What is it about?
Aqueous hydrogen peroxide oxidizes hydrocarbons in very mild conditions when Ti-zeolites are used as catalysts. This is at the basis of important, widespread and environmentally friendly large-scale industrial processes, such as the epoxidation of olefins with Ti-silicalite zeolite. In spite of the relevance of this process, the role and the behavior of the titanium active centers in the oxidative cycle were still unknown in microscopic detail. Therefore, we modeled a titanium zeolite, simulated the epoxidation reaction with an accurate methodology and validated results against a broad set of experimental data obtained with different techniques. Our work uncovered the mechanism of the catalytic action and provided a consistent picture of the complex behavior of such a relevant class of materials.
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Why is it important?
We explained the full catalytic oxidation cycle of an olefin inside a Ti-zeolite in the presence of aqueous hydrogen peroxide. At the end of the cycle, ethylene epoxide and water (a sustainable by-product, indeed!) are obtained, and the catalyst is recovered. One of the novelties of our study is that we predicted the active, oxidizing intermediate to be a peroxo species actually inserted in the zeolite framework. This means that the true catalyst is the whole zeolite, not only its titanium sites, because the framework truly plays a key active role in the catalytic process.
Perspectives
I vividly recall our surprise when we found out that the elusive "active intermediate" of the catalytic cycle was literally inserted iinto the walls of the catalyst. Before our study, no one had hypothesized that this could even possibly occur. Anyway, our data on this intermediate were in agreement with all the previous experimental findings on this catalyst available in the literature. This validated the proposed mechanism, which helped to reconcile apparently contrasting observations collected on this catalyst in many years of previous studies. Beside molecular-level understanding of the catalytic oxidation cycle, the active participation of the zeolite framework to the catalytic process also explained the aging and deactivation mechanism of this very important class of industrial catalysts.
Gloria Tabacchi
university of insubria
Read the Original
This page is a summary of: On the Role of Ti(IV) as a Lewis Acid in the Chemistry of Titanium Zeolites: Formation, Structure, Reactivity, and Aging of Ti−Peroxo Oxidizing Intermediates. A First Principles Study, The Journal of Physical Chemistry, November 2006, American Chemical Society (ACS),
DOI: 10.1021/jp065494m.
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