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 VO x /Al 2 O 3 and VO x /ZrO 2 . C 2 H 6 −C 2 D 6 −O 2 and C 2 H 6 −D 2 O−O 2 react to form alkenes and CO x without concurrent formation of C 2 H 6 - x D x or C 2 H 4 - x D x 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 ( k C - H / k C - D = 2.4); similar values were measured for ethane and ethene combustion (1.9 and 2.8, respectively). 16 O 2 − 18 O 2 −C 2 H 6 reactions on supported V 16 O x domains led to the initial appearance of 16 O from the lattice in H 2 O, CO, and CO 2, consistent with the involvement of lattice oxygen in C−H bond activation steps. Isotopic contents are similar in H 2 O, CO, and CO 2, suggesting that ODH and combustion reactions use similar lattice oxygen sites. No 16 O 18 O isotopomers were detected during reactions of 16 O 2 − 18 O 2 −C 2 H 6 mixtures, as expected if dissociative O 2 chemisorption steps were irreversible. The alkyl species formed in these steps desorb irreversibly as ethene and the resulting O−H groups recombine to form H 2 O and reduced V centers in reversible desorption steps. These reduced V centers reoxidize by irreversible dissociative chemisorption of O 2 . 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 C 2 H 6 and O 2 pressures. This kinetic analysis suggests that surface oxygen, OH groups, and oxygen vacancies are the most abundant reactive intermediates during ethane ODH on active VO x domains.