Ethane Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts

Kinetic and isotopic tracer and exchange measurements were used to determine the identity and reversibility of elementary steps involved in ethane oxidative dehydrogenation (ODH) on VOx/Al2O3 and VOx/ZrO2. C2H6−C2D6−O2 and C2H6−D2O−O2 react to form alkenes and COx without concurrent formation of C2H6-xDx or C2H4-xDx isotopomers, suggesting that C−H bond cleavage in ethane and ethene is an irreversible and kinetically relevant step in ODH and combustion reactions. Primary ethane ODH reactions show normal kinetic isotopic effects (kC-H/kC-D = 2.4); similar values were measured for ethane and ethene combustion (1.9 and 2.8, respectively). 16O2−18O2−C2H6 reactions on supported V16Ox domains led to the initial appearance of 16O from the lattice in H2O, CO, and CO2, consistent with the involvement of lattice oxygen in C−H bond activation steps. Isotopic contents are similar in H2O, CO, and CO2, suggesting that ODH and combustion reactions use similar lattice oxygen sites. No 16O18O isotopomers were detected during reactions of 16O2−18O2−C2H6 mixtures, as expected if dissociative O2 chemisorption steps were irreversible. The alkyl species formed in these steps desorb irreversibly as ethene and the resulting O−H groups recombine to form H2O and reduced V centers in reversible desorption steps. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. A pseudo-steady state analysis of these elementary steps together with these reversibility assumptions led to a rate expression that accurately describes the observed inhibition of ODH rates by water and the measured kinetic dependence of ODH rates on C2H6 and O2 pressures. This kinetic analysis suggests that surface oxygen, OH groups, and oxygen vacancies are the most abundant reactive intermediates during ethane ODH on active VOx domains.