Acidic properties of [Al], [Ga] and [Fe] isomorphously substituted zeolites. Density functional model cluster study of the complexes with a probe CO molecule

The Brønsted acid strength and related characteristics of bridging hydroxyl groups in Al]-, Ga]- and Fe]-framework-substituted zeolites have been studied using a model cluster density functional approach based 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. The present quantification is based on the calculated deprotonation energy 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.