Extraframework material

Catalyst Structural Characteristics. Structural features of AFS and USY materials have been characterized in this work in terms of unit cell size, presence of extraframework material, active-site distributions, and pore-size distributions. These features are similar for both sets of USY and AFS samples which indicates that structural characteristics are not related to the source of Y zeolite. 

As-synthesized AFS zeolites do not contain extraframework aluminum as evidenced by Al NMR. As-synthesized USY zeolites contain appreciable amounts of extraframework material as seen by comparing framework and bulk silica-alumina ratios and by examining 27A1 spectra. Upon calcination both AFS and USY materials contain extraframework aluminum. The amount of extraframework aluminum in both AFS and USY materials increases on steaming. 

Catalyst Performance Relationships. Hexadecane cracking activity of AFS and USY zeolites, when corrected for deactivation effects, shows little or no dependence on framework composition. Rather, as shown in Figure 6, activity appears to be a function of total aluminum content independent of the method of dealumination. This result implies that hexadecane cracking occurs over both framework and extraframework acid sites and that it is the total number of such sites which determines catalytic activity. Hence, extraframework material in the USY samples makes a significant contribution to catalyst activity as reported by others(18.19). 

Gasoline selectivity is influenced by both the method of dealumination and steam treatment and, hence, depends on framework acid sites and on presence of extraframework material. Both framework and extraframework sites contribute to the overall zeolite acidity. Framework Al(IV) acid sites are associated with Bronsted acidity whereas extraframework Al(VI) acid sites are associated with Lewis acidity(21). Calcined AFS samples are reported (22) to contain greater Bronsted acidity than USY samples from infrared characterization and to have stronger acidity as measured by ammonia desorption. As a result, the stronger acidity of calcined AFS samples cracks hexadecane to lower molecular weight products than USY. After steaming, the acidities of both AFS and USY are reduced to similar levels and lead to similar gasoline selectivities which are increased relative to the calcined zeolites. 

Gasoline selectivity and octane performance are functions of not only unit cell size but also factors such as extraframework material and spatial distribution of active sites. These factors influence... 

Figure 9.5 FTIR spectra of a H-ZSM5 sample containing extraframework material, after activation by outgassing at 673 K, and cooling at 120 K (a), in contact with CO at 120 K (b), and after outgassing at 120 K (c)...

The perspectives of the use of basic zeolites as catalysts has been reviewed recently by Davis [287], Extraframework material free, alkali exchanged zeolites are used as quite mild basic catalysts. As seen earlier, light alkali-metal zeolites, such as Na-X and Na-Y, have a mild Lewis acid behavior and do not appear to have strong basic character [245], However, heavy-alkali metal zeolites like Cs-Y act actually as basic catalysts, or better as acid-base catalysts, for example, for toluene side chain alkylation. Stronger basic character arises from impregnation of alkah-zeolites with alkali salts, later decomposed to occluded alkali oxide, as evidenced by CO2 adsorption microcalorimetry. The characterization of such materials is still quite poor. 

In an attempt to produce TS-1 at low cost, alternative, cheaper sources of Ti and Si and other bases such as binary mixtures of (tetrabutylammonium and tetraethylammonium hydroxides), (tetrabutylphosphonium and tetraethylpho-sphonium hydroxides), (tetrapropylammonium bromide and ammonia, water, hexanediamine, n-butylamine, diethylamine, ethylenediamine, or triethanolamine) in place of TPAOH have been used (284—294). TS-1 was synthesized in the presence of fluoride ions but the material thus formed contained extraframework Ti species (295-297). 

Experiments to further demonstrate the critical role of extraframework Al, or another polyvalent cation, have recently been carried out in our laboratory (19.20). A series of faujasite-type zeolites was prepared that had Alf concentrations between 21 and 54 per u.c. At the low end of the range, AHF was used to remove the framework Al, and an H-ZSM-20 zeolite with 42 Alf/u.c. was synthesized. ZSM-20 is an intergrowth of the cubic faujasite structure and the hexagonal variant know as Breck s structure six (BSS) (21). Thus, it is a faujasite-like material. The catalytic activities of these zeolites for hexane cracking are compared in Figure 5 (lower data set) with the activities of zeolites prepared by steaming or by treatment with SiClA (upper data set). The solid lines represent N(0) distributions. The samples without extraframework Al exhibited very modest activity, even though some of them had a favorable N(0) concentration. 

