability of these materials was evaluated by DTA/TG (Fig.2). It is seen that the exothermic peak (indicating the collapse of the faujasite lattice) around 1133 K in NaY( Sample A) progressively shifts to lower temperataures with increasing iron content in the zeolite reaching 1033 K for Sample E indicating that the presence of iron reduces the thermal stability of the lattice... 

The thermal stabilities of hydrogen faujasites and mordenites with different Si/Al ratios are reported. The temperature fields are outlined which characterize the thermal resistance of the lattice, framework Al, hydroxyl coverage, and the active sites. By choosing the proper conditions for activation of hydrogen zeolites, it is possible to induce the release of Al from the framework and in this way to promote the formation of strong add sites and enhance the catalytic activity. 

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. 

Figure 1 shows the variations in the relative crystallinity of faujasites and mordenites with different framework compositions pretreated at temperatures between 400 and 1000°C. It can be seen that the thermal stability of zeolites increases with decreasing A1 content or with increasing Si/Al ratio. For the zeolites with Si/Al less than 2.8, temperatures only as high as 500-550°C can quantify the onset of structural degradation. By contrast, the structure of the siliceous varieties with Si/A1 > 2.8 disintegrates only above 700°C. The improve-... 

Table 1 summarizes the data on the thermal stability of the hydroxyl groups in faujasites and mordenites. The table contains the results derived from the measurements of the relative intensity of the hydroxyl bands at 3640 cm l and 3610 cm l as a function of the calcination temperature for faujasites and mordenites, respectively, vrith different amounts of the framework Al. Included also are the data calculated from the concentration of OH groups foimd by H-D exchange and from the high temperature weight loss based on thermogravimet-ric analysis. 

The key structural feature of the molecular sieves is the narrow, uniform, continuous channel system that becomes available after the zeolitic water has been driven off by heating and evacuation. Great thermal stability after dehydration has been observed in the rigid lattices of X- and Y-type faujasites, zeolite A, mordenite, and chabazite. The geometry of the internal channel and cavity system is characteristic of the individual zeolite. Entrance to the intracrystalline volume is through orifices (ranging from 3 to 9 A in the various zeolites) located periodically throughout the structure. It is thus apparent that access to the intrazeolitic environment is limited to molecules whose dimensions are less than a certain critical size. 

These dealumination procedures result in Al deficient zeolites of the high thermal stability. In the case of faujasite Y, the so called ultra-stable Y zeolite (US-Y) is used as a catalyst for cracking catalysis (47). It was reported that the total acidity of (48) Al-deficient zeoITtes was less than that of the parent zeolite buT with stronger acidic sites (49). For mordenite, this acidity decreases linearly with Al content (50, 51). However calorimetric measurement of the NH- heat of adsorption has shown that when the total number of acidic sites decreases regularly with dealumination, as could be reasonably expected, the strength of the strongest acid sites is enhanced (52). 

Ambs and Flank (2) have concluded on the basis of limited data that the thermal stability of synthetic faujasite is dependent only on the level of sodium present. They further assert that no difference exists between decationated Y and ultrastable materials. 

Intermediates in high-temperature processes have been stabilized at low temperature after y irradiation of metal oxides and zeolites. Important early examples were oxygen anions. O, 02 and O. Some of their reactions with small molecules were also elucidated by EPR. Metal cluster ions have also been produced by radiolysis and stabilized in zeolites. Examples include alkali metal cation clusters in faujasites and silver cation clusters in zeolite A and in silicoaluminophosphate molecular sieves. Detailed information was obtained from EPR studies about their structure, thermal stability and formation of adducts. 

Since the thermal stability of hydrogen forms of faujasite-type zeolites turned out to be crucial in their important industrial application as cracking catalysts [401, 402], techniques were developed for stability improvement, as for instance the treatment via steaming of the catalysts [403-406]. This procedure of treatment of NH4- or H-forms of faujasite-type catalysts requires the presence of water vapor at elevated temperatures [404-407]. It proved to reduce the density of strong Bronsted sites (indicated by the bands at ca. 3640 and 3550 cm ), to increase the concentration of non-acid silanol groups (i.e., the intensity of the 3740 cm -band) and to generate additional OH groups different from those... 

Chemical and thermal stability of dealuminated faujasite-type zeolites in gaseous and aqueous phases". Applied Catal., 2S. 71-87. 

Zeolite catalysts have also been proposed for stationary SCR applications, mainly in gas-fired cogeneration plants. Zeolites in the acid form, in which transition metal ions (eg, Fe, Co, Cu, Ni) are introduced in the structure to improve the SCR activity, guarantee high de-NOx activity even at high temperatures to a maximum of 600° C, where metal oxide based catalysts are thermally unstable. The use of metal-exchanged zeolite-based catalysts with distinct structures has been proposed, for example, mordenite, faujasite (both of X and Y types), and ZSM-5 (21,22). Techniques to remove the aluminum oxide from the crystal matrix can be conveniently applied to increase the Si/Al ratio and accordingly the thermal stability of the zeolite and at the same time to limit its tendency to sulfatation. 

An important group of catalysts for NOx reduction through anunonia SCR is represented by copper exchanged zeolites [1-9]. Many different zeolites have been investigated, for example, Cu-ZSM-5 [10-13], Cu-faujasite [14], Cu-Beta [15,16], and Cu-Y [17]. Recently, copper zeolites with chabazite (CHA) structure have received great attention due to their high thermal stability and hydro carbon resistance [18]. Both Cu-SAPO-34 [19] and Cu-SSZ-13 [18] have a CHA structure... 

In 1967, McDaniel and Maher [61] reported a method to increase the thermal stability of Y zeolite. This so-called ultrastabilization procedure immediately became a matter of considerable interest because of the technical importance of Y zeolite as a catalyst. The process consists of two major steps, (1) the practically complete removal of sodium ions by a two-step ammonium ion exchange with intermittent heating and (2) the conversion of this material by heat treatment at 800 °C or above to a faujasite-type zeolite resistant to the influence of heat up to about 1000 °C. Although McDaniel and Maher noticed that ultrastabilization is associated with a decrease of the ion-exchange capacity and unit-cell size, the question of the framework aluminum content of modified Y zeolite was not explicitely raised in their paper. 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

The open frameworks of zeolites are slightly less stable than the corresponding condensed structures [15,16] into which they will transform during severe thermal treatment. Nevertheless, the difference in energy between a-quartz, the most stable polymorph of silica, and siliceous faujasite, one of the most open and least stable, is only about 15 kj mol k The extensive occurrence of aluminosilicate zeolites and their widespread utility in industry therefore depend heavily upon both the strengths of their T-O bonds (e.g. Si-O 466 kJ mol ), which render them stable with respect to framework rearrangement. The challenge with many of the newer materials is that their stability with respect to transformation into alternative condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of the many open-framework families of materials can be rendered truly nanoporous,... 

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