A density functional theory study of the effects of metal cations on the Brønsted acidity of H-ZSM-5

Density functional theory calculations have been carried out to establish the influence of mono- and polyvalent cations on the - Brønsted acidity of H-ZSM-5. The zeolite was modeled as a cluster containing 41-45 atoms, in the center of which is an Al⁽¹⁾(OH)SiOAl⁽²⁾(OM) unit, where M+ = H⁺, Li⁺,Na⁺, K⁺,Ca(OH), A1O, A1(OH)⁺₂. The local geometry of the Brønsted acid site is affected by the nature of M⁺ and this in turn causes a change in the value of the proton affinity (PA) for the site. The highest value of PA is 330 kcal/mol for M⁺ = H⁺ and the lowest value of PA is 305 kcal/rnol for M⁺ = AlO⁺. No correlation was found between the value of PA and the Mulliken charge on Al⁽¹⁾. With the exception of the case where M⁺ = A1O⁺, the binding energy of CO with the Brønsted acid proton is approximately 8.8 kcal/mol, independent of the nature of M⁺. When M⁺ = A1O⁺, the binding energy for CO is 11.1 kcal/mol. The present calculations suggest that different factors affect proton affinity and the binding energy for CO adsorption.