Combined Experimental and Computational Approaches To Elucidate the Structures of Silver Clusters inside the ZSM-5 Cavity

Abstract

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.

Utilizing super-atom orbital ideas to understand properties of silver clusters inside ZSM-5 zeolite

“…47 Previously, we combined UV-vis measurements with density functional theory (DFT) calculations to elucidate the state of silver atoms within ZSM-5 zeolites, whose ten-membered rings cavities are on the nanometre scale. 48,49 UV-vis measurements in previous studies found maximum peaks ranging from 300-320 nm. 43,49 These peaks can be assigned by electronic transitions of triangle Ag 3 and buttery Ag 4 clusters inside ZSM-5, whose structures were optimized by DFT calculations (Fig.…”

“…48,49 UV-vis measurements in previous studies found maximum peaks ranging from 300-320 nm. 43,49 These peaks can be assigned by electronic transitions of triangle Ag 3 and buttery Ag 4 clusters inside ZSM-5, whose structures were optimized by DFT calculations (Fig. S1 (ESI †)).…”

Synthesis of an unexpected [Zn2]2+ species utilizing an MFI-type zeolite as a nano-reaction pot and its manipulation with light and heat

“…Previously, our studies confirmed that this B3LYP-optimized MFI zeolite model comprised of 10-MR in cavities reproduced well the experimentally observed data. [12][13][14]22,[24][25][26][27][28][29][30][31] Taking our previous studies into consideration, we used the M7 site for the Al substitution position; 32 and we defined the Al configuration as the [M7-S2] arrangement. [12][13][14]30 To optimize fully the larger MFI geometry with a 10-MR cavity, we used the 6-311G(d) basis set for the zinc atom and the 6-31G (d) basis set for the Al atom, as well as four O atoms that were connected to the substituted Al atom.…”

Characterization of metallic species on porous materials by in situ XAS

“…dispersed Ag + cations256 is reestablished and well visualized for AgCHA-R1O and AgCHA-R2O samples. On the other hand, for all RHO-based samples the typical band of Ag + species is shown while a quite similar band centered at 355 nm attributed as Ag 0 nanoparticles 261 can be seen.…”

“…The AgCHA RED400 catalyst maintains roughly the same UV-Vis spectrum as in the reduced state after reduction, while the spectrum of AgRHO RED400 was completely changed after reaction. The band present in reduced sample disappeared giving rise to the formation of typical band at 220 nm characteristic of Ag + in exchange positions within the zeolite 256.…”