NiO–W 2 O 3 /Al 2 O 3 catalysts for the production of ecological gasoline: Effect of both NiO and the preparation method on the isobutene oligomerization selectivity F. Tzompantzi a , A. Mantilla b,c , G. Del Angel a , J.M. Padilla a , J.L. Ferna ´ ndez b , J.A.I. Dı ´az-Go ´ ngora b , R. Go ´ mez a, * a UAM-Iztapalapa, Depto. Quı´mica, EcoCata ´lisis Av. Sn. Rafael Atlixco 186, Me ´xico 09340, D.F., Mexico b CICATA-LEGARIA,IPN, Av. Legaria 694 Col. Irrigacio ´n, Me ´xico 11500, D.F., Mexico c UAM-Azcapotzalco, Depto. Quı´mica, Aplicada Av. Sn. Pablo No 180, Me ´xico 02200, D.F., Mexico 1. Introduction An attractive option to produce high octane ecological gasoline, free of pollutant components like aromatic hydrocarbons and low sulfur content is the oligomerization of the C 3 –C 4 paraffin/olefin fraction of the FCC units, in order to produce iso-octene/iso-octane fractions, which after a single hydrogenation step produce gasoline with a high research octane number, RON 105 [1,2]. Typically, the catalysts used in this process are liquid acids such as phosphoric acid impregnated in solid supports [3,4]; however, the disposal of this kind of catalysts produces important pollution and the substitution of them by cleaner solid acid catalysts is a challenge around the world. In the olefin dimerization, the undesirable formation of high molecular weight oligomers provokes important deactivation problems [5]: these oligomers act as the precursors of the growth of carbonaceous slots on the catalyst surface, leading to the deactivation process [6–11]. The design of new catalysts for this reaction is then focused on reaching the highest selectivity towards C 8 ¼ olefins, avoiding the formation of heavier olefinic products (þC 12 ¼ ). Some of the developed solid acid catalysts include: Ziegler–Natta-based catalysts [12], benzyl sulfonic acid on silica [13], zeolites [14,15], montmorillonite [16], zirconium oxide [17], sulfated titania [6,7] and sulfated zirconia [18,19]; however, low selectivity to the dimer and deactivation problems remain in all of them. According to the afore mentioned, it has been recently reported that the addition of NiO to sulfated ZrO 2 increases the stability and selectivity towards the C 8 ¼ fraction, drastically diminishing the formation of C 12 ¼ and heavier olefins in the isobutene oligomerization [20]. Looking for an improved solid acid catalyst, in the present work NiO–W 2 O 3 /Al 2 O 3 catalysts were prepared by the impregnation and sol–gel methods, since it has been reported that W 2 O 3 is an important solid acid catalyst [21– 23] and NiO improves the selectivity of the reaction towards lower molecular weight olefinic fractions [20]. The catalysts were characterized by N 2 adsorption, XRD, and FTIR pyridine adsorp- tion; evaluated in the isobutene oligomerization reaction. 2. Experimental 2.1. Preparation of catalysts The g-alumina support prepared by the sol–gel method was obtained following the general procedure reported by Deng et al. [24]: 1.2 g of cetyl tert-butyl ammonium (CTBA) were mixed with 150 mL of a butanol/water solution (ratio 30:70, v/v) at 60 8C with vigorous stirring and reflux until a homogeneous mixture was obtained. Then, 6.0 g of the aluminum precursor (aluminum sec- butoxide) were added. The resulting solution was gelled with an ammonium hydroxide solution in order to maintain pH 9. The solution was kept under stirring for 24 h. After gelling, the solid Catalysis Today 143 (2009) 132–136 ARTICLE INFO Article history: Available online 5 November 2008 Keywords: Isobutene dimerization Tungsten oxide Nickel–tungsten oxides Solid acid catalysts Oligomerization ABSTRACT The results obtained for the oligomerization of C 4 olefins in order to produce high octane ecological gasoline by using NiO–W 2 O 3 /Al 2 O 3 catalysts prepared by both the wetness impregnation and sol–gel methods, are reported. High selectivity towards C 8 ¼ olefins, avoiding the formation of heavier olefinic products, as well as an excellent stability were reached by means of the NiO–W 2 O 3 /Al 2 O 3 impregnated catalyst. The characterization of the catalysts by XRD, total specific surface area and FTIR pyridine adsorption are also reported. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +52 55 58044668; fax: +52 55 5804 4666. E-mail address: gomr@xanum.uam.mx (R. Go ´ mez). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2008.09.022