Computational Investigation of the Lewis Acidity in Three-Dimensional and Corresponding Two-Dimensional Zeolites: UTL vs IPC-1P

The
adsorption and catalytic properties of three-dimensional zeolite UTL
were investigated computationally along with properties of its two-dimensional
analogue IPC-1P that can be obtained from UTL by a removal of D4R
units. Adsorption properties and Lewis acidity of extra-framework
Li+ sites were investigated for both two- and three-dimensional
forms of UTL using the carbon monoxide as a probe molecule. The CO
adsorption enthalpies, calculated with various dispersion-corrected
DFT methods, including DFT/CC, DFT-D2, and vdW-DF2, and the CO stretching
frequencies obtained with the νCO/rCO correlation method are compared for corresponding Li+ sites in 3D and 2D UTL zeolite. For the majority of framework
Al positions the Li+ cation is preferably located in one
of the channel wall sites and such sites remains unchanged upon the
3D → 2D UTL transformation; consequently, the adsorption enthalpies
become only slightly smaller in 2D UTL (less than 3 kJ mol–1) due to the missing part of dispersion interactions and νCO becomes also only up to 5 cm–1 smaller
in 2D UTL due to the missing repulsion with framework oxygen atoms
from the opposite site of the zeolite channel (effect from the top).
However, when Li+ is located in the intersection site in
3D UTL (about 20% probability), its coordination with the framework
is significantly increased in 2D UTL and that is accompanied by significant
decrease of both νCO (about 20 cm–1) and adsorption enthalpy (about 20 kJ mol–1).
Because the intersection sites in 3D UTL are the most active adsorption
and catalytic Lewis sites, the results reported herein suggest that
the 3D → 2D transformation of UTL zeolite is connected with
partial decrease of zeolite activity in processes driven by Lewis
acid sites.