Journal of Catalysis 189, 253–262 (2000) doi:10.1006/jcat.1999.2513, available online at http://www.idealibrary.com on A Study of the Kinetics and Mechanism of the Adsorption and Anaerobic Partial Oxidation of n-Butane over a Vanadyl Pyrophosphate Catalyst B. H. Sakakini, ∗ Y. H. Taufiq-Yap,† and K. C. Waugh ∗ ∗ Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, United Kingdom; and †Department of Chemistry, Universiti Putra Malaysia, 43400 UPM Serdang, Malaysia Received September 23, 1998; revised February 3, 1999; accepted March 29, 1999 The interaction of n-butane with a ((VO) 2 P 2 O 7 ) catalyst has been investigated by temperature-programmed desorption and anaero- bic temperature-programmed reaction. n-Butane has been shown to adsorb on the (VO) 2 P 2 O 7 to as a butyl–hydroxyl pair. When adsorp- tion is carried out at 223 K, upon temperature programming some of the butyl–hydroxyl species recombine resulting in butane desorp- tion at 260 K. However, when adsorption is carried out at 423 K, the hydroxyl species of the butyl–hydroxyl pair migrate away from the butyl species during the adsorption, forming water which is detected in the gas phase.Butane therefore is not observed to des- orb at 260 K after we lowered the temperature to 223 K under the butane/helium from the adsorption temperature of 423 K priorto temperature programming from that temperature to 1100 K under a helium stream. Anaerobic temperature-programmed oxidation of n-butane produces butene and butadiene at a peak maximum temperature of 1000 K; this is exactly the temperature at which, upon temperature programming, oxygen evolves from the lattice and desorbs as O 2 . This, and the fact that the amount of oxygen desorbing from the (VO) 2 P 2 O 7 at ∼1000 K is the same as that re- quired for the oxidation of the n-butane to butene and butadiene, strongly suggests (i) that lattice oxygen as it emerges at the surface is the selective oxidant and (ii) that its appearance at the surface is the rate-determining step in the selective oxidation of n-butane. The surface of the (VO) 2 P 2 O 7 catalyst on which this selective oxida- tion takes place has had approximately two monolayers of oxygen removed from it by unselective oxidation of the n-butane to CO, CO 2 , and H 2 O between 550 and 950 K and has had approximately one monolayer of carbon deposited on it at ∼1000 K. It is apparent, therefore, that the original crystallography of the (VO) 2 P 2 O 7 cata- lyst will not exist during this selective oxidation and that theories that relate selectivity in partial oxidation to the (100) face of the (VO) 2 P 2 O 7 catalyst cannot apply in this case. c  2000 Academic Press Key Words: partial oxidation; vanadyl pyrophosphate catalyst; n-butane; kinetics; mechanism; adsorption. INTRODUCTION Selectivities (product formed/reactant consumed) of the order of 60–80% are routinely achieved in the partial oxi- dation of hydrocarbons (benzene, n -butane, butenes, etc.) over vanadium-based catalysts. The origin of these selectiv- ities has been the subject of considerable investigation and of consequent debate (1–10, 17–20). In the partial oxidation of benzene to maleic anhydride over VO 2 O 5 /MoO 3 (3:1) catalyst, it has been argued that the selective reactive pathway occurred when an electroni- cally perturbed oxygen molecule (an O − 2 species produced by the molecular oxygen chemisorbing on a V 4+ site form- ing an O − 2 V 5+ dimer) added para across the electronically perturbed benzene ring.Thisisproduced when the benzene molecule adsorbes on a V 5+ site giving a C 6 H + 6 V 4+ dimer (1, 2). Orbital symmetry conservation arguments showed this to be an allowed reaction (3). The adduct (adduct 1) so obtained is and wasthought to rearrange to hydroquinone,which after an identical para addition of O − 2 and rearrangement pro- duced maleic anhydride. Subsequent quantum mechanical calculations by Haber and co-workers showed this to be a feasible low-energy pathway (4, 5). The fundamental thesis of these papers was that reaction of the benzene with the oxygen ions of the V 2 O 5 /MoO 3 lattice would probably be unspecific and lead to the formation of CO and CO 2 . However, a considerable body of opinion holds that it is the oxygen of the catalyst that is the oxidant and that selec- tivity to maleic anhydride derives from the configuration of the adsorbate imposed on it by the crystal field of the oxide. In the case of the oxidation of n -butane to maleican- hydride over a vanadium pyrophosphate catalyst, it is the (100) face of that catalyst that is though to be selective by its imposition of a maleic anhydride-like configuration on the adsorbed butane (6–10) (Fig. 1). The mechanism is essentially Mars and Van Krevelen, in which the gas phase oxygen is used to replace the catalyst (11). Recently it has been reported that the anaerobic 253 0021-9517/00 $35.00 Copyright c  2000 by Academic Press All rights of reproduction in any form reserved.