Mechanistic Aspects and Reaction Pathways for Oxidative Coupling of Methane on Mn/Na2WO4/SiO2 Catalysts

ors that form stronger bonds with H-atoms react with lower sensitivity to the strength of the C-H bond being activated. We have previously shown that reactions with energetic H-abstractors, such as OH radicals, react less selectively with the weaker C-H bonds in HCHO (relative to stronger C-H bonds in CH4) than more stable radicals, such as O and HO2. OH radicals are among the most thermodynamically unstable and reactive H-abstractors; they form very strong O-H bonds within the stable H2O molecules formed in such processes.18 The ratio of the rate constants for H-abstraction from ethane and methane by gas-phase OH radicals (kC2H6 OH /kCH4 OH ) is 3.3-4.4.14,38 OH-mediated pathways in OCM reactions give values slightly higher than these (6.8; Table 2), but much smaller than for surface-mediated pathways (30). We surmise that measured kC2H6 OH /kCH4 OH ratios are higher than reported for Habstraction by OH radicals,14,38 because of parallel contributions from CH3 + C2H6 f CH4 + C2H5 reactions and perhaps also from reactions with O and HO2 radicals, which are less reactive than OH radicals. Contributions from these less reactive radicals to C-H bond activation would lead to KIE values larger than from exclusive OH-mediated H-abstraction from CH4 (Table 1), as mentioned in section 3.4. C2H4 conversion rate constants (k5 ′ + k6 ′) were ∼5 times larger than for CH4 conversion (k1 ′ + k2 ′) for surface-mediated pathways prevalent under anhydrous conditions, even though C-H bonds in C2H4 are significantly stronger than in CH4 reactants. This high C2H4 reactivity appears to reflect the specific and preferential binding of C2H4 on oxide surfaces,41 compared with CH4 and C2H6. In contrast, C2H4 was found to be less reactive than CH4 via OH-mediated pathways ((k5 ′ + k6 ′)/(k1 ′ + k2 ′) ∼ 0.7; Table 2). This reflects the homolytic C-H bond activation routes involved in OH-mediated pathways, which rigorously reflect the relative energies of the C-H bonds involved in the activation of CH4 and C2H4. The reported rate of homogeneous Habstraction from C2H4 by OH radicals is also smaller than for CH4, and the ratio of C2H4 to CH4 conversion rates reported here (0.7 at 1073 K) is very similar to that reported for gasphase radical reactions (kC2H4 OH /kCH4 OH ) 0.8).14,38 These low kC2H4/kCH4 ratios reflect the stronger C-H bonds in C2H4 compared with CH4, as well as the introduction of homogeneous pathways that do not involve surface sites and the concomitant preferential binding of C2H4. Surface-mediated pathways that prevail under anhydrous conditions account for the high reactivity of C2H4 during OCM reactions and for the preferential removal of these valuable products as CH4 conversion increases. Figure 10. Ratio of rate constants for conversion of C2H6 (k3′ + k4′) and C2H4 (k5 ′ + k6 ′) to that for conversion of CH4 (k1 ′ + k2 ′) as a function of C-H bond dissociation energies. ([) Surface-mediated pathway; (9) OH-mediated pathway; (O) H abstraction by OH radicals in the gas-phase.38 10140 J. Phys. Chem. C, Vol. 113, No. 23, 2009 Takanabe and Iglesia