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Anchoring Fe Ions to Amorphous and Crystalline Oxides: A Means To Tune the Degree of Fe Coordination
Author(s) -
Berlier Gloria,
Bonino Francesca,
Zecchina Adriano,
Bordiga Silvia,
Lamberti Carlo
Publication year - 2003
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200300769
Subject(s) - degree of unsaturation , amorphous solid , chemistry , catalysis , crystallography , adsorption , zeolite , organic chemistry
We report on an IR spectroscopic study on the room‐temperature adsorption of NO on different iron( II )‐containing siliceous matrices. Fe 2+ hosted inside the channels of MFI‐type zeolites (Fe‐ZSM‐5 and Al‐free Fe‐silicalite) exhibits pronounced coordinative unsaturation, as witnessed by the capability to form, at 300 K, [Fe 2+ (NO)], [Fe 2+ (NO) 2 ] and [Fe 2+ (NO) 3 ] complexes with increasing NO equilibrium pressure. Fe 2+ hosted on amorphous supports (high surface area SiO 2 and MCM‐41) sinks more deeply into the surface of the siliceous support and thus exhibits less pronounced coordinative unsaturation: only [Fe 2+ (NO) 2 ] complexes were observed, even at the highest investigated NO equilibrium pressures. Activation at higher temperature (1073 K) of the Al‐free Fe‐silicalite sample resulted in the appearance of Fe 2+ species similar to those observed on SiO 2 and MCM‐41, and this suggests that local (since not detectable by X‐ray diffraction) amorphisation of the environment around Fe 2+ anchoring sites occurs. The fact that this behaviour is not observed on the Fe‐ZSM‐5 sample activated at the same temperature suggests that framework Al species (and their negatively charged oxygen environment) have an important role in anchoring extraframework Fe 2+ species. Such an anchoring phenomenon will prevent a random migration of iron species, with subsequent aggregation and loss of coordinative unsaturation. These observations can thus explain the higher catalytic activity of the Fe‐ZSM‐5 system in one‐step benzene to phenol conversion when compared with the parent, Al‐free, Fe‐silicalite system with similar Fe content. The nature of the support and the activation temperature can therefore be used as effective means to tune the degree of Fe coordination.