Abstrakt
The zeolite activity in processes driven by Lewis acid sites is determined by the distribution of the extra-framework cations in the zeolite and their coordination with the framework (the stronger is the cation coordination, the weaker is its Lewis acidity). This study aims to test how much the transformation from 3D zeolite to its corresponding 2D layered form changes the extra-framework cation distribution and coordination and thus how much it affects the zeolite Lewis acid strength.
Zeolites with two distinct topologies, MWW and MFI, which form layers with considerably different silanol density (1.1 and 2.9 silanols/nm(2), respectively) were considered. To probe the Lewis acidity of extra-framework Li+ sites the carbon monoxide stretching frequencies and adsorption enthalpies were calculated, employing the dispersion-corrected density functional theory and the results are in good agreement with available experimental data.
No or negligible differences in the Lewis acidity of Li+ sites in 2D and 3D form of zeolites with MWW topology are found; both, the extra-framework cation distribution and the coordination with the framework remain the same. On the contrary, there are significant differences in Lewis acidic properties of 3D and 2D zeolites with MFI topology.
The Li+ cation coordination in 2D-MFI is considerably improved compared to 3D-MFI due to the large concentration of surface silanols and due to an increased flexibility of the material. Consequently, Lewis acidity of 2D-MFI is lower compared to corresponding 3D-MFI material.
This Lewis acidity decrease also reflects the fact that the strongest Lewis acid sites found in 3D zeolite - the channel-intersection sites (coordinated with two framework oxygens only) - are not present in 2D-MFI. The results reported herein suggest that the flexibility of the layer, which increases with silanol density and varies for each zeolite topology, significantly influences its Lewis acidity; the larger is the flexibility of the layer, the better is the cation coordination with the framework and thus smaller is its Lewis acidity.