Journal of Natural Gas Chemistry 18(2009) 319–324 Oxidation of propane over substituted Keggin phosphomolybdate salts T. Mazari 1 , C. Roch. Marchal 2 , S. Hocine 1 , N. Salhi 1 , C. Rabia 1∗ 1. Laboratoire de Chimie du Gaz Naturel, Facult ´ e de Chimie, Universit ´ e des Sciences et de la Technologie Houari Boumedi` ene, USTHB, BP32, El-Alia, Bab-Ezzouar, Alger, Alg´ erie; 2. ILV-UMR 8180 CNRS-Universit ´ e de Versailles-St Quentin-en-Yvelines, B ˆ atiment Lavoisier, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France [ Received February 4, 2009; Revised March 31, 2009; Available online July 20, 2009 ] Abstract Ammonium salts, (NH 4 ) 6 HPMo 11 MO 40 (M = Ni, Co, Fe), have been investigated for the oxidation of propane, with molecular oxygen, at temperature ranging between 380 ◦ C and 420 ◦ C after in-situ pre-treatment performed at two heating rate of 5 or 9 ◦ C/min. They were characterized by BET method, XRD, 31 P NMR, UV-Vis and IR techniques. The catalysts were found active in the propane oxidation and selective to propene or acrolein, in particular for samples pre-treated with the heating rate of 9 ◦ C/min. Key words propane oxidation; propene; acrolein; polyoxometalates; phosphomolybdate 1. Introduction Liquified petroleum gas (LPG) can be used as a source of propane at lower cost, then oxidative reaction of propane with molecular oxygen into more valuable products as propylene, aldehyde (acrolein) and acids (acrylic, acetic), was proved to be a challenging task due to the tendency of the alkane to produce carbon oxides. This type of reaction requires acid and redox properties of the catalyst [1-4]. Therefore the reducibility and reoxidizability of the catalyst are criti- cal in the determination of their performances. On the other hand, an appropriate acid-base character of catalyst is im- portant for the activation of alkane molecule and for facil- itating desorption of some reaction products. In this con- text, several works have demonstrated the potential of Keggin- type polyoxometalates, and phosphomolybdates in particular, for converting propane to propylene and acrylic acid. These materials present the advantage to have both acid and ox- idative properties. Many studies have focused on the effect of the cation nature in the polyoxometalate catalytic perfor- mances. Among the tested catalysts in the propane oxida- tion, Fe 0.08 Cs 2.5 H 1.26 PMo 11 VO 40 [5,6] and pyridinium (pyr) salts of H 3 PMo 12 acid [7,8] appear effective to acrylic acid, and HPW 11 modified with a transition metal ion (Fe 3+ , Fe 2+ or Mn 2+ ) [9], Cs 2.5 H 6x-y M 1-x PMo 11-x VM x O 40 (M = Co, Fe, Ni, Ga, Sb or Zn), Cs 2.5 H 1.5 PVMo 11-x W x O 40 [10-12], CuPMo 11 V [13,14] and NbPMo 12 (V)pyr [15] favour propene formation. On the other hand, it has been reported that am- monium based salts of phosphomolybdic acid efficiently cat- alyze the oxidation of isobutane into methacrylic acid and methacrolein [16-21]. In these works, it has been observed that the presence of transition metal as addenda atoms and/or ammonium or pyri- dinium as conter-ions in the phosphomolybdate compounds can play a significant role in the determination of the redox processes and thus may improve the catalytic performances. It has been also found that the partially reduced state of the catalyst was stable under the operating conditions and pointed out the importance to have a reduced Keggin anion in order to get better selectivities to the products of partial oxidation. Such reduced state provides not only a coordinative unsatu- rated Mo for the adsorption or stabilization of intermediate species but also basic lattice oxygen that may only be formed near the reduced Mo sites [22]. Additionally these reduced catalysts were found to be less active to total combustion. The presence of both ammonium ions and transition metal in heteropolycompounds appears to be important for catalytic activity and selectivity in the alkane oxidation reaction. In order to achieve these goals, the salts, (NH 4 ) 6 HPMo 11 MO 40 with M = Ni 2+ , Co 2+ and (NH 4 ) 6 PMo 11 Fe(H 2 O)O 39 (de- noted as PMo 11 M) were prepared and tested in the reaction of propane oxidation in the temperature range 380-420 ◦ C after in-situ pre-treatment performed at two heating rates of 5 or 9 ◦ C/min. The catalysts were characterized by the BET method, XRD, 31 P NMR, UV-Vis and IR techniques. ∗ Corresponding author. E-mail: c rabia@yahoo.fr Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(08)60111-5