Probing the Brønsted and Lewis acidity of Fe-silicalite by FTIR spectroscopy of H 2 adsorbed at 20 K: Evidences for the formation of Fe 3+/H 2 and Fe 2+/H 2 molecular adducts

Abstract

We report on the spectroscopic characterisation of a Fe-silicalite sample in terms of Brønsted and Lewis acidity using 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) surface of zeolites: Brønsted, silanols, and metal ions. The spectroscopic manifestations of the different H 2 adducts are found in distinct spectral regions depending on the nature and strength of the adducts: ν ¯ HH modes of OH⋯H 2 adducts are found in the 4170–4100 cm −1 region, and those of Fe x+ ⋯H 2 adducts are found in the 4100–3900 cm −1 region. Four distinct Fe x+ ⋯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 Fe 2+ and two Fe 3+ distinct sites, interacting with H 2 mainly by electrostatic forces. The importance of these sites as active species for the selective oxidation of hydrocarbons with N 2O is discussed.