Dynamical Equilibrium between Brønsted and Lewis Sites in Zeolites: Framework-Associated Octahedral Aluminum.

While the structures of Brønsted acid sites (BAS) in zeolites are well understood, those of Lewis acid sites (LAS) remain an active area of investigation. Under hydrated conditions, the reversible formation of framework-associated octahedral aluminum has been observed in zeolites in the acidic form. However, the structure and formation mechanisms are currently unknown. In this work, combined experimental 27Al NMR spectroscopy and computational data reveal for the first time the details of the zeolite framework-associated octahedral aluminium. The octahedral LAS site becomes kinetically allowed and thermodynamically stable under wet conditions in the presence of multiple nearby BAS sites. The critical condition for the existence of such octahedral LAS appears to be the availability of three protons: at lower proton concentration, either by increasing the Si/Al or by ion-exchange to non-acidic form, the tetrahedral BAS becomes thermodynamically more stable. This work resolves the question about the nature and reversibility of framework-associated octahedral aluminium in zeolites.