Mechanistic pathways and role of oxygen in oxidative coupling of methane derived from transient kinetic studies

Abstract

Oxidative coupling of methane (OCM) is a promising industrial process to upgrade natural gas to high value chemicals. In this study, Temporal Analysis of Products (TAP) and steady-state experiments were conducted to distinguish how the composition of surface and gas phase oxygen influence mechanistic details of the selective conversion of CH4 to C2H4 over the Mn-Na2WO4/SiO2 catalyst. The results from TAP studies indicate that methane activation on this catalyst proceeds predominantly via a short-lived, transient surface oxygen species and there is a competition for this species to form either CO or methyl radicals on the surface. This active species has a total lifetime of 3 s and is identified to have a dioxygen (e.g., O22- or O2-) form. We show that the concentration of the transient surface oxygen species significantly impacts the OCM performance. Oxygen attributed to the catalyst lattice (in a singular form e.g., O-), is found to activate methane to a lesser degree, but exclusively forms CO2. Evidence for these surface pathways for methyl radical, CO and CO2 formation identified by TAP are also validated through steady-state experiments. By distinguishing different catalyst oxygen species and their role in selective/nonselective pathways, important screening criteria have been identified for the advancement of superior catalyst formulations.