NOx-mediated homogeneous pathways for the synthesis of formaldehyde from CH4-O2 mixtures

A detailed kinetic network for homogeneous CH4-O-2-NOx reactions is used to estimate maximum attainable formaldehyde (and methanol) yields and to identify elementary steps that lead to the observed enhancement effects of NOx on CH4 oxidation rates, to HCHO yield limits, and to NOx losses to unreactive N-compounds. NOx was shown previously to increase CH4 oxidation rates and HCHO yields in CH4-O-2 reactions, but maximum yields were low (< 10%) and intrinsic kinetic limits were not rigorously examined. We show here that the CH4 oxidation rate increases because NO2 reacts with CH4 during an initial induction period. NO and NO2 lead to similar effects, except that residence times required for a given yield are higher for NO feeds because NO-NO2 interconversion must first occur. CH4 leads to supra-equilibrium NO2 concentrations because HO2 formed during HCHO oxidation reacts with NO to form OH and NO2 faster than NO2 can decompose to NO. Oxygenate selectivities decrease with increasing CH4 conversion, because weaker C-H bonds in HCHO and CH3OH relative to CH4 lead to their fast sequential oxidation to CO and CO2. Rate-of-formation analyses show that NOx, molecules introduce more effective elementary steps for the formation of CH3O intermediates and for its conversion to HCHO, but H-abstraction from CH4 and HCHO remains the predominant step in controlling rates and selectivities in the presence or absence of NOx. Without NOx, OH radicals account for all H-abstraction reactions from CH4, while HCHO reacts with OH but also with less reactive H and HO2 radicals. NOx increases HCHO yields by converting these less reactive H and HO2 radicals to OH radicals, which become the predominant H-abstractor for both CH4 and HCHO and which react less selectively with HCHO than do H and HO2. Kinetic selectivity, based on C-H bond energy differences between CH4 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 CH4 and HCHO at equal rates, but radical species prevalent during CH4-O-2-NOx, 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 Cl-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 NOx losses to less reactive N-compounds.