Density Functional Theory Investigations of the Direct Oxidation of Methane on an Fe-Exchanged Zeolite

The reactions of methane with [FeO₂]⁺ and [OFeO)]⁺ cations exchanged into ZSM-5 have been investigated using density functional theory. Experimental evidence for the latter cation has recently been reported on the basis of EXAFS experiments performed on Fe-ZSM-5 with Fe/Al = 0.17-0.80. Of the two iron-containing species investigated, Z[OFeO] [Z here represents the cation-exchange site in the zeolite] is lower in energy by 7.7 kcal/mol, assuming a spin multiplicity M = 6. The activation energy for the conversion of Z[FeO₂] and Z[OFeO] is 10.0 kcal/mol. The activation of methane occurs preferentially on Z[OFeO]. Weakly adsorbed methane reacts with Z[OFeO] to produce a weakly bound CH₃· free radical. The activation barrier for this process is 15.9 kcal/mol. The methyl radical then reacts via a barrierless process to form Z[(OH)Fe(OCH ₃)]. This product is very stable thermally, but can be converted to adsorbed methanol or formaldehyde via processes exhibiting high activation barriers (∼39.3 kcal/mol in both cases). Hydrolysis of Z[(OH)Fe(OCH ₃)] to form adsorbed methanol is practically thermoneutral and has an activation barrier of 6.2 kcal/mol. The desorption of the adsorbed methanol is endothermic by 18.8 kal/mol, but the formation of water from the resulting Z[Fe(OH)₂] has a moderately high activation barrier of 37.9 kcal/mol. If N₂O is present in the gas phase, the activation barrier for the formation of H₂O decreases to 18.1 kcal/mol. The results of the present investigation are qualitatively consistent with recent experimental observations.