Abstract
The direct functionalization of methane into platform chemicals is arguably one of the holy grails in chemistry. The actual active sites for methane activation are intensively debated. By correlating a wide variety of characterization results with catalytic performance data we have been able to identify mononuclear Fe species as the active site in the Fe/ZSM-5 zeolites for the mild oxidation of methane with H(2)O(2) at 50 °C. The 0.1% Fe/ZSM-5 catalyst with dominant mononuclear Fe species possess an excellent turnover rate (TOR) of 66 mol(MeOH) mol(Fe) (-1) h(-1), approximately 4 times higher compared to the state-of-the-art dimer-containing Fe/ZSM-5 catalysts. Based on a series of advanced in situ spectroscopic studies and (1)H- and (13)C- nuclear magnetic resonance (NMR), we found that methane activation initially proceeds on the Fe site of mononuclear Fe species. With the aid of adjacent Brønsted acid sites (BAS), methane can be first oxidized to CH(3)OOH and CH(3)OH, and then subsequently converted into HOCH(2)OOH and consecutively into HCOOH. These findings will facilitate the search towards new metal-zeolite combinations for the activation of C-H bonds in various hydrocarbons, for light alkanes and beyond.