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by a large shift of IR band of oxygen ring which was deformed by the interaction with Cu+ and relaxed when Cu+ was withdrawn from the ring by alkene.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

The Variety of Carbon-Metal Bonds inside Cu-ZSM-5 Zeolites: A Density Functional Theory Study

“…Note that CC stretching vibrational modes of free C 2 H 4 and C 2 H 2 span A g and Σ g + , respectively and thus the modes are IR-inactive [54,55]. Similar symmetry lowering can be seen in the symmetric CH stretching mode in the adsorbed C 2 H 2 , and thus a new IR peak appears around 3273 cm -1 after the C 2 H 2 binding into the monocopper cation [56,57,58]. …”

Elucidating the mechanism of heterogeneous Wacker oxidation over Pd-Cu/zeolite Y by transient XAS

“…NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-14982-x ARTICLE undesired carbon dioxide. Although ethylene is known to adsorb on Cu(I) 40 , this interaction is insufficient to lead to full oxidation at low oxygen partial pressures.…”

IR Studies of the Cu Ions in Cu-Faujasites

“…Cu + ions in zeolites were also found to activate not only N=O bonds but also the multiple bonds in organic molecules, resulting in a significant red shift of the C=C double bond in alkenes (ethene, propene, butenes) [20], a triple C≡C bond in ethyne [21], C=O bonds in acetone and formaldehyde [22,23], as well as the activation of the aromatic ring in benzene [24]. Quantum chemical Density Functional Theory (DFT) calculations showed [25,26,27] that the activation of multiple bonds in these molecules was realized according to a mechanism similar to that of NO activation, i.e., by the donation of electrons from Cu + and the zeolite framework to antibonding π* orbitals of molecules.…”