Aqueous‐Phase Selective Oxidation of Methane with Oxygen over Iron Salts and Pd/C in the Presence of Hydrogen

Abstract

Direct conversion of methane into value‐added chemicals is a challenging but worthwhile subject. In this communication, the direct conversion of methane into methane oxygenates was achieved in an aqueous solution at room temperature using iron salts and Pd/C as catalysts and hydrogen peroxide as an oxidant. The hydrogen peroxide can be directly added or generated in situ from hydrogen and oxygen. The Pd/C catalyst greatly enhanced the reaction rate together with the iron salts. The effect of some parameters such as reaction temperature, reaction time, pH, acid type, and catalyst amount on the yields of the methane oxygenates was also investigated. When hydrogen peroxide was directly added, the turnover frequency (TOF), defined as the moles of methane oxygenates per moles of Fe per unit time, at 293 K was 29 h−1 at pH=2.3. The TOF at 293 K was 42 h−1 at pH=1.3 with in situ generated hydrogen peroxide from hydrogen and oxygen.

Selective Oxidation of Methane over Fe-Zeolites by In Situ Generated H2O2

“…% Pd/AC under the same reaction conditions as those employed for the heterogeneous catalyst. Similar to the previous results [35], the total product yield increased but the selectivity to the methane oxygenates decreased with increasing concentration of FeSO4 (Table S10). Figure 3 shows that the total product yield was proportional to the amount of homogeneous Fe species in the reaction system.…”

“…Generally, a better catalytic activity implies worse catalyst stability ( Table 1 and Table S2). Further study with Fe-ZSM-5 confirmed that the effect of some reaction variables such as reaction temperature, pH, and amount of catalyst was similar to that observed when the homogeneous Fe catalyst was used, as reported previously [35]. The ICP analysis of the liquid after the heterogeneous reaction conducted under different conditions supported the observation that the catalytic activity is closely related to the amount of leached Fe species.…”

“…The Fe-ZSM-5 catalyst has been reported to oxidize methane with H 2 O 2 in water [25]. On the other hand, FeSO 4 itself is not active for partial oxidation of methane with H 2 O 2 in water as reported in our previous work [35]. This implies that these two catalysts have different reaction mechanisms for the aqueous-phase oxidation of methane.…”

“…A heterogeneous catalyst is preferred over a homogeneous catalyst from a practical point of view as long as both catalysts exhibit similar catalytic performance. Based on the report [35] that a homogenous Fe catalyst was able to oxidize methane in water at low temperatures with H 2 O 2 generated in situ from H 2 and O 2 , we applied heterogeneous Fe-zeolites instead of a homogeneous Fe catalyst in this study. As this reaction is conducted in acidified water, each Fe-zeolite shows different catalytic activity and stability.…”

“…In our previous study, we reported the aqueous-phase partial oxidation of methane using homogeneous Fe catalysts and Pd/C, where H 2 O 2 was generated in situ from H 2 and O 2 [35]. In this study, Fe-zeolites were prepared via an ion-exchange method to immobilize the homogeneous Fe catalysts into the zeolite matrix, and were applied to the liquid-phase partial oxidation of methane.…”