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 CC 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.