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

Abstract

The interaction of carbon monoxide with H-faujasite (H-FAU) and metal-exchange 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 evaluated to be −1.90 kcal/mol for the H-FAU/CO complex. Inclusion of the Madelung potential field from the zeolite framework has an effect of lengthening the OH distance, hence enhancing the binding energies of the H-FAU/CO (−3.20 kcal/mol). For adsorption of CO on the metal-exchanged zeolites, the Madelung potential was found to differentiate the different types of zeolites (ZSM-5 and FAU), that cannot be drawn from the typical 3T-quantum cluster. This finding clearly demonstrates that acidity does not depend only on the Brønsted group center, but also on the dimension of the channel where the Brønsted group is located. The adsorption energy of the embedded cluster model (−6.69 kcal/mol) lies between those of the bare quantum cluster model (−5.81 kcal/mol) and the simple naked Li(I)/CO system (−13.14 kcal/mol). Correction of the 3T cluster model to take into account the long range contribution of the electrostatic potential of the zeolite crystal is found to agree with the experimental observation.