Probing the Bronsted and Lewis acidity of Fe-silicalite by FTIR spectroscopy of H2 adsorbed at 20 K:: Evidences for the formation of Fe3+/H2 and Fe2+/H2 molecular adducts

We report on the spectroscopic characterisation of it Fe-silicalite sample in terms of Bronsted and Lewis acidity ushw H-2 as a probe molecule at low temperature (20 K). At 20 K, H-2 is able to form adducts with surface sites present in the internal (and external) SUrfaceofzeolites: Bronsted, silanols, and metal ions. The spectroscopic manifestations of the different H-2 adducts are found in distinct spectral re.-ions dependin.g on the nature region, and those of Fex+center dot center dot center dot H-2 adducts are found and strength of the adducts (v) over bar (HH) modes of OH center dot center dot center dot H-2 adducts ire found in the 4170-4100 cm(-1) in the 4100-3900 cm(-1) region. Four distinct Fex+center dot center dot center dot H-2 adductS (at 4050, 4028, 3990, and 3960 cm(-1)) were formed on extra-framework Fe sites, created by controlled migration of Fe from the framework as a consequence of thermal treatments. The dependence on red-ox treatments of the relative concentration of these sites allowed identification of two Fe2+ and two Fe3+ distinct sites, interacting with H-2 mainly by electrostatic Forces. The importance of these sites its active species for the selective oxidation of hydrocarbons with N2O is discussed. (c) 2006 Elsevier Inc. All rights reserved.