A theoretical study of the confinement effects on the energetics and vibrational properties of 4,4′-bipyridine adsorption on H-ZSM-5 zeolite

The confinement effects of a zeolite framework on the adsorption of a bidentate 4,4′-bipyridine (44BPY) ligand on the straight channel of H-ZSM-5 have been investigated by density functional theory calculations using B3LYP, B3LYP-D3, M06-2X, M06-2X-D3 functionals, and the MP2 method with two basis sets 6-31+G* and 6-31++G(2d,2p). The straight channel is simulated by a realistic cluster model of 32 tetrahedra (T), having two aluminum atoms located in the straight and intersection regions, respectively. The potential energy surface of the double proton transfer reaction from the Brønsted acid sites of H-ZSM-5 to 44BPY is characterized by three minima corresponding to two monodentate ion pair complexes 44BPYH+/32T− and one bidentate ion pair complex 44BPYH22+/32T2− formed consecutively via two distinct pathways. No energy minimum is found for a neutral hydrogen bonding structure. The relative stabilities of these 44BPY adsorption complexes and the transition states connecting them do not exceed 4.9 and 4.2 kcal mol−1, respectively. Consequently equilibrium between the mono and bidentate complexes could be established. Our results clearly show that the adsorption energy for all complexes is substantially governed by the confinement effects executed through steric constraints and dispersive van der Waals interactions. The calculated vibrational frequencies and frequency shifts of 44BPY adsorbed as mono or diprotonated species are in good agreement with our Raman spectra of 44BPY occluded in H-ZSM-5 of different Si/Al ratios.