Extraframework aluminum species

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. 

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). 

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. 

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. 

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... 

Min investigated the reaction mechanism of this dealumination approach.[24] Breck and Skeels had found that, in the ammonium hexafluorosilicate solution, the extraframework silicon atoms could substitute for the aluminum atoms in the crystal lattice of the zeolite framework, but the mechanism of the whole reaction process was not clear. Through 29Si,27Al-MAS-NMR and IR characterization techniques, Min and coworkers investigated the removal of A1 atoms from the zeolite framework, the chemical environment of the extraframework silicon atoms, and the species of the silicon that can be inserted into the Al-removal framework vacancies during the reaction process, and finally they proposed the reaction mechanism [Equations (6.18)-(6.25)] as follows. 

The Si, A1, and H spectroscopy were used to characterize (Fe,Al)-FER, synthesized in the presence of ethylene glycol and ethylene diamine [lOCl]. Some Si and Al MAS NMR spectra are depicted in Fig. 46. The Al NMR spectra show that most of aluminum occupies tetrahedral positions, characterized by a hne at 54 ppm. The line at <5 = 14 ppm and the broad contribution at <5 = 35 ppm were assigned to deformed extraframework A1 and Al-oxide species [97S2]. The Si NMR spectra have been analyzed based on the presence of two different crystallographic Si atoms, A and B [97B3]. The parameters determined by the analyses of the spectra are listed in Table 11. The Si NMR lines are broader compared to samples without iron, suggesting that the Fe ions were incorporated in the framework. The NMR spectra of the samples synthesized in the presence of ethylene diamine exhibited a small line of amine at S= 39.3 ppm, but most of these initial molecules were transformed into oligomers and aromatic molecules. The rather broad bands at <5 = 120 and 130 ppm are... 

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