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.