Effects of reducibility on propane oxidative dehydrogenation over  -Al 2 O 3 -supported chromium oxide-based catalysts B.Y. Jibril a; *, S.M. Al-Zahrani a , A.E. Abasaeed a , and R. Hughes b a Chemical Engineering Department, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia b Chemical Engineering, University of Salford, Maxwell Building, The Crescent, Salford, Manchester M5 4WT, UK Received 28 October 2002; accepted 27 January 2003 Alumina-supported chromium oxide and binary mixed oxide catalysts of the form Cr–M oxide/-Al 2 O 3 (where M is Ni, Co, Mo, W, Ho, La, Li, Cs or Bi) were found to catalyze the oxidative dehydrogenation of propane at 300–450 8C. The basic characters of the metals were found to determine partly the selectivity to propylene. Cr–Mo/-Al 2 O 3 proved to be the most promising. It exhibited a propylene yield of 10.3% at 450 8C. The connections between the selectivity and reducibility of the catalyst were explored. TPR results showed that addition of molybdenum to chromium increased the temperature of reduction maxima. Thus the selectivity to propylene was improved by a decrease in the tendency of the catalyst to undergo a redox cycle. Further, an X-ray photoelectron spectroscopy study conducted on a sample of the catalysts showed that the basicity of the catalysts increased with increase in molybdenum. Catalysts with appropriate Cr/Mo ratios exhibited lower selectivity to over-oxidation product than those containing either chromium or molybdenum alone. KEY WORDS: oxidative dehydrogenation; propane; propylene; chromium oxide; molybdenum oxide; TPR; XPS. 1. Introduction Oxidative dehydrogenation of lower alkanes is a potential alternative or complementary route to alkenes. The reaction is exothermic whereas the nonoxidative dehydrogenation is endothermic. Therefore, the latter reaction takes place at high temperatures for favorable thermodynamic equilibrium. The high temperatures often lead to cracking and formation of coke. This makes it necessary to frequently shut down the reactor for decoking and/or catalyst regeneration. These difficul- ties are minimized in oxidative dehydrogenation since there are no thermodynamic equilibrium limitations and the reaction takes place at much lower temperature. How- ever, it is still difficult to design a catalytic system that gives high yield in the reaction because both alkanes and alkenes have tendencies towards oxidation to CO x [1]. This situation poses a challenge and serves as a motivation for developing new and/or improving the existing catalytic systems for the reaction. To this end, catalytic propane oxidative dehydrogenation (POD) has been the subject of extensive research efforts that have led to the discovery of several promising catalysts. The most studied catalytic systems for the reaction are based on vanadium [2–4], metal tungstates [5], metal molybdates [6,7] and metal phosphates [8,9]. Recently, rare earth vanadates have been reported to have better performance than the much-studied V–Mg–O [10]. Ni–Co–Mo was also shown to be as promising as V-based catalysts [6]. In a previous study, we reported the performances of some supported metal oxide catalysts for the reaction, among which chromium oxide was found to be promis- ing [11]. This study focuses on the attempt to modify some of the catalyst’s properties in order to improve its performance. It is generally accepted that oxidative dehydrogenation takes place through the classical Mars–Van Krevelen mechanism where the catalyst donates lattice oxygen to take part in the oxidation reaction [3,12]. The gas-phase oxygen reoxidizes the reduced catalyst. This concept of a redox cycle in selective oxidation catalysis was propounded earlier by Grasselli and co-workers at SOHIO/BP [13]. The redox character of the catalyst is among the important factors upon which optimum catalyst activity and selectivity are based. Therefore, the reducibility of the catalyst surface is critical in determining the catalyst’s performance [14,15]. In addition, it is important to have a catalyst of appropriate basicity to facilitate the desorption of olefins [6,14]. The redox character could be modified by addition of some metals to the base metal [16]. This is because the lattice oxygen migration could be influenced by such addition. Mixing certain metals with the base catalyst might also improve the selectivity to propylene (i) by modifying the oxidation state and electronic structure of the chromium center, (ii) by modifying its behavior through structural site isolation or (iii) due to stabilization of a lower chromium oxidation state. Therefore, the optimized catalyst should consist of a group of elements that are able to change their valance states on appropriate supports to give a desired redox and acid–base character Catalysis Letters Vol. 87, Nos. 3–4, April 2003 (# 2003) 121 1011-372X/03/0400-0121/0 # 2003 Plenum Publishing Corporation * To whom correspondence should be addressed. E-mail: baba@ksu.edu.sa