Activity pattern of low-loaded FeO x /SiO 2 catalysts in the selective oxidation of C 1 and C 3 alkanes with oxygen F. Arena a, * , G. Gatti b , L. Stievano c , G. Martra b , S. Coluccia b , F. Frusteri d , L. Spadaro d , A. Parmaliana a a Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Universita ` degli Studi di Messina, Salita Sperone 31, 98166 S. Agata (Messina), Italy b Dipartimento di Chimica IFM and NIS Center of Excellence, Universita ` degli Studi di Torino, Via P. Giuria 7, 10125 Torino, Italy c Laboratoire de Re ´activite ´ de Surface, Universite ´ Paris VI, UMR 7609, Place Jussieu 4, 75252 Paris, France d Istituto CNR-ITAE ‘‘Nicola Giordano’’, Salita S. Lucia 39, I-98126 S. Lucia (Messina), Italy Available online 27 June 2006 Abstract The structure and redox properties of low-loaded (0.015–0.73 wt% Fe) FeO x /SiO 2 catalysts obtained by adsorption–precipitation of Fe 2+ were probed by Mo ¨ssbasuer and TPR techniques. Oxide dispersion data, obtained by deconvolution of Mo ¨ssbasuer and TPR spectra, signal the speciation of the active phase into ‘‘isolated’’ FeO x sites, ‘‘2-d FeO x patches’’ and ‘‘3-d Fe 2 O 3 clusters’’. The reactivity of FeO x /SiO 2 catalysts in the selective oxidation of CH 4 and C 3 H 8 in the range 475–650 8C has been assessed. Basic relationships amongst dispersion, specific rate of alkane conversion and product formation signal that the selective oxidation functionality depends upon local environment and oxygen bond strength of active sites, their distribution being closely related to the efficiency of synthesis route. # 2006 Elsevier B.V. All rights reserved. Keywords: FeO x /SiO 2 catalysts; Light paraffins; Preparation method; Selective oxidation; Dispersion 1. Introduction The direct conversion of natural gas (NG) streams into commodity chemicals constitutes nowadays one of the hottest topics in catalysis [1–3]. Potential alkanes activation routes based on novel catalytic approaches along with low-cost NG feedstock would in fact result in significant advantages over current technologies in terms of reduced dependence from oil, improved process economics and better environmental compatibility [1]. Various catalyst formulations for the selective oxy- functionalization of C1–C5 alkanes have been so far proposed [1–10], however the low productivity remains still the major drawback for industrial exploitation [1,2]. Although the oxy- functionalization of C1–C5 alkanes entails operating condi- tions and catalyst features specifically ‘‘tuned’’ on the reactivity of the substrate and required product(s) [7–10], these catalysts share some basic requirements mostly consisting in a high dispersion of active phases (site isolation) and ‘‘smoothed’’ acid–base characteristics in order to minimize product adsorption and over-oxidation phenomena [1–4]. A superior performance of FeO x /SiO 2 catalysts in the selective oxidation of CH 4 to HCHO (MPO) has been recently recognised [2–4] yet, dispersion and local environment of Fe 3+ centres decisively affect the overall activity–selectivity pattern [2–7]. Therefore, this paper shows the effects of the preparation method and loading on speciation, dispersion and reducibility of the active phase in low-loaded FeO x /SiO 2 catalysts, addressing direct relationships amongst dispersion, activity and apparent activation energy which prove the key-role of dispersion on the oxidation path of both CH 4 and C 3 H 8 substrates. 2. Experimental 2.1. Catalyst preparation Commercial F5 and Si 4-5P silica samples (AKZO NOBEL) were used for preparation of FeO x /SiO 2 catalysts either by adsorption–precipitation (AP) or incipient wetness (IW) routes [2–7], respectively. The list of the studied samples is given in Table 1. www.elsevier.com/locate/cattod Catalysis Today 117 (2006) 75–79 * Corresponding author. Tel.: +39 090 676 5606; fax: +39 090 391 518. E-mail address: Francesco.Arena@unime.it (F. Arena). 0920-5861/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2006.05.047