Dear Sara,

Hope the enclosed article can help.

Best wishes,

Carlos

Dear Sara ,

The dealumination of the crystal structure results in a contraction of the unit cell . The dealumination process is not uniform throughout the crystal, hence the creation of an aluminum gradient which can have a significant influence on the overall acidity and on the distribution of acid sites. The number and distribution of these acid sites depend on the dealumination mode; dealumination by hydrothermal treatment or with SiCl4 leads to preferential dealumination inside the crystal while mineral acids and hexafluorosilicate tend to remove aluminum from the framework in the vicinity of the surface. The activity of zeolitic catalysts therefore depends not only on the Si / Al ratio but also on the dealumination mode. It is generally accepted that during dealumination by hydrothermal treatment, extra-lattice octahedral coordinate aluminum species (EFAlVI) are formed; some EFAls also exist in tetrahedral or pentahedral form. In addition, EFAls of various kinds have been reported by different authors. These species can exist in the form of cations (eg Al (OH) 2+, Al (OH) 2+) or different types of aluminum oxide (eg Al2O3, Boehmite). The nature of the EFAl species and their distribution may depend on the conditions for the preparation of dealuminated zeolites: temperature, hydrothermal treatment time, water pressure. The ultra-stabilized zeolites exhibit certain very acidic EFAl species which are active in cracking n-heptane. The EFAl species enhance the catalytic activity by cracking isobutane. Other studies have also shown a positive effect of EFAls on the activity of USYs in cracked gas oils. A positive effect of the EFAls present in the USY has also been observed by other authors. These results clearly demonstrate the presence of acidity in EFAls and their direct participation in the reaction catalyzed by dealuminated Y zeolites.

Other studies suggest that superacid sites would be created by the interaction of the Lewis EFAl sites and the Bronsted zeolite sites that a significant increase in activity would result from this interaction. Also a hydrothermal treatment of a ZSM5 zeolite can multiply by 2.5 its activity in cracking n-heptane. It should however be noted that certain authors have shown that EFAls do not always have a positive effect on activity, and find that the elimination of EFAls from highly dealuminated Y zeolites (Si / Al from the framework> 20) causes a increased activity of these zeolites for the transformation of toluene and m-xylene.

It has been found that the extra-lattice aluminum species are reinserted into the frame after re-aluminization of the dealuminated zeolite. Another study has shown that the treatment with H2SO4 of a dealuminated zeolite by hydrothermal treatment, eliminates the extra-lattice aluminum located in the supercages without attacking the framework.

Extraframework (non-tetrahedral) Al -located mostly in octahedral positions into the channels- is well-known and it can be checked by in situ & ex situ 27Al MAS NMR , please see e.g: https://onlinelibrary.wiley.com/doi/abs/10.1002/1521-3765%2820010903%297%3A17%3C3705%3A%3AAID-CHEM3705%3E3.0.CO%3B2-5

thank you all

Structure des Aluminosilicates présence des ions de Si ,O , la presence d'Aluminium 3+,OH - de cautions Na+ beaucoup de cristallinite , K+ ,Mg2+,Ca2+ le sens de donneurs-accepteurs,electr

Le sens donneurs et accepteurs,electro-neutralité. Rapport cautions et anions, forme géométrique, nombre de coordination, fondamentale tétraèdrique de liaisons covalentes,les liaisons varient beaucoup... le sens d'electronegativite...Photo difficile, Votre mail je vous envoie Training TOPIC 2 Prof Joseph DAVIDOVITS speaker.

Aluminium est de donner le sialate système, jouant aussi de coordination fold aussi.