Kinetic conjugation effects in oxidation of C1-C2 hydrocarbons: Experiment and modeling

Oxidation of individual C-1-C-2 alkanes and their mixture (CH4:C2H6 = 8:1) was studied in an empty flow reactor and in the presence of two model catalysts used for oxidative coupling of methane (OCM) and oxidative transformations of C2+ alkanes, namely NaWMn/SiO2 and Ca-La/Al2O3. Ethane is much more reactive in homogeneous oxidation due to a high rate of free radical chain branching. Methane inhibits both homogeneous and catalytic oxidation of ethane, while ethane accelerates the conversion of methane. Methane reactivity is higher over the Ca-La/Al2O3 catalyst. In the presence of NaWMn/SiO2 ethane reacts faster, and the 'conversion vs. contact time' kinetic curves preserve an S-shape character typical for homogeneous branching chain processes. The observed features were rationalized using the numerical simulations in the framework of heterogeneous-homogeneous kinetic scheme that accounted both free-radical gas phase processes and reactions of molecular and radical species with catalytic active sites. It was shown that typical OCM catalysts play a dual role: they activate hydrocarbons by capturing H-atoms from C-H bonds and also efficiently terminate chain reactions by trapping free radicals. Consequently, over the catalyst that is more active in methane oxidation (Ca-La/Al2O3) ethane reacts slower due to the elimination of free radical chain branching and development. During the oxidation of ethane, methane - as an efficient radical scavenger - improves the selectivity to ethylene due to the inhibition of its secondary transformations (mainly to CO via HCO center dot radicals).