Combined experimental and computational approaches to elucidate the structures of silver clusters inside the ZSM-5 cavity

A combination of experimental and computational analyses suggested the presence of Ag₃ or Ag₄ clusters inside a nanometer-sized cavity in Ag-ZSM-5 zeolites, which were formed from H-ZSM-5 using the conventional ion-exchange method in an aqueous silver nitrate solution. During the experimental analyses, we investigated the structural and absorption properties of Ag-ZSM-5 through UV-vis diffuse reflectance and X-ray absorption fine structure (XAFS) measurements. The results from the extended XAFS (EXAFS) analysis indicated that clusters contained in the ZSM-5 cavity have Ag-Ag separations of approximately 2.6 Å. The UV-vis measurements indicated that the clusters located in the cavity present three prominent absorption bands centered at approximately 255, 287, and 331 nm for the sample treated at 473 K. The Ag-ZSM-5 treated at 573 or 673 K presents new UV-vis bands at approximately 303 and 319 nm. The experimental results regarding the structural and absorption properties of Ag-ZSM-5 could be well-reproduced by DFT calculations when a model large enough to represent a 10-membered ring of ZSM-5 was used. DFT optimization indicated that the ZSM-5 cavity can accommodate a triangular Ag₃ cluster and a butterfly Ag₄ cluster whose Ag-Ag separations range from 2.7 to 2.9 Å. According to time-dependent DFT calculations, these clusters have electronic transitions from a completely symmetric 5s-based orbital to a 5s-based orbital with one node. The electronic excitations between the 5s-based orbitals are modulated by the ZSM-5 encapsulation through the resulting deformation of the cluster and interactions between the cluster and framework oxygen atoms. The electronic transitions between the 5s-based orbitals that appropriately explain the UV-vis absorption properties would become fingerprints for identifying the shapes and sizes of clusters inside a zeolite cavity. Our multidisciplinary analyses conclusively determined the origin of the absorption peaks in Ag-ZSM-5 and successfully obtained atomistic information about the states of silver clusters inside a ZSM-5 cavity. The findings of this study will provide useful information for elucidating the structures of active sites in Ag-ZSM-5 and their important role in catalytic reactions such as C-H bond activation in hydrocarbons.