The activation of C=O bond in acetone by Cu+ cations in zeolites:: IR studies and quantum chemical DFT calculations

Our previous IR studies as well as quantum chemical DFT calculations evidenced that Cu+ ions in zeolites were able to activate C=C double bond in alkenes by pi back donation of d electrons of Cu+ to pi* antibonding orbitals of alkenes. It resulted in a distinct weakening of C=C bond. The present study was undertaken in order to answer the question if Cu+ ions in zeolites were also able to activate another than C=C double bond. We studied therefore the interaction of acetone with Cu+ in zeolites CuZSM-5, CuMCM-41, CuX and CuY. IR studies showed a weakening of C=O bond and a red shift of IR band by 39-51 cm(-1). The experiments in which the small doses of acetone were adsorbed as well as desorption studies evidenced that Cu+ bonded acetone more strongly than other adsorption sites, such as hydroxyl groups. DFT modeling of the adsorption of the acetone molecule on copper site gave the information about the function of the zeolite as a host for transition metal cationic sites and as an electron reservoir. Calculated values of electron affinity and HOMO energy allowed to explain that, on the basis of the charge distribution analysis, electrons flowed from the acetone molecule to the copper site, so that the molecule was positive. It was evidenced by the analysis of the charge distribution. Therefore, the activation upon adsorption has been justified by the electronic structure of the molecule, its electron affinity as well as the ability of the site to pi back donation. Moreover, the calculated value of the red shift of the IR band of the activated C=O bond was similar as experimental one. (c) 2005 Elsevier B.V. All rights reserved.