Selective oxidation of methane with H2O2 over Fe-silicalite-1: An investigation of the influence of crystal sizes, calcination temperatures and acidities

[Display omitted] •A crystal size around 400 nm was preferable as diffusion limitations occur in the secondary particle (200 μm) when crystal size is below 400 nm.•Increasing calcination temperatures led to a growing fraction of extra-framework Fe species.•Extra-framework Fe species are responsible for the decomposition of H2O2 and over-oxidation.•The formation of active Fe sites were already completed upon removal of the template at a calcination temperature of 370 °C.•Brønsted acid sites do not play a role for the activation of methane but strengthening the decomposition of H2O2 and over-oxidation. Selective oxidation of methane to hydrocarbon oxygenates using H2O2 as oxidant over Fe-silicalite-1 catalysts was conducted in a micro fixed-bed reactor. The influence of different crystal sizes, different calcination temperatures and different acidities of the catalysts on the reaction was investigated. A sub-micrometer crystal size around 400 nm was preferable because diffusion limitations occur also in the secondary particle having a size of 200 μm when primary particle size is below 400 nm. After calcination, framework Fe species were found partially migrated to extra-framework positions. By increasing calcination temperatures, a growing fraction of extra-framework Fe species and concurrently a growing variety of types and structures of such species was observed. These newly formed extra-framework Fe species seem to be just responsible for the decomposition of H2O2 and over-oxidation of the oxygenates. Similarly, Brønsted acid sites also seem only to enhance the decomposition of H2O2 and over-oxidation of oxygenates. The formation of Fe sites responsible for selective methane oxidation were already completed upon removal of the template at a calcination temperature of 370 °C.