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 E. Broclawik,* ,† J. Zalucka, ‡ P. Kozyra, ‡ M. Mitoraj, ‡ and J. Datka ‡ Institute of Catalysis, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Krako ´w, Poland, and Faculty of Chemistry, Jagiellonian UniVersity, ul. Ingardena 3, 30-060 Krako ´w, Poland ReceiVed: January 11, 2010; ReVised Manuscript ReceiVed: April 14, 2010 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. Introduction Zeolites are a group of compounds of great importance for science as well as for industry. They are used as catalysts in such processes as cracking, alkylation, and polymerization. Both natural and synthetic zeolites are used also as sorbents and ion exchangers. Zeolites with transition metal ions in exchangeable positions are of special significance. Such cationic sites are catalytically active in denox and other redox reactions. Our earlier IR studies and DFT quantum chemical calculations evidenced that Cu(I) ions in zeolites CuZSM-5, CuX, or CuY and also in CuMCM-41 material were able to activate multiple bonds in organic molecules such as alkenes (ethene, propene, and butenes), ethyne, benzene, acetone, and formaldehyde. 1 This activation was evidenced by significant red-shift of the IR multiple bond stretching frequency. High activity of copper sites toward NO or alkenes, promoted by their ability to donate electrons to π antibonding orbitals of the adsorbate, is well- known both from experiment and theoretical modeling. 1-5 Our former IR studies showed that silver sites in zeolites did also activate multiple bonds in organic molecules, similarly to copper ions; however, this effect for Ag was smaller than for Cu ions: the red-shifts of IR bands of stretching of multiple bonds for the Ag site was about half of the value observed for the Cu site. 6 Thus in this study Cu(I) sites are examined parallel with Ag(I) ones to disclose factors essential for high activity of copper sites in MFI zeolite. Ethene is of interest here as it forms complexes with various cationic centers, where the CdC bond can be either shortened or elongated, depending not only on the cation but also on its next neighbors. 7 It is used in many industrial processes like production of propene over Ag(I) exchanged zeolites, 8 produc- tion of aromatic compounds 9 and oxygenated compounds, 10 and production of isoprene and methacrylic acid used in polymer synthesis. 11 Molecules with triple C≡C bond have also wide range of application in many processes, e.g., Diels-Alder reaction, 12 di- and oligomerization, 7,13 Reppe’s cycle, 14 and synthesis of N-nonsubstituted 1,2,3-triazols. 15 Ethyne is a good representative of this group. In many processes ethene and ethyne undergo transformations along similar reaction mecha- nisms, involving preliminary activation of the CC bond. Moreover, ethene and ethyne are good candidates for probe molecules to examine zeolite properties: they are small and highly symmetric and exhibit large effects while interacting with a center, such as geometry changes and IR frequency shifts (significant especially for ethyne). 1,6,16-19 Coinage metal cations, bare or coordinated by selected model ligands, have already been studied in the context of their interaction with alkene or alkyne molecules by several groups, including theoretical and/or experimental researches (see, e.g., refs 20-23 and references therein). Electron rich molecules like ethene and ethyne form complexes with cationic sites where they are good electron donors to the central ion (e.g., Cu(I) or Ag(I)) as well as good electron acceptors. Both of these processes are important for activation, but π-backdonation is believed to be crucial in the case of molecules with multiple bonds. 1,21,23-26 For the purpose of quantitative estimation of donation and backdonation effects energy decomposition schemes are very often used. 27-29 This is achieved by the decomposition of the orbital interaction term into different irreducible repre- sentations. However, for molecules with no symmetry, such approaches hardly provide separate information on donation and backdonation processes. Along this line, our work presents the first screening application of the recently proposed tool for charge flow decomposition, invoked to discriminate Cu and Ag cations embedded in a realistic model of the MFI framework. To this end, the widely accepted mechanism for adsorbed molecule activation by donation and backdonation is examined by means * Corresponding author. E-mail: broclawi@chemia.uj.edu.pl. † Polish Academy of Sciences. ‡ Jagiellonian University. J. Phys. Chem. C 2010, 114, 9808–9816 9808 10.1021/jp1002676  2010 American Chemical Society Published on Web 05/07/2010