Adsorption of methanol on the nanocrystalline H–zeolite and alkali metal exchanged M–zeolites: Energetic, NBO and QTAIM analyses

Adsorption of methanol on the H–zeolite and ion-exchanged M–zeolites (M = Li, Na, and K) has been investigated using 3T cluster model at the B3LYP/6-31++G(d,p) and MP2/6-31++G(d,p) levels and 5T cluster model at B3LYP/6-31++G(d,p) level of theory. The adsorption energy of methanol on the M–zeolites (M = Li, Na, and K) decreases when the cations (M+) are changed from Li+ to K+, in agreement with the weaker acidity of K+ ion compared to the Li+ ion. The proton jump between neighboring oxygen atoms of Al center has been studied in five cluster models. The electron density (ρ), Laplacian of electron density (∇2ρ) and energy density (H) estimated by AIM calculations, showed that both HM⋯OZ and OM⋯HZ hydrogen bonds are partially covalent and partially electrostatic in nature, whereas both OZ–HZ and OM–HM have covalent character. ∇2ρ and H(r) values at M⋯OM (M = Li, Na, and K) BCP of complexes are positive and decrease on going from complex Li+ to complex K+. As a result, electrostatic nature of M⋯OM interaction decreases on going from complex Li+ to complex K+. Also, the interaction between methanol and the ZSM-5 zeolites has been examined using the natural bond orbital (NBO) analysis.