Potential for C-H activation in CH₄ utilizing a cumfi-type zeolite as a catalyst

We have found that a copper ion-exchanged MFI (Mobil Five)-type zeolite, having an ion exchange capacity of 112% (CuMFI-112), exhibits a specific interaction with CH₄, even at room temperature. The observed properties are very remarkable compared with those found in the adsorbed amount of sodium ion-exchanged MFI and in the interaction with the sodium ion in MFI as well as in the adsorbed amounts for other types of copper ion-exchanged zeolites. The specificity of Cu⁺ in MFI is clearly evident. The most remarkable feature of the present system is the distinct appearance of infrared bands (IR) at 2661 and 2619 cm^-1 due to adsorbed CH₄ at 300 K and the change in the X-ray absorption fine structure spectra of CuMFI before and after CH₄ adsorption at 300 K, indicating the existence of substantial and specific interaction in this system. In addition, calculations based on the density functional theory were carried out by utilizing a Cu-AlSi₉₁O₁₅₁H₆₆ cluster model, which is made up of a 10-membered ring of CuMFI. As a result, the calculations gave absorption bands that can be explained reasonably by the observed novel IR bands, indicating the existence of two types of η²(H, H)-type interactions between Cu⁺ and CH₄: (1) hydrogen interacted with a Cu cation coordinating to two oxygen atoms near an intersection of straight and sinusoidal channels, and (2) hydrogen interacted with a Cu cation located above a 5-membered ring on a wall of a straight channel. The nature of such bonding can be described by the formation of Cu-C bonds, which causes the deformation the T_d structure of CH₄. Such interaction results in a weakening of the C-H bond and thus a lower frequency shift of the C-H bond interacting with Cu⁺.