Existence of dual species composed of Cu⁺ in CuMFI being bridged by C₂H₂

The interaction of ethyne (C₂H₂), as well as of carbon dioxide (CO₂), with copper-ion-exchanged MFI zeolite (CuMFI) at room temperature was examined. It was found that CuMFI preferentially adsorbs C₂H₂, while this material does not respond to CO₂. To clarify the specificity of CuMFI, a combination of various experimental techniques and theoretical calculations was adopted. Distinctive interaction energies of 140 and 110 kJ mol^-1 were clearly observed at the initial stage of C₂H₂ adsorption on CuMFI, suggesting the presence of two types of adsorbed C₂H₂. Two distinct IR bands at 1620 and 1814 cm^-1 appeared, which were assigned to the C=C stretching vibration modes of C₂H₂ differing in their adsorbed state. Both photoluminescence and X-ray absorption spectra showed that cuprous ions (Cu⁺) in CuMFI act as efficient sites for a marked C₂H₂ adsorption. From the analysis of the latter spectra and the calculational results based on the density functional theory, the formation of dual Cu⁺…(C₂H₂)…Cu⁺ complexes was indicated for the first time for CuMFI, and such a special configuration of the Cu⁺ sites contributed to the extremely strong adsorption of C₂H₂. In contrast, it was necessary for the linear CO₂ molecule to take a bent structure to be adsorbed on Cu⁺ in CuMFI. It was concluded that the difference in the adsorption response of Cu⁺ in CuMFI towards C₂H₂ and CO₂ is due to the chemistry between the nature of electron donation of Cu⁺ and the hybrid orbitals of the respective molecules. This work promotes further understanding of the states of active centres in CuMFI for C₂H₂ activation, as well as for N₂ fixation.