Synthesis, Characterization, and Spectroscopic Characteristics of Chromium(6+) and -(4+) Silicalite-2 (ZSM-11) Materials

The systematic incorporation of Cr ions into a phase-pure silicalite-2 lattice was accomplished through hydrothermal synthesis using 3,5-dimethylpiperidinium as a templating agent. The Cr ions, after calcination to remove the template, were in the 6+ oxidation state, with their incorporation into the lattice verified by the systematic expansion of the unit cell as a function of Cr loading. The structures of these materials as revealed by electronic spectroscopy and X-ray absorption near-edge spectroscopy (XANES) were consistent with the dioxo structure typically exhibited by Cr6+ in an amorphous silica matrix. These materials were highly luminescent, with the emission spectra showing an unusually well-resolved vibronic structure characteristic of an emissive site with little inhomogeneous broadening. The site was reduced under flowing CO to Cr4+, as characterized by XANES. The reduction of Cr from 6+ to 4+ resulted in unit-cell volumes that are systematically smaller than those observed with Cr6+, even though the ionic radius of Cr is larger. This is attributed to the fact that the Cr6+ site is not a simple metal ion but a significantly larger [CrO2](2+) unit, requiring a larger lattice expansion to accommodate it. Through analysis of the XANES preedge and assignment of the ligand-field spectrum of the Cr4+ ions, it is possible to establish isomorphic substitution into the silicalite lattice.