JOURNAL OF CATALYSIS 161, 132–142 (1996) ARTICLE NO. 0170 Selectivity to Olefins of Fe/SiO 2 –MgO Catalysts in the Fischer–Tropsch Reaction N. G. Gallegos, A. M. Alvarez, M. V. Cagnoli, J. F. Bengoa, S. G. Marchetti, R. C. Mercader, and A. A. Yeramian Centro de Investigaci ´ on y Desarrollo en Procesos Catal´ ıticos (CINDECA, CONICET), Facultad de Ciencias Exactas (UNLP), Facultad de Ingenier´ ıa (UNLP), and Comisi ´ on de Investigaciones Cient´ ıficas de la Pcia.de Bs.As. (CIC), Calle 47, 257, La Plata 1900, Argentina Received May 31, 1995; revised January 4, 1996; accepted February 5, 1996 SiO 2 covered with MgO has been used as support of iron cata- lysts in the Fischer–Tropsch reaction. Catalysts of 5% (w/w) iron concentration and 2, 4, and 8% (w/w) of MgO on SiO 2 were pre- pared. Selective chemisorption of CO, volumetric oxidation, and M¨ ossbauer spectroscopy were used to characterize the type of iron species and the metallic crystal sizes. MgO covers the SiO 2 surface and modifies the metallic crystal size. The activity to total hydrocar- bons increases with the amount of MgO added. An optimal concen- tration of about 4% (w/w) was found to have the highest selectivity to olefins. c  1996 Academic Press, Inc. INTRODUCTION The use of supported metallic catalysts in the Fischer– Tropsch synthesis has been extensively studied in the past years (1–16). Nevertheless, a catalyst selective to some in- teresting products like light olefins and with an acceptable activity is still an unresolved problem. Previously (17), we have been able to corroborate that a basic support like MgO has an improved selectivity to light olefins compared to Fe catalysts supported on typical materials like SiO 2 and Al 2 O 3 (17, 18). However, MgO alone presents drawbacks as a support. Its low surface area leads to a poor dispersion of the active phase, and the very easy way in which it is carbonated or hydroxilated, once in contact with air, leads to changes in its main catalytic features. With the intention of keeping the good selectivity to olefins, while increasing the surface exposed to the reac- tants and improving the stability of the solid, iron catalysts supported on silica covered with different amounts of MgO have been prepared. In this paper we report the study of the activity, selectivity and basicity, and particle size effects of these catalysts. METHODS Preparation of the Catalysts Five precursors of the catalysts of approximately 5% (w/w) of Fe were prepared: Fe supported on SiO 2 (p-0), Fe supported on SiO 2 onto which 2, 4, and 8% (w/w) of MgO were added (p-2, p-4, and p-8) and, as a test sample, a precursor of Fe supported on MgO (p-t). The p-0 was made by dry impregnation of Kieselgel-100 (Merck) silica gel, with 400 m 2 /g specific area, 1 cm 3 /g pore volume and 0.06–0.20 mm particle size. An impregnating solution of pH 0.5 of Fe(NO 3 ) 3 · 9H 2 O with a high enough concentration to yield a catalyst of ca. 5% (w/w) was used. The precursor was air dried at room temperature for 20 days and then calcinated in air at 698 ± 5 K for 8 h. The precursors p-2, p-4, and p-8 were prepared in two steps: (a) dry impregnation of silica gel with a solution of Mg(NO 3 ) 2 and air calcination at 698 ± 5 K during 8 h, (b) dry impegnation of the resulting SiO 2 –MgO support with Fe(NO 3 ) 3 · 9H 2 O. Further drying and calcination were carried out in identical conditions as for p-0. All samples were introduced into the oven once its temperature had reached 698 ± 5 K. The precursor of Fe/MgO was prepared by a technique recommended by Boudart et al. (19). A suspension of mag- nesium hydroxycarbonate (MHC, Carlo Erba) in deminer- alized water was mechanically stirred and heated at 337 K. A solution of Fe(NO 3 ) 3 · 9H 2 O, prepared with a concentra- tion calculated to get a catalyst of ca. 5% (w/w) of iron, was added to that suspension. The slurry was kept at 337 K during 30 min with stirring. Once this period was over, the slurry was centrifuged and the solid washed with deminer- alized water. Afterward, it was calcinated in air at 698 ± 5K for 8 h (p-t). All precursors were reduced in H 2 stream according to a program reported in Ref. (18). Catalysts c-0, c-2, c-4, c-8, and c-t are the final products of the reduction. Characterization Techniques CO chemisorption. Measurements were made on the catalysts in a conventional static volumetric equipment with grease-free vacuum valves using CO as titration reagent as described in (18). Volumetric oxidation. Volumetric oxidation experi- ments were performed in the adsorption equipment 0021-9517/96 $18.00 Copyright c  1996 by Academic Press, Inc. All rights of reproduction in any form reserved. 132