Effects of O 2 Concentration on the Rate and Selectivity in Oxidative Dehydrogenation of Ethane Catalyzed by Vanadium Oxide: Implications for O 2 Staging and Membrane Reactors Toshio Waku, † Morris D. Argyle, Alexis T. Bell,* and Enrique Iglesia* Chemical Sciences Division, E. O. Lawrence Berkeley National Laboratory, and Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720 Staged-O 2 introduction and the effects of O 2 concentration on primary and secondary reactions were examined during oxidative dehydrogenation on V 2 O 5 /γ-Al 2 O 3 containing predominately isolated monovanadates. Cofeed and staged-O 2 introduction modes led to similar ethane dehydrogenation and combustion rates, despite significant differences in the average O 2 concentrations, as expected from their zero-order O 2 dependences. The rate of ethene conversion to CO x , however, was lower when O 2 coreactants were introduced gradually as ethane conversion increased. These effects reflect inhibition of homogeneous ethene combustion pathways, which in contrast with their heterogeneous counterparts show a positive dependence in O 2 . Axial O 2 distribution using multiple injectors or membranes will therefore influence alkene yields only by decreasing homogeneous alkene oxidation rates. Homogeneous contributions are much smaller in large reactors, because catalyst-to-volume ratios are greater than those in laboratory reactors. Other oxidation reactions occurring via redox cycles with lattice oxygens as the most abundant intermediates are expected to exhibit a response similar to that of O 2 staging. Introduction Many oxidation reactions on oxides occur via catalytic sequences requiring lattice oxygen atoms, as suggested first by Mars and van Krevelen for partial oxidation of aromatics on V 2 O 5 1 and later confirmed for many oxidation reactions. 2-10 As a result, oxides can catalyze oxidation reactions even without gas-phase O 2 , as long as lattice O atoms are available as stoichiometric coreactants. 1,5 Oxidative dehydrogenation (ODH) of alkanes involves such redox cycles and proceeds via the parallel and sequential pathways shown (Scheme 1). 2,9,10 Dehydrogenation (reaction 1) occurs in parallel with alkane conversion to CO and CO 2 (reaction 2), which also form in secondary alkene reactions (reaction 3). Rate constants for alkene combustion (k 3 ) are typically much larger than those for dehydrogenation (k 1 ), and maximum alkene yields are consequently low. 2,3,9,10 These kinetic constraints and the need for alkene yield improvements have led to an extensive search for improved catalysts and for operating strategies that influence the relative rates of dehydrogenation and combustion. These steps appear to require similar types of sites, and their relative rates predominately depend on the relative C-H bond energies of alkanes and alkenes. Therefore, several studies have explored alter- nate approaches that minimize O 2 concentrations by separating the hydrocarbon and O 2 coreactants tempo- rally or spatially, using cyclic and membrane reactors, respectively. Vrieland and Murchison reported high C 4 H 8 selectiv- ity (∼80%) at relatively high n-C 4 H 10 conversions (∼50%) by alternate introduction of O 2 /He and C 4 H 10 /He mix- tures on MoO 3 /MgO at 783 K. 5 Similar cyclic operation for C 3 H 8 reactions led to high C 3 H 6 selectivity (∼80%) at low conversions (∼5%) on V-Mg-O at 783-823 K 6,7 and to high C 3 H 6 selectivity (∼70%) at moderate con- versions (<50%) on V-Si-O at 823 K. 8 In all three cases, it was suggested that the coexistence of hydro- carbons and O 2 led to undesired combustion reactions, even though lattice oxygens are involved in both selec- tive and unselective pathways. Lower O 2 concentrations can be maintained during steady-state ODH reactions in tubular reactors by staging the introduction of the required O 2 coreactant along the reactor. In this manner, the stoichiometric coreactant is supplied, but O 2 concentrations throughout the reactor length remain much lower than those in cofeed mode. In some cases, staging also allows the introduction of O 2 reactant requirements that would lead to explosive mixtures in cofeed mode. Oxygen transport membranes or multiple injection points can be used to implement these methods. 11-15 Membranes provide the additional advantage of separating O 2 from air, thus decreasing compression or reactor volume requirements. Tonkovich et al. reported 53% C 2 H 4 selectivity at 95% C 2 H 6 conversion using porous R-alu- mina membranes and MgO/LiO/Sm 2 O 3 at 873 K; C 2 H 4 selectivities were only 8% at similar conversion and temperature in a cofeed tubular reactor. 11 Wang et al. reported much higher C 2 H 4 selectivity (80%) at 84% * To whom correspondence should be addressed. E-mail: iglesia@cchem.berkeley.edu, bell@cchem.berkeley.edu. † Permanent address: Central Technical Research Labora- tory, Nippon Oil Corp., Yokohama 231-0815, Japan. Scheme 1. Primary and Secondary Reaction Pathways in ODH of Ethane 5462 Ind. Eng. Chem. Res. 2003, 42, 5462-5466 10.1021/ie0304661 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003