It is observed that as the Si/Al ratio increases, acidity decreases. This indicates that aluminium genarates acidity. Due to its valency, there is an overall negative charge (due to oxygen) on the zeolite which is balanced by a cation. If it is a proton, the zeolite acts a bronsted acid.
When aluminum substitutes for silicon in a tetrahedral zeolite framework, a cation is required to satisfy the Al tetrahedron. If this cation is a proton (which is quite often the case), the proton is a strong Bronsted acid. Thus, increasing the Al/Si ratio (or decreasing the Si/Al ratio) in the zeolite framework increases the number of acid sites if all the cation sites are populated by protons. However, each acid site becomes a somewhat weaker Bronsted site as the population of protons increases. This is due to proton crowding in the zeolite pores.
Lewis acidity due to aluminum in a zeolite generally results from extra-framework aluminum. Extra-framework aluminum is aluminum which is not tetrahedrally bound in the framework. You could think of this as islands of Al2O3 often dispersed in the zeolite pore structure, but it can also appear on the exterior of the zeolite crystals or even at the pore mouths. Extra-framework aluminum is often an artifact of the synthesis process whereby excess aluminum in the synthesis mixture does not get incorporated into the zeolite crystals. Extra-framework aluminum can also be created by purposely steaming a zeolite to remove Al from the framework. This process is used to increase the Si/Al ratio of zeolite-Y for cracking catalysts.
Generally, Lewis acidity is much weaker in zeolites than Bronsted acidity if the zeolites are in their form.
I hope this might help
The aluminum acts as a lewis acid site, accepting electron pairs with the acidity depending on coordination with its neighboring atoms. When dehydroxylated, sites of coordinated unsaturation occur, causing this acidity.
See link:
http://pubs.acs.org/doi/citedby/10.1021/j100075a024
The effect of the aluminum also acts to increase the acidity of the Bronsted acid (hydroxyl)sites.
"Lewis acid sites actingas electron pair acceptors are extraframework species, e.g.,formed by cation exchange or caused by steaming to createattice defects and extraframework aluminum clusters.3 The
change in the distribution of framework aluminum atoms affects
the acid strength of the hydroxyl groups in zeolite. Framework
aluminum atoms with no second-neighbor aluminum atoms are
responsible for strong Brønsted acid sites."
http://itc-web.itc.uni-stuttgart.de/hunger/bilder/Paper154b.pdf
When aluminum substitutes for silicon in a tetrahedral zeolite framework, a cation is required to satisfy the Al tetrahedron. If this cation is a proton (which is quite often the case), the proton is a strong Bronsted acid. Thus, increasing the Al/Si ratio (or decreasing the Si/Al ratio) in the zeolite framework increases the number of acid sites if all the cation sites are populated by protons. However, each acid site becomes a somewhat weaker Bronsted site as the population of protons increases. This is due to proton crowding in the zeolite pores.
Lewis acidity due to aluminum in a zeolite generally results from extra-framework aluminum. Extra-framework aluminum is aluminum which is not tetrahedrally bound in the framework. You could think of this as islands of Al2O3 often dispersed in the zeolite pore structure, but it can also appear on the exterior of the zeolite crystals or even at the pore mouths. Extra-framework aluminum is often an artifact of the synthesis process whereby excess aluminum in the synthesis mixture does not get incorporated into the zeolite crystals. Extra-framework aluminum can also be created by purposely steaming a zeolite to remove Al from the framework. This process is used to increase the Si/Al ratio of zeolite-Y for cracking catalysts.
Generally, Lewis acidity is much weaker in zeolites than Bronsted acidity if the zeolites are in their form.
In reactions involving zeolites what kind of acidity is responsible for activity? Lewis/Bronsted or both??
Dear Geoffrey,
I need your explanation on Phosphoric acid impregnated zeolites
When aluminium cation replaces a silicon cation a additional positive charge is required to maintain the charge neutral. When the proton H+ is added it caused acidity.
The second source of acidity ion zeolite is Lewis acid site related to the electron accepting capacity of the sites. Generally, the new aluminum species created during the activation process (calcination of parent zeolite at higher temperature) are the source of this Lewis acidity.
Al atom has vacant p-orbital which try to withdraw of electron from O atom in [Al-O(H)Si] group, and thus polarization of the O-H bond, as a result Bronsted acid site in presence of Al.
Determined the relation of Si/Al is the most important to understand the reaction mechanism in zeolites with the molecules. The temperature, the zeolite / reactant ratio, and the reaction time are decisive for the understanding of the transformation processes that use acid solids such as zeolites.
According to Prof. Geoffrey L. Price, Lewis acidity in a zeolite generally results from extra-framework aluminum. But how extra-framework aluminum is bridged with Zeolite?
Hi Sanchari Basu
When a zeolite is dealuminated the Si/Al decrease but the acid strengh per acid site and the separation between then is greater, which leads to diferent and controllable reaction mechanisms.
can you please explain density and distribution of acidic site in zeolites?
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