Anchoring Fe Ions to Amorphous and Crystalline Oxides: A Means To Tune the Degree of Fe Coordination

Authors

  • Gloria Berlier Dr.,

    1. Dipartimento di Chimica IFM, Via P. Giuria 7, 10125 Torino, Italy
    2. INSTM Research Unit of Turin University, Italy, Fax: (+39) 011-6707855
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  • Francesca Bonino,

    1. Dipartimento di Chimica IFM, Via P. Giuria 7, 10125 Torino, Italy
    2. INSTM Research Unit of Turin University, Italy, Fax: (+39) 011-6707855
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  • Adriano Zecchina Prof. Dr.,

    1. Dipartimento di Chimica IFM, Via P. Giuria 7, 10125 Torino, Italy
    2. INSTM Research Unit of Turin University, Italy, Fax: (+39) 011-6707855
    3. INFM UdR di Torino Università, Italy
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  • Silvia Bordiga Prof. Dr.,

    1. Dipartimento di Chimica IFM, Via P. Giuria 7, 10125 Torino, Italy
    2. INSTM Research Unit of Turin University, Italy, Fax: (+39) 011-6707855
    3. INFM UdR di Torino Università, Italy
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  • Carlo Lamberti Dr.

    1. Dipartimento di Chimica IFM, Via P. Giuria 7, 10125 Torino, Italy
    2. INSTM Research Unit of Turin University, Italy, Fax: (+39) 011-6707855
    3. INFM UdR di Torino Università, Italy
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Abstract

We report on an IR spectroscopic study on the room-temperature adsorption of NO on different iron(II)-containing siliceous matrices. Fe2+ 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, [Fe2+(NO)], [Fe2+(NO)2] and [Fe2+(NO)3] complexes with increasing NO equilibrium pressure. Fe2+ hosted on amorphous supports (high surface area SiO2 and MCM-41) sinks more deeply into the surface of the siliceous support and thus exhibits less pronounced coordinative unsaturation: only [Fe2+(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 Fe2+ species similar to those observed on SiO2 and MCM-41, and this suggests that local (since not detectable by X-ray diffraction) amorphisation of the environment around Fe2+ 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 Fe2+ 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.

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