New Insights into Charge Flow Processes and Their Impact on the Activation of Ethene and Ethyne by Cu(I) and Ag(I) Sites in MFI

This paper concerns the activation of ethene and ethyne molecules on two cationic sites (Cu(I) and Ag(I)) in ZSM-5 zeolite. QM/MM calculations were carried out to obtain geometric structure and vibrational frequencies. A novel analysis tool, NOCV (natural orbitals for chemical valence) supported by an ETS energy decomposition scheme, was applied to characterize charge flow between adsorbed molecules and the cationic site in ZSM-5 zeolite. The ETS-NOCV method allows for separating independent components of differential electron density into donation and backdonation channels, responsible for the substrate activation. It also helps to evaluate the importance of particular density transfer channels in the activation process. Two partition schemes into two subsystems are proposed here to extract complete information on the electronic balance between the molecule, the cation, and the zeolite framework. Both cationic sites (Cu(I) and Ag(I)) and both molecules (ethene and ethyne) are compared and the differences in the red-shift of CC stretching frequency are rationalized in terms of donation and backdonation charge transfer processes. They are shown to depend as well on metal specific properties as on the interaction between the metal and the framework.