Uniform Catalytic Site in Sn-β-Zeolite Determined Using X-ray Absorption Fine Structure

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Abstract
The Sn silicate zeolite, Sn-β, has been shown to be an efficient, selective heterogeneous catalyst for Baeyer−Villiger oxidations. Using primarily a multishell fit to extended X-ray absorption fine structure (EXAFS) data, we show that the Sn does not randomly insert into the β-zeolite structure but rather occupies identical, specific, crystallographic sites. These sites are the T5/T6 sites in the six-membered rings. Moreover, the Sn is substituted in pairs on opposite sides of these six-membered rings. We believe that it is the specific, uniform crystallographic location of the Sn in the β crystal structure that leads to sites with uniform catalytic activity, and consequently to the high chemical selectivity demonstrated for this catalyst. This manifests itself in the almost enzyme-like selectivity of this catalyst in Baeyer−Villiger oxidations. This uniform site distribution of the Sn suggests that there is likely a symbiotic relationship between the structure-directing agent in the zeolite synthesis and the Sn heteroatoms during the framework formation.