Adsorption of carbon monoxide on H-FAU and Li-FAU zeolites: an embedded cluster approach

The interaction of carbon monoxide with H-faujasite (H-FAU) and metal-excha nge Li-FAU zeolites has been investigated by means of cluster and embedded cluster approaches at the HF/6-31G(d,p) level of theory. In the case of the protonated zeolite, the adsorption energy of the bare quantum cluster is e valuated to be -1.90 kcal/mol for the H-FAU/CO complex. Inclusion of the Ma delung potential field from the zeolite framework has an effect. of lengthe ning the OH distance, hence enhancing the binding energies of the H-FAU/CO (-3.10 kcal/mol). For adsorption of CO on the metal-exchanged zeolites, the Madelung potential was found to differentiate the different types of zeoli tes (ZSM-5 and FAU), that cannot be drawn from the typical 3T-quantum clust er. This finding clearly demonstrates that acidity does not depend only on the Bronsted group center, but also on the dimension of the channel where t he Bronsted group is located. The adsorption energy of the embedded cluster model (-6.69 kcal/mol) lies between those of the bare quantum duster model (-5.81 kcal/mol) and the simple naked Li(I)/CO system (-13.14 kcal/mol). C orrection of the 3T cluster model to take into account the long range contr ibution of the electrostatic potential of the zeolite crystal is found to a gree with the experimental observation. (C) 2000 Elsevier Science B.V. All rights reserved.