CHEMPHYSCHEM 2003, 4, 1073±1078 DOI: 10.1002/cphc.200300769 ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1073 Anchoring Fe Ions to Amorphous and Crystalline Oxides: A Means To Tune the Degree of Fe Coordination GloriaBerlier,* [a, b] Francesca Bonino, [a, b] AdrianoZecchina, [a, b, c] Silvia Bordiga, [a, b, c] and Carlo Lamberti* [a, b, c] 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 300K, [Fe 2 (NO)], [Fe 2 (NO) 2 ] and [Fe 2 (NO) 3 ] complexes with increasing NO equili- brium 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. 1. Introduction The one-step conversion of benzene to phenol with N 2 O as the oxidising agent is a promising and environmentally friendly alternative to the commonly used three-step process via cumene. [1, 2] This reaction is catalysed by Fe-MFI zeolites [3] (Fe- silicalite and Fe-ZSM-5), which are synthesised with accommo- dation of small amounts of Fe 3 ions in framework positions. [4, 5] The catalyst becomes active only after severe thermal treatment (up to 1100 K), which causes the migration of iron to extraframe- work positions and its reduction to Fe 2 . [4±14] The extraframework iron species thus formed are thought to be the active sites where N 2 O decomposes to form so-called a-oxygen, which is the active site in partial oxidation reactions. [5, 6, 15, 16] However, in spite of the great interest in these materials, the characterisation of extra- framework Fe species has represented an extremely difficult task because of the very low iron concentration in the active catalyst. [5] Recently, we reported a multitechnique approach (electron paramagnetic resonance, X-ray absorption near-edge spectro- scopy (XANES), extended X-ray absorption fine structure (EX- AFS), UV/Vis and IR spectroscopy of adsorbed CO, N 2 O and NO) for the characterisation of extraframework iron species formed in Fe-silicalite on activation in vacuum at 773 ± 973 K. [11±14,16] Infra- red spectroscopy of adsorbed NO has been the most informative technique. It allows the recognition of coordinatively unsatu- rated Fe 2 species that, at room temperature, can form [Fe 2 (NO)], [Fe 2 (NO) 2 ] and [Fe 2 (NO) 3 ] complexes, relative concentrations of which can be modulated by means of the NO equilibrium pressure P NO . Fe-silicalite samples activated above 1073 K showed a more complex spectroscopy: in addition to the bands of the above-mentioned complexes new nitrosyl bands ap- peared, whose interpretation needed further investigations. [11] Herein, we further investigate the influence of the siliceous host matrix on the capability of grafted iron species to form NO adducts. Based on previous experience, IR spectroscopy was used as the characterisation technique. To this end, the formation of iron nitrosyl adducts on high surface area silica and on mesoporous MCM-41 was studied. Similarities were found between the IR spectra obtained on the two amorphous systems and those recorded on Fe-silicalite activated at 1073 K. [a] Dr. G. Berlier, Dr. C. Lamberti, F. Bonino, Prof.Dr. A. Zecchina, Prof. Dr. S. Bordiga Dipartimento di Chimica IFM, Via P. Giuria 7 10125 Torino (Italy) E-mail: gloria.berlier@unito.it, carlo.lamberti@unito.it [b] Dr. G. Berlier, Dr. C. Lamberti, F. Bonino, Prof.Dr. A. Zecchina, Prof. Dr. S. Bordiga INSTM Research Unit of Turin University (Italy) Fax: ( 39)011-6707855 [c] Dr. C. Lamberti, Prof. Dr. A. Zecchina, Prof. Dr. S. Bordiga INFM UdR di Torino Universita ¡ (Italy)