Possibility of Copper-Ion-Exchanged MFI-Type Zeolite as C-H Bond Activation Material for Propane and the Driving Force for Activation

The dehydrogenation of propane (C₃H₈) utilizing solid catalysts is expected to be a promising method for producing propene (C₃H₆) selectively and economically. Propene is useful as a raw material to produce other valuable materials. To push forward the dehydrogenation reaction of C₃H₈, it is essential to find catalysts that work effectively in the activation of the C-H bonds in C₃H₈. To this end, we investigated the interaction of C₃H₈ with the copper-ion-exchanged MFI-type zeolite (CuMFI) at room temperature. The C₃H₈ adsorption properties of CuMFI were remarkable, compared with those of the sodium-form MFI-type zeolite (NaMFI); CuMFI exhibited irreversible adsorption, even at room temperature, whereas NaMFI exhibited reversible adsorption. Correspondingly, the CuMFI had an adsorption energy of 110 kJ mol^-1 in the initial adsorption region, whereas the NaMFI had an adsorption energy of 50 kJ mol^-1. Furthermore, a peculiar feature was observed in the appearance of the characteristic infrared (IR) absorption bands at 2740, 2659, and 2624 cm^-1 and, particularly, the band that appeared at 2555 cm^-1. They were noticeably shifted toward the lower wavenumber side, compared with the C-H vibration bands for a gaseous C₃H₈ molecule. This was a result of the interaction of C₃H₈ with the Cu⁺ ions formed in CuMFI. The application of density functional theory calculations to the model, comprising the interaction between the dual-Cu⁺ site in CuMFI and C₃H₈, well explains the observation of the appearance of these four types of bands toward the lower wavenumber side and also the adsorption energy. The cause is the dominant contribution of the back-donation from Cu⁺ to C₃H₈, rather than the donation from C₃H₈ to Cu⁺, which results in a weakening of the C-H bonds in the C₃H₈ molecule. The Cu⁺ site taking a dimeric structure, not various types of single-Cu⁺ sites, plays a key role in activating C₃H₈ effectively, even at around 300 K. Results of this study should contribute to not only understanding the critical role of the dimeric Cu⁺ species in MFI but also developing new materials with superior activity in the activation of the C-H bond in alkanes.