Extraframework aluminum

Tetrahedral and Octahedral Extraframework Aluminum in Ultrastable Y Zeolites... 

It is generally accepted that Lewis acidity in zeolites is due mainly to extraframework aluminum (16,17,18). Consequently, Lewis acid sites measured by pyridine adsorption must correlate with extra-framework aluminum. In Table I, the amount of pyridine coordinated to Lewis sites decreases for samples with the lowest Si/Al ratio, showing that, after thermal treatment, the amount of extraframework aluminum decreases with Si/Al ratio of the Beta zeolite. 

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

Infrared bands of dealuminated zeolites in the 0-H stretching region have been extensively studied (14-17) however, emphasis will be placed on the study by Fritz and Lunsford (10), which describes the effect of Na+ on these infrared bands. Perhaps the most informative example is the SiClA-treated zeolite (series A of Figure 3). The SiCl4-treated zeolites have less extraframework aluminum than do the steamed samples therefore, the infrared spectra in the 0-H stretching region are less complicated. 

Evidence that something more than a proper Alf distribution is needed to create strong acidity first came from the experiments of Beyerlein et al. (7). Using the acid-catalyzed conversion of isobutane as a measure of strong acidity, they found that a dealuminated zeolite prepared by treatment with ammonium hexafluorosilicate (AHF) exhibited much less carbonium ion activity than might be expected, based on the number of Alf atoms. This treatment leaves very little extraframework Al in the zeolite. When the sample was mildly steamed, the activity became considerably greater. The authors concluded that the enhanced acidity was a result of a synergism between the framework Bronsted sites and the Lewis sites associated with extraframework aluminum. 

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... 

Very clean and highly dealuminated samples have been prepared that have removed some of the variables in determining the state of the framework and extraframework aluminum. 

Extraframework aluminum species created by mild steaming were shown to increase the catalytic activity of zeolites. This increase in activity was ascribed to the creation of sites exhibiting enhanced acidity through interaction of bridging hydroxyl groups (BrOnsted sites) and neighboring small extraframework aluminum species (Lewis acid sites) (16). Their exact nature is still a matter of debate, but is schematically represented as ... 

Pt-mordenite catalyst for isomerizing nCs-Q alkanes with an optimal acidity (presence of very strong protonic sites due to interaction of framework hydroxyl groups with extraframework aluminum species)... 

All these aspects were thoroughly discussed by lecturers and participants during the round table organized during the Poitiers School on The Future Trends in Zeolite Applications . Special emphasis was placed on the role played by the sites at the external surface (pockets, etc.) or at the pore mouth, by mesopores, extraframework aluminum species, as well as by the polarity of reactant and product molecules. Other important topics dealt with the remarkable catalytic properties of BEA zeolites for fine chemical synthesis, the potential of mesoporous molecular sieves, zeolitic membranes and the role of combinatorial catalysis in the development of zeolite catalysts. It is our hope that the fruits of these discussions will appear in the literature or even better as new and environmentally friendly products or processes. 

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. 

As shown in Figure 7, calcined zeolites with similar unit cell sizes demonstrate different octane-producing behavior. Hence, structural characteristics such as extraframework aluminum and spatial distribution of acid sites are important factors affecting zeolite acidity. After steaming, the octane behaviors of AFS and USY zeolites are similar reflecting similarities in zeolite acidities. 

Extraframework aluminum contributes to the observed catalytic behavior in both activity and selectivity. It is interesting to note that although steamed USY contains more extraframework aluminum than steamed AFS, both AFS and USY give similar product selectivities. Observed product selectivities from steamed zeolites are insensitive to the amount of extraframework aluminum present within the composition range investigated in this study. 

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. 

The extraframework aluminum (EFAL) content was estimated as the difference between the total aluminum content determined by XRF and framework content estimated from the Fichtner-Schmittler equation. 

Deactivation and the Role of Extraframework Aluminum in the Reactions of 2-Methylpentane on Ultra-Stabilized MY Zeolite... 

McDaniel, Maher, Kerr and Shipman have studied[21,22] framework dealumination under high-temperature conditions for (A) in detail, and they concluded that the dealumination product is complicated and in the extraframework channels, cages, and on the surface there are various extraframework aluminum species (generally denoted EFAL) (Figure 6.11). 

These zeolites after hydrothermal treatment and dealumination were examined by XRD for crystallinity and structural stability. The nature and extent of extraffamework aluminum (AEFAL) as well as framework aluminum (Al F), acid soluble extraframework aluminum (AIEFAL) assessed by chemical analysis and XRF for total aluminum (ALT), Si NMR for framework aluminum, acid extraction for acid soluble extrafarmework aluminum. From the above mentioned measurements acid insoluble extra framework aluminum (AIEFAL) was derived. The amounts of various alumina as a function of severity of hydrothermal treatment are presented in (Table-1). It can be noted that acid insoluble extraframework aluminum (AIEFL) per unit cell passes through a maximum at a steaming temperature of400 C. 

In the first case the secondary synthesis is performed on a sample of dealuminated ZSM-5 and the substitution presumably takes place on the sites made vacant from the removal of aluminum. In the second case the synthesis is performed on a ZSM-5 not dealuminated and the treatment with TiCl does not reduce the aluminum content in this case, if titanium replace some lattice aluminum, extraframework aluminum oxide must be formed in the zeolite crystals. In both the above mentioned works, samples treated with TiCl have an IR absorbance band at 960 cm . Ferrini observed the appearance of such a band also on Ti-contalning Beta and Y samples prepared by secondary synthesis. 

The effect of the synthesis medium, OH or F, on the nature and amount of add sites present in Al,Si-MFl zeolites, has also been examined by microcalorimetry of NH3 adsorption and infrared spectroscopy [182]. Both techniques revealed that H-MFI (F ) with Si/Al < 30 contained extraframework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Bronsted acidity. In contrast, MFI (F ) characterized by Si/Al > 30 behaved very much as H-MFI (OH") [182]. 

A more in-depth study on FAU-structiu e zeolites with high siUca content by Beyerlein et al. [108] addresses the influence of extra-framework species (EFAL) on the catalytic activity for the isobutane conversion. Overall, the cracking activity increased linearly with the concentration of framework aluminum sites for all catalysts. However, FAU samples dealuminated with ammonium hexafluorosilicate (AHF) showed a lower activity compared to conventionally hydrothermally treated catalysts. Steaming of the chemically dealuminated catalysts led to an increased activity in cracking isobutane compared to the parent FAU. This is interpreted as an indication that extraframework aluminum species, typically acting as Lewis add sites, are interacting with Bronsted acid sites to form new and stronger addic sites. 

For the hydroconversion of -heptane and n-decane, the group of Jacobs [96,165-169] reported a hnear relation between the rates of hydroisomerization normalized to the concentration of tetrahedral A1 and the concentration of pentacoordinated and tetrahedrally distorted A1 (analyzed by Al MAS NMR) [166]. This emphasizes that the interaction of extraframework aluminum species with Bronsted acid sites creates more active sites. The work by Blomsma et al. [96,165,167] shows that hydroconversion of n-heptane over Pd/H-Beta zeolites is a combination of classic bifimctional hydrocracking and cracking of dimerized C7 species. 

H.-M. Kao, Y.-C. Chen, C.-C. Ting, P. T. Chen, J.-C. Jiang, Characterization of extraframework aluminum in H-mordenite dealuminated with ammonium hexafluorosilicate, Catal. Today, 97, 13-23 (2004). 

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