Reaction network and kinetics of propane oxydehydrogenation over nickel cobalt molybdate

Reaction kinetics and a proposed mechanism for the oxydehydrogenation of propane over Ni0.5Co0.5MoO4/SiO2 are described. The reaction pathway proceeds by propane oxydehydrogenation yielding propylene as the exclusive primary product. The propylene thus formed oxidizes further primarily to acrolein, which oxides still further to waste products CO and CO2, and acrylic acid. The relative rate of acrolein formation from propylene is 3.5 times that of propylene formation from propane, the rate of COx formation from acrolein is 13 times that of acrolein formation from propylene, and the rate of COx formation from acrolein is 46 times that of propylene formation from propane. Kinetic isolation of intermediates is therefore imperative for the recovery of practical amounts of useful products, and might be achievable through dioxygen limitation in the feed or utilization of cocatalysts to produce more stable intermediates. The selective oxidation of propane to propylene and propylene to acrolein are both zero order in oxygen and first order in hydrocarbon (propane and propylene, respectively). Deep oxidation of propane (to CO and CO2) is half order in oxygen and first order in propane, while deep oxidation of propylene exhibits Langmuir type dependence on hydrocarbon and is half order in oxygen. Propane/propylene competition experiments reveal that propylene competes for the same metal oxide sites on which propane activation occurs, Their respective effectivenesses are of the same order of magnitude, with propylene being favored by a factor of 2.3 at equimolar concentration. These results are consistent with a direct pathway (i.e., surface mediated reaction) for the formation of useful products from both propane and propylene, and consecutive overoxidation of sobbed intermediates leading to deep oxidation. Kinetic isotope effects for both propane (k(H)/k(D))C-3 degrees = 1.7) and propylene ((k(H)/k(D))C-3(=) = 1.9) activation reveal methylene hydrogen abstraction and allylic hydrogen abstraction, respectively to be the rate determining steps. (C) 1997 Academic Press.