IR and TPD studies of the interaction of alkenes with Cu+ sites in CuNaY and CuNaX zeolites of various Cu content.: The heterogeneity of Cu+ sites

Cu+ ions in zeolites activate organic molecules containing pi electrons by pi back donation, which results in a distinct weakening of multiple bonds. In this study, we followed the activation of alkenes (ethene and propene) by Cu+ ions in CuY and CuX zeolites of various Cu content. We also studied the strength of bonding of alkenes to Cu+ ions. IR studies have shown that there are two kinds of Cu+ sites of various electron donor properties. We suppose that they could be attributed to the presence of Cu+ ions of various number of oxygen atoms surrounding the cation. IR studies have shown that Cu ions introduced into Y and X zeolites in the first-order (at low Cu content) form Cu+ ions of stronger electron donor properties (i.e. activate alkenes to larger extend) than Cu ions introduced in the next order (at higher Cu content). IR and TPD studies of alkenes desorption evidenced that Cu+ ions of stronger electron donor properties bond alkenes stronger than less electron donor ones. It suggests that pi back donation has more important contribution to the strength of bonding alkenes to cation than pi donation. (c) 2006 Elsevier B.V. All rights reserved.