Spectroscopic Determination of the Site in MFI Zeolite where Cobalt(I) Performs Two-Electron Reduction of O₂ at Room Temperature

Abstract

Zeolites have well-defined pores of molecular dimensions to stabilize the molecularly dispersed reactive oxygen species (ROS) responsible for actual key intermediates in a variety of desirable catalytic transformation processes. Defining the local environment to create such an important class of ROS is undoubtedly important for designing more efficient zeolite-based catalysts; however, it remains challenging due to the obscure distribution of the framework Al atoms. In the present study, we defined the local environment of MFI zeolite to create a novel O 2 activation property of the cobalt ion. We found that the Co III −peroxo species is formed through two-electron reduction of O 2 at room temperature by an extra lattice Co I species that corresponds to an abnormal oxidation state of Co. This is the first example of the experimental detection of the O 2 -derived mononuclear metal−peroxo species in zeolites. The change in the skeletal vibrational mode associated with the Al site, ν Al−O−Si , upon the O 2 activation process was experimentally observed. It provides the information about the position of the framework Al atom. By applying B3LYP-D3 density functional theory (DFT) calculations, we demonstrated that the positioning of the framework Al atom at an interface between straight and sinusoidal channels of MFI zeolite is essential for the formation of the Co III −peroxo species. The experimentally calibrated DFT model features the S = 1 square planar Co III −peroxo species bound to the interface region by two anionic lattice oxygen atoms. The Co III −peroxo species is stabilized only by weak donor O ligands, never previously identified. This unique state is demonstrated by combined experimental and computational analyses of the vibronic progression found for the first time in this system, which provides detailed information about the site geometry and electronic structure. The findings of the present study assist us to understand how the zeolite lattice creates the abnormal O 2 activation property of metal ions.

17O-ESR Evidence for Zeolite Matrix Isolation of a Square Planar ZnO3 Ring Radical with C2v Symmetry

“…This is indicative that the electronic structure of the ZnO 3 ring radical is highly localized near the framework Al site, which is not inconsistent with the observation of the well-resolved vibrational fine structure (Figure ). , Similar cluster size independency was previously observed for the Co III –peroxo, Ni II –superoxo, Zn II –oxyl, − Ga III –ozonide, and Ga III –oxyl species isolated in the zeolite matrix, where a variety of vibronic progression features were observed even at RT. The effect of the Al position on the local geometry of the ZnO 3 core was also examined by periodic DFT calculations.…”

“…The validity of our experimental setup was verified in our previous work. 28 During the preparation of ozonide and UV−vis measurements, the samples were treated in situ cells without exposure to air.…”

How to Constrain Metal–Oxyl Bonds on a Solid Surface? Lesson from Isovalent Zn(II)–Oxyl and Ga(III)–Oxyl Bonds Isolated in Zeolite Matrix

“…It is worth noting that such a comparison is particularly helpful in evaluating the accuracy of the theoretically proposed structures of the metal− oxygen radicals in the zeolite matrix. 27,28,42,43 The side-on ozonide model provided a vibronic FC progression having a vibronic origin at 19990 cm −1 and a spacing of ca. 900 cm −1 derived from the stretching O−O−O vibrational structure of the nb → π* O−O−O excited state (Figure 3B, red line).…”

“…1000 cm −1 are within the error range for the DFT-based FC analyses. 27,28,42,43 The slight difference in intensity distributions of the vibronic progressions of the Ga III −ozonide species, which was peculiarly observed in the higher energy region, originates from the Franck−Condon method, which is implemented in a Gaussian software, as the method does not consider the anharmonic term. Accordingly, we can reasonably say that our model well accounted for the vibronic progression features.…”

“…To validate our calculated lowest-energy geometries and reaction mechanism, we computed the vibronic Franck–Condon (FC) progressions of oxyl and side-on ozonide to compare with the experimental data. It is worth noting that such a comparison is particularly helpful in evaluating the accuracy of the theoretically proposed structures of the metal–oxygen radicals in the zeolite matrix. ,,, The side-on ozonide model provided a vibronic FC progression having a vibronic origin at 19990 cm –1 and a spacing of ca. 900 cm –1 derived from the stretching O–O–O vibrational structure of the nb → π* O–O–O excited state (Figure B, red line).…”