Selective oxidation of n-butane over iron-doped vanadyl pyrophosphate prepared from lamellar vanadyl n-hexylphosphate Original Research Article

Intercalation of Fe(acac)3 into lamellar vanadyl n-hexylphosphate, VO{(n-C6H13)0.6/H0.4}PO4·(n-C6H13OH)0.5 followed by calcination, has been investigated for preparation of Fe-doped (VO)2P2O7 catalyst; selective oxidation of n-butane to maleic anhydride was performed. Fe(acac)3 was successfully intercalated into the vanadyl n-hexylphosphate by heating in toluene. The obtained precursor was transformed to single phase (VO)2P2O7 by a calcination at 823 K in the presence of a mixture of 5% n-butane and 20% O2 in N2 (balance). The doped Fe ions uniformly spread in the bulk of the catalyst and were substituted for V4+ in the (VO)2P2O7 crystal. The catalytic activity for maleic anhydride formation per unit surface area on the Fe-doped catalyst was higher than that on the Fe-free catalyst prepared by calcination of the vanadyl n-hexylphosphate. Rates of reduction with n-butane and re-oxidation with air in the top few surface layers of the catalysts were estimated from weight decreases and increases detected on a TG/DTA apparatus. The Fe-doped catalysts gave high rates of both the reduction and re-oxidation, especially re-oxidation. XPS studies confirmed that the surface of the Fe-doped catalysts was in a higher oxidation state under the steady state of the reaction. In addition, the Fe-doped catalysts showed a smaller reaction order on partial pressure of O2. Therefore, the doped Fe can enhance the redox ability, especially the re-oxidation, of the catalyst, resulting in the high catalytic performance.