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

Abstract

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.

On the nature of spin- and orbital-resolved Cu+–NO charge transfer in the gas phase and at Cu(I) sites in zeolites

“…As we have shown in this contribution and postulated previously for other molecules [28][29][30], in order to fully understand the nature of electron transfer between the active site and a ligand total differential density must be resolved into independent electron transfer channels since various symmetries and transfer directions may coexist, either corroborating or opposing activation. All the more for the NO ligand, which due to its specific open-shell electronic structure is a special case, and thus requires NOCV analysis with spin-resolution and careful selection of initial fragment density, prerequisite for defining proper deformation density.…”

“…On the other hand, it was suggested, after analyzing atomic population changes upon ligand adsorption on a bare cation versus the cation embedded in a model environment that the unusual catalytic activity of transition-metal exchanged zeolites should be due to the activation of the transition-metal ion by the zeolite framework which functions as a specific ligand. Theoretical studies devoted to the interaction of small ligands with multiple bonds bound by zeolitic sites clearly indicated that the charge flow upon adsorption involves several transfer channels, strongly dependent on a zeolite framework type and a cation position [23,[26][27][28][29][30]. Populations within orbital resolution and devised selection schemes, frequently supported by topological analyses of electron density, were able to discriminate distinct electron transfers for a model metal-ligand bond [23,26,27].…”

“…This method may also serve to follow changes in electron transfer processes imposed by metal environment. ETS-NOCV methodology has already been applied by us with flexible strategy of dividing multicomponent zeolitic active site into fragments and shown to perform well for copper (I) sites interacting with closed-shell ligands like ethene, ethyne, benzene, or formaldehyde [28][29][30]35].…”

Electronic propensity of Cu(II) versus Cu(I) sites in zeolites to activate NO — Spin- and orbital-resolved Cu–NO electron transfer

“…We have previously studied distinct electron density transfer channels resulting from natural orbitals for chemical valence (NOCV) analysis (described in detail elsewhere) 8-10 performed for molecules coordinated to Cu(I) systems via a C-C multiple bond (benzene, ethene, or ethyne) [11][12][13] or an oxygen lone pair (formaldehyde). 13 NOCV orbitals diagonalize differential electron density (the difference between the electron density of a complex system and superimposed densities of non-interacting fragments, here a Cu(I) site and a ligand), and thus decompose global density flow accompanying ligand bonding by the site into independent density transfer channels.…”

On the asymmetry in molybdenum–oxygen bonding in the MoO3 structure: ETS–NOCV analysis

“…In the present account we will describe the Mo-O bonding in the Mo 7 O 30 H 18 cluster using the recently developed extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach [26] based on NOCV [27,28] and the ZieglerRauk bond-energy partitioning (ETS) [29]. This approach was successfully applied in characterizing various types of chemical bonds in inorganic and organic compounds, involving donor-acceptor, ionic and covalent bonds as well as relatively weak interactions (hydrogen bonds, agostic bonds) [26,[30][31][32]. The main advantage of the NOCVbased approach is that it allows one to separate the contributions to the deformation density originating from different components of the bond (r-, p-, d-), providing the corresponding charge-flow and energy estimates [26][27][28].…”