IR studies of the activation of C=C bond in alkenes by Cu+ ions in zeolites

IR studies show that Cu+ ions in zeolites CuX, CuY, and CuZSM-5 are able to activate C=C bond in alknenes (ethene, propene, but-1-ene, and cis-but-2-ene). The stretching frequency of the C=C- band decreased by 78-115 cm(-1), indicating a significant weakening of the double bond. In ethene, the IR inactive C=C stretching became IR active when interacting with Cu+ as the result of the loss of symmetry. The band of C=C interacting with alkenes was split into two submaxima suggesting that Cu+ ions, activate alkenes to a different extent. The IR bands corresponding to C-H stretching, deformation, and bending of the =CH2 and =C-H groups near the double bond also shifted to lower frequencies. At high loadings, some of the Cu+ ions were able to bond two alkene molecules. The interaction of alkenes with Cu+ was so strong that alkenes replaced CO and NO in relatively stable Cu+CO and Cu+(NO)(2). Very strong interaction of alkenes with Cu+ was also evidenced by a large shift of IR band of oxygen ring which was deformed by the interaction with Cu+ and relaxed when Cu+ was withdrawn from the ring by alkene. Most probably the mechanism of the activation of alkenes by Cu+ is similar to the mechanism of activation of NO, that is, involves the pi-back-donation of d electrons of Cu+ to pi* antibonding orbitals of the alkenes.