Further evidence for the existence of a dual-Cu⁺ site in MFI working as the efficient site for C₂H₆ adsorption at room temperature

We have recently clarified the following point: a dual-type site, which is composed of a pair of monovalent copper ions (Cu⁺) formed in a copper-ion-exchanged MFI-type zeolite (CuMFI), functions as the active center for strong ethane (C₂H₆) adsorption even at room temperature rather than a single-type site composed of a Cu⁺ ion. However, the character of the dual-Cu⁺ site in a CuMFI is not yet fully understood. In this study, we have elucidated the nature of the active sites for C₂H₆ based on infrared (IR) and calorimetric data. On the basis of the results obtained, we came to the conclusion that the dual-Cu⁺ site composed of Cu⁺ ions giving the adsorption energy of 100 kJ mol^-1 and the absorption band at 2151 cm ^-1 for carbon monoxide (used as a probe molecule) at room temperature functions as an adsorption site for C₂H₆. We also evaluated, for the first time, the interaction between the dual-Cu⁺ site and C₂H₆ energetically, by the direct measurement of heat of adsorption. The value of 67 kJ mol^-1 that we recorded was higher than that for the single-Cu⁺ site in this sample and also for other samples, such as NaMFI and HMFI.