Bathochromic Effects in Electronic Excitation Spectra of Hydrated Ti Zeolites: A Theoretical Characterization

Ettore Fois, Aldo Gamba, Gloria Tabacchi
  • ChemPhysChem, March 2008, Wiley
  • DOI: 10.1002/cphc.200700719

Water effects in titanium-zeolites

What is it about?

Titanium-containing zeolites are widely used in industry as catalysts for oxidation in mild conditions. In zeolites, titanium is normally coordinated to four oxygen atoms of the framework. However, titanium may expand its coordination from four to six, acting as a Lewis acid with respect to bases present in the channels and cages of zeolites. Water is one of those bases. How does water influence the structure of the Titanium sites? Is it possible to get this information from the electronic spectrum of the material? We have found an answer to these questions using theoretical models of hydrated titanium zeolites.

Why is it important?

We have simulated the electronic bands detected by UV/Vis spectra, and we have explained how these bands depend on the number of molecules bonded to the titanium center. With this information, we can determine not only the water content of the sample, but also the coordination number and the structure of the titanium sites from the UV/VIs bands detected experimentally.

Perspectives

Gloria Tabacchi
university of insubria

We have seen that in hydrated titanium zeolites, the coordination number of Ti increases with the number of water molecules. This affects the corresponding UV-Vis bands, which become red-shifted by increasing the water content of the zeolite. In fact, when water binds to the Ti site, the other Ti-O bonds become longer and the tetrahedral geometry is distorted. Water quenches somehow this effect by donating charge to titanium and lowering its electron-attractor character. I believe that the explanation of the effect of water on the Ti sites is important for further progress in the chemistry of these catalysts.

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http://dx.doi.org/10.1002/cphc.200700719

The following have contributed to this page: Gloria Tabacchi