NO_x-Mediated Homogeneous Pathways for the Synthesis of Formaldehyde from CH₄−O₂ Mixtures

Abstract

A detailed kinetic network for homogeneous CH 4 -O 2 -NO x reactions is used to estimate maximum attainable formaldehyde (and methanol) yields and to identify elementary steps that lead to the observed enhancement effects of NO x on CH 4 oxidation rates, to HCHO yield limits, and to NO x losses to unreactive N-compounds. NO x was shown previously to increase CH 4 oxidation rates and HCHO yields in CH 4 -O 2 reactions, but maximum yields were low (<10%) and intrinsic kinetic limits were not rigorously examined. We show here that the CH 4 oxidation rate increases because NO 2 reacts with CH 4 during an initial induction period. NO and NO 2 lead to similar effects, except that residence times required for a given yield are higher for NO feeds because NO-NO 2 interconversion must first occur. CH 4 leads to supra-equilibrium NO 2 concentrations because HO 2 formed during HCHO oxidation reacts with NO to form OH and NO 2 faster than NO 2 can decompose to NO. Oxygenate selectivities decrease with increasing CH 4 conversion, because weaker C-H bonds in HCHO and CH 3 OH relative to CH 4 lead to their fast sequential oxidation to CO and CO 2 . Rate-of-formation analyses show that NO x molecules introduce more effective elementary steps for the formation of CH 3 O intermediates and for its conversion to HCHO, but H-abstraction from CH 4 and HCHO remains the predominant step in controlling rates and selectivities in the presence or absence of NO x . Without NO x , OH radicals account for all H-abstraction reactions from CH 4 , while HCHO reacts with OH but also with less reactive H and HO 2 radicals. NO x increases HCHO yields by converting these less reactive H and HO 2 radicals to OH radicals, which become the predominant H-abstractor for both CH 4 and HCHO and which react less selectively with HCHO than do H and HO 2 . Kinetic selectivity, based on C-H bond energy differences between CH 4 and HCHO, becomes weaker with increasing radical reactivity and increasing reaction temperature. Maximum HCHO yields of 37% are theoretically possible for radicals that abstract H from CH 4 and HCHO at equal rates, but radical species prevalent during CH 4 -O 2 -NO x reactions lead to maximum HCHO yields below 10% under all conditions. Higher yields appear unlikely with more reactive radicals, because their reactivity would lead to cascade reactions that form species with greater kinetic sensitivity to C-H bond energies. Maximum C 1 -oxygenate yields increase with increasing O 2 pressure, suggesting that the O 2 distribution along a reactor will not improve HCHO yields but may prove useful to inhibit NO x losses to less reactive N-compounds.

Rate and Selectivity Enhancements Mediated by OH Radicals in the Oxidative Coupling of Methane Catalyzed by Mn/Na2WO4/SiO2

“…21 This ratio increases as the RH products become more stable (R + H → RH, more exothermic). These abstraction reactions become less sensitive (${k{{{\rm R}\hfill \atop {\rm CH}{_{4}}\hfill}}}$ /${k{{R\hfill \atop {\rm C}{_{2}}{\rm H}{_{6}}\hfill}}}$ is larger) to differences in energy between the CH bonds in CH 4 and C 2 H 6 (CH 4 : 439 kJ mol −1 ; C 2 H 6 : 423 kJ mol −1 )20 as the abstraction products become more stable, as is also the case for the relative rates of HCHO and CH 4 oxidation via homogeneous pathways 22. OH radicals lead to very exothermic H‐abstraction reactions and to the highest ${k{{{\rm R}\hfill \atop {\rm CH}{_{4}}\hfill}}}$ /${k{{R\hfill \atop {\rm C}{_{2}}{\rm H}{_{6}}\hfill}}}$ ratios (Figure 3).…”

Oxidation of zeolite acid sites in NO/O2 mixtures and the catalytic properties of the new site in NO oxidation

“…NO oxidation also plays a significant role in NO x -mediated homogenous pathways of CH 4 oxidation to produce formaldehyde (HCHO). Iglesia and coworkers [18] have shown that CH 4 -O 2 mixtures in the presence of NO x , particularly NO 2 , exhibit significant reaction rates increases and HCHO yield. Lastly, NO oxidation is part of nitric acid synthesis via the Ostwald process (eq.…”

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

“…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 CH 4 ) than more stable radicals, such as O and HO 2 . 40 OH radicals are among the most thermodynamically unstable and reactive H-abstractors; they form very strong O-H bonds within the stable H 2 O molecules formed in such processes. 18 The ratio of the rate constants for H-abstraction from ethane and methane by gas-phase OH radicals (k C 2 H 6 OH /k CH 4 OH ) is 3.3-4.4.…”