Synthesis, Structure, and Reactivity of Iron-Sulfur Species in Zeolites.

Previous studies have shown that a variety of iron-oxo species, including nanoclusters and dimers, can be formed within the pore structure of ZSM-5 and other porous materials. We report that these species can readily be sulfided by exposure to hydrogen sulfide at 623 K when formed in ZSM-5, mordenite, and MCM-41. The iron-sulfur entities produced have been characterized by XPS and EXAFS spectroscopy. Iron is in the oxidation state +2 and the nearest neighbor iron-sulfur interatomic distance is ∼2.25 Å, which is close to that in iron(II) sulfide. Sulfidation appears to be at least 90% complete. In ZSM-5 the same iron-sulfur species, which contains isolated iron atoms or very small clusters, is formed irrespective of whether the iron was introduced by aqueous exchange or chemical vapor deposition. The sulfided ZSM-5 material is found to be weakly active for the hydration of acrylonitrile to acrylamide in tetrahydrofuran solution at 338 K, with the best materials achieving about 0.5 turnovers per iron species per day. [ABSTRACT FROM AUTHOR]