Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234 Catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of saturated and unsaturated hydrocarbons Susana Mart´ ınez-M´ endez 1 , Yurgenis Henr´ ıquez, Olgioly Dom´ ınguez 2 , Lindora D’Ornelas ∗ , Heinz Krentzien Laboratorio de S´ ıntesis Organomet´ alica, Centro de Qu´ ımica Organomet´ alica y Macromolecular, Escuela de Qu´ ımica, Facultad de Ciencias, Universidad Central de Venezuela, Av. Los Ilustres, Los Chaguaramos, Apdo Postal 47778, Caracas 1040, Venezuela Received 13 December 2005; received in revised form 10 February 2006; accepted 14 February 2006 Available online 3 April 2006 Abstract The catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of different organic substrates (cyclohexane, cyclohexene, 1-hexene) using tertbutylhydroperoxide (TBHP) and molecular oxygen as the oxidizing agents was stud- ied. Titanium (1.9 nm), vanadium (2.3 nm) and niobium (1.6 nm) oxide nanoparticles stabilized on silica were synthesized by the reduction of TiCl 4 ·2THF, VCl 3 ·3THF and NbCl 4 ·2THF with K[BEt 3 H]. These materials were characterized by inductive coupled plasma-optical emission spectroscopy (ICP-OES), Fourier transformed infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analyses. The solids obtained were employed as heterogeneous catalysts. For 1-hexene and cyclohexene, the titanium oxide nanostructured material showed 100% selectivity towards the epoxidation product, with conversions above 50%. In the case of cyclohexane, the titanium oxide nanoparticles are 100% selective towards the desired oxidation products (cyclohexanol and cyclohexanone). The highest oxidation activity was achieved with the vanadium oxide nanoparticles for the three substrates studied: cyclohexane, cyclohexene and 1-hexene, TN 562, 878 and 1190 mol of product/mol of metal, respectively, after 6 h of reaction time at 80 ◦ C. Nevertheless, with this metal the selectivity is different to the one obtained for with the titanium oxide nanostructured system. The niobium catalyst was less active than the titanium and vanadium oxide catalysts, although it proved to be more selective towards the formation of alcohols. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Oxidation catalysts; Vanadium; Niobium; Titanium; Epoxidation 1. Introduction The oxidation of organic substrates represents one of the most important industrial chemical reactions [1], explaining the sig- nificant effort invested in the research and development of new heterogeneous catalysts with increased activities and selectivi- ties in these types of reactions. In particular, alkene epoxidation ∗ Corresponding author. Tel.: +58 212 605 1343; fax: +58 212 605 1256. E-mail address: ldornela@strix.ciens.ucv.ve (L. D’Ornelas). 1 Permanent address: PDVSA-INTEVEP, Apartado 76343, Caracas 1070, Venezuela. 2 Permanent address: Departamento the Quimica Aplicada, Fac. de Ingenier´ ıa, Universidad Central de Venezuela, Av. Los Ilustres, Los Chaguaramos, Caracas 1040, Venezuela. is one of the main aims in this area [2,3]. The oxidation of cyclo- hexane and cyclohexene to cyclohexanol and cyclohexanone is the key reaction in the synthesis of adipic acid, which is an essential precursor in the production of nylon 6 and nylon 66 [4]. It is well known that amongst the most active homogeneous and heterogeneous catalysts for alkene and alkane oxidation, are those based on transition metals of groups 4 (Ti), 5 (V and Nb) and 6 (Mo) [5–10]. In the case of heterogeneous catalysts, it has been proved that a proper selection of preparation conditions is essential to obtain a homogeneous metal dispersion on the support, which in turn, generates highly active and selective catalysts towards oxidation reactions [9–14]. Over the last decade, the research and development of transi- tion metal nanoparticles has received a lot of attention in many 1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2006.02.041