Computational investigation of the lewis acidity in three-dimensional and corresponding two-dimensional zeolites: UTL vs IPC-1P
Abstract
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 v(CO)/r(CO) 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 v(CO) 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 v(CO) (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.