Selective Oxidation of n -Pentane Over V 2 O 5 Supported on Hydroxyapatite
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Selective Oxidation of n-Pentane Over V2O5 Supported on Hydroxyapatite Sooboo Singh · Sreekanth B. Jonnalagadda
Received: 4 June 2008 / Accepted: 31 July 2008 / Published online: 3 September 2008 © Springer Science+Business Media, LLC 2008
Abstract The catalytic efficiency of V2O5 supported on hydroxyapatite in controlled oxidation of n-pentane to phthalic anhydride and maleic anhydride is investigated as function of flux rates of the reactants and temperature in the gas- phase in a fixed- bed stainless steel microreactor under steady state conditions. The hydroxyapatite was prepared by co-precipitation and the loaded catalysts by wet impregnation using NH4VO3 solution. Selectivity towards the products was influenced by the total flow rate, reaction temperature and V2O5 loadings. Good selectivities towards the anhydrides (MA 40% and PA 25%) is obtained with 5.0 and 7.5 wt.% of V2O5 at 360 °C. Keywords V2O5 · Hydroxyapatite · Selective oxidation of n-pentane · Phthalic anhydride · Maleic anhydride 1 Introduction Supported vanadium oxide catalysts have become an important class of catalysts because they exhibit excellent activity and selectivity for a variety of oxidation reactions including the selective oxidation of benzene, butane or pentane to maleic anhydride [1–3] and the conversion of o-xylene or naphthalene to phthalic anhydride [4, 5]. Phthalic anhydride (PA) and maleic anhydride (MA) are important chemical precursors used extensively for the synthesis of plasticizers required for the production of flexible PVC products, alkyd resins used in the manufacture of paints, dyes and pigments and also unsaturated polyester resins. It is generally accepted that o-xylene and n-butane are, S. Singh · S. B. Jonnalagadda (&) School of Chemistry, University of KwaZulu-Natal, Westville Campus, Chiltern Hills, Durban, South Africa e-mail: [email protected]
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respectively the preferred feed stocks for the production of PA and MA. However, the scope of n-pentane as feed stock has been recently explored. The oxidation of n-pentane over VPO, 12-molybdophosphoric acid, vanadomolybdophosphates, and vanadium-substituted molybdophosphoric acid resulted in more oxygenated products compared to only 10% of oxygenates found in the products for the oxidation of n-butane [6, 7]. This prompted Michalakos et al. [8] to study the variation in the selectivity for n-pentane on supported vanadia catalysts. In the preparation of these catalysts, vanadia was introduced by impregnating Al2O3 with a methanol solution of vanadium triisopropoxide and for SiO2, an aqueous oxalic acid solution of ammonium metavanadate was used. The vanadia loadings for the Al2O3 samples were 8.2 and 23.4 wt.% V2O5 and for the SiO2 samples were 1 and 10 wt.%. The Al2O3-supported catalysts were more active than the SiO2supported ones. For samples of the same support, the activity per mole depended slightly on the loading. At low pentane conversions, the 1 wt.% V/SiO2 produced pentenes and 1,4-pentadiene, whereas the 10 wt.% V/SiO2 produced, mainly carbon oxi
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