If aluminum substitutes for silicon in a tetrahedral zeolite framework and a cation (Proton) is used for neutralization, it will create strong Bronsted acid.
Yes, you are right. But,
1) If a cation other than a proton ( e. g., an alkali-metal cation) is used for charge neutralization you will have there a Lewis acid site.
2) Often, some co-ordinatively unsaturated Al(3+)-oxide species remain ouside the zeolite framework; and they can also act as Lewis acid sites.
Best wishes,
Carlos
In "pure" (only Si and Al) zeolite, Lewis acid sites are defect sites (unlike Bronsted acid sites they cannot be described based on the crystallographic structure of the zeolite). Their exact nature is still controversial, but one can safely say that they are formed by partial or complete extraction of an Al from a framework site : for example if you considere an extraframework Al3+ in a zeolite cage, if your sample is hydrated it will form [Al(H2O)6]3+, with no Lewis acidity, but if you strongly dehydrate your zeolite, you will form Al3+ that could act as a strong Lewis acid. Beside these strong Lewis sites you can also have the formation of weak Lewis acid sites that are usually associated with the formation of alumina-like debris (this association is based on the fact that their Lewis acidity is similar to that of gamma-alumina). For more information regarding the possible structure of strong Lewis sites in zeolite, you could have a look at the following article (but there are many others !):
Article Ab Initio Simulation of Lewis Sites in Mordenite and Compara...
Prof. Carlos Otero Areán ,
Thank you very much for your answer. Would you please provide your/any paper exploring your answer that I can cite?
Aluminum is a trivalent element, and if placed in a tetrahedral postion a negative charge is created, think that Al3+ is replacing Si4+ in the zeolite framework. The negative charge is formed by bonding with oxygen atoms (4 in the tetrahedral position). Thus the charge needs to be counterbalanced by a cation/proton. The acidity is formed when a proton is present. The strong acidity comes from chemical and structural considerations. The latter occurs due to the size of the microporous system, which can strongly adsorb the base and stabilize the formation of the conjugate acid. The strong adsorption observed in microporous system is because of an overlap of surface forces. The former is because the charge can be distributed and stabilized by crystalline structure, stabilizing the conjugate base.
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