A new insight in the unusual adsorption properties of Cu+ cations in Cu-ZSM-5 zeolite

ZSM-5 zeolites modified with Cu+ ions were prepared either by the high-temperature chemical reaction of hydrogen form with CuCl vapour or by the wet ion exchange with subsequent reduction of the modified samples in CO at 873 K. Adsorption of H-2, N-2 or C2H6 by Cu+ ions was studied by DRIFTS and by volumetric technique. The conclusions about the structure of adsorption complexes were supported by the DFT cluster quantum chemical calculations. The obtained results indicated that in addition to the previously reported strong adsorption of nitrogen, the univalent copper also unusually strongly adsorbs molecular hydrogen and ethane. Adsorption of hydrogen is the most amazing since the observed low-frequency shifts of the H-H stretching vibrations were as high as about 1000 cm(-1). This is quite different from much weaker H-2 perturbation by Cu2+ cations. Adsorption of ethane by Cu+ ions also resulted in the low-frequency shifts of some of C-H IR stretching bands up to 400 cm(-1). The DFT cluster modelling indicated that both adsorption of hydrogen and ethane could be explained by interaction with the isolated Cu+ ions localized at the alpha sites of the ZSM-5 framework. Quantum chemical calculations indicated the important role in the bonding of adsorbed hydrogen and ethane of electron back donation from d(pi)-orbitals of Cu+ ions to the sigma*H-H or C-H orbitals. The overall yield of Cu+ sites of the strong H2 or N2 adsorption is about twice lower than the total copper content. (c) 2005 Elsevier B.V. All rights reserved.