The evidence to date suggests that the development of strong Bronsted acidity in Y-type zeolites requires both isolated Al atoms and the presence of extraframework aluminum. Extraframework aluminum may, of course, exist in several forms, including a boehmite-like material... 

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

Hexadecane cracking activity correlates with total aluminum content USY materials are more active than AFS materials before and after steaming. Extraframework aluminum contributes to catalytic cracking activity. 

Zeolite catalysts are frequently applied after treatments that tend to increase their stability and also to further enhance surface acidity and shape selectivity effects. These treatments, such as steam dealumination, can cause a decrease in the framework A1 content and the release of aluminum-containing species from the framework. This can contribute to the stability of the framework, but extraframework species can also contain additional catalyticaUy active acid sites. These particles can also narrow the size of the zeolite charmels or of their mouths, so improving the shape selectivity effects. Extra-framework material (EF) can also... 

The second aspect which makes the interest of the study of zeolites by XPS is that it allows to understand many unexpected aspects of this spectroscopy, in particular when applied to solids. Zeolites in many respects are materials which may be regarded as model solids and not only because of their regular pore structure. Indeed by varying the Si/Al ratio, the nature of counter cations, the ion exchange level, the occlusion of extraframework phases, such properties as the density and strength of acidic or basic sites may be varied in a predictable fashion. All these variations are reflected in the XPS spectra yielding data... 

In the practical syntheses of ferrisilicates, isomorphous substitution of iron usually is not complete, thus the starting material may already contain some amount of extraframework ions. At the following steps (e.g. at the conversion of the synthesized Na form to H-form, or at the activation or during catalytic processess) the portion of the... 

Aluminophosphates (AIPO4S) are a class of materials which, like the aluminosilicate zeolites, assume open framework structures containing channels of molecular dimensions with molecular sieve properties. Since the AIPO4 structures contain equal numbers of AIO4 and PO4 units there is no necessity for charge-balancing extraframework cations and consequently no sites to provide acid catalytic properties. 

UV-vis spectrum of the calcinated material showed an adsorption band at about 205 nm, characteristic of titanium silicalites. The absence of signal beyond 275 nm indicated that the material was free from extraframework oxide species. S.E.M. pictures revealed that the sample was in the form of very small uniform crystals of about 0.3 pm in size. 

The main feature of a zeolite structure is its framework type, which describes the arrangement of the cages, the dimensionality of the channel system and the approximate size of the pore openings. A few framework types, selected for their industrial relevance and/or to illustrate some of the more common structural nomenclature, have been presented. However, there are many more, and for more information about a specific framework type, the reader is referred to the relevant references in the Atlas and the Collection. To fully understand the properties of a real zeolitic material though, not only the framework type, but also the composition and true geometry of the framework, the location and nature of the extraframework species, and the number and type of defects must be investigated. 

In the extraframework octahedral aluminium (EFAL ) (Fig. 2) there are species appearing at 1.4 and higher ppm s whereas it is usually reported from -0.5 to 0 ppm (6). This is an indication that these EFAL species are polymerized, forming an alumina type material. Therefore, in order to explain the cracking activity and selectivity of highly dealuminated zeolites we have to take into... 

A1 mas NMR spectra of all the samples (not shown) show only one line corresponding to aluminum atoms situated at 60 ppm assigned to tetrahedral position in the framework. No peak for six coordinated (extra-framework) aluminium which occurs at 0 ppm was observed. This shows that dealumination by H4EDTA treatment removes aluminium atoms not only from the framework position but also removes extrafiamework aluminum species from the interstitial positions. It may be mentioned here that the other methods of dealumination like hydrothermal [17] or SiCU treatment [23] results in a material which contains some amount of extraframework aluminium, specially in the former case. 

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