Partial oxidation of methane has been proposed as one of methods in the direct conversion of methane which is a major component in natural gas. Especially, liquid-phase selective oxidation of methane has attracted much attention because of its high product selectivity under mild reaction conditions. Until now, various homogeneous and heterogeneous catalyst systems have been reported. However, Fe/ZSM-5 catalyst has its own unique role in this reaction because of its high catalytic activity and similar structure with an active site in the enzyme. In this study, various Fe-based zeolites were prepared, characterized, and evaluated to understand this catalyst system for the aqueous phase selective oxidation of methane with H2O2.
First, Fe-zeolites with different frameworks were prepared with an ion-exchange method and compared in order to find out the role of the zeolite structure in this reaction. Besides ZSM-5, the other zeolites (e.g., ZSM-11, SSZ-57, and SSZ-74) which have t-mel-1 zeolite tiles were found to be active. These different catalytic activities appear to be closely related to the activation of H2O2.
The effect of the catalyst preparation method on the catalytic activity was examined over Fe/ZSM-5 catalysts. Various preparation methods such as solid-state ion-exchange (SIE), wet impregnation (WI), chemical vapor impregnation (CVI), ion-exchange (IE), and hydrothermal synthesis (HT) were compared. Among them, the ion-exchange method was confirmed to form preferentially Fe species in the extra-framework which is active site for this reaction.
The effect of the Fe precursor on the catalytic activity was also investigated over Fe/ZSM-5 catalysts. The divalent Fe precursors were better than the trivalent Fe precursors in terms of the catalytic activity. The Fe species in the extra-framework and inactive iron oxides were observed in the Fe/ZSM-5 catalysts prepared from divalent Fe and trivalent Fe precursors, respectively.
Finally, the promotional effect of Cu was examined over Cu-Fe/ZSM-5 catalysts. The preferential formation of methanol was confirmed over Cu-Fe/ZSM-5 catalyst. However, the instability of this catalyst system was observed due to Cu leaching.
