Density functional study for estimating Bronsted acid site strength in isomorphously substituted ZSM-5

We use local density functional theory to study the Bronsted acid strength of a series of clusters that model a catalytic acid site within zeolite ZSM-5. We consider models of the formula (A)(3)Si-OH-T(A)(3) where A = OH and OSi(OH)(3); T = Si, Al, B, Ga or Fe. We have studied the effect of replacing Si with trivalent metal cations on the structure and acid strength of the zeolite ZSM-5. Our results also show the effect of cluster size on the calculated properties. It is suggested that a more accurate descriptor of the decreased electron density around the hydroxyl hydrogen would be the electrostatic potential calculated on the total electron density surface. We studied the molecular electrostatic potential maps for the cluster models to test this fact. It is observed that the calculation results predict that acid strength increases in the order B-ZSM-5 < Fe-ZSM-5 < Ga-ZSM-5 < Al-ZSM-5, which shows a good agreement with experimental results. The results show a prospective trend to predict properties of metal substituted zeolites which a special emphasis on Bronsted acid strength of zeolites. (C) 1998 Elsevier Science B.V. All rights reserved.