A comparative theoretical study of cluster and periodic models by DFT calculations for pyridine adsorption in H-ZSM-5...

To improve the performance of aromatic reactions using zeolite catalysts, a fundamental understanding of adsorption properties at the molecular scale by reliable theoretical methods is needed. Our aim in this study is specifically to estimate the different component contributions to the adsorption energy. For this purpose, we have investigated the adsorption of pyridine (PY) on Brønsted acid sites (BAS) of H-ZSM-5 (ZOH) zeolite cavity in the framework of cluster and periodic model approaches, both using PBE-D3 in the density functional theory calculations. Two zeolite models, a cluster model of 32 tetrahedral centers and a periodic model of 96 tetrahedral centers, were used. The substitution of one to four Si atoms in four crystallographic T-sites by Al atoms within ZSM-5 has been considered in both models. The effect of the Si/Al ratio of 32T clusters with different positions and distributions of one to four Al atoms, as well as the confinement effects resulting from van der Waals dispersion interactions and steric constraints, on the energetic properties of PY adsorption in the intersection region and in the narrow region situated between two intersections of the straight channel of H-ZSM-5 has been thoroughly examined and compared with those of the periodic model. This comparative study allows to estimate the contributions of the long range electrostatic and dispersive interactions to the adsorption energies. In all cases, upon adsorption on BAS, the ion pair complexes PYH⁺/ZO^- are spontaneously formed. The average calculated adsorption energy value of -44.8 kcal mol^-1 for 32T cluster model in the intersection region is 5.3 kcal mol^-1 smaller than the average periodic model value of -50.1 kcal mol^-1, in good agreement with experiment (-47.8 kcal mol^-1). These PBE-D3 adsorption energy differences between both models are due to the long range dispersive (-2.9 kcal mol^-1) and electrostatic (-2.4 kcal mol^-1) interactions for the intersection region. In the narrow region, the average calculated adsorption energies are significantly smaller, with values of -29.7 and -39.6 kcal mol^-1 for cluster and periodic models, respectively. The PBE-D3 difference between adsorption energy values calculated by two models is due, besides long range dispersive (-2.5 kcal mol^-1) and electrostatic (-2.2 kcal mol^-1) interactions, to the important steric interactions (5.2 kcal mol^-1).