Theoretical Methods for Estimating Brönsted Acid Site Strength in Zeolites: Application to Isomorphously Substituted Systems

Abstract

Abstract Parameters derived from molecular orbital calculations on cluster models corresponding to zeolite acid sites are discussed with regard to their correlation with intrinsic Bronsted acid site strength. While no correlation of hydroxyl stretching frequency with acid site strength can be expected there is likely to be a direct correlation between acid site strength and the calculated partial charge on the hydroxyl hydrogen. The limitations on the concept of partial charge is discussed and 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. As this parameter is directly calculable from the wavefunction it is thought to give a more accurate description of molecular polarity and eliminates sometimes arbitrary electron density partitioning schemes associated with partial charge determinations. Combined with molecular graphics methods this parameter can be used to give a visual demonstration of acid site strength variation. Using the above parameters, the intrinsic acidity of cluster units representing bridged Si/Al, Si/Ga, Si/B and Ge/Al hydroxyl groups in zeolites is estimated from ab initio molecular orbital methods. Comparison with acid parameters derived from more classical acids allows one to rank zeolite acid site strength on a broad acidity scale.