Journal of Catalysis 218 (2003) 307–314 www.elsevier.com/locate/jcat Oxidative conversion of propane over lithium-promoted magnesia catalyst II. Active site characterization and hydrocarbon activation L. Leveles, a K. Seshan, a J.A. Lercher, b and L. Lefferts a,∗ a Faculty of Chemical Technology, University of Twente, Postbus 217, 7500 AE, Enschede, The Netherlands b Institute for Chemical Technology, Department of Chemistry, Technische Universität München, Lichtenbergstrasse 4, D-85747 Garching, Germany Received 9 October 2002; revised 26 February 2003; accepted 7 March 2003 Abstract Activation of propane over Li/MgO catalyst has been investigated. It is shown that a small fraction of the oxygen ions in Li/MgO catalysts can be removed from the catalyst by reduction treatment in H 2 at 600 ◦ C. Catalytic activity of Li/MgO exhibits a strong correlation to the amount of oxygen that is removed. It is proposed that the sites containing removable oxygen are responsible for the activation of propane. About 70 propane molecules were converted after consumption of one such oxygen site, in the absence of gas-phase oxygen, implying a mechanism in which propane molecules are activated on the catalyst resulting in propyl radicals that are released to the gas phase where they undergo chain propagation reactions, resulting in the products observed. The active O site is consumed by conversion into an OH group, as the oxygen is not removed from the catalyst with propane. The oxidative conversion of propane over Li/MgO catalysts follows a mixed heterogeneous-homogeneous radical chemistry where the catalyst acts as an initiator. At low propane partial pressures (0.1 bar), the catalyst surface area to volume ratio of the catalytic reactor does not influence the chain length in the propagation step. At higher propane partial pressures (> 0.3 bar), favoring extensive gas-phase reactions, the catalyst affects conversion and selectivity also via quenching and chain termination.  2003 Elsevier Inc. All rights reserved. Keywords: Li/MgO; Alkanes oxidative dehydrogenation; Propene; Propylene; Ethene; Ethylene 1. Introduction Oxidative conversion of alkanes to olefins, especially de- hydrogenation propane to propene, has been and continues to be an important research subject. Despite the efforts, most of the redox-type catalyst systems reported in the lit- erature gave low yields of propene (< 30%) due to com- bustion of propene to carbon oxides. On the other hand, nonredox catalysts such as Li-promoted magnesia resulted in olefin yields in the range of 50%, as a mixture of ethene and propene [1,2]. Although there are only a few studies of propane oxidative conversion, propane selective oxidation without use of a catalyst appears to produce better olefin selectivities than catalytically. Burch and Crabb [3] com- pared catalytic and noncatalytic performances of propane oxidative conversion and concluded that the combination of heterogeneous and homogeneous reactions offers a better * Corresponding author. E-mail address: l.lefferts@utwente.nl (L. Lefferts). opportunity for obtaining commercially acceptable yields of olefins than a purely catalytic reaction. It is unclear from the literature whether noncatalytic contributions are impor- tant during catalytic propane conversion, unlike in methane oxidative coupling where the role of catalytic and homo- geneous reactions is well established [4,5]. Some authors explain their results of propane conversion to olefins in terms of catalytic reactions only, without taking into account pos- sible homogeneous gas-phase contribution [6–8], while oth- ers describe their results in terms of radical reactions in the gas-phase initiated on the catalyst, and radical-surface inter- actions [9,10]. In the preceding paper (part I) we have suggested a reac- tion mechanism that involves a sequence in which propane is first activated on the [Li + O - ] active sites of Li/MgO catalysts. The resulting radical then desorbs and initiates a gas-phase chain propagation reaction. The conditions under which catalytic activation prevails over homogeneous acti- vation were also defined [11]. It was observed that at low propane partial pressures catalytic activation prevails, while 0021-9517/03/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9517(03)00113-1