Catching Zeolites in Action

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Science  02 Apr 2004:
Vol. 304, Issue 5667, pp. 18
DOI: 10.1126/science.304.5667.18c

X-ray techniques for studying heterogeneous catalytic reactions can probe some of the structural properties of species that accumulate over a working active site. Van Bokhoven et al. used extended x-ray absorption fine structure (EXAFS) of the Al K edge to determine local structure during the oligomerization of ethylene over zeolite Y (which has a silicon/aluminum ratio of 2.6). Reduction of the NH4-Y zeolite to the H-Y form created one longer Al-O bond of 1.89 Å. During reaction at 50°C, this Al-O bond shortened to 1.81 Å, and the authors attribute this change to the adsorption of a carbenium ion at the Brønsted acid site on the hydroxyl groups that bridge Al and Si atoms, forming an alkoxy species. At higher temperatures (about 350°C), they observed an increase in the Al-O bond to near that for H-Y that they attribute to desorption of the intermediate. — PDS

J. Am. Chem. Soc. 10.1021/ja031755j (2004).

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