ACIDIC PROPERTIES OF [AL], [GA] AND [FE] ISOMORPHOUSLY SUBSTITUTED ZEOLITES - DENSITY-FUNCTIONAL MODEL CLUSTER STUDY OF THE COMPLEXES WITH A PROBE CO MOLECULE

The Bronsted acid strength and related characteristics of bridging hyd roxyl groups in [Al]-, [Ga]- and [Fe]-framework-substituted zeolites h ave been studied using a model cluster density functional approach bas ed on a gradient-corrected exchange-correlation energy functional. The acidity is found to decrease in the order Al(OH)Si > Ga(OH)Si > Fe(OH )Si in agreement with existing experimental and theoretical results. T he present quantification is based on the calculated deprotonation ene rgy of H3Si(OH)TH3, on the adsorption energy of a CO probe molecule as well as on the changes of the vibrational frequencies and absolute IR intensities of the O-H and of the C-O modes induced by CO adsorption. The vibrational parameters of the carbonyl adsorption complex in [Fe] -zeolites are predicted.