Density functional theory study of the effects of metal cations on the Bronsted acidity of H-ZSM-5

Density functional theory calculations have been carried out to establish the influence of mono-and polyvalent cations on the Bronsted 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)(+), AlO+, Al(OH)(2)(+). The local geometry of the Bronsted 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/mol 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+ = AlO+, the binding energy of CO with the Bronsted acid proton is approximately 8.8 kcal/mol, independent of the nature of M+. When M+ = AlO+, 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.