IR Studies of the Activation of CC Bond in Alkenes by Cu⁺ Ions in Zeolites

Abstract

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 CH 2 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by a large shift of IR band of oxygen ring which was deformed by the interaction with Cu + and relaxed when Cu + was withdrawn from the ring by alkene.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 π-back-donation of d electrons of Cu + to π* antibonding orbitals of the alkenes.