In its element: Zn(2+) at the M7 site of MFI-type zeolites activates H(2), via ZnH and OH species, and leads to Zn(0) species. The Zn(0) species returns to its original state, a Zn(2+) ion, upon evacuation of the zeolite at 873 K (see picture). The formation of the Zn(0) species is supported by DFT calculations.

In this work, we used both experimental and density functional theory (DFT) calculation methods to examine the mechanism of CH 4 activation taking place on the Zn 2+ ion exchanged MFI-type zeolite (ZnMFI). The heterolytic dissociation of CH 4 on ZnMFI around 300 K was observed experimentally, causing the appearance of IR bands at 3615, 2930, and 2892 cm −1 . The first band can be assigned to the OH stretching vibration associated with the formation of the Brønsted acid site and the latter to the C−H stretching modes ascribable to the −[ZnCH 3 ] + species. Combining the IR spectroscopy with a DFT calculation, it is apparent that the heterolytic C−H bond dissociation of CH 4 has an activation energy of 15 kJ mol −1 and takes place on a monomeric Zn 2+ at the M7S2 site. The M7S2 site has a specific Al arrangement in MFI and exhibits a pronounced reactivity for the H−H bond cleavage of H 2 , even at room temperature. In addition, to our knowledge, we are the first to succeed in explaining the dissociation process of CH 4 by applying natural bond orbital (NBO) and interaction localized orbital (ILO) analyses to the present system; the donation interaction from the CH 4 −σ(C−H) orbital to the Zn−4s orbital triggers the cleavage of the C−H bond of CH 4 under mild conditions.

For the first time, the paramagnetic Zn(+) species was prepared successfully by the excitation with ultraviolet light in the region ascribed to the absorption band resulting from the 4s-4p transition of an atomic Zn(0) species encapsulated in an MFI-type zeolite. The formed species gives a specific electron spin resonance band at g = 1.998 and also peculiar absorption bands around 38,000 and 32,500 cm(-1) which originate from 4s-4p transitions due to the Zn(+) species with paramagnetic nature that is formed in MFI. The transformation process (Zn(0) → Zn(+)) was explained by considering the mechanism via the excited triplet state ((3)P) caused by the intersystem crossing from the excited singlet state ((1)P) produced through the excitation of the 4s-4p transition of an atomic Zn(0) species grafted in MFI by UV light. The transformation process was well reproduced with the aid of a density functional theory calculation. The thus-formed Zn(+) species which has the doublet spin state exhibits characteristic reaction nature at room temperature for an O2 molecule having a triplet spin state in the ground state, forming an η(1) type of Zn(2+)-O2(-) species. These features clearly indicate the peculiar reactivity of Zn(+) in MFI, whereas Zn(0)-(H(+))2MFI hardly reacts with O2 at room temperature. The bonding nature of [Zn(2+)-O2(-)] species was also evidenced by ESR measurements and was also discussed on the basis of the results obtained through DFT calculations.

We present clear IR and density functional theory (DFT) evidence demonstrating that the electron-accepting nature of Zn 2+ ion exchanged in MFItype zeolite (ZnMFI) plays a dominant role in CH 4 activation. The IR study revealed that the heterolytic dissociation of CH 4 takes place on the Zn 2+ ion exchanged in MFI under a CH 4 atmosphere even near room temperature, whereas a similar reaction scarcely occurred on Mg 2+ ion exchanged in MFI, although the ionic radius and charge of Mg 2+ are almost the same as those of Zn 2+ . These data indicate that the dissociation reaction of CH 4 on Zn 2+ in MFI is facilitated not only by the electrostatic interaction but also by the electrontransfer interaction. This interpretation was clearly evidenced by the observed v 1 mode of the C−H symmetric stretching vibration, i.e., a larger band shift toward lower wavenumbers, for the molecular CH 4 adsorbed on ZnMFI, compared with those for a gaseous CH 4 molecule. Additional experiments were also performed by the IR method utilizing CO as a probe molecule that has an electron-donating nature. All experimental data presented were successfully explained in terms of the superior electron-accepting nature of Zn 2+ exchanged in MFI. Furthermore, the DFT calculation method completely explained all experimental data by adopting the M7S2 model, which was truncated from the ZnMFI structure; the electron-accepting nature is dominant in the heterolytic activation of CH 4 in the Zn 2+ ion in MFI in comparison with that of Mg 2+ exchanged at the same site. We have thus shown that the electron-transfer interaction between Zn 2+ and CH 4 plays a key role in the heterolytic CH 4 activation process: the σ donation from the σ(C−H) orbital of CH 4 toward the Zn 4s orbital through overlapping with each orbital.

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.