Catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of saturated and unsaturated hydrocarbons

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 studied. Titanium (1.9nm), vanadium (2.3nm) and niobium (1.6nm) oxide nanoparticles stabilized on silica were synthesized by the reduction of TiCl4·2THF, VCl3·3THF and NbCl4·2THF with K[BEt3H]. 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 1190mol of product/mol of metal, respectively, after 6h 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.