Mechanistic aspects and reaction pathways for oxidative coupling of methane on Mn/Na ₂WO ₄/SiO ₂ catalysts

Kinetic and isotopic methods were used to determine the identity, rate constants, and reversibility of elementary steps for primary and secondary reactions involved in the oxidative coupling of methane (OCM) on Mn/ Na ₂WO ₄/SiO ₂. We provide evidence in this study for parallel C-H bond activation pathways, in which H-abstraction is mediated by either oxygen species on surfaces or by OH radicals formed via H ₂O/O ₂ equilibration on catalyst surfaces. OCM rates and C ₂₊ yields are higher when H ₂O is present and OH-mediated pathways prevail, because of the high reactivity of OH radicals and of their lesser sensitivity to the energy of the C-H bond containing the hydrogen abstracted. These coupled homogeneous-catalytic sequences account for all observed kinetic effects of O ₂, CH ₄, and H ₂O on rates and selectivities for both CH ₄ conversion and for subsequent reactions of C ₂H ₆, C ₂H ₄ and C ₃ products; they are also consistent with measured kinetic and thermodynamic isotope effects for C-H bond activation mediated by surface and OH radicals. Kinetic isotope effects and isotopic scrambling studies (CD ₄/CH ₄; D ₂O/H ₂O; ¹⁸O ₂/ ¹⁶O ₂) indicate that C-H bond activation is irreversible and kinetically-relevant. O ₂ dissociation is quasi-equilibrated, but becomes irreversible as H ₂O/ O ₂ ratios increase with increasing conversion and residence time. Competitive reactions of ¹³CH ₄/O ₂ with ¹²C ₂H ₆, ¹²C ₂H ₄, and ¹²C ₃H ₆ with and without added H ₂O show that H-abstraction from hydrocarbons is much less sensitive to C-H bond strength when OH radicals are used to abstract hydrogen instead of oxide surfaces. Maximum C ₂₊ yields require conditions that favor OH-mediated pathways while maintaining equilibrium oxygen surface coverages and OH radical concentrations. OH-mediated pathways are more sensitive to O ₂ pressure than surface-mediated pathways; thus, low O ₂ pressures and staging strategies that maintain stoichiometric O ₂ requirements and low local O ₂ pressures can improve C ₂₊ selectivities but only when OH radicals are maintained at equilibrium concentrations via catalytic H ₂O-O ₂ reactions. These findings and interpretations indicate that intermediate O ₂ pressures give maximum C ₂₊ yields, but that their optimal value depends sensitively on prevalent H ₂O concentrations as they vary with conversion along the reactor. These predictions about the consequences of various operating strategies have become feasible because of the detailed and quantitative nature of the mechanism-based kinetic networks reported here for the first time.