Influence of isomorphous substitution on acidity of zeolites: ab initio and density functional studies

The influence of chemical composition on acidity of zeolite clusters and their interaction with CO molecules were investigated by quantum chemical methods. We used both Hartree-Fock and density functional theory to study the acidity of zeolite cluster models of the form (H3Si-OH-TH2(OSiH3)), where T = Al, Ga, or B. The acidity is found to decrease in the order Al(OH)Si > Ga(OH)Si > B(OH)Si in agreement with existing experimental and theoretical results. The present quantification is based on the calculated charge on hydrogen atom, bond length of bridging hydroxyl groups and their vibrational frequencies. We also studied the change in the vibrational frequencies of the CO molecule upon adsorption on the acidic sites. The CO molecule bound to the acidic hydrogen via the oxygen center is found to exhibit an adsorption-induced red shift of the stretching frequency, in contrast to the blue shift of the carbon bound species. (C) 2001 Elsevier Science B.V. All rights reserved.