Journal of Catalysis 182, 264–269 (1999) Article ID jcat.1998.2336, available online at http://www.idealibrary.com on RESEARCH NOTE Transient Kinetic Analysis of the Oxidative Dehydrogenation of Propane D. Creaser,‡ B. Andersson,† R. R. Hudgins, ∗,1 and P. L. Silveston ∗ ∗ Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada; †Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96, G ¨ oteborg, Sweden;and ‡Chemical Technology, L ule˚ a University of Technology, S-971 87 L ule˚ a, Sweden Received May 11, 1998; revised November 18, 1998; accepted November 19, 1998 Oxidative dehydrogenation of propane was studied using various transient techniques. Results support a redox mechanism in which propane reduces the catalyst, which is reoxidized by gas-phase oxygen. Only lattice oxygen participates in propene formation. Desorbable oxygen is a major source of poor selectivity, although lattice oxygen also causes total oxidation. Consequently, propene selectivity in the absence of gas-phase O 2 is superior to co-feed, steady-state selectivity at the same propane conversion. Propene selectivity is furtherimproved by increasing the degree of catalyst reduction. c  1999 Academic Press Key Words: propane oxidative dehydrogenation;V–Mg–O;tran- sient analysis. Poor selectivityin the oxidative dehydrogenation ofalka- neshasbeen a problem preventingcommercialexploitation ofthisprocess.A better understandingofthe mechanism of oxidative dehydrogenation would permit identification of factors limiting the selectivity. Most researchers favor a Mars–Van Krevelen redox mechanism involving reduction of the metal oxide surface by alkane with the formation of alkene and water, followed by reoxidation of the sur- face through gas-phase oxygen. Zanthoff et al. (1) summa- rize mechanisms and surface properties proposed in the recent literature for selective oxidative dehydrogenation. In an earlier steady-state study (2), the authors observed that several Mars–Van Krevelen redox models would fit the measurements but were unable to decide which model was most appropriate. Our inability to resolve the models is not surprising, since it is difficult to deduce mechanism from kinetic data collected at steady state. Transient data are more effective.The use oftransient response for investi- gation of mechanism has been reviewed by Kobayashi and Kobayashi (3), among others. These methods have been applied to to oxidative dehydrogenation. Sloczynski (4) ex- amined reduction and reoxidation reactions from cyclic ex- periments for the oxidative dehydrogenation of C 3 H 8 over 1 To whom correspondence should be addressed. V 2 O 5 /TiO 2 , while Pantazidis and Mirodatos (5) used tran- sient and isotope exchange experiments to propose a qual- itative mechanism for the reaction over Mg–V–O catalyst. In this contribution, various transient techniques such as startup,reaction interruption,and pulsingare used to iden- tify steps in the reaction mechanism, estimate amounts of intermediates or reaction by-products adsorbed, and de- termine the degree of reduction of the catalyst surface. A practical goal of our study was to evaluate whether an un- steady-state operation could minimize complete oxidation of C 3 H 8 . The same batch of catalyst used in our earlier study of the steady-state kinetics of oxidative dehydrogenation (2) was used for this investigation. It contained 60 wt% MgO and 40 wt% V 2 O 5 .X-raydiffraction indicated that the prin- cipal phases were magnesium oxide and magnesium ortho - vanadate [Mg 3 (VO 4 ) 2 ]. Experiments were performed in a quartz microreactor containing 15 to 40 mg of catalyst. Details of reactor design have been published (3). All experiments were conducted isothermally at 510 ◦ C. Total flow rate was 50 ml (STP)/min; pressure was close to atmospheric. A quadrupole mass spectrometer with a computerized data acquisition system was used to monitor the transient response of each component and allowance was made for cross-contributionsamongcomponent peaks.Detailsofthe mass spectrometry (MS) data analysis are given by (6). To verify the MS results, concentrations of carbon-containing products were measured in parallel by gas chromatogra- phy (GC) at a much lower frequency. GC analysis estab- lished that the onlycarbon-containingproductswere C 3 H 6 , CO 2 ,and CO.Creaser and Andersson (2) described the GC analysis. Figure 1 shows results of a step-change from an initial feed containing 6% O 2 to a reaction mixture of 6% O 2 and 3% C 3 H 8 employing 21 mg of fresh catalyst. The step-change occurs at time zero. From the delay in the appearance of C 3 H 8 in the off-gas, the transport lag due to gas holdup for the reactor system was about 9 s. Buildup of 0021-9517/99 $30.00 Copyright c  1999 by Academic Press All rights of reproduction in any form reserved. 264